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Author name code: aschwanden
ADS astronomy entries on 2022-09-14
author:"Aschwanden, Markus J."
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Title: Interface Region Imaging Spectrograph (IRIS) Observations of
the Fractal Dimension in the Solar Atmosphere
Authors: Aschwanden, Markus J.; Vilangot Nhalil, Nived
2022arXiv220712894A Altcode:
While previous work explored the fractality and self-organized
criticality (SOC) of flares and nanoflares in wavelengths emitted in the
solar corona (such as in hard X-rays, soft X-rays, and EUV wavelenghts),
we focus here on impulsive phenomena in the photosphere and transition
region, as observed with the {\sl Interface Region Imaging Spectrograph
(IRIS)} in the temperature range of $T_e \approx 10^4-10^6$ K. We find
the following fractal dimensions (in increasing order): $D_A=1.21 \pm
0.07$ for photospheric granulation, $D_A=1.29 \pm 0.15$ for plages
in the transition region, $D_A=1.54 \pm 0.16$ for sunspots in the
transition region, $D_A=1.59 \pm 0.08$ for magnetograms in active
regions, $D_A=1.56 \pm 0.08$ for EUV nanoflares, $D_A=1.76 \pm 0.14$
for large solar flares, and up to $D_A=1.89 \pm 0.05$ for the largest
X-class flares. We interpret low values of the fractal dimension
($1.0 \lapprox D_A \lapprox 1.5$) in terms of sparse curvi-linear flow
patterns, while high values of the fractal dimension ($1.5 \lapprox
D_A \lapprox 2.0$) indicate near space-filling transport processes,
such as chromospheric evaporation. Phenomena in the solar transition
region appear to be consistent with SOC models, based on their size
distributions of fractal areas $A$ and (radiative) energies $E$,
which show power law slopes of $\alpha_A^{obs}=2.51 \pm 0.21$ (with
$\alpha_A^{theo}=2.33$ predicted), and $\alpha_E^{obs}=2.03 \pm 0.18$
(with $\alpha_E^{theo}=1.80$ predicted).
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Title: Reconciling Power-law Slopes in Solar Flare and Nanoflare
Size Distributions
Authors: Aschwanden, Markus J.
2022ApJ...934L...3A Altcode: 2022arXiv220312484A
We unify the power laws of size distributions of solar flare and
nanoflare energies. We present three models that predict the power-law
slopes α <SUB> E </SUB> of flare energies defined in terms of the
2D and 3D fractal dimensions (D <SUB> A </SUB>, D <SUB> V </SUB>):
(i) the spatiotemporal standard self-organized criticality model,
defined by the power-law slope α <SUB> E1</SUB> =1 + 2/(D <SUB>
V </SUB> + 2) = (13/9) ≍ 1.44; (ii) the 2D thermal energy model,
α <SUB> E2</SUB> = 1 + 2/D <SUB> A </SUB> = (7/3) ≍ 2.33; and
(iii) the 3D thermal energy model, α <SUB> E3</SUB> = 1 + 2/D <SUB>
V </SUB> = (9/5) ≍ 1.80. The theoretical predictions of energies
are consistent with the observational values of these three groups,
i.e., α <SUB> E1</SUB> = 1.47 ± 0.07, α <SUB> E2</SUB> = 2.38 ±
0.09, and α <SUB> E3</SUB> = 1.80 ± 0.18. These results corroborate
that the energy of nanoflares does not diverge at small energies,
since (α <SUB> E1</SUB> < 2) and (α <SUB> E3</SUB> < 2),
except for the 2D model (α <SUB> E2</SUB> > 2). Thus, while this
conclusion does not support nanoflare scenarios of coronal heating
from a dimensionality point of view, magnetic reconnection processes
with quasi-1D or quasi-2D current sheets cannot be ruled out.
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Title: The Fractality and Size Distributions of Astrophysical
Self-Organized Criticality Systems
Authors: Aschwanden, Markus J.
2022ApJ...934...33A Altcode:
The statistics of nonlinear processes in avalanching systems, based
on the self-organized criticality (SOC) concept of Bak et al. (1988),
predicts power-law-like size (or occurrence frequency) distribution
functions. Following up on previous work, we define a standard SOC
model in terms of six assumptions: (i) area fractality, (ii) volume
fractality, (iii) the flux-volume proportionality, (iv) classical
diffusion, (v) the Euclidean maximum at the event peak time, and (vi)
the spatiotemporal fluence or energy of an avalanche event. We gather
data of the fractal dimension and power-law slopes from 162 publications
and assemble them in 28 groups (for instance, solar flare energies, or
stellar flare energies), from which we find that 75% of the groups are
consistent with the standard SOC model. Alternative SOC models (Lévy
flight, flat-world, nonfractal) are slightly less correlated with the
data. Outliers are attributed to small number statistics, background
definition problems, inadequate fitting ranges, and deviations from
ideal power laws.
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Title: New insight into the nature and origin of switchbacks thanks
to a comprehensive catalogue of events
Authors: Dudok de Wit, Thierry; Aschwanden, Markus; Bale, Stuart;
Froment, Clara; Krasnoselskikh, Vladimir; Larosa, Andrea; MacDowall,
Robert; Raouafi, Nour
2021AGUFMSH44B..05D Altcode:
One of the most intriguing observations made by Parker Solar Probe
is the omnipresence of sudden deflections of the magnetic field,
called switchbacks or jets. One of the pathways towards understanding
the nature and the origin of these structures consists in studying
their statistical properties. This can be pursued only if we have a
means for detecting and extracting each individual switchback. Here
we provide a robust technique that allows to automatically detect
and identify switchbacks based on their sudden deflection from the
Parker spiral. This allows us to build a comprehensive catalogue,
with thousands of events per solar encounter. Most importantly,
this catalogue does not only include full reversals, but also smaller
deflections, whose properties have been overlooked so far while they
are essential for building a complete picture. Using this catalogue
we provide new constraints on the origin of these structures and their
radial evolution.
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Title: Global energetics of solar flares. XIII. The Neupert effect
and acceleration of coronal mass ejections
Authors: Aschwanden, Markus J.
2021arXiv211207759A Altcode:
Our major aim is a height-time model $r(t)$ of the propagation
of {\sl Coronal Mass Ejections (CMEs)}, where the lower corona is
self-consistently connected to the heliospheric path. We accomplish this
task by using the Neupert effect to derive the peak time, duration,
and rate of the CME acceleration phase, as obtained from the time
derivative of the {\sl soft X-ray (SXR)} light curve. This novel
approach offers the advantage to obtain the kinematics of the CME
height-time profile $r(t)$, the CME velocity profile $v(t)=dr(t)/dt$,
and the CME acceleration profile $a(t)=dv(t)/dt$ from {\sl Geostationary
Orbiting Earth Satellite (GOES)} and white-light data, without the
need of {\sl hard X-ray (HXR)} data. We apply this technique to a
data set of 576 (GOES X and M-class) flare events observed with GOES
and the {\sl Large Angle Solar Coronagraph (LASCO)}. Our analysis
yields acceleration rates in the range of $a_A = 0.1-13$ km s$^{-2}$,
acceleration durations of $\tau_A = 1.2-45$ min, and acceleration
distances in the range of $d_A = 3-1063$ Mm, with a median of $d_A=39$
Mm, which corresponds to the hydrostatic scale height of a corona with
a temperature of $T_e \approx 0.8$ MK. The results are consistent
with standard flare/CME models that predict magnetic reconnection
and synchronized (primary) acceleration of CMEs in the low corona
(at a height of ~0.1 R_sun), while secondary (weaker) acceleration
may occur further out at heliospheric distances.
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Title: Memory and Scaling Laws in the Dynamics of Solar Flares
Authors: Johnson, Jay; Nurhan, Yosia; Aschwanden, Markus; Homan,
Jonathan; Ccopa Rivera, Elmer Alberto; Wing, Simon
2021AGUFMSH25E2131J Altcode:
The solar magnetic activity cycle provides energy input that is
released in intense bursts of radiation known as solar flares. As
such, the dynamics of the activity cycle is embedded in the sequence
of times between the flare events. Recent analysis [Snelling et al.,
2020, Ashwanden and Johnson, 2021] shows that solar flares exhibit
memory on different timescales. Information theory analysis shows
that the time ordering of flare events is not random, but rather
there is dependence between successive flares. The increased mutual
information results from the clustering of flares, which we demonstrate
by comparing the cumulative distribution function of successive flares
with the cumulative distribution function of surrogate sequences of
flares obtained by random permutation of flares within rate-variable
Bayesian blocks during which it is assumed that the flare rate is
constant. Differences between the cumulative distribution functions
is substantial on a timescale around 3 hours, suggesting that flare
recurrence on that timescale is more likely than would be expected if
the waiting time were drawn from a nonstationary Poisson process. At
longer waiting times, the waiting time distribution of flares exhibits
a power law form. The power laws also reveal memory in the nonlinear
time dependence of the flaring rate. We discuss how time variability in
the underlying driver of flares leads to power laws, and in particular
discuss how sinusoidal or impulsive driving affects the waiting time
distribution of flares.
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Title: The Poissonian Origin of Power Laws in Solar Flare Waiting
Time Distributions
Authors: Aschwanden, Markus J.; Johnson, Jay R.; Nurhan, Yosia I.
2021ApJ...921..166A Altcode: 2021arXiv210713065A
In this study we aim for a deeper understanding of the power-law slope,
α, of waiting time distributions. Statistically independent events
with linear behavior can be characterized by binomial, Gaussian,
exponential, or Poissonian size distribution functions. In contrast,
physical processes with nonlinear behavior exhibit spatiotemporal
coherence (or memory) and "fat tails" in their size distributions that
fit power-law-like functions, as a consequence of the time variability
of the mean event rate, as demonstrated by means of Bayesian block
decomposition in the work of Wheatland et al. In this study we conduct
numerical simulations of waiting time distributions N(τ) in a large
parameter space for various (polynomial, sinusoidal, Gaussian) event
rate functions λ(t), parameterized with an exponent p that expresses
the degree of the polynomial function λ(t) ∝ t<SUP>p</SUP>. We
derive an analytical exact solution of the waiting time distribution
function in terms of the incomplete gamma function, which is similar
to a Pareto type II function and has a power-law slope of α = 2 +
1/p, in the asymptotic limit of large waiting times. Numerically
simulated random distributions reproduce this theoretical prediction
accurately. Numerical simulations in the nonlinear regime (p ≥ 2)
predict power-law slopes in the range of 2.0 ≤ α ≤ 2.5. The
self-organized criticality model yields a prediction of α =
2. Observations of solar flares and coronal mass ejections (over at
least a half solar cycle) are found in the range of α<SUB>obs</SUB>
≍ 2.1-2.4. Deviations from strict power-law functions are expected
due to the variability of the flare event rate λ(t), and deviations
from theoretically predicted slope values α occur due to the Poissonian
weighting bias of power-law fits.
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Title: The Solar Memory from Hours to Decades
Authors: Aschwanden, Markus J.; Johnson, Jay R.
2021ApJ...921...82A Altcode: 2021arXiv210713621A
Waiting-time distributions allow us to distinguish at least
three different types of dynamical systems, including (i) linear
random processes (with no memory); (ii) nonlinear, avalanche-type,
nonstationary Poisson processes (with memory during the exponential
growth of the avalanche rise time); and (iii) chaotic systems in the
state of a nonlinear limit cycle (with memory during the oscillatory
phase). We describe the temporal evolution of the flare rate λ(t)
∝ t<SUP>p</SUP> with a polynomial function, which allows us to
distinguish linear (p ≍ 1) from nonlinear (p ≳ 2) events. The
power-law slopes α of the observed waiting times (with full solar
cycle coverage) cover a range of α = 2.1-2.4, which agrees well with
our prediction of α = 2.0 + 1/p = 2.3-2.6. The memory time can also be
defined with the time evolution of the logistic equation, for which we
find a relationship between the nonlinear growth time τ<SUB>G</SUB>
= τ<SUB>rise</SUB>/(4p) and the nonlinearity index p. We find a
nonlinear evolution for most events, in particular for the clustering
of solar flares (p = 2.2 ± 0.1), partially occulted flare events (p =
1.8 ± 0.2), and the solar dynamo (p = 2.8 ± 0.5). The Sun exhibits
memory on timescales of ≲2 hr to 3 days (for solar flare clustering),
6-23 days (for partially occulted flare events), and 1.5 month to 1 yr
(for the rise time of the solar dynamo).
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Title: Role of the Solar Minimum in the Waiting Time Distribution
Throughout the Heliosphere
Authors: Nurhan, Yosia I.; Johnson, Jay R.; Homan, Jonathan R.; Wing,
Simon; Aschwanden, Markus J.
2021GeoRL..4894348N Altcode: 2021arXiv210505939N
Many processes throughout the heliosphere such as flares, coronal
mass ejections (CMEs), storms and substorms have abrupt onsets. The
waiting time between these onsets provides key insights as to the
underlying dynamical processes. We explore the tail of these waiting
time distributions (WTDs) in the context of random processes driven by
the solar magnetic activity cycle, which we approximate by a sinusoidal
driver. Analytically, we find that the distribution of large waiting
times of such a process approaches a power law slope of −2.5,
which is primarily controlled by the conditions when the driving is
minimum. We find that the asymptotic behavior of WTDs of solar flares,
CMEs, geomagnetic storms, and substorms exhibit power laws that are in
reasonable agreement with a sinusoidally driven nonstationary Poisson
process. However, the WTD of substorms during solar minimum may be
more consistent with prolonged periods of weak driving followed by
abrupt increase in the rate.
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Title: Correlation of the Sunspot Number and the Waiting-time
Distribution of Solar Flares, Coronal Mass Ejections, and Solar Wind
Switchback Events Observed with the Parker Solar Probe
Authors: Aschwanden, Markus J.; Dudok de Wit, Thierry
2021ApJ...912...94A Altcode: 2021arXiv210202305A
Waiting-time distributions of solar flares and coronal mass ejections
(CMEs) exhibit power-law-like distribution functions with slopes in
the range of α<SUB>τ</SUB> ≍ 1.4-3.2, as observed in annual data
sets during four solar cycles (1974-2012). We find a close correlation
between the waiting-time power-law slope α<SUB>τ</SUB> and the
sunspot number (SN), i.e., α<SUB>τ</SUB> = 1.38 + 0.01 × SN. The
waiting-time distribution can be fitted with a Pareto-type function of
the form N(τ) = N<SUB>0</SUB> ${({\tau }_{0}+\tau )}^{-{\alpha }_{\tau
}}$ , where the offset τ<SUB>0</SUB> depends on the instrumental
sensitivity, the detection threshold of events, and pulse pileup
effects. The time-dependent power-law slope α<SUB>τ</SUB>(t) of
waiting-time distributions depends only on the global solar magnetic
flux (quantified by the sunspot number) or flaring rate, which is
not predicted by self-organized criticality or magnetohydrodynamic
turbulence models. Power-law slopes of α<SUB>τ</SUB> ≍ 1.2-1.6
were also found in solar wind switchback events, as observed with the
Parker Solar Probe during the solar minimum, while steeper slopes are
predicted during the solar maximum. We find that the annual variability
of switchback events in the heliospheric solar wind and solar flare
and CME rates (originating in the photosphere and lower corona) are
highly correlated.
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Title: VizieR Online Data Catalog: Global energetics of solar
flares. IX. (Aschwanden, 2019)
Authors: Aschwanden, M. J.
2021yCat..18850049A Altcode:
We analyzed the same data set of 173 solar flares presented in PaperI,
which includes all M- and X-class flares observed with the Solar
Dynamics Observatory (SDO) during the first 3.5yr of the mission (2010
June 1 to 2014 January 31). We use the 45s line-of-sight magnetograms
from Helioseismic and Magnetic Imager (HMI/SDO) and make use of all
coronal Extreme Ultraviolet Variability (EUV) channels of Atmospheric
Imaging Assembly (AIA/SDO) (in the six wavelengths 94, 131, 171, 193,
211, 335Å), in the temperature range of T~0.6-16MK. The spatial
resolution is ~1.6" for AIA, and the pixel size of HMI is 0.5". <P
/>(1 data file).
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Title: Finite System-size Effects in Self-organized Criticality
Systems
Authors: Aschwanden, Markus J.
2021ApJ...909...69A Altcode: 2021arXiv210103124A
We explore upper limits for the largest avalanches or catastrophes
in nonlinear energy dissipation systems governed by self-organized
criticality. We generalize the idealized "straight" power-law
size distribution and Pareto distribution functions in order to
accommodate incomplete sampling, limited instrumental sensitivity,
finite system-size effects, and "Black Swan" and "Dragon King"
extreme events. Our findings are as follows. (i) Solar flares show
no finite system-size limits up to L ≲ 200 Mm, but solar flare
durations reveal an upper flare duration limit of ≲6 hr. (ii)
Stellar flares observed with Kepler exhibit inertial ranges of E ≍
10<SUP>34</SUP>-10<SUP>37</SUP> erg, finite system-size ranges of
E ≍ 10<SUP>37</SUP>-10<SUP>38</SUP> erg, and extreme events at E
≍ (1-5) × 10<SUP>38</SUP> erg. (iii) The maximum flare energies
of different spectral type stars (M, K, G, F, A, giants) reveal
a positive correlation with the stellar radius, which indicates a
finite system-size limit imposed by the stellar surface area. Fitting
our finite system-size models to terrestrial data sets (earthquakes,
wildfires, city sizes, blackouts, terrorism, words, surnames, web
links) yields evidence (in half of the cases) for finite system-size
limits and extreme events, which can be modeled with dual power-law
size distributions.
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Title: Self-organized Criticality in Stellar Flares
Authors: Aschwanden, Markus J.; Güdel, Manuel
2021ApJ...910...41A Altcode: 2021arXiv210606490A
Power-law size distributions are the hallmarks of nonlinear
energy dissipation processes governed by self-organized criticality
(SOC). Here we analyze 75 data sets of stellar flare size distributions,
mostly obtained from the Extreme-Ultraviolet Explorer and the Kepler
mission. We aim to answer the following questions for size distributions
of stellar flares. (i) What are the values and uncertainties of
power-law slopes? (ii) Do power-law slopes vary with time? (iii)
Do power-law slopes depend on the stellar spectral type? (iv) Are
they compatible with solar flares? (v) Are they consistent with SOC
models? We find that the observed size distributions of stellar flare
fluences (or energies) exhibit power-law slopes of α<SUB>E</SUB> =
2.09 ± 0.24 for optical data sets observed with Kepler. The observed
power-law slopes do not show much time variability and do not depend
on the stellar spectral type (M, K, G, F, A, giants). In solar
flares, we find that background subtraction lowers the uncorrected
value of α<SUB>E</SUB> = 2.20 ± 0.22 to α<SUB>E</SUB> = 1.57 ±
0.19. Furthermore, most of the stellar flares are temporally not
resolved in low-cadence (30 minutes) Kepler data, which causes an
additional bias. Taking these two biases into account, the stellar
flare data sets are consistent with the theoretical prediction
$N(x)\propto {x}^{-{\alpha }_{x}}$ of SOC models, i.e., α<SUB>E</SUB>
= 1.5. Thus, accurate power-law fits require automated detection of
the inertial range and background subtraction, which can be modeled
with the generalized Pareto distribution, finite-system size effects,
and extreme event outliers.
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Title: VizieR Online Data Catalog: Global energetics of solar
flares. VIII. (Aschwanden+, 2019)
Authors: Aschwanden+; Aschwanden, M. J.; Kontar, E. P.; Jeffrey,
N. L. S.
2020yCat..18810001A Altcode:
We use the same Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
data of 191 flare events as previously analyzed in Paper III, using
the Object Spectral Executive (OSPEX) software. <P />(1 data file).
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Title: Global Energetics of Solar Flares. XII. Energy Scaling Laws
Authors: Aschwanden, Markus J.
2020ApJ...903...23A Altcode: 2020arXiv200704419A
In this study we test 18 versions of five fundamental energy scaling
laws that operate in large solar flares. We express scaling laws
in terms of the magnetic potential field energy E<SUB>p</SUB>,
the mean potential field strength B<SUB>p</SUB>, the free energy
E<SUB>free</SUB>, the dissipated magnetic flare energy E<SUB>diss</SUB>,
the magnetic length scale L, the thermal length scale L<SUB>th</SUB>,
the mean helically twisted flux tube radius R, the sunspot radius r,
the emission-measure-weighted flare temperature T<SUB>e</SUB>, the
electron density n<SUB>e</SUB>, and the total emission measure EM,
measured from a data set of 173 GOES M- and X-class flare events. The
five categories of physical scaling laws include (i) a scaling law of
the potential field energy, (ii) a scaling law for helical twisting,
(iii) a scaling law for Petschek-type magnetic reconnection, (iv) the
Rosner-Tucker-Vaiana scaling law, and (v) the Shibata-Yokoyama scaling
law. We test the self-consistency of these theoretical scaling laws
with observed parameters by requiring two criteria: a cross-correlation
coefficient of CCC > 0.5 between the theoretically predicted scaling
laws and observed values, and a linear regression fit with a slope of
α ≍ 1 within one standard deviation σ. These two criteria enable us
(i) to corroborate some existing (or modified) scaling laws, (ii) to
reject other scaling laws that are not consistent with the observations,
(iii) to probe the dimensionality of flare geometries, and (iv) to
predict various energy parameters based on tested scaling laws.
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Title: Global Energetics of Solar Flares. XI. Flare Magnitude
Predictions of the GOES Class
Authors: Aschwanden, Markus J.
2020ApJ...897...16A Altcode: 2020arXiv200704413A
In this study we determine scaling relationships of observed solar
flares that can be used to predict upper limits of the Geostationary
Orbiting Earth Satellite (GOES)-class magnitude of solar flares. The
flare prediction scheme is based on the scaling of the slowly
varying potential energy E<SUB>p</SUB>(t), which is extrapolated
in time over an interval of Δt ≤ 24 hr. The observed scaling of
the dissipated energy E<SUB>diss</SUB> scales with the potential
field energy as ${E}_{\mathrm{diss}}\propto {E}_{p}^{1.32}$ . In
addition, the observed scaling relationship of the flare volume,
$V\propto {E}_{\mathrm{diss}}^{1.17}$ , the multi-thermal energy,
E<SUB>th</SUB> ∝ V<SUP>0.76</SUP>, the flare emission measure
$(\mathrm{EM})\propto {E}_{\mathrm{th}}^{0.79}$ , the EM-weighted
temperature T<SUB>w</SUB>, and the GOES flux, ${F}_{8}{(t)\propto
{E}_{p}(t)}^{0.92}$ , allows us then to predict an upper limit of
the GOES-class flare magnitude in the extrapolated time window. We
find a good correlation (cross-correlation coefficient (CCC) ≍ 0.7)
between the observed and predicted GOES-class flare magnitudes (in
172 X- and M-class events). This is the first algorithm that employs
observed scaling laws of physical flare parameters to predict GOES
flux upper limits, an important capability that complements previous
flare prediction methods based on machine-learning algorithms used in
space-weather forecasting.
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Title: Global Energetics of Solar Flares. X. Petschek Reconnection
Rate and Alfvén Mach Number of Magnetic Reconnection Outflows
Authors: Aschwanden, Markus J.
2020ApJ...895..134A Altcode: 2020arXiv200704404A
We investigate physical scaling laws for magnetic energy dissipation
in solar flares, in the framework of the Sweet-Parker model and the
Petschek model. We find that the total dissipated magnetic energy
E<SUB>diss</SUB> in a flare depends on the mean magnetic field
component B<SUB>f</SUB> associated with the free energy E<SUB>f</SUB>,
the length scale L of the magnetic area, the hydrostatic density scale
height λ of the solar corona, the Alfvén Mach number M<SUB>A</SUB> =
V<SUB>1</SUB>/V<SUB>A</SUB> (the ratio of the inflow speed V<SUB>1</SUB>
to the Alfvénic outflow speed V<SUB>A</SUB>), and the flare duration
τ<SUB>f</SUB>, I.e., ${E}_{\mathrm{diss}}=(1/4\pi ){B}_{f}^{2}\ L\
\lambda \ {V}_{{\rm{A}}}\ {M}_{{\rm{A}}}\ {\tau }_{f}$ , where the
Alfvén speed depends on the nonpotential field strength B<SUB>np</SUB>
and the mean electron density n<SUB>e</SUB> in the reconnection
outflow. Using MDI/Solar Dynamics Observatory (SDO) and AIA/SDO
observations and 3D magnetic field solutions obtained with the
vertical-current approximation non-linear force-free field code we
measure all physical parameters necessary to test scaling laws, which
represents a new method to measure Alfvén Mach numbers M<SUB>A</SUB>,
the reconnection rate, and the total free energy dissipated in solar
flares.
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Title: Torsional Alfvénic Oscillations Discovered in the Magnetic
Free Energy during Solar Flares
Authors: Aschwanden, Markus J.; Wang, Tongjiang
2020ApJ...891...99A Altcode: 2020arXiv200110103A
We report the discovery of torsional Alfvénic oscillations in solar
flares, which modulate the time evolution of the magnetic free energy
E<SUB>f</SUB>(t), while the magnetic potential energy E<SUB>p</SUB>(t)
is uncorrelated, and the nonpotential energy varies as E<SUB>np</SUB>(t)
= E<SUB>p</SUB> + E<SUB>f</SUB>(t). The mean observed time period of
the torsional oscillations is P<SUB>obs</SUB> = 15.1 ± 3.9 minutes,
the mean field line length is L = 135 ± 35 Mm, and the mean phase
speed is V<SUB>phase</SUB> = 315 ± 120 km s<SUP>-1</SUP>, which we
interpret as torsional Alfvénic waves in flare loops with enhanced
electron densities. Most of the torsional oscillations are found to be
decay-less, but exhibit a positive or negative trend in the evolution of
the free energy, indicating new emerging flux (if positive), magnetic
cancellation, or flare energy dissipation (if negative). The time
evolution of the free energy has been calculated in this study with
the Vertical-current Approximation (Version 4) Non-linear Force-free
Field code, which incorporates automatically detected coronal loops in
the solution and bypasses the non-force-freeness of the photospheric
boundary condition, in contrast to traditional NLFFF codes.
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Title: Nonstationary Fast-driven, Self-organized Criticality in
Solar Flares
Authors: Aschwanden, Markus J.
2019ApJ...887...57A Altcode: 2019arXiv190908673A
The original concept of self-organized criticality, applied to
solar flare statistics, assumed a slow-driven and stationary flaring
rate, which implies timescale separation (between flare durations
and interflare waiting times). The concept reproduces power-law
distributions for flare peak fluxes and durations, but predicts an
exponential waiting time distribution. In contrast to these classical
assumptions, we observe (i) multiple energy dissipation episodes during
most flares, (ii) violation of the principle of timescale separation,
(iii) a fast-driven and nonstationary flaring rate, (iv) a power-law
distribution for waiting times Δt, with a slope of α <SUB>Δt </SUB>
≈ 2.0, as predicted from the universal reciprocality between mean
flaring rates and mean waiting times, and (v) pulses with rise times
and decay times of the dissipated magnetic free energy on timescales
of 12 ± 6 minutes, and up to 13 times in long-duration (≲4 hr)
flares. These results are inconsistent with coronal long-term energy
storage, but require photospheric-chromospheric current injections
into the corona.
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Title: Self-Organized Criticality in Solar and Stellar Flares. Are
There Dragon-King Events ?
Authors: Aschwanden, M. J.
2019AGUFMSH34A..07A Altcode:
We search for outliers in extreme events of statistical size
distributions of astrophysical data sets, motivated by the
Dragon-King hypothesis of Sornette (2009), which suggests that the
most extreme events in a statistical distribution may belong to
a different population, and thus may be generated by a different
physical mechanism, in contrast to the strict power law behavior of
self-organized criticality (SOC) models. Identifying such disparate
outliers is important for space weather predictions. Possible physical
mechanisms to produce such outliers could be generated by sympathetic
flaring. However, we find that Dragon-King events are not common in
solar and stellar flares, identified in 4 out of 25 solar and stellar
flare data sets only. Consequently, small, large, and extreme flares
are essentially scale-free and can be modeled with a single physical
mechanism. In very large data sets (N > 10,000), we find significant
deviations from ideal power laws in almost all data sets. Neverthess,
the fitted power law slopes constrain physcial scaling laws in terms
of flare areas and volumes, which have the highest nonlinearity in
their scaling laws.
---------------------------------------------------------
Title: Global Energetics of Solar Flares and Coronal Mass Ejections
Authors: Aschwanden, Markus J.; Caspi, Amir; Cohen, Christina M. S.;
Holman, Gordon; Jing, Ju; Kretzschmar, Matthieu; Kontar, Eduard
P.; McTiernan, James M.; Mewaldt, Richard A.; O'Flannagain, Aidan;
Richardson, Ian G.; Ryan, Daniel; Warren, Harry P.; Xu, Yan
2019JPhCS1332a2002A Altcode:
We investigate the global energetics and energy closure of various
physical processes that are energetically important in solar flares
and coronal mass ejections (CMEs), which includes: magnetic energies,
thermal energies, nonthermal energies (particle acceleration),
direct and indirect plasma heating processes, kinetic CME energies,
gravitational CME energies, aerodynamic drag of CMEs, solar
energetic particle events, EUV and soft X-ray radiation, white-light,
and bolometric energies. Statistics on these forms of energies is
obtained from 400 GOES M- and X-class events during the first 3.5
years of the Solar Dynamics Observatory (SDO) mission. A primary
test addressed in this study is the closure of the various energies,
such as the equivalence of the dissipated magnetic energies and the
primary dissipated are energies (accelerated particles, direct heating,
CME acceleration), which faciliate the energy of secondary processes
(plasma heating, shock acceleration) and interactions with the solar
wind (aerodynamic drag). Our study demonstrates energy closure in the
statistical average, while individual events may have considerable
uncertainties, requiring improved nonlinear force-free field models,
and particle acceleration models with observationally constrained
low-energy cutoffs.
---------------------------------------------------------
Title: Global Energetics of Solar Flares. IX. Refined Magnetic
Modeling
Authors: Aschwanden, Markus J.
2019ApJ...885...49A Altcode: 2019arXiv190908672A
A more accurate analytical solution of the vertical-current
approximation nonlinear force-free field (VCA3-NLFFF) model is presented
that includes, besides the radial (B <SUB> r </SUB>) and azimuthal
(B <SUB> φ </SUB>) magnetic field components, a poloidal component
({B}<SUB>θ </SUB>\ne 0) as well. This new analytical solution
is of second-order accuracy in the divergence-freeness condition
and of third-order accuracy in the force-freeness condition. We
reanalyze the sample of 173 GOES M- and X-class flares observed
with the Atmospheric Imaging Assembly and Helioseismic and Magnetic
Imager on board the Solar Dynamics Observatory (SDO). The new code
reproduces helically twisted loops with a low winding number below
the kink instability consistently, avoiding unstable, highly twisted
structures of the Gold-Hoyle flux rope type. The magnetic energies
agree within {E}<SUB>VCA</SUB>3}/{E}<SUB>W</SUB>=0.99+/- 0.21 with
the Wiegelmann (W-NLFFF) code. The time evolution of the magnetic
field reveals multiple, intermittent energy buildup and releases
in most flares, contradicting both the Rosner-Vaiana model (with
gradual energy storage in the corona) and the principle of timescale
separation (τ <SUB>flare</SUB> ≪ τ <SUB>storage</SUB>) postulated
in self-organized criticality models. The mean dissipated flare energy
is found to amount to 7% ± 3% of the potential energy, or 60% ± 26%
of the free energy, a result that can be used for predicting flare
magnitudes based on the potential field of active regions.
---------------------------------------------------------
Title: Self-organized Criticality in Solar and Stellar Flares:
Are Extreme Events Scale-free?
Authors: Aschwanden, Markus J.
2019ApJ...880..105A Altcode: 2019arXiv190605840A
We search for outliers in extreme events of statistical size
distributions of astrophysical data sets, motivated by the Dragon-King
hypothesis of Sornette, which suggests that the most extreme events
in a statistical distribution may belong to a different population,
and thus may be generated by a different physical mechanism,
in contrast to the strict power-law behavior of self-organized
criticality models. Identifying such disparate outliers is important
for space weather predictions. Possible physical mechanisms to produce
such outliers could be generated by sympathetic flaring. However,
we find that Dragon-King events are not common in solar and stellar
flares, identified in 4 out of 25 solar and stellar flare data sets
only. Consequently, small, large, and extreme flares are essentially
scale-free and can be modeled with a single physical mechanism. In very
large data sets (N ≳ 10<SUP>4</SUP>) we find significant deviations
from ideal power laws in almost all data sets. Nevertheless, the fitted
power-law slopes constrain physical scaling laws in terms of flare areas
and volumes, which have the highest nonlinearity in their scaling laws.
---------------------------------------------------------
Title: Global Energetics of Solar Flares. VIII. The Low-energy Cutoff
Authors: Aschwanden, Markus J.; Kontar, Eduard P.; Jeffrey, Natasha
L. S.
2019ApJ...881....1A Altcode: 2019arXiv190605835A
One of the key problems in solar flare physics is the determination
of the low-energy cut-off: the value that determines the energy of
nonthermal electrons and hence flare energetics. We discuss different
approaches to determine the low-energy cut-off in the spectrum of
accelerated electrons: (i) the total electron number model, (ii) the
time-of-flight model (based on the equivalence of the time-of-flight
and the collisional deflection time), (iii) the warm target model of
Kontar et al., and (iv) the model of the spectral cross-over between
thermal and nonthermal components. We find that the first three models
are consistent with a low-energy cutoff with a mean value of ≈10 keV,
while the cross-over model provides an upper limit for the low-energy
cutoff with a mean value of ≈21 keV. Combining the first three models
we find that the ratio of the nonthermal energy to the dissipated
magnetic energy in solar flares has a mean value of q <SUB> E </SUB>
= 0.57 ± 0.08, which is consistent with an earlier study based on
the simplified approximation of the warm target model alone (q <SUB>
E </SUB> = 0.51 ± 0.17). This study corroborates the self-consistency
between three different low-energy cutoff models in the calculation
of nonthermal flare energies.
---------------------------------------------------------
Title: Global Energetics of Solar Flares. VII. Aerodynamic Drag in
Coronal Mass Ejections
Authors: Aschwanden, Markus J.; Gopalswamy, Nat
2019ApJ...877..149A Altcode: 2019arXiv190605804A
The free energy that is dissipated in a magnetic reconnection process
of a solar flare, generally accompanied by a coronal mass ejection
(CME), has been considered as the ultimate energy source of the global
energy budget of solar flares in previous statistical studies. Here
we explore the effects of the aerodynamic drag force on CMEs, which
supplies additional energy from the slow solar wind to a CME event,
besides the magnetic energy supply. For this purpose, we fit the
analytical aerodynamic drag model of Cargill and Vršnak et al. to
the height-time profiles r(t) of LASCO/SOHO data in 14,316 CME events
observed during the first 8 yr (2010-2017) of the Solar Dynamics
Observatory era (ensuring EUV coverage with AIA). Our main findings
are (1) a mean solar wind speed of w = 472 ± 414 km s<SUP>-1</SUP>,
(2) a maximum drag-accelerated CME energy of E <SUB>drag</SUB> ≲
2 × 10<SUP>32</SUP> erg, (3) a maximum flare-accelerated CME energy
of E <SUB>flare</SUB> ≲ 1.5 × 10<SUP>33</SUP> erg, (4) the ratio
of the summed kinetic energies of all flare-accelerated CMEs to the
drag-accelerated CMEs amounts to a factor of 4, (5) the inclusion
of the drag force slightly lowers the overall energy budget of CME
kinetic energies in flares from ≈7% to ≈4%, and (6) the arrival
times of CMEs at Earth can be predicted with an accuracy of ≈23%.
---------------------------------------------------------
Title: Helical Twisting Number and Braiding Linkage Number of Solar
Coronal Loops
Authors: Aschwanden, Markus J.
2019ApJ...874..131A Altcode: 2019arXiv190210612A
Coronal loops in active regions are often characterized by
quasi-circular and helically twisted (sigmoidal) geometries, which are
consistent with dipolar potential field (PF) models in the former case,
and with nonlinear force-free field models with vertical currents
in the latter case. Alternatively, Parker-type nanoflare models
of the solar corona hypothesize that a braiding mechanism operates
between unresolved loop strands, which is a more complex topological
model. In this study we use the vertical-current approximation of a
nonpotential magnetic field solution (that fulfils the divergence-free
and force-free conditions) to characterize the number of helical
turns N <SUB>twist</SUB> in twisted coronal loops. We measure the
helical twist in 15 active regions observed with Atmospheric Imaging
Assembly and Helioseismic and Magnetic Imager/SDO (Solar Dynamic
Observatory) and find a mean nonpotentiality angle (between the
potential and nonpotential field directions) of μ <SUB>NP</SUB>
= 15° ± 3°. The resulting mean rotational twist angle is φ =
49° ± 11°, which corresponds to N <SUB>twist</SUB> = φ/360° =
0.14 ± 0.03 turns with respect to the untwisted PF, with an absolute
upper limit of N <SUB>twist</SUB> ≲ 0.5, which is far below the kink
instability limit of | {N}<SUB>twist</SUB>}| ≳ 1. The number of twist
turns N <SUB>twist</SUB> corresponds to the Gauss linkage number N
<SUB>link</SUB> in braiding topologies. We conclude that any braided
topology (with | {N}<SUB>link</SUB>}| ≥slant 1) cannot explain the
observed stability of loops in a force-free corona, nor the observed low
twist number. Parker-type nanoflaring can thus occur in non-force-free
environments only, such as in the chromosphere and transition region.
---------------------------------------------------------
Title: New Millennium Solar Physics
Authors: Aschwanden, Markus J.
2019ASSL..458.....A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Minimum Energy Principle Applied to Parker's Coronal
Braiding and Nanoflaring Scenario
Authors: Aschwanden, Markus; van Ballegooijen, A. A.
2018csc..confE..52A Altcode: 2018arXiv180805269A
Parker's coronal braiding and nanoflaring scenario predicts the
development of tangential discontinuities and highly misaligned
magnetic field lines, as a consequence of random buffeting of their
footpoints due to the action of sub-photospheric convection. The
increased stressing of magnetic field lines is thought to become
unstable above some critical misalignment angle and to result into
local magnetic reconnection events, which is generally referred to
as Parker's “nanoflaring scenario”. In this study we show that
the minimum (magnetic) energy principle leads to a bifurcation of
force-free field solutions for helical twist angles at |phi(t)| =
pi, which prevents the build-up of arbitrary large free energies
and misalignment angles. The minimum energy principle predicts that
neighbored magnetic field lines are almost parallel (with misalignment
angles of Delta mu 1.6-1.8 deg, and do not reach a critical misalignment
angle prone to nanoflaring. Consequently, no nanoflares are expected
in the divergence-free and force-free parts of the solar corona, while
they are more likely to occur in the chromosphere and transition region.
---------------------------------------------------------
Title: Convection-driven Generation of Ubiquitous Coronal Waves
Authors: Aschwanden, Markus J.; Gošic, Milan; Hurlburt, Neal E.;
Scullion, Eamon
2018ApJ...866...73A Altcode:
We develop a new method to measure the 3D kinematics of the
subphotospheric motion of magnetic elements, which is used to study
the coupling between the convection-driven vortex motion and the
generation of ubiquitous coronal waves. We use the method of decomposing
a line-of-sight magnetogram from MDI/SDO into unipolar magnetic charges,
which yields the (projected) 2D motion [x(t), y(t)] and the (half) width
evolution w(t) of an emerging magnetic element from an initial depth
of d ≲ 1500 km below the photosphere. A simple model of rotational
vortex motion with magnetic flux conservation during the emergence
process of a magnetic element predicts the width evolution, i.e.,
w(t)/w <SUB>0</SUB> = [B(t)/B <SUB>0</SUB>]<SUP>-1/2</SUP>, and an
upper limit of the depth variation d(t) ≤ 1.3 w(t). While previous
2D tracing of magnetic elements provided information on advection
and superdiffusion, our 3D tracing during the emergence process of a
magnetic element is consistent with a ballistic trajectory in the upward
direction. From the estimated Poynting flux and lifetimes of convective
cells, we conclude that the Coronal Multi-channel Polarimeter-detected
low-amplitude transverse magnetohydrodynamic waves are generated by
the convection-driven vortex motion. Our observational measurements
of magnetic elements appear to contradict the theoretical random-walk
braiding scenario of Parker.
---------------------------------------------------------
Title: Toward a Quantitative Comparison of Magnetic Field
Extrapolations and Observed Coronal Loops
Authors: Warren, Harry P.; Crump, Nicholas A.; Ugarte-Urra, Ignacio;
Sun, Xudong; Aschwanden, Markus J.; Wiegelmann, Thomas
2018ApJ...860...46W Altcode: 2018arXiv180500281W
It is widely believed that loops observed in the solar atmosphere
trace out magnetic field lines. However, the degree to which magnetic
field extrapolations yield field lines that actually do follow loops
has yet to be studied systematically. In this paper, we apply three
different extrapolation techniques—a simple potential model, a
nonlinear force-free (NLFF) model based on photospheric vector data,
and an NLFF model based on forward fitting magnetic sources with
vertical currents—to 15 active regions that span a wide range of
magnetic conditions. We use a distance metric to assess how well each
of these models is able to match field lines to the 12202 loops traced
in coronal images. These distances are typically 1″-2″. We also
compute the misalignment angle between each traced loop and the local
magnetic field vector, and find values of 5°-12°. We find that the
NLFF models generally outperform the potential extrapolation on these
metrics, although the differences between the different extrapolations
are relatively small. The methodology that we employ for this study
suggests a number of ways that both the extrapolations and loop
identification can be improved.
---------------------------------------------------------
Title: Order out of Randomness: Self-Organization Processes in
Astrophysics
Authors: Aschwanden, Markus J.; Scholkmann, Felix; Béthune, William;
Schmutz, Werner; Abramenko, Valentina; Cheung, Mark C. M.; Müller,
Daniel; Benz, Arnold; Chernov, Guennadi; Kritsuk, Alexei G.; Scargle,
Jeffrey D.; Melatos, Andrew; Wagoner, Robert V.; Trimble, Virginia;
Green, William H.
2018SSRv..214...55A Altcode: 2017arXiv170803394A
Self-organization is a property of dissipative nonlinear processes
that are governed by a global driving force and a local positive
feedback mechanism, which creates regular geometric and/or
temporal patterns, and decreases the entropy locally, in contrast
to random processes. Here we investigate for the first time a
comprehensive number of (17) self-organization processes that
operate in planetary physics, solar physics, stellar physics,
galactic physics, and cosmology. Self-organizing systems create
spontaneous " order out of randomness", during the evolution from an
initially disordered system to an ordered quasi-stationary system,
mostly by quasi-periodic limit-cycle dynamics, but also by harmonic
(mechanical or gyromagnetic) resonances. The global driving force
can be due to gravity, electromagnetic forces, mechanical forces
(e.g., rotation or differential rotation), thermal pressure, or
acceleration of nonthermal particles, while the positive feedback
mechanism is often an instability, such as the magneto-rotational
(Balbus-Hawley) instability, the convective (Rayleigh-Bénard)
instability, turbulence, vortex attraction, magnetic reconnection,
plasma condensation, or a loss-cone instability. Physical models
of astrophysical self-organization processes require hydrodynamic,
magneto-hydrodynamic (MHD), plasma, or N-body simulations. Analytical
formulations of self-organizing systems generally involve coupled
differential equations with limit-cycle solutions of the Lotka-Volterra
or Hopf-bifurcation type.
---------------------------------------------------------
Title: Alfvén wave dissipation in the solar chromosphere
Authors: Grant, Samuel D. T.; Jess, David B.; Zaqarashvili, Teimuraz
V.; Beck, Christian; Socas-Navarro, Hector; Aschwanden, Markus J.;
Keys, Peter H.; Christian, Damian J.; Houston, Scott J.; Hewitt,
Rebecca L.
2018NatPh..14..480G Altcode: 2018arXiv181007712G
Magnetohydrodynamic Alfvén waves<SUP>1</SUP> have been a focus of
laboratory plasma physics<SUP>2</SUP> and astrophysics<SUP>3</SUP>
for over half a century. Their unique nature makes them ideal energy
transporters, and while the solar atmosphere provides preferential
conditions for their existence<SUP>4</SUP>, direct detection has proved
difficult as a result of their evolving and dynamic observational
signatures. The viability of Alfvén waves as a heating mechanism relies
upon the efficient dissipation and thermalization of the wave energy,
with direct evidence remaining elusive until now. Here we provide the
first observational evidence of Alfvén waves heating chromospheric
plasma in a sunspot umbra through the formation of shock fronts. The
magnetic field configuration of the shock environment, alongside the
tangential velocity signatures, distinguish them from conventional
umbral flashes<SUP>5</SUP>. Observed local temperature enhancements
of 5% are consistent with the dissipation of mode-converted Alfvén
waves driven by upwardly propagating magneto-acoustic oscillations,
providing an unprecedented insight into the behaviour of Alfvén waves
in the solar atmosphere and beyond.
---------------------------------------------------------
Title: Self-organizing systems in planetary physics: Harmonic
resonances of planet and moon orbits
Authors: Aschwanden, Markus J.
2018NewA...58..107A Altcode: 2017arXiv170108181A
The geometric arrangement of planet and moon orbits into a regularly
spaced pattern of distances is the result of a self-organizing
system. The positive feedback mechanism that operates a self-organizing
system is accomplished by harmonic orbit resonances, leading to
long-term stable planet and moon orbits in solar or stellar systems. The
distance pattern of planets was originally described by the empirical
Titius-Bode law, and by a generalized version with a constant geometric
progression factor (corresponding to logarithmic spacing). We find that
the orbital periods T<SUB>i</SUB> and planet distances R<SUB>i</SUB>
from the Sun are not consistent with logarithmic spacing, but rather
follow the quantized scaling (R<SUB>i + 1</SUB> /R<SUB>i</SUB>)
=<SUP>(T<SUB>i + 1</SUB> /T<SUB>i</SUB>) 2 / 3</SUP> =<SUP>(H<SUB>i
+ 1</SUB> /H<SUB>i</SUB>) 2 / 3</SUP> , where the harmonic ratios
are given by five dominant resonances, namely (H<SUB>i + 1</SUB>
:H<SUB>i</SUB>) =(3 : 2) ,(5 : 3) ,(2 : 1) ,(5 : 2) ,(3 : 1) . We find
that the orbital period ratios tend to follow the quantized harmonic
ratios in increasing order. We apply this harmonic orbit resonance model
to the planets and moons in our solar system, and to the exo-planets of
55 Cnc and HD 10180 planetary systems. The model allows us a prediction
of missing planets in each planetary system, based on the quasi-regular
self-organizing pattern of harmonic orbit resonance zones. We predict 7
(and 4) missing exo-planets around the star 55 Cnc (and HD 10180). The
accuracy of the predicted planet and moon distances amounts to a few
percents. All analyzed systems are found to have ≈ 10 resonant zones
that can be occupied with planets (or moons) in long-term stable orbits.
---------------------------------------------------------
Title: Statistical Properties of Photospheric Magnetic Elements
Observed by the Helioseismic and Magnetic Imager onboard the Solar
Dynamics Observatory
Authors: Javaherian, M.; Safari, H.; Dadashi, N.; Aschwanden, M. J.
2017SoPh..292..164J Altcode:
Magnetic elements of the solar surface are studied (using the 6173
Å Fe I line) in magnetograms recorded with the high-resolution
Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager
(HMI). To extract some statistical and physical properties of these
elements (e.g. filling factors, magnetic flux, size, and lifetimes),
we employed the region-based method called Yet Another Feature Tracking
Algorithm (YAFTA). An area of 400<SUP>″</SUP>×400<SUP>″</SUP>
was selected to investigate the magnetic characteristics in 2011. The
correlation coefficient between filling factors of negative and positive
polarities is 0.51. A broken power-law fit was applied to the frequency
distribution of size and flux. Exponents of the power-law distributions
for sizes smaller and greater than 16 arcsec<SUP>2</SUP> were found
to be −2.24 and −4.04, respectively. The exponents of power-law
distributions for fluxes lower and greater than 2.63 ×10<SUP>19</SUP>Mx
were found to be −2.11 and −2.51, respectively. The relationship
between the size [S ] and flux [F ] of elements can be expressed
by a power-law behavior of the form of S ∝F<SUP>0.69</SUP>. The
lifetime and its relationship with the flux and size of quiet-Sun (QS)
elements during three days were studied. The code detected patches with
lifetimes of about 15 hours, which we call long-duration events. We
found that more than 95% of the magnetic elements have lifetimes shorter
than 100 minutes. About 0.05% of the elements had lifetimes of more
than six hours. The relationships between size [S ], lifetime [T ],
and flux [F ] for patches in the QS yield power-law relationships S
∝T<SUP>0.25</SUP> and F ∝T<SUP>0.38</SUP>, respectively. Executing
a detrended-fluctuation analysis of the time series of new emerged
magnetic elements, we found a Hurst exponent of 0.82, which implies
a long-range temporal correlation in the system.
---------------------------------------------------------
Title: Exoplanet Predictions Based on Harmonic Orbit Resonances
Authors: Aschwanden, Markus; Scholkmann, Felix
2017Galax...5...56A Altcode: 2017arXiv170507138A
The current exoplanet database includes 5454 confirmed and candidate
planets observed with the Kepler mission. We find 932 planet pairs
from which we extract distance and orbital period ratios. While
earlier studies used a logarithmic spacing, which lacks a physical
model, we employ here the theory of harmonic orbit resonances, which
contains quantized ratios instead, to explain the observed planet
distance ratios and to predict undetected exoplanets. We find that
the most prevailing harmonic ratios are (2:1), (3:2), and (5:3) in
73% of the cases, while alternative harmonic ratios of (5:4), (4:3),
(5:2), and (3:1) occur in the other 27% of the cases. Our orbital
predictions include 171 exoplanets, 2 Jupiter moons, 1 Saturn moon, 3
Uranus moons, and 4 Neptune moons. The accuracy of the predicted planet
distances amounts to a few percent, which fits the data significantly
better than the logarithmic spacing. This information may be useful
for targeted exoplanet searches with Kepler data and to estimate the
number of live-carrying planets in habitable zones.
---------------------------------------------------------
Title: Global Energetics of Solar Flares. VI. Refined Energetics of
Coronal Mass Ejections
Authors: Aschwanden, Markus J.
2017ApJ...847...27A Altcode: 2017arXiv170401993A
In this study, we refine the coronal mass ejection (CME) model
that was presented in an earlier study of the global energetics of
solar flares and associated CMEs and apply it to all (860) GOES M-
and X-class flare events observed during the first seven years
(2010-2016) of the Solar Dynamics Observatory (SDO) mission. The
model refinements include (1) the CME geometry in terms of a 3D volume
undergoing self-similar adiabatic expansion, (2) the solar gravitational
deceleration during the propagation of the CME, which discriminates
between eruptive and confined CMEs, (3) a self-consistent relationship
between the CME center-of-mass motion detected during EUV dimming
and the leading-edge motion observed in white-light coronagraphs,
(4) the equipartition of the CME’s kinetic and thermal energies,
and (5) the Rosner-Tucker-Vaiana scaling law. The refined CME model is
entirely based on EUV-dimming observations (using Atmospheric Imager
Assembly (AIA)/SDO data) and complements the traditional white-light
scattering model (using Large-Angle and Spectrometric Coronagraph
Experiment (LASCO)/Solar and Heliospheric Observatory data), and
both models are independently capable of determining fundamental
CME parameters. Comparing the two methods, we find that (1) LASCO
is less sensitive than AIA in detecting CMEs (in 24% of the cases),
(2) CME masses below {m}<SUB>{cme</SUB>}≲ {10}<SUP>14</SUP> g are
underestimated by LASCO, (3) AIA and LASCO masses, speeds, and energies
agree closely in the statistical mean after the elimination of outliers,
and (4) the CME parameters speed v, emission measure-weighted flare peak
temperature T <SUB> e </SUB>, and length scale L are consistent with the
following scaling laws: v\propto {T}<SUB>e</SUB><SUP>1/2</SUP>, v\propto
{({m}<SUB>{cme</SUB>})}<SUP>1/4</SUP>, and {m}<SUB>{cme</SUB>}\propto
{L}<SUP>2</SUP>.
---------------------------------------------------------
Title: Global Energetics in Solar Flares and Coronal Mass Ejections
Authors: Aschwanden, Markus J.
2017SPD....4820002A Altcode:
We present a statistical study of the energetics of coronal mass
ejections (CME) and compare it with the magnetic, thermal, and
nonthermal energy dissipated in flares. The physical parameters of CME
speeds, mass, and kinetic energies are determined with two different
independent methods, i.e., the traditional white-light scattering
method using LASCO/SOHO data, and the EUV dimming method using AIA/SDO
data. We analyze all 860 GOES M- and X-class flare events observed
during the first 7 years (2010-2016) of the SDO mission. The new
ingredients of our CME modeling includes: (1) CME geometry in terms
of a self-similar adiabatic expansion, (2) DEM analysis of CME mass
over entire coronal temperature range, (3) deceleration of CME due to
gravity force which controls the kinetic and potentail CME energy as
a function of time, (4) the critical speed that controls eruptive and
confined CMEs, (5) the relationship between the center-of-mass motion
during EUV dimming and the leading edge motion observed in white-light
coronagraphs. Novel results are: (1) Physical parameters obtained
from both the EUV dimming and white-light method can be reconciled;
(2) the equi-partition of CME kinetic and thermal flare energy; (3)
the Rosner-Tucker-Vaiana scaling law. We find that the two methods
in EUV and white-light wavelengths are highly complementary and yield
more complete models than each method alone.
---------------------------------------------------------
Title: Statistical Properties of Photospheric Magnetic Elements
Observed by SDO/HMI
Authors: Javaherian, Mohsen; Safari, Hossein; Dadashi, Neda;
Aschwanden, Markus Josef
2017arXiv170709291J Altcode:
Magnetic elements of the solar surface are studied in magnetograms
recorded with the high-resolution Solar Dynamics Observatory /
Helioseismic and Magnetic Imager . To extract some statistical
and physical properties of these elements (e.g., filling factors,
magnetic flux, size, lifetimes), the Yet Another Feature Tracking
Algorithm (YAFTA), a region-based method, is employed. An area
with 400$^{\prime\prime}\times$400$^{\prime\prime}$ was selected to
investigate the magnetic characteristics during the year 2011. The
correlation coefficient between filling factors of negative and
positive polarities is 0.51. A broken power law fit was applied to the
frequency distribution of size and flux. Exponents of the power-law
distributions for sizes smaller and greater than 16 arcsec$^2$ were
found to be -2.24 and -4.04, respectively. The exponents of power$-$law
distributions for fluxes smaller and greater than 2.63$\times$10$^{19}$
Mx were found to be -2.11 and -2.51, respectively. The relationship
between the size ($S$) and flux ($F$) of elements can be expressed
by a power-law behavior in the form of $S\propto F~^{0.69}$. The
lifetime and its relationship with the flux and size of quiet-Sun (QS)
elements are studied during three days. The code detected patches with
lifetimes of about 15 hours, which we call long-duration events. It is
found that more than 95\% of the magnetic elements have lifetimes of
less than 100 minutes. About 0.05\% of the elements were found with
lifetimes of more than 6 hours. The relationships between the size
(S), lifetime (T), and the flux (F) for patches in the QS, indicate the
power$-$law relationships $S\propto T~^{0.25}$ and $F\propto T~^{0.38}$,
respectively. Executing a detrended fluctuation analysis of the time
series of new emerged magnetic elements, we find a Hurst exponent of
0.82, which implies long-range temporal correlation in the system.
---------------------------------------------------------
Title: The Width Distribution of Loops and Strands in the Solar
Corona—Are We Hitting Rock Bottom?
Authors: Aschwanden, Markus J.; Peter, Hardi
2017ApJ...840....4A Altcode: 2017arXiv170101177A
In this study, we analyze Atmospheric Imaging Assembly (AIA) and Hi-C
images in order to investigate absolute limits for the finest loop
strands. We develop a model of the occurrence-size distribution function
of coronal loop widths, characterized by the lower limit of widths w
<SUB>min</SUB>, the peak (or most frequent) width w <SUB> p </SUB>, the
peak occurrence number n <SUB> p </SUB>, and a power-law slope a. Our
data analysis includes automated tracing of curvilinear features with
the OCCULT-2 code, automated sampling of the cross-sectional widths of
coronal loops, and fitting of the theoretical size distribution to the
observed distribution. With Monte Carlo simulations and variable pixel
sizes {{Δ }}x, we derive a first diagnostic criterion to discriminate
whether the loop widths are unresolved ({w}<SUB>p</SUB>/{{Δ }}x≈
2.5+/- 0.2) or fully resolved (if {w}<SUB>p</SUB>/{{Δ }}x≳ 2.7). For
images with resolved loop widths, we can apply a second diagnostic
criterion that predicts the lower limit of loop widths as a function
of the spatial resolution. We find that the loop widths are marginally
resolved in AIA images but are fully resolved in Hi-C images, where
our model predicts a most frequent (peak) value at {w}<SUB>p</SUB>≈
550 {km}, in agreement with recent results of Brooks et al. This result
agrees with the statistics of photospheric granulation sizes and thus
supports coronal heating mechanisms operating on the macroscopic scale
of photospheric magneto-convection, rather than nanoflare braiding
models on unresolved microscopic scales.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Global energetics of solar
flares. III. (Aschwanden+, 2016)
Authors: Aschwanden, M. J.; Holman, G.; O'Flannagain, A.; Caspi, A.;
McTiernan, J. M.; Kontar, E. P.
2017yCat..18320027A Altcode:
This study entails the third part of a global flare energetics project,
in which Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data
of 191 M and X-class flare events from the first 3.5yrs of the Solar
Dynamics Observatory mission are analyzed. We fit a thermal and a
nonthermal component to RHESSI spectra, yielding the temperature
of the differential emission measure (DEM) tail, the nonthermal
power-law slope and flux, and the thermal/nonthermal cross-over energy
e<SUB>co</SUB>. From these parameters, we calculate the total nonthermal
energy E<SUB>nt</SUB> in electrons with two different methods: (1)
using the observed cross-over energy e<SUB>co</SUB> as low-energy
cutoff, and (2) using the low-energy cutoff e<SUB>wt</SUB> predicted
by the warm thick-target bremsstrahlung model of Kontar et al. Based
on a mean temperature of T<SUB>e</SUB>=8.6MK in active regions, we
find low-energy cutoff energies of e<SUB>wt</SUB>=6.2+/-1.6keV for the
warm-target model, which is significantly lower than the cross-over
energies e<SUB>co</SUB>=21+/-6keV. Comparing with the statistics
of magnetically dissipated energies E<SUB>mag</SUB> and thermal
energies E<SUB>th</SUB> from the two previous studies, we find the
following mean (logarithmic) energy ratios with the warm-target model:
E<SUB>nt</SUB>=0.41E<SUB>mag</SUB>, E<SUB>th</SUB>=0.08E<SUB>mag</SUB>,
and E<SUB>th</SUB>=0.15E<SUB>nt</SUB>. The total dissipated magnetic
energy exceeds the thermal energy in 95% and the nonthermal energy in
71% of the flare events, which confirms that magnetic reconnection
processes are sufficient to explain flare energies. The nonthermal
energy exceeds the thermal energy in 85% of the events, which largely
confirms the warm thick-target model. <P />(1 data file).
---------------------------------------------------------
Title: Global Energetics of Solar Flares. V. Energy Closure in Flares
and Coronal Mass Ejections
Authors: Aschwanden, Markus J.; Caspi, Amir; Cohen, Christina M. S.;
Holman, Gordon; Jing, Ju; Kretzschmar, Matthieu; Kontar, Eduard
P.; McTiernan, James M.; Mewaldt, Richard A.; O'Flannagain, Aidan;
Richardson, Ian G.; Ryan, Daniel; Warren, Harry P.; Xu, Yan
2017ApJ...836...17A Altcode: 2017arXiv170101176A
In this study we synthesize the results of four previous studies
on the global energetics of solar flares and associated coronal
mass ejections (CMEs), which include magnetic, thermal, nonthermal,
and CME energies in 399 solar M- and X-class flare events observed
during the first 3.5 yr of the Solar Dynamics Observatory (SDO)
mission. Our findings are as follows. (1) The sum of the mean
nonthermal energy of flare-accelerated particles ({E}<SUB>{nt</SUB>}),
the energy of direct heating ({E}<SUB>{dir</SUB>}), and the
energy in CMEs ({E}<SUB>{CME</SUB>}), which are the primary
energy dissipation processes in a flare, is found to have a ratio of
({E}<SUB>{nt</SUB>}+{E}<SUB>{dir</SUB>}+{E}<SUB>{CME</SUB>})/{E}<SUB>{mag</SUB>}=0.87+/-
0.18, compared with the dissipated magnetic free energy
{E}<SUB>{mag</SUB>}, which confirms energy closure within the
measurement uncertainties and corroborates the magnetic origin of
flares and CMEs. (2) The energy partition of the dissipated magnetic
free energy is: 0.51 ± 0.17 in nonthermal energy of ≥slant 6 {keV}
electrons, 0.17 ± 0.17 in nonthermal ≥slant 1 {MeV} ions, 0.07 ±
0.14 in CMEs, and 0.07 ± 0.17 in direct heating. (3) The thermal
energy is almost always less than the nonthermal energy, which is
consistent with the thick-target model. (4) The bolometric luminosity
in white-light flares is comparable to the thermal energy in soft
X-rays (SXR). (5) Solar energetic particle events carry a fraction
≈ 0.03 of the CME energy, which is consistent with CME-driven shock
acceleration. (6) The warm-target model predicts a lower limit of the
low-energy cutoff at {e}<SUB>c</SUB>≈ 6 {keV}, based on the mean peak
temperature of the differential emission measure of T <SUB>e</SUB> =
8.6 MK during flares. This work represents the first statistical study
that establishes energy closure in solar flare/CME events.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Global energetics of solar
flares. IV. CME (Aschwanden, 2016)
Authors: Aschwanden, M. J.
2017yCat..18310105A Altcode:
We analyze observations from AIA/SDO for the 399 flare events of the
primary data set that consists of all solar GOES M- and X-class flare
events observed with Atmospheric Imaging Assembly (AIA)/SDO during
the first 3.5 years of the SDO mission (2010 June-2014 January),
which is identical to those analyzed in Papers I (Aschwanden+,
2014, J/ApJ/797/50), II (Aschwanden+ 2015SoPh..290.2733A) and III
(Aschwanden+, 2016, J/ApJ/832/27) for other forms of energies. <P />(1
data file).
---------------------------------------------------------
Title: Global Energetics of Solar Flares. IV. Coronal Mass Ejection
Energetics
Authors: Aschwanden, Markus J.
2016ApJ...831..105A Altcode: 2016arXiv160504952A
This study entails the fourth part of a global flare energetics project,
in which the mass m <SUB>cme</SUB>, kinetic energy E <SUB>kin</SUB>,
and the gravitational potential energy E <SUB>grav</SUB> of coronal
mass ejections (CMEs) is measured in 399 M and X-class flare events
observed during the first 3.5 years of the Solar Dynamics Observatory
(SDO) mission, using a new method based on the EUV dimming effect. EUV
dimming is modeled in terms of a radial adiabatic expansion process,
which is fitted to the observed evolution of the total emission measure
of the CME source region. The model derives the evolution of the mean
electron density, the emission measure, the bulk plasma expansion
velocity, the mass, and the energy in the CME source region. The EUV
dimming method is truly complementary to the Thomson scattering method
in white light, which probes the CME evolution in the heliosphere
at r ≳ 2 R <SUB>⊙</SUB>, while the EUV dimming method tracks the
CME launch in the corona. We compare the CME parameters obtained in
white light with the LASCO/C2 coronagraph with those obtained from EUV
dimming with the Atmospheric Imaging Assembly onboard the SDO for all
identical events in both data sets. We investigate correlations between
CME parameters, the relative timing with flare parameters, frequency
occurrence distributions, and the energy partition between magnetic,
thermal, nonthermal, and CME energies. CME energies are found to be
systematically lower than the dissipated magnetic energies, which is
consistent with a magnetic origin of CMEs.
---------------------------------------------------------
Title: Global Energetics of Solar Flares. III. Nonthermal Energies
Authors: Aschwanden, Markus J.; Holman, Gordon; O'Flannagain, Aidan;
Caspi, Amir; McTiernan, James M.; Kontar, Eduard P.
2016ApJ...832...27A Altcode: 2016arXiv160706488A
This study entails the third part of a global flare energetics project,
in which Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data
of 191 M and X-class flare events from the first 3.5 years of the
Solar Dynamics Observatory mission are analyzed. We fit a thermal and
a nonthermal component to RHESSI spectra, yielding the temperature
of the differential emission measure (DEM) tail, the nonthermal
power-law slope and flux, and the thermal/nonthermal cross-over energy e
<SUB>co</SUB>. From these parameters, we calculate the total nonthermal
energy E <SUB>nt</SUB> in electrons with two different methods: (1)
using the observed cross-over energy e <SUB>co</SUB> as low-energy
cutoff, and (2) using the low-energy cutoff e <SUB>wt</SUB> predicted by
the warm thick-target bremsstrahlung model of Kontar et al. Based on a
mean temperature of T <SUB> e </SUB> = 8.6 MK in active regions, we find
low-energy cutoff energies of {e}<SUB>{wt</SUB>}=6.2+/- 1.6 {keV} for
the warm-target model, which is significantly lower than the cross-over
energies {e}<SUB>{co</SUB>}=21+/- 6 {keV}. Comparing with the statistics
of magnetically dissipated energies E <SUB>mag</SUB> and thermal
energies E <SUB>th</SUB> from the two previous studies, we find the
following mean (logarithmic) energy ratios with the warm-target model:
{E}<SUB>{nt</SUB>}=0.41 {E}<SUB>{mag</SUB>}, {E}<SUB>{th</SUB>}=0.08
{E}<SUB>{mag</SUB>}, and {E}<SUB>{th</SUB>}=0.15 {E}<SUB>{nt</SUB>}. The
total dissipated magnetic energy exceeds the thermal energy in 95%
and the nonthermal energy in 71% of the flare events, which confirms
that magnetic reconnection processes are sufficient to explain flare
energies. The nonthermal energy exceeds the thermal energy in 85%
of the events, which largely confirms the warm thick-target model.
---------------------------------------------------------
Title: The Global Energetics of Solar Flares and CMEs
Authors: Aschwanden, Markus
2016usc..confE..58A Altcode:
We present statistical results of a global flare energetics project,
in which the mass, the kinetic energy, and the gravitational potential
energy of coronal mass ejections (CMEs) is measured in 399 M and X-class
flare events observed during the first 3.5 yrs of the Solar Dynamics
Observatory (SDO) mission, using a new method based on the EUV dimming
effect. The EUV dimming is modeled in terms of a radial adiabatic
expansion process, which is fitted to the observed evolution of the
total emission measure of the CME source region. The model derives the
evolution of the mean electron density, the emission measure, the bulk
plasma expansion velocity, the mass, and the energy in the CME source
region. The EUV dimming method is truly complementary to the Thomson
scattering method in white light, which probes the CME evolution in
the heliosphere at r >2 R_sun, while the EUV dimming method tracks
the CME launch in the corona. We compare the CME parameters obtained
in white light with the LASCO/C2 coronagraph with those obtained from
EUV dimming with AIA) onboard SDO for all identical events in both data
sets. We investigate correlations between CME parameters, the relative
timing with flare parameters, frequency occurrence distributions,
and the energy partition between magnetic, thermal, non-thermal, and
CME energies. CME energies are found to be systematically lower than
the dissipated magnetic energies, which is consistent with a magnetic
origin of CMEs.
---------------------------------------------------------
Title: Tracing the Chromospheric and Coronal Magnetic Field with AIA,
IRIS, IBIS, and ROSA Data
Authors: Aschwanden, Markus J.; Reardon, Kevin; Jess, Dave B.
2016ApJ...826...61A Altcode: 2016arXiv160202119A
The aim of this study is to explore the suitability of
chromospheric images for magnetic modeling of active regions. We
use high-resolution images (≈ 0\buildrel{\prime\prime}\over{.}
2{--}0\buildrel{\prime\prime}\over{.} 3), from the Interferometric
Bidimensional Spectrometer in the Ca II 8542 Å line, the Rapid
Oscillations in the Solar Atmosphere instrument in the Hα 6563 Å
line, the Interface Region Imaging Spectrograph in the 2796 Å line,
and compare non-potential magnetic field models obtained from those
chromospheric images with those obtained from images of the Atmospheric
Imaging Assembly in coronal (171 Å, etc.) and in chromospheric (304
Å) wavelengths. Curvi-linear structures are automatically traced in
those images with the OCCULT-2 code, to which we forward-fitted magnetic
field lines computed with the Vertical-current Approximation Nonlinear
Force Free Field code. We find that the chromospheric images: (1)
reveal crisp curvi-linear structures (fibrils, loop segments, spicules)
that are extremely well-suited for constraining magnetic modeling; (2)
that these curvi-linear structures are field-aligned with the best-fit
solution by a median misalignment angle of {μ }<SUB>2</SUB>≈ 4^\circ
-7° (3) the free energy computed from coronal data may underestimate
that obtained from cromospheric data by a factor of ≈ 2-4, (4) the
height range of chromospheric features is confined to h≲ 4000 km,
while coronal features are detected up to h = 35,000 km; and (5) the
plasma-β parameter is β ≈ {10}<SUP>-5</SUP>{--}{10}<SUP>-1</SUP>
for all traced features. We conclude that chromospheric images reveal
important magnetic structures that are complementary to coronal
images and need to be included in comprehensive magnetic field models,
something that is currently not accomodated in standard NLFFF codes.
---------------------------------------------------------
Title: The Vertical-current Approximation Nonlinear Force-free Field
Code—Description, Performance Tests, and Measurements of Magnetic
Energies Dissipated in Solar Flares
Authors: Aschwanden, Markus J.
2016ApJS..224...25A Altcode: 2016arXiv160200635A
In this work we provide an updated description of the Vertical-Current
Approximation Nonlinear Force-Free Field (VCA-NLFFF) code, which is
designed to measure the evolution of the potential, non-potential,
free energies, and the dissipated magnetic energies during solar
flares. This code provides a complementary and alternative method
to existing traditional NLFFF codes. The chief advantages of the
VCA-NLFFF code over traditional NLFFF codes are the circumvention of the
unrealistic assumption of a force-free photosphere in the magnetic field
extrapolation method, the capability to minimize the misalignment angles
between observed coronal loops (or chromospheric fibril structures)
and theoretical model field lines, as well as computational speed. In
performance tests of the VCA-NLFFF code, by comparing with the NLFFF
code of Wiegelmann, we find agreement in the potential, non-potential,
and free energy within a factor of ≲ 1.3, but the Wiegelmann code
yields in the average a factor of 2 lower flare energies. The VCA-NLFFF
code is found to detect decreases in flare energies in most X, M, and
C-class flares. The successful detection of energy decreases during a
variety of flares with the VCA-NLFFF code indicates that current-driven
twisting and untwisting of the magnetic field is an adequate model
to quantify the storage of magnetic energies in active regions and
their dissipation during flares. The VCA-NLFFF code is also publicly
available in the Solar SoftWare.
---------------------------------------------------------
Title: 25 Years of Self-organized Criticality: Numerical Detection
Methods
Authors: McAteer, R. T. James; Aschwanden, Markus J.; Dimitropoulou,
Michaila; Georgoulis, Manolis K.; Pruessner, Gunnar; Morales, Laura;
Ireland, Jack; Abramenko, Valentyna
2016SSRv..198..217M Altcode: 2015SSRv..tmp...31M; 2015arXiv150608142M
The detection and characterization of self-organized criticality
(SOC), in both real and simulated data, has undergone many
significant revisions over the past 25 years. The explosive
advances in the many numerical methods available for detecting,
discriminating, and ultimately testing, SOC have played a critical
role in developing our understanding of how systems experience and
exhibit SOC. In this article, methods of detecting SOC are reviewed;
from correlations to complexity to critical quantities. A description
of the basic autocorrelation method leads into a detailed analysis
of application-oriented methods developed in the last 25 years. In
the second half of this manuscript space-based, time-based and
spatial-temporal methods are reviewed and the prevalence of power
laws in nature is described, with an emphasis on event detection and
characterization. The search for numerical methods to clearly and
unambiguously detect SOC in data often leads us outside the comfort
zone of our own disciplines—the answers to these questions are often
obtained by studying the advances made in other fields of study. In
addition, numerical detection methods often provide the optimum link
between simulations and experiments in scientific research. We seek
to explore this boundary where the rubber meets the road, to review
this expanding field of research of numerical detection of SOC systems
over the past 25 years, and to iterate forwards so as to provide some
foresight and guidance into developing breakthroughs in this subject
over the next quarter of a century.
---------------------------------------------------------
Title: 25 Years of Self-Organized Criticality: Solar and Astrophysics
Authors: Aschwanden, Markus J.; Crosby, Norma B.; Dimitropoulou,
Michaila; Georgoulis, Manolis K.; Hergarten, Stefan; McAteer, James;
Milovanov, Alexander V.; Mineshige, Shin; Morales, Laura; Nishizuka,
Naoto; Pruessner, Gunnar; Sanchez, Raul; Sharma, A. Surja; Strugarek,
Antoine; Uritsky, Vadim
2016SSRv..198...47A Altcode: 2014arXiv1403.6528A; 2014SSRv..tmp...29A
Shortly after the seminal paper "Self-Organized Criticality: An
explanation of 1/ f noise" by Bak et al. (1987), the idea has been
applied to solar physics, in "Avalanches and the Distribution of Solar
Flares" by Lu and Hamilton (1991). In the following years, an inspiring
cross-fertilization from complexity theory to solar and astrophysics
took place, where the SOC concept was initially applied to solar flares,
stellar flares, and magnetospheric substorms, and later extended to
the radiation belt, the heliosphere, lunar craters, the asteroid belt,
the Saturn ring, pulsar glitches, soft X-ray repeaters, blazars,
black-hole objects, cosmic rays, and boson clouds. The application
of SOC concepts has been performed by numerical cellular automaton
simulations, by analytical calculations of statistical (powerlaw-like)
distributions based on physical scaling laws, and by observational
tests of theoretically predicted size distributions and waiting
time distributions. Attempts have been undertaken to import physical
models into the numerical SOC toy models, such as the discretization
of magneto-hydrodynamics (MHD) processes. The novel applications
stimulated also vigorous debates about the discrimination between SOC
models, SOC-like, and non-SOC processes, such as phase transitions,
turbulence, random-walk diffusion, percolation, branching processes,
network theory, chaos theory, fractality, multi-scale, and other
complexity phenomena. We review SOC studies from the last 25 years
and highlight new trends, open questions, and future challenges,
as discussed during two recent ISSI workshops on this theme.
---------------------------------------------------------
Title: 25 Years of Self-organized Criticality: Space and Laboratory
Plasmas
Authors: Sharma, A. Surjalal; Aschwanden, Markus J.; Crosby, Norma
B.; Klimas, Alexander J.; Milovanov, Alexander V.; Morales, Laura;
Sanchez, Raul; Uritsky, Vadim
2016SSRv..198..167S Altcode:
Studies of complexity in extended dissipative dynamical systems, in
nature and in laboratory, require multiple approaches and the framework
of self-organized criticality (SOC) has been used extensively in the
studies of such nonequilibrium systems. Plasmas are inherently nonlinear
and many ubiquitous features such as multiscale behavior, intermittency
and turbulence have been analyzed using SOC concepts. The role of
SOC in advancing our understanding of space and laboratory plasmas as
nonequilibrium systems is reviewed in this article. The main emphasis
is on how SOC and related approaches have provided new insights and
models of nonequilibrium plasma phenomena. Among the natural plasmas
the magnetosphere, driven by the solar wind, is a prominent example
and extensive data from ground-based and space-borne instruments have
been used to study phenomena of direct relevance to space weather,
viz. geomagnetic storms and substorms. During geomagnetically
active periods the magnetosphere is far from equilibrium, due
to its internal dynamics and being driven by the turbulent solar
wind, and substorms are prominent features of the complex driven
system. Studies using solar wind and magnetospheric data have shown
both global and multiscale features of substorms. While the global
behavior exhibits system-wide changes, the multiscale behavior shows
scaling features. Along with the studies based on observational data,
analogue models of the magnetosphere have advanced the understanding
of space plasmas as well as the role of SOC in natural systems. In
laboratory systems, SOC has been used in modeling the plasma behavior
in fusion experiments, mainly in tokamaks and stellarators. Tokamaks
are the dominant plasma confinement system and modeling based on SOC
have provided a complementary approach to the understanding of plasma
behavior under fusion conditions. These studies have provided insights
into key features of toroidally confined plasmas, e.g., the existence
of critical temperature gradients above which the transport rates
increase drastically. The SOC models address the transport properties
from a more general approach, compared to those based on turbulence
arising from specific plasma instabilities, and provide a better
framework for modeling features such as superdiffusion. The studies
of space and laboratory plasmas as nonequilibrium systems have been
motivated by features such as scaling and critical behavior, and have
provided new insights by highlighting the properties that are common
with other systems.
---------------------------------------------------------
Title: Preface
Authors: Aschwanden, Markus J.
2016SSRv..198....1A Altcode: 2015SSRv..tmp..112A
No abstract at ADS
---------------------------------------------------------
Title: A Self-Critique of Self-Organized Criticality in Astrophysics
Authors: Aschwanden, Markus J.
2016IAUFM..29B.735A Altcode:
The concept of “self-organized criticality” (SOC) was originally
proposed as an explanation of 1/f-noise by Bak, Tang, and Wiesenfeld
(1987), but turned out to have a far broader significance for scale-free
nonlinear energy dissipation processes occurring in the entire
universe. Over the last 30 years, an inspiring cross-fertilization from
complexity theory to solar and astrophysics took place, where the SOC
concept was initially applied to solar flares, stellar flares, and
magnetospheric substorms, and later extended to the radiation belt,
the heliosphere, lunar craters, the asteroid belt, the Saturn ring,
pulsar glitches, soft X-ray repeaters, blazars, black-hole objects,
cosmic rays, and boson clouds. The application of SOC concepts has been
performed by numerical cellular automaton simulations, by analytical
calculations of statistical (powerlaw-like) distributions based on
physical scaling laws, and by observational tests of theoretically
predicted size distributions and waiting time distributions. Attempts
have been undertaken to import physical models into numerical SOC toy
models. The novel applications stimulated also vigorous debates about
the discrimination between SOC-related and non-SOC processes, such as
phase transitions, turbulence, random-walk diffusion, percolation,
branching processes, network theory, chaos theory, fractality,
multi-scale, and other complexity phenomena. We review SOC models
applied to astrophysical observations, attempt to describe what physics
can be captured by SOC models, and offer a critique of weaknesses and
strengths in existing SOC models.
---------------------------------------------------------
Title: Thresholded Power law Size Distributions of Instabilities
in Astrophysics
Authors: Aschwanden, Markus J.
2015ApJ...814...19A Altcode: 2015arXiv151001987A
Power-law-like size distributions are ubiquitous in astrophysical
instabilities. There are at least four natural effects that cause
deviations from ideal power law size distributions, which we model
here in a generalized way: (1) a physical threshold of an instability;
(2) incomplete sampling of the smallest events below a threshold
x<SUB>0</SUB>; (3) contamination by an event-unrelated background
x<SUB>b</SUB>; and (4) truncation effects at the largest events due to a
finite system size. These effects can be modeled in the simplest terms
with a “thresholded power law” distribution function (also called
generalized Pareto [type II] or Lomax distribution), N(x){dx}\propto
{(x+{x}<SUB>0</SUB>)}<SUP>-a</SUP>{dx}, where x<SUB>0</SUB> >
0 is positive for a threshold effect, while x<SUB>0</SUB> < 0 is
negative for background contamination. We analytically derive the
functional shape of this thresholded power law distribution function
from an exponential growth evolution model, which produces avalanches
only when a disturbance exceeds a critical threshold x<SUB>0</SUB>. We
apply the thresholded power law distribution function to terrestrial,
solar (HXRBS, BATSE, RHESSI), and stellar flare (Kepler) data sets. We
find that the thresholded power law model provides an adequate fit
to most of the observed data. Major advantages of this model are
the automated choice of the power law fitting range, diagnostics of
background contamination, physical instability thresholds, instrumental
detection thresholds, and finite system size limits. When testing
self-organized criticality models that predict ideal power laws,
we suggest including these natural truncation effects.
---------------------------------------------------------
Title: Benchmark Test of Differential Emission Measure Codes and
Multi-thermal Energies in Solar Active Regions
Authors: Aschwanden, Markus J.; Boerner, Paul; Caspi, Amir; McTiernan,
James M.; Ryan, Daniel; Warren, Harry
2015SoPh..290.2733A Altcode: 2015arXiv150907546A; 2015SoPh..tmp..146A
We compare the ability of 11 differential emission measure (DEM)
forward-fitting and inversion methods to constrain the properties
of active regions and solar flares by simulating synthetic data
using the instrumental response functions of the Solar Dynamics
Observatory/Atmospheric Imaging Assembly (SDO/AIA) and EUV Variability
Experiment (SDO/EVE), the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI), and the Geostationary Operational Environmental
Satellite/X-ray Sensor (GOES/XRS). The codes include the single-Gaussian
DEM, a bi-Gaussian DEM, a fixed-Gaussian DEM, a linear spline DEM,
the spatial-synthesis DEM, the Monte-Carlo Markov Chain DEM, the
regularized DEM inversion, the Hinode/X-Ray Telescope (XRT) method, a
polynomial spline DEM, an EVE+GOES, and an EVE+RHESSI method. Averaging
the results from all 11 DEM methods, we find the following accuracies
in the inversion of physical parameters: the EM-weighted temperature
T<SUB>w</SUB><SUP>fit</SUP>/T<SUB>w</SUB><SUP>sim</SUP>=0.9
±0.1 , the peak emission measure
EM<SUB>p</SUB><SUP>fit</SUP>/EM<SUB>p</SUB><SUP>sim</SUP>=0.6
±0.2 , the total emission measure
EM<SUB>t</SUB><SUP>fit</SUP>/EM<SUB>t</SUB><SUP>sim</SUP>=0.8
±0.3 , and the multi-thermal energies
E<SUB>th</SUB><SUP>fit</SUP>/EM<SUB>th</SUB><SUP>approx</SUP>=1.2
±0.4 . We find that the AIA spatial-synthesis, the EVE+GOES, and the
EVE+RHESSI method yield the most accurate results.
---------------------------------------------------------
Title: Blind Stereoscopy of the Coronal Magnetic Field
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Malanushenko,
Anna
2015SoPh..290.2765A Altcode: 2015SoPh..tmp..147A; 2015arXiv150604713A
We test the feasibility of 3D coronal-loop tracing in stereoscopic
EUV image pairs, with the ultimate goal of enabling efficient 3D
reconstruction of the coronal magnetic field that drives flares and
coronal mass ejections (CMEs). We developed an automated code designed
to perform triangulation of coronal loops in pairs (or triplets) of EUV
images recorded from different perspectives. The automated (or blind)
stereoscopy code includes three major tasks: i) automated pattern
recognition of coronal loops in EUV images, ii) automated pairing of
corresponding loop patterns from two different aspect angles, and iii)
stereoscopic triangulation of 3D loop coordinates. We perform tests
with simulated stereoscopic EUV images and quantify the accuracy of
all three procedures. In addition we test the performance of the
blind-stereoscopy code as a function of the spacecraft-separation
angle and as a function of the spatial resolution. We also test the
sensitivity to magnetic non-potentiality. The automated code developed
here can be used for analysis of existing Solar TErrestrial RElationship
Observatory (STEREO) data, but primarily serves for a design study
of a future mission with dedicated diagnostics of non-potential
magnetic fields. For a pixel size of 0.6<SUP>″</SUP> (corresponding
to the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly
(AIA) spatial resolution of 1.4<SUP>″</SUP>), we find an optimum
spacecraft-separation angle of α<SUB>s</SUB>≈5<SUP>∘</SUP>.
---------------------------------------------------------
Title: A Self-Critique of Self-Organized Criticality in Astrophysics
Authors: Aschwanden, Markus J.
2015IAUGA..2255145A Altcode:
The concept of “self-organized criticality” (SOC) was originally
proposed as an explanation of 1/f-noise by Bak, Tang, and Wiesenfeld
(1987), but turned out to have a far broader significance for scale-free
nonlinear energy dissipation processes occurring in the entire
universe. Over the last 30 years, an inspiring cross-fertilization from
complexity theory to solar and astrophysics took place, where the SOC
concept was initially applied to solar flares, stellar flares, and
magnetospheric substorms, and later extended to the radiation belt,
the heliosphere, lunar craters, the asteroid belt, the Saturn ring,
pulsar glitches, soft X-ray repeaters, blazars, black-hole objects,
cosmic rays, and boson clouds. The application of SOC concepts has been
performed by numerical cellular automaton simulations, by analytical
calculations of statistical (powerlaw-like) distributions based on
physical scaling laws, and by observational tests of theoretically
predicted size distributions and waiting time distributions. Attempts
have been undertaken to import physical models into numerical SOC toy
models. The novel applications stimulated also vigorous debates about
the discrimination between SOC-related and non-SOC processes, such as
phase transitions, turbulence, random-walk diffusion, percolation,
branching processes, network theory, chaos theory, fractality,
multi-scale, and other complexity phenomena. We review SOC models
applied to astrophysical observations, attempt to describe what physics
can be captured by SOC models, and offer a critique of weaknesses and
strengths in existing SOC models.
---------------------------------------------------------
Title: VizieR Online Data Catalog: Global energetics of solar
flares. II. (Aschwanden+, 2015)
Authors: Aschwanden, M. J.; Boerner, P.; Ryan, D.; Caspi, A.;
McTiernan, J. M.; Warren, H. P.
2015yCat..18020053A Altcode:
The dataset we are analyzing for this project on the global energetics
of flares includes all M- and X-class flares observed with the Solar
Dynamics Observatory (SDO) during the first 3.5yr of the mission
(2010 June 1 to 2014 January 31), which amounts to 399 flare events,
as described in Paper I (Aschwanden et al. 2014, J/ApJ/797/50). We
attempt to calculate the thermal energies in all 399 cataloged
events, but we encountered eight events with incomplete or corrupted
Atmospheric Imaging Assembly (AIA) data, so that we are left with
391 events suitable for thermal data analysis. <P />AIA provides EUV
images corresponding to an effective spatial resolution of ~1.6". <P
/>(1 data file).
---------------------------------------------------------
Title: VizieR Online Data Catalog: Global energetics of solar
flares. I. (Aschwanden+, 2014)
Authors: Aschwanden, M. J.; Xu, Y.; Jing, J.
2015yCat..17970050A Altcode:
The data set we are analyzing for this project on the global energetics
of flares includes all M- and X-class flares observed with the Solar
Dynamics Observatory (SDO) during the first 3.5yr of the mission (2010
June 1 to 2014 January 31), which amounts to 399 flare events. Magnetic
energies are determined for events that have a heliographic longitude
of <~45° (177 events), of which 5 events contained incomplete or
corrupted Atmospheric Imaging Assembly (AIA) data, so that we are left
with 172 events suitable for magnetic data analysis. <P />The analyzed
SDO data set includes EUV images observed with the AIA, as well as
magnetograms from the Helioseismic and Magnetic Imager (HMI). The SDO
started observations on 2010 March 29 and has produced essentially
continuous data of the full Sun since then. <P />(1 data file).
---------------------------------------------------------
Title: Magnetic Energy Dissipation during the 2014 March 29 Solar
Flare
Authors: Aschwanden, Markus J.
2015ApJ...804L..20A Altcode: 2015arXiv150403301A
We calculated the time evolution of the free magnetic energy during
the 2014 March 29 flare (SOL2014 March 29T17:48), the first X-class
flare detected by the Interface Region Imaging Spectrograph (IRIS). The
free energy was calculated from the difference between the nonpotential
field, constrained by the geometry of observed loop structures, and the
potential field. We use Atmospheric Imager Assembly (AIA)/SDO and IRIS
images to delineate the geometry of coronal loops in EUV wavelengths,
as well as to trace magnetic field directions in UV wavelengths in the
chromosphere and transition region. We find an identical evolution
of the free energy for both the coronal and chromospheric tracers,
as well as agreement between AIA and IRIS results, with a peak
free energy of {{E}<SUB>free</SUB>}({{t}<SUB>peak</SUB>})≈ (45+/-
2)× {{10}<SUP>30</SUP>} erg, which decreases by an amount of {Δ
}{{E}<SUB>free</SUB>}≈ (29+/- 3)× {{10}<SUP>30</SUP>} erg during
the flare decay phase. The consistency of free energies measured from
different EUV and UV wavelengths for the first time here, demonstrates
that vertical electric currents (manifested in form of helically
twisted loops) can be detected and measured from both chromospheric
and coronal tracers.
---------------------------------------------------------
Title: Multi-thermal Energies of Solar Flares
Authors: Ryan, Daniel; Aschwanden, Markus; Boerner, Paul; Caspi,
Amir; McTiernan, James; Warren, Harry
2015TESS....130215R Altcode:
Measuring energy partition in solar eruptions is key to understanding
how different processes affect their evolution. In order to improve
our knowledge on this topic, we are participating in a multi-study
project to measure the energy partition of 400 M- and X-class flares
and associated coronal mass ejections (CMEs). In this study we focus
on the flare thermal energies of 391 of these events. We improve upon
previous studies in the following ways: 1) We determine thermal energy
using spatially resolved multi-thermal differential emission measures
(DEMs) determined from AIA (Atmospheric Imaging Assembly) rather than
relying on the isothermal assumption; 2) We determine flare volumes
by thresholding these DEM maps rather than relying on single passband
observations which may not show the full flare volume; 3) We analyze
a greater number of events than previous similar studies to increase
the statistical reliability of our results. We find that the thermal
energies of these flares lie in the range 10^26.8—10^32 erg. These
results are compared to those of Aschwanden et al. (2014) who examined
a subset of these events. They determined the dissipated non-potential
magnetic energy which is thought to be the total energy available
to drive solar eruptions. For the 171 events common to both studies,
we find that the ratio of flare thermal energy to dissipated magnetic
energy ranges from 2%—40%. This is an order of magnitude higher than
previously found by Emslie et al. (2012). This may be because Emslie et
al. (2012) had to assume the amount of non-potential magnetic energy,
or that they relied on the isothermal assumption to determine flare
thermal energies. The improved results found here will help us better
understand the role played by flare thermal processes in dissipating
the overall energy of solar eruptions.
---------------------------------------------------------
Title: Magnetic and Hydrodynamic Energy Scaling Laws in Solar Flares
Authors: Aschwanden, Markus; Boerner, Paul; Xu, Yan; Ju, Jing; Ryan,
Dan; Caspi, Amir; McTiernan, James; Warren, Harry
2015TESS....140603A Altcode:
We determine the dissipated non-potential magnetic energy and measure
the multi-thermal energy in a sample of about 400 M and X-class
flares observed with AIA and HMI during the first 4 years of the SDO
mission. The free energy is determined with two nonlinear force-free
field (NLFFF) models, one is based on the 3D vectorphotospheric magnetic
field and the other uses forward-fitting of a vertical-current model to
automatically traced coronal loops.The multi-thermal energy is measured
with a spatial-synthesis differential emission measure (DEM) code,
which yields a more comprehensive multi-thermal energy (being larger
by an averagefactor of 14) than iso-thermal estimates. We show how the
correlations and powerlaw-like size distributions of energies and other
geometrical and physical parameters reveal magnetic and hydrodynamic
scaling lawsthat are in agreement with recent statistical models of
nonlinear dissipative systems governed by self-organized criticality.
---------------------------------------------------------
Title: Global Energetics of Solar Flares: II. Thermal Energies
Authors: Aschwanden, Markus J.; Boerner, Paul; Ryan, Daniel; Caspi,
Amir; McTiernan, James M.; Warren, Harry P.
2015ApJ...802...53A Altcode: 2015arXiv150205941A
We present the second part of a project on the global energetics of
solar flares and coronal mass ejections that includes about 400 M-
and X-class flares observed with the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO) during the
first 3.5 yr of its mission. In this Paper II we compute the
differential emission measure (DEM) distribution functions and
associated multithermal energies, using a spatially-synthesized
Gaussian DEM forward-fitting method. The multithermal DEM function
yields a significantly higher (by an average factor of ≈14),
but more comprehensive (multi-)thermal energy than an isothermal
energy estimate from the same AIA data. We find a statistical
energy ratio of {{E}<SUB>th</SUB>}/{{E}<SUB>diss</SUB>} ≈ 2-40%
between the multithermal energy E<SUB>th</SUB> and the magnetically
dissipated energy E<SUB>diss</SUB>, which is an order of magnitude
higher than the estimates of Emslie et al. 2012. For the analyzed
set of M- and X-class flares we find the following physical
parameter ranges: L={{10}<SUP>8.2</SUP>}{{-10}<SUP>9.7</SUP>}
cm for the length scale of the flare areas,
{{T}<SUB>p</SUB>}={{10}<SUP>5.7</SUP>}{{-10}<SUP>7.4</SUP>}
K for the DEM peak temperature,
{{T}<SUB>w</SUB>}={{10}<SUP>6.8</SUP>}{{-10}<SUP>7.6</SUP>}
K for the emission measure-weighted temperature,
{{n}<SUB>p</SUB>}={{10}<SUP>10.3</SUP>}-{{10}<SUP>11.8</SUP>}
cm<SUP>-3</SUP> for the average electron density,
E{{M}<SUB>p</SUB>}={{10}<SUP>47.3</SUP>}-{{10}<SUP>50.3</SUP>}
cm<SUP>-3</SUP> for the DEM peak emission measure, and
{{E}<SUB>th</SUB>}={{10}<SUP>26.8</SUP>}-{{10}<SUP>32.0</SUP>} erg
for the multithermal energies. The deduced multithermal energies
are consistent with the RTV scaling law {{E}<SUB>th,RTV</SUB>}=7.3×
{{10}<SUP>-10</SUP>} T<SUB>p</SUB><SUP>3</SUP>L<SUB>p</SUB><SUP>2</SUP>,
which predicts extremal values of {{E}<SUB>th,max </SUB>}≈ 1.5×
{{10}<SUP>33</SUP>} erg for the largest flare and {{E}<SUB>th,min
</SUB>}≈ 1× {{10}<SUP>24</SUP>} erg for the smallest coronal
nanoflare. The size distributions of the spatial parameters exhibit
powerlaw tails that are consistent with the predictions of the
fractal-diffusive self-organized criticality model combined with the
RTV scaling law.
---------------------------------------------------------
Title: Magnetic Energies in Solar Active Regions and Flares Calculated
from Automated Coronal Loop Tracing
Authors: Aschwanden, M. J.
2014AGUFMSH13A4073A Altcode:
Magnetic energies contained in solar active regions or dissipated
in flares can now be calculated from coronal images (such as from
AIA/SDO) and line-of-sight magnetograms (such as from HM I/SDO). The
magnetogram provides a potential field solution, while automated
tracing of coronal loops in different EUV wavelengths provide the
misalignment angles between potential field lines and non-potential
field lines. We present an automated code that uses data from AIA and
HMI to calculate the free energy and dissipated energy in solar flares,
based on a nonlinear force-free field approximation in terms of vertical
currents that produce helical twists of coronal loops. We study the
time evolution of free energy and energy dissipation during some 200
solar flares and compare it with the global energetics of flare and CME
energies. The occurrence frequency distributions of dissipated magnetic
energies follow closely the predicted powerlaw distribution functions of
self-organized criticality models. The presented results provide for the
first time statistics on magnetic energies dissipated in solar flares.
---------------------------------------------------------
Title: Global Energetics of Solar Flares. I. Magnetic Energies
Authors: Aschwanden, Markus J.; Xu, Yan; Jing, Ju
2014ApJ...797...50A Altcode: 2014arXiv1410.8013A
We present the first part of a project on the global energetics of
solar flares and coronal mass ejections that includes about 400 M-
and X-class flares observed with Atmospheric Imaging Assembly (AIA)
and Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory (SDO). We calculate the potential (E<SUB>p</SUB> ), the
nonpotential (E <SUB>np</SUB>) or free energies (E <SUB>free</SUB>
= E <SUB>np</SUB> - E<SUB>p</SUB> ), and the flare-dissipated
magnetic energies (E <SUB>diss</SUB>). We calculate these magnetic
parameters using two different NLFFF codes: the COR-NLFFF code uses
the line-of-sight magnetic field component B<SUB>z</SUB> from HMI to
define the potential field, and the two-dimensional (2D) coordinates of
automatically detected coronal loops in six coronal wavelengths from
AIA to measure the helical twist of coronal loops caused by vertical
currents, while the PHOT-NLFFF code extrapolates the photospheric
three-dimensional (3D) vector fields. We find agreement between
the two codes in the measurement of free energies and dissipated
energies within a factor of <~ 3. The size distributions of magnetic
parameters exhibit powerlaw slopes that are approximately consistent
with the fractal-diffusive self-organized criticality model. The
magnetic parameters exhibit scaling laws for the nonpotential energy,
E<SUB>np</SUB> \propto E_p<SUP>1.02</SUP>, for the free energy,
E<SUB>free</SUB> \propto E_p<SUP>1.7</SUP> and E<SUB>free</SUB> \propto
B<SUB>\varphi </SUB><SUP>1.0</SUP> L<SUP>1.5</SUP>, for the dissipated
energy, E<SUB>diss</SUB> \propto E_p<SUP>1.6</SUP> and E<SUB>diss</SUB>
\propto E<SUB>free</SUB><SUP>0.9</SUP>, and the energy dissipation
volume, V \propto E<SUB>diss</SUB><SUP>1.2</SUP>. The potential
energies vary in the range of E<SUB>p</SUB> = 1 × 10<SUP>31</SUP>-4
× 10<SUP>33</SUP> erg, while the free energy has a ratio of E
<SUB>free</SUB>/E<SUB>p</SUB> ≈ 1%-25%. The Poynting flux amounts
to F <SUB>flare</SUB> ≈ 5 × 10<SUP>8</SUP>-10<SUP>10</SUP> erg
cm<SUP>-2</SUP> s<SUP>-1</SUP> during flares, which averages to F
<SUB>AR</SUB> ≈ 6 × 10<SUP>6</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>
during the entire observation period and is comparable with the coronal
heating rate requirement in active regions.
---------------------------------------------------------
Title: Homologous flare-CME events and their metric type II radio
burst association
Authors: Yashiro, S.; Gopalswamy, N.; Mäkelä, P.; Akiyama, S.;
Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Chandra, R.; Manoharan,
P. K.; Mahalakshmi, K.; Dwivedi, V. C.; Jain, R.; Awasthi, A. K.;
Nitta, N. V.; Aschwanden, M. J.; Choudhary, D. P.
2014AdSpR..54.1941Y Altcode:
Active region NOAA 11158 produced many flares during its disk
passage. At least two of these flares can be considered as homologous:
the C6.6 flare at 06:51 UT and C9.4 flare at 12:41 UT on February
14, 2011. Both flares occurred at the same location (eastern edge of
the active region) and have a similar decay of the GOES soft X-ray
light curve. The associated coronal mass ejections (CMEs) were slow
(334 and 337 km/s) and of similar apparent widths (43° and 44°), but
they had different radio signatures. The second event was associated
with a metric type II burst while the first one was not. The COR1
coronagraphs on board the STEREO spacecraft clearly show that the
second CME propagated into the preceding CME that occurred 50 min
before. These observations suggest that CME-CME interaction might be
a key process in exciting the type II radio emission by slow CMEs.
---------------------------------------------------------
Title: Determining the 3D Structure of the Corona Using Vertical
Height Constraints on Observed Active Region Loops
Authors: Gary, G. Allen; Hu, Qiang; Lee, Jong Kwan; Aschwanden,
Markus J.
2014SoPh..289.3703G Altcode: 2014SoPh..tmp...87G
The corona associated with an active region is structured by
high-temperature, magnetically dominated closed and open loops. The
projected 2D geometry of these loops is captured in EUV filtergrams. In
this study using SDO/AIA 171 Å filtergrams, we expand our previous
method to derive the 3D structure of these loops, independent of
heliostereoscopy. We employ an automated loop recognition scheme
(Occult-2) and fit the extracted loops with 2D cubic Bézier
splines. Utilizing SDO/HMI magnetograms, we extrapolate the magnetic
field to obtain simple field models within a rectangular cuboid. Using
these models, we minimize the misalignment angle with respect to
Bézier control points to extend the splines to 3D (Gary, Hu, and Lee
2014). The derived Bézier control points give the 3D structure of
the fitted loops. We demonstrate the process by deriving the position
of 3D coronal loops in three active regions (AR 11117, AR 11158, and
AR 11283). The numerical minimization process converges and produces
3D curves which are consistent with the height of the loop structures
when the active region is seen on the limb. From this we conclude that
the method can be important in both determining estimates of the 3D
magnetic field structure and determining the best magnetic model among
competing advanced magnetohydrodynamics or force-free magnetic-field
computer simulations.
---------------------------------------------------------
Title: The Compatibility of Flare Temperatures Observed with AIA,
GOES, and RHESSI
Authors: Ryan, Daniel F.; O'Flannagain, Aidan M.; Aschwanden, Markus
J.; Gallagher, Peter T.
2014SoPh..289.2547R Altcode: 2014SoPh..tmp...31R; 2014arXiv1401.4098R
We test the compatibility and biases of multi-thermal flare DEM
(differential emission measure) peak temperatures determined with
AIA with those determined by GOES and RHESSI using the isothermal
assumption. In a set of 149 M- and X-class flares observed
during the first two years of the SDO mission, AIA finds DEM peak
temperatures at the time of the peak GOES 1 - 8 Å flux to have
an average of T<SUB>p</SUB>=12.0±2.9 MK and Gaussian DEM widths of
log<SUB>10</SUB>(σ<SUB>T</SUB>)=0.50±0.13. From GOES observations of
the same 149 events, a mean temperature of T<SUB>p</SUB>=15.6±2.4
MK is inferred, which is systematically higher by a factor of
T<SUB>GOES</SUB>/T<SUB>AIA</SUB>=1.4±0.4. We demonstrate that
this discrepancy results from the isothermal assumption in the
inversion of the GOES filter ratio. From isothermal fits to photon
spectra at energies of ϵ≈6 - 12 keV of 61 of these events,
RHESSI finds the temperature to be higher still by a factor of
T<SUB>RHESSI</SUB>/T<SUB>AIA</SUB>=1.9±1.0. We find that this is
partly a consequence of the isothermal assumption. However, RHESSI
is not sensitive to the low-temperature range of the DEM peak,
and thus RHESSI samples only the high-temperature tail of the DEM
function. This can also contribute to the discrepancy between AIA and
RHESSI temperatures. The higher flare temperatures found by GOES and
RHESSI imply correspondingly lower emission measures. We conclude that
self-consistent flare DEM temperatures and emission measures require
simultaneous fitting of EUV (AIA) and soft X-ray (GOES and RHESSI)
fluxes.
---------------------------------------------------------
Title: The Compatibility of Flare Temperatures Observed with AIA,
GOES, and RHESSI
Authors: Ryan, Daniel; Aschwanden, Markus J.; O'Flannagain, Aidan M;
Gallagher, Peter T
2014AAS...22412337R Altcode:
In this talk we compare multi-thermal flare DEM peak temperatures
determined with SDO/AIA with those determined by GOES/XRS and RHESSI
using the isothermal assumption. In a set of 149 M- and X-class flares,
AIA finds an average DEM peak temperature at the time of the GOES long
channel peak of 12.0±2.9 MK and Gaussian DEM widths of log10(σT )
= 0.50±0.13. From GOES observations of the same 149 events, a mean
temperature of 15.6±2.4 MK is inferred, which is higher by a factor
of TGOES/TAIA = 1.4±0.4. We demonstrate that this discrepancy results
from the isothermal assumption in the inversion of the GOES filter
ratio. From isothermal fits to photon spectra at energies of 6-12
keV of 61 of these events, RHESSI finds the temperature to be higher
(TRHESSI/TAIA = 1.9±1.0). We find that this is partly a consequence
of the isothermal assumption. However, RHESSI is not sensitive to
the low-temperature range of the DEM peak, and thus only samples the
DEM’s high-temperature tail. This is expected to be the cause of
further discrepancies. We conclude that self-consistent flare DEM
temperatures require simultaneous fitting of EUV and SXR fluxes.
---------------------------------------------------------
Title: Magnetic Energy Dissipation in 200 Solar Flares Measured
with SDO
Authors: Aschwanden, Markus J.
2014AAS...22412306A Altcode:
We present the first statistical study of magnetic energetics
in solar flares. The amount of dissipated magnetic energy during
solar flares provides the fundamental limit on the flare energy
budget that is partitioned into the kinetic and potential energy of
CMEs, acceleration of nonthermal particles, and radiation in soft
X-rays, EUV, UV, and bolometric luminosity. The determination of
the dissipated magnetic energy requires the calculation of nonlinear
force-free field (NLFFF) solutions during flares,which can quantify the
nonpotential E_N(t), the potential E_P(t), and the free magnetic energy
E_{free}(t)=E_N(t)-E_P(t), which itself represents an upper limit on the
magnetic energy dE_diss that can be dissipated during a flare. Here we
developed a NLFFF forward-fitting code that fits a nonpotential field
in terms of vertical currents with helically twisted field lines to
automatically traced coronal loops from 7 AIA wavelength filters and
apply it to 200 M- and X-class flares that havebeen observed during
the first 4 years of the Solar Dynamics Observatory (SDO) mission. We
cacluate the free energy with a cadence of 6 minutes during all 200
flares, and find significantmagnetic energy decreases dE_diss in almost
all flares, in the order of E_diss ~ 10(31)-10(32) erg, which amounts
to a fraction of dE_diss/E_P ~ 0.01-0.3 of the potential magnetic
energy E_P. We find that the dissipated energy dE_diss cannot simply
be determined by an energy difference before and after the flare,
because the hydrodynamic evolution causes brightenings and dimmings of
helically twisted loops (sigmoids) in the flare core region, which acts
as a time-dependent illumination effect of nonpotential loop structures.
---------------------------------------------------------
Title: The Association of Solar Flares with Coronal Mass Ejections
During the Extended Solar Minimum
Authors: Nitta, N. V.; Aschwanden, M. J.; Freeland, S. L.; Lemen,
J. R.; Wülser, J. -P.; Zarro, D. M.
2014SoPh..289.1257N Altcode: 2013arXiv1308.1465N
We study the association of solar flares with coronal mass ejections
(CMEs) during the deep, extended solar minimum of 2007 - 2009, using
extreme-ultraviolet (EUV) and white-light (coronagraph) images from the
Solar Terrestrial Relations Observatory (STEREO). Although all of the
fast (v>900 km s<SUP>−1</SUP>), wide (θ>100<SUP>∘</SUP>) CMEs
are associated with a flare that is at least identified in GOES soft
X-ray light curves, a majority of flares with relatively high X-ray
intensity for the deep solar minimum (e.g. ≳1×10<SUP>−6</SUP>
W m<SUP>−2</SUP> or C1) are not associated with CMEs. Intense
flares tend to occur in active regions with a strong and complex
photospheric magnetic field, but the active regions that produce
CME-associated flares tend to be small, including those that have no
sunspots and therefore no NOAA active-region numbers. Other factors
on scales similar to and larger than active regions seem to exist that
contribute to the association of flares with CMEs. We find the possible
low coronal signatures of CMEs, namely eruptions, dimmings, EUV waves,
and Type III bursts, in 91 %, 74 %, 57 %, and 74 %, respectively, of
the 35 flares that we associate with CMEs. None of these observables
can fully replace direct observations of CMEs by coronagraphs.
---------------------------------------------------------
Title: The Magnetic Field of Active Region 11158 during the 2011
February 12-17 Flares: Differences between Photospheric Extrapolation
and Coronal Forward-Fitting Methods
Authors: Aschwanden, Markus J.; Sun, Xudong; Liu, Yang
2014ApJ...785...34A Altcode: 2014arXiv1402.5340A
We developed a coronal nonlinear force-free field (COR-NLFFF)
forward-fitting code that fits an approximate nonlinear force-free
field (NLFFF) solution to the observed geometry of automatically
traced coronal loops. In contrast to photospheric NLFFF codes,
which calculate a magnetic field solution from the constraints of the
transverse photospheric field, this new code uses coronal constraints
instead, and this way provides important information on systematic
errors of each magnetic field calculation method, as well as on the
non-force-freeness in the lower chromosphere. In this study we applied
the COR-NLFFF code to NOAA Active Region 11158, during the time interval
of 2011 February 12-17, which includes an X2.2 GOES-class flare plus
35 M- and C-class flares. We calculated the free magnetic energy with
a 6 minute cadence over 5 days. We find good agreement between the two
types of codes for the total nonpotential E<SUB>N</SUB> and potential
energy E<SUB>P</SUB> but find up to a factor of 4 discrepancy in the
free energy E <SUB>free</SUB> = E<SUB>N</SUB> - E<SUB>P</SUB> and
up to a factor of 10 discrepancy in the decrease of the free energy
ΔE <SUB>free</SUB> during flares. The coronal NLFFF code exhibits a
larger time variability and yields a decrease of free energy during the
flare that is sufficient to satisfy the flare energy budget, while the
photospheric NLFFF code shows much less time variability and an order of
magnitude less free-energy decrease during flares. The discrepancy may
partly be due to the preprocessing of photospheric vector data but more
likely is due to the non-force-freeness in the lower chromosphere. We
conclude that the coronal field cannot be correctly calculated on the
basis of photospheric data alone and requires additional information
on coronal loop geometries.
---------------------------------------------------------
Title: STEREO/ Extreme Ultraviolet Imager (EUVI) Event Catalog 2006
- 2012
Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre; Nitta, Nariaki
V.; Lemen, James R.; Freeland, Sam; Thompson, William T.
2014SoPh..289..919A Altcode: 2013arXiv1306.3180A
We generated an event catalog with an automated detection algorithm
based on the entire EUVI image database observed with the two Solar
Terrestrial Relations Observatory (STEREO)-A and -B spacecraft over
the first six years of the mission (2006 - 2012). The event catalog
includes the heliographic positions of some 20 000 EUV events,
transformed from spacecraft coordinates to Earth-based coordinates,
and information on associated GOES flare events (down to the level
of GOES A5-class flares). The 304 Å wavelength turns out to be the
most efficient channel for flare detection (79 % of all EUVI event
detections), while the 171 Å (4 %), 195 Å (10 %), and the 284 Å
channel (7 %) retrieve substantially fewer flare events, partially
due to the suppressing effect of EUV dimming, and partially due
to the lower cadence in the later years of the mission. Due to the
Sun-circling orbits of STEREO-A and -B, a large number of flares have
been detected on the farside of the Sun, invisible from Earth, or seen
as partially occulted events. The statistical size distributions of
EUV peak fluxes (with a power-law slope of α<SUB>P</SUB>=2.5±0.2)
and event durations (with a power-law slope of α<SUB>T</SUB>=2.4±0.3)
are found to be consistent with the fractal-diffusive self-organized
criticality model. The EUVI event catalog is available on-line at
secchi.lmsal.com/EUVI/euvi_autodetection/euvi_events.txt and may serve
as a comprehensive tool to identify stereoscopically observed flare
events for 3D reconstruction and to study occulted flare events.
---------------------------------------------------------
Title: A Macroscopic Description of a Generalized Self-organized
Criticality System: Astrophysical Applications
Authors: Aschwanden, Markus J.
2014ApJ...782...54A Altcode: 2013arXiv1310.4191A
We suggest a generalized definition of self-organized criticality (SOC)
systems: SOC is a critical state of a nonlinear energy dissipation
system that is slowly and continuously driven toward a critical
value of a system-wide instability threshold, producing scale-free,
fractal-diffusive, and intermittent avalanches with power law-like
size distributions. We develop here a macroscopic description of
SOC systems that provides an equivalent description of the complex
microscopic fine structure, in terms of fractal-diffusive transport
(FD-SOC). Quantitative values for the size distributions of SOC
parameters (length scales L, time scales T, waiting times Δt, fluxes F,
and fluences or energies E) are derived from first principles, using
the scale-free probability conjecture, N(L)dLvpropL <SUP>-d </SUP>,
for Euclidean space dimension d. We apply this model to astrophysical
SOC systems, such as lunar craters, the asteroid belt, Saturn ring
particles, magnetospheric substorms, radiation belt electrons, solar
flares, stellar flares, pulsar glitches, soft gamma-ray repeaters,
black-hole objects, blazars, and cosmic rays. The FD-SOC model predicts
correctly the size distributions of 8 out of these 12 astrophysical
phenomena, and indicates non-standard scaling laws and measurement
biases for the others.
---------------------------------------------------------
Title: Multiwavelength diagnostics of the precursor and main phases
of an M1.8 flare on 2011 April 22
Authors: Awasthi, A. K.; Jain, R.; Gadhiya, P. D.; Aschwanden, M. J.;
Uddin, W.; Srivastava, A. K.; Chandra, R.; Gopalswamy, N.; Nitta,
N. V.; Yashiro, S.; Manoharan, P. K.; Choudhary, D. P.; Joshi, N. C.;
Dwivedi, V. C.; Mahalakshmi, K.
2014MNRAS.437.2249A Altcode: 2013arXiv1310.6029A; 2013MNRAS.tmp.2720A
We study the temporal, spatial and spectral evolution of the M1.8 flare,
which occurred in the active region 11195 (S17E31) on 2011 April 22,
and explore the underlying physical processes during the precursor
phase and their relation to the main phase. The study of the source
morphology using the composite images in 131 Å wavelength observed by
the Solar Dynamics Observatory/Atmospheric Imaging Assembly and 6-14
keV [from the Reuven Ramaty High Energy Solar Spectroscopic Imager
(RHESSI)] revealed a multiloop system that destabilized systematically
during the precursor and main phases. In contrast, hard X-ray emission
(20-50 keV) was absent during the precursor phase, appearing only from
the onset of the impulsive phase in the form of foot-points of emitting
loops. This study also revealed the heated loop-top prior to the loop
emission, although no accompanying foot-point sources were observed
during the precursor phase. We estimate the flare plasma parameters,
namely temperature (T), emission measure (EM), power-law index (γ)
and photon turn-over energy (ɛ<SUB>to</SUB>), and found them to be
varying in the ranges 12.4-23.4 MK, 0.0003-0.6 × 10<SUP>49</SUP>
cm<SUP>-3</SUP>, 5-9 and 14-18 keV, respectively, by forward fitting
RHESSI spectral observations. The energy released in the precursor
phase was thermal and constituted ≈1 per cent of the total energy
released during the flare. The study of morphological evolution of
the filament in conjunction with synthesized T and EM maps was carried
out, which reveals (a) partial filament eruption prior to the onset of
the precursor emission and (b) heated dense plasma over the polarity
inversion line and in the vicinity of the slowly rising filament during
the precursor phase. Based on the implications from multiwavelength
observations, we propose a scheme to unify the energy release during
the precursor and main phase emissions in which the precursor phase
emission was originated via conduction front that resulted due to the
partial filament eruption. Next, the heated leftover S-shaped filament
underwent slow-rise and heating due to magnetic reconnection and finally
erupted to produce emission during the impulsive and gradual phases.
---------------------------------------------------------
Title: Soft X-ray Fluxes of Major Flares Far Behind the Limb as
Estimated Using STEREO EUV Images
Authors: Nitta, N. V.; Aschwanden, M. J.; Boerner, P. F.; Freeland,
S. L.; Lemen, J. R.; Wuelser, J. -P.
2013SoPh..288..241N Altcode: 2013arXiv1304.4163N
With increasing solar activity since 2010, many flares from the backside
of the Sun have been observed by the Extreme Ultraviolet Imager (EUVI)
on either of the twin STEREO spacecraft. Our objective is to estimate
their X-ray peak fluxes from EUVI data by finding a relation of the EUVI
with GOES X-ray fluxes. Because of the presence of the Fe XXIV line at
192 Å, the response of the EUVI 195 Å channel has a secondary broad
peak around 15 MK, and its fluxes closely trace X-ray fluxes during
the rise phase of flares. If the flare plasma is isothermal, the EUVI
flux should be directly proportional to the GOES flux. In reality,
the multithermal nature of the flare and other factors complicate
the estimation of the X-ray fluxes from EUVI observations. We discuss
the uncertainties, by comparing GOES fluxes with the high cadence EUV
data from the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamics Observatory (SDO). We conclude that the EUVI 195 Å data can
provide estimates of the X-ray peak fluxes of intense flares (e.g.,
above M4 in the GOES scale) to small uncertainties. Lastly we show
examples of intense flares from regions far behind the limb, some of
which show eruptive signatures in AIA images.
---------------------------------------------------------
Title: Multi-wavelength Observations of the Spatio-temporal Evolution
of Solar Flares with AIA/SDO. II. Hydrodynamic Scaling Laws and
Thermal Energies
Authors: Aschwanden, Markus J.; Shimizu, Toshifumi
2013ApJ...776..132A Altcode: 2013arXiv1308.5198A
In this study we measure physical parameters of the same set of 155
M- and X-class solar flares observed with AIA/SDO as analyzed in
Paper I, by performing a differential emission measure analysis to
determine the flare peak emission measure EM <SUB>p</SUB> , peak
temperature T<SUB>p</SUB> , electron density n<SUB>p</SUB> , and
thermal energy E <SUB>th</SUB>, in addition to the spatial scales L,
areas A, and volumes V measured in Paper I. The parameter ranges
for M- and X-class flares are log (EM<SUB> p </SUB>) = 47.0-50.5,
T<SUB>p</SUB> = 5.0-17.8 MK, n<SUB>p</SUB> = 4 × 10<SUP>9</SUP>-9 ×
10<SUP>11</SUP> cm<SUP>-3</SUP>, and thermal energies of E <SUB>th</SUB>
= 1.6 × 10<SUP>28</SUP>-1.1 × 10<SUP>32</SUP> erg. We find that these
parameters obey the Rosner-Tucker-Vaiana (RTV) scaling law T_p^2 \propto
n_p L and HvpropT <SUP>7/2</SUP> L <SUP>-2</SUP> during the peak time
t<SUB>p</SUB> of the flare density n<SUB>p</SUB> , when energy balance
between the heating rate H and the conductive and radiative loss rates
is achieved for a short instant and thus enables the applicability
of the RTV scaling law. The application of the RTV scaling law
predicts power-law distributions for all physical parameters, which
we demonstrate with numerical Monte Carlo simulations as well as with
analytical calculations. A consequence of the RTV law is also that
we can retrieve the size distribution of heating rates, for which we
find N(H)vpropH <SUP>-1.8</SUP>, which is consistent with the magnetic
flux distribution N(Φ)vpropΦ<SUP>-1.85</SUP> observed by Parnell
et al. and the heating flux scaling law F<SUB>H</SUB> vpropHLvpropB/L
of Schrijver et al.. The fractal-diffusive self-organized criticality
model in conjunction with the RTV scaling law reproduces the observed
power-law distributions and their slopes for all geometrical and
physical parameters and can be used to predict the size distributions
for other flare data sets, instruments, and detection algorithms.
---------------------------------------------------------
Title: A Nonlinear Force-Free Magnetic Field Approximation Suitable
for Fast Forward-Fitting to Coronal Loops. III. The Free Energy
Authors: Aschwanden, Markus J.
2013SoPh..287..369A Altcode: 2012arXiv1211.1708A; 2012SoPh..tmp..329A
An analytical approximation of a nonlinear force-free magnetic field
(NLFFF) solution was developed in Paper I, while a numerical code
that performs fast forward-fitting of this NLFFF approximation to
a line-of-sight magnetogram and coronal 3D loops has been described
and tested in Paper II. Here we calculate the free magnetic energy
E<SUB>free</SUB>=E<SUB>N</SUB>−E<SUB>P</SUB>, i.e., the difference of
the magnetic energies between the non-potential field and the potential
field. A second method to estimate the free energy is obtained from the
mean misalignment angle change Δμ=μ<SUB>P</SUB>−μ<SUB>N</SUB>
between the potential and non-potential field, which scales as
E<SUB>free</SUB>/E<SUB>P</SUB>≈tan<SUP>2</SUP>(Δμ). For the four
active regions observed with STEREO in 2007 we find free energies in the
range of q<SUB>free</SUB>=(E<SUB>free</SUB>/E<SUB>P</SUB>)≈1 % - 10 %,
with an uncertainty of less than ± 2 % between the two methods, while
the free energies obtained from 11 other NLFFF codes exhibit a larger
scatter of about ± 10 %. We also find a correlation between the free
magnetic energy and the GOES flux of the largest flare that occurred
during the observing period, which can be quantified by an exponential
relationship, F<SUB>GOES</SUB>∝exp(q<SUB>free</SUB>/0.015), implying
an exponentiation of the dissipated currents.
---------------------------------------------------------
Title: A Nonlinear Force-Free Magnetic Field Approximation Suitable
for Fast Forward-Fitting to Coronal Loops. I. Theory
Authors: Aschwanden, Markus J.
2013SoPh..287..323A Altcode: 2012arXiv1207.2780A; 2012SoPh..tmp..181A
We derive an analytical approximation of nonlinear force-free
magnetic field solutions (NLFFF) that can efficiently be used for
fast forward-fitting to solar magnetic data, constrained either by
observed line-of-sight magnetograms and stereoscopically triangulated
coronal loops, or by 3D vector-magnetograph data. The derived NLFFF
solutions provide the magnetic field components B<SUB>x</SUB>(x),
B<SUB>y</SUB>(x), B<SUB>z</SUB>(x), the force-free parameter α(x),
the electric current density j(x), and are accurate to second-order
(of the nonlinear force-free α-parameter). The explicit expressions of
a force-free field can easily be applied to modeling or forward-fitting
of many coronal phenomena.
---------------------------------------------------------
Title: A Nonlinear Force-Free Magnetic Field Approximation Suitable
for Fast Forward-Fitting to Coronal Loops. II. Numeric Code and Tests
Authors: Aschwanden, Markus J.; Malanushenko, Anna
2013SoPh..287..345A Altcode: 2012arXiv1207.2783A; 2012SoPh..tmp..182A
Based on a second-order approximation of nonlinear force-free
magnetic field solutions in terms of uniformly twisted field lines
derived in Paper I, we develop here a numeric code that is capable
to forward-fit such analytical solutions to arbitrary magnetogram (or
vector magnetograph) data combined with (stereoscopically triangulated)
coronal loop 3D coordinates. We test the code here by forward-fitting to
six potential field and six nonpotential field cases simulated with our
analytical model, as well as by forward-fitting to an exactly force-free
solution of the Low and Lou (Astrophys. J.352, 343, 1990) model. The
forward-fitting tests demonstrate: i) a satisfactory convergence
behavior (with typical misalignment angles of μ≈1<SUP>∘</SUP> -
10<SUP>∘</SUP>), ii) relatively fast computation times (from seconds
to a few minutes), and iii) the high fidelity of retrieved force-free
α-parameters (α<SUB>fit</SUB>/α<SUB>model</SUB>≈0.9 - 1.0 for
simulations and α<SUB>fit</SUB>/α<SUB>model</SUB>≈0.7±0.3 for
the Low and Lou model). The salient feature of this numeric code is
the relatively fast computation of a quasi-force-free magnetic field,
which closely matches the geometry of coronal loops in active regions,
and complements the existing nonlinear force-free field (NLFFF) codes
based on photospheric magnetograms without coronal constraints.
---------------------------------------------------------
Title: Multi-wavelength Observations of the Spatio-temporal Evolution
of Solar Flares with AIA/SDO. I. Universal Scaling Laws of Space
and Time Parameters
Authors: Aschwanden, Markus J.; Zhang, Jie; Liu, Kai
2013ApJ...775...23A Altcode: 2013arXiv1308.4936A
We extend a previous statistical solar flare study of 155 GOES M- and
X-class flares observed with AIA/SDO to all seven coronal wavelengths
(94, 131, 171, 193, 211, 304, and 335 Å) to test the wavelength
dependence of scaling laws and statistical distributions. Except for
the 171 and 193 Å wavelengths, which are affected by EUV dimming
caused by coronal mass ejections (CMEs), we find near-identical size
distributions of geometric (lengths L, flare areas A, volumes V, and
fractal dimension D <SUB>2</SUB>), temporal (flare durations T), and
spatio-temporal parameters (diffusion coefficient κ, spreading exponent
β, and maximum expansion velocities v <SUB>max</SUB>) in different
wavelengths, which are consistent with the universal predictions of the
fractal-diffusive avalanche model of a slowly driven, self-organized
criticality (FD-SOC) system, i.e., N(L)vpropL <SUP>-3</SUP>, N(A)vpropA
<SUP>-2</SUP>, N(V)vpropV <SUP>-5/3</SUP>, N(T)vpropT <SUP>-2</SUP>,
and D <SUB>2</SUB> = 3/2, for a Euclidean dimension d = 3. Empirically,
we find also a new strong correlation κvpropL <SUP>0.94 ± 0.01</SUP>
and the three-parameter scaling law Lvpropκ T <SUP>0.1</SUP>, which
is more consistent with the logistic-growth model than with classical
diffusion. The findings suggest long-range correlation lengths in
the FD-SOC system that operate in the vicinity of a critical state,
which could be used for predictions of individual extreme events. We
find also that eruptive flares (with accompanying CMEs) have larger
volumes V, longer flare durations T, higher EUV and soft X-ray fluxes,
and somewhat larger diffusion coefficients κ than confined flares
(without CMEs).
---------------------------------------------------------
Title: Nonlinear Force-Free Magnetic Fields of Active Regions based
on automated loop tracing in AIA/SDO images with DEM discrimination
of chromospheric and coronal features
Authors: Aschwanden, Markus J.; Boerner, P.; AIA/SDO Team
2013SPD....4420104A Altcode:
We developed a forward-fitting code that computes a nonlinear
force-free magnetic field (NLFFF) solution constrained by line-of-sight
magnetograms from HMI/SDO and by coronal loop structures detected in EUV
images from AIA/SDO. The 2D coordinates of coronal loop structures are
detected with an improved version of the Oriented Coronal CUrved Loop
Tracing (OCCULT-2) code, an automated pattern recognition algorithm that
has demonstrated a quality and fidelity in loop tracing that matches
visual perception. One fundamental limitation in the completeness
of detecting coronal loops comes from the background confusion
of coronal loop EUV emission with low-temperature (T=10^4-10^6 K)
emission from the chromosphere and transition region, as well as T ~
1.0 MK emission from reticulated “moss structure” that stems from the
footpoints of hotter (T 2-8 MK) coronal loops. We employ a pixel-wise
differential emission measure (DEM) analysis using the 7 coronal AIA
filters in order to produce uncontaminated emission measure maps in
coronal temperature ranges, which allows an improved performance of
automated loop tracing. A nonlinear force-free magnetic field solution
is then computed by forward-fitting of an analytical NLFFF solution
of twisted coronal field lines to the automatically traced coronal
loop coordinates. We demonstrate the performance of this magnetic
field modeling for a number of solar active regions observed with
SDO. The developed method is able to calculate the most realistic
magnetic field models of solar active regions that match all available
observable constraints.
---------------------------------------------------------
Title: A multiwavelength study of eruptive events on January 23,
2012 associated with a major solar energetic particle event
Authors: Joshi, N. C.; Uddin, W.; Srivastava, A. K.; Chandra, R.;
Gopalswamy, N.; Manoharan, P. K.; Aschwanden, M. J.; Choudhary, D. P.;
Jain, R.; Nitta, N. V.; Xie, H.; Yashiro, S.; Akiyama, S.; Mäkelä,
P.; Kayshap, P.; Awasthi, A. K.; Dwivedi, V. C.; Mahalakshmi, K.
2013AdSpR..52....1J Altcode: 2013arXiv1303.1251J
We use multiwavelength data from space and ground based instruments
to study the solar flares and coronal mass ejections (CMEs) on January
23, 2012 that were responsible for one of the largest solar energetic
particle (SEP) events of solar cycle 24. The eruptions consisting of two
fast CMEs (≈1400 km s<SUP>-1</SUP> and ≈2000 km s<SUP>-1</SUP>) and
M-class flares that occurred in active region 11402 located at ≈N28
W36. The two CMEs occurred in quick successions, so they interacted
very close to the Sun. The second CME caught up with the first one
at a distance of ≈11-12 R<SUB>sun</SUB>. The CME interaction may be
responsible for the elevated SEP flux and significant changes in the
intensity profile of the SEP event. The compound CME resulted in a
double-dip moderate geomagnetic storm (Dst∼-73nT). The two dips are
due to the southward component of the interplanetary magnetic field in
the shock sheath and the ICME intervals. One possible reason for the
lack of a stronger geomagnetic storm may be that the ICME delivered
a glancing blow to Earth.
---------------------------------------------------------
Title: Optimization of Curvilinear Tracing Applied to Solar Physics
and Biophysics
Authors: Aschwanden, Markus; De Pontieu, Bart; Katrukha, Eugene
2013Entrp..15.3007A Altcode: 2013arXiv1307.5046A
We developed an automated pattern recognition code that is particularly
well suited to extract one-dimensional curvi-linear features from
two-dimensional digital images. A former version of this {\sl Oriented
Coronal CUrved Loop Tracing (OCCULT)} code was applied to spacecraft
images of magnetic loops in the solar corona, recorded with the NASA
spacecraft {\sl Transition Region And Coronal Explorer (TRACE)} in
extreme ultra-violet wavelengths. Here we apply an advanced version of
this code ({\sl OCCULT-2}) also to similar images from the {\sl Solar
Dynamics Observatory (SDO)}, to chromospheric H-$\alpha$ images obtained
with the {\sl Swedish Solar Telescope (SST)}, and to microscopy images
of microtubule filaments in live cells in biophysics. We provide a full
analytical description of the code, optimize the control parameters,
and compare the automated tracing with visual/manual methods. The
traced structures differ by up to 16 orders of magnitude in size,
which demonstrates the universality of the tracing algorithm.
---------------------------------------------------------
Title: Self-Organized Criticality Systems
Authors: Aschwanden, M. J.
2013socs.book.....A Altcode:
Contents: (1) Introduction - Norma B. Crosby --- <P />(2) Theoretical
Models of SOC Systems - Markus J. Aschwanden --- <P />(3) SOC and
Fractal Geometry - R. T. James McAteer --- <P />(4) Percolation Models
of Self-Organized Critical Phenomena - Alexander V. Milovanov ---
<P />(5) Criticality and Self-Organization in Branching Processes:
Application to Natural Hazards - Álvaro Corral, Francesc Font-Clos ---
<P />(6) Power Laws of Recurrence Networks - Yong Zou, Jobst Heitzig,
Jürgen Kurths --- <P />(7) SOC computer simolations - Gunnar Pruessner
--- <P />(8) SOC Laboratory Experiments - Gunnar Pruessner --- <P />(9)
Self-Organizing Complex Earthquakes: Scaling in Data, Models, and
Forecasting - Michael K. Sachs et al. --- <P />(10) Wildfires and the
Forest-Fire Model - Stefan Hergarten --- <P />(11) SOC in Landslides -
Stefan Hergarten --- <P />(12) SOC and Solar Flares - Paul Charbonneau
--- <P />(13) SOC Systems in Astrophysics - Markus J. Aschwanden ---
---------------------------------------------------------
Title: Height of shock formation in the solar corona inferred from
observations of type II radio bursts and coronal mass ejections
Authors: Gopalswamy, N.; Xie, H.; Mäkelä, P.; Yashiro, S.; Akiyama,
S.; Uddin, W.; Srivastava, A. K.; Joshi, N. C.; Chandra, R.; Manoharan,
P. K.; Mahalakshmi, K.; Dwivedi, V. C.; Jain, R.; Awasthi, A. K.;
Nitta, N. V.; Aschwanden, M. J.; Choudhary, D. P.
2013AdSpR..51.1981G Altcode: 2013arXiv1301.0893G
Employing coronagraphic and EUV observations close to the solar surface
made by the Solar Terrestrial Relations Observatory (STEREO) mission,
we determined the heliocentric distance of coronal mass ejections
(CMEs) at the starting time of associated metric type II bursts. We
used the wave diameter and leading edge methods and measured the CME
heights for a set of 32 metric type II bursts from solar cycle 24. We
minimized the projection effects by making the measurements from a
view that is roughly orthogonal to the direction of the ejection. We
also chose image frames close to the onset times of the type II bursts,
so no extrapolation was necessary. We found that the CMEs were located
in the heliocentric distance range from 1.20 to 1.93 solar radii (Rs),
with mean and median values of 1.43 and 1.38 Rs, respectively. We
conclusively find that the shock formation can occur at heights
substantially below 1.5 Rs. In a few cases, the CME height at type
II onset was close to 2 Rs. In these cases, the starting frequency
of the type II bursts was very low, in the range 25-40 MHz, which
confirms that the shock can also form at larger heights. The starting
frequencies of metric type II bursts have a weak correlation with the
measured CME/shock heights and are consistent with the rapid decline
of density with height in the inner corona.
---------------------------------------------------------
Title: Kelvin-Helmholtz Instability of the CME Reconnection Outflow
Layer in the Low Corona
Authors: Foullon, Claire; Verwichte, Erwin; Nykyri, Katariina;
Aschwanden, Markus J.; Hannah, Iain G.
2013ApJ...767..170F Altcode:
New capabilities for studying the Sun allow us to image for the first
time the magnetic Kelvin-Helmholtz (KH) instability developing at the
surface of a fast coronal mass ejecta (CME) less than 150 Mm above the
solar surface. We conduct a detailed observational investigation of this
phenomenon, observed off the east solar limb on 2010 November 3, in the
EUV with SDO/AIA. In conjunction with STEREO-B/EUVI, we derive the CME
source surface position. We ascertain the timing and early evolution
of the CME outflow leading to the instability onset. We perform image
and spectral analysis, exploring the CME plasma structuring and its
parabolic flow pattern. As we evaluate and validate the consistency
of the observations with theoretical considerations and predictions,
we take the view that the ejecta layer corresponds to a reconnection
outflow layer surrounding the erupting flux rope, accounting for
the timing, high temperature (~11.6 MK), and high flow shear (~680 km
s<SUP>-1</SUP>) on the unstable CME northern flank and for the observed
asymmetry between the CME flanks. From the irregular evolution of the
CME flow pattern, we infer a shear gradient consistent with expected
spatial flow variations across the KH-unstable flank. The KH phenomenon
observed is tied to the first stage of a linked flare-CME event.
---------------------------------------------------------
Title: Automated Temperature and Emission Measure Analysis of
Coronal Loops and Active Regions Observed with the Atmospheric
Imaging Assembly on the Solar Dynamics Observatory (SDO/AIA)
Authors: Aschwanden, Markus J.; Boerner, Paul; Schrijver, Carolus J.;
Malanushenko, Anna
2013SoPh..283....5A Altcode: 2011SoPh..tmp..384A
We developed numerical codes designed for automated analysis of
SDO/AIA image datasets in the six coronal filters, including: i)
coalignment test between different wavelengths with measurements of
the altitude of the EUV-absorbing chromosphere, ii) self-calibration by
empirical correction of instrumental response functions, iii) automated
generation of differential emission measure [DEM] distributions
with peak-temperature maps [T<SUB>p</SUB>(x,y)] and emission measure
maps [EM<SUB>p</SUB>(x,y)] of the full Sun or active region areas,
iv) composite DEM distributions [dEM(T)/dT] of active regions or
subareas, v) automated detection of coronal loops, and vi) automated
background subtraction and thermal analysis of coronal loops, which
yields statistics of loop temperatures [T<SUB>e</SUB>], temperature
widths [σ<SUB>T</SUB>], emission measures [EM], electron densities
[n<SUB>e</SUB>], and loop widths [w]. The combination of these
numerical codes allows for automated and objective processing of
numerous coronal loops. As an example, we present the results of an
application to the active region NOAA 11158, observed on 15 February
2011, shortly before it produced the largest (X2.2) flare during the
current solar cycle. We detect 570 loop segments at temperatures in the
entire range of log(T<SUB>e</SUB>)=5.7 - 7.0 K and corroborate previous
TRACE and AIA results on their near-isothermality and the validity of
the Rosner-Tucker-Vaiana (RTV) law at soft X-ray temperatures (T≳2
MK) and its failure at lower EUV temperatures.
---------------------------------------------------------
Title: Hydrodynamic Scaling Laws and Solar Flare Statistics from AIA
Authors: Aschwanden, Markus J.
2013enss.confE..75A Altcode:
We present a statistical solar flare study of 155 GOES M- and X-class
flares observed with AIA/SDO in all 7 coronal wavelengths (94, 131,
171, 193, 211, 304, 335 A) and investigate the wavelength-dependence of
scaling laws and statistical distributions. Except for the 171 and 193
A wavelengths, which are affected by EUV dimming caused by coronal mass
ejections (CMEs), we find near-identical size distributions of geometric
(flare size L, area A, volume V, fractal dimension D2), temporal (flare
duration D), and spatio-temporal parameters (diffusion coefficient,
spreading exponent, and maximum expansion velocity) in different
wavelengths, which are consistent with the universal predictions of the
fractal-diffusive avalanche model of a slowly-driven self-organized
criticality (FD-SOC) system, i.e., N(L) L^(-3), N(A) A^(-2), N(V)
V^(-5/3), N(D) D^(-2), D2=3/2, for a Euclidean dimension d=3. We perfom
also a differential emission measure (DEM) analysis in all flares to
determine the flare peak emission measure EM_p, peak temperature T_p,
electron density n_p, and thermal energy E_th. We find that these
parameters obey the Rosner-Tucker-Vaiana (RTV) scaling law T_p^2 n_p L
and H T^(7/2) L^(-2) during the flare peak time t_p of maximum density
n_p, when energy balance between the heating rate H and the conductive
and radiative loss rates is achieved for a short instant, and thus
enables the applicability of the RTV scaling law. The application of
the RTV scaling law predicts powerlaw distributions for all physical
parameters, which we demonstrate with numerical Monte-Carlo simulations
as well as with analytical calculations. A consequence of the RTV law
is also that we can retrieve the size distribution of heating rates, for
which we find N(H) H^(-1.8), which is consistent with the magnetic flux
distribution N(Phi) Phi^(-1.85) observed by Parnell et al. (2009) and
the heating flux scaling law F_H H L B/L of Schrijver et al. (2004). The
fractal-diffusive self-organized criticality model in conjunction with
the RTV scaling law reproduces the observed powerlaw distributions
and their slopes for all geometrical and physical parameters and can
be used to predict the size distributions for other flare datasets,
instruments, and detection algorithms.
---------------------------------------------------------
Title: Recent Advances in Observations of Coronal EUV Waves
Authors: Liu, Wei; Ofman, Leon; Aschwanden, Markus J.; Nitta, Nariaki;
Zhao, Junwei; Title, Alan M.
2013enss.confE..67L Altcode:
MHD waves can be used as seismological tools to decipher otherwise
elusive physical parameters of the solar corona, such as the magnetic
field strength and plasma density. Recent high cadence, high resolution,
full-disk imaging observations from SDO/AIA have opened a new chapter
in understanding these waves. Various types of EUV waves associated with
flares/CMEs have been discovered or observed in unprecedented detail. In
this talk, we will review such new observations, focusing on the
following topics and their interrelationships: (1) quasi-periodic fast
waves traveling along coronal funnels within CME bubbles at speeds up
to 2000 km/s, associated with flare pulsations at similar frequencies;
(2) quasi-periodic wave trains within broad, diffuse pulses of global
EUV waves (so-called "EIT waves") running ahead of CME fronts; (3)
interactions of global EUV waves with local coronal structures on
their paths, such as flux-rope coronal cavities and their embedded
filaments (kink oscillations) and coronal holes or active regions
(deflections). We will discuss the implications of these observations
on coronal seismology, on their roles in transporting energy through
different parts of the solar atmosphere, and on understanding their
associated eruptive flares/CMEs.
---------------------------------------------------------
Title: Nonlinear Force-free Magnetic Field Fitting to Coronal Loops
with and without Stereoscopy
Authors: Aschwanden, Markus J.
2013ApJ...763..115A Altcode: 2012arXiv1212.2996A
We developed a new nonlinear force-free magnetic field (NLFFF)
forward-fitting algorithm based on an analytical approximation of
force-free and divergence-free NLFFF solutions, which requires as
input a line-of-sight magnetogram and traced two-dimensional (2D) loop
coordinates of coronal loops only, in contrast to stereoscopically
triangulated three-dimensional loop coordinates used in previous
studies. Test results of simulated magnetic configurations and from four
active regions observed with STEREO demonstrate that NLFFF solutions
can be fitted with equal accuracy with or without stereoscopy, which
relinquishes the necessity of STEREO data for magnetic modeling
of active regions (on the solar disk). The 2D loop tracing method
achieves a 2D misalignment of μ<SUB>2</SUB> = 2.°7 ± 1.°3 between
the model field lines and observed loops, and an accuracy of ≈1.0%
for the magnetic energy or free magnetic energy ratio. The three times
higher spatial resolution of TRACE or SDO/AIA (compared with STEREO)
also yields a proportionally smaller misalignment angle between model
fit and observations. Visual/manual loop tracings are found to produce
more accurate magnetic model fits than automated tracing algorithms. The
computation time of the new forward-fitting code amounts to a few
minutes per active region.
---------------------------------------------------------
Title: Multi-wavelength diagnostics of thermal and non-thermal
characteristics in 22 April 2011 confined flare
Authors: Awasthi, Arun K.; Jain, Rajmal; Aschwanden, Markus J.; Uddin,
Wahab; Srivastava, Abhishek K.; Chandra, Ramesh; Gopalswamy, Nat;
Nitta, Nariaki; Yashiro, Seiji; Manoharan, P. K.; Prasad Choudhary,
Debi; Joshi, N. C.; Dwivedi, Vidya Charan; Mahalakshmi, K.
2013ASInC...9...71A Altcode:
We study the spatial, spectral and temporal characteristics of thermal
and non-thermal emission in an M1.8 flare, which occurred in NOAA
AR 11195 (S17E31) on 22 April 2011. This study quantifies spatial
and temporal correlation of thermal and non-thermal emissions in
precursor, impulsive as well as gradual phase of energy release
employing multi-wavelength observation from SDO, HESSI and SOXS
missions. Based on spectral fitting analysis performed on the X-ray
emission observed by RHESSI as well as SOXS missions in low energy
and high energy respectively, we define that <20 keV emission
corresponds to thermal and >20 keV emission to be non-thermal
counterpart of the emission. Therefore, we construct X-ray images
employing RHESSI observation in energy bands 6-20 and 20-100 keV
over the time integration of 30s. We report co-spatial X-ray emission
in various phases of emission. We also report absence of non-thermal
counterpart in the X-ray emission in precursor phase however visible at
the commencement of main phase. To characterize thermal and non-thermal
signatures, we overlay the X-ray image contours on the Hα and EUV
observations from GONG and SDO/AIA respectively. We report thermal
emission in the precursor phase to be co-spatial to UV counterpart. In
contrast, we report absence of emission in the EUV wavebands i.e. 1600
and 1700 Å which, in principle, correspond to temperature minimum
zone and photosphere during the precursor phase. This confirms the
absence of non-thermal emission as appeared in X-ray emission during
the precursor phase. Further, during the impulsive as well as in
gradual phase, thermal and non-thermal emissions have been found to
be originated from a compact source, co-spatial in nature. Analysis
of Line of sight (LOS) magnetic field observations from SDO/HMI does
not reveal noticeable changes in the positive and negative fluxes
as well as magnetic-field gradient during this event. In contrast,
Hα emission observed by GONG has revealed the filament eruption as
the trigger of flare. This suggests filament eruption to be driver of
this event, consistent with the CSHKP model of solar flare.
---------------------------------------------------------
Title: Coronal Seismology with ATST
Authors: Aschwanden, M. J.
2012ASPC..463..133A Altcode:
We give a brief summary on the current status of coronal seismology and
anticipate research opportunities for ATST in this discipline. Given
the optical/infrared spectral range and the high-resolution magnetic
field capabilities of ATST (≍ 0.05″-0.1″), the potential of
exploring coronal seismology includes: (1) Optical detection of
coronal waves and oscillations, (2) high-resolution magnetic field
modeling with accurate determination of Alfvénic speeds, and (3)
correlative studies that investigate the coupling between photospheric
waves (detected in optical wavelengths) and coronal waves, which will
provide insights into the generation mechanism of coronal waves, the
origin and efficiency of coronal heating by waves, and diagnostics on
flare and CME processes by global waves.
---------------------------------------------------------
Title: Quantifying Coronal Dimming as Observed in EUV and X-ray
Images in Eruptive Events
Authors: Nitta, N. V.; Aschwanden, M. J.; Boerner, P.; Hill, S. M.;
Lemen, J. R.; Liu, W.; Schrijver, C.; Wuelser, J.
2012AGUFMSH41A2097N Altcode:
Data from SOHO have shown that coronal dimming is closely related
with coronal mass ejections (CMEs). In particular, dimming areas in
EIT 195 A images often match the lateral extension of the associated
CMEs. In this presentation, we summarize how CMEs compare with dimming
as identified at different wavelengths and by other instruments, such as
Yohkoh SXT, TRACE, GOES (12-15) SXI, STEREO EUVI and SDO AIA. Emphasis
is placed on recent data, since the combination of AIA and STEREO
data can lead us to better characterize CMEs and to more accurately
estimate how much mass is ejected. We discuss technical issues that
arise when quantifying dimming as a proxy for a CME. The issues include
instrument calibration, effects of heating and cooling and integration
along the line of sight. We also touch on the relation of dimming with
globally propagating coronal fronts, which are routinely isolated in
running difference images, and its implications on the magnitudes of
the associated CMEs.
---------------------------------------------------------
Title: Measurements of the Mass of Coronal Mass Ejections from the
EUV Dimming Observed with STEREO
Authors: Aschwanden, M. J.
2012AGUFMSH44A..02A Altcode:
The masses of Coronal Mass Ejections (CMEs) have traditionally
been determined from white-light coronagraphs (based on Thomson
scattering of electrons), as well as from EUV dimming observed with
one spacecraft. Here we report on an improved method of measuring
CME masses based on EUV dimming observed with the dual STEREO/EUVI
spacecraft in multiple temperature filters that includes 3D volume
and density modeling in the dimming region and background corona. As
a test we investigate 8 CME events with previous mass determinations
from STEREO/COR2, of which 6 cases are reliably detected with EUVI
using our automated multi-wavelength detection code. We find CME masses
in the range of m_CME = (2-7) * 10^(15) g. The agreement between the
two EUVI/A and B spacecraft is m_A/m_B =1.3+/-0.6$ and the consistency
with white-light measurements by COR2 is m_EUVI/m_COR2 = 1.1+/-0.3. The
consistency between EUVI and COR2 implies no significant mass backflows
(or inflows) at r < 4 R_sun and adequate temperature coverage
for the bulk of the CME mass in the range of T = 0.5-3.0 MK. The
temporal evolution of the EUV dimming allows us also to model the
evolution of the CME density n_e(t), volume V(t), height-time h(t),
and propagation speed v(t) in terms of an adiabatically expanding
self-similar geometry. We determine e-folding EUV dimming times of
t_D=1.3+/-1.4 hrs. We test the adiabatic expansion model in terms of
the predicted detection delay dt = 0.7 hr between EUVI and COR2 for
the fastest CME event (2008-Mar-25) and find good agreement with the
observed delay dt = 0.8 hr.
---------------------------------------------------------
Title: Solar Stereoscopy with STEREO/EUVI A and B Spacecraft from
Small (6<SUP>∘</SUP>) to Large (170<SUP>∘</SUP>) Spacecraft
Separation Angles
Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre; Nitta, Nariaki;
Lemen, James
2012SoPh..281..101A Altcode: 2012SoPh..tmp..197A; 2012arXiv1207.2787A
We performed for the first time stereoscopic
triangulation of coronal loops in active regions
over the entire range of spacecraft separation angles
(α<SUB>sep</SUB>≈6<SUP>∘</SUP>,43<SUP>∘</SUP>,89<SUP>∘</SUP>,127<SUP>∘</SUP>,and
170<SUP>∘</SUP>). The accuracy of stereoscopic correlation depends
mostly on the viewing angle with respect to the solar surface
for each spacecraft, which affects the stereoscopic correspondence
identification of loops in image pairs. From a simple theoretical model
we predict an optimum range of α<SUB>sep</SUB>≈22<SUP>∘</SUP> -
125<SUP>∘</SUP>, which is also experimentally confirmed. The best
accuracy is generally obtained when an active region passes the central
meridian (viewed from Earth), which yields a symmetric view for both
STEREO spacecraft and causes minimum horizontal foreshortening. For
the extended angular range of α<SUB>sep</SUB>≈6<SUP>∘</SUP>
- 127<SUP>∘</SUP> we find a mean 3D misalignment angle
of μ<SUB>PF</SUB>≈21<SUP>∘</SUP> - 39<SUP>∘</SUP> of
stereoscopically triangulated loops with magnetic potential-field
models, and μ<SUB>FFF</SUB>≈15<SUP>∘</SUP> - 21<SUP>∘</SUP>
for a force-free field model, which is partly caused by stereoscopic
uncertainties μ<SUB>SE</SUB>≈9<SUP>∘</SUP>. We predict optimum
conditions for solar stereoscopy during the time intervals of 2012 -
2014, 2016 - 2017, and 2021 - 2023.
---------------------------------------------------------
Title: GeV Particle Acceleration in Solar Flares and Ground Level
Enhancement (GLE) Events
Authors: Aschwanden, Markus J.
2012SSRv..171....3A Altcode: 2012SSRv..tmp....5A; 2010arXiv1005.0029A; 2012SSRv..tmp....7A
Ground Level Enhancement (GLE) events represent the most energetic
class of solar energetic particle (SEP) events, requiring acceleration
processes to boost ≳1 GeV ions in order to produce showers
of secondary particles in the Earth's atmosphere with sufficient
intensity to be detected by ground-level neutron monitors, above the
background of cosmic rays. Although the association of GLE events with
both solar flares and coronal mass ejections (CMEs) is undisputed, the
question arises about the location of the responsible acceleration site:
coronal flare reconnection sites, coronal CME shocks, or interplanetary
shocks? To investigate the first possibility we explore the timing
of GLE events with respect to hard X-ray production in solar flares,
considering the height and magnetic topology of flares, the role of
extended acceleration, and particle trapping. We find that 50% (6
out of 12) of recent (non-occulted) GLE events are accelerated during
the impulsive flare phase, while the remaining half are accelerated
significantly later. It appears that the prompt GLE component,
which is observed in virtually all GLE events according to a recent
study by Vashenyuk et al. (Astrophys. Space Sci. Trans. 7(4):459-463,
2011), is consistent with a flare origin in the lower corona, while
the delayed gradual GLE component can be produced by both, either by
extended acceleration and/or trapping in flare sites, or by particles
accelerated in coronal and interplanetary shocks.
---------------------------------------------------------
Title: The Spatio-temporal Evolution of Solar Flares Observed with
AIA/SDO: Fractal Diffusion, Sub-diffusion, or Logistic Growth?
Authors: Aschwanden, Markus J.
2012ApJ...757...94A Altcode: 2012arXiv1208.1527A
We explore the spatio-temporal evolution of solar flares by fitting
a radial expansion model r(t) that consists of an exponentially
growing acceleration phase, followed by a deceleration phase that is
parameterized by the generalized diffusion function r(t)vpropκ(t -
t <SUB>1</SUB>)<SUP>β/2</SUP>, which includes the logistic growth
limit (β = 0), sub-diffusion (β = 0-1), classical diffusion
(β = 1), super-diffusion (β = 1-2), and the linear expansion
limit (β = 2). We analyze all M- and X-class flares observed with
Geostationary Operational Environmental Satellite and Atmospheric
Imaging Assembly/Solar Dynamics Observatory (SDO) during the first two
years of the SDO mission, amounting to 155 events. We find that most
flares operate in the sub-diffusive regime (β = 0.53 ± 0.27), which
we interpret in terms of anisotropic chain reactions of intermittent
magnetic reconnection episodes in a low plasma-β corona. We find a
mean propagation speed of v = 15 ± 12 km s<SUP>-1</SUP>, with maximum
speeds of v <SUB>max</SUB> = 80 ± 85 km s<SUP>-1</SUP> per flare,
which is substantially slower than the sonic speeds expected for thermal
diffusion of flare plasmas. The diffusive characteristics established
here (for the first time for solar flares) is consistent with the
fractal-diffusive self-organized criticality model, which predicted
diffusive transport merely based on cellular automaton simulations.
---------------------------------------------------------
Title: First Three-dimensional Reconstructions of Coronal Loops
with the STEREO A+B Spacecraft. IV. Magnetic Modeling with Twisted
Force-free Fields
Authors: Aschwanden, Markus J.; Wuelser, Jean-Pierre; Nitta, Nariaki
V.; Lemen, James R.; DeRosa, Marc L.; Malanushenko, Anna
2012ApJ...756..124A Altcode: 2012arXiv1207.2790A
The three-dimensional coordinates of stereoscopically triangulated
loops provide strong constraints for magnetic field models of active
regions in the solar corona. Here, we use STEREO/A and B data from some
500 stereoscopically triangulated loops observed in four active regions
(2007 April 30, May 9, May 19, and December 11), together with SOHO/MDI
line-of-sight magnetograms. We measure the average misalignment angle
between the stereoscopic loops and theoretical magnetic field models,
finding a mismatch of μ = 19°-46° for a potential field model,
which is reduced to μ = 14°-19° for a non-potential field model
parameterized by twist parameters. The residual error is commensurable
with stereoscopic measurement errors (μ<SUB>SE</SUB> ≈ 8°-12°). We
developed a potential field code that deconvolves a line-of-sight
magnetogram into three magnetic field components (B<SUB>x</SUB> ,
B<SUB>y</SUB> , B<SUB>z</SUB> ), as well as a non-potential field
forward-fitting code that determines the full length of twisted loops
(L ≈ 50-300 Mm), the number of twist turns (median N <SUB>twist</SUB>
= 0.06), the nonlinear force-free α-parameter (median α ≈ 4 ×
10<SUP>-11</SUP> cm<SUP>-1</SUP>), and the current density (median
j<SUB>z</SUB> ≈ 1500 Mx cm<SUP>-2</SUP> s<SUP>-1</SUP>). All twisted
loops are found to be far below the critical value for kink instability,
and Joule dissipation of their currents is found to be far below the
coronal heating requirement. The algorithm developed here, based on an
analytical solution of nonlinear force-free fields that is accurate to
second order (in the force-free parameter α), represents the first
code that enables fast forward fitting to photospheric magnetograms
and stereoscopically triangulated loops in the solar corona.
---------------------------------------------------------
Title: Automated Solar Flare Statistics in Soft X-Rays over 37 Years
of GOES Observations: The Invariance of Self-organized Criticality
during Three Solar Cycles
Authors: Aschwanden, Markus J.; Freeland, Samuel L.
2012ApJ...754..112A Altcode: 2012arXiv1205.6712A
We analyzed the soft X-ray light curves from the Geostationary
Operational Environmental Satellites over the last 37 years (1975-2011)
and measured with an automated flare detection algorithm over 300,000
solar flare events (amounting to ≈5 times higher sensitivity than the
NOAA flare catalog). We find a power-law slope of α<SUB> F </SUB> =
1.98 ± 0.11 for the (background-subtracted) soft X-ray peak fluxes
that is invariant through three solar cycles and agrees with the
theoretical prediction α<SUB> F </SUB> = 2.0 of the fractal-diffusive
self-organized criticality (FD-SOC) model. For the soft X-ray flare
rise times, we find a power-law slope of α<SUB> T </SUB> = 2.02 ±
0.04 during solar cycle minima years, which is also consistent with
the prediction α<SUB> T </SUB> = 2.0 of the FD-SOC model. During
solar cycle maxima years, the power-law slope is steeper in the
range of α<SUB> T </SUB> ≈ 2.0-5.0, which can be modeled by
a solar-cycle-dependent flare pile-up bias effect. These results
corroborate the FD-SOC model, which predicts a power-law slope of
α<SUB> E </SUB> = 1.5 for flare energies and thus rules out significant
nanoflare heating. While the FD-SOC model predicts the probability
distribution functions of spatio-temporal scaling laws of nonlinear
energy dissipation processes, additional physical models are needed
to derive the scaling laws between the geometric SOC parameters and
the observed emissivity in different wavelength regimes, as we derive
here for soft X-ray emission. The FD-SOC model also yields statistical
probabilities for solar flare forecasting.
---------------------------------------------------------
Title: On the Oscillatory and Non-oscillatory Loop Systems and
Dynamical Processes during the X2.1 Solar Flare on 06 September 2011
Authors: Srivastava, Abhishek K.; Jain, Rajmal; Prasad Choudhary, Debi;
Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy,
Nat; Awasthi, Arun Kumar; Chandra, Ramesh; Kayshap, Pradeep; Joshi,
N. C.; Manoharan, P. K.; Norris, Max; Makela, Pertti; Mahalakshmi,
K.; Elamathi, E.
2012cosp...39.1882S Altcode: 2012cosp.meet.1882S
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Flux Imbalance in Active Regions NOAA 11283 and
NOAA 11302
Authors: Prasad Choudhary, Debi; Jain, Rajmal; Charan Dwivedi, Vidya;
Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat; Awasthi, Arun
Kumar; Chandra, Ramesh; Srivastava, Abhishek K.; Manoharan, P. K.;
Norris, Max; Mahalakshmi, K.; Elamathi, E.; Uddin, Wahab; Yashiro,
Seiji
2012cosp...39..334P Altcode: 2012cosp.meet..334P
We investigate the magnetic flux imbalance of two active regions NOAA
11302 and NOAA 11283 during their disk passage. The active region NOAA
11302 appeared in the east limb on September 23, 2011 as beta-gamma
complexity and produced 73 c-class, 27 M-class and 2 X-class flares
many of which were associated with CMEs during the disk passage. The
active region NOAA 11283 appeared on the east limb on September 1,
2011 as beta-gamma complexity and produced 16 c-class, 9 m-class and 2
x-class flares and CMEs. Both these active regions were of similar size
but the evolution of magnetic complexity during their disk passage
was very different. None of them made second disk passage. These
two active regions represent two different class of activity. Among
several reasons, the magnetic flux imbalance of the active regions
result due to the presence of electric current with in the active
regions. The high cadence full disk magnetograms obtained using the
GONG and SDO-HMI instruments serve as the primary data source of this
investigation. We relate the change in the magnetic flux imbalance
with the flare occurrence in these two contrasting active regions.
---------------------------------------------------------
Title: Height of Shock Formation in the Solar Corona Inferred from
Observations of Type II Radio Bursts and Coronal Mass Ejections
Authors: Gopalswamy, Nat; Jain, Rajmal; Prasad Choudhary, Debi;
Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Awasthi,
Arun Kumar; Srivastava, Abhishek K.; Joshi, N. C.; Manoharan, P. K.;
Makela, Pertti; Mahalakshmi, K.; Elamathi, E.; Uddin, Wahab; Yashiro,
Seiji; Akiyam, Sachiko
2012cosp...39..653G Altcode: 2012cosp.meet..653G
No abstract at ADS
---------------------------------------------------------
Title: Quasi-periodic Fast-mode Wave Trains within a Global EUV Wave
and Sequential Transverse Oscillations Detected by SDO/AIA
Authors: Liu, Wei; Ofman, Leon; Nitta, Nariaki V.; Aschwanden, Markus
J.; Schrijver, Carolus J.; Title, Alan M.; Tarbell, Theodore D.
2012ApJ...753...52L Altcode: 2012arXiv1204.5470L
We present the first unambiguous detection of quasi-periodic wave
trains within the broad pulse of a global EUV wave (so-called EIT wave)
occurring on the limb. These wave trains, running ahead of the lateral
coronal mass ejection (CME) front of 2-4 times slower, coherently
travel to distances >~ R <SUB>⊙</SUB>/2 along the solar surface,
with initial velocities up to 1400 km s<SUP>-1</SUP> decelerating to
~650 km s<SUP>-1</SUP>. The rapid expansion of the CME initiated at
an elevated height of 110 Mm produces a strong downward and lateral
compression, which may play an important role in driving the primary
EUV wave and shaping its front forwardly inclined toward the solar
surface. The wave trains have a dominant 2 minute periodicity that
matches the X-ray flare pulsations, suggesting a causal connection. The
arrival of the leading EUV wave front at increasing distances produces
an uninterrupted chain sequence of deflections and/or transverse (likely
fast kink mode) oscillations of local structures, including a flux-rope
coronal cavity and its embedded filament with delayed onsets consistent
with the wave travel time at an elevated (by ~50%) velocity within
it. This suggests that the EUV wave penetrates through a topological
separatrix surface into the cavity, unexpected from CME-caused magnetic
reconfiguration. These observations, when taken together, provide
compelling evidence of the fast-mode MHD wave nature of the primary
(outer) fast component of a global EUV wave, running ahead of the
secondary (inner) slow component of CME-caused restructuring.
---------------------------------------------------------
Title: Coronal Mass Ejections and Type II Radio Bursts from Active
Region 11158
Authors: Yashiro, Seiji; Jain, Rajmal; Prasad Choudhary, Debi; Charan
Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat;
Awasthi, Arun Kumar; Chandra, Ramesh; Srivastava, Abhishek K.; Kayshap,
Pradeep; Joshi, N. C.; Manoharan, P. K.; Makela, Pertti; Mahalakshmi,
K.; Elam, E.
2012cosp...39.2205Y Altcode: 2012cosp.meet.2205Y
The NOAA active region (AR) 11158 emerged at around S20E50 on 2011
February 10 as two bipoles and quickly developed into a large complex
region. During 2011 February 13-17, AR 11158 produced 48 flares
(>C1 level) including the first X-class flare of 15 February,
2011 in solar cycle 24. The 48 flares can be divided into four groups
based on their location within the AR. We examined their associations
of coronal mass ejections (CMEs) and metric type II radio bursts in
order to find preferred locations of both the phenomena. We found that,
out of 48 flares, 15 had associated CMEs, occurring frequently at the
eastern edge of the AR. We also found that six flares were associated
with type II radio bursts and all of them were associated with CMEs
also. No type II was associated with the CME-less flare. This suggests
that the CME association is a necessary condition for a flare to be
associated with a metric type II burst.
---------------------------------------------------------
Title: Multi-wavelength diagnostics of thermal and non-thermal
sources in the 22 April 2011 flare event
Authors: Awasthi, Arun Kumar; Jain, Rajmal; Prasad Choudhary, Debi;
Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy,
Nat; Chandra, Ramesh; Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi,
N. C.; Manoharan, P. K.; Norris, Max; Mahalakshmi, K.; Elamathi, E.;
Uddin, Wahab
2012cosp...39...75A Altcode: 2012cosp.meet...75A
No abstract at ADS
---------------------------------------------------------
Title: A Study of the 12 June 2010 C6.1/SF flare associated with a
CME, surge and energetic particles
Authors: Uddin, Wahab; Jain, Rajmal; Manoharan, P. K.; Prasad
Choudhary, Debi; Charan Dwivedi, Vidya; Aschwanden, Markus; Nitta,
Nariaki; Gopalswamy, Nat; Awasthi, Arun Kumar; Chandra, Ramesh;
Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi, N. C.; Norris, Max;
Makela, Pertti; Mahalaksh, K.
2012cosp...39.2026U Altcode: 2012cosp.meet.2026U
In this paper, we present the multiwavelength analysis of the C6.1/SF
flare on 12 June 2010 from NOAA AR 11081. The flare was observed by
various ground based (ARIES H-alpha; HIRAS Radio) and space borne
observatories (SDO, STEREO, SOHO, GOES). The flare was accompanied by
a spray/surge and a slow coronal mass ejection (CME) that propagated
with a speed of ~382 km/s. The eruption was associated with a weak
solar energetic particle (SEP) event. The solar source of the eruption
was a rapid emerging flux region. The eruption was also associated
with the three major types of radio bursts (type II, III and IV). The
interesting observation is the shock production (type II burst and
SEP event) by a relatively slow CME. We interpret the results in the
light of existing theories.
---------------------------------------------------------
Title: Solar Energetic Particle Events and Associated CMEs during
the Rising Phases of Solar Cycle 23 and 24 - A Comparative study
Authors: Chandra, Ramesh; Jain, Rajmal; Prasad Choudhary, Debi; Charan
Dwivedi, Vidya; Aschwanden, Markus; Nitta, Nariaki; Gopalswamy, Nat;
Awasthi, Arun Kumar; Srivastava, Abhishek K.; Kayshap, Pradeep; Joshi,
N. C.; Manoharan, P. K.; Makela, Pertti; Mahalakshmi, K.; Elamathi,
E.; Uddi, Wahab
2012cosp...39..303C Altcode: 2012cosp.meet..303C
No abstract at ADS
---------------------------------------------------------
Title: Self-Organized Criticality Systems in Astrophysics (Chapter 13)
Authors: Aschwanden, Markus J.
2012arXiv1207.4413A Altcode:
Chapter 13: SOC Systems in Astrophysics --- Content list: 13.1
Theory -- 13.1.1 The Sacle-Free Probability Theorem - 13.1.2
The Fractal-Diffusive Spatio-Temporal Relationship - 13.1.3 Size
Distributions of Astrophysical Observables - 13.1.4 Scaling Laws for
Thermal Emission of Astrophysical Plasmas - 13.1.5 Scaling Laws for
Astrophysical Acceleration Mechanisms - 13.2 Observations -- 13.2.1
Lunar Craters - 13.2.2 Asteroid Belt - 13.2.3 Saturn Ring - 13.2.4
Magnetospheric Substorms and Auroras - 13.2.5 Solar Flares - 13.2.6
Stellar Flares - 13.2.7 Pulsars - 13.2.8 Soft Gamma-Ray Repeaters -
13.2.9 Black-Hole Objects - 13.2.10 Blazars - 13.2.11 Cosmic Rays -
13.3 Conclusions
---------------------------------------------------------
Title: Flares in the Crab Nebula Driven by Untwisting Magnetic Fields
Authors: Sturrock, Peter; Aschwanden, Markus J.
2012ApJ...751L..32S Altcode: 2012arXiv1205.0039S
The recent discovery of PeV electrons from the Crab Nebula, produced
on rapid timescales of one day or less with a sharply peaked gamma-ray
spectrum without hard X-rays, challenges traditional models of diffusive
shock acceleration followed by synchrotron radiation. Here, we outline
an acceleration model involving a DC electric field parallel to the
magnetic field in a twisted toroidal field around the pulsar. Sudden
developments of resistivity in localized regions of the twisted field
are thought to drive the particle acceleration, up to PeV energies,
resulting in flares. This model can reproduce the observed timescales
of T ≈ 1 day, the peak photon energies of U <SUB>Φ, rr </SUB> ≈
1 MeV, maximum electron energies of U <SUB> e, rr </SUB> ≈ 1 PeV,
and luminosities of L ≈ 10<SUP>36</SUP> erg s<SUP>-1</SUP>.
---------------------------------------------------------
Title: SDO/AIA Detection of Quasi-periodic Wave Trains Within Global
EUV ("EIT") Waves and Their Coronal Seismology Implications
Authors: Liu, Wei; Ofman, L.; Aschwanden, M. J.; Nitta, N.; Schrijver,
C. J.; Title, A. M.; Tarbell, T. D.
2012AAS...22051501L Altcode:
The nature of global EUV waves (so-called "EIT waves") has long
been under debate because of instrumental limitations and projection
effects when viewed on the solar disk. We present here high cadence
SDO/AIA observations of global EUV waves occurring on the limb. We
report newly discovered quasi-periodic wave trains located in the low
corona within a broad, diffuse pulse of the global EUV wave ahead of
the lateral CME front/flank. These waves coherently travel to large
distances on the order of 1 solar radii with initial velocities up
to 1400 km/s. They have dominant 1-3 minute periodicities that often
match the X-ray pulsations of the accompanying flare, suggestive of
a causal connection. In addition, recently discovered quasi-periodic
fast propagating (QFP) waves of 1000-2000 km/s (Liu, Title, Zhao et
al. 2011 ApJL) are found in the funnel of coronal loops rooted at the
flare kernel. These waves are spatially confined within the CME bubble
and rapidly disappear while approaching the CME front, suggestive
of strong damping and/or dispersion. These observations provide new
evidence of the fast-mode wave nature of the primary, fast component
of a global EUV wave, running ahead of a secondary, slow component
of CME-caused restructuring of the coronal magnetic field. We suggest
that the two types of quasi-periodic waves are both integral parts of
global coronal dynamics manifested as a CME/flare eruption, and they
have important implications for global and local coronal seismology.
---------------------------------------------------------
Title: Flares in the Crab Nebula Driven by Untwisting Magnetic Fields
Authors: Sturrock, Peter A.; Aschwanden, M. J.
2012AAS...22040708S Altcode:
The recent discovery of PeV electrons from the Crab nebula, produced on
rapid time scales of one day or less with a sharply peaked gamma-ray
spectrum without hard X-rays, challenges traditional diffusive shock
acceleration models followed by synchrotron radiation. Here we outline
an acceleration model involving a DC electric field, parallel to the
magnetic field, in a twisted toroidal field in the nebula.. Sudden
developments of resistivity in localized regions of the twisted field
are thought to drive the flares and associated particle acceleration up
to PeV energies. This model can reproduce the observed time scales of
about 1 day, the peak photon energies of about 1 MeV, maximum electron
energies of about 1 PeV, and a luminosity of about 1036 erg s-1.
---------------------------------------------------------
Title: Constraining Differential Emission Measure and Energy Estimates
for Microflares and Active Regions by Combining SDO/AIA and RHESSI
Authors: Inglis, Andrew; Christe, S.; Aschwanden, M.
2012AAS...22050905I Altcode:
Direct diagnostics of the fundamental parameters of solar coronal
phenomena remains an active and challenging goal. Constrained, spatially
resolved values of many parameters, such as the magnetic field strength,
temperature, emission measure, and energy, are often difficult to
achieve. However, the advent of the Atmospheric Imaging Assembly (AIA)
on board SDO provides us with greater opportunity to pursue these
measurements. <P />Here, we present differential emission measure
(DEM) analysis of a selection of recent microflares and hot active
regions, utilising a combination of high temperature RHESSI data, and
forward-fitting temperature mapping, a procedure developed by Achwanden
et al. (2011) for SDO/AIA. This procedure models the plasma temperature
as a Gaussian distribution, and uses the instrument response functions
to find the distribution which best reproduces the observed fluxes in
each AIA wavelength. The accuracy of the method is examined, including
an investigation of uncertainties and the temperature range over which
the DEM is reasonably constrained. We also study the ability of the
method to accurately represent the flux in all of the AIA wavelengths
simultaneously. This technique is combined with RHESSI spectral data
to produce complementary time-dependent DEM measurements for four
microflares from June - August 2011, and one set of post-flare loops
from 2010 October 16. In general, the high temperature emission measure
fitted by RHESSI has a steeper spectral index than that observed with
AIA. However, we illustrate how the high temperature RHESSI data can
be used to further inform the AIA fitting procedure, improving the
results. We also expand on the AIA forward fitting to produce spatial
energy maps of microflares and active regions, also allowing estimates
of the total energy of these regions to be made.
---------------------------------------------------------
Title: Force-Free Magneto-Stereoscopy of Coronal Loops
Authors: Aschwanden, Markus J.; Malanushenko, A.; Wuelser, J.; Nitta,
N.; Lemen, J. R.; DeRosa, M.
2012AAS...22041103A Altcode:
We derive an analytical approximation of nonlinear force-free
magnetic field solutions (NLFFF) that can efficiently be used for
fast forward-fitting to solar magnetic data, constrained either by
observed line-of-sight magnetograms and stereoscopically triangulated
coronal loops, or by 3D vector-magnetograph data. We test the code by
forward-fitting to simulated data, to force-free solutions derived by
Low and Lou (1990), and to active regions observed with STEREO/EUVI and
SOHO/MDI. The forward-fitting tests demonstrate: (i) a satisfactory
convergence behavior (with typical misalignment angles of 1-10 deg),
(ii) a high fidelity of retrieved force-free alpha-parameters, and
(iii) relatively fast computation times (from seconds to minutes). The
novel feature of this NLFFF code is the derivation of a quasi-forcefree
field based on coronal constraints, which bypasses the non-forcefree
photosphere of standard magnetograms. Applications range from magnetic
modeling of loops to the determnination of electric currents, twist,
helicity, and free (non-potential) energy in active regions.
---------------------------------------------------------
Title: Theoretical Models of Self-Organized Criticality (SOC) Systems
Authors: Aschwanden, Markus J.
2012arXiv1204.5119A Altcode:
In this chapter 2 of the e-book "Self-Organized Criticality Systems"
we summarize the classical cellular automaton models, which consist of a
statistical aspect that is universal to all SOC systems, and a physical
aspect that depends on the physical definition of the observable. Then
we derive some general analytical formulations of SOC processes, such as
the exponential-growth SOC model and the fractal-diffusive SOC model,
which also have universal validity for SOC processes, while specific
applications to observations require additional physical scaling laws
(e.g., for astrophysical or geophysical observations). Finally we
discuss alternative SOC processes, SOC-related, or non-SOC processes,
such as: self-organization (without criticality), forced SOC model,
Brownian motion or classical diffusion, hyper-diffusion and Levy
flight, nonextensive Tsallis entropy, turbulence, percolation, phase
transitions, network systems, and chaotic systems. We synthesize a
metrics that specifies which observational SOC properties are shared
by these processes.
---------------------------------------------------------
Title: Force-free Magnetic Fields and Electric Currents inferred
from Coronal Loops and Stereoscopy
Authors: Aschwanden, Markus J.; Boerner, P.; Schrijver, C. J.;
Malanushenko, A.
2012decs.confE.105A Altcode:
Force-free magnetic fields are considered to be a natural state of the
low plasma-beta corona. There exist about a dozen of numerical nonlinear
force-free field (NLFFF) computation codes that are able to caclulate
a divergence-free and force-free solution of the magnetic field, by
extrapolation from a lower boundary condition that is specified with
3D vector magnetograph data. However, significant differences in the
solutions have been found among the different NLFFF codes, as well as in
comparison with stereoscopically triangulated 3D coordinates of coronal
loops, exhibiting field misalignment angles of 20-40 degrees. Each
calculation of a NLFFF solution is computing-intensive and no code is
fast enough to enable forward-fitting to observations. Here we derive
an analytical approximation of NLFFF solutions that is accurate to
second order and can efficiently be used for forward-fitting to coronal
loops. We demonstrate the accurcay of the NLFFF forward-fitting code by
reproducing the Low and Lou (1990) analytical model withg an accuracy
of <5 degres. Further, we show examples of fitted NLFFF solutions to
STEREO observations of coronal loops. Future NLFFF fits are expected
based on line-of-sight magnetograms and automated loop tracings only,
without requiring vector field and STEREO data.
---------------------------------------------------------
Title: A statistical fractal-diffusive avalanche model of a
slowly-driven self-organized criticality system
Authors: Aschwanden, M. J.
2012A&A...539A...2A Altcode: 2011arXiv1112.4859A
<BR /> Aims: We develop a statistical analytical model that predicts
the occurrence frequency distributions and parameter correlations
of avalanches in nonlinear dissipative systems in the state of
a slowly-driven self-organized criticality (SOC) system. <BR />
Methods: This model, called the fractal-diffusive SOC model, is based
on the following four assumptions: (i) the avalanche size L grows as a
diffusive random walk with time T, following L ∝ T<SUP>1/2</SUP>;
(ii) the energy dissipation rate f(t) occupies a fractal volume
with dimension D<SUB>S</SUB>; (iii) the mean fractal dimension of
avalanches in Euclidean space S = 1,2,3 is D<SUB>S</SUB> ≈ (1 +
S)/2; and (iv) the occurrence frequency distributions N(x) ∝ x<SUP>
- α<SUB>x</SUB></SUP> based on spatially uniform probabilities in a
SOC system are given by N(L) ∝ L<SUP> - S</SUP>, with S being the
Eudlidean dimension. We perform cellular automaton simulations in three
dimensions (S = 1,2,3) to test the theoretical model. <BR /> Results:
The analytical model predicts the following statistical correlations:
F ∝ L<SUP>D<SUB>S</SUB></SUP> ∝ T<SUP>D<SUB>S</SUB>/2</SUP> for
the flux, P ∝ L<SUP>S</SUP> ∝ T<SUP>S/2</SUP> for the peak energy
dissipation rate, and E ∝ FT ∝ T<SUP>1 + D<SUB>S</SUB>/2</SUP> for
the total dissipated energy; the model predicts powerlaw distributions
for all parameters, with the slopes α<SUB>T</SUB> = (1 + S)/2,
α<SUB>F</SUB> = 1 + (S - 1)/D<SUB>S</SUB>, α<SUB>P</SUB> = 2 - 1/S,
and α<SUB>E</SUB> = 1 + (S - 1)/(D<SUB>S</SUB> + 2). The cellular
automaton simulations reproduce the predicted fractal dimensions,
occurrence frequency distributions, and correlations within a
satisfactory agreement within ≈ 10% in all three dimensions. <BR />
Conclusions: One profound prediction of this universal SOC model is
that the energy distribution has a powerlaw slope in the range of
α<SUB>E</SUB> = 1.40 - 1.67, and the peak energy distribution has a
slope of α<SUB>P</SUB> = 1.67 (for any fractal dimension D<SUB>S</SUB>
= 1,...,3 in Euclidean space S = 3), and thus predicts that the
bulk energy is always contained in the largest events, which rules
out significant nanoflare heating in the case of solar flares. <P
/>Movie included with Fig. 1 is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: SDO/AIA Observations of Various Coronal EUV Waves Associated
with Flares/CMEs and Their Coronal Seismology Implications
Authors: Liu, Wei; Ofman, Leon; Aschwanden, Markus J.; Nitta, Nariaki;
Zhao, Junwei; Title, Alan M.
2012decs.confE..87L Altcode:
MHD waves can be used as diagnostic tools of coronal seismology to
decipher otherwise elusive critical physical parameters of the solar
corona, such as the magnetic field strength and plasma density. They
are analogous to acoustic waves used in helioseismology, but with
complexities arising from the magnetic field and nonlinearity. Recent
high cadence, high resolution, full-disk imaging observations from
SDO/AIA have opened a new chapter in understanding these waves. Various
types of EUV waves associated with flares/CMEs have been discovered
or observed in unprecedented detail. In this presentation, we will
review such new AIA observations, focusing on the following topics and
their interrelationships: (1) quasi-periodic fast waves traveling along
coronal funnels within CME bubbles at speeds up to 2000 km/s, associated
with flare pulsations at similar frequencies; (2) quasi-periodic wave
trains within broad, diffuse pulses of global EUV waves (so-called
EIT waves) running ahead of CME fronts; (3) interactions of global EUV
waves with local coronal structures on their paths, such as flux-rope
coronal cavities and their embedded filaments (kink oscillations)
and coronal holes/active regions (deflections). We will discuss the
implications of these observations on coronal seismology, on their roles
in transporting energy through different parts of the solar atmosphere,
and on understanding their associated eruptive flares/CMEs.
---------------------------------------------------------
Title: SDO/AIA Observations of Quasi-periodic Fast (~1000 km/s)
Propagating (QFP) Waves as Evidence of Fast-mode Magnetosonic Waves
in the Low Corona: Statistics and Implications
Authors: Liu, W.; Ofman, L.; Title, A. M.; Zhao, J.; Aschwanden, M. J.
2011AGUFMSH33A2043L Altcode:
Recent EUV imaging observations from SDO/AIA led to the discovery of
quasi-periodic fast (~2000 km/s) propagating (QFP) waves in active
regions (Liu et al. 2011). They were interpreted as fast-mode
magnetosonic waves and reproduced in 3D MHD simulations (Ofman
et al. 2011). Since then, we have extended our study to a sample
of more than a dozen such waves observed during the SDO mission
(2010/04-now). We will present the statistical properties of these waves
including: (1) Their projected speeds measured in the plane of the sky
are about 400-2200 km/s, which, as the lower limits of their true speeds
in 3D space, fall in the expected range of coronal Alfven or fast-mode
speeds. (2) They usually originate near flare kernels, often in the wake
of a coronal mass ejection, and propagate in narrow funnels of coronal
loops that serve as waveguides. (3) These waves are launched repeatedly
with quasi-periodicities in the 30-200 seconds range, often lasting
for more than one hour; some frequencies coincide with those of the
quasi-periodic pulsations (QPPs) in the accompanying flare, suggestive
a common excitation mechanism. We obtained the k-omega diagrams and
dispersion relations of these waves using Fourier analysis. We estimate
their energy fluxes and discuss their contribution to coronal heating
as well as their diagnostic potential for coronal seismology.
---------------------------------------------------------
Title: A Statistical Fractal-Diffusive Avalanche Model of a
Slowly-Driven Self-Organized Criticality System
Authors: Aschwanden, M. J.
2011AGUFMSH51C2018A Altcode:
We develop a statistical analytical model that predicts the
occurrence frequency distributions and parameter correlations
of avalanches in nonlinear dissipative systems in the state
of slowly-driven self-organized criticality (SOC). This model,
called the fractal-diffusive SOC model, is based on the following
four assumptions: (i) The avalanche size L grows as a diffusive
random walk with time T, following L ∝ T<SUP>1/2</SUP>; (ii) The
instantaneous energy dissipation rate P occupies a fractal volume
with dimension D<SUB>S</SUB>, which predicts the relationships P
∝ L<SUP>D<SUB>S</SUB> ∝ T<SUP>D<SUB>S/2</SUB> and the total
dissipated energy E ∝ P T ∝ T<SUP>1+D<SUB>S/2</SUB>; (iii)
The mean fractal dimension of avalanches in Euclidean space
S=1,2,3 is D<SUB>S</SUB> ≈ (1+S)/2; and (iv) The occurrence
frequency distributions N(x) ∝ x<SUP>-α <SUB>x</SUB> based
on spatially uniform probabilities in a SOC system are given by
N(L) ∝ L<SUP>-S</SUP>, which predicts powerlaw distributions
for all parameters, with the slopes α <SUB>T=(1+S)/2</SUB>, α
<SUB>P=1+(S-1)/D_S</SUB>, and α <SUB>E=1+(S-1)/(D_S+2)</SUB>. We test
the predicted fractal dimensions, occurrence frequency distributions,
and correlations with numerical simulations of cellular automaton
models in three dimensions S=1,2,3 and find satisfactory agreement
within ≈ 10%. One profound prediction of this universal SOC model
is that the energy distribution has a powerlaw slope in the range
of α <SUB>E=1.40-1.67</SUB> (for any fractal dimension) and thus
predicts that the bulk energy is always contained in the largest
events, which rules out significant nanoflare heating in the case
of solar flares.<br /><br /><img class="jpg" border=0
width=600px src="/meetings/fm11/program/images/SH51C-2018_A.jpg">
<P /></SUP></SUP></SUP></SUP>
---------------------------------------------------------
Title: The State of Self-organized Criticality of the Sun during
the Last Three Solar Cycles. II. Theoretical Model
Authors: Aschwanden, Markus J.
2011SoPh..274..119A Altcode: 2010arXiv1010.0986A
The observed power-law distributions of solar-flare parameters can be
interpreted in terms of a nonlinear dissipative system in a state of
self-organized criticality (SOC). We present a universal analytical
model of an SOC process that is governed by three conditions: i) a
multiplicative or exponential growth phase, ii) a randomly interrupted
termination of the growth phase, and iii) a linear decay phase. This
basic concept approximately reproduces the observed frequency
distributions. We generalize it to a randomized exponential growth
model, which also includes a (log-normal) distribution of threshold
energies before the instability starts, as well as randomized decay
times, which can reproduce both the observed occurrence-frequency
distributions and the scatter of correlated parameters more
realistically. With this analytical model we can efficiently perform
Monte-Carlo simulations of frequency distributions and parameter
correlations of SOC processes, which are simpler and faster than
the iterative simulations of cellular automaton models. Solar-cycle
modulations of the power-law slopes of flare-frequency distributions
can be used to diagnose the thresholds and growth rates of magnetic
instabilities responsible for solar flares.
---------------------------------------------------------
Title: The State of Self-organized Criticality of the Sun During
the Last Three Solar Cycles. I. Observations
Authors: Aschwanden, Markus J.
2011SoPh..274...99A Altcode: 2010arXiv1006.4861A
We analyze the occurrence-frequency distributions of peak fluxes
[P], total fluxes [E], and durations [T] of solar flares over
the last three solar cycles (during 1980 - 2010) from SMM/HXRBS,
CGRO/BATSE, and RHESSI hard X-ray data. From the synthesized data we
find powerlaw slopes with mean values of α<SUB>P</SUB>=1.73±0.07
for the peak flux, α<SUB>E</SUB>=1.62±0.12 for the total flux, and
α<SUB>T</SUB>=1.99±0.35 for flare durations. We find a tendency of
an anti-correlation of the powerlaw slope of peak fluxes with the
flare rate or sunspot number as a function of the solar cycle. The
occurrence powerlaw slope is always steeper by Δα≈0.1 during a
solar-cycle minimum compared with the previous solar-cycle maximum,
but the relative amplitude varies for each cycle or instrument. Since
each solar cycle has been observed with a different instrument, part
of the variation could be attributed to instrumental characteristics
and different event selection criteria used in generating the event
catalogs. The relatively flatter powerlaw slopes during solar maxima
could indicate more energetic flares with harder electron-energy
spectra, probably due to a higher magnetic complexity of the solar
corona. This would imply a non-stationarity (or solar-cycle dependence)
of the coronal state of self-organized criticality.
---------------------------------------------------------
Title: Transient phenomena and thermal evolution during the Boss'
Day flare of 2010 October 16 with AIA, EVE and RHESSI
Authors: Inglis, A.; Christe, S.; Aschwanden, M. J.; Dennis, B. R.
2011AGUFMSH41A1902I Altcode:
On 2010 October 16th, Boss' Day, SDO/AIA observed an M1.6 flare,
its first flare using automatic exposure control. This flare also
exhibited a number of interesting features, including a faint wavefront,
visible in the 171 A and 193 A channels of AIA with an estimated speed
of ~700-900 km/s, a CME with a much lower velocity of ~300 km/s,
a large plasma ribbon ignited by the flare, late-phase post-flare
loops situated away from the main flare kernel, and lastly a vertically
polarised kink mode excited far from the flare kernel which was subject
to a recent coronal seismological study by Aschwanden & Schrijver
(2011). We present an investigation of a number of these features. We
find that the wavefront observed in AIA is most likely a signature of
the CME observed at later times with LASCO, thus indicating that the CME
experiences significant deceleration. A deceleration scenario is also
supported by the fact that the observed wavefront velocity, which is
only visible some time after the flare, is too slow to be consistent
with the timing of the flare eruption. The differential emission
measure and energetics of the late-phase post-flare loop are also
investigated using coordinated AIA, EVE, and RHESSI observations. We
find that the temperature in the late-phase loops is initially modelled
well by a single dominant temperature profile in each pixel. However,
this model breaks down as the loop cools suggesting a broadening of
the differential emission measure as a function of time. This case
study illustrates the limitations of single-temperature modelling
of the solar corona, but also reveals the diagnostic potential of
multi-wavelength AIA studies.
---------------------------------------------------------
Title: Modeling Super-fast Magnetosonic Waves Observed by SDO in
Active Region Funnels
Authors: Ofman, L.; Liu, W.; Title, A.; Aschwanden, M.
2011ApJ...740L..33O Altcode:
Recently, quasi-periodic, rapidly propagating waves have been observed
in extreme ultraviolet by the Solar Dynamics Observatory/Atmospheric
Imaging Assembly (AIA) instrument in about 10 flare/coronal mass
ejection (CME) events thus far. A typical example is the 2010 August 1
C3.2 flare/CME event that exhibited arc-shaped wave trains propagating
in an active region (AR) magnetic funnel with ~5% intensity variations
at speeds in the range of 1000-2000 km s<SUP>-1</SUP>. The fast
temporal cadence and high sensitivity of AIA enabled the detection
of these waves. We identify them as fast magnetosonic waves driven
quasi-periodically at the base of the flaring region and develop
a three-dimensional MHD model of the event. For the initial state
we utilize the dipole magnetic field to model the AR and include
gravitationally stratified density at coronal temperature. At the
coronal base of the AR, we excite the fast magnetosonic wave by
periodic velocity pulsations in the photospheric plane confined to a
funnel of magnetic field lines. The excited fast magnetosonic waves
have similar amplitude, wavelength, and propagation speeds as the
observed wave trains. Based on the simulation results, we discuss the
possible excitation mechanism of the waves, their dynamical properties,
and the use of the observations for coronal MHD seismology.
---------------------------------------------------------
Title: Solar Stereoscopy and Tomography
Authors: Aschwanden, Markus J.
2011LRSP....8....5A Altcode:
We review stereoscopic and tomographic methods used in the solar
corona, including ground-based and space-based measurements, using
solar rotation or multiple spacecraft vantage points, in particular
from the STEREO mission during 2007-2010. Stereoscopic and tomographic
observations in the solar corona include large-scale structures,
streamers, active regions, coronal loops, loop oscillations, acoustic
waves in loops, erupting filaments and prominences, bright points, jets,
plumes, flares, CME source regions, and CME-triggered global coronal
waves. Applications in the solar interior (helioseismic tomography)
and reconstruction and tracking of CMEs from the outer corona and into
the heliosphere (interplanetary CMEs) are not included.
---------------------------------------------------------
Title: Coronal Seismology in the SDO Era: AIA Observations of Various
Coronal Waves Associated with CMEs/Flares
Authors: Liu, Wei; Ofman, Leon; Aschwanden, Markus J.; Nitta, Nariaki;
Zhao, Junwei; Title, Alan M.
2011sdmi.confE..49L Altcode:
MHD waves, as critical diagnostic tools of coronal seismology, can be
used to decipher otherwise elusive physical parameters of the solar
corona, such as the magnetic field strength and plasma density. They
are analogous to acoustic waves used in helioseismology. Recent high
cadence, high resolution, full-disk imaging observations from SDO/AIA
have opened a new chapter in understanding these waves. Various types
of waves associated with flares and/or CMEs have been discovered. In
this presentation, we will review such new AIA observations, focusing
on the following topics: (1) fine structures in CME-related global EUV
waves (so-called EIT waves), including a diffuse pulse superimposed
with multiple sharp fronts or "ripples" (Liu et al. 2010, ApJL); (2)
quasi-periodic fast waves traveling in coronal funnels at speeds up to
2000 km/s and associated with flares pulsating at similar frequencies
(Liu et al. 2011, ApJL); (3) interaction of global EUV waves with local
coronal structures on their paths, such as flux-rope coronal cavities
(triggered kink oscillations, Liu et al. in preparation) and coronal
holes/active regions (deflection). We will discuss the implications
of these observations on coronal seismology and on understanding their
associated flares and CMEs. We also anticipate to exchange ideas with
helioseismologists at this workshop, in a hope to bring together coronal
seismology and helioseismology techniques to advance our understanding
of solar oscillations from the interior to the upper atmosphere.
---------------------------------------------------------
Title: Implications of X-ray Observations for Electron Acceleration
and Propagation in Solar Flares
Authors: Holman, G. D.; Aschwanden, M. J.; Aurass, H.; Battaglia, M.;
Grigis, P. C.; Kontar, E. P.; Liu, W.; Saint-Hilaire, P.; Zharkova,
V. V.
2011SSRv..159..107H Altcode: 2011SSRv..tmp..162H; 2011SSRv..tmp..242H; 2011SSRv..tmp..260H;
2011SSRv..tmp...86H; 2011arXiv1109.6496H
High-energy X-rays and γ-rays from solar flares were discovered
just over fifty years ago. Since that time, the standard for
the interpretation of spatially integrated flare X-ray spectra
at energies above several tens of keV has been the collisional
thick-target model. After the launch of the Reuven Ramaty High
Energy Solar Spectroscopic Imager ( RHESSI) in early 2002, X-ray
spectra and images have been of sufficient quality to allow a greater
focus on the energetic electrons responsible for the X-ray emission,
including their origin and their interactions with the flare plasma
and magnetic field. The result has been new insights into the flaring
process, as well as more quantitative models for both electron
acceleration and propagation, and for the flare environment with
which the electrons interact. In this article we review our current
understanding of electron acceleration, energy loss, and propagation
in flares. Implications of these new results for the collisional
thick-target model, for general flare models, and for future flare
studies are discussed.
---------------------------------------------------------
Title: Coronal Loop Oscillations Observed with Atmospheric Imaging
Assembly—Kink Mode with Cross-sectional and Density Oscillations
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.
2011ApJ...736..102A Altcode: 2011arXiv1105.2191A
A detailed analysis of a coronal loop oscillation event is presented,
using data from the Atmospheric Imaging Assembly on board the Solar
Dynamics Observatory (SDO) for the first time. The loop oscillation
event occurred on 2010 October 16, 19:05-19:35 UT and was triggered
by an M2.9 GOES-class flare, located inside a highly inclined cone
of a narrow-angle coronal mass ejection. This oscillation event had a
number of unusual features: (1) excitation of kink-mode oscillations in
vertical polarization (in the loop plane), (2) coupled cross-sectional
and density oscillations with identical periods, (3) no detectable
kink amplitude damping over the observed duration of four kink-mode
periods (P=6.3 minutes), (4) multi-loop oscillations with slightly
(≈10%) different periods, and (5) a relatively cool loop temperature
of T ≈ 0.5 MK. We employ a novel method of deriving the electron
density ratio external and internal to the oscillating loop from the
ratio of Alfvénic speeds deduced from the flare trigger delay and the
kink-mode period, i.e., n<SUB>e</SUB> /n<SUB>i</SUB> = (v<SUB>A</SUB>
/v <SUB>Ae</SUB>)<SUP>2</SUP> = 0.08 ± 0.01. The coupling of the
kink mode and cross-sectional oscillations can be explained as a
consequence of the loop length variation in the vertical polarization
mode. We determine the exact footpoint locations and loop length
with stereoscopic triangulation using STEREO/EUVI/A data. We model
the magnetic field in the oscillating loop using Helioseismic and
Magnetic Imager/SDO magnetogram data and a potential-field model and
find agreement with the seismological value of the magnetic field,
B <SUB>kink</SUB> = 4.0 ± 0.7 G, within a factor of two.
---------------------------------------------------------
Title: Direct Imaging of Quasi-periodic Fast Propagating Waves of
~2000 km s<SUP>-1</SUP> in the Low Solar Corona by the Solar Dynamics
Observatory Atmospheric Imaging Assembly
Authors: Liu, Wei; Title, Alan M.; Zhao, Junwei; Ofman, Leon;
Schrijver, Carolus J.; Aschwanden, Markus J.; De Pontieu, Bart;
Tarbell, Theodore D.
2011ApJ...736L..13L Altcode: 2011arXiv1106.3150L
Quasi-periodic propagating fast mode magnetosonic waves in the solar
corona were difficult to observe in the past due to relatively low
instrument cadences. We report here evidence of such waves directly
imaged in EUV by the new Atmospheric Imaging Assembly instrument
on board the Solar Dynamics Observatory. In the 2010 August 1 C3.2
flare/coronal mass ejection event, we find arc-shaped wave trains of
1%-5% intensity variations (lifetime ~200 s) that emanate near the
flare kernel and propagate outward up to ~400 Mm along a funnel of
coronal loops. Sinusoidal fits to a typical wave train indicate a phase
velocity of 2200 ± 130 km s<SUP>-1</SUP>. Similar waves propagating
in opposite directions are observed in closed loops between two flare
ribbons. In the k-ω diagram of the Fourier wave power, we find a
bright ridge that represents the dispersion relation and can be well
fitted with a straight line passing through the origin. This k-ω
ridge shows a broad frequency distribution with power peaks at 5.5,
14.5, and 25.1 mHz. The strongest signal at 5.5 mHz (period 181 s)
temporally coincides with quasi-periodic pulsations of the flare,
suggesting a common origin. The instantaneous wave energy flux
of (0.1-2.6) × 10<SUP>7</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>
estimated at the coronal base is comparable to the steady-state heating
requirement of active region loops.
---------------------------------------------------------
Title: A New Method for Modeling the Coronal Magnetic Field with
STEREO and Submerged Dipoles
Authors: Sandman, A. W.; Aschwanden, M. J.
2011SoPh..270..503S Altcode: 2011SoPh..tmp..109S
Recent magnetic modeling efforts have shown substantial misalignment
between theoretical models and observed coronal loop morphology as
observed by STEREO/EUVI, regardless of the type of model used. Both
potential field and non-linear force-free field (NLFFF) models
yielded overall misalignment angles of 20 - 40 degrees, depending on
the complexity of the active region (Sandman et al., Solar Phys.259,
1, 2009; DeRosa et al., Astrophys. J.696, 1780, 2009) We demonstrate
that with new, alternative forward-fitting techniques, we can achieve a
significant reduction in the misalignment angles compared with potential
field source surface (PFSS) models and NLFFF models. Fitting a series
of submerged dipoles to the field directions of stereoscopically
triangulated loops in four active regions (30 April, 9 May, 19 May,
and 11 December 2007), we find that 3 - 5 dipoles per active region
yield misalignment angles of ∼ 11° - 18°, a factor of two smaller
than those given by previously established extrapolation methods. We
investigate the spatial and temporal variation of misalignment angles
with subsets of loops for each active region, as well as loops observed
prior to and following a flare and filament eruption, and find that the
spatial variation of median misalignment angles within an active region
(up to 75%) exceeds the temporal variation associated with the flare
(up to 40%). We also examine estimates of the stereoscopic error of our
analysis. The corrected values yield a residual misalignment of 7° -
13°, which is attributed to the non-potentiality due to currents in
the active regions.
---------------------------------------------------------
Title: 3-D reconstruction of active regions with STEREO
Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre
2011JASTP..73.1082A Altcode:
We review data analysis and physical modeling related to the 3-D
reconstruction of active regions in the solar corona, using stereoscopic
image pairs from the STEREO/EUVI instrument. This includes the 3-D
geometry of coronal loops (with measurements of the loop inclination
plane, coplanarity, circularity, and hydrostaticity), the 3-D electron
density and temperature distribution (which enables diagnostics of
hydrostatic, hydrodynamic, and heating processes), the 3-D magnetic
field (independent of any theoretical model based on photospheric
extrapolations), as well as the 3-D reconstruction of CME phenomena,
such as EUV dimming, CME acceleration, CME bubble expansion, and
associated Lorentz forces that excite MHD kink-mode oscillations in the
surroundings of a CME launch site. The mass of CMEs, usually measured
from white-light coronagraphs, can be determined independently from
the EUV dimming in the CME source region. The detailed 3-D density
and temperature structure of an active region can be modeled using
the method of instant stereoscopic tomography with orders of magnitude
higher spatial resolution than with standard solar-rotation tomography.
---------------------------------------------------------
Title: SDO/AIA Observations of a Global EUV Disturbance Traveling
into a Coronal Cavity and Its Subsequent Oscillations: New Evidence
of Fast Mode MHD Waves
Authors: Liu, Wei; Aschwanden, M. J.; Ofman, L.; Nitta, N. V.; Tarbell,
T. D.
2011SPD....42.0906L Altcode: 2011BAAS..43S.0906L
We report new SDO/AIA observations of a global EUV disturbance
that propagates at 600 km/s and sweeps through a coronal cavity,
instigating its bodily transverse oscillations. The high temporal
resolution and large FOV of AIA allow us to clearly see, for the first
time, the timing coincidence between the onsets of the oscillations
and the arrival of the disturbance at increasing distances covering
300 Mm in the neighborhood of the cavity. There is a time delay of
the oscillations from the near side to the far side of the cavity,
which is consistent with the travel time of the global perturbation. In
addition, we find a fine structure consisting of evenly spaced pulses
of periods 100-120 s within the global disturbance. In contrast, the
CME loop expansion falls behind the global disturbance at a smaller
velocity of 200 km/s. These observations suggests that this global
disturbance is a real fast mode MHD wave that continues propagating
into the cavity, rather than an apparent wave caused by CME expulsion
that is not expected to penetrate through a topological separatrix,
including the flux rope cavity boundary here. The cavity and its
hosted prominence have oscillation amplitudes of 20 km/s and periods
of 20-30 minutes. Such unusually long periods, compared with a few
minutes commonly observed in coronal loops, likely reflect kink mode
oscillations of the long cavity flux rope of a large length (a fraction
of the solar radius).
---------------------------------------------------------
Title: Energy-Dependent Timing of Thermal Emission in Solar Flares
Authors: Jain, Rajmal; Awasthi, Arun Kumar; Rajpurohit, Arvind Singh;
Aschwanden, Markus J.
2011SoPh..270..137J Altcode: 2011SoPh..tmp...60J; 2011SoPh..tmp...83J; 2011arXiv1103.5546J
We report solar flare plasma to be multi-thermal in nature based
on the theoretical model and study of the energy-dependent timing of
thermal emission in ten M-class flares. We employ high-resolution X-ray
spectra observed by the Si detector of the "Solar X-ray Spectrometer"
(SOXS). The SOXS onboard the Indian GSAT-2 spacecraft was launched
by the GSLV-D2 rocket on 8 May 2003. Firstly we model the spectral
evolution of the X-ray line and continuum emission flux F(ε) from
the flare by integrating a series of isothermal plasma flux. We
find that the multi-temperature integrated flux F(ε) is a power-law
function of ε with a spectral index (γ)≈−4.65. Next, based on
spectral-temporal evolution of the flares we find that the emission in
the energy range E=4 - 15 keV is dominated by temperatures of T=12 -
50 MK, while the multi-thermal power-law DEM index (δ) varies in
the range of −4.4 and −5.7. The temporal evolution of the X-ray
flux F(ε,t) assuming a multi-temperature plasma governed by thermal
conduction cooling reveals that the temperature-dependent cooling time
varies between 296 and 4640 s and the electron density (n<SUB>e</SUB>)
varies in the range of n<SUB>e</SUB>=(1.77 - 29.3)×10<SUP>10</SUP>
cm<SUP>−3</SUP>. Employing temporal evolution technique in the current
study as an alternative method for separating thermal from nonthermal
components in the energy spectra, we measure the break-energy point,
ranging between 14 and 21±1.0 keV.
---------------------------------------------------------
Title: Multi-thermal observations of the 2010 October 16 flare:heating
of a ribbon via loops, or a blast wave?
Authors: Christe, Steven; Inglis, A.; Aschwanden, M.; Dennis, B.
2011SPD....42.2237C Altcode: 2011BAAS..43S.2237C
On 2010 October 16th SDO/AIA observed its first flare using automatic
exposure control. Coincidentally, this flare also exhibited a large
number of interesting features. Firstly, a large ribbon significantly
to the solar west of the flare kernel was ignited and was visible in
all AIA wavelengths, posing the question as to how this energy was
deposited and how it relates to the main flare site. A faint blast
wave also emanates from the flare kernel, visible in AIA and observed
traveling to the solar west at an estimated speed of 1000 km/s. This
blast wave is associated with a weak white-light CME observed with
STEREO B and a Type II radio burst observed from Green Bank Observatory
(GBSRBS). One possibility is that this blast wave is responsible for the
heating of the ribbon. However, closer scrutiny reveals that the flare
site and the ribbon are in fact connected magnetically via coronal
loops which are heated during the main energy release. These loops
are distinct from the expected hot, post-flare loops present within
the main flare kernel. RHESSI spectra indicate that these loops are
heated to approximately 10 MK in the immediate flare aftermath. Using
the multi-temperature capabilities of AIA in combination with RHESSI,
and by employing the cross-correlation mapping technique, we are able
to measure the loop temperatures as a function of time over several
post-flare hours and hence measure the loop cooling rate. We find
that the time delay between the appearance of loops in the hottest
channel, 131 A, and the cool 171 A channel, is 70 minutes. Yet the
causality of this event remains unclear. Is the ribbon heated via
these interconnected loops or via a blast wave?
---------------------------------------------------------
Title: Solar Corona Loop Studies with the Atmospheric Imaging
Assembly. I. Cross-sectional Temperature Structure
Authors: Aschwanden, Markus J.; Boerner, Paul
2011ApJ...732...81A Altcode: 2011arXiv1103.0228A
We present a first systematic study on the cross-sectional temperature
structure of coronal loops using the six coronal temperature filters
of the Atmospheric Imaging Assembly (AIA) instrument on the Solar
Dynamics Observatory (SDO). We analyze a sample of 100 loop snapshots
measured at 10 different locations and 10 different times in active
region NOAA 11089 on 2010 July 24, 21:00-22:00 UT. The cross-sectional
flux profiles are measured and a cospatial background is subtracted
in six filters in a temperature range of T ≈ 0.5-16 MK, and
four different parameterizations of differential emission measure
(DEM) distributions are fitted. We find that the reconstructed DEMs
consist predominantly of narrowband peak temperature components with
a thermal width of σ<SUB>log (T)</SUB> <= 0.11 ± 0.02, close
to the temperature resolution limit of the instrument, consistent
with earlier triple-filter analysis from the Transition Region and
Coronal Explorer by Aschwanden & Nightingale and from EIS/Hinode by
Warren et al. or Tripathi et al. We find that 66% of the loops could
be fitted with a narrowband single-Gaussian DEM model, and 19% with a
DEM consisting of two narrowband Gaussians (which mostly result from
pairs of intersecting loops along the same line of sight). The mostly
isothermal loop DEMs allow us also to derive an improved empirical
response function of the AIA 94 Å filter, which needs to be boosted by
a factor of q <SUB>94</SUB> = 6.7 ± 1.7 for temperatures at log (T)
<~ 6.3. The main result of near-isothermal loop cross-sections is
not consistent with the predictions of standard nanoflare scenarios,
but can be explained by flare-like heating mechanisms that drive
chromospheric evaporation and upflows of heated plasma coherently over
loop cross-sections of w ≈ 2-4 Mm.
---------------------------------------------------------
Title: Direct Imaging by SDO/AIA of Quasi-periodic Propagating Fast
Mode Magnetosonic Waves of 2000 km/s in the Solar Corona
Authors: Liu, Wei; Title, A. M.; Zhao, J.; Ofman, L.; Schrijver,
C. J.; Aschwanden, M. J.; De Pontieu, B.; Tarbell, T. D.
2011SPD....42.2114L Altcode: 2011BAAS..43S.2114L
Quasi-periodic, propagating fast mode magnetosonic waves in the
corona were difficult to observe in the past due to relatively low
instrument cadences. We report here unprecedented evidence of such
waves directly imaged in EUV by the new SDO/AIA instrument. In the 2010
August 1 C3.2 flare/CME event, we find arc-shaped wave trains of 1-5%
intensity variations emanating near the flare kernel and propagating
outward along a funnel of coronal loops. Sinusoidal fits to a typical
wave train indicate a phase velocity of 2350 +/- 210 km/s. Similar
waves propagating in opposite directions are observed in closed loops
between two flare ribbons. In the k-omega diagram of the Fourier wave
power, we find a bright ridge that represents the dispersion relation
and can be well fitted with a straight line passing through the
origin, giving an equal phase and group velocity of 1630 +/- 760 km/s
averaged over the event. This k-omega ridge shows a broad frequency
distribution with prominent power at four non-harmonic frequencies,
5.5, 14.5, 25.1, and 37.9 mHz, among which the 14.5 mHz (period:
69 s) signal is the strongest. The signal at 5.5 mHz (period: 181 s,
same as chromospheric 3-minute oscillations) temporally coincides with
flare pulsations, suggesting a common origin of possibly quasi-periodic
magnetic reconnection. The instantaneous wave energy flux of (0.1-2.6)e7
ergs/cm<SUP>2</SUP>/s estimated at the coronal base is comparable to
the steady-state heating requirement of active region loops.
---------------------------------------------------------
Title: Temperature Analysis of Coronal Loop Cross-Sections: Monolithic
vs. Nanoflare Heating
Authors: Aschwanden, Markus J.; Boerner, P.
2011SPD....42.0504A Altcode: 2011BAAS..43S.0504A
We present a first systematic study on the cross-sectional temperature
structure of coronal loops using the six coronal temperature
filters of the Atmospheric Imaging Assembly (AIA) instrument on the
Solar Dynamics Observatory (SDO). We analyze a sample of 100 loop
snapshots measured at 10 different locations and 10 different times
in active region NOAA 11089 on 2010 July 24, 21:00-22:00 UT. <P />The
cross-sectional flux profiles are measured and a cospatial background
is subtracted in 6 filters in a temperature range of T ≈ 0.5-16 MK,
and 4 different parameterizations of differential emission measure
(DEM) distributions are fitted. We find that the reconstructed DEMs
consist predominantly of narrowband peak temperature components with
a thermal width of &#963<SUB>log(T)</SUB> ≤ 0.11±0.02, close
to the temperature resolution limit of the instrument, consistent with
earlier triple-filter analysis from TRACE by Aschwanden and Nightingale
(2005) and from EIS/Hinode by Warren et al. (2008) or Tripathi et
al. (2009). We find that 66% of the loops could be fitted with a
narrowband single-Gaussian DEM model, and 19% with a DEM consisting of
two narrowband Gaussians (which mostly result from pairs of intersecting
loops along the same line-of-sight). The mostly isothermal loop DEMs
allow us also to derive an improved empirical response function of
the AIA 94 [[Unable to Display Character: &#506]] filter, which
needs to be boosted by a factor of q<SUB>94</SUB> = 6.7± 1.7 for
temperatures at log(T) </≈ 6.3. The main result of near-isothermal
loop cross-sections is not consistent with the predictions of standard
nanoflare scenarios, but can be explained by flare-like heating
mechanisms that drive chromospheric evaporation and upflows of heated
plasma coherently over loop cross-sections of w ≈ 2-4 Mm.
---------------------------------------------------------
Title: Modeling Fast Magnetosonic Waves Observed by SDO in Active
region Funnels
Authors: Ofman, Leon; Liu, W.; Title, A.; Aschwanden, M.
2011SPD....42.2104O Altcode: 2011BAAS..43S.2104O
Recently, quasi-periodic, propagating waves have been observed in EUV by
the SDO/AIA instrument in about 10 flare/CME events thus far. A typical
example is the waves associated with the 2010 August 1 C3.2 flare/CME
that exhibited arc-shaped wave trains propagating in an active region
magnetic funnel with 5% intensity variations at speeds in the range
of 1000-2000 km/s. The fast temporal cadence and high sensitivity of
AIA enabled the detection of these waves. We identify them as fast
magnetosonic waves driven quasi-periodically at the base of the flaring
region, and develop a three-dimensional MHD model of the event. For
the initial state we utilize the dipole magnetic field to model the
active region, and include gravitationally stratified density at coronal
temperature. At the coronal base of the active region we excite the fast
magnetosonic wave by periodic velocity pulsations in the photospheric
plane confined to the funnel of magnetic field line. The excited fast
magnetosonic waves have similar amplitude, wavelength and propagation
speeds as the observed wave trains. Based on the simulation results, we
discuss the possible excitation mechanism of the waves, their dynamical
properties, and the use of the event for coronal MHD seismology.
---------------------------------------------------------
Title: Simulating Coronal Emission in Six AIA Channels Using
Quasi-Static Atmosphere Models and Non-Linear Magnetic Field Models
Authors: Malanushenko, Anna; Schrijver, C.; DeRosa, M.; Aschwanden,
M.; Wheatland, M. S.; van Ballegooijen, A. A.
2011SPD....42.2116M Altcode: 2011BAAS..43S.2116M
We present the results of simulations of the EUV coronal emission in
AIA channels. We use a non-linear force-free model of magnetic field
constructed in such a way that its field lines resemble the observed
coronal loops in EUV. We then solve one-dimensional quasi-steady
atmosphere model along the magnetic field lines (Schrijver &
Ballegooijen, 2005). Using coronal abundances from CHIANTI and AIA
response functions we then simulate the emission that would be observed
in AIA EUV channels. The resulting intensities are compared against the
real observations in a manner similar to that in Aschwanden et. al.,
2011. The study is similar to those by Lindquist et. al., 2008, with a
few important differences. We use a model of the coronal magnetic field
that resembles the topology observed in EUV, we study EUV emission of
cool loops (rather than SXR) and we make use of high resolution and
cadence AIA and HMI data.
---------------------------------------------------------
Title: Determining the Structure of Solar Coronal Loops Using Their
Evolution
Authors: Mulu-Moore, Fana M.; Winebarger, Amy R.; Warren, Harry P.;
Aschwanden, Markus J.
2011ApJ...733...59M Altcode:
Despite significant progress in understanding the dynamics of the
corona, there remain several unanswered questions about the basic
physical properties of coronal loops. Recent observations from
different instruments have yielded contradictory results about some
characteristics of coronal loops, specifically as to whether the
observed loops are spatially resolved. In this paper, we examine the
evolution of coronal loops through two extreme-ultraviolet filters
and determine if they evolve as a single cooling strand. We measure
the temporal evolution of eight active region loops previously
studied and found to be isothermal and resolved by Aschwanden &
Nightingale. All eight loops appear in "hotter" TRACE filter images
(Fe XII 195 Å) before appearing in the "cooler" (Fe IX/Fe X 171 Å)
TRACE filter images. We use the measured delay between the two filters
to calculate a cooling time and then determine if that cooling time is
consistent with the observed lifetime of the loop. We do this twice:
once when the loop appears (rise phase) and once when it disappears
(decay phase). We find that only one loop appears consistent with a
single cooling strand and hence could be considered to be resolved by
TRACE. For the remaining seven loops, their observed lifetimes are
longer than expected for a single cooling strand. We suggest that
these loops could be formed of multiple cooling strands, each at a
different temperature. These findings indicate that the majority of
loops observed by TRACE are unresolved.
---------------------------------------------------------
Title: Self-Organized Criticality in Astrophysics
Authors: Aschwanden, Markus J.
2011soca.book.....A Altcode:
Chapter 1: Self-organized criticality phenomena <P />Chapter 2:
Numerical SOC models <P />Chapter 3: Anaytical SOC models <P />Chapter
4: Statistics of random processes <P />Chapter 5: Waiting time
distributions <P />Chapter 6: Event detection methods <P />Chapter 7:
Occurrence frequency distributions <P />Chapter 8: Fractal geometry
<P />Chapter 9: Physical SOC models <P />Chapter 10: SOC-like models
---------------------------------------------------------
Title: First Results on Coronal Loop Analysis with AIA/SDO
Authors: Aschwanden, M. J.
2010AGUFMSH11A1599A Altcode:
A decisive question with regard to coronal heating is whether
the cross-sectional structure of coronal loops is isothermal or
multi-thermal. If the cross-section of an observed coronal loop has an
isothermal structure, this indicates macroscopically resolved heating
mechanisms, such as chromospheric heating by a flare-like process that
channels upflow of heated plasma through the diverging canopy geometry
in the transition region and fills coronal fluxtubes with diameters
corresponding to the separation of photospheric magneto-convection
cells, observed as granulation with spatial scales of about 1000
km. Such diameters of coronal fluxtubes can be resolved with AIA/SDO. On
the other side, if the observed loop cross-sections have a multi-thermal
structure, this could indicate a substructure with unresolved loop
strands with different temperatures, as they would be produced by
theoretical nanoflare heating scenarios. Using the 6 coronal filters
of AIA/SDO we model the differential emission measure distribution
of the smallest resolved coronal loops in active region NOAA 11089,
observed on 2010 July 24. We present the first results of DEM modeling
that reveals statistics of iso-thermal and multi-thermal loops and
their dependence on other physical parameters (temperature, density,
loop widths). We discuss the consequences for coronal heating theories.
---------------------------------------------------------
Title: Uncovering Mechanisms of Coronal Magnetism via Advanced 3D
Modeling of Flares and Active Regions
Authors: Fleishman, Gregory; Gary, Dale; Nita, Gelu; Alexander,
David; Aschwanden, Markus; Bastian, Tim; Hudson, Hugh; Hurford,
Gordon; Kontar, Eduard; Longcope, Dana; Mikic, Zoran; DeRosa, Marc;
Ryan, James; White, Stephen
2010arXiv1011.2800F Altcode:
The coming decade will see the routine use of solar data of
unprecedented spatial and spectral resolution, time cadence, and
completeness. To capitalize on the new (or soon to be available)
facilities such as SDO, ATST and FASR, and the challenges they present
in the visualization and synthesis of multi-wavelength datasets,
we propose that realistic, sophisticated, 3D active region and flare
modeling is timely and critical, and will be a forefront of coronal
studies over the coming decade. To make such modeling a reality, a
broad, concerted effort is needed to capture the wealth of information
resulting from the data, develop a synergistic modeling effort, and
generate the necessary visualization, interpretation and model-data
comparison tools to accurately extract the key physics.
---------------------------------------------------------
Title: Science Objectives for an X-Ray Microcalorimeter Observing
the Sun
Authors: Laming, J. Martin; Adams, J.; Alexander, D.; Aschwanden, M;
Bailey, C.; Bandler, S.; Bookbinder, J.; Bradshaw, S.; Brickhouse,
N.; Chervenak, J.; Christe, S.; Cirtain, J.; Cranmer, S.; Deiker, S.;
DeLuca, E.; Del Zanna, G.; Dennis, B.; Doschek, G.; Eckart, M.; Fludra,
A.; Finkbeiner, F.; Grigis, P.; Harrison, R.; Ji, L.; Kankelborg,
C.; Kashyap, V.; Kelly, D.; Kelley, R.; Kilbourne, C.; Klimchuk, J.;
Ko, Y. -K.; Landi, E.; Linton, M.; Longcope, D.; Lukin, V.; Mariska,
J.; Martinez-Galarce, D.; Mason, H.; McKenzie, D.; Osten, R.; Peres,
G.; Pevtsov, A.; Porter, K. Phillips F. S.; Rabin, D.; Rakowski, C.;
Raymond, J.; Reale, F.; Reeves, K.; Sadleir, J.; Savin, D.; Schmelz,
J.; Smith, R. K.; Smith, S.; Stern, R.; Sylwester, J.; Tripathi, D.;
Ugarte-Urra, I.; Young, P.; Warren, H.; Wood, B.
2010arXiv1011.4052L Altcode:
We present the science case for a broadband X-ray imager with
high-resolution spectroscopy, including simulations of X-ray spectral
diagnostics of both active regions and solar flares. This is part of
a trilogy of white papers discussing science, instrument (Bandler et
al. 2010), and missions (Bookbinder et al. 2010) to exploit major
advances recently made in transition-edge sensor (TES) detector
technology that enable resolution better than 2 eV in an array that
can handle high count rates. Combined with a modest X-ray mirror, this
instrument would combine arcsecondscale imaging with high-resolution
spectra over a field of view sufficiently large for the study of
active regions and flares, enabling a wide range of studies such as
the detection of microheating in active regions, ion-resolved velocity
flows, and the presence of non-thermal electrons in hot plasmas. It
would also enable more direct comparisons between solar and stellar
soft X-ray spectra, a waveband in which (unusually) we currently have
much better stellar data than we do of the Sun.
---------------------------------------------------------
Title: Bootstrapping the Coronal Magnetic Field with STEREO: Unipolar
Potential Field Modeling
Authors: Aschwanden, Markus J.; Sandman, Anne W.
2010AJ....140..723A Altcode: 2010arXiv1004.4920A
We investigate the recently quantified misalignment of α<SUB>mis</SUB>
≈ 20°-40° between the three-dimensional geometry of stereoscopically
triangulated coronal loops observed with STEREO/EUVI (in four active
regions (ARs)) and theoretical (potential or nonlinear force-free)
magnetic field models extrapolated from photospheric magnetograms. We
develop an efficient method of bootstrapping the coronal magnetic
field by forward fitting a parameterized potential field model to the
STEREO-observed loops. The potential field model consists of a number
of unipolar magnetic charges that are parameterized by decomposing
a photospheric magnetogram from the Michelson Doppler Imager. The
forward-fitting method yields a best-fit magnetic field model with
a reduced misalignment of α<SUB>PF</SUB> ≈ 13°-20°. We also
evaluate stereoscopic measurement errors and find a contribution
of α<SUB>SE</SUB> ≈ 7°-12°, which constrains the residual
misalignment to α<SUB>NP</SUB> ≈ 11°-17°, which is likely due
to the nonpotentiality of the ARs. The residual misalignment angle,
α<SUB>NP</SUB>, of the potential field due to nonpotentiality is
found to correlate with the soft X-ray flux of the AR, which implies
a relationship between electric currents and plasma heating.
---------------------------------------------------------
Title: A Universal Scaling Law for the Fractal Energy Dissipation
Domain in Self-Organized Criticality Systems
Authors: Aschwanden, Markus J.
2010arXiv1008.0873A Altcode:
Nonlinear dissipative systems in the state of self-organized
criticality release energy sporadically in avalanches of all sizes,
such as in earthquakes, auroral substorms, solar and stellar flares,
soft gamma-ray repeaters, and pulsar glitches. The statistical
occurrence frequency distributions of event energies $E$ generally
exhibit a powerlaw-like function $N(E)\propto E^{-\alpha_E}$ with a
powerlaw slope of $\alpha_E \approx 1.5$. The powerlaw slope $\alpha_E$
of energies can be related to the fractal dimension $D$ of the spatial
energy dissipation domain by $D=3/\alpha_E$, which predicts a powerlaw
slope $\alpha_E=1.5$ for area-rupturing or area-spreading processes
with $D=2$. For solar and stellar flares, 2-D area-spreading dissipation
domains are naturally provided in current sheets or separatrix surfaces
in a magnetic reconnection region. Thus, this universal scaling law
provides a useful new diagnostic on the topology of the spatial energy
dissipation domain in geophysical and astrophysical observations.
---------------------------------------------------------
Title: Reconciliation of Waiting Time Statistics of Solar Flares
Observed in Hard X-rays
Authors: Aschwanden, Markus J.; McTiernan, James M.
2010ApJ...717..683A Altcode: 2010arXiv1002.4869A
We study the waiting time distributions of solar flares observed in
hard X-rays with ISEE-3/ICE, HXRBS/SMM, WATCH/GRANAT, BATSE/CGRO,
and RHESSI. Although discordant results and interpretations have been
published earlier, based on relatively small ranges (<2 decades) of
waiting times, we find that all observed distributions, spanning over
6 decades of waiting times (Δt ≈ 10<SUP>-3</SUP>-10<SUP>3</SUP>
hr), can be reconciled with a single distribution function, N(Δt)
vprop λ<SUB>0</SUB>(1 + λ<SUB>0</SUB>Δt)<SUP>-2</SUP>, which has a
power-law slope of p ≈ 2.0 at large waiting times (Δt ≈ 1-1000 hr)
and flattens out at short waiting times Δt <~ Δt <SUB>0</SUB> =
1/λ<SUB>0</SUB>. We find a consistent breakpoint at Δt <SUB>0</SUB>
= 1/λ<SUB>0</SUB> = 0.80 ± 0.14 hr from the WATCH, HXRBS, BATSE,
and RHESSI data. The distribution of waiting times is invariant
for sampling with different flux thresholds, while the mean waiting
time scales reciprocically with the number of detected events, Δt
<SUB>0</SUB> vprop 1/n <SUB>det</SUB>. This waiting time distribution
can be modeled with a nonstationary Poisson process with a flare
rate λ = 1/Δt that varies as f(λ) vprop λ<SUP>-1</SUP>exp -
(λ/λ<SUB>0</SUB>). This flare rate distribution requires a highly
intermittent flare productivity in short clusters with high rates,
separated by relatively long quiescent intervals with very low flare
rates.
---------------------------------------------------------
Title: Determining the Temperature Structure of Solar Coronal Loops
using their Temporal Evolution
Authors: Mulu, Fana; Winebarger, A. R.; Warren, H. P.; Aschwanden,
M. J.; Klimchuk, J. A.
2010AAS...21630001M Altcode:
Despite much progress toward understanding the dynamics of the
corona, the physical properties of coronal loops are not yet fully
understood. Recent investigations and observations from different
instruments have yielded contradictory results about the true physical
properties of coronal loops, specifically as to whether the observed
loops are isothermal structures or the convolution of several
multi-thermal strands. In this talk, we introduce a new technique
to determine if an observed loop is isothermal or multi-thermal. We
determine the evolution of ten selected loops in multiple filter
images from the Transition Region and Coronal Explorer (TRACE). Our
new technique calculates the delay, calculates a cooling time,
and determines if that cooling time is consistent with the observed
lifetime. If the observational lifetime of the loop agrees with the
calculated lifetime, then we can conclude that the loop is a single
"monolithic” structure that heats and cools as a homogeneous unit,
with isothermal temperature over the cross-section. If not, the
loop must be a bundle of multiple multi-thermal strands, all being
heated and cooling independently. In the second part of the talk,
we utilize the concept of nanoflare storms to understand the reason
behind the extended lifetimes. By simulating the observed light curves
of the loops using EBTEL (Enthalpy-Based Thermal Evolution of Loops),
we find that the longer observed lifetimes can be reproduced by using
a set of small-scale impulsively heated strands.
---------------------------------------------------------
Title: Bootstrapping the Coronal Magnetic Field with STEREO/EUVI
Authors: Aschwanden, Markus; Sandman, Anne
2010EGUGA..1214426A Altcode:
The 3D coronal magnetic field obtained from stereoscopically
triangulated loops has been compared with standard photospheric
magnetogram extrapolations. We found a large misalignment of 20-40
deg, depending on the complexity of an AR (Sandman et al. 2009;
DeRosa et al. 2009). These studies prove that the magnetic field in
the photosphere is not force-free and fundamentally cannot reproduce
the coronal magnetic field. Bootstrapping with coronal loop 3D
geometries are required to improve modeling of the coronal field. Such
coronal field bootstrapping methods are currently developed using
stereoscopically triangulated loops from STEREO/EUVI and preliminary
results show already a significantly reduced misalignment of 10-20 deg.
---------------------------------------------------------
Title: Coronal Dimming And Waves Observed In Flare-Associated CMEs
Authors: Nitta, Nariaki; Aschwanden, M.; Freeland, S.; Lemen, J.;
Wuelser, J.; Zarro, D.
2010AAS...21640614N Altcode: 2010BAAS...41..882N
The relationship between solar flares and coronal mass ejections
(CMEs) is still an active area of research. It is studied from
various aspects. Our goal is to understand the importance of magnetic
reconnection in launching CMEs and that of magnetic field environment
of the flaring regions to determine how eruptive flares are. We have
studied the association of solar flares during 2007-2009 with CMEs,
using primarily extreme-ultraviolet (EUV) and inner coronagraphic
images from the Solar Terrestrial Relations Observatory (STEREO). While
energetic CMEs tend to accompany a flare, flares with relatively high
soft X-ray intensity for the extended solar minimum conditions are often
found without an associated CME, even though the underlying photospheric
magnetic field is strong and complex. In contrast, some of the regions
hosting flare-associated CMEs have weak photospheric field, sometimes
not even classified as active regions because of no sunspots. Out of
several signatures in low coronal images previously raised as proxies
for CMEs, large-scale dimming that persists for at least an hour is
found to be a sufficient condition. Waves in EUV images, on the other
hand, may not necessarily signal an appreciable CME that is still
clearly observed beyond, for example, 5 Rsun, unless the concurrent
dimming is substantial. This suggests that waves detected in EUV images
may have more than one origins. We present the result of the survey
and discuss a number of well-observed cases focusing on the properties
of the flares and CMEs with respect to the extents of dimming and wave.
---------------------------------------------------------
Title: Bootstrapping the Coronal Magnetic Field with STEREO
Authors: Aschwanden, Markus J.
2010AAS...21630601A Altcode:
The 3D coronal magnetic field obtained from stereoscopically
triangulated loops has been compared with standard photospheric
magnetogram extrapolations. We found a large misalignment of 20-40
deg, depending on the complexity of an AR (Sandman et al. 2009;
DeRosa et al. 2009). These studies prove that the magnetic field in
the photosphere is not force-free and fundamentally cannot reproduce
the coronal magnetic field. Bootstrapping with coronal loop 3D
geometries are required to improve modeling of the coronal field. Such
coronal field bootstrapping methods are currently developed using
stereoscopically triangulated loops from STEREO/EUVI and preliminary
results show already a significantly reduced misalignment of 10-20 deg.
---------------------------------------------------------
Title: A Code for Automated Tracing of Coronal Loops Approaching
Visual Perception
Authors: Aschwanden, Markus J.
2010SoPh..262..399A Altcode: 2010SoPh..tmp...45A; 2010SoPh..tmp...57A
We develop a new numerical code with automated feature extraction,
customized for tracing of coronal loops, a method we call Oriented
Coronal CUrved Loop Tracing (OCCULT), which for the first time
breaks even with the results of visual tracing. The method used
is based on oriented-directivity tracing of curvi-linear features,
but in contrast to other general feature-extraction algorithms, it
is customized for solar EUV and SXR images by taking advantage of
the specific property that coronal loops have large curvature radii
compared with their widths. We evaluate the performance of this new
code by comparing the cumulative distribution of loop lengths, the
median and maximum loop lengths, the completeness of detection, and
the congruency of the detected features with other numerical codes
and visual tracings. We find that the new code closely approaches
the results of visual perception and outperforms the other existing
numerical codes. This algorithm is useful for the 3D reconstruction
of the geometry, motion, and oscillations of coronal loops, with
single or stereoscopic spacecraft, as well as for modeling of the loop
hydrodynamics and the coronal magnetic field.
---------------------------------------------------------
Title: Image Processing Techniques and Feature Recognition in
Solar Physics
Authors: Aschwanden, Markus J.
2010SoPh..262..235A Altcode:
This review presents a comprehensive and systematic overview of
image-processing techniques that are used in automated feature-detection
algorithms applied to solar data: i) image pre-processing procedures,
ii) automated detection of spatial features, iii) automated detection
and tracking of temporal features (events), and iv) post-processing
tasks, such as visualization of solar imagery, cataloguing, statistics,
theoretical modeling, prediction, and forecasting. For each aspect
the most recent developments and science results are highlighted. We
conclude with an outlook on future trends.
---------------------------------------------------------
Title: Self-Organized Criticality in Solar Physics and Astrophysics
Authors: Aschwanden, Markus J.
2010arXiv1003.0122A Altcode:
The concept of "self-organized criticality" (SOC) has been introduced
by Bak, Tang, and Wiesenfeld (1987) to describe the statistics of
avalanches on the surface of a sandpile with a critical slope, which
produces a scale-free powerlaw size distribution of avalanches. In the
meantime, SOC behavior has been identified in many nonlinear dissipative
systems that are driven to a critical state. On a most general level,
SOC is the statistics of coherent nonlinear processes, in contrast to
the Poisson statistics of incoherent random processes. The SOC concept
has been applied to laboratory experiments (of rice or sand piles),
to human activities (population growth, language, economy, traffic
jams, wars), to biophysics, geophysics (earthquakes, landslides, forest
fires), magnetospheric physics, solar physics (flares), stellar physics
(flares, cataclysmic variables, accretion disks, black holes, pulsar
glitches, gamma ray bursts), and to galactic physics and cosmology.
---------------------------------------------------------
Title: CME-related Phenomena and Solar Flares
Authors: Nitta, Nariaki; Aschwanden, Markus; Freeland, Samuel; Lemen,
James; Wuelser, Jean-Pierre; Zarro, Dominic
2010cosp...38.1792N Altcode: 2010cosp.meet.1792N
The relationship between solar flares and coronal mass ejections
(CMEs) is still an active area of research. It is studied from
various aspects. Our goal is to understand the importance of magnetic
reconnection in launching CMEs and that of magnetic field environment
of the flaring regions to determine how eruptive flares are. We have
studied the association of solar flares dur-ing 2007-2009 with CMEs,
using primarily extreme-ultraviolet (EUV) and inner coronagraphic
images from the Solar Terrestrial Relations Observatory (STEREO). While
energetic CMEs tend to accompany a flare, flares with relatively high
soft X-ray intensity for the extended solar minimum conditions are often
found without an associated CME, even though the underlying photospheric
magnetic field is strong and complex. In contrast, some of the regions
hosting flare-associated CMEs have weak photospheric field, sometimes
not even classified as active regions because of no sunspots. Out of
several signatures in low coronal images previously raised as proxies
for CMEs, large-scale dimming that persists for at least an hour is
found to be a sufficient condition. Waves in EUV images, on the other
hand, may not necessarily signal an appreciable CME that is still
clearly observed beyond, for example, 5 Rsun, unless the concur-rent
dimming is substantial. This suggests that waves detected in EUV images
may have more than one origins. We present the result of the survey
and discuss a number of well-observed cases focusing on the properties
of the flares and CMEs with respect to the extents of dimming and wave.
---------------------------------------------------------
Title: Energy-dependent timing of thermal emission in solar flares
Authors: Jain, Rajmal; Rajpurohit, Arvind; Awasthi, Arun; Aschwanden,
Markus
2010cosp...38.2965J Altcode: 2010cosp.meet.2965J
A study of thermal emission in solar flares using high-resolution X-ray
spectra observed by the Si detector onboard "Solar X-ray Spectrometer"
(SOXS) has been conducted. The SOXS onboard GSAT-2 Indian spacecraft was
launched by GSLV-D2 rocket on 08 May 2003. With this we investigate the
energy dependent timing of thermal emission in solar flares. Firstly
we model the spectral-temporal evolution of the X-ray flux F(e,t)
assuming multi-temperature plasma governed by thermal conduction
cooling. This model is found in agreement with the temperature and
emission measure derived from the fitting of the spectra observed by
the Si detector. We investigate 10 M-class flares and found that the
emission in the energy range e=6 -20 keV is dominated by temperatures
T=15 -50 MK while the power-law index (gamma) of the thermal spectrum
varies over 2.7 -4.3. The temperature-dependent cooling time varies
between 22 and 310 s. The electron density (ne) obtained for the flares
under investigation ranges between 0.03 and 5X1011 cm-3 suggests that
conduction cooling of thermal X-ray plasma dominates over radiative
cooling in the initial phase of the flare. The current study also
provides an alternative method for separating thermal from non-thermal
spectra, which enables us to measure the break-energy point to be
varying between 17 and 220.7 keV.
---------------------------------------------------------
Title: Joint STEREO-Hinode Observations of Coronal Dimming and Waves
Associated with a CME/Flare Event
Authors: Nitta, Nariaki; Aschwanden, Markus; Frank, Zoe; Slater,
Gregory; Tarbell, Theodore; Zarro, Dominic
2010cosp...38.2928N Altcode: 2010cosp.meet.2928N
During the solar minimum between cycles 23 and 24, a number of
minor flares from unimpressive active regions were associated with
large-scale dimming and waves as observed by the EUV Imager (EUVI) on
STEREO. We present a detailed study on one of the CME/flare events that
was observed also by the instruments on Hinode. We analyze SOT Ca II
data to explore the origin of the disturbances and EIS slit spectra to
determine the timings of the upflow and associated turbulence that are
presumably correlated with coronal dimming. The sequence of phenomena
as captured by different instruments on STEREO, Hinode and SOHO helps
us identify the key physical processes that gave rise to the event.
---------------------------------------------------------
Title: Spatial Scaling Law in Solar Flares
Authors: Aschwanden, Markus
2010cosp...38.2995A Altcode: 2010cosp.meet.2995A
How do the observed macroscopic scales observed in solar flares relate
to the theoretically implied microscopic scales of energy release
and particle acceleration processes. We approach this question by
investigating the fast milli-second time scales observed during
the impulsive flare phase in hard X-ray and radio wavelengths and
their relation to the spatial scales observed in hard and soft X-ray
images. Previous studies with Yohkoh and CGRO data established a
scaling law between the temporal and spatial scales. We investigate
also the fractal spatial patterns of flares observed at arc-second
spatial resolution with TRACE and deduce the fractal dimension in
2D and filling factors in 3D. We discuss the implications of these
observational results for the spatial scaling of the underlying magnetic
reconnection processes.
---------------------------------------------------------
Title: The 3D Geometry, Motion, and Hydrodynamic Aspects of
Oscillating Coronal Loops
Authors: Aschwanden, Markus J.
2009SSRv..149...31A Altcode:
We transition from two-dimensional (2D) imaging observations of
kink-mode loop oscillations in the solar corona to three-dimensional
(3D) reconstructions by exploring two new methods: (1) De-projection of
2D loop tracings using the strategy of curvature radius maximization in
3D space, based on the assumption of force-free magnetic fields; and (2)
stereoscopic triangulation of epipolar loop coordinates using coaligned
images from the STEREO EUVI/A and B spacecraft. Both methods reveal
new features of oscillating loops: non-circularity, non-planarity,
and helical geometries. We extend the 3D reconstruction techniques
into the time domain and find indications of circularly polarized
(helical) kink-mode oscillations, in contrast to linearly polarized
modes assumed previously. We discuss also hydrodynamic effects of
coronal loops in non-equilibrium state that are essential for the
detection and modeling of kink-mode oscillations.
---------------------------------------------------------
Title: 4D Modeling of CME Expansion and EUV Dimming Observed with
STEREO/EUVI
Authors: Aschwanden, M. J.
2009AGUFMSH41A1644A Altcode:
This is the first attempt to model the kinematics of a CME launch
and the resulting EUV dimming quantitatively with a self-consistent
model. Our 4D-model assumes self-similar expansion of a spherical CME
geometry that consists of a CME front with density compression and a
cavity with density rarefaction, satisfying mass conservation of the
total CME and swept-up corona. The model contains 14 free parameters
and is fitted to the 2008 March 25 CME event observed with STEREO/A
and B. Our model is able to reproduce the observed CME expansion and
related EUV dimming during the initial phase from 18:30 UT to 19:00
UT. The CME kinematics can be characterized by a constant acceleration
(i.e., a constant magnetic driving force). While the observations
of EUVI/A are consistent with a spherical bubble geometry, we detect
significant asymmetries and density inhomogeneities with EUVI/B. This
new forward-modeling method demonstrates how the observed EUV dimming
can be used to model physical parameters of the CME source region,
the CME geometry, and CME kinematics.
---------------------------------------------------------
Title: Hydrodynamic Modeling of Coronal Loops with Hinode and STEREO
Authors: Aschwanden, M. J.
2009ASPC..415..234A Altcode:
The hydrodynamic evolution of impulsively-heated coronal loops and
their subsequent cooling can now be modeled with multi-wavelength
imaging instruments in soft X-ray (SXR) and extreme ultraviolet (EUV)
wavelengths. Using analytical approximations to the hydrodynamic
evolution of the density n(s,t) and temperature T(s,t) of an
impulsively-heated loop (as a function of the loop length coordinate s
and time t) we show an example how lightcurves observed with HINODE/XRT,
EIS, GOES, and STEREO/EUVI can be modeled with a forward-fitting method
in order to infer the maximum heating rate, the heating duration,
and the cooling time of a heated loop during a small B1-class flare
on 2007 February 1, previously analyzed by Warren et al. (2007).
---------------------------------------------------------
Title: The Hydrodynamic Evolution of Impulsively Heated Coronal Loops:
Explicit Analytical Approximations
Authors: Aschwanden, Markus J.; Tsiklauri, David
2009ApJS..185..171A Altcode:
We derive simple analytical approximations (in explicit form)
for the hydrodynamic evolution of the electron temperature T(s,
t) and electron density n(s, t), for one-dimensional coronal loops
that are subject to impulsive heating with subsequent cooling. Our
analytical approximations are derived from first principles, using (1)
the hydrodynamic energy balance equation, (2) the loop scaling laws
of Rosner-Tucker-Vaiana and Serio, (3) the Neupert effect, and (4)
the Jakimiec relationship. We compare our analytical approximations
with 56 numerical cases of time-dependent hydrodynamic simulations
from a parametric study of Tsiklauri et al., covering a large parameter
space of heating rates, heating timescales, heating scale heights, loop
lengths, for both footpoint and apex heating, mostly applicable to flare
conditions. The average deviations from the average temperature and
density values are typically ≈20% for our analytical expressions. The
analytical approximations in explicit form provide an efficient tool
to mimic time-dependent hydrodynamic simulations, to model observed
soft X-rays and extreme-ultraviolet light curves of heated and cooling
loops in the solar corona and in flares by forward fitting, to model
microflares, to infer the coronal heating function from light curves
of multi-wavelength observations, and to provide physical models of
differential emission measure distributions for solar and stellar
flares, coronae, and irradiance.
---------------------------------------------------------
Title: First Measurements of the Mass of Coronal Mass Ejections from
the EUV Dimming Observed with STEREO EUVI A+B Spacecraft
Authors: Aschwanden, Markus J.; Nitta, Nariaki V.; Wuelser,
Jean-Pierre; Lemen, James R.; Sandman, Anne; Vourlidas, Angelos;
Colaninno, Robin C.
2009ApJ...706..376A Altcode:
The masses of coronal mass ejections (CMEs) have traditionally been
determined from white-light coronagraphs (based on Thomson scattering
of electrons), as well as from extreme ultraviolet (EUV) dimming
observed with one spacecraft. Here we develop an improved method
of measuring CME masses based on EUV dimming observed with the dual
STEREO/EUVI spacecraft in multiple temperature filters that includes
three-dimensional volume and density modeling in the dimming region
and background corona. As a test, we investigate eight CME events with
previous mass determinations from STEREO/COR2, of which six cases are
reliably detected with the Extreme Ultraviolet Imager (EUVI) using our
automated multi-wavelength detection code. We find CME masses in the
range of m <SUB>CME</SUB> = (2-7) × 10<SUP>15</SUP> g. The agreement
between the two EUVI/A and B spacecraft is m<SUB>A</SUB> /m<SUB>B</SUB>
= 1.3 ± 0.6 and the consistency with white-light measurements by COR2
is m <SUB>EUVI</SUB>/m <SUB>COR2</SUB> = 1.1 ± 0.3. The consistency
between EUVI and COR2 implies no significant mass backflows (or inflows)
at r < 4 R <SUB>sun</SUB> and adequate temperature coverage for the
bulk of the CME mass in the range of T ≈ 0.5-3.0 MK. The temporal
evolution of the EUV dimming allows us to also model the evolution
of the CME density n<SUB>e</SUB> (t), volume V(t), height-time h(t),
and propagation speed v(t) in terms of an adiabatically expanding
self-similar geometry. We determine e-folding EUV dimming times of
t<SUB>D</SUB> = 1.3 ± 1.4 hr. We test the adiabatic expansion model
in terms of the predicted detection delay (Δt ≈ 0.7 hr) between
EUVI and COR2 for the fastest CME event (2008 March 25) and find good
agreement with the observed delay (Δt ≈ 0.8 hr).
---------------------------------------------------------
Title: Comparison of STEREO/EUVI Loops with Potential Magnetic
Field Models
Authors: Sandman, A. W.; Aschwanden, M. J.; DeRosa, M. L.; Wülser,
J. P.; Alexander, D.
2009SoPh..259....1S Altcode:
The Solar Terrestrial Relations Observatory (STEREO) provides the
first opportunity to triangulate the three-dimensional coordinates of
active region loops simultaneously from two different vantage points in
space. Three-dimensional coordinates of the coronal magnetic field have
been calculated with theoretical magnetic field models for decades,
but it is only with the recent availability of STEREO data that a
rigorous, quantitative comparison between observed loop geometries and
theoretical magnetic field models can be performed. Such a comparison
provides a valuable opportunity to assess the validity of theoretical
magnetic field models. Here we measure the misalignment angles between
model magnetic fields and observed coronal loops in three active
regions, as observed with the Extreme Ultraviolet Imager (EUVI) on
STEREO on 30 April, 9 May, and 19 May 2007. We perform stereoscopic
triangulation of some 100 - 200 EUVI loops in each active region and
compute extrapolated magnetic field lines using magnetogram information
from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric
Observatory (SOHO). We examine two different magnetic extrapolation
methods: (1) a potential field and (2) a radially stretched potential
field that conserves the magnetic divergence. We find considerable
disagreement between each theoretical model and the observed loop
geometries, with an average misalignment angle on the order of 20°
- 40°. We conclude that there is a need for either more suitable
(coronal rather than photospheric) magnetic field measurements or more
realistic field extrapolation models.
---------------------------------------------------------
Title: 4-D modeling of CME expansion and EUV dimming observed with
STEREO/EUVI
Authors: Aschwanden, M. J.
2009AnGeo..27.3275A Altcode: 2009arXiv0908.1913A
This is the first attempt to model the kinematics of a CME launch
and the resulting EUV dimming quantitatively with a self-consistent
model. Our 4-D-model assumes self-similar expansion of a spherical CME
geometry that consists of a CME front with density compression and a
cavity with density rarefaction, satisfying mass conservation of the
total CME and swept-up corona. The model contains 14 free parameters
and is fitted to the 25 March 2008 CME event observed with STEREO/A
and B. Our model is able to reproduce the observed CME expansion and
related EUV dimming during the initial phase from 18:30 UT to 19:00
UT. The CME kinematics can be characterized by a constant acceleration
(i.e., a constant magnetic driving force). While the observations
of EUVI/A are consistent with a spherical bubble geometry, we detect
significant asymmetries and density inhomogeneities with EUVI/B. This
new forward-modeling method demonstrates how the observed EUV dimming
can be used to model physical parameters of the CME source region,
the CME geometry, and CME kinematics.
---------------------------------------------------------
Title: Can We Use STEREO/EUVI to Improve Boundary Conditions for
Magnetic Modeling?
Authors: Sandman, Anne; Aschwanden, Markus J.; Alexander, David
2009shin.confE..19S Altcode:
The STEREO mission provides a unique opportunity to observe
active region loops simultaneously from two different vantage
points in space. Using the stereoscopic data, we can triangulate
3D coordinates for active region loops and use them to perform
quantitative comparisons between observed loop geometries and
theoretical magnetic field models. These comparisons provide some
insight into the validity of magnetic field models, but recent work
(DeRosa et al. 2009, Sandman et al. 2009) has shown that potential and
non-potential models yield equally poor agreement with observed loop
structures. In order to improve the results of magnetic modeling we
must resolve the discrepancy between the typically force-free domain
(the corona) and non-force-free boundary condition (the photosphere
or chromosphere). We seek to address this discrepancy by 'correcting'
the boundary condition using observational constraints. The magnetic
field in the low corona cannot currently be mapped directly, but using
STEREO 3D loop coordinates we can constrain the orientation of the
magnetic field in the low corona. In a previous study we measured the
misalignment angles between model magnetic fields and observed coronal
loops in three active regions, as observed with STEREO/EUVI on April 30,
May 9, and May 19, 2007. We now attempt to use these misalignment angles
to modify the magnetogram input to the magnetic field model such that
the modified model has minimal misalignment with the observed coronal
loops near the base of the corona. We present the results of a test
case exploring the potential and limitations of this technique.
---------------------------------------------------------
Title: The possible role of vortex shedding in the excitation of
kink-mode oscillations in the solar corona
Authors: Nakariakov, V. M.; Aschwanden, M. J.; van Doorsselaere, T.
2009A&A...502..661N Altcode:
We propose a model for the excitation of horizontally polarised
transverse (kink) magnetoacoustic oscillations of solar coronal loops
by upflows associated with coronal mass ejections. If the magnetic
field in the plasma that is dragged in the vertical direction by the
flow is parallel to the loop, the phenomenon of vortex shedding causes
the appearance of a quasi-periodic horizontal force that is applied to
alternating sides of the loop. The period of the force is determined
by the flow speed and the loop's minor radius. The oscillations are
excited the most effectively when the force is in resonance with the
natural frequency of the kink oscillations. This model can explain
the selectivity of the excitation of the oscillations and the initial
growth of the oscillation amplitude.
---------------------------------------------------------
Title: Seismology of a Large Solar Coronal Loop from EUVI/STEREO
Observations of its Transverse Oscillation
Authors: Verwichte, E.; Aschwanden, M. J.; Van Doorsselaere, T.;
Foullon, C.; Nakariakov, V. M.
2009ApJ...698..397V Altcode:
The first analysis of a transverse loop oscillation observed by both
Solar TErrestrial RElations Observatories (STEREO) spacecraft is
presented, for an event on the 2007 June 27 as seen by the Extreme
Ultraviolet Imager (EUVI). The three-dimensional loop geometry is
determined using a three-dimensional reconstruction with a semicircular
loop model, which allows for an accurate measurement of the loop
length. The plane of wave polarization is found from comparison with
a simulated loop model and shows that the oscillation is a fundamental
horizontally polarized fast magnetoacoustic kink mode. The oscillation
is characterized using an automated method and the results from
both spacecraft are found to match closely. The oscillation period
is 630 ± 30 s and the damping time is 1000 ± 300 s. Also, clear
intensity variations associated with the transverse loop oscillations
are reported for the first time. They are shown to be caused by the
effect of line-of-sight integration. The Alfvén speed and coronal
magnetic field derived using coronal seismology are discussed. This
study shows that EUVI/STEREO observations achieve an adequate accuracy
for studying long-period, large-amplitude transverse loop oscillations.
---------------------------------------------------------
Title: Nonlinear Force-Free Magnetic Field Modeling of AR 10953:
A Critical Assessment
Authors: De Rosa, Marc L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
Lites, B. W.; Aschwanden, M. J.; Amari, T.; Canou, A.; McTiernan,
J. M.; Régnier, S.; Thalmann, J. K.; Valori, G.; Wheatland, M. S.;
Wiegelmann, T.; Cheung, M. C. M.; Conlon, P. A.; Fuhrmann, M.;
Inhester, B.; Tadesse, T.
2009SPD....40.3102D Altcode:
Nonlinear force-free field (NLFFF) modeling seeks to provide accurate
representations of the structure of the magnetic field above solar
active regions, from which estimates of physical quantities of interest
(e.g., free energy and helicity) can be made. However, the suite of
NLFFF algorithms have failed to arrive at consistent solutions when
applied to (thus far, two) cases using the highest-available-resolution
vector magnetogram data from Hinode/SOT-SP (in the region of the
modeling area of interest) and line-of-sight magnetograms from
SOHO/MDI (where vector data were not available). One issue is that
NLFFF models require consistent, force-free vector magnetic boundary
data, and vector magnetogram data sampling the photosphere do not
satisfy this requirement. Consequently, several problems have arisen
that are believed to affect such modeling efforts. We use AR 10953
to illustrate these problems, namely: (1) some of the far-reaching,
current-carrying connections are exterior to the observational field
of view, (2) the solution algorithms do not (yet) incorporate the
measurement uncertainties in the vector magnetogram data, and/or (3)
a better way is needed to account for the Lorentz forces within the
layer between the photosphere and coronal base. In light of these
issues, we conclude that it remains difficult to derive useful and
significant estimates of physical quantities from NLFFF models.
---------------------------------------------------------
Title: A Critical Assessment of Nonlinear Force-Free Field Modeling
of the Solar Corona for Active Region 10953
Authors: De Rosa, Marc L.; Schrijver, Carolus J.; Barnes, Graham;
Leka, K. D.; Lites, Bruce W.; Aschwanden, Markus J.; Amari, Tahar;
Canou, Aurélien; McTiernan, James M.; Régnier, Stéphane; Thalmann,
Julia K.; Valori, Gherardo; Wheatland, Michael S.; Wiegelmann, Thomas;
Cheung, Mark C. M.; Conlon, Paul A.; Fuhrmann, Marcel; Inhester,
Bernd; Tadesse, Tilaye
2009ApJ...696.1780D Altcode: 2009arXiv0902.1007D
Nonlinear force-free field (NLFFF) models are thought to be viable
tools for investigating the structure, dynamics, and evolution of
the coronae of solar active regions. In a series of NLFFF modeling
studies, we have found that NLFFF models are successful in application
to analytic test cases, and relatively successful when applied
to numerically constructed Sun-like test cases, but they are less
successful in application to real solar data. Different NLFFF models
have been found to have markedly different field line configurations
and to provide widely varying estimates of the magnetic free energy in
the coronal volume, when applied to solar data. NLFFF models require
consistent, force-free vector magnetic boundary data. However,
vector magnetogram observations sampling the photosphere, which is
dynamic and contains significant Lorentz and buoyancy forces, do not
satisfy this requirement, thus creating several major problems for
force-free coronal modeling efforts. In this paper, we discuss NLFFF
modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT,
STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process
illustrate three such issues we judge to be critical to the success of
NLFFF modeling: (1) vector magnetic field data covering larger areas
are needed so that more electric currents associated with the full
active regions of interest are measured, (2) the modeling algorithms
need a way to accommodate the various uncertainties in the boundary
data, and (3) a more realistic physical model is needed to approximate
the photosphere-to-corona interface in order to better transform the
forced photospheric magnetograms into adequate approximations of nearly
force-free fields at the base of the corona. We make recommendations
for future modeling efforts to overcome these as yet unsolved problems.
---------------------------------------------------------
Title: Tracking The 3-d Structure Of Erupting Filaments With The
Stereo/secchi Euvi
Authors: Wuelser, Jean-Pierre; Aschwanden, M.; Lemen, J.; Nitta, N.;
Sandman, A.
2009SPD....40.2605W Altcode:
In May 2007, the STEREO/SECCHI EUVI observed several erupting filaments
that were associated with a CME. The stereoscopic observations in both
He II and Fe IX allow a detailed study of the 3-D geometry and dynamics
of the filaments as they erupt. Deconvolution of the EUVI imagery with
the instrument point spread function enhances image contrast and detail,
and improves the accuracy of the 3-D analysis. The quantitative results
of this analysis may potentially lead to a better understanding of
the early CME process.
---------------------------------------------------------
Title: Can We Use STEREO/EUVI to Improve Boundary Conditions for
Magnetic Modeling?
Authors: Sandman, Anne; Aschwanden, M. J.; Alexander, D.
2009SPD....40.1220S Altcode:
The STEREO mission provides a unique opportunity to observe
active region loops simultaneously from two different vantage
points in space. Using the stereoscopic data, we can triangulate
3D coordinates for active region loops and use them to perform
quantitative comparisons between observed loop geometries and
theoretical magnetic field models. These comparisons provide some
insight into the validity of magnetic field models, but recent work
(DeRosa et al. 2009, Sandman et al. 2009) has shown that potential and
non-potential models yield equally poor agreement with observed loop
structures. In order to improve the results of magnetic modeling we
must resolve the discrepancy between the typically force-free domain
(the corona) and non-force-free boundary condition (the photosphere or
chromosphere). We seek to address this discrepancy by "correcting”
the boundary condition using observational constraints. The magnetic
field in the low corona cannot currently be mapped directly, but using
STEREO 3D loop coordinates we can constrain the orientation of the
magnetic field in the low corona. In a previous study we measured the
misalignment angles between model magnetic fields and observed coronal
loops in three active regions, as observed with STEREO/EUVI on April 30,
May 9, and May 19, 2007. We now use these misalignment angles to modify
the magnetogram input to the magnetic field model. The resulting model
field has minimal misalignment with the observed coronal loops near the
base of the corona, allowing for a more realistic field extrapolation.
---------------------------------------------------------
Title: Solar Flare and CME Observations with STEREO/EUVI
Authors: Aschwanden, M. J.; Wuelser, J. P.; Nitta, N. V.; Lemen, J. R.
2009SoPh..256....3A Altcode:
STEREO/EUVI observed 185 flare events (detected above the GOES class C1
level or at > 25 keV with RHESSI) during the first two years of the
mission (December 2006 - November 2008), while coronal mass ejections
(CMEs) were reported in about a third of these events. We compile a
comprehensive catalog of these EUVI-observed events, containing the peak
fluxes in soft X rays, hard X rays, and EUV, as well as a classification
and statistics of prominent EUV features: 79% show impulsive EUV
emission (coincident with hard X rays), 73% show delayed EUV emission
from postflare loops and arcades, 24% represent occulted flares, 17%
exhibit EUV dimming, 5% show loop oscillations or propagating waves,
and at least 3% show erupting filaments. We analyze an example of each
EUV feature by stereoscopic modeling of its 3D geometry. We find that
EUV emission can be dominated by impulsive emission from a heated,
highly sheared, noneruptive filament, in addition to the more common
impulsive EUV emission from flare ribbons or the delayed postflare
EUV emission that results from cooling of the soft-X-ray-emitting
flare loops. Occulted flares allow us to determine CME-related coronal
dimming uncontaminated from flare-related EUV emission. From modeling
the time evolution of EUV dimming we can accurately quantify the
initial expansion of CMEs and determine their masses. Further, we
find evidence that coronal loop oscillations are excited by the rapid
initial expansion of CMEs. These examples demonstrate that stereoscopic
EUV data provide powerful new methods to model the 3D aspects in the
hydrodynamics of flares and kinematics of CMEs.
---------------------------------------------------------
Title: Classifying Coronal Loops as Isothermal or Multi-thermal
Using the Loops' Evolution
Authors: Mulu, Fana; Winebarger, A. R.; Warren, H. P.; Aschwanden,
M. J.
2009SPD....40.1218M Altcode:
Despite much progress toward understanding the dynamics of the
corona, the physical properties of coronal loops are not yet fully
understood. Recent investigations and observations from different
instruments have yielded contradictory results about the true
physical properties of coronal loops, specifically as to whether the
observed loops are isothermal structures or the convolution of several
multi-thermal strands. In this poster, we introduce a new technique
to determine if an observed loop is isothermal or multi-thermal. We
will determine the evolution of loops in multiple filter images from
the Transition Region and Coronal Explorer (TRACE). Our new technique
will calculate the delay of the loop between different filter images,
calculate a cooling time, and determine if that cooling time is
consistent with the observed lifetime. We will present preliminary
results that show if the temperature structure across coronal loops
is isothermal or multi-thermal.
---------------------------------------------------------
Title: First Measurements of the Mass of Coronal Mass Ejections from
the EUV Dimming Observed with Stereo EUVI A and B Spacecraft
Authors: Aschwanden, Markus J.; Nitta, N. V.; Wuelser, J.; Lemen,
J. R.; Sandman, A.; Vourlidas, A.; Colaninno, R. C.
2009SPD....40.2116A Altcode:
The masses of Coronal Mass Ejections (CMEs) have traditionally
been determined from white-light coronagraphs, based on the Thomson
scattering of electrons. Here we develop a new method of measuring CME
masses from the EUV dimming seen with EUV imaging telescopes in multiple
temperature filters. As a test we compare the CME masses measured by
STEREO/EUVI A and B with those previously determined by STEREO/COR2, for
a set of 8 CME events of which we detected 7 with EUVI and determined
the masses in 6 cases. We find CME masses in the range of m = (2-7)
x 10(15) g. The agreement between the two EUVI/A and B spacecraft is
mA/mB =1.3 +/- 0.6 and the consistency with white-light measurements
by COR2 is mEUVI/mCOR2 = 1.1 +/- 0.3. The consistency between EUVI
and COR2 implies no significant mass backflows (or inflows) at r <
4 R and adequate temperature coverage for the bulk of the CME mass
in the range of T = 0.5-3.0 MK. The temporal evolution of the EUV
dimming allows us also to model the evolution of the CME density,
volume, height-time, and propagation speed in terms of an adiabatically
expanding self-similar geometry. We test this model with the predicted
detection delay between EUVI and COR2 for the 2008-Mar-25 event.
---------------------------------------------------------
Title: The CME-Flare Relation Revisited With STEREO Observations
Authors: Nitta, Nariaki; Aschwanden, M.; Freeland, S.; Lemen, J.;
Wuelser, J.; Zarro, D.
2009SPD....40.2105N Altcode:
We study the association of solar flares since March 2007 with coronal
mass ejections (CMEs), using images taken by the EUV Imager (EUVI),
COR1 and COR2 coronagraphs on board STEREO. This is done by searching
EUVI data for low coronal signatures attributable to CMEs, such as
dimming, EUV waves and eruptions, following them to COR1 and COR2
fields of view. Base and running difference images (after correcting
for differential rotation) as well as raw images in all the four
filters of EUVI on STEREO A and STEREO B are viewed as movies to find
the CME-related signatures. The COR1 data are particularly helpful for
connecting the EUVI signatures with CMEs observed by COR2. Only 2 (out
of 11) M-class flares and 7 (out of 64) C-class flares are convincingly
associated with CMEs traceable beyond 5 Rs. There are also a handful
of less intense (B-class and A-class) flares associated with CMEs. We
discuss the "calibration" of the low coronal signatures with actual
CMEs, quantitatively re-defining them to be used as reliable proxies
for CMEs. Radio observations are also found to be of use to distinguish
flares associated and not associated with CMEs. Lastly we consider the
CME association of flares in terms of the the following items about the
flaring active regions: their basic properties, their relations with
more global field, and local (spatial or temporal) changes therein. This
study may help us understand the effect of (reconnection-driven)
flare processes on the initiation and subsequent dynamics of CMEs.
---------------------------------------------------------
Title: First Three-Dimensional Reconstructions of Coronal Loops with
the STEREO A+B Spacecraft. III. Instant Stereoscopic Tomography of
Active Regions
Authors: Aschwanden, Markus J.; Wuelser, Jean-Pierre; Nitta, Nariaki
V.; Lemen, James R.; Sandman, Anne
2009ApJ...695...12A Altcode:
Here we develop a novel three-dimensional (3D) reconstruction method
of the coronal plasma of an active region by combining stereoscopic
triangulation of loops with density and temperature modeling of
coronal loops with a filling factor equivalent to tomographic volume
rendering. Because this method requires only a stereoscopic image
pair in multiple temperature filters, which are sampled within ≈1
minute with the recent STEREO/EUVI instrument, this method is about
four orders of magnitude faster than conventional solar rotation-based
tomography. We reconstruct the 3D density and temperature distribution
of active region NOAA 10955 by stereoscopic triangulation of 70 loops,
which are used as a skeleton for a 3D field interpolation of some
7000 loop components, leading to a 3D model that reproduces the
observed fluxes in each stereoscopic image pair with an accuracy
of a few percents (of the average flux) in each pixel. With the
stereoscopic tomography we infer also a differential emission
measure distribution over the entire temperature range of T ≈
10<SUP>4</SUP>-10<SUP>7</SUP>, with predictions for the transition
region and hotter corona in soft X-rays. The tomographic 3D model
provides also large statistics of physical parameters. We find that
the extreme-ultraviolet loops with apex temperatures of T<SUB>m</SUB>
lsim 3.0 MK tend to be super-hydrostatic, while hotter loops with
T<SUB>m</SUB> ≈ 4-7 MK are near-hydrostatic. The new 3D reconstruction
model is fully independent of any magnetic field data and is promising
for future tests of theoretical magnetic field models and coronal
heating models.
---------------------------------------------------------
Title: New Aspects on Particle Acceleration in Solar Flares from
RHESSI Observations
Authors: Aschwanden, M. J.
2009AsJPh..17..423A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Mass Ejections Associated With Impulsive Solar Flares -
Observations With SECCHI EUVI On STEREO
Authors: Nitta, N. V.; Lemen, J. R.; Wuelser, J.; Aschwanden, M. J.;
Freeland, S. L.; Zarro, D. M.
2008AGUFMSH13B1538N Altcode:
Long-duration flares, sometimes referred to as Long Decay Events (LDEs),
are known to be unmistakable signatures of coronal mass ejections
(CMEs), and often of fast and large ones. Short-duration or impulsive
flares, on the other hand, do not as frequently accompany CMEs,
even though X-ray plasmoid ejections seen in some of these flares may
suggest that all flares are eruptive irrespective of durations. Some
of these ejections in X-ray or EUV images could be failed ejections,
however, meaning that they do not move into interplanetary medium. A
complementary, and perhaps more reliable signature of a CME in the
low corona may be large-scale dimming typically observed at 1-2
MK. We report on high cadence observations of SECCHI EUVI on STEREO
that show this phenomenon in weak impulsive flares more frequently
than expected. We systematically study flare periods with good data
coverage. In order to avoid false dimming, we use both base and running
difference images after carefully co-aligning the image pairs. Some
of the dimming events were observed in more than one channel and
at two widely separated view angles, letting us better understand
the nature of dimming especially in terms of the associated CME. We
discuss how the properties of dimming are reflected in CME parameters,
how to distinguish the impulsive flares with large- scale effects from
those that are confined, and whether similar events could account for
orphan ICMEs without a clearly associated CME near the Sun.
---------------------------------------------------------
Title: Nonlinear Force-Free Magnetic Field Modeling of the Solar
Corona: A Critical Assessment
Authors: De Rosa, M. L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
Lites, B. W.; Aschwanden, M. J.; McTiernan, J. M.; Régnier, S.;
Thalmann, J.; Valori, G.; Wheatland, M. S.; Wiegelmann, T.; Cheung,
M.; Conlon, P. A.; Fuhrmann, M.; Inhester, B.; Tadesse, T.
2008AGUFMSH41A1604D Altcode:
Nonlinear force-free field (NLFFF) modeling promises to provide accurate
representations of the structure of the magnetic field above solar
active regions, from which estimates of physical quantities of interest
(e.g., free energy and helicity) can be made. However, the suite of
NLFFF algorithms have so far failed to arrive at consistent solutions
when applied to cases using the highest-available-resolution vector
magnetogram data from Hinode/SOT-SP (in the region of the modeling
area of interest) and line-of-sight magnetograms from SOHO/MDI (where
vector data were not been available). It is our view that the lack of
robust results indicates an endemic problem with the NLFFF modeling
process, and that this process will likely continue to fail until (1)
more of the far-reaching, current-carrying connections are within the
observational field of view, (2) the solution algorithms incorporate
the measurement uncertainties in the vector magnetogram data, and/or
(3) a better way is found to account for the Lorentz forces within
the layer between the photosphere and coronal base. In light of these
issues, we conclude that it remains difficult to derive useful and
significant estimates of physical quantities from NLFFF models.
---------------------------------------------------------
Title: Using STEREO/EUVI to Study Active Region Magnetic Fields
Authors: Sandman, A.; Aschwanden, M.; Wuelser, J.; De Rosa, M.;
Alexander, D.
2008AGUFMSH13B1523S Altcode:
We examine the effect of linear transformations on the misalignment
between model magnetic fields and coronal loops in active regions,
as observed with STEREO/EUVI on three separate occasions between
April 30 and May 19, 2007. We perform stereoscopic triangulation of
some 100 EUVI loops in each active region, and identify the tangent
vectors along every loop. Using magnetogram information from SOHO/MDI
we compute a 3D potential field and interpolate the magnetic field
vector at every position along the EUVI loops. The angle between the
loop tangent vector and the magnetic field vector provides a measure
of the misalignment angle between the observed field configuration
and the model. We then transform the field in a way that preserves
the divergence-free condition while injecting electric currents into
the system. With this modified field we repeat our calculation of the
misalignment angles between the magnetic field vectors and the EUV
loop tangent vectors, quantifying the improvement of the transformed
magnetic field model. Results of this type of magnetic modeling are
presented for three active regions.
---------------------------------------------------------
Title: Tracking the 3-D Structure of an Erupting Filament With the
STEREO/SECCHI EUVI
Authors: Wuelser, J.; Aschwanden, M. J.; Lemen, J. R.; Nitta, N.;
Sandman, A.
2008AGUFMSH12A..01W Altcode:
On 2007 May 19, the STEREO/SECCHI EUVI observed an erupting filament
that was associated with a CME. The stereoscopic observations in both He
II and Fe IX allow a detailed study of the 3-D geometry and dynamics of
the filament as it erupts. Deconvolution of the EUVI imagery with the
instrument point spread function enhances image contrast and detail,
and improves the accuracy of the 3-D analysis. The quantitative results
of such an analysis may lead to a better understanding of the early
CME process.
---------------------------------------------------------
Title: Instant Stereoscopic Tomography of Active Regions with
STEREO/EUVI
Authors: Aschwanden, M. J.; Wuelser, J.; Nitta, N.; Lemen, J.;
Sandman, A.
2008AGUFMSH13B1555A Altcode:
We develop a novel 3D reconstruction method of the coronal plasma of
an active region by combining stereoscopic triangulation of loops with
density and temperature modeling of coronal loops with a filling factor
equivalent to tomographic volume rendering. Because this method requires
only a stereoscopic image pair in multiple temperature filters, which
are sampled within ~1 minute with the recent STEREO/EUVI instrument,
this method is about 4 orders of magnitude faster than conventional
solar rotation-based tomography. We reconstruct the 3D density and
temperature distribution of active region NOAA 10955 by stereoscopic
triangulation of 70 loops, which are used as a skeleton for a 3D field
interpolation of some 7000 loop components, leading to a 3D model that
reproduces the observed fluxes in each stereosocpic image pair with an
accuracy of a few percent (of the average flux) in each pixel. With the
stereoscopic tomography we infer also a differential emission measure
(DEM) distribution over the entire temperature range of T~0.01-10 MK,
with predictions for the transition region and hotter corona in soft
X-rays. The tomographic 3D model provides also large statistics of
physical parameters. We find that the EUV loops with apex temperatures
of T = 1- 3 MK tend to be super-hydrostatic, while hotter loops with T =
4-7 MK are near-hydrostatic. The new 3D reconstruction model is fully
independent of any magnetic field data and is promising for future
tests of theoretical magnetic field models and coronal heating models.
---------------------------------------------------------
Title: The Effect of Radiative Cooling on Coronal Loop Oscillations
Authors: Aschwanden, Markus J.; Terradas, Jaume
2008ApJ...686L.127A Altcode:
Coronal loops that exhibit kink-mode oscillations have generally been
assumed to have a constant density and temperature during the observed
time interval. Analyzing their intensities in an EUV wave band, however,
clearly shows that their brightness varies in a way that is consistent
with a temperature cooling through the EUV passband, which limits
their detection time, observed damping time, and number of observable
periods. We study kink-mode oscillations of eight loops observed during
the so-called harmonica event on 2001 April 15, 21:58-22:27 UT in the
171 Å band. We find loop densities of n<SUB>e</SUB> = (1.4 +/- 0.6)
× 10<SUP>9</SUP> cm<SUP>-3</SUP>, loop widths of w = 2.0 +/- 2.6 Mm,
and e-folding cooling times of τ<SUB>cool</SUB> = 17 +/- 7 minutes,
when they cool through the peak temperature T = 0.95 MK of the 171 Å
band. We conclude that oscillations of a single loop cannot be detected
longer than 10-20 minutes in one single filter and appropriate light
curve modeling is necessary to disentangle the subsequent oscillation
phases of multiple near-cospatial loops.
---------------------------------------------------------
Title: First 3D Reconstructions of Coronal Loops with the STEREO
A+B Spacecraft. II. Electron Density and Temperature Measurements
Authors: Aschwanden, Markus J.; Nitta, Nariaki V.; Wuelser,
Jean-Pierre; Lemen, James R.
2008ApJ...680.1477A Altcode:
Using the stereoscopically derived three-dimensional (3D) geometry
of 30 loops observed with STEREO EUVI (described in Paper I) we
determine here the electron density profiles n<SUB>e</SUB>(s) and
electron temperature profiles T<SUB>e</SUB>(s) from a triple-filter
analysis of the stereoscopic images taken in the wavelengths of λ =
171, 195, and 284 Å. The statistical results of our analysis of
seven complete loops are: observed loop widths w<SUB>obs</SUB> =
2.6 +/- 0.1 Mm, corresponding to effective loop widths of w = 1.1
+/- 0.3 Mm if corrected for the instrumental point-spread function;
loop flux ratios f<SUB>loop</SUB>/f<SUB>total</SUB> = 0.11 +/- 0.04;
mean loop (DEM peak) temperatures T<SUB>p</SUB> = 1.1 +/- 0.2 MK;
DEM temperature Gaussian widths σ<SUB>DEM</SUB> = 0.35 +/- 0.04 MK;
temperature variations along loops σ<SUB>T</SUB>/T<SUB>p</SUB> = 0.24
+/- 0.05; (resolution-corrected) loop base densities n<SUB>e</SUB> =
(2.2 +/- 0.5) × 10<SUP>9</SUP> cm<SUP>-3</SUP> loop lengths of L = 130
+/- 67 Mm; and all quantities are found to agree between STEREO A and
B within a few percent. The temperature profiles T(s) along loops are
found to be nearly constant, within the uncertainties of the background
subtraction. The density profiles n<SUB>e</SUB>(s) are consistent with
the gravitational stratification of hydrostatic loops, n<SUB>e</SUB>(h)
= n<SUB>base</SUB>exp (- h/λ<SUB>T</SUB>) , defined by the temperature
scale heights λ<SUB>T</SUB> and stereoscopically measured from the
height profiles h(s) . The stereoscopic 3D reconstruction allows us for
the first time to accurately measure the loop length L and to test loop
scaling laws. We find that the observations are not consistent with
equilibrium solutions, but rather display the typical overpressures
of loops that have been previously heated to higher temperatures and
cool down in a nonequilibrium state, similar to earlier EIT and TRACE
measurements.
---------------------------------------------------------
Title: First Three-Dimensional Reconstructions of Coronal Loops with
the STEREO A and B Spacecraft. I. Geometry
Authors: Aschwanden, Markus J.; Wülser, Jean-Pierre; Nitta, Nariaki
V.; Lemen, James R.
2008ApJ...679..827A Altcode:
We present one of the first triangulations and 3D reconstructions
of coronal loops, using the EUVI telescopes of the two STEREO A and
B spacecraft. The first triangulation of coronal loops was performed
in an active region, observed with STEREO A and B on 2007 May 9 with a
spacecraft separation angle of α<SUB>sep</SUB> = 7.3°, at a wavelength
of 171 Å. We identify 30 loop structures (7 complete loops and 23
partial segments) and compute their 3D coordinates (x,y,z) (the full
3D coordinates are available as an electronic file). We quantify the
height range, the stereoscopic height measurement errors, the loop
plane inclination angles, and the coplanarity and circularity of the
analyzed loops. The knowledge of the exact 3D geometry of a loop with
respect to the observer's line of sight has important consequences
for determining the correct vertical density scale height (used in
hydrostatic models), the aspect angle of loop cross sections (used in
inferring electron densities from optically thin emission measures),
the absolute flow speeds (used in siphon flow models), the correct
loop length (used in loop scaling laws), and the 3D vectors of the
coronal magnetic field (used in testing theoretical magnetic field
extrapolation models). The hydrodynamic and magnetic modeling of the
analyzed loops will be described in subsequent papers.
---------------------------------------------------------
Title: Exploring large-scale coronal magnetic field over extended
longitudes by STEREO/EUVI and its effect on solar wind prediction
Authors: Nitta, N. V.; De Rosa, M. L.; Zarro, D. M.; Wuelser, J.;
Aschwanden, M. J.; Lemen, J. R.
2008AGUSMSH23A..06N Altcode:
The potential field source surface (PFSS) model forms the basis of
a wide range of heliospheric science and applications, including
prediction of the solar wind speed near Earth. Experience shows that
the model sometimes works quite well, but not always. Possible reasons
for failure include deviation of the interplanetary magnetic field from
the nominal Parker spiral, violation of the assumptions used in the
model such as the discontinuity at the source surface, and the lack of
simultaneous full-surface magnetograms. Here we study the impact of the
lack of simultaneous full-surface magnetograms, using observations of
the corona over an extended longitude range made possible by the EUVI
on board the STEREO mission. In spite of the lack of magnetographs on
STEREO, EUVI data with a growing separation angle between spacecraft
A and B at least allow us to locate major active regions and coronal
holes in the area not seen from Earth. The PFSS extrapolations and their
input synoptic maps are compared with EUVI data to measure how well the
model fits the observations. These comparisons are discussed in terms of
the solar wind speed predicted by the model and observed at L1 by ACE.
---------------------------------------------------------
Title: Tracing the 3-D coronal structure during CMEs with
STEREO/SECCHI EUVI observations
Authors: Wuelser, J.; Aschwanden, M.; De Rosa, M.; Lee, C.; Lemen,
J.; Nitta, N.; Sandman, A.
2008AGUSMSH31A..05W Altcode:
STEREO/SECCHI EUVI observations of solar coronal loops, filaments,
and dimming regions provide unique information on the 3-D topology
of the coronal magnetic field above active regions and its evolution
during coronal mass ejections (CMEs). Active Region #10956 produced
several CMEs during its passage across the solar disk in May 2007,
some of them showing filament eruptions and dimming. The SECCHI/EUVI
instrument on STEREO obtained high cadence observations in multiple
lines simultaneously from both STEREO spacecraft. 3-D reconstructions
of coronal features over the course of a CME show significant changes
of the field topology. Comparisons with the potential field topology
from magnetic field extrapolations show the degree of non-potentiality
of the real field and changes in the vicinity of the CME onset. We
present initial results of this study.
---------------------------------------------------------
Title: Solar Flares and CMEs observed with STEREO/EUVI
Authors: Aschwanden, M. J.; Wuelser, J.; Nitta, N. V.; Lemen, J. R.
2008AGUSMSP44A..01A Altcode:
STEREO/EUVI observed a respectable number of small flares and CMEs
during the first year of its mission. A comprehensive survey between
Dec 2006 and Jan 2008 yields: 10 GOES >M1-class flares, 20 GOES
>C5-class flares, 80 >C1-class flares, 180 flares with >25 keV
hard X-ray emission observed by RHESSI, at least 35 flare events with
associated CME reports by LASCO or STEREO COR-1, and 25 flare events
are occulted by either STEREO A or B. Some flares associated with CMEs
clearly show an EUV dimming in the active region below the launched
CME, for which we determine for the first time the 3D geometry with
the two STEREO A and B spacecraft, allowing us to model the volume,
density, and ejected mass from the corona. Some flares show impulsive
signatures in EUV, simultaneously occurring with hard X-rays pulses,
indicative of the primary heating of the chromospheric plasma. In
larger flares, the bulk of the EUV emission is always substantially
delayed to the soft X-ray emission, as a result of the plasma cooling
from soft X-ray to EUV temperatures, from which we can estimate the
total thermal flare energy using hydrodynamic models.
---------------------------------------------------------
Title: Fundamental physical processes in coronae: Waves, turbulence,
reconnection, and particle acceleration
Authors: Aschwanden, Markus J.
2008IAUS..247..257A Altcode: 2007arXiv0711.0007A; 2007IAUS..247..257A
Our understanding of fundamental processes in the solar corona has
been greatly progressed based on the space observations of SMM, Yohkoh,
Compton GRO, SOHO, TRACE, RHESSI, and STEREO. We observe now acoustic
waves, MHD oscillations, turbulence-related line broadening, magnetic
configurations related to reconnection processes, and radiation from
high-energy particles on a routine basis. We review a number of key
observations in EUV, soft X-rays, and hard X-rays that innovated our
physical understanding of the solar corona, in terms of hydrodynamics,
MHD, plasma heating, and particle acceleration processes.
---------------------------------------------------------
Title: Comparison of Five Numerical Codes for Automated Tracing of
Coronal Loops
Authors: Aschwanden, Markus J.; Lee, Jong Kwan; Gary, G. Allen; Smith,
Michael; Inhester, Bernd
2008SoPh..248..359A Altcode:
The three-dimensional (3D) modeling of coronal loops and filaments
requires algorithms that automatically trace curvilinear features in
solar EUV or soft X-ray images. We compare five existing algorithms
that have been developed and customized to trace curvilinear features
in solar images: i) the oriented-connectivity method (OCM), which is
an extension of the Strous pixel-labeling algorithm (developed by Lee,
Newman, and Gary); ii) the dynamic aperture-based loop-segmentation
method (developed by Lee, Newman, and Gary); iii) unbiased detection of
curvilinear structures (developed by Steger, Raghupathy, and Smith); iv)
the oriented-direction method (developed by Aschwanden); and v) ridge
detection by automated scaling (developed by Inhester). We test the
five existing numerical codes with a TRACE image that shows a bipolar
active region and contains over 100 discernable loops. We evaluate the
performance of the five codes by comparing the cumulative distribution
of loop lengths, the median and maximum loop length, the completeness
or detection efficiency, the accuracy, and flux sensitivity. These
algorithms are useful for the reconstruction of the 3D geometry of
coronal loops from stereoscopic observations with the STEREO spacecraft,
or for quantitative comparisons of observed EUV loop geometries with
(nonlinear force-free) magnetic field extrapolation models.
---------------------------------------------------------
Title: Theoretical modeling for the stereo mission
Authors: Aschwanden, Markus J.; Burlaga, L. F.; Kaiser, M. L.; Ng,
C. K.; Reames, D. V.; Reiner, M. J.; Gombosi, T. I.; Lugaz, N.;
Manchester, W.; Roussev, I. I.; Zurbuchen, T. H.; Farrugia, C. J.;
Galvin, A. B.; Lee, M. A.; Linker, J. A.; Mikić, Z.; Riley, P.;
Alexander, D.; Sandman, A. W.; Cook, J. W.; Howard, R. A.; Odstrčil,
D.; Pizzo, V. J.; Kóta, J.; Liewer, P. C.; Luhmann, J. G.; Inhester,
B.; Schwenn, R. W.; Solanki, S. K.; Vasyliunas, V. M.; Wiegelmann, T.;
Blush, L.; Bochsler, P.; Cairns, I. H.; Robinson, P. A.; Bothmer,
V.; Kecskemety, K.; Llebaria, A.; Maksimovic, M.; Scholer, M.;
Wimmer-Schweingruber, R. F.
2008SSRv..136..565A Altcode: 2006SSRv..tmp...75A
We summarize the theory and modeling efforts for the STEREO mission,
which will be used to interpret the data of both the remote-sensing
(SECCHI, SWAVES) and in-situ instruments (IMPACT, PLASTIC). The
modeling includes the coronal plasma, in both open and closed magnetic
structures, and the solar wind and its expansion outwards from the Sun,
which defines the heliosphere. Particular emphasis is given to modeling
of dynamic phenomena associated with the initiation and propagation
of coronal mass ejections (CMEs). The modeling of the CME initiation
includes magnetic shearing, kink instability, filament eruption, and
magnetic reconnection in the flaring lower corona. The modeling of CME
propagation entails interplanetary shocks, interplanetary particle
beams, solar energetic particles (SEPs), geoeffective connections,
and space weather. This review describes mostly existing models of
groups that have committed their work to the STEREO mission, but is by
no means exhaustive or comprehensive regarding alternative theoretical
approaches.
---------------------------------------------------------
Title: Solar flare physics enlivened by TRACE and RHESSI
Authors: Aschwanden, Markus J.
2008JApA...29..115A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Keynote address: Outstanding problems in solar physics
Authors: Aschwanden, Markus J.
2008JApA...29....3A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar Flare Geometries. I. The Area Fractal Dimension
Authors: Aschwanden, Markus J.; Aschwanden, Pascal D.
2008ApJ...674..530A Altcode:
In this study we investigate for the first time the fractal dimension
of solar flares and find that the flare area observed in EUV
wavelengths exhibits a fractal scaling. We measure the area fractal
dimension D<SUB>2</SUB>, also called the Hausdorff dimension, with
a box-counting method, which describes the fractal area as A(L)
~ L<SUP>D<SUB>2</SUB></SUP>. We apply the fractal analysis to a
statistical sample of 20 GOES X- and M-class flares, including
the Bastille Day 2000 July 14 flare, one of the largest flares
ever recorded. We find that the fractal area (normalized by the
time-integrated flare area A<SUB>f</SUB>) varies from near zero at the
beginning of the flare to a maximum of A(t)/A<SUB>f</SUB> = 0.65 +/-
0.12 after the peak time of the flare, which corresponds to an area
fractal dimension in the range of 1.0lesssim D<SUB>2</SUB>(t) lesssim
1.89 +/- 0.05. We find that the total EUV flux F<SUB>tot</SUB>(t) is
linearly correlated with the fractal area A(t) . From the area fractal
dimension D<SUB>2</SUB>, the volume fractal dimension D<SUB>3</SUB>
can be inferred (subject of Paper II), which is crucial to inferring
a realistic volume filling factor, which affects the derived electron
densities, thermal energies, and cooling times of solar and stellar
flares.
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Title: Solar Flare Geometries. II. The Volume Fractal Dimension
Authors: Aschwanden, Markus J.; Aschwanden, Pascal D.
2008ApJ...674..544A Altcode:
Based on the area fractal dimension D<SUB>2</SUB> of solar flares
measured in Paper I, we carry out modeling of the three-dimensional
(3D) flare volume here and derive an analytical relation between
the volume fractal scaling V(L) ~ L<SUP>D<SUB>3</SUB></SUP> and the
area fractal scaling A(L) ~ L<SUP>D<SUB>2</SUB></SUP>. The 3D volume
model captures a flare arcade with a variable number of flare loops;
its fractal structure is not isotropic, but consists of aligned
one-dimensional substructures. The geometry of the arcade model has
three free parameters and makes some simplifying assumptions, such as
semicircular loops, east-west orientation, location near the equator,
and no magnetic shear. The analytical model predicts the scaling of the
area filling factor q<SUB>A</SUB>(n<SUB>loop</SUB>) and volumetric
filling factor q<SUB>V</SUB>(n<SUB>loop</SUB>) as a function of
the number of loops n<SUB>loop</SUB>, and allows one to predict the
volume filling factor q<SUB>V</SUB>(q<SUB>A</SUB>) and volume fractal
dimension D<SUB>3</SUB>(D<SUB>2</SUB>) from the observationally measured
parameters q<SUB>A</SUB> and D<SUB>2</SUB>. We also corroborate the
analytical model with numerical simulations. We apply this fractal
model to the 20 flares analyzed in Paper I and find maximum volume
filling factors with a median range of q<SUB>V</SUB> ≈ 0.03-0.08
(assuming solid filling for loop widths of lesssim1 Mm). The fractal
nature of the flare volume has important consequences for correcting
electron densities determined from flare volume emission measures and
density-dependent physical quantities, such as the thermal energy or
radiative cooling time. The fractal scaling has also far-reaching
consequences for frequency distributions and scaling laws of solar
and stellar flares.
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Title: Solar Active Regions: A Transition from Morphological
Observations to Physical Modeling (Opening Keynote Address)
Authors: Aschwanden, M. J.
2008ASPC..383....1A Altcode:
Although solar active regions can easily be identified from their
sunspot groups, they keep some of the most challenging secrets of
solar physics, regarding their subphotospheric origin, photospheric
emergence, magnetic field structure, magnetic helicity, wave generation,
propagation and dissipation, plasma heating and cooling, plasma flows,
fractal geometry and intermittency, and magnetic instabilities leading
to flares, CMEs, and coronal dimming. In this opening talk we review
how morphological observations of active regions have gradually evolved
into a more physics-based modeling approach over the last decades.
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Title: An Observational Test That Disproves Coronal Nanoflare
Heating Models
Authors: Aschwanden, Markus J.
2008ApJ...672L.135A Altcode:
All theoretical models of nanoflare heating in the solar corona predict
loops with an unresolved multistrand structure, which is in discrepancy
with the quasi-isothermal cross sections of the finest coronal loop
structures observed with TRACE, which have spatially resolved widths
of w ≈ 1000-2000 km. We suggest modifying the theoretical models
by relocating the hypothetical nanoflare events from their coronal
location down to the chromosphere/transition region.
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Title: Scaling Laws of Solar and Stellar Flares
Authors: Aschwanden, Markus J.; Stern, Robert A.; Güdel, Manuel
2008ApJ...672..659A Altcode: 2007arXiv0710.2563A
In this study we compile for the first time comprehensive data sets of
solar and stellar flare parameters, including flare peak temperatures
T<SUB>p</SUB>, flare peak volume emission measures EM<SUB>p</SUB>,
and flare durations τ<SUB>f</SUB> from both solar and stellar data,
as well as flare length scales L from solar data. Key results are
that both the solar and stellar data are consistent with a common
scaling law of EM<SUB>p</SUB> propto T<SUP>4.7</SUP><SUB>p</SUB>,
but the stellar flares exhibit ≈250 times higher emission measures
(at the same flare peak temperature). For solar flares we observe
also systematic trends for the flare length scale L(T<SUB>p</SUB>)
propto T<SUP>0.9</SUP><SUB>p</SUB> and the flare duration
τ<SUB>F</SUB>(T<SUB>p</SUB>) propto T<SUP>0.9</SUP><SUB>p</SUB>
as a function of the flare peak temperature. Using the theoretical
RTV scaling law and the fractal volume scaling observed for solar
flares, i.e., V(L) propto L<SUP>2.4</SUP>, we predict a scaling
law of EM<SUB>p</SUB> propto T<SUP>4.3</SUP><SUB>p</SUB>, which is
consistent with observations, and a scaling law for electron densities
in flare loops, n<SUB>p</SUB> propto T<SUP>2</SUP><SUB>p</SUB>/L propto
T<SUP>1.1</SUP><SUB>p</SUB>. The RTV-predicted electron densities were
also found to be consistent with densities inferred from total emission
measures, n<SUB>p</SUB> = (EM<SUB>p</SUB>/q<SUB>V</SUB>V)<SUP>1/2</SUP>,
using volume filling factors of q<SUB>V</SUB> = 0.03-0.08 constrained
by fractal dimensions measured in solar flares. Solar and stellar
flares are expected to have similar electron densities for equal flare
peak temperatures T<SUB>p</SUB>, but the higher emission measures
of detected stellar flares most likely represent a selection bias
of larger flare volumes and higher volume filling factors, due to
low detector sensitivity at higher temperatures. Our results affect
also the determination of radiative and conductive cooling times,
thermal energies, and frequency distributions of solar and stellar
flare energies.
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Title: 3D Geometry of Coronal Loops Measured with STEREO / EUVI
Authors: Aschwanden, M. J.; Wuelser, J.; Nitta, N.; Lemen, J.
2007AGUFMSH41B..04A Altcode:
Using images from the STEREO/EUVI A and B spacecraft we developed
an accurate method that performs stereoscopic triangulation and
reconstruction of the 3D geometry of curvi-linear structures in
the solar corona, such as loops, filaments, prominence threads, or
wave features. We test the coalignment of stereoscopic images and
establish an accuracy of better than <0.1 pixels in east-west
direction, <0.3 pixels in north-south direction, and <0.05
degrees in roll angle. We reconstruct the 3D geometry of some 100
coronal loops loops in active regions in May 2007, when the spacecraft
had a separation angle of ~10 degrees. We find that complete loops or
incomplete segments of loops can only be reconstructed up to altitudes
of about one hydrostatic scale height, which is h<50 Mm at a coronal
temperature of T=1 MK. The determination of the 3D geometry of coronal
loops is an important and necessary step to model their hydrodynamic
structure. We show also that this method can be used to determine
quantitatively the eigen-motion, oscillation, twisting, expansion,
acceleration, or other dynamics of coronal loops, erupting filaments,
and MHD wave fronts, in particular in association with flares and CMEs.
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Title: 3D Topology of Prominences Measured with STEREO/EUVI
Authors: Slater, G. L.; Aschwanden, M. J.
2007AGUFMSH32A0771S Altcode:
Using images from the STEREO/EUVI A and B spacecraft and stereoscopic
triangulation methods developed by Aschwanden et al for the
reconstruction of the 3D geometry of curvi-linear structures in the
solar corona, we determine three dimensional model reconstructions of
several long-lived prominences as well as their topology - in particular
the twist and number of helical turns. The derived geometries are
analyzed together with photospheric magnetograms and potential field
extrapolations (PFFS) and compared to various models for the magnetic
confinement and support of prominence plasmas.
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Title: Stereoscopic Observations of Low Coronal Ejections With and
Without CMEs
Authors: Nitta, N. V.; Wülser, J.; Aschwanden, M. J.; Lemen, J. R.
2007AGUFMSH32A0775N Altcode:
Yohkoh soft X-ray images of solar flares have frequently shown
characteristic ejections during the impulsive phase. They are
thought to be plasmoids that hold important information on magnetic
reconnection. These ejections are intimately associated with coronal
mass ejections (e.g., Nitta & Akiyama 1999; Kim et al. 2005). They
probably represent internal structures of CMEs, i.e., high-temperature
counterparts of filament eruptions. However, their relation with
ejections seen at low temperatures has not been studied systematically,
although TRACE has revealed many beautiful examples. In this work we
study ejections observed by the SECCHI EUVI on STEREO. Some of them are
associated with CMEs, and others are not. Using pairs of EUVI images
from spacecraft A and B, the trajectories of ejecta in individual
channels (with representative temperatures 0.1-2 MK) are reconstructed
in 3D. We discuss these ejections at different temperatures in the
context of CMEs. Specifically, we ask what kinetic properties are
correlated with CMEs and how they are related with CME manifestations
in the low corona such as dimming and waves.
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Title: 3-D reconstruction of CME related transient coronal phenomena
observed with the STEREO/SECCHI Extreme Ultraviolet Imager
Authors: Wuelser, J.; Aschwanden, M. J.; Lemen, J. R.; Nitta, N.
2007AGUFMSH32A0772W Altcode:
One of the primary objectives of the SECCHI investigation on STEREO
is to study the initiation of CMEs in the low corona, and to better
understand CME related changes of the three-dimensional coronal
structure. The SECCHI Extreme Ultraviolet Imagers (EUVI) have been
observing the solar corona from two significantly different vantage
points since about March 2007. They have since captured several
CMEs, including a few during the SECCHI campaign in May 2007, at an
observatory separation angle of about 7-8 degrees. Observations at
relatively small separation angles allow for easier identification of
features in two views, which is critical for visually aided tie- point
tools, as well as for more automated 3-D reconstruction methods. EUVI
movies taken during the early onset of a CME show a range of transient
phenomena, including coronal ejecta that can be tracked into the
coronagraph fields of view, erupting filaments that trail the coronal
ejecta, displacement of active region loops, coronal dimming, and "EIT"
waves. We present preliminary results of our first 3-D reconstruction
attempts on a selection of such CME related phenomena, with emphasis
on coronal ejecta and active region loop displacements.
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Title: Early Evolution of CMEs as Observed by SECCHI EUVI on STEREO
Authors: Lemen, J. R.; Nitta, N. V.; Wülser, J.; Aschwanden, M. J.
2007AGUFMSH32A0768L Altcode:
We report on two eruptions associated with small (C1 and B8) flares that
occurred in AR 956 on 2007 May 19 and 20, as observed stereoscopically
by EUVI. The separation of the two spacecraft was approximately 9
degrees. Pairs of images in 171~Å\ and 304~Å\ from two view angles
are used to constrain the trajectories of the ejecta or filaments
that appear to be responsible for the associated CMEs; they were not
homologous. We study how the 3D motions of the ejecta in the low corona
correspond to the CMEs at higher altitudes. We also discuss the possible
relation between the early CME propagation and the coronal magnetic
field topology inferred from EUV loops in EUVI and TRACE 171~Å\ images.
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Title: 3D Magnetic Modeling of Active Regions Using STEREO/EUVI
Authors: Sandman, A.; Aschwanden, M. J.; Alexander, D.; Wuelser, J.
2007AGUFMSH32A0770S Altcode:
With the recent availability of stereoscopic data from the Extreme
Ultraviolet Imager (EUVI) on the Solar Terrestrial Relations Observatory
(STEREO) we have an unprecedented opportunity to investigate the
accuracy of 3D magnetic field models. These data will be put to
best use by modeling techniques that make no assumptions about the
nature of the field (that it is potential, force-free, etc.). The
Gary-Alexander radial stretching method use a series of transformations
to map a simple potential field to a more complicated target field, and
compare the transformed field lines with observed coronal structures
in the EUV. Unlike many other simulation techniques, this approach
requires only that the field remain divergence-free and continuous
at the photosphere. Here we apply this transformation method to
STEREO/EUVI data. We obtain a 3D potential field extrapolation using
an MDI magnetogram, and utilize stereoscopy to derive the 3D field
line coordinates from pairs of EUV images at 171Å. By comparing the
3D coordinates of the transformed model field lines with those of
the real field lines as seen by EUVI, we can place constraints on the
distribution of magnetic field and current in an active region.
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Title: Astrophysics in 2006
Authors: Trimble, Virginia; Aschwanden, Markus J.; Hansen, Carl J.
2007SSRv..132....1T Altcode: 2007arXiv0705.1730T
The fastest pulsar and the slowest nova; the oldest galaxies and the
youngest stars; the weirdest life forms and the commonest dwarfs; the
highest energy particles and the lowest energy photons. These were
some of the extremes of Astrophysics 2006. We attempt also to bring
you updates on things of which there is currently only one (habitable
planets, the Sun, and the Universe) and others of which there are
always many, like meteors and molecules, black holes and binaries.
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Title: RHESSI Timing Studies: Multithermal Delays
Authors: Aschwanden, Markus J.
2007ApJ...661.1242A Altcode:
We investigate the energy-dependent timing of thermal emission
in solar flares using high-resolution spectra and demodulated
time profiles from the RHESSI instrument. We model for the first
time the spectral-temporal hard X-ray flux f(ɛ,t) in terms of a
multitemperature plasma governed by thermal conduction cooling. In
this quantitative model we characterize the multitemperature
differential emission measure distribution (DEM) and nonthermal
spectra with power-law functions. We fit this model to the spectra
and energy-dependent time delays of a representative data set of 89
solar flares observed with RHESSI during 2002-2005. Eliminating weak
flares, we find 65 events suitable for fitting and obtain in 44 events
(68%) a satisfactory fit that is consistent with the theoretical
model. The best-fit results yield a thermal-nonthermal crossover
energy of ɛ<SUB>th</SUB>=18.0+/-3.4 keV, nonthermal spectral indices
of γ<SUB>nth</SUB>=3.5+/-1.1 (at ~30-50 keV), thermal multispectral
indices of γ<SUB>th</SUB>=6.9+/-0.9 (at ~10-20 keV), and thermal
conduction cooling times of τ<SUB>c0</SUB>=10<SUP>1.6+/-0.6</SUP>
s at ɛ<SUB>th</SUB>=1 keV (or T<SUB>0</SUB>=11.6 MK), which
scale with temperature as τ<SUB>c</SUB>(T)~T<SUP>-β</SUP> with
β=2.7+/-1.2, consistent with the theoretically expected scaling of
τ<SUB>c</SUB>(T)~T<SUP>-5/2</SUP> for thermal conduction cooling. The
(empirical) Neupert effect is consistent with this theoretical model in
the asymptotic limit of long cooling times. This study provides clear
evidence that all analyzed flares are consistent with the model of a
multitemperature plasma distribution and with thermal conduction as
dominant cooling mechanism (at flare temperatures of T>~10 MK). Our
modeling of energy-dependent time delays provides an alternative method
for separating multithermal from nonthermal spectral components based
on information in the time domain, in contrast to previous spectral
fitting methods.
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Title: The Third Solar Dimension (Invited Parker Lecture)
Authors: Aschwanden, Markus J.
2007AAS...21010401A Altcode: 2007BAAS...39..230A
Over the last two decades we acquired stunning images of the Sun in EUV,
soft X-ray, and hard X-ray wavelengths, which show us magnetic loops
and arcades in the solar corona, quiescent and eruptive filaments,
flares and coronal mass ejections (CMEs). Physical modeling of these
phenomena requires a reconstruction of the 3-dimensional (3D) geometry,
which was mostly accomplished with 3D extrapolations of the photospheric
field, using theoretical models of magnetic potential fields and
force-free fields. The <P />3D distribution of the coronal plasma
could also be reconstructed by means of solar rotation stereoscopy and
tomography. The most recent solar space mission is STEREO, launched
in 2006, which provides us true stereoscopic images of unprecedented
clarity and should reveal us the full 3D magnetic topology of flares
and CMEs. The ULYSSES spacecraft, as well as the future missions Solar
Orbiter and Sentinels, will fly out of the ecliptic plane and will
provide us a 3D perspective of the heliosphere.
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Title: First 3d Triangulation Of Coronal Loops With Stereo/euvi
Authors: Aschwanden, Markus J.; Wuelser, J.; Lemen, J.; Nitta, N.
2007AAS...210.2810A Altcode: 2007BAAS...39..137A
The orbits of the two STEREO spacecraft A(head) and B(ehind) move to
progressively larger stereoscopic separation angles a, with a=1.2 deg
on March 1, a=3.0 deg on April 1, a=6.0 deg on May 1, and a=10.5 deg
on June 1. This range of small-angle separation enables the first 3D
triangulation of coronal features. Active region loops at 1 MK have
a scale height of 50 Mm, for which the parallax effect amounts up
to 7 EUVI pixels at a 10 deg separation angle. We present the first
triangulations of active region loops, with the goal to reconstruct
the 3D geometry along entire loop lengths. Such 3D reconstructions
yield the inclination angles of loop planes, which allow us to test
the relation between projected and vertical hydrostatic density scale
heights. Another important application is how the 3D geometry of the
stereoscopically reconstructed loops relates to theoretical (potential,
linear and nonlinear force-free) magnetic field extrapolations. We
attempt also to reconstruct the 3D geometry of filaments and to
track their motion in 3D before eruption and onset of flares and
CMEs. - This work is supported by the NASA STEREO under NRL contract
N00173-02-C-2035.
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Title: First Assessments Of EUVI Performance On STEREO SECCHI
Authors: Lemen, James; Wuelser, J. P.; Nitta, N.; Aschwanden, M.
2007AAS...210.2801L Altcode: 2007BAAS...39..135L
The SECCHI investigation on the STEREO mission contains two Extreme
Ultraviolet Imagers (EUVI), one on the ahead spacecraft and one on the
behind spacecraft. EUVI views the solar disk using multilayer-coated
normal-incidence optics that image onto 1.6 arcsec-per-pixel
back-thinned CCD detectors. Four wavelength bandpasses are observed
in series, 17.1 nm (Fe IX), 19.5 nm (Fe XII), 28.4 nm (Fe XV),
and 30.4 nm (He II), covering the chromospheric and coronal plasma
temperatures.Science operations began in January 2007 and both EUVIs
are working very well. The fine pointing system effectively removes
low frequency spacecraft pointing jitter, so the image resolution
quality is very good, approaching 3.5 arcsec. We present early on-orbit
assessments of the performance of both EUVIs and updated predictions
for the temperature dependent instrument response functions, which
are compared to early observations. The lunar transit observed by
the EUVI on the behind spacecraft is used to assess the point spread
function. <P />This work is supported by the NASA STEREO mission under
NRL contract N00173-02-C-2035.
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Title: Low Coronal Manifestations Of Coronal Mass Ejections As
Observed By STEREO EUVI
Authors: Nitta, Nariaki; Wuelser, J.; Lemen, J.; Aschwanden, M.;
Attrill, G.
2007AAS...210.2805N Altcode: 2007BAAS...39..136N
Data from SOHO/EIT have tremendously advanced our knowledge about
the initiation of coronal mass ejections (CMEs) as observed by
SOHO/LASCO. The unequivocal EUV manifestations of CMEs include
dimming and waves, typically observed at EIT's 195 A channel. In
this presentation, we report mainly on these two observables in two
similar events as observed on 2007 January 24 and 25 by the Extreme
Ultraviolet Imagers (EUVI), which are part of SECCHI on STEREO. The
source region of the two events was most likely AR 0940, which was
located 20 degrees and 10 degrees behind the east limb at the times of
the events. Images in 171 A and 195 A were taken at a basic cadence of
10 minutes, slightly better than that of the EIT movie sequence. But
what is remarkable is the availability of nearly simultaneous (dt =
11 sec) full-disk images in the two wavelengths. We give detailed
comparisons of the wave propagations and dimming regions as observed
at 171 A and 195 A, and discuss their relations with the white-light
CMEs and their associated flares. This work has been supported by NASA
STEREO mission contract N00173-02-C-2035 through NRL.
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Title: Stereo Impact Investigation Early Mission Observations of The
Solar Wind Structure, Its Sources and Its Interplanetary Consequences
Authors: Luhmann, Janet; Schroeder, P.; Lin, R. P.; Larson, D. E.;
Lee, C. O.; Sauvaud, J.; Acuna, M. H.; von Rosenvinge, T.; Mewaldt,
R. A.; Davis, A. J.; Mueller-Mellin, R.; Mason, G. M.; Russell, C. T.;
Jian, L.; Galvin, A. B.; Howard, R. A.; Aschwanden, M.; Arge, C. N.;
MacNeice, P.; Chulaki, A.; Petrie, G.
2007AAS...21011907L Altcode: 2007BAAS...39..244L
STEREO carries two main in-situ investigations to measure the solar wind
plasma and interplanetary energetic particles and magnetic fields at 1
AU at the increasingly separated STEREO sites. These measurements have
provided the opportunity to study the connections between the quiet
corona and the solar wind structure during the first months of the
mission's operation. In addition to the possibility of observing the
same features at multiple sites, in combination with the SECCHI imaging
investigation and SOHO and ACE the data allow us to explore specific
questions regarding solar wind source regions and stream structure
geometry. For example, not all high speed-low speed stream interaction
regions have similar character; some exhibit more substructure and some
have stronger locally accelerated particle signatures than others. We
investigate this and other features taking into account the coronal
source differences.
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Title: STEREO's in-situ perspective on the solar minimum solar
wind structure
Authors: Luhmann, J. G.; Larson, D.; Schroeder, P.; Lee, C. O.;
Sauvaud, J.; Acuna, M. H.; Galvin, A. B.; Russell, C. T.; Jian, L.;
Arge, C. N.; Odstrcil, D.; Riley, P.; Howard, R. A.; Aschwanden, M.;
MacNeice, P.; Chulaki, A.
2007AGUSMSH34A..04L Altcode:
STEREO multipoint measurements of the solar wind structure with the
IMPACT and PLASTIC investigations, near Earth but off the Sun-Earth
line, allow its sources and structure to be examined at solar minimum
when such studies are particularly straightforward. With the aid of 3D
models of the heliosphere available at the CCMC, we map the in-situ
observations to their solar sources using a combination of the open
field regions inferred from the SECCHI EUVI imagers and SOHO EIT, and
the magnetogram-based models of the corona and solar wind. Our ultimate
goal is the continuous tracking of solar wind source regions as the
STEREO mission progresses, as well as the use of the mappings to deduce
the distinctive properties of solar wind from different types of sources
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Title: The Coronal Heating Paradox
Authors: Aschwanden, Markus J.; Winebarger, Amy; Tsiklauri, David;
Peter, Hardi
2007ApJ...659.1673A Altcode:
The “coronal heating problem” has been with us over 60 years, and
hundreds of theoretical models have been proposed without an obvious
solution in sight. In this paper we point out that observations show no
evidence for local heating in the solar corona, but rather for heating
below the corona in the transition region and upper chromosphere,
with subsequent chromospheric evaporation as known in flares. New
observational evidence for this scenario comes from (1) the temperature
evolution of coronal loops, (2) the overdensity of hot coronal loops,
(3) upflows in coronal loops, (4) the Doppler shift in coronal loops,
(5) upward propagating waves, (6) the energy balance in coronal loops,
(7) the magnetic complexity in the transition region, (8) the altitude
of nanoflares and microflares, (9) the cross section of elementary
loops, as well as (10) 3D MHD simulations of coronal heating. The phrase
“coronal heating problem” is therefore a paradoxical misnomer for
what should rather be addressed as the “chromospheric heating problem”
and “coronal loop filling process.” This paradigm shift substantially
reduces the number of relevant theoretical models for coronal heating
in active regions and the quiet Sun, but our arguments do not apply
to coronal holes and the extended heliospheric corona.
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Title: A Statistical Model of the Inhomogeneous Corona Constrained
by Triple-Filter Measurements of Elementary Loop Strands with TRACE
Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Boerner, Paul
2007ApJ...656..577A Altcode:
Here we present the first quantitative model of the inhomogeneous
solar corona, which we call the composite and elementary loops
in a thermally inhomogeneous corona (CELTIC) model. We develop a
self-consistent statistical model that quantifies the distributions
of physical parameters, i.e., the distributions of loop widths,
N(w,T<SUB>e</SUB>), electron densities, N(n<SUB>e</SUB>,T<SUB>e</SUB>),
electron temperatures, N(T<SUB>e</SUB>), and statistical correlations
between them. The parameterized distributions are constrained by
the observed triple-filter fluxes of the EUV corona measured at
some 18,000 loop positions with TRACE in the temperature range of
T<SUB>e</SUB>~0.7-2.7 MK, as well as by the individual loop parameters
(w,n<SUB>e</SUB>,T<SUB>e</SUB>) measured at these positions in ~240
detected loops, mostly sampled in active regions. The CELTIC model
is inverted from the TRACE data and reproduces both the fluxes of
the composite (active region and quiet Sun) background corona and the
distributions of loop parameters in a self-consistent way. The best-fit
values constrain a statistical correlation between the density and
temperature, i.e., e>~e><SUP>α</SUP>, with α=0.9+/-0.6, and
between the loop width and temperature, i.e., ~e><SUP>β</SUP>,
with β=1.3+/-0.7, which can be related to the thermal pressure in a
regime with a high plasma-β parameter. A possible explanation is a
heating process located in the lower transition region or the upper
chromosphere (e.g., as reproduced in the recent MHD simulations of
Gudiksen and Nordlund), which produces sufficiently high electron
densities, high plasma-β parameters, and thermally homogeneous loop
cross sections as observed in elementary loop strands.
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Title: The Localization of Particle Acceleration Sites in Solar
Flares and CMEs
Authors: Aschwanden, Markus J.
2007sdeh.book..361A Altcode:
No abstract at ADS
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Title: The Sun
Authors: Aschwanden, Markus J.
2007ess..book...71A Altcode:
The Sun is the central body and energy source of our solar system. The
Sun is our nearest star, but otherwise it represents a fairly typical
star in our galaxy, classified as G2-V spectral type, with a radius
of r <SUB>o</SUB> ≈700,000 km, a mass of m <SUB>o</SUB>≈ 21033 g,
a luminosity of L <SUB>o</SUB>≈ 3.8 10 <SUP>26</SUP> W, and an age of
t <SUB>o</SUB>≈ 4.6 10 <SUP>9</SUP> years (Table 1). The distance from
the Sun to our Earth is called an astronomical unit (AU) and amounts to
R150 106 km. The Sun lies in a spiral arm of our galaxy, the Milky Way,
at a distance of 8.5 kiloparsecs from the galactic center. Our galaxy
contains R10 <SUP>12</SUP> individual stars, many of which are likely
to be populated with similar solar systems, according to the rapidly
increasing detection of extrasolar planets over the last years; the
binary star systems are very unlikely to harbor planets because of their
unstable, gravitationally disturbed orbits. The Sun is for us humans of
particular significance, first because it provides us with the source of
all life, and second because it furnishes us with the closest laboratory
for astrophysical plasma physics, magneto-hydrodynamics (MHD), atomic
physics, and particle physics. The Sun still represents the only star
from which we can obtain spatial images, in many wavelengths.
---------------------------------------------------------
Title: From solar nanoflares to stellar giant flares: Scaling laws
and non-implications for coronal heating
Authors: Aschwanden, Markus J.
2007AdSpR..39.1867A Altcode:
In this study we explore physical scaling laws applied to solar
nanoflares, microflares, and large flares, as well as to stellar giant
flares. Solar flare phenomena exhibit a fractal volume scaling, V( L)
≲ L<SUP>1.9</SUP>, with L being the flare loop length scale, which
explains the observed correlation EM∝Tp4 between the total emission
measure EM<SUB>p</SUB> and flare peak temperature T<SUB>p</SUB> in
both solar and stellar flares. However, the detected stellar flares
have higher emission measures EM<SUB>p</SUB> than solar flares at the
same flare peak temperature T<SUB>p</SUB>, which can be explained by a
higher electron density that is caused by shorter heating scale height
ratios s<SUB>H</SUB>/ L ≈ 0.04-0.1. Using these scaling laws we
calculate the total radiated flare energies E<SUB>X</SUB> and thermal
flare energies E<SUB>T</SUB> and find that the total counts C are a
good proxy for both parameters. Comparing the energies of solar and
stellar flares we find that even the smallest observed stellar flares
exceed the largest solar flares, and thus their observed frequency
distributions are hypothetically affected by an upper cutoff caused
by the maximum active region size limit. The powerlaw slopes fitted
near the upper cutoff can then not reliably be extrapolated to the
microflare regime to evaluate their contribution to coronal heating.
---------------------------------------------------------
Title: Particle Acceleration in Solar Flares and Escape into
Interplanetary Space
Authors: Aschwanden, Markus J.
2006GMS...165..189A Altcode:
We review the physics of particle acceleration and kinematics in solar
flares under the particular aspect of their escape and propagation
into interplanetary space. The topics include the magnetic topology
in acceleration regions, the altitude of flare acceleration regions,
evidence for bi-directional acceleration, the asymmetry of upward versus
downward acceleration, and particle access to interplanetary space.
---------------------------------------------------------
Title: Astrophysics in 2005
Authors: Trimble, Virginia; Aschwanden, Markus J.; Hansen, Carl J.
2006PASP..118..947T Altcode: 2006astro.ph..6663T
We bring you, as usual, the Sun and Moon and stars, plus some galaxies
and a new section on astrobiology. Some highlights are short (the
newly identified class of gamma-ray bursts, and the Deep Impact on
Comet 9P/Tempel 1), some long (the age of the universe, which will
be found to have the Earth at its center), and a few metonymic, for
instance the term “down-sizing” to describe the evolution of star
formation rates with redshift.
---------------------------------------------------------
Title: The Localization of Particle Acceleration Sites in Solar
Flares and CMES
Authors: Aschwanden, Markus J.
2006SSRv..124..361A Altcode: 2006SSRv..tmp..109A
We review the particular aspect of determining particle acceleration
sites in solar flares and coronal mass ejections (CMEs). Depending on
the magnetic field configuration at the particle acceleration site,
distinctly different radiation signatures are produced: (1) If charged
particles are accelerated along compact closed magnetic field lines,
they precipitate to the solar chromosphere and produce hard X-rays,
gamma rays, soft X-rays, and EUV emission; (2) if they are injected into
large-scale closed magnetic field structures, they remain temporarily
confined (or trapped) and produce gyrosynchrotron emission in radio
and bremsstrahlung in soft X-rays; (3) if they are accelerated along
open field lines they produce beam-driven plasma emission with a metric
starting frequency; and (4) if they are accelerated in a propagating
CME shock, they can escape into interplanetary space and produce
beam-driven plasma emission with a decametric starting frequency. The
latter two groups of accelerated particles can be geo-effective if
suitably connected to the solar west side. Particle acceleration sites
can often be localized by modeling the magnetic topology from images in
different wavelengths and by measuring the particle velocity dispersion
from time-of-flight delays.
---------------------------------------------------------
Title: Temperature and Electron Density Along Elementary Coronal
Loop Strands as Observed by TRACE
Authors: Nightingale, Richard W.; Aschwanden, M. J.
2006SPD....37.0104N Altcode: 2006BAAS...38..215N
Using the measured data of our initial triple-filter study originally
observed by TRACE (with its < 1" resolution) in all three
extreme-ultraviolet (EUV) filters at 171, 195, and 284 A (Aschwanden and
Nightingale, 2005), where we disentangled the multi-thread structure of
78 coronal loops into elementary threads with isothermal cross-sections,
we analyze now the 1-dimensional functions of temperature T(s),
electron density n(s), and loop width w(s) along these finest observed
loop threads. These functions of physical parameters determined with
triple-filter data and subarcsecond resolution are believed to be the
least background-contaminated measurements of coronal loop structures
available today and thus provide suitable input for hydrodynamic
loop models in the temperature range of T=0.7-2.7 MK. This work was
supported in part by NASA under the TRACE contract NAS5-38099.
---------------------------------------------------------
Title: Coronal magnetohydrodynamic waves and oscillations:
observations and quests
Authors: Aschwanden, Markus J.
2006RSPTA.364..417A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Stellar magnetic energy release at small-scales - microflares
and their relevance for coronal heating
Authors: Aschwanden, M.
2006cosp...36.3417A Altcode: 2006cosp.meet.3417A
Recent data analysis of EUV images from TRACE and SoHO EIT have
provided abundant statistics on physical parameters and their
occurrence distributions of small-scale phenomena in the solar corona
and transition region The observations indicate that many small-scale
phenomena occur in the transition region and lowest region of the solar
corona often revealing a signature of cooling plasma in the 1-2 MK
temperature range that is confined in small-scale loops like miniature
versions of larger flares These phenomena have therefore been aptly
been named microflares and nanoflares also supported by the signatures
of nonthermal electrons that accompany them - On the theoretical side
there is the notion that nanoflares occur throughout the corona in
small magnetic reconnection events driven by the braiding of coronal
loops as consequence of the random footpoint motion as envisioned
by Gene Parker In this talk we will discuss the discrepancy between
theoretical expectations and observational constraints as well as
discuss future ways to reconcile theory with observations
---------------------------------------------------------
Title: Physics of the Solar Corona. An Introduction with Problems
and Solutions (2nd edition)
Authors: Aschwanden, Markus J.
2005psci.book.....A Altcode:
This paperback is the second edition of the original textbook published
in Aug 2004, with an addition of some 170 problems and solutions,
written for graduate students, post-Docs and researchers. It provides
a systematic introduction into all phenomena of the solar corona,
including the Quiet Sun, flares, and CMEs, covering the latest results
from Yohoh, SoHO, TRACE, and RHESSI. The contents are: <P />1 -
Introduction <P />2 - Thermal Radiation <P />3 - Hydrostatics <P />4 -
Hydrodynamics <P />5 - Magnetic Fields <P />6 - Magnetohydrodynamics
(MHD) <P />7 - MHD Oscillations <P />8 - Propagating MHD Waves <P />9
- Coronal Heating <P />10 - Magnetic Reconnection <P />11 - Particle
Acceleration <P />12 - Particle Kinematics <P />13 - Hard X-rays <P
/>14 - Gamma-Rays <P />15 - Radio Emission <P />16 - Flare Plasma
Dynamics <P />17 - Coronal Mass Ejections
---------------------------------------------------------
Title: Discriminating Composite from Elementary Coronal Loops
Authors: Aschwanden, M. J.
2005AGUFMSH13B..01A Altcode:
We establish three simple criteria to discriminate between
elementary and composite loop structures, based on observations from
high-resolution (TRACE) and low-resolution (EIT) imagers. Both datasets
contain triple-filter data with a temperature diagnostic in the range
of T=0.7-2.8 MK, which we use for forward-fitting of a differential
emission measure (DEM) distribution to constrain the temperature
range of each structure. From the TRACE dataset we find for the finest
structures a mean width of w=1.5±0.4 Mm, a flux-to-background contrast
of c=0.18±0.13, and a temperature range of dT=0.07±0.10 MK, which
we identify as elementary strands, defined in terms of their thermal
homogeneity. The loop structures observed with EIT have loop widths
w, flux contrasts c, and temperature ranges dT that are all about
an order of magnitude larger, and thus clearly constitute composite
structures, consisting of many loop strands. These two contrasting
observations resolve previous controversies about the basic thermal
structure of coronal loops and yield a simple discrimination rule:
Elementary loop strands (1) are near-isothermal (dT < 0.15 MK), (2)
have a small width (w < 2 Mm), and (3) have a faint contrast (c <
0.3), while virtually all wider and higher-contrast loop features are
most likely to be multi-thermal composites and have a broad DEM.
---------------------------------------------------------
Title: Examples of Elementary Coronal Loop Strands as Observed
by TRACE
Authors: Nightingale, R. W.; Aschwanden, M. J.
2005AGUFMSH41B1127N Altcode:
A large study has recently been completed (in press at
Astrophys. Jour.) of coronal loop structures observed by TRACE (with
its < 1" resolution) in all three extreme-ultraviolet (EUV) filters
at 171, 195, and 284 A. One of the results is that all of the fitted,
uncontaminated loops with widths < 2000 km are consistent with a
single temperature cross-section. This, in turn, leads to the conclusion
that TRACE has resolved "elementary" or "monolithic" loop strands, in
terms of thermal homogeneity over the loop cross-section. This further
implies, due to the inhibition of cross-field transport in the corona
(because the plasma-beta parameter is low), that the loop heating
occurs below the corona in the transition region or chromosphere, where
the plasma-beta values are higher allowing for possible cross-field
transport. Several cases of measurements and results of acceptable
isothermal fits of "elementary" loop strands (not shown in the original
paper) will be presented. Also to be displayed are comparisons between
extracted original images and high-pass filtered images utilized in
the analysis and conclusions above. This work was supported in part
by NASA under the TRACE contract NAS5-38099.
---------------------------------------------------------
Title: Three Criteria to Discriminate between Elementary and Composite
Coronal Loops
Authors: Aschwanden, Markus J.
2005ApJ...634L.193A Altcode:
In this Letter, we establish three simple criteria to discriminate
between elementary and composite loop structures, based on observations
from high-resolution (TRACE) and low-resolution (EIT) imagers. Both
data sets contain triple-filter data with a temperature diagnostic
in the range of T~0.7-2.8 MK, which we use for forward-fitting of
a differential emission measure (DEM) distribution to constrain
the temperature range of each structure. From the TRACE data set,
we find for the finest structures a mean width of w=1.4+/-0.3 Mm, a
flux-to-background contrast of c~0.18+/-0.13, and a temperature range
of dT=0.07+/-0.10 MK, which we identify as elementary strands, defined
in terms of their thermal homogeneity. The loop structures observed with
EIT have loop widths w, flux contrasts c, and temperature ranges dT that
are all about an order of magnitude larger and thus clearly constitute
composite structures, consisting of many loop strands. These two
contrasting observations resolve previous controversies about the basic
thermal structure of coronal loops and yield a simple discrimination
rule: elementary loop strands (1) are near-isothermal (dT<~0.2 MK),
(2) have a small width (w<~2 Mm), and (3) have a faint contrast
(c<~0.3), while virtually all wider and higher contrast loop features
are most likely multithermal composites and have a broad DEM.
---------------------------------------------------------
Title: Elementary Loop Structures in the Solar Corona Analyzed from
TRACE Triple-Filter Images
Authors: Aschwanden, Markus J.; Nightingale, Richard W.
2005ApJ...633..499A Altcode:
This study represents the first quantitative analysis of the multithread
structure of coronal loops. We analyzed a set of 234 fine loop threads
observed with TRACE triple-filter images at wavelengths of 171, 195,
and 284 Å. The cross-sectional flux profiles are simultaneously
forward fitted in all three filters with the superposition of a
cospatial Gaussian and linear background functions. We fit a general
multitemperature differential emission measure (DEM) distribution to
each cross section, as well as the special case of a single-temperature
or isothermal DEM. We perform these forward fits at ~18,000 loop
positions and find that this geometric model could be fitted in ~3500
cases (within a χ<SUB>red</SUB><=1.5), while all other cases require
more complex geometric models of the loop cross section, secondary
loops, and background. The major result of this study is that the vast
majority (84%) of the acceptable DEM fits are isothermal. Temperatures
are measured over the whole sensitivity range of 0.7-2.8 MK, but with a
higher probability near the peak sensitivities of the three filters. We
conclude that we indeed resolve “elementary” or “monolithic” loop
strands with TRACE, in terms of isothermal homogeneity. Virtually all
earlier studies detected ensembles of multiple strands, while our
detected loop strands exhibit much smaller widths (w~1.4+/-0.3 Mm)
and also smaller signal-to-background ratios (14%+/-10%). We suggest
that the temperature homogeneity of coronal loops up to widths of
w<~2000 km is related to their magnetic mapping to photospheric
granulation (convection) cells.
---------------------------------------------------------
Title: Foreword
Authors: Gallagher, Peter; Berghmans, David; Aschwanden, Markus
2005SoPh..228....1G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Elementary Structure of Coronal Loops
Authors: Aschwanden, M. J.
2005AGUSMSP13B..02A Altcode:
There is the question whether the plasma of coronal loops consists
of unresolved strands or not, given the best spatial resolution
of <1" we have today with EUV telescopes such as TRACE. Since
the hypothetical loop strands would be thermally insulated from each
other, due to the inhibited cross-field diffusion in the coronal plasma
that has a plasma-beta parameter significantly smaller than unity, we
expect that loop strands have arbitrarily different heating rates and
temperatures. Consequently, both resolved as well as unresolved loop
strands are expected not to be co-spatial in different temperature
filters. We present the results of a triple-filter analysis of a
larger number of coronal loop structures observed with TRACE in the
171, 195, and 284 A filters, where we measure the cospatiality of
loop substructures with sub-arcsecond accuracy in near-simultaneous
measurements with triple filters. In virtually all loop structures
we identify multiple resolved substructures that are not cospatial
in triple temperature filters, but are cospatial in dual temperature
filters and consistent with a single temperatures over partial loop
length segments. From these results we conclude that the cross-section
of heated loop strands is resolved on ~1" spatial scale, but no
thermal equilibrium is achieved over the entire length of coronal
loops, and thus loops are always asymmetric in their temperature and
density structure.
---------------------------------------------------------
Title: 2D Feature Recognition And 3d Reconstruction In Solar Euv
Images
Authors: Aschwanden, Markus J.
2005SoPh..228..339A Altcode:
EUV images show the solar corona in a typical temperature range of T
>rsim 1 MK, which encompasses the most common coronal structures:
loops, filaments, and other magnetic structures in active regions,
the quiet Sun, and coronal holes. Quantitative analysis increasingly
demands automated 2D feature recognition and 3D reconstruction,
in order to localize, track, and monitor the evolution of such
coronal structures. We discuss numerical tools that "fingerprint"
curvi-linear 1D features (e.g., loops and filaments). We discuss
existing finger-printing algorithms, such as the brightness-gradient
method, the oriented-connectivity method, stereoscopic methods,
time-differencing, and space-time feature recognition. We discuss
improved 2D feature recognition and 3D reconstruction techniques
that make use of additional a priori constraints, using guidance
from magnetic field extrapolations, curvature radii constraints,
and acceleration and velocity constraints in time-dependent image
sequences. Applications of these algorithms aid the analysis of
SOHO/EIT, TRACE, and STEREO/SECCHI data, such as disentangling, 3D
reconstruction, and hydrodynamic modeling of coronal loops, postflare
loops, filaments, prominences, and 3D reconstruction of the coronal
magnetic field in general.
---------------------------------------------------------
Title: Astrophysics in 2004
Authors: Trimble, Virginia; Aschwanden, Markus
2005PASP..117..311T Altcode:
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars
(§ 4), the Moon and Planets (§ 3), a truly binary pulsar (§ 5),
a kinematic apology (§ 6), the whole universe (§§ 7 and 8),
reconsideration of old settled (§ 9) and unsettled (§ 10) issues,
and some things that happen only on Earth, some indeed only in these
reviews (§§ 10 and 11).
---------------------------------------------------------
Title: Coronal heating and the appearance of solar and stellar coronae
Authors: Schrijver, C. J.; Sandman, A. W.; Aschwanden, M. J.; De Rosa,
M. L.
2005ESASP.560...65S Altcode: 2005csss...13...65S
No abstract at ADS
---------------------------------------------------------
Title: The Nature and Excitation Mechanisms of Acoustic Oscillations
in Solar and Stellar Coronal Loops
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.; Aschwanden,
M. J.
2004ESASP.575..114T Altcode: 2004soho...15..114T; 2004astro.ph..9556T
In the recent work of Nakariakov et al. (2004), it has been shown
that the time dependences of density and velocity in a flaring loop
contain pronounced quasi-harmonic oscillations associated with the
2nd harmonic of a standing slow magnetoacoustic wave. That model
used a symmetric heating function (heat deposition was strictly at
the apex). This left outstanding questions: A) is the generation of
the 2nd harmonic a consequence of the fact that the heating function
was symmetric? B) Would the generation of these oscillations occur if
we break symmetry? C) What is the spectrum of these oscillations? Is
it consistent with a 2nd spatial harmonic? The present work (and
partly Tsiklauri et al. (2004b)) attempts to answer these important
outstanding questions. Namely, we investigate the physical nature
of these oscillations in greater detail: we study their spectrum
(using periodogram technique) and how heat positioning affects the
mode excitation. We found that excitation of such oscillations is
practically independent of location of the heat deposition in the
loop. Because of the change of the background temperature and density,
the phase shift between the density and velocity perturbations is
not exactly a quarter of the period, it varies along the loop and is
time dependent, especially in the case of one footpoint (asymmetric)
heating. We also were able to model successfully SUMER oscillations
observed in hot coronal loops.
---------------------------------------------------------
Title: The Role of Observed MHD Oscillations and Waves for Coronal
Heating
Authors: Aschwanden, M. J.
2004ESASP.575...97A Altcode: 2004soho...15...97A
No abstract at ADS
---------------------------------------------------------
Title: STEREO/SECCHI Simulations of CMEs and Flares using TRACE Images
Authors: Aschwanden, M. J.; Lemen, J.; Nitta, N.; Metcalf, T.; Wuelser,
J.; Alexander, D.
2004AGUFMSH22A..02A Altcode:
We simulate 3D models of EUV images of flare and CME events, using
TRACE EUV movies. TRACE movies show 2D images in projection along a
particular line-of-sight. We simulate 3D models of erupting filaments,
flare loops, and postflare loops using: (1) a “finger printing”
technique to trace linear structures in 2D images; (2) geometric 3D
models based on force-free fields and curvature radius maximization
of flare loop and flux rope structures; (3) conservation of velocity
and acceleration parameters; (4) multi-temperature plasma filling
according to hydrodyamic scaling laws; and (5) 2D projections from
secondary line-of-sights that correspond to viewpoints of the secondary
STEREO spacecraft. From such simulations we envision to illustrat
3D time-dependent models, what would be observed at the two STEREO
spacecraft positions as well as from a near-Earth spacecraft such as
SoHO. These simulations are used to test STEREO analysis software and
to investigate what physical parameters and geometric 3D reconstructions
can be retrieved from STEREO/SECCHI data.
---------------------------------------------------------
Title: The Coronal Heating Mechanism as Identified by Full-Sun
Visualizations
Authors: Schrijver, Carolus J.; Sandman, Anne W.; Aschwanden, Markus
J.; De Rosa, Marc L.
2004ApJ...615..512S Altcode:
We constrain the properties of the mechanism(s) responsible for the
bulk of the heating of the corona of the Sun by simulating, for the
first time, the appearance of the entire solar corona. Starting from
full-sphere magnetic field maps for 2000 December 1 and 8, when
the Sun was moderately active, we populate nearly 50,000 coronal
field lines with quasi-static loop atmospheres. These atmospheres
are based on heating flux densities F<SUB>H</SUB> that depend in
different ways on the loop half-length L, the field strength B at
the chromospheric base, the loop expansion with height, and the
heating scale height. The best match to X-ray and EUV observations
of the corona over active regions and their environs is found for
F<SUB>H</SUB>~4×10<SUP>14</SUP>B<SUP>1.0+/-0.3</SUP>/L<SUP>1.0+/-0.5</SUP>
(in ergs cm<SUP>-2</SUP> s<SUP>-1</SUP> for B in Mx cm<SUP>-2</SUP> and
L in cm), while allowing for substantial loop expansion with increasing
height, and for a heating scale height that is at least a sizeable
fraction of the loop length. This scaling for coronal heating points
to DC reconnection at tangential discontinuities as the most likely
coronal heating mechanism, provided that the reconnection progresses
proportional to the Alfvén velocity. The best-fit coronal filling
factor equals unity, suggesting that most of the corona is heated most
of the time. We find evidence that loops with half-lengths exceeding
~100,000 km are heated significantly more than suggested by the above
scaling, possibly commensurate with the power deposited in the open
field of coronal holes.
---------------------------------------------------------
Title: Tomographic 3D-Modeling of the Solar Corona with FASR
Authors: Aschwanden, Markus J.; Alexander, David; de Rosa, Marc L.
2004ASSL..314..243A Altcode: 2003astro.ph..9501A
The Frequency-Agile Solar Radiotelescope (FASR) literally opens up
a new dimension, in addition to the 3D Euclidian geometry—the
frequency dimension. The 3D geometry is degenerated to 2D in all
images from astronomical telescopes, but the additional frequency
dimension allows us to retrieve the missing third dimension by means of
physical modeling. We call this type of 3D reconstruction Frequency
Tomography. In this study we simulate a realistic 3D model of an
active region, composed of 500 coronal loops with the 3D geometry
[x(s), y(s), z(s)] constrained by magnetic field extrapolations and
the physical parameters of the density n<SUB>e</SUB>(s) and temperature
T<SUB>e</SUB>(s) given by hydrostatic solutions. We simulate a series
of 20 radio images in a frequency range of ν=0.1-10 GHz, anticipating
the capabilities of FASR, and investigate what physical information
can be retrieved from such a dataset. We discuss also forward-modeling
of the chromospheric and Quiet Sun density and temperature structure,
another primary goal of future FASR science.
---------------------------------------------------------
Title: Physics of the Solar Corona. An Introduction
Authors: Aschwanden, Markus J.
2004psci.book.....A Altcode:
This textbook is written for graduate students, post-Docs, and
researchers. It provides a systematic introduction into all phenomena
of the solar corona, including the Quiet Sun, flares, and CMEs,
covering the latest results from Yohoh, SoHO, TRACE, and RHESSI. The
contents are: <P />1 - Introduction <P />2 - Thermal Radiation <P
/>3 - Hydrostatics <P />4 - Hydrodynamics <P />5 - Magnetic Fields <P
/>6 - Magnetohydrodynamics (MHD) <P />7 - MHD Oscillations <P />8 -
Propagating MHD Waves <P />9 - Coronal Heating <P />10 - Magnetic
Reconnection <P />11 - Particle Acceleration <P />12 - Particle
Kinematics <P />13 - Hard X-rays <P />14 - Gamma-Rays <P />15 -
Radio Emission <P />16 - Flare Plasma Dynamics <P />17 - Coronal
Mass Ejections
---------------------------------------------------------
Title: Flare-generated acoustic oscillations in solar and stellar
coronal loops
Authors: Tsiklauri, D.; Nakariakov, V. M.; Arber, T. D.; Aschwanden,
M. J.
2004A&A...422..351T Altcode: 2004astro.ph..2261T
Long period longitudinal oscillations of a flaring coronal loop are
studied numerically. In the recent work of Nakariakov et al. (2004)
it has been shown that the time dependence of density and velocity in a
flaring loop contain pronounced quasi-harmonic oscillations associated
with the 2nd harmonic of a standing slow magnetoacoustic wave. In
this work we investigate the physical nature of these oscillations
in greater detail, namely, their spectrum (using the periodogram
technique) and how heat positioning affects mode excitation. We found
that excitation of such oscillations is practically independent of the
location of the heat deposition in the loop. Because of the change of
the background temperature and density, the phase shift between the
density and velocity perturbations is not exactly a quarter of the
period; it varies along the loop and is time dependent, especially in
the case of one footpoint (asymmetric) heating.
---------------------------------------------------------
Title: Pulsed Particle Injection in a Reconnection-Driven Dynamic
Trap Model in Solar Flares
Authors: Aschwanden, Markus J.
2004ApJ...608..554A Altcode:
The time structure of hard X-ray emission during solar flares
shows subsecond pulses, which have an energy-dependent timing that
is consistent with electron time-of-flight delays. The inferred
time-of-flight distances imply an injection height about 50% above
the soft X-ray-bright flare loops, where electrons are injected
in a synchronized way. No physical injection mechanism is known
that can account for the energy synchronization and duration of
subsecond pulses. Here we propose a model in terms of dynamic loss
cone angle evolution of newly reconnected magnetic field lines that
relax from the cusp at the reconnection point into a force-free
configuration, which can explain the subsecond pulse structure of
injected particles as well as the observed correlation between the
pulse duration and flare loop size. This quantitative model predicts
that the pulse duration of hard X-ray or radio pulses scales with
t<SUB>w</SUB>~2L<SUB>B</SUB>/v<SUB>A</SUB> (s), which is the Alfvénic
transit time through the magnetic outflow region of a Petschek-type
X-point with magnetic length scale L<SUB>B</SUB>, and thus provides
a direct diagnostic of the magnetic reconnection geometry. It also
demonstrates that the observed pulse durations are primarily controlled
by the injection time rather than by the acceleration timescale.
---------------------------------------------------------
Title: Radiative hydrodynamic modeling of the Bastille-Day flare
(14 July, 2000). I. Numerical simulations
Authors: Tsiklauri, D.; Aschwanden, M. J.; Nakariakov, V. M.; Arber,
T. D.
2004A&A...419.1149T Altcode: 2004astro.ph..2260T
A 1D loop radiative hydrodynamic model that incorporates the effects
of gravitational stratification, heat conduction, radiative losses,
external heat input, presence of helium, and Braginskii viscosity is
used to simulate elementary flare loops. The physical parameters for the
input are taken from observations of the Bastille-Day flare of 2000 July
14. The present analysis shows that: a) the obtained maximum values of
the electron density can be considerably higher (4.2 × 10<SUP>11</SUP>
cm<SUP>-3</SUP> or more) in the case of footpoint heating than in
the case of apex heating (2.5 × 10<SUP>11</SUP> cm<SUP>-3</SUP>);
b) the average cooling time after the flare peak takes less time in
the case of footpoint heating than in the case of apex heating; c)
the peak apex temperatures are significantly lower (by about 10 MK)
for the case of footpoint heating than for apex heating (for the
same average loop temperature of about 30 MK). This characteristic
would allow to discriminate between different heating positioning; d)
in both cases (of apex and footpoint heating), the maximum obtained
apex temperature T<SUP>max</SUP> is practically independent of the
heating duration σ<SUB>t</SUB>, but scales directly with the heating
rate E<SUB>H0</SUB>; e) the maximum obtained densities at the loop
apex, n<SUB>e</SUB><SUP>max</SUP>, increase with the heating rate
E<SUB>H0</SUB> and heating duration σ<SUB>t</SUB> for both footpoint
and apex heating. In Paper II we will use the outputs of these
hydrodynamic simulations, which cover a wide range of the parameter
space of heating rates and durations, as an input for forward-fitting
of the multi-loop arcade of the Bastille-day flare.
---------------------------------------------------------
Title: Solar Coronal Heating Inferred from Full-disk Models of
Coronal Emission
Authors: Schrijver, C. J.; Sandman, A. W.; De Rosa, M. L.; Aschwanden,
M. J.
2004AAS...204.9501S Altcode: 2004BAAS...36Q.826S
The appearance of the corona as viewed by different instruments, as
well as its global spectral irradiance, sensitively depends on how
coronal heating scales with the properties of the coronal magnetic
field. We explore a variety of scaling dependences by simulating the
appearance of the full-disk solar corona as viewed by SOHO/EIT and by
YOHKOH/SXT, based on observed photospheric magnetic fields combined with
a potential-field source-surface model. This leads us to conclude that
the best match to X-ray and EUV observations of the corona over active
regions and their environments is found for a heating flux density going
into the corona that scales linearly with the field strength at the
coronal base and roughly inversely with loop length. This scaling points
to DC reconnection at tangential discontinuities as the most likely
coronal heating mechanism, provided that the reconnection progresses
at a rate proportional to the Alfven velocity. We also find that the
best-fit coronal filling factor equals unity, suggesting that most of
the corona is heated most of the time. We find evidence that loops with
half lengths exceeding approximately 100,000 km are heated significantly
more than suggested by the above scaling, possibly commensurate with
the power deposited in the open field of coronal holes.
---------------------------------------------------------
Title: TRACE Triple-Filter Analysis - Spatial Widths of Monolithic
Coronal Loops
Authors: Aschwanden, M. J.
2004AAS...204.9506A Altcode: 2004BAAS...36..827A
We focus on the question of elementary plasma structures in the solar
corona. The crucial issue under investigation is whether we observe with
current high-resolution instrumentation (<1", such as with TRACE)
monolithic structures in the corona, i.e., elementary loops or loop
threads that are homogeneous in temperature and density in any given
cross-section. The search and identification of monolithic structures
has fundamental consequences in theory (hydrodynamic modeling, coronal
heating function, cross-field diffusion processes) and for the planning
and design of future high-resolution EUV and soft X-ray telescopes. The
identification of monolithic structures is a necessary prerequisite
to apply hydrodynamic models, to determine the time-dependent heating
function and cooling processes, and to measure the iron abundance and
first-ionization potential (FIP) effect. We analyze a number of loop
structures from TRACE triple-filter data (171, 195, 284 A) and present
quantitative results about the temperature width of the differential
emission measure distribution of the finest resolved coronal loops
in the temperature range of T=0.7-2.5 MK. We measure the width range
of monolithic loop structures, for which we find upper limits in the
range of 1"-2". A consequence is that loop structures observed with
instruments of lower resolution (e.g., with Yohkoh, EIT, CDS) represent
composite temperature structures and have to be modeled as such. -We
acknowledge support from NASA for TRACE, LWS, and SEC projects.
---------------------------------------------------------
Title: Astrophysics in 2003
Authors: Trimble, Virginia; Aschwanden, Markus J.
2004PASP..116..187T Altcode:
Five coherent sections appear this year, addressing solar physics,
cosmology (with WMAP highlights), gamma-ray bursters (and their
association with Type Ia supernovae), extra-solar-system planets,
and the formation and evolution of galaxies (from reionization to
assemblage of Local Group galaxies). There are also eight incoherent
sections that deal with other topics in stellar, galactic, and planetary
astronomy and the people who study them.
---------------------------------------------------------
Title: Acoustic Oscillations in Solar and Stellar Flaring Loops
Authors: Tsiklauri, D.; Nakariakov, V. M.; Kelly, A.; Aschwanden,
M. J.; Arber, T. D.
2004ESASP.547..473T Altcode: 2004soho...13..473T
No abstract at ADS
---------------------------------------------------------
Title: Solar Magnetic Loops Observed with TRACE and EIT
Authors: Aschwanden, M. J.; Title, A. M.
2004IAUS..219..503A Altcode: 2003IAUS..219E.219A
We review major discoveries and new physical results that have
been obtained from the TRACE mission over the last 4 years such as:
(1) the temperature and density inhomogeneity of the coronal plasma
(2) hydrostatic and non-hydrostatic loops (3) plasma flows in loops
(4) transverse oscillations in the MHD kink-mode (5) the spatial
heating function of loops (6) intermittent heating and cooling time
scales (7) iron abundance enhancements (8) magnetic nullpoints and
separator regions (9) highly fragmented postflare loop arcades and
(10) nanoflare loop phenomena. We transform the physical properties
as measured by TRACE for the Sun to stellar coronae and show how
information on heating and cooling processes can be obtained from
stellar differential emission measure (DEM) distributions.
---------------------------------------------------------
Title: On the Photometric Accuracy of RHESSI Imaging and Spectrosocopy
Authors: Aschwanden, Markus J.; Metcalf, Thomas R.; Krucker, Säm;
Sato, Jun; Conway, Andrew J.; Hurford, G. J.; Schmahl, Edward J.
2004SoPh..219..149A Altcode: 2003astro.ph..9499A
We compare the photometric accuracy of spectra and images in flares
observed with the Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI) spacecraft. We test the accuracy of the photometry by
comparing the photon fluxes obtained in different energy ranges from the
spectral-fitting software SPEX with those fluxes contained in the images
reconstructed with the Clean, MEM, MEM-Vis, Pixon, and Forward-fit
algorithms. We quantify also the background fluxes, the fidelity of
source geometries, and spatial spectra reconstructed with the five image
reconstruction algorithms. We investigate the effects of grid selection,
pixel size, field of view, and time intervals on the quality of image
reconstruction. The detailed parameters and statistics are provided
in an accompanying CD-ROM and web page. We find that Forward-fit,
Pixon, and Clean have a robust convergence behavior and a photometric
accuracy in the order of a few percent, while MEM does not converge
optimally for large degrees of freedom (for large field of view and/or
small pixel sizes), and MEM-Vis suffers in the case of time-variable
sources. This comparative study documents the current status of the
RHESSI spectral and imaging software, one year after launch.
---------------------------------------------------------
Title: 2D MHD Modelling of Heated Coronal Loops Compared to TRACE
Observations
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H. C.; Tsinganos,
K.; Aschwanden, M. J.
2004hell.conf...31P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Acoustic oscillations in solar and stellar flaring loops
Authors: Nakariakov, V. M.; Tsiklauri, D.; Kelly, A.; Arber, T. D.;
Aschwanden, M. J.
2004A&A...414L..25N Altcode: 2004astro.ph..2223N
Evolution of a coronal loop in response to an impulsive energy release
is numerically modelled. It is shown that the loop density evolution
curves exhibit quasi-periodic perturbations with the periods given
approximately by the ratio of the loop length to the average sound
speed, associated with the second standing harmonics of an acoustic
wave. The density perturbations have a maximum near the loop apex. The
corresponding field-aligned flows have a node near the apex. We
suggest that the quasi-periodic pulsations with periods in the range
10-300 s, frequently observed in flaring coronal loops in the radio,
visible light and X-ray bands, may be produced by the second standing
harmonic of the acoustic mode.
---------------------------------------------------------
Title: Magnetohydrodynamic Sausage-Mode Oscillations in Coronal Loops
Authors: Aschwanden, Markus J.; Nakariakov, Valery M.; Melnikov,
Victor F.
2004ApJ...600..458A Altcode: 2003astro.ph..9493A
A recent study by Nakariakov and coworkers pointed out that the
dispersion relation of MHD sausage-mode oscillations has been
incorrectly applied to coronal loops, neglecting the highly dispersive
nature of the phase speed and the long-wavelength cutoff of the
wavenumber. In the light of these new insights, we revisit previous
observations that have been interpreted in terms of MHD sausage-mode
oscillations in coronal loops and come to the following conclusions:
(1) fast sausage MHD-mode oscillations require such a high electron
density imposed by the wavenumber cutoff that they can only occur in
flare loops, and (2) in the previously reported radio observations
(ν~100 MHz to 1 GHz) with periods of P~0.5-5 s, the fast sausage
MHD-mode oscillation is likely to be confined to a small segment
(corresponding to a high harmonic node) near the apex of the loop,
rather than involving a global oscillation over the entire loop
length. The recent microwave and soft X-ray observations of fast periods
(P~6-17 s) by Asai and coworkers and Melnikov and coworkers, however,
are consistent with fast sausage MHD oscillations at the fundamental
harmonic.
---------------------------------------------------------
Title: 3D Numerical Simulations of Impulsively Generated MHD Waves
in Solar Coronal Loops
Authors: Selwa, M.; Murawski, K.; Kowal, G.; Nakariakov, V.;
Aschwanden, M.; Oliver, R.; Ballester, J. L.
2004ESASP.547..495S Altcode: 2004soho...13..495S
Impulsively generated magnetohydrodynamic waves in a typical EUV solar
coronal loop are studied numerically with a use of the three-dimensional
FLASH code. Our results reveal several 3D effects such as distinctive
time signatures which are collected at a detection point inside the
loop. A slow magnetosonic wave generates a significant variation in a
mass density profile with a time-scale of the order of s. A fast kink
wave affects a mass density too but its magnitude is much lower than
in the case of a slow wave. Time-scales which are associated with the
fast kink wave are generally lower than in the case of a slow wave;
they are in the range of a dozen or so seconds. Temporal signatures
of a fast sausage wave reveal s oscillations in the quasi-periodic
phase. Impulses which are launched outside the loop excite few seconds
oscillations in the mass density. Time-signatures depend on a position
of the detection point; they are usually more complex further out from
the exciter.
---------------------------------------------------------
Title: Communications, Navigation, and Timing Constraints for the
Solar Imaging Radio Array (SIRA)
Authors: Lemaster, E. A.; Byler, E. A.; Aschwanden, M. J.
2003AGUFMSH42C0553L Altcode:
The Solar Imaging Radio Array (SIRA) is a proposed NASA mission to
measure solar radio emissions in the 30kHz to 30MHz region of the
electromagnetic spectrum. The baseline design consists of 16 separated
spacecraft in an irregular pattern several kilometers across. Each
spacecraft is equipped with a pair of crossed dipole antennas that
together form a 16-element radio interferometer for Fourier-type
image reconstruction (120 baselines in the UV-plane). The required
close coordination between this formation of spacecraft places many
unique constraints on the SIRA communications, navigation, control,
and timing architectures. Current specifications call for knowledge of
the relative locations of the spacecraft to approximately meter-level
accuracy in order to maintain primary instrument resolution. Knowledge
of the relative timing differences between the clocks on the spacecraft
must likewise be maintained to tens of nanoseconds or better. This in
turn sets a minimum bound on the regularity of communications updates
between spacecraft. Although the actual positions of the spacecraft are
not tightly constrained, enough control authority and system autonomy
must be present to keep the spacecraft from colliding due to orbital
perturbations. Each of these constraints has an important effect
on the design of the architecture for the entire array. This paper
examines the engineering requirements and design tradeoffs for the
communications, navigation, and timing architectures for SIRA. Topics
include the choice of navigation sensor, communications methodology
and modulation schemes, and clock type to meet the overall system
performance goals while overcoming issues such as communications
dynamic range, bandwidth limitations, power constraints, available
antenna beam patterns, and processing limitations. In addition, this
paper discusses how the projected use of smaller spacecraft buses with
their corresponding payload and cost limits has important consequences
for the overall system design.
---------------------------------------------------------
Title: Observational Tests of Damping by Resonant Absorption in
Coronal Loop Oscillations
Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Andries,
Jesse; Goossens, Marcel; Van Doorsselaere, Tom
2003ApJ...598.1375A Altcode: 2003astro.ph..9470A
One of the proposed damping mechanisms of coronal (transverse)
loop oscillations in the kink mode is resonant absorption as a
result of the Alfvén speed variation at the outer boundary of
coronal loops. Analytical expressions for the period and damping
time exist for loop models with thin nonuniform boundaries. They
predict a linear dependency of the ratio of the damping time to the
period on the thickness of the nonuniform boundary layer. Ruderman and
Roberts used a sinusoidal variation of the density in the nonuniform
boundary layer and obtained the corresponding analytical expression
for the damping time. Here we measure the thickness of the nonuniform
layer in oscillating loops for 11 events, by forward-fitting of the
cross-sectional density profile n<SUB>e</SUB>(r) and line-of-sight
integration to the cross-sectional fluxes F(r) observed with
TRACE 171 Å. This way we model the internal (n<SUB>i</SUB>) and
external electron density (n<SUB>e</SUB>) of the coronal plasma in
oscillating loops. This allows us to test the theoretically predicted
damping rates for thin boundaries as a function of the density ratio
χ=n<SUB>e</SUB>/n<SUB>i</SUB>. Since the observations show that
the loops have nonuniform density profiles, we also use numerical
results for damping rates to determine the value of χ for the
loops. We find that the density ratio predicted by the damping time,
χ<SUB>LEDA</SUB>=0.53+/-0.12, is a factor of ~1.2-3.5 higher than the
density ratio estimated from the background fluxes, χ=0.30+/-0.16. The
lower densities modeled from the background fluxes are likely to be
a consequence of the neglected hotter plasma that is not detected
with the TRACE 171 Å filter. Taking these corrections into account,
resonant absorption predicts damping times of kink-mode oscillations
that are commensurable with the observed ones and provides a new
diagnostic of the density contrast of oscillating loops.
---------------------------------------------------------
Title: Turbulence, Waves and Instabilities in the Solar Plasma
Authors: Erdélyi, R.; Petrovay, K.; Roberts, B.; Aschwanden, M.
2003twis.book.....E Altcode:
Significant advances have been made recently in both the theoretical
understanding and observation of small-scale turbulence in different
layers of the Sun, and in the instabilities that give rise to them. The
general development of solar physics, however, has led to such a
degree of specialization as to hinder interaction between workers in
the field. This book therefore presents studies of different layers
and regions of the Sun, but from the same aspect, concentrating on
the study of small-scale motions. The main emphasis is on the common
theoretical roots of these phenomena, but the book also contains
an extensive treatment of the observational aspects. <P />Link: <A
href="http://www.springer.com/east/home?SGWID=5-102-22-3362=5696-0&changeHeader=true">http://www.springer.com/east/home?SGWID=5-102-22-3362=5696-0&changeHeader=true</A>
---------------------------------------------------------
Title: Radio Coverage from Chromosphere to Earth: FASR-LOFAR-SIRA
Synergy
Authors: Gary, D. E.; Kassim, N.; Gopalswamy, N.; Aschwanden, M. J.
2003AGUFMSH42E..02G Altcode:
Radio emission is uniquely sensitive to a number of key plasma
parameters (magnetic field, temperature, density, high-energy
electrons, and various plasma waves) over heights ranging without
gaps from the chromosphere, throughout the corona and heliosphere, to
the Earth. Two ground-based radio arrays, the Frequency Agile Solar
Radiotelescope (FASR) and the Low Frequency Array (LOFAR), together
with the space-based Solar Imaging Radio Array (SIRA) are planned
that will for the first time provide direct imaging of disturbances
over this vast height range through interferometric imaging over their
equally impressive frequency range of 24 GHz to 30 kHz. We describe the
science goals of these instruments, focusing especially on the science
addressed jointly by all three instruments. Among the examples are
(1) simultaneous imaging of CMEs, flaring loops, and shock-associated
(type II) emission and (2) imaging the propagation of electrons on
open field lines (type III), from their acceleration point through
the corona and heliosphere to the point where they are measured in
situ by near-Earth spacecraft. In addition to spatially relating the
different phenomena, the spectral information is rich in quantitative
diagnostics. We give some examples of the revolutionary results we
can expect from the combined instruments.
---------------------------------------------------------
Title: Coronal heating and the appearance of the solar corona
Authors: Schrijver, C. J.; Sandman, A.; De Rosa, M. L.; Aschwanden,
M. J.
2003AGUFMSH32A1104S Altcode:
The details of the dependence of coronal heating on the conditions
within the corona determine the appearance of the corona as viewed
by different instruments. For example, strong fields at the base of
short loops cause relatively hot, X-ray bright loops, whereas the
much weaker fields over the quiet Sun result in cooler, EUV bright
loops. Any dependence of the volume heating rates on local conditions
(such as height or field strength) has a signature in the thermal
profiles along the loops, translating into an appearance that depends
on the instrumental pass band. In this preliminary study, we explore
how such dependences of coronal heating on coronal conditions affect
the appearance of the solar corona, and investigate the consequences
for the global EUV and X-ray spectral irradiance. These results will
eventually be used to compute the solar spectral irradiance in the
EUV and X-rays for quiescent conditions throughout the solar cycle.
---------------------------------------------------------
Title: Probing Solar Energetic Particles with SIRA
Authors: Aschwanden, M. J.; Nitta, N.; Lemaster, E.; Byler, E.; Gary,
D.; Kassim, N.; Gopalswamy, N.
2003AGUFMSH42C0555A Altcode:
The space-based SIRA (Solar Imaging Radio Array) will provide a powerful
capability to track high energy particles from solar flare and CME sites
through interplanetary/heliospheric space all the way to Earth. Together
with two other overlapping planned radio interferometers, i.e., FASR
(Frequency-Agile Solar Radiotelescope) and LOFAR (Low-Frequency Array)
the entire plasma frequency range from 30 GHz all the way down to
the plasma frequency cutoff of 30 kHz at 1 AU will be covered. These
instruments will track the magnetic trajectory of high energy particles,
beam-driven radio emission, and localize the acceleration sites in
the corona or interplanetary shocks. We simulate some CME and type III
events, as they will be mapped with these instruments, using realistic
scattering functions of radio waves on coronal and heliospheric density
inhomogeneities.
---------------------------------------------------------
Title: 2D MHD modelling of compressible and heated coronal loops
obtained via nonlinear separation of variables and compared to TRACE
and SoHO observations
Authors: Petrie, G. J. D.; Gontikakis, C.; Dara, H. C.; Tsinganos,
K.; Aschwanden, M. J.
2003A&A...409.1065P Altcode: 2003astro.ph..3373P
An analytical MHD model of coronal loops with compressible flows and
including heating is compared to observational data. The model is
constructed via a systematic nonlinear separation of the variables
method used to calculate several classes of exact MHD equilibria in
Cartesian geometry and uniform gravity. By choosing a particularly
versatile solution class with a large parameter space we are able to
calculate models whose loop length, shape, plasma density, temperature
and velocity profiles are fitted to loops observed with TRACE, SoHO/CDS
and SoHO/SUMER. Synthetic emission profiles are also calculated and
fitted to the observed emission patterns. An analytical discussion is
given of the two-dimenional balance of the Lorentz force and the gas
pressure gradient, gravity and inertial forces acting along and across
the loop. These models are the first to include a fully consistent
description of the magnetic field, 2D geometry, plasma density and
temperature, flow velocity and thermodynamics of loops. The consistently
calculated heating profiles which are largely dominated by radiative
losses and concentrated at the footpoints are influenced by the flow
and are asymmetric, being biased towards the upflow footpoint.
---------------------------------------------------------
Title: Review of Coronal Oscillations - An Observer's View
Authors: Aschwanden, Markus J.
2003astro.ph..9505A Altcode:
Recent observations show a variety of oscillation modes in the
corona. Early non-imaging observations in radio wavelengths showed a
number of fast-period oscillations in the order of seconds, which have
been interpreted as fast sausage mode oscillations. TRACE observations
from 1998 have for the first time revealed the lateral displacements of
fast kink mode oscillations, with periods of ~3-5 minutes, apparently
triggered by nearby flares and destabilizing filaments. Recently,
SUMER discovered with Doppler shift measurements loop oscillations
with longer periods (10-30 minutes) and relatively short damping times
in hot (7 MK) loops, which seem to correspond to longitudinal slow
magnetoacoustic waves. In addition, propagating longitudinal waves
have also been detected with EIT and TRACE in the lowest density
scale height of loops near sunspots. All these new observations seem
to confirm the theoretically predicted oscillation modes and can now
be used as a powerful tool for “coronal seismology” diagnostic.
---------------------------------------------------------
Title: A New Method to Constrain the Iron Abundance from Cooling
Delays in Coronal Loops
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Winebarger,
Amy R.; Warren, Harry P.
2003ApJ...588L..49A Altcode: 2003astro.ph..9506A
Recent observations with the Transition Region and Coronal Explorer
reveal that the time delay between the appearance of a cooling loop in
different EUV temperature filters is proportional to the loop length,
Δt<SUB>12</SUB>~L. We model this cooling delay in terms of radiative
loss and confirm this linear relationship theoretically. We derive an
expression that can be used to constrain the coronal iron enhancement
α<SUB>Fe</SUB>=A<SUP>cor</SUP><SUB>Fe</SUB>/A<SUP>ph</SUP><SUB>Fe</SUB>
relative to the photospheric value as function of the cooling delay
Δt<SUB>12</SUB>, flux F<SUB>2</SUB>, loop width w, and filling factor
q<SUB>w</SUB><=1. With this relation, we find upper limits on
the iron abundance enhancement of α<SUB>Fe</SUB><=4.8+/-1.7 for
10 small-scale nanoflare loops, and α<SUB>Fe</SUB><=1.4+/-0.4
for five large-scale loops, in the temperature range of T~1.0-1.4
MK. This result supports the previous finding that low first ionization
potential elements, including Fe, are enhanced in the corona. The
same relation constitutes also a lower limit for the filling factor,
which is q<SUB>w</SUB>>=0.2+/-0.1 and q<SUB>w</SUB>>=0.8+/-0.2
for the two groups of coronal loops.
---------------------------------------------------------
Title: A New Method to Constrain the Iron Abundance from Cooling
Delays in Coronal Loops
Authors: Aschwanden, M. J.; Schrijver, C. J.; Winebarger, A. R.;
Warren, H. P.
2003SPD....34.1701A Altcode: 2003BAAS...35..837A
Recent observations with TRACE reveal that the time delay between
the appearance of a cooling loop in different EUV temperature
filters is proportional to the loop length, dt<SUB>12</SUB>
∼ L . We model this cooling delay in terms of radiative loss
and confirm this linear relationship theoretically. We derive an
expression that can be used to constrain the coronal iron enhancement
A<SUB>Fe</SUB>=A<SUB>Fe</SUB><SUP>cor</SUP>/A<SUB>Fe</SUB><SUP>Ph</SUP>
relative to the photospheric value as function of the cooling delay
dt<SUB>12</SUB>, flux F<SUB>2</SUB>, loop width w, and filling factor
q<SUB>w</SUB> < 1. With this relation we find upper limits on
the iron abundance enhancement of A<SUB>Fe</SUB> < 4.8 +/- 1.7
for 10 small-scale nanoflare loops, and A<SUB>Fe</SUB> < 1.4 +/-
0.4 for 5 large-scale loops, in the temperature range of T ∼ 1.0-1.4
MK. This result supports the previous finding that low-FIP elements,
including Fe, are enhanced in the corona. The same relation constitutes
also a lower limit for the filling factor, which is q<SUB>w</SUB> >
0.2 +/- 0.1 and q<SUB>w</SUB> > 0.8 +/- 0.2 for the two groups of
coronal loops.
---------------------------------------------------------
Title: Astrophysics in 2002
Authors: Trimble, Virginia; Aschwanden, Markus J.
2003PASP..115..514T Altcode:
This has been the Year of the Baryon. Some low temperature ones were
seen at high redshift, some high temperature ones were seen at low
redshift, and some cooling ones were (probably) reheated. Astronomers
saw the back of the Sun (which is also made of baryons), a possible
solution to the problem of ejection of material by Type II supernovae
(in which neutrinos push out baryons), the production of R Coronae
Borealis stars (previously-owned baryons), and perhaps found the missing
satellite galaxies (whose failing is that they have no baryons). A
few questions were left unanswered for next year, and an attempt is
made to discuss these as well.
---------------------------------------------------------
Title: Flare electron energy budgets - what is RHESSI telling us?
Authors: Brown, J. C.; Kontar, E.; MacKinnon, A. L.; Aschwanden, M. J.
2002ESASP.506..253B Altcode: 2002ESPM...10..253B; 2002svco.conf..253B
We address the idea that energetic particles may play a key role in the
dissipation and transport of energy in flares. After three decades of
predictions of spatial, spectral and temporal distributions of hard X-
and γ-rays, the various models can now be quantitatively tested against
RHESSI high resolution spectral imaging data. It is shown that RHESSI
results for a number of HXR flares are in very good agreement with
predictions of the basic thick target model (Brown 1971) regarding
source height as a function of energy and of global HXR spectrum. A
single power-law injection spectrum and purely collisional transport
(no wave generation) fit well the decrease of source peak height
with increasing energy for very plausible chromospheric density
structures. When the target ionisation drop across the transition one
is included, the global HXR spectrum agrees well with observed "knee"
spectra without any feature added to a scale-less power-law electron
injection spectrum. This result favours statistically distributed, as
opposed to single large scale, E-field acceleration. Whether energetic
electron beams actually dominate flare energy transport still depends
on accurate inference of the low energy thermal/nonthermal spectral
transition though RHESSI results to date support the idea. The ion
energy budget is also briefly mentioned.
---------------------------------------------------------
Title: Chromospheric density and height measurements of the
2002-Feb-20 flare observed with RHESSI
Authors: Aschwanden, Markus J.; Brown, John C.; Kontar, Eduard P.
2002ESASP.506..275A Altcode: 2002ESPM...10..275A; 2002svco.conf..275A
We present the first chromospheric density and height measurements
made with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI)
spacecraft during the flare of 2002-Feb-22, 11:06 UT. Thanks to the
high energy resolution of the germanium-cooled hard X-ray detectors on
RHESSI we can measure the flare source positions with a high accuracy
as a function of energy. Using a forward-fitting algorithm for image
reconstruction, we find a systematic decrease in the altitudes of the
source centroids z(ɛ) as a function of increasing hard X-ray energy
ɛ, as expected in the thick-target bremsstrahlung model of Brown. The
altitude of hard X-ray emission as a function of photon energy ɛ can
be characterized by a powerlaw function in the ɛ = 15-50 keV energy
range, viz. z(ɛ) ≍ 2.3 (ɛ/20 keV)<SUP>-1.3</SUP> Mm. Based on a
purely collisional 1-D thick-target model, this height dependence can
be inverted into a chromospheric density model n(z), which follows
the powerlaw function n<SUB>e</SUB>(z) = 1.25×10<SUP>13</SUP> (z/1
Mm)<SUP>-2.5</SUP> cm<SUP>-3</SUP>. This density is comparable with
models based on optical/UV spectrometry in the chromospheric height
range, while at a height of h≍1000-2500 km, it is more consistent
with the "spicular extended-chromosphere model" inferred from radio
sub-mm observations. In coronal heights of the flare loop, the RHESSI
inferred desities are comparable with soft X-ray and radio observations.
---------------------------------------------------------
Title: Coronal Loops Heated by Magnetohydrodynamic Turbulence. I. A
Model of Isobaric Quiet-Sun Loops with Constant Cross Sections
Authors: Chae, Jongchul; Poland, Arthur I.; Aschwanden, Markus J.
2002ApJ...581..726C Altcode:
Several recent papers have presented new observational results
indicating that many coronal loops in active regions are nearly
isothermal. It is expected that quiet-Sun loops may have similar thermal
structures, since quiet-Sun differential emission measures look similar
to those in active regions. In the quiet Sun, it is well known from
observations that the nonthermal velocity inferred from the excess
broadening of a line over thermal broadening reaches a peak of about
30 km s<SUP>-1</SUP> around 3×10<SUP>5</SUP> K and then decreases
with temperature, having a value of about 20 km s<SUP>-1</SUP> at
1×10<SUP>6</SUP> K. In the present work, we make the assumption
that the observed nonthermal velocities are a manifestation of
magnetohydrodynamic (MHD) turbulence and present a model of static,
isobaric coronal loops heated by turbulence. Instead of solving the
MHD equations, we adopt simple energy spectra in MHD turbulence and
infer the heating rate as a function of temperature from the observed
nonthermal velocities. By solving the steady state energy equation
of a loop in which temperature monotonically increases with height,
we obtain the following results: (1) The heating rate is predominantly
near the footpoints and decreases with the loop arc length. (2) There
is a critical temperature above which the loop cannot be maintained
in a steady state. (3) The loop is denser and is more isothermal than
uniformly heated loops, being compatible with recent observations. (4)
The theoretical differential emission measures are in good agreement
with the empirical values at temperatures above 10<SUP>5</SUP> K. Below
this temperature, we still have a large discrepancy. (5) It is possible
to explain the observed strong correlation between intensity and
nonthermal velocity of a spectral line in the quiet Sun. Our results
support the idea that quasi-statically driven MHD turbulence of the
direct current (DC) type in the stratified medium (transition region
and corona) is a viable mechanism for coronal heating.
---------------------------------------------------------
Title: Overview of the US system for post-docs, contractors, and
careers in solar physics (Invited review)
Authors: Aschwanden, Markus J.
2002ESASP.506..991A Altcode: 2002svco.conf..991A; 2002ESPM...10..991A
This article is intended to offer young physicists specific information
on job opportunities and careers in solar physics in the United
States (US), a traditional post-Doc country for many young European
physicists. There live about 500 solar physicists in the United States,
which are employed at universities and colleges (43%), in government
laboratories (35%), in private companies (8%), or have no official
affiliation (14%). We provide a brief overview of the affiliations of
the US solar physicists, where the academic institutions are located,
what the government laboratories consist of, and what private companies
have the largest contracts in the solar physics business. We compile
also some demographic and sociological statistics from the larger
groups of US astronomers and physicists, that may be of interest for
prospective post-Docs.
---------------------------------------------------------
Title: Damping of coronal loop oscillations by resonant absorption
of quasi-mode kink oscillations
Authors: Goossens, M.; Andries, J.; Aschwanden, M. J.
2002ESASP.506..629G Altcode: 2002svco.conf..629G; 2002ESPM...10..629G
Damped quasi-mode kink oscillations in cylindrical flux tubes are
capable of explaining the observed rapid damping of the coronal loop
oscillations when the ratio of the inhomogneity length scale to the
radius of the loop is allowed to vary from loop to loop. They do not
need to invoke anomalously low Reynolds numbers. The theoretical
expressions for the decay time by Hollweg & Yang (1988) and
Ruderman & Roberts (2002) are used to estimate the ratio of
the length scale of inhomogneity compared to the loop radius for a
collection of loop oscillations.
---------------------------------------------------------
Title: Chromospheric Height and Density Measurements in a Solar
Flare Observed with RHESSI II. Data Analysis
Authors: Aschwanden, Markus J.; Brown, John C.; Kontar, Eduard P.
2002SoPh..210..383A Altcode:
We present an analysis of hard X-ray imaging observations from one
of the first solar flares observed with the Reuven Ramaty High-Energy
Solar Spectroscopic Imager (RHESSI) spacecraft, launched on 5 February
2002. The data were obtained from the 22 February 2002, 11:06 UT flare,
which occurred close to the northwest limb. Thanks to the high energy
resolution of the germanium-cooled hard X-ray detectors on RHESSI
we can measure the flare source positions with a high accuracy as
a function of energy. Using a forward-fitting algorithm for image
reconstruction, we find a systematic decrease in the altitudes of
the source centroids z(ε) as a function of increasing hard X-ray
energy ε, as expected in the thick-target bremsstrahlung model of
Brown. The altitude of hard X-ray emission as a function of photon
energy ε can be characterized by a power-law function in the ε=15-50
keV energy range, viz., z(ε)≈2.3(ε/20 keV)<SUP>−1.3</SUP>
Mm. Based on a purely collisional 1-D thick-target model, this
height dependence can be inverted into a chromospheric density model
n(z), as derived in Paper I, which follows the power-law function
n<SUB>e</SUB>(z)=1.25×10<SUP>13</SUP>(z/1 Mm)<SUP>−2.5</SUP>
cm<SUP>−3</SUP>. This density is comparable with models based on
optical/UV spectrometry in the chromospheric height range of h≲1000
km, suggesting that the collisional thick-target model is a reasonable
first approximation to hard X-ray footpoint sources. At h≈1000-2500
km, the hard X-ray based density model, however, is more consistent
with the `spicular extended-chromosphere model' inferred from radio
sub-mm observations, than with standard models based on hydrostatic
equilibrium. At coronal heights, h≈2.5-12.4 Mm, the average flare
loop density inferred from RHESSI is comparable with values from
hydrodynamic simulations of flare chromospheric evaporation, soft
X-ray, and radio-based measurements, but below the upper limits set
by filling-factor insensitive iron line pairs.
---------------------------------------------------------
Title: The Differential Emission Measure Distribution in the
Multiloop Corona
Authors: Aschwanden, Markus J.
2002ApJ...580L..79A Altcode:
This is a rebuttal of a recent Letter on the inadequacy of temperature
measurements in the solar corona through narrowband filter and line
ratios by Martens et al. We simulate the differential emission measure
(DEM) distribution of a multiloop corona and find that the temperature
profile of individual loops can be retrieved with narrowband filter
ratios. The apparently flat DEM distributions constructed from Coronal
Diagnostics Spectrometer line fluxes by Schmelz et al. are an artifact
of a smoothing function (in temperature), while the unsmoothed DEM
distribution reveals multiple peaks of near-isothermal loops.
---------------------------------------------------------
Title: Chromospheric Height and Density Measurements in a Solar
Flare Observed with RHESSI I. Theory
Authors: Brown, John C.; Aschwanden, Markus J.; Kontar, Eduard P.
2002SoPh..210..373B Altcode:
We obtain a theoretical description of the height (z) distribution
of flare hard X-rays in the collisional thick-target model as a
function of photon energy ε. This depends on the target atmosphere
density structure n(z) and on the beam spectral index δ. We
show that by representing the data in terms of the 1-D function
z(ε) defining where the emission peaks as a function of ε it is
possible to derive n(z) from data on z(ε). This is done first on
the basis of a simple stopping depth argument then refined to allow
for the dependence on spectral index δ. The latter is worked out
in detail for the case of a parameterization n(z)=n<SUB>0</SUB>
(z/z<SUB>0</SUB>)<SUP>−b</SUP> which yields numerical results for
z(ε) well fit by z(ε)∼ε<SUP>−α</SUP>, with α dependent on δ,
which is also found to fit well to actual observations. This enables
derivation of flare loop n(z) in terms of n<SUB>0</SUB>,b from RHESSI
data in an entirely novel way, independent of other density diagnostic
methods, and also of how n(z) varies with time in flares such as by
evaporation, as detailed in companion Paper II.
---------------------------------------------------------
Title: Reconstruction of RHESSI Solar Flare Images with a Forward
Fitting Method
Authors: Aschwanden, Markus J.; Schmahl, Ed; Team, The Rhessi
2002SoPh..210..193A Altcode:
We describe a forward-fitting method that has been developed
to reconstruct hard X-ray images of solar flares from the Ramaty
High-Energy Solar Spectroscopic Imager (RHESSI), a Fourier imager with
rotation-modulated collimators that was launched on 5 February 2002. The
forward-fitting method is based on geometric models that represent a
spatial map by a superposition of multiple source structures, which are
quantified by circular gaussians (4 parameters per source), elliptical
gaussians (6 parameters), or curved ellipticals (7 parameters),
designed to characterize real solar flare hard X-ray maps with a
minimum number of geometric elements. We describe and demonstrate the
use of the forward-fitting algorithm. We perform some 500 simulations
of rotation-modulated time profiles of the 9 RHESSI detectors, based
on single and multiple source structures, and perform their image
reconstruction. We quantify the fidelity of the image reconstruction,
as function of photon statistics, and the accuracy of retrieved source
positions, widths, and fluxes. We outline applications for which the
forward-fitting code is most suitable, such as measurements of the
energy-dependent altitude of energy loss near the limb, or footpoint
separation during flares.
---------------------------------------------------------
Title: The RHESSI Imaging Concept
Authors: Hurford, G. J.; Schmahl, E. J.; Schwartz, R. A.; Conway,
A. J.; Aschwanden, M. J.; Csillaghy, A.; Dennis, B. R.; Johns-Krull,
C.; Krucker, S.; Lin, R. P.; McTiernan, J.; Metcalf, T. R.; Sato,
J.; Smith, D. M.
2002SoPh..210...61H Altcode:
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI)
observes solar hard X-rays and gamma-rays from 3 keV to 17 MeV
with spatial resolution as high as 2.3 arc sec. Instead of focusing
optics, imaging is based on nine rotating modulation collimators that
time-modulate the incident flux as the spacecraft rotates. Starting from
the arrival time of individual photons, ground-based software then uses
the modulated signals to reconstruct images of the source. The purpose
of this paper is to convey both an intuitive feel and the mathematical
basis for this imaging process. Following a review of the relevant
hardware, the imaging principles and the basic back-projection method
are described, along with their relation to Fourier transforms. Several
specific algorithms (Clean, MEM, Pixons and Forward-Fitting) applicable
to RHESSI imaging are briefly described. The characteristic strengths
and weaknesses of this type of imaging are summarized.
---------------------------------------------------------
Title: Coronal loop oscillations. An interpretation in terms of
resonant absorption of quasi-mode kink oscillations
Authors: Goossens, M.; Andries, J.; Aschwanden, M. J.
2002A&A...394L..39G Altcode:
Damped quasi-mode kink oscillations in cylindrical flux tubes are
capable of explaining the observed rapid damping of the coronal
loop oscillations when the ratio of the inhomogeneity length scale
to the radius of the loop is allowed to vary from loop to loop,
without the need to invoke anomalously low Reynolds numbers. The
theoretical expressions for the decay time by Hollweg & Yang
(\cite{hollweg1988}) and Ruderman & Roberts (\cite{ruderman2002})
are used to estimate the ratio of the length scale of inhomogeneity
compared to the loop radius for a collection of loop oscillations.
---------------------------------------------------------
Title: The Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI)
Authors: Lin, R. P.; Dennis, B. R.; Hurford, G. J.; Smith, D. M.;
Zehnder, A.; Harvey, P. R.; Curtis, D. W.; Pankow, D.; Turin, P.;
Bester, M.; Csillaghy, A.; Lewis, M.; Madden, N.; van Beek, H. F.;
Appleby, M.; Raudorf, T.; McTiernan, J.; Ramaty, R.; Schmahl, E.;
Schwartz, R.; Krucker, S.; Abiad, R.; Quinn, T.; Berg, P.; Hashii,
M.; Sterling, R.; Jackson, R.; Pratt, R.; Campbell, R. D.; Malone,
D.; Landis, D.; Barrington-Leigh, C. P.; Slassi-Sennou, S.; Cork, C.;
Clark, D.; Amato, D.; Orwig, L.; Boyle, R.; Banks, I. S.; Shirey,
K.; Tolbert, A. K.; Zarro, D.; Snow, F.; Thomsen, K.; Henneck,
R.; Mchedlishvili, A.; Ming, P.; Fivian, M.; Jordan, John; Wanner,
Richard; Crubb, Jerry; Preble, J.; Matranga, M.; Benz, A.; Hudson,
H.; Canfield, R. C.; Holman, G. D.; Crannell, C.; Kosugi, T.; Emslie,
A. G.; Vilmer, N.; Brown, J. C.; Johns-Krull, C.; Aschwanden, M.;
Metcalf, T.; Conway, A.
2002SoPh..210....3L Altcode:
RHESSI is the sixth in the NASA line of Small Explorer (SMEX)
missions and the first managed in the Principal Investigator mode,
where the PI is responsible for all aspects of the mission except
the launch vehicle. RHESSI is designed to investigate particle
acceleration and energy release in solar flares, through imaging and
spectroscopy of hard X-ray/gamma-ray continua emitted by energetic
electrons, and of gamma-ray lines produced by energetic ions. The
single instrument consists of an imager, made up of nine bi-grid
rotating modulation collimators (RMCs), in front of a spectrometer
with nine cryogenically-cooled germanium detectors (GeDs), one behind
each RMC. It provides the first high-resolution hard X-ray imaging
spectroscopy, the first high-resolution gamma-ray line spectroscopy,
and the first imaging above 100 keV including the first imaging of
gamma-ray lines. The spatial resolution is as fine as ∼ 2.3 arc sec
with a full-Sun (≳ 1°) field of view, and the spectral resolution
is ∼ 1-10 keV FWHM over the energy range from soft X-rays (3 keV)
to gamma-rays (17 MeV). An automated shutter system allows a wide
dynamic range (>10<SUP>7</SUP>) of flare intensities to be handled
without instrument saturation. Data for every photon is stored in a
solid-state memory and telemetered to the ground, thus allowing for
versatile data analysis keyed to specific science objectives. The
spin-stabilized (∼ 15 rpm) spacecraft is Sun-pointing to within ∼
0.2° and operates autonomously. RHESSI was launched on 5 February
2002, into a nearly circular, 38° inclination, 600-km altitude orbit
and began observations a week later. The mission is operated from
Berkeley using a dedicated 11-m antenna for telemetry reception and
command uplinks. All data and analysis software are made freely and
immediately available to the scientific community.
---------------------------------------------------------
Title: Analytical Approximations to Hydrostatic Solutions and Scaling
Laws of Coronal Loops
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.
2002ApJS..142..269A Altcode:
We derive accurate analytical approximations to hydrostatic solutions
of coronal loop atmospheres, applicable to uniform and nonuniform
heating in a large parameter space. The hydrostatic solutions of the
temperature T(s), density n<SUB>e</SUB>(s), and pressure profile p(s)
as a function of the loop coordinate s are explicitly expressed in
terms of three independent parameters: the loop half-length L, the
heating scale length s<SUB>H</SUB>, and either the loop-top temperature
T<SUB>max</SUB> or the base heating rate E<SUB>H0</SUB>. The analytical
functions match the numerical solutions with a relative accuracy of
<~10<SUP>-2</SUP>-10<SUP>-3</SUP>. The absolute accuracy of the
scaling laws for loop base pressure p<SUB>0</SUB>(L, s<SUB>H</SUB>,
T<SUB>max</SUB>) and base heating rate E<SUB>H0</SUB>(L, s<SUB>H</SUB>,
T<SUB>max</SUB>), previously derived for uniform heating by Rosner
et al., and for nonuniform heating by Serio et al., is improved to a
level of a few percent. We generalize also our analytical approximations
for tilted loop planes (equivalent to reduced surface gravity) and for
loops with varying cross sections. There are many applications for such
analytical approximations: (1) the improved scaling laws speed up the
convergence of numeric hydrostatic codes as they start from better
initial values, (2) the multitemperature structure of coronal loops
can be modeled with multithread concepts, (3) line-of-sight integrated
fluxes in the inhomogeneous corona can be modeled with proper correction
of the hydrostatic weighting bias, (4) the coronal heating function
can be determined by forward-fitting of soft X-ray and EUV fluxes, or
(5) global differential emission measure distributions dEM/dT of solar
and stellar coronae can be simulated for a variety of heating functions.
---------------------------------------------------------
Title: Observations and models of coronal loops: from Yohkoh to TRACE
Authors: Aschwanden, Markus J.
2002ESASP.505..191A Altcode: 2002IAUCo.188..191A; 2002solm.conf..191A
We review highlights of recent observations of coronal loops in EUV
and soft X-rays from Yohkohand TRACE. We review (1) hydrostatic loops,
(2) non-hydrostatic loops, (3) oscillating loops, (4) nanoflare loops,
and (5) coronal heating, in the light of new observations, with a
critical discussion of previous interpretations and theoretical models.
---------------------------------------------------------
Title: Particle Acceleration and Kinematics in Solar Flares
Authors: Aschwanden, Markus J.
2002paks.book.....A Altcode:
This book presents a synthesis of what we learned about particle
acceleration and kinematics from recent solar flare observations
with the Yohkoh, SoHO, TRACE, CGRO spacecraft and radio instruments
over the last decade. It deals with the topology of magnetic
reconnection regions, discusses the geometry, small-scale dynamics,
and electromagnetic fields of acceleration region in solar flares,
provides a systematic description of the relativistic kinematics
of particle acceleration, propagation, time-of-flight measurements,
particle trapping, precipitation, and the resulting emissions in gamma
rays, hard X-rays, and radio wavelengths. It is the first monograph on
these solar flare topics, written for the level of graduate students
and researchers in the field of solar physics, astrophysics, and
magnetospheric physics. Link: http://www.wkap.nl/prod/b/1-4020-0725-6
---------------------------------------------------------
Title: Damping Time Scaling of Coronal Loop Oscillations Deduced
from Transition Region and Coronal Explorer Observations
Authors: Ofman, L.; Aschwanden, M. J.
2002ApJ...576L.153O Altcode:
The damping mechanism of recently discovered coronal loop transverse
oscillations provides clues to the mechanism of coronal heating. We
determine the scaling of the damping time with the parameters of the
loops observed in extreme ultraviolet by the Transition Region and
Coronal Explorer. We find excellent agreement of the scaling power
to the power predicted by phase mixing and poor agreement with the
power predicted by the wave leakage or ideal decay of the cylindrical
kink mode mechanisms. Phase mixing leads to rapid dissipation of the
Alfvén waves due to the variation of the Alfvén speed across the
wave front and formation of small scales. Our results suggest that
the loop oscillations are dissipated by phase mixing with anomalously
high viscosity.
---------------------------------------------------------
Title: Nanoflare Statistics from First Principles: Fractal Geometry
and Temperature Synthesis
Authors: Aschwanden, Markus J.; Parnell, Clare E.
2002ApJ...572.1048A Altcode:
We derive universal scaling laws for the physical parameters
of flarelike processes in a low-β plasma, quantified in terms
of spatial length scales l, area A, volume V, electron density
n<SUB>e</SUB>, electron temperature T<SUB>e</SUB>, total emission
measure M, and thermal energy E. The relations are specified as
functions of two independent input parameters, the power index a
of the length distribution, N(l)~l<SUP>-a</SUP>, and the fractal
Haussdorff dimension D between length scales l and flare areas,
A(l)~l<SUP>D</SUP>. For values that are consistent with the data,
i.e., a=2.5+/-0.2 and D=1.5+/-0.2, and assuming the RTV scaling
law, we predict an energy distribution N(E)~E<SUP>-α</SUP> with a
power-law coefficient of α=1.54+/-0.11. As an observational test,
we perform statistics of nanoflares in a quiet-Sun region covering a
comprehensive temperature range of T<SUB>e</SUB>~1-4 MK. We detected
nanoflare events in extreme-ultraviolet (EUV) with the 171 and 195
Å filters from the Transition Region and Coronal Explorer (TRACE),
as well as in soft X-rays with the AlMg filter from the Yohkoh soft
X-ray telescope (SXT), in a cospatial field of view and cotemporal time
interval. The obtained frequency distributions of thermal energies of
nanoflares detected in each wave band separately were found to have
power-law slopes of α~1.86+/-0.07 at 171 Å (T<SUB>e</SUB>~0.7-1.1 MK),
α~1.81+/-0.10 at 195 Å (T<SUB>e</SUB>~1.0-1.5 MK), and α~1.57+/-0.15
in the AlMg filter (T<SUB>e</SUB>~1.8-4.0 MK), consistent with earlier
studies in each wavelength. We synthesize the temperature-biased
frequency distributions from each wavelength and find a corrected
power-law slope of α~1.54+/-0.03, consistent with our theoretical
prediction derived from first principles. This analysis, supported by
numerical simulations, clearly demonstrates that previously determined
distributions of nanoflares detected in EUV bands produced a too
steep power-law distribution of energies with slopes of α~2.0-2.3
mainly because of this temperature bias. The temperature-synthesized
distributions of thermal nanoflare energies are also found to be more
consistent with distributions of nonthermal flare energies determined
in hard X-rays (α~1.4-1.6) and with theoretical avalanche models
(α~1.4-1.5).
---------------------------------------------------------
Title: Astrophysics in 2001
Authors: Trimble, Virginia; Aschwanden, Markus J.
2002PASP..114..475T Altcode:
During the year, astronomers provided explanations for solar topics
ranging from the multiple personality disorder of neutrinos to
cannibalism of CMEs (coronal mass ejections) and extra-solar topics
including quivering stars, out-of-phase gaseous media, black holes of
all sizes (too large, too small, and too medium), and the existence of
the universe. Some of these explanations are probably possibly true,
though the authors are not betting large sums on any one. The data ought
to remain true forever, though this requires a careful definition of
“data” (think of the Martian canals).
---------------------------------------------------------
Title: The Electron Number Problem Revisited with RHESSI Flare
Observations
Authors: Aschwanden, M. J.; Alexander, D.; Metcalf, T.; Nitta, N.
2002AAS...200.7608A Altcode: 2002BAAS...34..776A
The Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) observed
a number of solar flares since its launch on February 5, 2002. We
analyze first images and spectra from this new mission, using a
variety of image reconstruction methods, such as Clean, Pixon, Maximum
Entropy (MEM), and Forward-Fitting. We obtain context images of the
flare regions in EUV from SoHO/EIT and TRACE, and soft X-ray light
curves from GOES. From RHESSI we reconstruct hard X-ray images with
full uv-coverage in time intervals of the spin period ( 4 s). RHESSI
provides a substantially higher spatial resolution ( 2") and spectral
resolution than previous data from Yohkoh/HXT. A longstanding problem
in solar flare physics is the so-called electron number problem,
which challenges a reconciliation between (1) the rate of injected
electrons inferred from the observed hard X-ray photon spectrum in
the framework of the thick-target model and the (2) the replenishment
rate of electrons in the coronal acceleration volume. Thanks to the
high spectral resolution of RHESSI we can for the first time properly
separate the thermal and the non-thermal electron spectral components,
and this way determine the injection rate of nonthermal electrons
more accurately. In addition, imaging in many energy bands allows us
to trace propagation and energy loss between the coronal acceleration
site and the chromospheric thick-target site with higher accuracy,
taking into account the partial energy loss of trapped and precipitating
electrons. The goal of this investigation is to quantify the geometry,
density, and inflows in the coronal acceleration region, which provides
crucial constraints for the underlying magnetic reconnection process.
---------------------------------------------------------
Title: Radio Frequency-Tomography of Solar Flares
Authors: Aschwanden, M. J.
2002AAS...200.4905A Altcode: 2002BAAS...34..722A
The Frequency-Agile Solar Radiotelescope (FASR) is designed to produce
simultaneous images of solar phenomena at many frequencies. A data cube
with a stack of multiple frequency images can be used for tomographic
reconstruction of the 3D density and temperature distribution of
flares, based on the free-free emission at cm and mm wavelengths. We
simulate a set of multi-frequency images for the Bastille-Day flare
of 2000-July-14, based on EUV observations from TRACE and soft X-ray
observations from Yohkoh. The 3D model consists of some 200 postflare
loops with observationally constrained densities and temperatures. The
temporal evolution involves flare plasma heating, a phase of conductive
cooling, followed by a phase of radiative cooling. The images simulated
at different microwave frequencies reveal a sequence of optically-thick
free-free emission layers, which can be "pealed off" like onion shells
with increasing radio frequency. We envision a tomographic method
that yields information on the density and temperature structure of
flare systems and their evolution. Comparison with EUV and soft X-ray
based 3D models will also allow to quantify wave scattering at radio
frequencies and provide information on small-scale inhomogeneities
and wave turbulence. Besides the thermal free-free emission, radio
images contain also information on coherent emission processes, such
as plasma emission from electron beams and loss-cone emission from
gyroresonant trapped particles, conveying information on particle
acceleration processes.
---------------------------------------------------------
Title: Transverse oscillations in coronal loops observed with
TRACE I. An Overview of Events, Movies, and a Discussion of Common
Properties and Required Conditions
Authors: Schrijver, Carolus J.; Aschwanden, Markus J.; Title, Alan M.
2002SoPh..206...69S Altcode:
We study transverse loop oscillations triggered by 17flares and filament
destabilizations; only 2 such cases have been reported in the literature
until now. Oscillation periods are estimated to range over a factor
of ∼15, with most values between 2 and 7 min. The oscillations are
excited by filament destabilizations or flares (in 6% of the 255 flares
inspected, ranging from about C3 to X2). There is no clear dependence
of oscillation amplitude on flare magnitude. Oscillations occur in
loops that close within an active region, or in loops that connect
an active region to a neighboring region or to a patch of strong
flux in the quiet Sun. Some magnetic configurations are particularly
prone to exhibit oscillations: two active regions showed two, and one
region even three, distinct intervals with loop oscillations. The
loop oscillations are not a resonance that builds up: oscillations
in loops that are excited along their entire length are likely to be
near the fundamental resonance mode because of that excitation profile,
but asymmetrically excited oscillations clearly show propagating waves
that are damped too quickly to build up a resonance, and some cases show
multiple frequencies. We discuss evidence that all oscillating loops lie
near magnetic separatrices that outline the large-scale topology of the
field. All magnetic configurations are more complicated than a simple
bipolar region, involving mixed-polarities in the interior or vicinity
of the region; this may reflect that the exciting eruptions occur only
in such environments, but this polarity mixing likely also introduces
the large-scale separatrices that are involved. Often the oscillations
occur in conjunction with gradual adjustments in loop positions in
response to the triggering event. We discuss the observations in
the context of two models: (a) transverse waves in coronal loops
that act as wave guides and (b) strong sensitivity to changes in
the field sources for field lines near separatrices. Properties
that favor model b are (1) the involvement of loops at or near
separatrices that outline the large-scale topology of the field,
(2) the combined occurrence of oscillations and loop translations,
(3) the small period spread and similar decay time scale in a set of
oscillating loops in one well-observed event, and (4) the existence
of loops oscillating in antiphase with footpoints close together in
two cases. All other properties are compatible with either model,
except the fact that almost all of the oscillations start away from
the triggering event, suggestive of an outward-pushing exciting wave
more in line with model a. The spread in periods from event to event
suggests that the oscillations may reflect the properties of some
driver mechanism that is related to the flare or mass ejection.
---------------------------------------------------------
Title: 3D coronal observations with the solar cycle
Authors: Portier-Fozzani, Fabrice; Inhester, Bernd; Papadopoulo,
Théodore; Bijaoui, Albert; Aschwanden, Markus
2002ESASP.477..159P Altcode: 2002scsw.conf..159P
The number of observed aurorae and variable disturbances in the
Earth magnetosphere is correlated with the 11 years solar cycle. These
phenomena result from 3D solar magnetic field variations and could have
many consequences such as loss of satellites control, uncertainties
in plane positions, high level of irradiance received by astronauts
with high particles, etc... Recent progress in 3D visualization and
reconstruction (with stereoscopy and tomography techniques) made
it possible to follow the evolution of coronal plasma structures
frozen in by the magnetic field. Loops geometries - including their
twists measurements -, arcade loops structures or filament shapes, are
determined by 3D techniques with SOHO, using the solar rotation. A 3D
analysis of the flare formation on April 4th, 1997 shows an emerging
flux near a sigmoid loop with magnetic fields reconnection. Other
observations emphasize the role of the helicity (observed for
example as a twist in many structures) in filament eruptions and CME
formations. Detwisting processes from EIT images to coronographic data
show ejections of material into the interplanetary space.
---------------------------------------------------------
Title: Transverse Oscillations in Coronal Loops Observed with TRACE
II. Measurements of Geometric and Physical Parameters
Authors: Aschwanden, Markus J.; De Pontieu, Bart; Schrijver, Carolus
J.; Title, Alan M.
2002SoPh..206...99A Altcode:
We measure geometric and physical parameters oftransverse oscillations
in 26 coronal loops, out of the 17 events described in Paper I by
Schrijver, Aschwanden, and Title (2002). These events, lasting
from 7 to 90 min, have been recorded with the Transition Region
and Coronal Explorer (TRACE) in the 171 and 195 Å wavelength
bands with a characteristic angular resolution of 1", with time
cadences of 15-75 seconds. We estimate the unprojected loop (half)
length L and orientation of the loop plane, based on a best-fit of a
circular geometry. Then we measure the amplitude A(t) of transverse
oscillations at the loop position with the largest amplitude. We
decompose the time series of the transverse loop motion into an
oscillating component A<SUB>osc</SUB>(t) and a slowly-varying trend
A<SUB>trend</SUB>(t). We find oscillation periods in the range of
P=2-33 min, transverse amplitudes of A=100-8800 km, loop half lengths
of L=37 000-291 000 km, and decay times of t<SUB>d</SUB>=3.2-21 min. We
estimate a lower limit of the loop densities to be in the range of
n<SUB>loop</SUB>=0.13-1.7×10<SUP>9</SUP> cm<SUP>−3</SUP>. The
oscillations show (1) strong deviations from periodic pulses, (2)
spatially asymmetric oscillation amplitudes along the loops, and
(3) nonlinear transverse motions of the centroid of the oscillation
amplitude. From these properties we conclude that most of the
oscillating loops do not fit the simple model of kink eigen-mode
oscillations, but rather manifest flare-induced impulsively generated
MHD waves, which propagate forth and back in the loops and decay
quickly by wave leakage or damping. In contrast to earlier work we
find that the observed damping times are compatible with estimates of
wave leakage through the footpoints, for chromospheric density scale
heights of ≈400-2400 km. We conclude that transverse oscillations
are most likely excited in loops that (1) are located near magnetic
nullpoints or separator lines, and (2) are hit by a sufficiently
fast exciter. These two conditions may explain the relative rarity of
detected loop oscillations. We show that coronal seismology based on
measurements of oscillating loop properties is challenging due to the
uncertainties in estimating various loop parameters. We find that a
more accurate determination of loop densities and magnetic fields,
as well as advanced numerical modeling of oscillating loops, are
necessary conditions for true coronal seismology.
---------------------------------------------------------
Title: Effects of Temperature Bias on Nanoflare Statistics
Authors: Aschwanden, Markus J.; Charbonneau, Paul
2002ApJ...566L..59A Altcode:
Statistics of solar flares, microflares, and nanoflares have been
gathered over an energy range of some 8 orders of magnitude, over
E~10<SUP>24</SUP>-10<SUP>32</SUP> ergs. Frequency distributions
of flare energies are always determined in a limited temperature
range, e.g., at T~1-2 MK if the 171 and 195 Å filters are used
from an extreme ultraviolet telescope (the Solar and Heliospheric
Observatory/EUV Imaging Telescope or the Transitional Region and
Coronal Explorer). Because the electron temperature T<SUB>e</SUB>
and the thermal energy E=3n<SUB>e</SUB>k<SUB>B</SUB>T<SUB>e</SUB>V
are statistically correlated in flare processes, statistics in
a limited temperature range introduce a bias in the frequency
distribution of flare energies, N(E)~E<SUP>-a<SUB>E</SUB></SUP>. We
demonstrate in this Letter that the power-law slope of nanoflare
energies, e.g., a<SUB>E</SUB>~1.9, as determined in a temperature
range of T~1.1-1.6 MK (195 Å), corresponds to a corrected value of
a<SUP>'</SUP><SUB>E</SUB>~1.4 in an unbiased, complete sample. This
corrected value is in much better agreement with predictions from
avalanche models of solar flares. However, it also implies that all
previously published power-law slopes of EUV nanoflares, covering
a range of a<SUB>E</SUB>~1.8-2.3, correspond to unbiased values of
a<SUB>E</SUB><2, which then poses a serious challenge to Parker's
hypothesis of coronal heating by nanoflares.
---------------------------------------------------------
Title: Constraining the Properties of Nonradiative Heating of the
Coronae of Cool Stars and the Sun
Authors: Schrijver, Carolus J.; Aschwanden, Markus J.
2002ApJ...566.1147S Altcode:
The dominant mechanism that heats the coronae of the Sun and of other
cool stars remains to be identified, despite numerous solar and stellar
studies. We address the problem from a statistical point of view,
by approximating the emission expected from the ensemble of loops
in stellar coronae. We develop a prototype of a global atmospheric,
empirical model that employs (1) simulations of the surface magnetic
field of the Sun and active stars throughout sunspot cycles, (2)
potential field computations of the corresponding coronal field, and
(3) an approximation of atmospheres for 2000 coronal loops for randomly
selected field lines in each flux configuration, representative of all
environments from very quiet to the interior of active regions. The
latter requires specification of the flux density P<SUB>H</SUB> that
passes through the base of the loops to heat the corona. We parameterize
P<SUB>H</SUB> as a function of the base field strength B<SUB>base</SUB>
(in G), loop half-length l (in Mm), and footpoint velocity v (in
km s<SUP>-1</SUP>). We find a best fit for a heating flux density
of P<SUB>H</SUB>~2×10<SUP>7</SUP>(B<SUB>base</SUB>/100)<SUP>1.0+/-
0.5</SUP>(l/24)<SUP>-0.7+/-0.3</SUP>(v/0.4)<SUP>0.0+/-0.5</SUP> ergs
cm<SUP>-2</SUP> s<SUP>-1</SUP> (the allowed ranges of the exponents
are shown). This parameterization matches the observed soft X-ray
losses from the coronae of the Sun and more active stars with rotation
periods down to 5 days, throughout their activity cycles, as well as
the characteristic coronal temperatures, and the relationships between
disk-averaged radiative and magnetic flux densities. We compare this
parameterization to models previously published in the literature
and find that dissipation of current layers and turbulence are the
most likely candidate heating mechanisms, for which both low-frequency
driving and high-frequency driving meet the criteria comparably well. We
find, moreover, that the heating scale length of ~20 Mm inferred from
solar observations matches the characteristic e-folding height of the
field strength over solar active regions, which suggests that coronal
heating depends on the local field strength. Our modeling suggests
that there is no need for a strong selection mechanism to determine
which loops are heated and which are not, but that the sensitive
dependence of the heating on the base field strength causes the
appearance of a corona that consists of bright loops embedded in less
bright environments. We compare the differential emission measures for
the simulated coronae to those of the Sun and more active cool stars,
and we also discuss the apparently weak velocity dependence of the
best-fit parameterization for P<SUB>H</SUB>.
---------------------------------------------------------
Title: Particle acceleration and kinematics in solar flares - A
Synthesis of Recent Observations and Theoretical Concepts (Invited
Review)
Authors: Aschwanden, Markus J.
2002SSRv..101....1A Altcode:
We review the physical processes of particle acceleration, injection,
propagation, trapping, and energy loss in solar flare conditions. An
understanding of these basic physical processes is inexorable to
interpret the detailed timing and spectral evolution of the radiative
signatures caused by nonthermal particles in hard X-rays, gamma-rays,
and radio wavelengths. In contrast to other more theoretically oriented
reviews on particle acceleration processes, we aim here to capitalize
on the numerous observations from recent spacecraft missions, such as
from the Compton Gamma Ray Observatory (CGRO), the Yohkoh Hard X-Ray
Telescope (HXT) and Soft X-Ray Telescope (SXT), and the Transition
Region and Coronal Explorer (TRACE). High-precision energy-dependent
time delay measurements from CGRO and spatial imaging with Yohkoh and
TRACE provide invaluable observational constraints on the topology of
the acceleration region, the reconstruction of magnetic reconnection
processes, the resulting electromagnetic fields, and the kinematics
of energized (nonthermal) particles.
---------------------------------------------------------
Title: Reconciliation of the Coronal Heating Function between Yohkoh
and TRACE
Authors: Aschwanden, Markus J.
2002mwoc.conf...57A Altcode:
Mixed results on the determination of the coronal heating function
are quoted in literature. There seems to be a polarization between
Yohkoh, SoHO/EIT, and TRACE results. Essentially, heating functions
E<SUB>H(s)</SUB> derived from the temperature-broadband instrument
SXT/Yohkoh yield best fits for uniform or looptop heating (Priest
et al. 1998, 2000; Wheatland, Sturrock, & Acton 1997), while
the same heating function derived from temperature-narrowband
instruments like EIT and TRACE yield best fits for footpoint heating
with scale heights of s<SUB>H</SUB> l ≅15 Mm. The problem seems to
be rooted in oversimplified modeling of filter-ratio temperatures,
using single-temperature models for every line-of-sight, although
the broadband response of Yohkoh/SXT is sensitive to emission
measure-weighted temperatures of the entire dEM(T)/dT distribution
above T > 1.5 MK. We revisit previous analyses of Yohkoh data and
demonstrate with forward-fitting methods of multi-temperature models
or continuous dEM(T)/dT distributions that both the Yohkoh/SXT and
TRACE data can be reconciled, both yielding coronal heating function
with scale heights of s<SUB>H</SUB> lapprox 15 Mm, for active region
loops as well as for quiet Sun region. This result has important
consequences for the identification of the long-sought physical
mechanisms responsible for coronal heating.
---------------------------------------------------------
Title: Transverse oscillations in coronal loops observed with TRACE
Authors: Schrijver, C. J.; Aschwanden, M. J.; De Pontieu, B.; Title,
A. M.
2001AGUFMSH11A0703S Altcode:
TRACE discovered transverse oscillations in coronal loops associated
with a flare three years ago, and until recently only two such events
were known. We have now identified a total of 17 events that trigger
some form of loop oscillations. Oscillation periods are estimated to
range over a factor of ~ 15, with most values between 2 and 7 min. The
oscillations are excited by filament destabilizations or flares (in 6%\
of the 255 flares inspected, ranging from about C3 to X2). Oscillations
occur in loops that close within an active region, or in loops that
connect an active region to a neighboring region or to a patch of strong
flux in the quiet Sun. Some magnetic configurations are particularly
prone to exhibit oscillations: two active regions showed two, and
one region even three, distinct intervals with loop oscillations. The
loop oscillations are not a resonance that builds up: oscillations in
loops that are excited along their entire length are likely to be near
the fundamental resonance mode because of that excitation profile, but
asymmetrically excited oscillations clearly show propagating waves that
are damped too quickly to build up a resonance, and some cases show
multiple frequencies. We discuss evidence that all oscillating loops
lie near magnetic separatrices that outline the large-scale topology
of the field. Often the oscillations occur in conjunction with gradual
adjustments in loop positions in response to the triggering event. We
discuss the observations in the context of two models, and evaluate
the contraints on coronal properties that can be deduced from them.
>http://vestige.lmsal.com/TRACE/POD/TRACEoscillations.html</a>
---------------------------------------------------------
Title: Evolution of Magnetic Flux Rope in the Active Region NOAA
9077 on 14 July 2000
Authors: Yan, Yihua; Aschwanden, Markus J.; Wang, Shujuan; Deng,
Yuanyong
2001SoPh..204...27Y Altcode:
The finite energy force-free magnetic fields of the active region NOAA
9077 on 14 July 2000 above the photosphere were reconstructed. We
study the evolution of the 3D magnetic field structures in AR 9077
and compare the reconstructed field lines with TRACE EUV 171 Å flare
loops during the flare maximum, which confirms the process that flaring
loops extended from lower sheared level to higher arcades. We also
demonstrate the 3D magnetic field evolution before the 3B/X5.7 flare
on 14 July and the magnetic structure after the flare on 15 July. This
shows that the helical magnetic structures were significantly changed,
suggesting that the flux rope was indeed erupted during the energetic
flare at 10:24 UT on 14 July.
---------------------------------------------------------
Title: Flare Plasma Cooling from 30 MK down to 1 MK modeled from
Yohkoh, GOES, and TRACE Observations during the Bastille-Day Event
(14 July 2000)
Authors: Aschwanden, M. J.; Alexander, D.
2001AGUFMSH32B..05A Altcode:
We present an analysis of the evolution of the thermal flare
plasma during the 2000-Jul-14, 10 UT, Bastille-Day flare event,
using spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and
TRACE. The spatial structure of this double-ribbon flare consists of
a curved arcade with some 100 post-flare loops which brighten up in
a sequential manner from highly-sheared low-lying to less-sheared
higher-lying bipolar loops. We reconstruct an instrument-combined,
average differential emission measure distribution dEM(T)/dT that
ranges from T=1 MK to 40 MK and peaks at T<SUB>0=10.9</SUB> MK. We
find that the time profiles of the different instrument fluxes peak
sequentially over 7 minutes with decreasing temperatures from T≈ 30
MK to 1 MK, indicating the systematic cooling of the flare plasma. From
these temperature-dependent relative peak times t<SUB>peak</SUB>(T)
we reconstruct the average plasma cooling function T(t) for loops
observed near the flare peak time, and find that their temperature
decrease is initially controlled by conductive cooling during the
first 188 s, T(t) ~ [1+(t/τ<SUB>cond</SUB>)]<SUP>-2/7</SUP>,
and then by radiative cooling during the next 592 s, T(t) ~
[1-(t/τ<SUB>rad</SUB>)]<SUP>3/5</SUP>. From the radiative cooling
phase we infer an average electron density of n<SUB>e=4.2x</SUB>
10<SUP>11</SUP> cm<SUP>-3</SUP>, which implies a filling factor
near 100% for the brightest observed 23 loops with diameters
of ≈ 1.8 Mm that appear simultaneously over the flare peak
time and are fully resolved with TRACE. We reproduce the time
delays and fluxes of the observed time profiles near the flare
peak self-consistently with a forward-fitting method of a fully
analytical model. The total integrated thermal energy of this
flare amounts to E<SUB>thermal</SUB>=2.6 x 10<SUP>31</SUP>
erg. >http://www.lmsal.com/~aschwand/publications/publ.html</a>
---------------------------------------------------------
Title: Flare Plasma Cooling from 30 MK down to 1 MK modeled from
Yohkoh, GOES, and TRACE observations during the Bastille Day Event
(14 July 2000)
Authors: Aschwanden, Markus J.; Alexander, David
2001SoPh..204...91A Altcode:
We present an analysis of the evolution of the thermal flare plasma
during the 14 July 2000, 10 UT, Bastille Day flare event, using
spacecraft data from Yohkoh/HXT, Yohkoh/SXT, GOES, and TRACE. The
spatial structure of this double-ribbon flare consists of a
curved arcade with some 100 post-flare loops which brighten up in
a sequential manner from highly-sheared low-lying to less-sheared
higher-lying bipolar loops. We reconstruct an instrument-combined,
average differential emission measure distribution dEM(T)/dT that
ranges from T=1 MK to 40 MK and peaks at T<SUB>0</SUB>=10.9 MK. We
find that the time profiles of the different instrument fluxes peak
sequentially over 7 minutes with decreasing temperatures from T≈30 MK
to 1 MK, indicating the systematic cooling of the flare plasma. From
these temperature-dependent relative peak times t<SUB>peak</SUB>(T)
we reconstruct the average plasma cooling function T(t) for loops
observed near the flare peak time, and find that their temperature
decrease is initially controlled by conductive cooling during the
first 188 s, T(t)∼[1+(t/τ<SUB>cond</SUB>)]<SUP>−2/7</SUP>,
and then by radiative cooling during the next 592 s,
T(t)∼[1−(t/τ<SUB>rad</SUB>)]<SUP>3/5</SUP>. From the
radiative cooling phase we infer an average electron density of
n<SUB>e</SUB>=4.2×10<SUP>11</SUP> cm<SUP>−3</SUP>, which implies
a filling factor near 100% for the brightest observed 23 loops with
diameters of ∼1.8 Mm that appear simultaneously over the flare
peak time and are fully resolved with TRACE. We reproduce the time
delays and fluxes of the observed time profiles near the flare peak
self-consistently with a forward-fitting method of a fully analytical
model. The total integrated thermal energy of this flare amounts to
E<SUB>thermal</SUB>=2.6×10<SUP>31</SUP> erg.
---------------------------------------------------------
Title: Measurement of coronal magnetic twists during loop emergence
of NOAA 8069
Authors: Portier-Fozzani, F.; Aschwanden, M.; Démoulin, P.; Neupert,
W.; EIT Team; Delaboudinière, J. -P.
2001SoPh..203..289P Altcode:
Emerging coronal loops were studied with extreme ultraviolet
observations performed by SOHO/EIT on 5 and 6 August 1997 for NOAA
8069. Physical parameters (size and twist) were determined by a new
stereoscopic method. The flux tubes were measured twisted when first
observed by EIT. After emerging, they de-twisted as they expanded,
which corresponds to a minimization of the energy. Different scenarios
which take into account the conservation of the magnetic helicity are
discussed in relation with structure and temperature variations.
---------------------------------------------------------
Title: Revisiting the Determination of the Coronal Heating Function
from Yohkoh Data
Authors: Aschwanden, Markus J.
2001ApJ...559L.171A Altcode:
Results on the coronal heating function seem to strongly depend
on the employed type of multitemperature modeling along the line
of sight. Instruments with broadband temperature filters cause more
temperature confusion than those with narrowband temperature filters. A
possible bias of broadband filters is the hydrostatic weighting of
multitemperature loop systems, which mimic a temperature increase
with altitude and thus yield a preference for looptop heating. In this
Letter we revisit a loop system previously analyzed by Priest et al.,
for which they found that the coronal heating is likely to be uniform
in the temperature range between 1.6 and 2.2 MK. As an alternative
scenario, we use standard hydrostatic solutions here (with vanishing
conductive flux in the transition region). We show that hydrostatic
solutions with a uniform heating function throughout the corona and
transition region lead to unphysical solutions for the column depth and
the altitude of the loop footpoints, while a footpoint-heating model
yields acceptable physical solutions for a heating scale height of
s<SUB>H</SUB>~13+/-1 Mm. The positive temperature gradient with height
(which is also found in filter-ratio temperatures of other Yohkoh data)
can be explained by the hydrostatic weighting bias resulting from hot
loops (T<SUB>max</SUB>~2.6 MK) embedded in a cooler (T<SUB>max</SUB>~1.0
MK) background corona.
---------------------------------------------------------
Title: An Evaluation of Coronal Heating Models for Active Regions
Based on Yohkoh, SOHO, and TRACE Observations
Authors: Aschwanden, Markus J.
2001ApJ...560.1035A Altcode:
Recent soft X-ray and EUV data from space observations with Yohkoh,
the Solar and Heliospheric Observatory (SOHO), and the Transition Region
and Coronal Explorer (TRACE) established three important observational
constraints for coronal heating models: (1) coronal loops in active
regions have an overdensity that can be supplied only by upflows of
heated chromospheric plasma, (2) chromospheric upflows have been
observed frequently in coronal loops, and (3) the coronal heating
function has been localized in the lower corona within a height range
of λ<SUB>H</SUB><~10 Mm above the photosphere. Although these
three observational facts have been derived from active region loops,
the part of the solar corona that is topologically connected to active
regions makes up >~80% of the heating energy requirement (at a
typical day around the maximum of the solar cycle) and thus constitutes
the majority of the energy budget of the coronal heating problem at
large. We discuss and compare a comprehensive set of theoretical models
of coronal heating under the aspect of whether they can satisfy these
observational constraints. We find that conventional direct current
(DC) and alternating current (AC) coronal heating models that consider
coronal loops as homogeneous flux tubes (in density and temperature) do
not predict these observed effects, while refined models that include
gravity and the transition region can reproduce them. In particular,
magnetic reconnection models that spawn chromospheric evaporation
satisfy the observational constraints the easiest. Our main conclusion
is that the coronal heating problem can be solved only by tapping
energization processes in the chromosphere and transition region.
---------------------------------------------------------
Title: Astrophysics in 2000
Authors: Trimble, Virginia; Aschwanden, Markus J.
2001PASP..113.1025T Altcode:
It was a year in which some topics selected themselves as important
through the sheer numbers of papers published. These include the
connection(s) between galaxies with active central engines and galaxies
with starbursts, the transition from asymptotic giant branch stars
to white dwarfs, gamma-ray bursters, solar data from three major
satellite missions, and the cosmological parameters, including dark
matter and very large scale structure. Several sections are oriented
around processes-accretion, collimation, mergers, and disruptions-shared
by a number of kinds of stars and galaxies. And, of course, there are
the usual frivolities of errors, omissions, exceptions, and inventories.
---------------------------------------------------------
Title: An Assessment of Coronal Heating Models based on Yohkoh,
SoHO, and TRACE Observations
Authors: Aschwanden, M. J.
2001AGUSM..SP61A08A Altcode:
Recent soft X-ray and EUV data from space observations with Yohkoh,
SoHO, and TRACE established three important observational constraints
for coronal heating models: (1) Coronal loops have an overdensity that
can only be supplied by upflows of heated chromospheric plasma, (2)
chromospheric upflows have been observed in a large number of coronal
loops, and (3) the coronal heating function has been localized in the
lower corona within a height range of λ<SUB>H</SUB> <~ 10-20 Mm
above the photosphere. We discuss and compare a comprehensive set of
theoretical models on coronal heating under the aspect whether they can
satisfy these observational constraints. We find that conventional DC
and AC coronal heating models that consider coronal loops as homogeneous
flux tubes (in density and temperature), detached from the transition
region, do not predict these observed effects, while refined models that
include gravity and chromospheric models can reproduce the observed
effects. Alternatively, most of the magnetic reconnection models
previously used for flares, can reproduce the observed effects, because
heating of the chromospheric plasma at the loop footpoints is involved
with subsequent chromospheric evaporation like in flares. Therefore,
coronal magnetic reconnection processes that work with lower heating
efficiency than in flares, as well as chromospheric and photospheric
reconnection processes should be considered as serious candidates for
coronal heating models. Our main conclusion is that the coronal heating
problem cannot be solved without including dynamic processes (and
related heating effects) in the chromosphere and transition region zone.
---------------------------------------------------------
Title: Critical Issues of Nanoflare Statistics
Authors: Aschwanden, M. J.
2001AGUSM..SP52B08A Altcode:
The accuracy of solar microflare and nanoflare statistics became
a critical issue because the extrapolation of their power-law
distribution at the lower flux or energy threshold is often used as
estimate of their total energy budget to meet the coronal heating
requirement. We review and quantify a number of aspects that play a
role in the determination of their frequency distribution: (1) event
definition and discrimination, (2) sampling completeness, (3) observing
cadence and exposure times, (4) pattern recognition algorithms, (5)
density and energy model, (6) line-of-sight integration, (7) physical
parameter limits, (8) wavelength dependence, (9) deviations from
power-law functions, and (10) error estimates of power-law slopes. We
discuss critical issues in the concept of event definitions, which
includes flares, subflare bursts, radio bursts, coherent as well as
intermittent temporal fine structure. Finally we review critical issues
in physical concepts of nanoflares, which range from Parker's coronal
tangential discontinuities down to miniature flare loops observed in
EUV in the transition region. We conclude that power-law slopes from
different data analyses cannot be properly compared without taking into
account the involved measurement biases and theoretical model concepts.
---------------------------------------------------------
Title: Modeling of Coronal EUV Loops Observed with
TRACE. I. Hydrostatic Solutions with Nonuniform Heating
Authors: Aschwanden, Markus J.; Schrijver, Carolus J.; Alexander, David
2001ApJ...550.1036A Altcode:
Recent observations of coronal loops in EUV wavelengths with
the Transition Region and Coronal Explorer (TRACE) and the
Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and
Heliospheric Observatory (SOHO) demonstrated three new results that
cannot be explained by most of the existing loop models: (1) EUV loops
are near-isothermal along their coronal segments, (2) they show an
overpressure or overdensity compared with the requirements of steady
state loops with uniform heating, and (3) the brightest EUV loops
exhibit extended scale heights up to 4 times the hydrostatic scale
height. These observations cannot be reconciled with the classical RTV
(Rosner, Tucker, & Vaiana) model, they do not support models with
uniform heating, and they even partially violate the requirements
of hydrostatic equilibrium. In this study we are fitting for the
first time steady state solutions of the hydrodynamic equations to
observed intensity profiles, permitting a detailed consistency test of
the observed temperature T(s) and density profiles n<SUB>e</SUB>(s)
with steady state models, which was not possible in previous studies
based on scaling laws. We calculate some 500 hydrostatic solutions,
which cover a large parameter space of loop lengths (L~4-300 Mm), of
nonuniform heating functions (with heating scale heights in the range
of λ<SUB>H</SUB>~1-300 Mm), approaching also the limit of uniform
heating (λ<SUB>H</SUB>>>L). The parameter space can be subdivided
into three regimes, which contain (1) solutions of stably stratified
loops, (2) solutions of unstably stratified loops (in the case of
short heating scale heights, λ<SUB>H,Mm</SUB>~L<SUB>Mm</SUB>),
and (3) a regime in which we find no numerical solutions (when
λ<SUB>H,Mm</SUB><~L<SUB>Mm</SUB>). Fitting the hydrostatic
solutions to 41 EUV loops observed with TRACE (selected by the
criterion of detectability over their entire length), we find that
only 30% of the loops are consistent with hydrostatic steady state
solutions. None of the observed EUV loops is consistent with a uniform
heating function while in quasi-steady state. Those loops compatible
with a steady state are found to be heated near the footpoints, with a
heating scale height of λ<SUB>H</SUB>=12+/-5 Mm, covering a fraction
λ<SUB>H</SUB>/L=0.2+/-0.1 of the loop length. These results support
coronal heating mechanisms operating in or near the chromosphere and
transition region.
---------------------------------------------------------
Title: Temperature Tomography of the Soft X-Ray Corona: Measurements
of Electron Densities, Tempuratures, and Differential Emission
Measure Distributions above the Limb
Authors: Aschwanden, Markus J.; Acton, Loren W.
2001ApJ...550..475A Altcode:
We analyze long-exposure and off-pointing Yohkoh/SXT data of
the solar corona observed on 1992 August 26. We develop a new
(temperature) tomography method that is based on a forward-fitting
method of a four-parameter model to the observed soft X-ray fluxes
F<SUB>1</SUB>(h) and F<SUB>2</SUB>(h) of two SXT wavelength filters
as a function of height h. The model is defined in terms of a
differential emission measure (DEM) distribution dEM(h, T)/dT,
which includes also a temperature dependence of density scale
heights λ<SUB>n</SUB>(T)=q<SUB>λ</SUB>λ<SUB>T</SUB> and allows
us to quantify deviations (q<SUB>λ</SUB>≠1) from hydrostatic
equilibrium (q<SUB>λ</SUB>=1). This parametrization facilitates a
proper line-of-sight integration and relates the widely used filter
ratio temperature T<SUB>FR</SUB> to the peak of the DEM distribution. A
direct consequence of the multi-scale height atmosphere is that the
filter ratio temperature T<SUB>FR</SUB>(h) is predicted to increase
with height, even if all magnetic field lines are isothermal. Our
model fitting reveals that coronal holes and quiet-Sun regions are
in perfect hydrostatic equilibrium but that coronal streamers have
a scale height that exceeds the hydrostatic scale height by a factor
of up to q<SUB>λ</SUB><~2.3, which underscores the dynamic nature
of coronal streamers. Our density measurements in coronal holes are
slightly lower than most of the white-light polarized brightness
inversions and seem to come closer to the requirements of solar wind
models. Our DEM model provides also a physical framework for the
semiempirical Baumbach-Allen formula and quantifies the temperature
ranges and degree of hydrostaticity of the K, L, and F coronae.
---------------------------------------------------------
Title: The New Solar Corona
Authors: Aschwanden, Markus J.; Poland, Arthur I.; Rabin, Douglas M.
2001ARA&A..39..175A Altcode:
We focus on new observational capabilities (Yohkoh, SoHO,
TRACE), observations, modeling approaches, and insights into
physical processes of the solar corona. The most impressive new
results and problems discussed in this article can be appreciated
from the movies available on the Annual Reviews website and at
http://www.lmsal.com/pub/araa/araa.html. "The Sun is new each
day." Heraclites (ca 530-475 BC) "Everything flows." Heraclites (ca
530-475 BC)
---------------------------------------------------------
Title: Solar Flares: Nonthermal Electrons
Authors: Aschwanden, M.
2000eaa..bookE2291A Altcode:
Solar flares energize particles to different levels, usually
subdivided into (1) thermal, (2) nonthermal or energetic and (3) high
energetic (figure 1). The lowest energetic level is a thermal particle
distribution (where particles have a Maxwellian velocity distribution),
produced by Coulomb collisions between the particles of the heated
flare plasma. Because the coronal plasma is heated to ...
---------------------------------------------------------
Title: Evidence for Nonuniform Heating of Coronal Loops Inferred
from Multithread Modeling of TRACE Data
Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Alexander,
David
2000ApJ...541.1059A Altcode:
The temperature T<SUB>e</SUB>(s) and density structure n<SUB>e</SUB>(s)
of active region loops in EUV observed with TRACE is modeled with a
multithread model, synthesized from the summed emission of many loop
threads that have a distribution of maximum temperatures and that
satisfy the steady state Rosner-Tucker-Vaiana (RTV) scaling law,
modified by Serio et al. for gravitational stratification (called
RTVS<SUB>p</SUB> in the following). In a recent Letter, Reale &
Peres demonstrated that this method can explain the almost isothermal
appearance of TRACE loops (observed by Lenz et al.) as derived from the
filter-ratio method. From model-fitting of the 171 and 195 Å fluxes
of 41 loops, which have loop half-lengths in the range of L=4-320
Mm, we find that (1) the EUV loops consist of near-isothermal loop
threads with substantially smaller temperature gradients than are
predicted by the RTVS<SUB>p</SUB> model; (2) the loop base pressure,
p<SUB>0</SUB>~0.3+/-0.1 dynes cm<SUP>-2</SUP>, is independent of
the loop length L, and it agrees with the RTVS<SUB>p</SUB> model
for the shortest loops but exceeds the RTVS<SUB>p</SUB> model up
to a factor of 35 for the largest loops; and (3) the pressure scale
height is consistent with hydrostatic equilibrium for the shortest
loops but exceeds the temperature scale height up to a factor of ~3
for the largest loops. The data indicate that cool EUV loops in the
temperature range of T<SUB>e</SUB>~0.8-1.6 MK cannot be explained with
the static steady state RTVS<SUB>p</SUB> model in terms of uniform
heating but are fully consistent with Serio's model in the case of
nonuniform heating (RTVS<SUB>ph</SUB>), with heating scale heights in
the range of s<SUB>H</SUB>=17+/-6 Mm. This heating function provides
almost uniform heating for small loops (L<~20 Mm), but restricts
heating to the footpoints of large loops (L~50-300 Mm).
---------------------------------------------------------
Title: Time Variability of the “Quiet” Sun Observed with
TRACE. II. Physical Parameters, Temperature Evolution, and Energetics
of Extreme-Ultraviolet Nanoflares
Authors: Aschwanden, Markus J.; Tarbell, Ted D.; Nightingale, Richard
W.; Schrijver, Carolus J.; Title, Alan; Kankelborg, Charles C.;
Martens, Piet; Warren, Harry P.
2000ApJ...535.1047A Altcode:
We present a detailed analysis of the geometric and physical
parameters of 281 EUV nanoflares, simultaneously detected with the
TRACE telescope in the 171 and 195 Å wavelengths. The detection and
discrimination of these flarelike events is detailed in the first paper
in this series. We determine the loop length l, loop width w, emission
measure EM, the evolution of the electron density n<SUB>e</SUB>(t) and
temperature T<SUB>e</SUB>(t), the flare decay time τ<SUB>decay</SUB>,
and calculate the radiative loss time τ<SUB>loss</SUB>, the conductive
loss time τ<SUB>cond</SUB>, and the thermal energy E<SUB>th</SUB>. The
findings are as follows: (1) EUV nanoflares in the energy range of
10<SUP>24</SUP>-10<SUP>26</SUP> ergs represent miniature versions
of larger flares observed in soft X-rays (SXR) and hard X-rays
(HXR), scaled to lower temperatures (T<SUB>e</SUB><~2 MK),
lower densities (n<SUB>e</SUB><~10<SUP>9</SUP> cm<SUP>-3</SUP>),
and somewhat smaller spatial scales (l~2-20 Mm). (2) The cooling
time τ<SUB>decay</SUB> is compatible with the radiative cooling
time τ<SUB>rad</SUB>, but the conductive cooling timescale
τ<SUB>cond</SUB> is about an order of magnitude shorter, suggesting
repetitive heating cycles in time intervals of a few minutes. (3)
The frequency distribution of thermal energies of EUV nanoflares,
N(E)~10<SUP>-46</SUP>(E/10<SUP>24</SUP>)<SUP>-1.8</SUP> (s<SUP>-1</SUP>
cm<SUP>-2</SUP> ergs<SUP>-1</SUP>) matches that of SXR microflares
in the energy range of 10<SUP>26</SUP>-10<SUP>29</SUP>, and exceeds
that of nonthermal energies of larger flares observed in HXR by a
factor of 3-10 (in the energy range of 10<SUP>29</SUP>-10<SUP>32</SUP>
ergs). Discrepancies of the power-law slope with other studies, which
report higher values in the range of a=2.0-2.6 (Krucker & Benz;
Parnell & Jupp), are attributed to methodical differences in the
detection and discrimination of EUV microflares, as well as to different
model assumptions in the calculation of the electron density. Besides
the insufficient power of nanoflares to heat the corona, we find also
other physical limits for nanoflares at energies <~10<SUP>24</SUP>
ergs, such as the area coverage limit, the heating temperature limit,
the lower coronal density limit, and the chromospheric loop height
limit. Based on these quantitative physical limitations, it appears
that coronal heating requires other energy carriers that are not
luminous in EUV, SXR, and HXR.
---------------------------------------------------------
Title: Time Variability of the “Quiet” Sun Observed with
TRACE. I. Instrumental Effects, Event Detection, and Discrimination
of Extreme-Ultraviolet Microflares
Authors: Aschwanden, Markus J.; Nightingale, Richard W.; Tarbell,
Ted D.; Wolfson, C. J.
2000ApJ...535.1027A Altcode:
The Transition Region and Coronal Explorer (TRACE) observed a
“quiet-Sun” region on 1999 February 17 from 02:15 UT to 3:00 UT
with full resolution (0.5" pixel size), high cadence (125 s), and deep
exposures (65 and 46 s) in the 171 Å and 195 Å wavelengths. We start
our investigation of the time variability of “quiet-Sun” images
with a detailed analysis of instrumental and nonsolar effects, such
as orbital temperature variations, filtering of particle radiation
spikes, spacecraft pointing drift, and solar rotation tracking. We
quantify the magnitude of various noise components (photon Poisson
statistics, data digitization, data compression, and readout noise)
and establish an upper limit for the data noise level, above which
temporal variability can safely be attributed to solar origin. We
develop a pattern recognition code that extracts spatiotemporal events
with significant variability, yielding a total of 3131 events in 171 Å
and 904 events in 195 Å. We classify all 904 events detected in 195 Å
according to flarelike characteristics and establish a numerical flare
criterion based on temporal, spatial, and dynamic cross-correlation
coefficients between the two observed temperatures (0.9 and 1.4
MK). This numerical criterion matches the visual flare classification
in 83% of the cases and can be used for automated flare search. Using
this flare discrimination criterion we find that only 35% (and 25%) of
the events detected in 171 (and 195) Å represent flarelike events. The
discrimination of flare events leads to a frequency distribution of
peak fluxes, N(ΔF)~ΔF<SUP>-1.83+/-0.07</SUP> at 195 Å, which is
significantly flatter than the distribution of all events. A sensitive
discrimination criterion of flare events is therefore important for
microflare statistics and for conclusions on their occurrence rate
and efficiency for coronal heating.
---------------------------------------------------------
Title: Multi-Thread Modeling of Coronal Loops with TRACE Data
Authors: Nightingale, R. W.; Aschwanden, M. J.; Alexander, D.; Reale,
F.; Peres, G.
2000SPD....31.0211N Altcode: 2000BAAS...32..812N
The temperature T<SUB>e(s)</SUB> and density structure n<SUB>e(s)</SUB>
of active region loops in EUV observed with TRACE is modeled with a
multi-thread model. The model loops are synthesized from the summed
emission of many loop threads that have a distribution of maximum
temperatures and that satisfy the steady-state Rosner-Tucker-Vaiana
(RTV) scaling law, modified by Serio et al. for gravitational
stratification (RTVS<SUB>p</SUB>). From model-fitting of the 171 and
195 Angstroms fluxes of 41 loops, which have loop half lengths in the
range of L=4-320 Mm, we find: (1) The EUV loops can be explained by
near-isothermal loop threads in the temperature range of T<SUB>e</SUB>
~ 0.8-1.6 MK with substantially smaller temperature gradients than
predicted by the RTVS<SUB>p</SUB> model, (2) the loop base pressure,
p<SUB>0</SUB> ~ 0.3+/- 0.1 dyne cm<SUP>-2</SUP>, is independent of
the loop length L, it agrees with the RTVS<SUB>p</SUB> model for
the shortest loops, but exceeds the RTVS<SUB>p</SUB> model up to a
factor of 35 for the largest loops, and (3) the pressure scale height
is consistent with hydrostatic equilibrium for the shortest loops,
but exceeds the temperature scale height up to a factor of ~ 3 for
the largest loops. This work was supported by the TRACE project at
LMSAL (contract NAS5-38099). Ref.: Aschwanden,M.J., Nightingale,R.W.,
Alexander,D., Reale,F., and Peres,G. 2000, ApJ, subm., “Evidence for
Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling
of TRACE Data”, URL="ftp://sag.lmsal.com/pub/aschwand/2000_reale.ps.gz"
---------------------------------------------------------
Title: What TRACE Observations tell us about Heating of Coronal Loops
Authors: Aschwanden, M. J.; Nightingale, R. W.; Alexander, D.; Reale,
F.; Peres, G.
2000SPD....31.0210A Altcode: 2000BAAS...32..812A
We analyzed the temperature T<SUB>e(s)</SUB> and density structure
n<SUB>e(s)</SUB> of active region loops in EUV observed with TRACE (see
SPD abstract by Nightingale et al.). The observational data indicate
that cool EUV loops with maximum temperatures of T<SUB>max</SUB> ~
0.8-1.6 MK cannot be explained with the static steady-state scaling
law of Rosner, Tucker, & Vaiana (1978) or Serio et al. (1981),
in terms of uniform heating. However, they are fully consistent with
Serio's model (which includes gravitation and a heating scale height)
in the case of nonuniform heating, with heating scale heights in the
range of s<SUB>H=17</SUB> +/- 6 Mm. This heating function provides
almost uniform heating for small loops (L < 20 Mm), but restricts
heating to the footpoints of large loops (L ~ 50-300 Mm). Another
observational result of cool EUV loops is that the pressure scale
height exceeds the hydrostatic scale height by a factor of q<SUB>l
~</SUB> 1-3. This suggests that the pressure balance of these EUV
loops may not be governed by hydrostatic equilibrium, but rather
indicates a non-steady state, e.g. caused by dynamic mass flows and/or
intermittent heating. Chromospheric upflows may explain the extended
scale heights as well as the quasi-isothermal temperature structure
of EUV loops. We review and discuss the differences between cool (T ~
1-2 MK) EUV loops and hot (T ~ 2-8 MK) soft X-ray loops concerning
loop scaling laws, radiative equilibrium, hydrostatic equilibrium,
and heating function. - This work is supported by the TRACE project at
LMSAL (contract NAS5-38099) Ref.: Aschwanden,M.J., Nightingale,R.W.,
Alexander,D., Reale,F., and Peres,G. 2000, ApJ, subm., “Evidence for
Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling
of TRACE Data”
---------------------------------------------------------
Title: Summary of Posters on Solar Physics
Authors: Aschwanden, M. J.
2000IAUS..195..447A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Effect of Hydrostatic Weighting on the Vertical Temperature
Structure of the Solar Corona
Authors: Aschwanden, Markus J.; Nitta, Nariaki
2000ApJ...535L..59A Altcode: 2000astro.ph..4093A
We investigate the effect of hydrostatic scale heights λ(T)
in coronal loops on the determination of the vertical temperature
structure T(h) of the solar corona. Every method that determines an
average temperature at a particular line of sight from optically
thin emission (e.g., in EUV or soft X-ray wavelengths) of a
mutlitemperature plasma is subject to the emission measure-weighted
contributions dEM(T)/dT from different temperatures. Because most
of the coronal structures (along open or closed field lines) are
close to hydrostatic equilibrium, the hydrostatic temperature scale
height introduces a height-dependent weighting function that causes
a systematic bias in the determination of the temperature structure
T(h) as function of altitude h. The net effect is that the averaged
temperature seems to increase with altitude, dT(h)/dh>0, even if
every coronal loop (of a multitemperature ensemble) is isothermal in
itself. We simulate this effect with differential emission measure
distributions observed by SERTS for an instrument with a broadband
temperature filter such as Yohkoh/Soft X-Ray Telescope and find that
the apparent temperature increase due to hydrostatic weighting is
of order ΔT~T<SUB>0</SUB>h/r<SUB>solar</SUB>. We suggest that this
effect largely explains the systematic temperature increase in the
upper corona reported in recent studies (e.g., by Sturrock et al.,
Wheatland et al., or Priest et al.), rather than being an intrinsic
signature of a coronal heating mechanism.
---------------------------------------------------------
Title: Electron Trapping and Precipitation in Asymmetric Solar
Flare Loops
Authors: Aschwanden, M. J.; Fletcher, L.; Sakao, T.; Kosugi, T.;
Hudson, H.
2000IAUS..195..375A Altcode:
Acceleration, propagation, and energy loss of particles energized in
solar flares cannot be studied separately because their radiative
signatures observed in the form of hard X-ray bremsstrahlung or
radio gyrosynchrotron emission represent a convolution of all these
processes. We analyze hard X-ray emission from solar flares using
a kinematic model that includes free-streaming electrons (having an
energy-dependent time-of-flight delay) as well as temporarily trapped
electrons (which are pitch-angle scattered by Coulomb collisional
scattering) to determine various physical parameters (trapping times,
flux asymmetry, loss-cone angles, magnetic mirror ratios) in flare
loops with asymmetric magnetic fields.
---------------------------------------------------------
Title: Electron Kinematics near the Loss-Cone
Authors: Fletcher, L.; Aschwanden, M. J.
2000SPD....31.0247F Altcode: 2000BAAS...32..819F
With the upcoming launch of the HESSI satellite, we expect that
problems of non-thermal electron transport and radiation signatures
will once more be the subject of some attention, since this is an
integral part of the calculation of the spectral and spatial behavior
of the radiative signatures which will be observed by HESSI. Problems
of particle transport in coronal magnetic traps are often treated by
making simple geometrical and timescale arguments for the fractions of
accelerated particles which are trapped and precipitate from coronal
loops. Such arguments are used to calculate the populations of,
for example, directly precipitating and trap-precipitating particles
(which can in principle be identified from hard X-ray time-series),
or coronal versus footpoint emission ratios (which can be studied from
spatially resolved HXR data). Using numerical simulation and analytic
arguments we have studied the dynamics of particles within coronal
traps, paying particular attention to the behavior in the vicinity
of the loss-cone. We find that over a broad range of normally-assumed
coronal parameters, such as mirror-ratio, loop length and loop density,
(a) electrons cannot pass easily from the trap region to the loss-cone,
so that (b) there is no collisionless trap-precipitating component and
(c) a large fraction of accelerated particles will lose their entire
energy budget within the coronal loop. We discuss what this means
for our current understanding of the solar flare environment and our
interpretation of radiative signatures. This work was supported by
the Yohkoh/SXT project at LMSAL (NASA grant NAS8-40801) and by the
U.K. Particle Physics and Astronomy Research Council.
---------------------------------------------------------
Title: Astrophysics in 1999
Authors: Trimble, Virginia; Aschwanden, Markus J.
2000PASP..112..434T Altcode:
The year 1999 saw the arrival of a star with three planets, a universe
with three parameters, and a solar corona that could be heated at
least three ways. In addition, there were at least three papers on
every question that has ever been asked in astrophysics, from “Will
the Universe expand forever?” to “Does mantle convection occur in
one or two layers?” The answers generally were, “Yes,” “No,” and
“None of the above,” to each of the questions. The authors have done
their best to organize the richness around centers defined by objects,
methods, and madnesses.
---------------------------------------------------------
Title: Three-dimensional Stereoscopic Analysis of Solar Active Region
Loops. II. SOHO/EIT Observations at Temperatures of 1.5-2.5 MK
Authors: Aschwanden, Markus J.; Alexander, David; Hurlburt, Neal;
Newmark, Jeffrey S.; Neupert, Werner M.; Klimchuk, J. A.; Gary,
G. Allen
2000ApJ...531.1129A Altcode:
In this paper we study the three-dimensional structure of hot
(T<SUB>e</SUB>~1.5-2.5 MK) loops in solar active region NOAA
7986, observed on 1996 August 30 with the Extreme-ultraviolet
Imaging Telescope (EIT) on board the Solar and Heliospheric
Observatory (SOHO). This complements a first study (Paper I) on
cooler (T<SUB>e</SUB>~1.0-1.5 MK) loops of the same active region,
using the same method of Dynamic Stereoscopy to reconstruct the
three-dimensional geometry. We reconstruct the three-dimensional
coordinates x(s), y(s), z(s), the density n<SUB>e</SUB>(s), and
temperature profile T<SUB>e</SUB>(s) of 35 individual loop segments
(as a function of the loop coordinate s) using EIT 195 and 284 Å
images. The major findings are as follows. (1) All loops are found
to be in hydrostatic equilibrium, in the entire temperature regime
of T<SUB>e</SUB>=1.0-2.5 MK. (2) The analyzed loops have a height of
2-3 scale heights, and thus only segments extending over about one
vertical scale height have sufficient emission measure contrast for
detection. (3) The temperature gradient over the lowest scale height
is of order dT/ds~1-10 K km<SUP>-1</SUP>. (4) The radiative loss
rate is found to exceed the conductive loss rate by about two orders
or magnitude in the coronal loop segments, implying that the loops
cannot be in quasi-static equilibrium, since standard steady-state loop
models show that radiative and conductive losses are comparable. (5) A
steady state could only be maintained if the heating rate E<SUB>H</SUB>
matches exactly the radiative loss rate in hydrostatic equilibrium,
requiring a heat deposition length λ<SUB>H</SUB> of the half density
scale height λ. (6) We find a correlation of p~L<SUP>-1</SUP> between
loop base pressure and loop length, which is not consistent with the
scaling law predicted from steady-state models of large-scale loops. All
observational findings indicate consistently that the energy balance
of the observed EUV loops cannot be described by steady-state models.
---------------------------------------------------------
Title: Hard X-Ray Timing Experiments with HESSI
Authors: Aschwanden, M. J.
2000ASPC..206..197A Altcode: 2000hesp.conf..197A
No abstract at ADS
---------------------------------------------------------
Title: Quadrupolar Magnetic Reconnection in Solar
Flares. I. Three-dimensional Geometry Inferred from Yohkoh
Observations
Authors: Aschwanden, Markus J.; Kosugi, Takeo; Hanaoka, Yoichiro;
Nishio, Masanori; Melrose, Donald B.
1999ApJ...526.1026A Altcode:
We analyze the three-dimensional geometry of solar flares that show
so-called interacting flare loops in soft X-ray, hard X-ray, and radio
emission, as previously identified by Hanaoka and Nishio. The two flare
loops that appear brightest after the flare are assumed to represent
the outcome of a quadrupolar magnetic reconnection process, during
which the connectivity of magnetic polarities is exchanged between
the four loop footpoints. We parameterize the three-dimensional
geometry of the four involved magnetic field lines with circular
segments, additionally constrained by the geometric condition that
the two pre-reconnection field lines have to intersect each other
at the onset of the reconnection process, leading to a 10 parameter
model. We fit this 10 parameter model to Yohkoh Soft and Hard X-Ray
Telescopes (SXT and HXT) data of 10 solar flares and determine in
this way the loop sizes and relative orientation of interacting field
lines before and after reconnection. We apply a flare model by Melrose
to calculate the magnetic flux transfer and energy released when two
current-carrying field lines reconnect to form a new current-carrying
system in a quadrupolar geometry. The findings and conclusions are
the following. (1) The pre-reconnection field lines always show
a strong asymmetry in size, consistent with the scenario of newly
emerging small-scale loops that reconnect with preexisting large-scale
loops. (2) The relative angle between reconnecting field lines is
nearly collinear in half of the cases, and nearly perpendicular in
the other half, contrary to the antiparallel configuration that is
considered to be most efficient for magnetic reconnection. (3) The
angle between interacting field lines is reduced by ~10°-50° after
quadrupolar reconnection. (4) The small-scale flare loop experiences a
shrinkage by a factor of 1.31+/-0.44, which is consistent with the
scaling law found from previous electron time-of-flight measurements,
suggesting that electron acceleration occurs near the cusp of
quadrupolar configurations. (5) The large-scale loop is found
to dominate the total induction between current-carrying loops,
providing a simple estimate of the maximum magnetic energy available
for flare energy release because of current transfer, which scales
as ΔE<SUP>I</SUP>~10<SUP>29.63</SUP>(r<SUB>2</SUB>/10<SUP>9</SUP>
cm)(I<SUB>2</SUB>/10<SUP>11</SUP>A)<SUP>2</SUP> (with r<SUB>2</SUB> the
curvature radius and I<SUB>2</SUB> the current of the large-scale loop)
and is found to correlate with observed flare energies deduced from
soft X-ray and hard X-ray fluxes. Most of the energy is transferred
to small-scale loops that have one-half of the large-scale current
(I<SUB>1</SUB>=I<SUB>2</SUB>/2). (6) The quadrupolar reconnection
geometry provides also a solution of Canfield's dilemma of the offset
between the maximum of vertical currents and the HXR flare loop
footpoints. (7) The quadrupolar geometry provides not only a framework
for interacting double-loop flares, but it can also be considered as
a generalized version of (cusp-shaped) single-loop flares.
---------------------------------------------------------
Title: Time Variability of EUV Brightenings in Coronal Loops Observed
with TRACE
Authors: Nightingale, Richard W.; Aschwanden, Markus J.; Hurlburt,
Neal E.
1999SoPh..190..249N Altcode:
We analyze coronal loops in active region 8272, observed with TRACE on
23 July 1998 during a 70-min interval with a cadence of 1.5 min, in the
temperature range of T≈0.9-1.6 MK. We focus on a compact dipolar loop
system with a linear size of ≈30 000 km. In this compact loop system
we detect about 20 EUV brightenings at any instant of time and in each
wavelength. We measure the centroid position of these EUV brightenings
and determine their cospatiality in subsequent time frames. We find
that EUV brightenings are not cospatial in subsequent time intervals
(Δt=90 s), but are almost randomly distributed in space and time.
---------------------------------------------------------
Title: Do EUV Nanoflares Account for Coronal Heating?
Authors: Aschwanden, Markus J.
1999SoPh..190..233A Altcode:
Recent observations with EUV imaging instruments such
as SOHO/EIT and TRACE have shown evidence for flare-like
processes at the bottom end of the energy scale, in the range of
E<SUB>th</SUB>≈10<SUP>24</SUP>-10<SUP>27</SUP> erg. Here we compare
these EUV nanoflares with soft X-ray microflares and hard X-ray flares
across the entire energy range. From the observations we establish
empirical scaling laws for the flare loop length, L(T)∼T, the electron
density, n<SUB>e</SUB>(T)∼T<SUP>2</SUP>, from which we derive scaling
laws for the loop pressure, p(T)∼T<SUP>3</SUP>, and the thermal
energy, E<SUB>th</SUB>∼T<SUP>6</SUP>. Extrapolating these scaling
laws into the picoflare regime we find that the pressure conditions in
the chromosphere constrain a height level for flare loop footpoints,
which scales with h<SUB>eq</SUB>(T)∼T<SUP>−0.5</SUP>. Based
on this chromospheric pressure limit we predict a lower cutoff of
flare loop sizes at L<SUB>∖min</SUB>≲5 Mm and flare energies
E<SUB>∖min</SUB>≲10<SUP>24</SUP> erg. We show evidence for such a
rollover in the flare energy size distribution from recent TRACE EUV
data. Based on this energy cutoff imposed by the chromospheric boundary
condition we find that the energy content of the heated plasma observed
in EUV, SXR, and HXR flares is insufficient (by 2-3 orders of magnitude)
to account for coronal heating.
---------------------------------------------------------
Title: Radio and Hard X-ray Observations of Flares and their Physical
Interpretation
Authors: Aschwanden, M.
1999spro.proc..307A Altcode:
We review a selection of observations in radio, hard X-rays (HXR) and
soft X-rays (SXR) that constrain geometrical and physical requirements
for solar flare models. Guided by observations of interacting flare
loops we discuss a flare model based on shear-driven quadrupolar
reconnection, which explains single-loop and double-loop flares in
a unified picture. We interpret various observational findings in
the light of this unified flare model: - topology and geometry of
interacting flare loops, - localization of particle acceleration
region, - scale invariance of electron time-of-flight path and
flare loop geometry, - density and magnetic field diagnostic in
acceleration region, - bi-directionality of injected electron beams,
- electron beam trajectories and correlated HXR pulses, - bifurcation
of directly-precipitating and trap-precipitating electrons, - density
and magnetic field diagnostic of trap region, - elementary time scales
and dynamics in acceleration region.
---------------------------------------------------------
Title: Particle Acceleration and Kinematics in Solar Flares and the
Solar Corona
Authors: Aschwanden, M. J.
1999ESASP.448.1015A Altcode: 1999ESPM....9.1015A; 1999mfsp.conf.1015A
No abstract at ADS
---------------------------------------------------------
Title: Coronal Loop Oscillations Observed with the Transition Region
and Coronal Explorer
Authors: Aschwanden, Markus J.; Fletcher, Lyndsay; Schrijver, Carolus
J.; Alexander, David
1999ApJ...520..880A Altcode:
We report here, for the first time, on spatial oscillations of coronal
loops, which were detected in extreme-ultraviolet wavelengths (171 Å)
with the Transition Region and Coronal Explorer, in the temperature
range of T<SUB>e</SUB>~1.0-1.5 MK. The observed loop oscillations
occurred during a flare that began at 1998 July 14, 12:55 UT and are
most prominent during the first 20 minutes. The oscillating loops
connect the penumbra of the leading sunspot to the flare site in
the trailing portion. We identified five oscillating loops with an
average length of L=130,000+/-30,000 km. The transverse amplitude
of the oscillations is A=4100+/-1300 km, and the mean period
is T=280+/-30 s. The oscillation mode appears to be a standing
wave mode (with fixed nodes at the footpoints). We investigate
different MHD wave modes and find that the fast kink mode with a
period τ=205(L/10<SUP>10</SUP> cm)(n<SUB>e</SUB>/10<SUP>9</SUP>
cm<SUP>-3</SUP>)<SUP>1/2</SUP>(B/10 G)<SUP>-1</SUP> s provides the
best agreement with the observed period. We propose that the onset
of loop oscillations in distant locations is triggered by a signal or
disturbance that propagates from the central flare site with a radial
speed of ~700 km s<SUP>-1</SUP>. Because the observed loop oscillation
periods are comparable to photospheric 5 minute oscillations, a resonant
coupling between the two systems is possible. We further find evidence
for global extreme-UV dimming in the entire active region possibly
associated with a coronal mass ejection.
---------------------------------------------------------
Title: Deconvolution of Directly Precipitating and Trap-precipitating
Electrons in Solar Flare Hard X-Rays. III.Yohkoh Hard X-Ray Telescope
Data Analysis
Authors: Aschwanden, Markus J.; Fletcher, Lyndsay; Sakao, Taro;
Kosugi, Takeo; Hudson, Hugh
1999ApJ...517..977A Altcode:
We analyze the footpoint separation d and flux asymmetry A of
magnetically conjugate double footpoint sources in hard X-ray images
from the Yohkoh Hard X-Ray Telescope (HXT). The data set of 54 solar
flares includes all events simultaneously observed with the Compton
Gamma Ray Observatory (CGRO) in high time resolution mode. From the CGRO
data we deconvolved the direct-precipitation and trap-precipitation
components previously (in Paper II). Using the combined measurements
from CGRO and HXT, we develop an asymmetric trap model that allows
us to quantify the relative fractions of four different electron
components, i.e., the ratios of direct-precipitating (q<SUB>P1</SUB>,
q<SUB>P2</SUB>) and trap-precipitating electrons (q<SUB>T1</SUB>,
q<SUB>T2</SUB>) at both magnetically conjugate footpoints. We find mean
ratios of q<SUB>P1</SUB>=0.14+/-0.06, q<SUB>P2</SUB>=0.26+/-0.10,
and q<SUB>T</SUB>=q<SUB>T1</SUB>+q<SUB>T2</SUB>=0.60+/-0.13. We
assume an isotropic pitch-angle distribution at the
acceleration site and double-sided trap precipitation
(q<SUB>T2</SUB>/q<SUB>T1</SUB>=q<SUB>P2</SUB>/q<SUB>P1</SUB>)
to determine the conjugate loss-cone angles
(α<SUB>1</SUB>=42<SUP>deg</SUP>+/-11<SUP>deg</SUP> and
α<SUB>2</SUB>=52<SUP>deg</SUP>+/-10<SUP>deg</SUP>) and magnetic
mirror ratiosat both footpoints (R<SUB>1</SUB>=1.6,...,4.0 and
R<SUB>2</SUB>=1.3,...,2.5). From the relative displacement of
footpoint sources we also measure altitude differences of hard
X-ray emission at different energies, which are found to decrease
systematically with higher energies, with a statistical height
difference of h<SUB>Lo</SUB>-h<SUB>M1</SUB>=980+/-250 km and
h<SUB>M1</SUB>-h<SUB>M2</SUB>=310+/-300 km between the three lower
HXT energy channels (Lo, M1, M2).
---------------------------------------------------------
Title: Quadrupolar Magnetic Reconnection in Solar Flares
Authors: Aschwanden, M. J.; Kosugi, T.; Hanaoka, Y.; Nishio, M.;
Melrose, D. B.
1999AAS...194.3107A Altcode: 1999BAAS...31..869A
We analyze the 3-dimensional (3D) geometry of solar flares that show
so-called interacting flare loops in soft X-ray, hard X-ray, and radio
emission, as previously identified by Hanaoka and Nishio. The two flare
loops that appear brightest after the flare are assumed to represent the
outcome of a quadrupolar magnetic reconnection process, during which
the connectivity of magnetic polarities is exchanged between the four
loop footpoints. We fit a 10-parameter 3D-model to Yohkoh SXT and HXT
data of 10 solar flares and determine this way the pre-reconnection
and post-flare geometry of interacting flare loops. We apply a flare
model of Melrose to calculate the magnetic flux transfer and energy
released when two current-carrying field lines reconnect to form a new
current-carrying system in a quadrupolar geometry. Some findings are:
(1) The pre-reconnection field lines always show a strong asymmetry in
size, consistent with the scenario of new-emerging small-scale loops
that reconnect with pre-existing large-scale loops. (2) The relative
angle between reconnecting field lines is near-collinear in half of
the cases, and near-perpendicular in the other half, contrary to the
anti-parallel configuration suggested in the model of Heyvaerts et
al. (3) The shear angle between interacting field lines reduces by
10-50 deg after quadrupolar reconnection. (4) The small-scale flare loop
experiences a shrinkage by a factor of 1.31+0.44, which is consistent
with the scaling law found from previous electron time-of-flight
measurements, suggesting that electron acceleration occurs near the
cusp of quadrupolar configurations. (5) The large-scale loop is found to
dominate the total induction between current-carrying loops, providing
a simple estimate of the maximum magnetic energy available for flare
energy release due to current transfer, which scales as E=10(29.63)
[r2/10(9) cm] [I2/10(11) A](2) , (with r2 the curvature radius and
I2 the current of the large-scale loop) and is found to correlate
with observed flare energies deduced from soft X-ray and hard X-ray
fluxes. Most of the energy is transferred to small-scale loops that
have half of the large-scale current I1=I2/2. (6) The quadrupolar
reconnection geometry provides also a solution of “Canfield's dilemma"
of the offset between the maximum of vertical currents and the HXR
flare loop footpoints.
---------------------------------------------------------
Title: YOHKOH Observations at the Y2K Solar Maximum
Authors: Aschwanden, M. J.
1999AAS...194.8003A Altcode: 1999BAAS...31..965A
Yohkoh will provide simultaneous co-aligned soft X-ray and hard X-ray
observations of solar flares at the coming solar maximum. The Yohkoh
Soft X-ray Telescope (SXT) covers the approximate temperature range
of 2-20 MK with a pixel size of 2.46\arcsec, and thus complements
ideally the EUV imagers sensitive in the 1-2 MK plasma, such as
SoHO/EIT and TRACE. The Yohkoh Hard X-ray Telescope (HXT) offers
hard X-ray imaging at 20-100 keV at a time resolution of down to 0.5
sec for major events. In this paper we review the major SXT and HXT
results from Yohkoh solar flare observations, and anticipate some of
the key questions that can be addressed through joint observations
with other ground and space-based observatories. This encompasses the
dynamics of flare triggers (e.g. emerging flux, photospheric shear,
interaction of flare loops in quadrupolar geometries, large-scale
magnetic reconfigurations, eruption of twisted sigmoid structures,
coronal mass ejections), the physics of particle dynamics during
flares (acceleration processes, particle propagation, trapping,
and precipitation), and flare plasma heating processes (chromospheric
evaporation, coronal energy loss by nonthermal particles). In particular
we will emphasize on how Yohkoh data analysis is progressing from a
qualitative to a more quantitative science, employing 3-dimensional
modeling and numerical simulations.
---------------------------------------------------------
Title: Time Variability of Coronal Loops observed by TRACE
Authors: Nightingale, R. W.; Aschwanden, M. J.; Hurlburt, N. E.
1999AAS...194.7802N Altcode: 1999BAAS...31..961N
We attempt the 3-dimensional reconstruction of a set of coronal loops
during the period of July 18 - 23, 1998, which has been observed
by TRACE with a cadence of a few minutes in the temperature range
of 1-1.5 MK. Using the method of “Dynamic Stereoscopy” and a
filter-ratio technique we obtain density n_e(s,t) and temperature
T_e(s,t) profiles along the loop length s with respect to time
t. Based on these measurements we calculate the radiative E_R(s,t)
and conductive E_C(s,t) losses, and attempt to constrain the heating
function E_H(s,t) as a function of loop coordinate and time. We test
whether the required heating function corresponds to a steady-state or
is governed by episodic heating. In particular we investigate which
time intervals are subject to continuous (or recurrent) heating and
which are dominated by radiative cooling. From this study we shed
some light on theoretical loop models (e.g., Rosner-Tucker-Vaiana
steady-state model) and the resulting scaling laws.
---------------------------------------------------------
Title: Three-dimensional Stereoscopic Analysis of Solar Active
Region Loops. I. SOHO/EIT Observations at Temperatures of (1.0-1.5)
× 10<SUP>6</SUP> K
Authors: Aschwanden, Markus J.; Newmark, Jeffrey S.; Delaboudinière,
Jean-Pierre; Neupert, Werner M.; Klimchuk, J. A.; Gary, G. Allen;
Portier-Fozzani, Fabrice; Zucker, Arik
1999ApJ...515..842A Altcode:
The three-dimensional structure of solar active region NOAA 7986
observed on 1996 August 30 with the Extreme-Ultraviolet Imaging
Telescope (EIT) on board the Solar and Heliospheric Observatory
(SOHO) is analyzed. We develop a new method of dynamic stereoscopy to
reconstruct the three-dimensional geometry of dynamically changing
loops, which allows us to determine the orientation of the mean
loop plane with respect to the line of sight, a prerequisite to
correct properly for projection effects in three-dimensional loop
models. With this method and the filter-ratio technique applied
to EIT 171 and 195 Å images we determine the three-dimensional
coordinates [x(s), y(s), z(s)], the loop width w(s), the electron
density n<SUB>e</SUB>(s), and the electron temperature T<SUB>e</SUB>(s)
as a function of the loop length s for 30 loop segments. Fitting the
loop densities with an exponential density model n<SUB>e</SUB>(h)
we find that the mean of inferred scale height temperatures,
T<SUP>λ</SUP><SUB>e</SUB>=1.22+/-0.23 MK, matches closely that of EIT
filter-ratio temperatures, T<SUP>EIT</SUP><SUB>e</SUB>=1.21+/-0.06
MK. We conclude that these cool and rather large-scale loops (with
heights of h~30-225 Mm) are in hydrostatic equilibrium. Most of the
loops show no significant thickness variation w(s), but we measure
for most of them a positive temperature gradient (dT/ds>0) across
the first scale height above the footpoint. Based on these temperature
gradients we find that the conductive loss rate is about 2 orders of
magnitude smaller than the radiative loss rate, which is in strong
contrast to hot active region loops seen in soft X-rays. We infer a
mean radiative loss time of τ<SUB>rad</SUB>~40 minutes at the loop
base. Because thermal conduction is negligible in these cool EUV
loops, they are not in steady state, and radiative loss has entirely
to be balanced by the heating function. A statistical heating model
with recurrent heating events distributed along the entire loop can
explain the observed temperature gradients if the mean recurrence time
is <~10 minutes. We computed also a potential field model (from
SOHO/MDI magnetograms) and found a reasonable match with the traced
EIT loops. With the magnetic field model we determined also the height
dependence of the magnetic field B(h), the plasma parameter β(h),
and the Alfvén velocity v<SUB>A</SUB>(h). No correlation was found
between the heating rate requirement E<SUB>H0</SUB> and the magnetic
field B<SUB>foot</SUB> at the loop footpoints.
---------------------------------------------------------
Title: Astrophysics in 1998
Authors: Trimble, Virginia; Aschwanden, Markus
1999PASP..111..385T Altcode:
From Alpha (Orionis and the parameter in mixing-length theory) to
Omega (Centauri and the density of the universe), the Greeks had a
letter for it. In between, we look at the Sun and planets, some very
distant galaxies and nearby stars, neutrinos, gamma rays, and some
of the anomalies that arise in a very large universe being studied by
roughly one astronomer per 10^7 Galactic stars.
---------------------------------------------------------
Title: Tetsuya Watanabe, Takeo Kosugi and Alphonse C. Sterling (eds.),
Observational Plasma Astrophysics: Five Years of Yohkoh and Beyond,
Astrophysics and Space Science Library 229
Authors: Aschwanden, Markus
1999SSRv...87..539A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Nonthermal Flare Emissions
Authors: Aschwanden, M. J.
1999mfs..conf..273A Altcode:
Statistical Aspects of Hard X-Ray Flares Variability during the Solar
Activity Cycle Periodicities during the Solar Activity Cycle Frequency
Distributions and Correlations The Concept of Self-Organized Criticality
Particle Acceleration and Injection Fast Time Structures in Hard X-Rays
The Concept of a "Statistical Flare" Electron Beam Signatures in Hard
X-Rays and Radio Pulsed Injection of Particles Second-Step Acceleration
Particle Trapping and Precipitation Incoherent Radiation from Trapped
Particles Temporal Aspects Spectral Analysis Imaging Data Coherent
Emission from Trapped Particles Beam-Driven Emission in the Trap
Loss-Cone-Driven Emission Pulsation Mechanisms in the Trap Precipitation
Signatures Flare Diagnostics from Hard X-Ray/Radio Observations
---------------------------------------------------------
Title: The Electron Injection Function and Energy-dependent Delays
in Thick-Target Hard X-Rays
Authors: Brown, John C.; Conway, Andrew J.; Aschwanden, Markus J.
1998ApJ...509..911B Altcode:
We analytically and numerically study the relationship
between an energy-dependent electron injection spectrum,
F<SUB>0</SUB>(E<SUB>0</SUB>, t), and the resulting bremsstrahlung
photon spectrum, J(ɛ, t), with the goal of exploring whether injection
functions could explain energy-dependent time delays observed in
solar flare hard X-rays (HXRs) without any time-of-flight effects. We
calculate the inversion of the bremsstrahlung photon spectrum (for
the Kramers cross section) and find that the timing of the electron
injection function depends on the time derivative of the second spectral
derivative of the photon spectrum. To match the observed delays, a
systematic softening of the electron injection spectrum is required
over the duration (~1 s) of individual HXR pulses. This requirement
is exactly the same as that which occurs in the time-of-flight model,
except there the softening is due to spatial dispersion of injected
electrons of different energy E<SUB>0</SUB>. We show that such a
softening injection rate is not consistent with acceleration models
where the electron acceleration times are comparable with the HXR pulse
lengths, but it can be consistent with models where the acceleration
times are very short since the injection spectrum variations are
then governed by spectral variations in the acceleration rate. We
conclude that acceleration mechanisms cannot be ruled out on the
basis of HXR light curves alone as an alternative to time-of-flight
effects. Observations of HXR images and of the relationship of HXRs to
soft X-ray loops strongly suggest, however, that time-of-flight effects
must be important and must be included in attempts to infer primary
accelerator properties from HXR light curves. We also conclude that the
agreement of the time-of-flight model with observed energy-dependent
HXR delays, and the properties of any acceleration model contributing
to this trend, puts strong constraints on the timescales involved in
the accelerator.
---------------------------------------------------------
Title: High-Energy Solar Spectroscopic Imager (HESSI) Small Explorer
mission for the next (2000) solar maximum
Authors: Lin, Robert P.; Hurford, Gordon J.; Madden, Norman W.;
Dennis, Brian R.; Crannell, Carol J.; Holman, Gordon D.; Ramaty,
Reuven; von Rosenvinge, Tycho T.; Zehnder, Alex; van Beek, H. Frank;
Bornmann, Patricia L.; Canfield, Richard C.; Emslie, A. Gordon; Hudson,
Hugh S.; Benz, Arnold; Brown, John C.; Enome, Shinzo; Kosugi, Takeo;
Vilmer, Nicole; Smith, David M.; McTiernan, Jim; Hawkins, Isabel;
Slassi-Sennou, Said; Csillaghy, Andre; Fisher, George; Johns-Krull,
Chris; Schwartz, Richard; Orwig, Larry E.; Zarro, Dominic; Schmahl,
Ed; Aschwanden, Markus; Harvey, Peter; Curtis, Dave; Pankow, Dave;
Clark, Dave; Boyle, Robert F.; Henneck, Reinhold; Michedlishvili,
Akilo; Thomsen, K.; Preble, Jeff; Snow, Frank
1998SPIE.3442....2L Altcode:
The primary scientific objective of the High Energy Solar Spectroscopic
Imager (HESSI) Small Explorer mission selected by NASA is to investigate
the physics of particle acceleration and energy release in solar
flares. Observations will be made of x-rays and (gamma) rays from
approximately 3 keV to approximately 20 MeV with an unprecedented
combination of high resolution imaging and spectroscopy. The HESSI
instrument utilizes Fourier- transform imaging with 9 bi-grid rotating
modulation collimators and cooled germanium detectors. The instrument
is mounted on a Sun-pointed spin-stabilized spacecraft and placed
into a 600 km-altitude, 38 degrees inclination orbit.It will provide
the first imaging spectroscopy in hard x-rays, with approximately
2 arcsecond angular resolution, time resolution down to tens of ms,
and approximately 1 keV energy resolution; the first solar (gamma)
ray line spectroscopy with approximately 1-5 keV energy resolution;
and the first solar (gamma) -ray line and continuum imaging,with
approximately 36-arcsecond angular resolution. HESSI is planned for
launch in July 2000, in time to detect the thousands of flares expected
during the next solar maximum.
---------------------------------------------------------
Title: Wavelet Analysis of Solar Flare Hard X-Rays
Authors: Aschwanden, Markus J.; Kliem, Bernhard; Schwarz, Udo; Kurths,
Jürgen; Dennis, Brian R.; Schwartz, Richard A.
1998ApJ...505..941A Altcode:
We apply a multiresolution analysis to hard X-ray (HXR) time profiles
f(t) of solar flares. This method is based on a wavelet transform
(with triangle-shaped wavelets), which yields a dynamic decomposition
of the power at different timescales T, the scalogram P(T, t). For
stationary processes, time-averaged power coefficients, the scalegram
S(T), can be calculated. We develop an algorithm to transform these
(multiresolution) scalegrams S(T) into a standard distribution function
of physical timescales, N(T). We analyze 647 solar flares observed with
the Compton Gamma Ray Observatory (CGRO), recorded at energies >=25
keV with a time resolution of 64 ms over 4 minutes in each flare. The
main findings of our wavelet analysis are: <P />1. In strong flares,
the shortest detected timescales are found in the range T<SUB>min</SUB>
~ 0.1-0.7 s. These minimum timescales are found to correlate with
the flare loop size r (measured from Yohkoh images in 46 flares),
according to the relation T<SUB>min</SUB>(r) ~ 0.5(r/10<SUP>9</SUP>
cm) s. Moreover, these minimum timescales are subject to a cutoff,
T<SUB>min</SUB>(n<SUB>e</SUB>) >~ T<SUB>Defl</SUB>(n<SUB>e</SUB>),
which corresponds to the electron collisional deflection time at the
loss-cone site of the flare loops (inferred from energy-dependent
time delays in CGRO data). <P />2. In smoothly varying flares, the
shortest detected timescales are found in the range T<SUB>min</SUB> ~
0.5-5 s. Because these smoothly varying flares exhibit also large trap
delays, the lack of detected fine structure is likely to be caused
by the convolution with trapping times. <P />3. In weak flares, the
shortest detected timescales cover a large range, T<SUB>min</SUB> ~
0.5-50 s, mostly affected by Poisson noise. <P />4. The scalegrams S(T)
show a power-law behavior with slopes of β<SUB>max</SUB> ~ 1.5-3.2
(for strong flares) over the timescale range of [T<SUB>min</SUB>,
T<SUB>peak</SUB>]. Dominant peaks in the timescale distribution N(T)
are found in the range T<SUB>peak</SUB> ~ 0.5-10<SUP>2</SUP> s, often
coinciding with the upper cutoff of N(T). These observational results
indicate that the fastest significant HXR time structures detected
with wavelets (in strong flares) are related to physical parameters
of propagation and collision processes. If the minimum timescale
T<SUB>min</SUB> is associated with an Alfvénic crossing time through
elementary acceleration cells, we obtain sizes of r<SUB>acc</SUB>
~ 75-750 km, which have a scale-invariant ratio r<SUB>acc</SUB>/r ~
0.03 to flare loops and are consistent with cell sizes inferred from
the frequency bandwidth of decimetric millisecond spikes.
---------------------------------------------------------
Title: Deconvolution of Directly Precipitating and Trap-Precipitating
Electrons in Solar Flare Hard X-Rays. I. Method and Tests
Authors: Aschwanden, Markus J.
1998ApJ...502..455A Altcode:
We develop and test a numerical code that provides a self-consistent
deconvolution of energy-dependent hard X-ray (HXR) time profiles
I(ɛ, t) into two HXR-producing electron components, i.e.,
directly precipitating and trap-precipitating electrons. These
two HXR components can be physically distinguished because their
energy-dependent time delays have an opposite sign. The deconvolution
is based on the following model assumptions: (1) nonthermal electrons
are injected from the acceleration site into coronal flare loops by an
injection function f(E, α, t) that consists of synchronized pulses
in energy E and pitch angle α, (2) electrons with initially small
pitch angles (α <= α<SUB>0</SUB>) precipitate directly to the
HXR emission site, (3) electrons with initially large pitch angles
(α >= α<SUB>0</SUB>) are temporarily trapped and precipitate
after the collisional deflection time, and (4) nonthermal electrons
lose their energy by Coulomb collisions and emit thick-target HXR
bremsstrahlung in a high-density (fully collisional) site (near
or inside the chromosphere). The numerical deconvolution provides
a self-consistent determination of three physical parameters:
(1) the electron time-of-flight distance l<SUP>TOF</SUP> between
the acceleration/injection site and the HXR emission site, (2) the
electron density n<SUB>e</SUB> in the trap region, and (3) the fraction
of HXR-emitting electrons that precipitate directly, q<SUP>prec</SUP>,
which relates to the loss cone angle by q<SUP>prec</SUP>(α<SUB>0</SUB>)
= (1 - cos α<SUB>0</SUB>) for isotropic pitch angle distributions. This
yields the magnetic mirror ratio R = B<SUP>loss</SUP>/B<SUP>inj</SUP>
= 1/sin<SUP>2</SUP> (α<SUB>0</SUB>) between the injection and loss
cone site. With this method, we measure for the first time magnetic
field ratios in coronal loops by means of HXR data. Based on this ratio,
together with the knowledge of the photospheric field at the footpoint,
a direct measurement of the magnetic field in the coronal acceleration
region can be obtained. We simulate energy-dependent HXR data I(ɛ,
t) with typical solar flare parameters (l<SUP>TOF</SUP> = 15,000 km,
n<SUB>e</SUB> = 10<SUP>11</SUP> cm<SUP>-3</SUP>, q<SUP>prec</SUP> = 0.5)
and test the accuracy of the inversion code. We perform the inversion
in 30 different simulations over the entire physically plausible
parameter space and demonstrate that a satisfactory inversion of
all three physical parameters l<SUP>TOF</SUP>, n<SUB>e</SUB>, and
q<SUP>prec</SUP> is achieved in a density range of n<SUB>e</SUB> =
10<SUP>10</SUP>-10<SUP>12</SUP> cm<SUP>-3</SUP> for precipitation
ratios of q<SUP>prec</SUP> = 0.1-0.9 and for signal-to-noise ratios
of >~100 (requiring HXR count rates of >~10<SUP>4</SUP> counts
s<SUP>-1</SUP>). Applications of this inversion method to solar flare
observations in hard X-rays (CGRO/BATSE, Yohkoh/Hard X-Ray Telescope)
and microwaves (Nobeyama) will be presented in subsequent papers.
---------------------------------------------------------
Title: Deconvolution of Directly Precipitating and Trap-Precipitating
Electrons in Solar Flare Hard X-Rays. II. Compton Gamma Ray
Observatory Data Analysis
Authors: Aschwanden, Markus J.; Schwartz, Richard A.; Dennis, Brian R.
1998ApJ...502..468A Altcode:
Based on the deconvolution method developed in the first paper of this
series, we present here the data analysis of 20-200 keV hard X-ray (HXR)
data from the Burst and Transient Source Experiment (BATSE) on board the
Compton Gamma Ray Observatory (CGRO) recorded during 103 solar flares
in 1991-1995. These are all of the flares simultaneously observed by
CGRO with high time resolution (64 ms) and by Yohkoh in flare mode. The
deconvolution method takes the measured HXR count rates as function of
energy and time, I(ɛ, t), and computes the following self-consistently:
the electron injection function n(E, t), the directly precipitating
electron flux n<SUP>prec</SUP>(E, t), the trapped-precipitating
flux n<SUP>trap</SUP>(E, t), the fraction of directly precipitating
electrons (q<SUP>prec</SUP>), the electron time-of-flight distance
(l<SUP>TOF</SUP>), and the electron density at the loss cone site of the
trap (n<SUB>e</SUB>). We find that the electron time-of-flight distances
(l<SUP>TOF</SUP> = 20.0 +/- 7.3 Mm) inferred with the deconvolution
method are fully consistent with those obtained earlier using a
Fourier filter method. The trap electron densities (n<SUB>e</SUB> =
10<SUP>10.96+/-0.57</SUP> cm<SUP>-3</SUP>) obtained from deconvolving
the e-folding decay times of HXR pulses (according to the trap model
of Melrose & Brown) are found to be statistically a factor of 1.5
lower than those inferred from cross-correlation delays. The fraction
q<SUP>prec</SUP> of directly precipitating electrons, measured for the
first time here, is found to have a mean (and standard deviation) of
q<SUP>prec</SUP> = 0.42 +/- 0.16. Based on this precipitation fraction,
we infer loss cone angles of α<SUB>0</SUB> ~ 20°-70° and magnetic
mirror ratios of R = B<SUP>loss</SUP>/B<SUP>inj</SUP> ~ 1.2 - 3 (with
a median value of R<SUB>median</SUB> = 1.6) between the loss cone site
and injection/acceleration site, assuming an isotropic pitch angle
distribution at the injection site. The TOF distances and mirror ratios
constrain magnetic scale heights in flare loops to λ<SUB>B</SUB> =
10-150 Mm. The fact that this two-component model (with free-streaming
and trapped electrons) satisfactorily fits the energy-dependent time
delays in virtually all flares provides strong evidence that electron
time-of-flight differences and collisional scattering times dominate
the observed HXR timing, while the injection of electrons appears to
be synchronized (independent of energy) and does not reveal the timing
of the acceleration process.
---------------------------------------------------------
Title: New features in type IV solar radio emission: combined effects
of plasma wave resonances and MHD waves
Authors: Chernov, G. P.; Markeev, A. K.; Poquerusse, M.; Bougeret,
J. L.; Klein, K. -L.; Mann, G.; Aurass, H.; Aschwanden, M. J.
1998A&A...334..314C Altcode:
An intense and complex type IV solar radio burst over the time period
1992 0217 08-12 was recorded simultaneously by 3 spectrographs, ARTEMIS
(100-500 MHz), OSRA (200-400 MHz) and IZMIRAN (180-270 MHz), and by
the Nan\c cay radioheliograph. For ~ 2 hours, the event exhibited
strong pulsations on various time scales and “zebra patterns” with
new features: sudden frequency shifts of the whole pattern, splitting
of individual zebra stripes into two stripes, structuration of the
upper-frequency split stripes into emission dots in phase with ~
0.2 s pulsations. Another new and spectacular feature was a ~ 10 MHz
bandwidth emission at the high frequency cut-off of the whole event,
oscillating between 350 and 450 MHz in phase with ~ 3 min pulsations,
and itself structured by the ~ 0.2 s pulsations. Another property
observed for the first time was that the circular polarization of
zebra patterns changed sign during the event, possibly due to magnetic
field reversal at some point of a long-lasting magnetic reconnection
process in the upper corona. According to a classical picture,
electrons are accelerated in the current sheet and trapped in the
magnetic arch. Pulsations are due to MHD waves affecting the whole
arch and electron beams as well. We confront two existing theories
to these new observational features. The l+w=> t model (Chernov
1976, 1989), based on Langmuir wave - whistler coupling at normal and
anomalous Doppler resonance, can account for all the observed fine
structures of zebra patterns, and gives a plausible magnetic field
of 11x 10(-4) T in the source. The Winglee & Dulk (1986) model,
based on electron-cyclotron maser emission of upper-hybrid waves at
double plasma resonance, seems the most adequate to account for the
evolving emission line, with its source in the dense current sheet.
---------------------------------------------------------
Title: G. M. Simnett, C. E. Alissandrakis, and L. Vlahos (eds.),
Solar and Heliospheric Plasma Physics, Proceedings of the 8th European
Meeting on Solar Physics
Authors: Aschwanden, Markus
1998SoPh..179..441A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Impulsively generated MHD waves and their detectability in
solar coronal loops
Authors: Murawski, K.; Aschwanden, M. J.; Smith, J. M.
1998SoPh..179..313M Altcode:
Impulsively generated magnetohydrodynamic waves in solar coronal loops,
with arbitrary plasma β, are studied numerically by a flux-corrected
transport algorithm. Numerical results show that the total reflection
which occurs in the region of low Alfvén speed leads to trapped fast
kink magnetosonic waves. These waves propagate along the slab and
exhibit periodic, quasi-periodic, and decay phases. As a consequence
of the difference in wave propagation speeds, the time signatures
of the slow magnetosonic waves are delayed in time in comparison to
the time signatures of the fast magnetosonic and Alfvén waves. An
interaction between the waves can generate a longer lasting and
complex quasi-periodic phase of the fast wave. We discuss also the
observational detectability of such MHD waves in optical, radio,
and soft X-ray wavelenghts.
---------------------------------------------------------
Title: Book Review: Solar and heliospheric plasma physics / Springer,
1997
Authors: Aschwanden, Markus
1998SoPh..179..443A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Logistic Avalanche Processes, Elementary Time Structures,
and Frequency Distributions in Solar Flares
Authors: Aschwanden, Markus J.; Dennis, Brian R.; Benz, Arnold O.
1998ApJ...497..972A Altcode:
We analyze the elementary time structures (on timescales of ~0.1-3.0
s) and their frequency distributions in solar flares using hard X-ray
(HXR) data from the Compton Gamma Ray Observatory (CGRO) and radio data
from the radio spectrometers of Eidgenoessische Technische Hochschule
(ETH) Zurich. The four analyzed data sets are gathered from over
600 different solar flares and include about (1) 10<SUP>4</SUP>
HXR pulses at >=25 and >=50 keV, (2) 4000 radio type III
bursts, (3) 4000 pulses of decimetric quasi-periodic broadband
pulsation events, and (4) 10<SUP>4</SUP> elements of decimetric
millisecond spike events. <P />The time profiles of resolved
elementary time structures have a near-Gaussian shape and can be
modeled with the logistic equation, which provides a quantitative
measurement of the exponential growth time τ<SUB>G</SUB> and the
nonlinear saturation energy level W<SUB>S</SUB> of the underlying
instability. Assuming a random distribution (Poisson statistics)
of saturation times t<SUB>S</SUB> (with an e-folding constant
t<SUB>Se</SUB>), the resulting frequency distribution of saturation
energies W<SUB>S</SUB> or peak energy dissipation rates F<SUB>S</SUB> =
(dW/dt)<SUB>t=t<SUB>S</SUB></SUB> has the form of a power-law function,
i.e., N(F<SUB>S</SUB>)~F<SUP>-α</SUP><SUB>S</SUB>, where the power-law
index α is directly related to the number of e-folding amplifications
by the relation α = (1 + τ<SUB>G</SUB>/t<SUB>Se</SUB>). The same
mathematical formalism is used to generate power-law distributions,
as in Rosner & Vaiana, but the distribution of energies released
in elementary flare instabilities is not related to the global energy
storage process. We assume Poissonian noise for the unamplified
energy levels in unstable flare cells, implying an exponential
frequency distribution of avalanche energies W<SUB>S</SUB> or fluxes
F<SUB>S</SUB> in the absence of coherent amplifications. Also, in
the case of coherent amplification, the Poissonian noise introduces
exponential rollovers of the power law at the low and high ends of the
frequency distributions. <P />We fit both power-law and exponential
functions to the observed frequency distributions of elementary pulse
fluxes N(F) in each flare separately. For HXR pulses, one-half of
the flares are better fitted with power-law frequency distributions,
demanding coherent amplification of the underlying energy dissipation
mechanism, e.g., current exponentiation occurring in the magnetic
tearing instability. The majority of type III burst flares are best
fitted with power-law distributions, consistent with the interpretation
in terms of beam-driven coherent emission. The frequency distributions
of decimetric pulsations and decimetric millisecond spikes are found to
fit exponential functions, in contrast to the expected power laws for
coherent emission mechanisms generally proposed for these radio burst
types. A coherent emission mechanism can be reconciled with the observed
exponential frequency distributions only if nonlinear saturation occurs
at a fixed amplification factor for all elementary pulses or spikes,
for example, if it is produced by an oscillatory compact source (in the
case of decimetric pulsations) or by a fragmented source with similar
spatial cell sizes (in the case of decimetric millisecond spikes).
---------------------------------------------------------
Title: Coordinated Observations with SOHO, YOHKOH and VLA
Authors: Aschwanden, Markus J.; Bastian, Tim S.; Nitta, Nariaki;
Newmark, Jeff; Thompson, Barbara J.; Harrison, Richard A.
1998ASPC..155..311A Altcode: 1998sasp.conf..311A
No abstract at ADS
---------------------------------------------------------
Title: 3-Dimensional Models of Active Region Loops
Authors: Aschwanden, M. J.; Neupert, W. M.; Newmark, J.; Thompson,
B. J.; Brosius, J. W.; Holman, G. D.; Harrison, R. A.; Bastian, T. S.;
Nitta, N.; Hudson, H. S.; Zucker, A.
1998ASPC..155..145A Altcode: 1998sasp.conf..145A
No abstract at ADS
---------------------------------------------------------
Title: 3D-Stereoscopic Analysis of Solar Active Region Loops Observed
with SOHO/EIT
Authors: Aschwanden, M. J.; Newmark, J. S.; Delaboudiniere, J. -P.;
Neupert, W. M.; Klimchuk, J. A.; Gary, G. Allen; Portier-Fozzani,
F.; Zucker, A.
1998cee..workE..19A Altcode:
The three-dimensional (3D) structure of solar active region NOAA
7986 observed on 1996 August 30 with the Extrem-ultraviolet Imaging
Telescope (EIT) onboard the Solar and Heliospheric Observatory
(SoHO) is analyzed. We develop a new method of Dynamic Stereoscopy to
reconstruct the 3D geometry of dynamically changing loops, which allows
us to determine the orientation of the loop plane with respect to
the line-of-sight, a prerequisite to correct properly for projection
effects in 3D loop models. With this method and the filter-ratio
technique applied to EIT 171 angle and 195 angle images we determine
the 3D coordinates [x(s),y(s),z(s)], the loop width w(s), the electron
density n_e(s), and the electron temperature T_e(s) as function of
the loop length s for 30 loop segments. Fitting the loop densities
with an exponential density model n_e(h) we find that the mean of
inferred scale height temperatures, T_e<SUP>lambda</SUP> = 1.22 plus or
minus 0.23 MK, matches closely that of EIT filter-ratio temperatures,
T_e<SUP>EIT</SUP> = 1.21 plus or minus 0.06 MK. We conclude that these
cool and rather large-scale loops (with heights of h ~30-225 Mm), which
dominate the EIT 171 angle images, are in hydrostatic equilibrium. Most
of the loops show no significant thickness variation w(s), but we can
measure for most of them a positive temperature gradient (dT/ds >
0) across the first scale height above the footpoint. Based on these
temperature gradients we find that the conductive loss rate is about
two orders of magnitude smaller than the radiative loss rate, which is
in strong contrast to hot active region loops seen in SXR. We infer a
mean radiative loss time of τ<SUB>rad</SUB> ~40 minutes. For steady
state models, the heating rate has to balance the radiative loss,
i.e. the heating rate has to scale with the squared density (E_H
propto n_e^2). From potential-field extrapolations we determine also
the magnetic field strength B(s), the plasma beta-parameter beta(s),
and the Alfven velocity v_A(s) along the loops, and discuss the findings
in the context of coronal heating models.
---------------------------------------------------------
Title: What did YOHKOH and Compton Change in Our Perception of
Particle Acceleration in Solar Flares?
Authors: Aschwanden, M. J.
1998ASSL..229..285A Altcode: 1998opaf.conf..285A
No abstract at ADS
---------------------------------------------------------
Title: Logistic Avalanche Processes, Elementary Time Structures,
Frequency Distributions, and Wavelet Analysis of Solar Flares
Authors: Aschwanden, M. J.; Dennis, B. R.; Schwartz, R. A.; Benz,
A. O.; Kliem, B.; Schwarz, U.; Kurths, J.
1998cee..workE..10A Altcode:
Do elementary time scales exist in solar flares that could provide
us a clue on the spatial fragmentation of the primary energy
release process? Or is there a continuous distribution, composed
of a hierarchy of unresolved time structures? We present results
of two recent studies on these fundamental questions. We applied a
multi-resolution analysis (using triangle-shaped wavelet transforms)
to 647 solar flares observed with the COMPTON Gamma Ray Observatory
(CGRO) at hard X-ray energies of >= 25 keV with a time resolution
of 64 ms. From the wavelet scalegrams we infer a distribution of time
scales N(T) for each flare and find a cutoff for the shortest detected
time scales T<SUB>min</SUB> that is independent of the Poisson noise for
strong flares. These shortest time scales T<SUB>min</SUB> are found to
correlate with the flare loop radius r (Fig.1), i.e. T<SUB>min</SUB> =
0.5 (r/10^9 cm)s (measured with Yohkoh) and the collisional deflection
time, T<SUB>min</SUB> > ~t<SUP>Defl</SUP>(n_e), (determined from the
electron density n_e of trapped electrons by measuring energy-dependent
time delays detected with CGRO). From these observations we infer
spatial sizes of r<SUB>acc</SUB> = 75-750 km for elementary acceleration
cells. In a second study we determined the frequency distributions
of elementary time structures in over 600 flares, based on some
10^4 hard X-ray pulses at 25 and 50 keV, 4000 radio type III bursts,
4000 decimetric quasi-periodic broadband pulsation events, and 10^4
decimetric millisecond spike events. All elementary time structures
have a quasi-Gaussian shape and can be modeled with the logistic
equation, which describes the exponential growth phase and nonlinear
saturation (caused by the limited amount of available free energy) of a
general instability. We derive a theoretical description of frequency
distributions in terms of this logistic avalanche model and find that
the power-law slope of observed frequency distributions provides a
powerful diagnostic on coherent versus incoherent instabilities.
---------------------------------------------------------
Title: Solar Active Region Movies seen by the SOHO Extreme-ultraviolet
Telescope
Authors: Newmark, J. S.; Thompson, B.; Gurman, J. B.; Delaboudiniere,
J. P.; Aschwanden, Markus; Mason, Helen
1997AAS...191.7307N Altcode: 1997BAAS...29.1321N
The Extreme-ultraviolet Imaging Telescope (EIT) on board the SOHO (SOlar
and Heliospheric Observatory) satellite provides wide-field images of
the corona and transition region on the solar disc and up to 1.4 solar
radii above the limb. Its normal incidence multilayer-coated optics
select spectral emission lines from FeIX (171 Ang), FeXII (195 Ang),
FeXV (284 Ang), and HeII (304 Ang) with 2.6 arcsecond resolution which
allow us to describe Solar activity over a wide temperature range. EIT
is providing unique EUV observations of the structure and evolution of
active regions. Here we show movies of active region 8059 from July
3-10, 1997. The high temporal variability of the AR loops is very
evident. Initial temperature and density diagnostics are explored as
well as a comparison with diagnostics from the SOHO-CDS instrument.
---------------------------------------------------------
Title: The Structure of the Solar Corona above Sunspots as Inferred
from Radio, X-Ray, and Magnetic Field Observations
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.
1997ApJ...489..403V Altcode:
We present observations of a solar active region, NOAA/USAF no. 7123,
during 1992 April 3-10. The database includes high-angular-resolution
radio, soft X-ray, magnetograph, and Hα observations. The radio
observations include VLA maps in the Stokes I and V parameters at 4.7
and 8.4 GHz. The soft X-ray observations were obtained by the Soft
X-Ray Telescope on board the Yohkoh satellite, the magnetograms were
obtained at Kitt Peak, Mt. Wilson, and Big Bear, and the Hα data were
obtained at Big Bear. <P />The lead sunspot in the active region is
studied here. In particular, the polarization properties and brightness
temperature spectrum are used to constrain the thermal structure of the
corona over the sunspot. It is found that the 4.7 GHz emission of the
sunspot is polarized in the sense of the ordinary mode, in contradiction
with simple gyroresonance models that predict that the spot should be
polarized in the sense of the extraordinary mode. We model the spectral
and temporal evolution of the polarization structure in two frequencies,
4.7 and 8.4 GHz, using gyroresonance models to fit one-dimensional
brightness temperature profiles across the spot in each polarization
and frequency. The constraints provided by the X-ray and magnetic
field observations help us to derive a qualitatively self-consistent
picture for the daily evolution of the spot. We attribute the excess
of the o-mode emission to the magnetic field configuration and to the
temperature inhomogeneities across the spot. Namely, we find that (1)
the umbral and penumbral environments are distinct, with the X-rays and
the o-mode radio emission coming from the hotter penumbral loops, while
the observed x-mode emission originates from the cooler umbral loops;
(2) there exist temperature inhomogeneities in both the radial and
vertical direction over the spot; and (3) the umbral magnetic field
remains more confined in the corona than that predicted by a dipole
model. Instead, a field configuration based on the magnetohydrostatic
equilibrium model of Low gives a better agreement with the observations.
---------------------------------------------------------
Title: Electron Trapping Times and Trap Densities in Solar Flare
Loops Measured with COMPTON and YOHKOH
Authors: Aschwanden, Markus J.; Bynum, Robert M.; Kosugi, Takeo;
Hudson, Hugh S.; Schwartz, Richard A.
1997ApJ...487..936A Altcode:
We measure energy-dependent time delays of ~20-200 keV hard X-ray
(HXR) emission from 78 flares observed simultaneously with the Compton
Gamma Ray Observatory and Yohkoh. Fast time structures (<~1 s)
are filtered out, because their time delays have been identified
in terms of electron time-of-flight (TOF) differences from directly
precipitating electrons (Aschwanden et al.). For the smooth HXR flux,
we find systematic time delays in the range of τ<SUB>S</SUB> = t<SUB>50
keV</SUB>-t<SUB>200 keV</SUB> ~ -(1 ... 10) s, with a sign opposite
to TOF delays, i.e., the high-energy HXRs lag the low-energy HXRs. <P
/>We interpret these time delays of the smooth HXR flux in terms of
electron trapping, and we fitted a model of the collisional deflection
time t<SUP>Defl</SUP>(E)~E<SUP>3/2</SUP>n<SUP>-1</SUP><SUB>e</SUB>
to the observed HXR delays in order to infer electron densities
n<SUP>Trap</SUP><SUB>e</SUB> in the trap. Independently,
we determine the electron density n<SUP>SXR</SUP><SUB>e</SUB>
in flare loops from soft X-ray (SXR) peak emission measures EM=
n<SUP>2</SUP><SUB>e</SUB>dh, using loop width (w) measurements to
estimate the column depth dh ~ w. Comparing the two independent
density measurements in HXR and SXR, we find a mean ratio of
q<SUB>e</SUB>=n<SUP>Trap</SUP><SUB>e</SUB>/n<SUP>SXR</SUP><SUB>e</SUB>~1,
with a relatively small scatter by a factor of ~2. Generally, it is
likely that the SXR-bright flare loops have a higher density than
the trapping regions (when q<SUB>e</SUB> < 1), but they also are
subject to filling factors less than unity (when q<SUB>e</SUB> >
1). Our measurements provide comprehensive evidence that electron
trapping in solar flares is governed in the weak-diffusion limit, i.e.,
that the trapping time corresponds to the collisional deflection time,
while pitch-angle scattering by resonant waves seems not to be dominant
in the 20-200 keV energy range. The measurements do not support a
second-step acceleration scenario for energies <=200 keV.
---------------------------------------------------------
Title: Joint Time-Frequency Analysis of Solar Flare Hard X-Ray
Observations
Authors: Dennis, B. R.; Aschwanden, M. J.
1997SPD....28.0161D Altcode: 1997BAAS...29..890D
Many flares have an impulsive phase in hard X-rays followed by a period
of more gradually varying emission. This general trend in the timescale
of the variations from <1 s early in a flare to as long as several
minutes in the later phases can tell us about the energy release and
particle acceleration processes if we know how to interpret it. We are
investigating several methods of quantifying this change including
the use of the short-time Fourier transform, the Gabor transform,
and various types of wavelet analysis tools. Preliminary results will
be presented from applying these techniques to HXRBS and BATSE hard
X-ray time profiles of several flares.
---------------------------------------------------------
Title: Electron Trapping in Flares Explored with COMPTON and YOHKOH
Authors: Aschwanden, M.
1997SPD....28.1802A Altcode: 1997BAAS...29..922A
We measure energy-dependent time delays of ~20-200 keV hard X-ray
(HXR) emission from 78 flares simultaneously observed with the Compton
Gamma Ray Observatory (CGRO) and Yohkoh. Fast time structures (≲1
s) are filtered out, because their time delays have been identified
in terms of electron time-of-flight (TOF) differences from directly
precipitating electrons. For the smooth HXR flux we find systematic
time delays in the range of τ_S=t_{50 keV}-t_{200 keV} ~ -(1...10)
s, with a sign opposite to TOF delays, i.e. the high-energy HXRs lag
the low-energy HXRs. We interpret these time delays of the smooth HXR
flux in terms of electron trapping and fit a model of the collisional
deflection time t(Defl) (E) ~ E(3/2) n_e(-1) to the observed HXR delays
to infer electron densities n_e(Trap) in the trap. Independently we
determine the electron density n_e(SXR) in flare loops from soft X-ray
(SXR) peak emission measures EM=int n_e(2) dh, using loop width (w)
measurements to estimate the column depth dh ~ w. Comparing the two
independent density measurements in HXR and SXR we find a mean ratio
of q_e=n_e(Trap}/n_e({SXR)) ~ 1, with a relatively small scatter
by a factor of ~ 2. It is likely that the SXR-bright flare loops
have generally a higher density than the trapping regions (when
q_e<1), but are also subject to filling factors less than unity
(when q_e>1). Our measurements provide comprehensive evidence that
electron trapping in solar flares is governed in the weak diffusion
limit, i.e. that the trapping time corresponds to the collisional
deflection time, while pitch-angle scattering by resonant waves seems
not to be dominant in the 20-200 keV energy range. The measurements
do not support a second-step acceleration scenario for energies <=
200 keV.
---------------------------------------------------------
Title: Electron Densities in Solar Flare Loops, Chromospheric
Evaporation Upflows, and Acceleration Sites
Authors: Aschwanden, Markus J.; Benz, Arnold O.
1997ApJ...480..825A Altcode:
We compare electron densities measured at three different locations
in solar flares: (1) in soft X-ray (SXR) loops, determined from SXR
emission measures and loop diameters from Yohkoh Soft X-Ray Telescope
maps [n<SUP>SXR</SUP><SUB>e</SUB>=(0.2-2.5)×10<SUP>11</SUP>
cm<SUP>-3</SUP>] (2) in chromospheric evaporation upflows,
inferred from plasma frequency cutoffs of decimetric radio
bursts detected with the 0.1-3 GHz spectrometer Phoenix of ETH
Zürich [n<SUP>upflow</SUP><SUB>e</SUB>=(0.3-11)×10<SUP>10</SUP>
cm<SUP>-3</SUP>] and (3) in acceleration sites, inferred from
the plasma frequency at the separatrix between upward-accelerated
(type III bursts) and downward-accelerated (reverse-drift bursts)
electron beams [n<SUP>acc</SUP><SUB>e</SUB>=(0.6-10)×10<SUP>9</SUP>
cm<SUP>-3</SUP>]. <P />The comparison of these density measurements,
obtained from 44 flare episodes (during 14 different flares),
demonstrates the compatibility of flare plasma density diagnostics with
SXR and radio methods. The density in the upflowing plasma is found to
be somewhat lower than in the filled loops, having ratios in a range
n<SUP>upflow</SUP><SUB>e</SUB>/n<SUP>SXR</SUP><SUB>e</SUB>=0.02-1.3,
and a factor of 3.6 higher behind the upflow front. The acceleration
sites are found to have a much lower density than the SXR-bright flare
loops, i.e., n<SUP>acc</SUP><SUB>e</SUB>/n<SUP>SXR</SUP><SUB>e</SUB>=
0.005-0.13, and thus must be physically displaced from the SXR-bright
flare loops. The scaling law between electron time-of-flight distances
l' and loop half-lengths s, i.e., l'/s = 1.4 +/- 0.3, recently
established by Aschwanden et al. suggests that the centroid of the
acceleration region is located above the SXR-bright flare loop, as
envisioned in cusp geometries (e.g., in magnetic reconnection models).
---------------------------------------------------------
Title: Relative Timing of Microwave and HXR Bursts
Authors: Bastian, T. S.; Aschwanden, M. J.
1997SPD....28.1803B Altcode: 1997BAAS...29..922B
The close correlation between microwave and hard X-ray (HXR) emission
during flares has often been cited as evidence that the same population
of energetic electrons is responsible for both types of emission. The
two emissions differ in detail, however. Imaging observations
have demonstrated that the two are not necessarily cospatial and
timing observations have demonstrated that the microwaves are often
significantly delayed with respect to HXR emission, typically by
several seconds, but occassionally by much longer times. Such delays
are in seeming conflict with the thick target model for HXR emission in
its simplest form, and with the idea that microwave and HXR emissions
result from essentially the same population of electrons. One way to
reconcile the delay between microwaves and HXRs in the thick target
model is to suppose that that electron trapping is significant (e.g.,
Cornell et al., ApJ, 279, 875). For a magnetic trap containing a
plasma of constant density, high energy electrons have a longer
lifetime against collisions than low energy electrons (tau_ {def} ~
E(3/2) ). Hence the energetic electrons responsible for the microwaves
remain in the trap longer and the microwave emission they emit peaks
later than the HXR emission. Another possibility is that higher energy
electrons are accelerated later than lower energy electrons (so-called
“second-step” acceleration models; e.g., Bai and Dennis 1985, ApJ,
292, 699). To explore the question in detail we have assembled a sample
of 16 flares observed simultaneously in microwaves by the Nobeyama
radioheliograph and in HXRs by the BATSE instrument on board the CGRO
in burst trigger mode. The former imaged the flares at 17 GHz with
an angular resolution of ~ 10” and a time resolution of either 50
msec or 1 sec. The latter obtained medium energy resolution spectra
(16 channels) between 20-200 keV with a time resolution of 16 or 64
msec. We present preliminary results of our analysis.
---------------------------------------------------------
Title: Coronal and Interplanetary Particle Beams
Authors: Aschwanden, Markus J.; Treumann, Rudolf A.
1997LNP...483..108A Altcode: 1997cprs.conf..108A
In this report we attempt to synthesize the results of a series of
working group discussions focused on the topics of particle beams
and particle acceleration in the solar corona and in interplanetary
space. We start our discussion of coronal beams with a standard
flare scenario, established on recent X-ray (Yohkoh, CGRO) and radio
observations, which constitutes a framework for the understanding of
upward and downward accelerated electron beams and their secondary
signatures, such as chromospheric evaporation. The second part is
dedicated to interplanetary electron and ion beams, with emphasis on
their relation to coronal beams, using recent spacecraft data from
Ulysses and Wind. Interplanetary electron beams can often be traced
back to coronal type III sources, while there is no such relation
for interplanetary ion beams. In the third part, we briefly review
acceleration mechanisms for coronal and interplanetary beams, separately
for electrons and ions.
---------------------------------------------------------
Title: YOHKOH observation of the source regions of solar narrowband,
millisecond spike events.
Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.
1997A&A...317..569K Altcode:
The source regions of metric spike events are investigated on Yohkoh
soft X-ray (SXR) maps. The spikes are identified by the spectrometer
Phoenix between 300MHz and 360MHz and are associated with groups of
type III bursts at lower frequencies reaching also the decametric
range. The Very Large Array (VLA) provides simultaneously spatial
information at 333MHz, 1446MHz and 4866MHz. Similar to the previous
VLA observation of a metric spike event, the new data are consistent
with a high altitude of the spike sources of about 5x10^10^cm above
the photosphere. The additionally available SXR data for one of the
presented events give the following new informations: (i) The spike
sources occur near open field lines and near regions of a slightly
enhanced SXR flux relative to the ambient plasma. (ii) Contrary to SXR
observations of type III bursts without metric spike activity, no SXR
jet is observed. (iii) At low altitude, a weak SXR enhancement occurs,
peaking about 60 s after the spike event. The SXR source and the spike
sources can be connected by potential field lines. The observations
corroborate a model in which a metric spike event is attributed to
an energy release region at high altitude, while upwards propagating
electrons produce type III bursts and downward moving electrons are
responsible for SXR emission of heated plasma.
---------------------------------------------------------
Title: The Scaling Law between Electron Time-of-Flight Distances
and Loop Lengths in Solar Flares
Authors: Aschwanden, Markus J.; Kosugi, Takeo; Hudson, Hugh S.; Wills,
Meredith J.; Schwartz, Richard A.
1996ApJ...470.1198A Altcode:
From the complete data set of solar flares simultaneously observed
with the Burst and Transient Source Experiment (BATSE) on board the
Compton Gamma Ray Observatory (CGRO) in the high-time resolution mode
(64 ms) and the Hard X-ray Telescope (HXT) on board Yohkoh, we were
able to determine the electron time-of-flight (TOF) distance l' and
the flare loop geometry in 42 events. The electron TOF distances were
determined from time delays (of ≍ 10-100 ms) of hard X-ray (HXR)
pulses (measured in 16 channel spectra over ≍ 20-200 keV), produced
by the velocity difference of the HXR-producing electrons. The flare
loops were mostly identified from double footpoint sources in ≥30
keV HXT images, with radii in the range r = 3000-25,000 km. We find
a scaling law between the electron TOF distance l' and the flare loop
half-length 5 = r(π/2), having a mean ratio (and standard deviation)
of l'/s = 1.4±0.3. In five flares, we observe coronal ≥ 30 keV HXR
sources of the Masuda type in the cusp region above the flare loop and
find that their heights are consistent with the electron TOF distance
to the footpoints. These results provide strong evidence that particle
acceleration in solar flares occurs in the cusp region above the flare
loop and that the coronal HXR sources discovered by Masuda et al. are
a signature of the acceleration site, probably controlled by a magnetic
reconnection process.
---------------------------------------------------------
Title: Electron versus Proton Timing Delays in Solar Flares
Authors: Aschwanden, Markus J.
1996ApJ...470L..69A Altcode: 1996astro.ph..7168A
Both electrons and ions are accelerated in solar flares and carry
nonthermal energy from the acceleration site to the chromospheric
energy-loss site, but the relative amount of energy carried by electrons
versus ions is subject of debate. In this Letter, we test whether the
observed energy-dependent timing delays of 20--200 keV hard X-ray (HXR)
emission can be explained in terms of propagating electrons versus
protons. For a typical flare, we show that the timing delays of fast
(<~1 s) HXR pulses is consistent with time-of-flight differences of
directly precipitating electrons, while the timing delays of the smooth
HXR flux is consistent with collisional deflection times of trapped
electrons. We show that these HXR timing delays cannot be explained
either by <=1 MeV protons (as proposed in a model by Simnett &
Haines), because of their longer propagation and trapping times, or by
~40 MeV protons (which have the same velocity as ~20 keV electrons),
because of their longer trapping times and the excessive fluxes required
to generate the HXRs. Thus, the HXR timing results clearly rule out
protons as the primary generators of >=20 keV HXR emission.
---------------------------------------------------------
Title: Electron Time-of-Flight Distances and Flare Loop Geometries
Compared from CGRO and YOHKOH Observations
Authors: Aschwanden, Markus J.; Wills, Meredith J.; Hudson, Hugh S.;
Kosugi, Takeo; Schwartz, Richard A.
1996ApJ...468..398A Altcode:
The distance between the coronal acceleration site and the chromospheric
hard X-ray (HXR) emission site can be determined from velocity-dependent
electron time-of-flight (TO F) differences in the framework of the
thick-target model. We determine these electron TOF distances 1 with
relative time delay measurements in the 30-300 keV energy range,
using 16 channel data from BATSE/CGRO for the eight largest flares
simultaneously observed with Yohkoh. We filter the HXR fine structure
from the smoothly varying HXR flux with a Fourier filter in order to
separate competing time delays. In the Yohkoh/HXT images we identify the
corresponding flare loops that show ≥30 keV HXR footpoint emission
and project the electron TOF distances into the loop plane, assuming
a semicircular shape (with radius r). The flare loop radii vary in
the range of r = 5600-17,000 km. In all eight flares we find that the
projected electron TOF distance l' exceeds the loop half-length s =
r(π/2), with a scale-invariant ratio of l'/s = 1.3±0.2. Projecting
the electron TOF distances onto an open field line that extends to the
cusp region above the flare loop, we find an average ratio of h/r =
1.7±0.4 for the height h of the acceleration site. This geometry is
compatible with acceleration mechanisms operating in the cusp region,
perhaps associated with magnetic reconnection processes above the flare
loop. Alternatively, acceleration sites inside the flare loop cannot
be ruled out (since l'/s < 2), but they do not provide a natural
explanation for the observed length ratio l'/s. Large-scale electric
DC field acceleration mechanisms are found to be less suitable to
explain the observed HXR timing and pulse durations.
---------------------------------------------------------
Title: Electron Time-of-Flight Measurements during the Masuda Flare,
1992 January 13
Authors: Aschwanden, Markus J.; Hudson, Hugh; Kosugi, Takeo; Schwartz,
Richard A.
1996ApJ...464..985A Altcode:
The solar flare of 1992 January 13, 1729 UT, has become famous because
Masuda's discovery of a hard X-ray loop-top source (Masuda 1994). Here
we analyze energy-dependent time delays occurring in 30-120 keV hard
X-ray (HXR) emission during this flare, observed by BATSE on board
the Compton Gamma Ray Observatory with a time resolution of 64 ms. The
purpose of this study is to reconstruct the kinematics of HXR-producing
electrons from energy-dependent HXR delays, and to relate the inferred
time-of-flight distance to the spatial geometry of the flare loop,
as observed by SXT and HXT on board Yohkoh. The findings are the
following: <P />1. The HXR flux can be decomposed into a sequence of
pulses with ≍2-3 s duration and into a smoothly varying envelope
that accounts for 90% of the ≥ 30 keV. flux. Cross-correlating the
pulses between five different energy channels in the 30-120 keV range,
we find that the HXR pulses are delayed (τ<SUB>P</SUB> = 40-220 ms)
at the lower energies with respect to the higher energies. For the
HXR envelopes, we find much larger delays (-τ<SUB>E</SUB> = 2.1-6.6
s) of opposite sign. <P />2. We fit kinematic models that quantify
electron acceleration and propagation times to the observed HXR timing,
for small-scale and large-scale accelerating fields, in semicircular
and cusplike flare loop geometries. We find that the acceleration
site is most likely located in an altitude of h = 44,000±6000 km,
in the cusp region above the SXR-emitting flare loop (h = 12,500 km),
and also significantly above Masuda's coronal HXR source (h = 22,100
km). This finding offers an interpretation of Masuda's HXR source in
terms of nonthermal bremsstrahlung by electrons partially confined
in the cusp region either by magnetic mirroring or by wave turbulence
in the reconnection outflow. <P />3.The delay of the smoothly varying
HXR flux is found to be consistent with trapping time differences in
terms of collisional deflection, based on estimates of the electron
density (n<SUB>e</SUB> ≤ 2 x 10<SUP>11</SUP> cm<SUP>-3</SUP>)
from SXT emission measure maps. <P />This study provides the first
quantitative localization of the electron acceleration site in a
solar flare, and demonstrates that energy-dependent HXR delays offer
a sensitive diagnostic for electron acceleration, propagation, and
trapping in solar flares.
---------------------------------------------------------
Title: The Inversion of Electron Time-of-Flight Distances from Hard
X-Ray Time Delay Measurements
Authors: Aschwanden, Markus J.; Schwartz, Richard A.
1996ApJ...464..974A Altcode:
The electron time-of-flight distance 1 between the acceleration
site and the chromosphere can be measured during solar flares from
energy-dependent hard X-ray (HXR) time delays τ(ɛ), based on the
applicability of the thick-target model. The determination of the path
length l represents an inversion problem because the time-dependent
electron injection spectrum at the acceleration site, N(E, t, x = 0),
is retarded by the propagation time t<SUP>prop</SUP>(E)=l/v(E) at the
thick-target site, i.e., N(E, t, x = l) = N[E, t - t<SUP>prop</SUP>(E),
x - 0], and has to be convolved with the bremsstrahlung cross
section σ(ɛ, E) and the instrumental detector response function
R<SUB>i</SUB>(ɛ) to reproduce the observed HXR time profiles
I(E<SUB>i</SUB>, t) (in different detector channels i), from which
the time delay differences τ(ɛ<SUB>i</SUB>) - τ(ɛ<SUB>i</SUB>)
can be measured. <P />In this study, we solve this inversion problem
by numerical forward integration of time-dependent electron injection
spectra N(E, t) with Gaussian pulse shapes to obtain the convolved
time-dependent HXR spectra I(ɛ, t), using specific detector response
functions from the Burst and Transient Source Experiment/Compton Gamma
Ray Observatory and the Hard X-Ray Burst Spectrometer/Solar Maximum
Mission. We find that the timing of HXR pulses can be accurately
represented with the (monoenergetic) photon energy ɛ<SUB>i</SUB> that
corresponds to the median of the channel count spectra C<SUB>i</SUB>(ɛ)
= I(ɛ)R<SUB>i</SUB>(ɛ). We compute numerical conversion factors
q<SUB>E</SUB>(ɛ, y, E<SUB>0</SUB>) that permit the conversion of the
timing of photon energies ɛ<SUB>i</SUB>(t) (for a power-law spectrum
with slope γ and upper cutoff energy E<SUB>0</SUB>) into electron
energies E<SUB>i</SUB>(t) = q<SUB>E</SUB>ɛ<SUB>i</SUB>(t), from
which kinematic parameters can be fitted to determine the electron
time-of-flight path length l. We test the inversion procedure with
numeric simulations and demonstrate that the inversion is accurate
within σ<SUB>l</SUB>/l ≤ 1% for noise-free data. This inversion
procedure is applied to the Masuda flare (in this volume) to localize
the electron acceleration region.
---------------------------------------------------------
Title: Hard X-ray timing
Authors: Aschwanden, Markus J.
1996AIPC..374..300A Altcode: 1996hesp.conf..300A
High-time resolution (64 ms) hard X-ray (HXR) data from BATSE/CGRO allow
us to study the energy-dependent timing of acceleration, propagation,
energy loss, and trapping of ≳20 keV HXR-emitting electrons during
solar flares. In many flares two different HXR flux components can be
distinguished: (1) the fine structure of a HXR time profile (containing
sequences of subsecond pulses) exhibits delays of ≊10-100 ms for
the low-energy electrons, while (2) the unmodulated HXR time profile
(a smooth lower envelope to the fine structure) shows a delay of
opposite sign and much larger magnitude, of typically 1-10 s. We model
the timing of various acceleration mechanisms and find that the delay
of the HXR pulses is dominated by time-of-flight differences rather
than by acceleration time scales, while the timing of the unmodulated
HXR flux is governed by trapping and collisional time scales.
---------------------------------------------------------
Title: Metric spikes and electron acceleration in the solar corona.
Authors: Benz, A. O.; Csillaghy, A.; Aschwanden, M. J.
1996A&A...309..291B Altcode:
Short and narrowband spikes near the starting frequency of metric
type III bursts have been searched and analyzed in Zurich radio
spectrometer data. We find that the probability for the occurrence
of metric spikes is related to the starting frequency of type III
bursts; the association rate increases to 34% for groups of type
III bursts with starting frequencies below 500MHz. The frequency
channel containing most of spikes has been cross-correlated in time
with the frequency channels of type III bursts. The correlation is
significant in all selected cases and is caused by the association
of individual type III bursts with groups of spikes, proving beyond
doubt that spikes and type III bursts are physically related. The
cross-correlation also defines the average drift rate of the type
III maximum and its relation to spikes. In the average, the peak
time of metric spikes coincides with the correlated type III burst,
extrapolated to the same frequency. This may be interpreted as the
spike radiation being emitted at the same characteristic frequency
as the type III emission. The generally higher polarization of spikes
helps to distinguish them from type III bursts. Contrary to spikes at
higher (decimetric) frequencies, 60% of metric spike events have the
same sign of circular polarization as the associated type III bursts,
33% are opposite, while the polarization was not measurable for the
rest of the type III bursts. Some of the metric spike events are
associated with weak flare activity in H<SUB>alpha</SUB>_, but none
has been found to be closely associated with a hard X-ray burst or
a microwave event. Metric spikes are consistent with energy release
at high altitudes producing low energy electron beams. If the spikes
are caused by the same electron population as the type III bursts,
its acceleration site must be below or in close proximity to the
spike source.
---------------------------------------------------------
Title: The Scaling Law between Electron Time-of-Flight Distances
and Loop Lengths in Solar Flares
Authors: Aschwanden, M. J.; Kosugi, T.; Hudson, H. S.; Wills, M. J.;
Schwartz, R. A.
1996AAS...188.2608A Altcode: 1996BAAS...28..858A
;}} ;}} From the complete dataset of solar flares simultaneously
observed with the Burst and Transient Source Experiment (BATSE) onboard
the Compton Gamma Ray Observatory (CGRO) in the high-time resolution
mode (64 ms) and the Hard X-Ray Telescope (HXT) onboard Yohkoh we
were able to determine the electron time-of-flight (TOF) distance
and the flare loop geometry in 42 events. The electron TOF distances
l' were determined from hard X-ray (HXR) time delays (~ 10-100 ms)
occurring in the 16-channel spectra (at ~ 20-200 keV), produced by the
velocity difference of the HXR-producing electrons. The flare loops
were mostly identified from double footpoint sources in >~ 30 keV
HXT images, with radii ranging from r=3000 to r=25,000 km. We find a
scaling law between the electron TOF distance l' and the flare loop
half length s=r(pi /2), having a mean ratio (and standard deviation)
of l'/s=1.41+/- 0.29. In 5 flares we observe coronal >~ 30 keV
HXR sources of the Masuda-type in the cusp region above the flare
loop, and find that their heights are consistent with the electron
TOF distance to the footpoints. These results provide strong evidence
that particle acceleration in solar flares occurs in the cusp region
above the flare loop and that the coronal HXR sources discovered by
Masuda are a signature of the acceleration site, probably controlled
by a magnetic reconnection process.
---------------------------------------------------------
Title: On the Peculiar Radio Polarization of a Sunspot and the
Distibution of the Coronal Plasma
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.
1996AAS...188.3602V Altcode: 1996BAAS...28..873V
A comprehensive set of microwave, soft x-ray and magnetic observations
of solar active region NOAA/USAF 7123 was obtained during 4--9 April,
1992. The observations show an unusual 4.7 GHz source which is polarized
in the sense of the ordinary mode over the umbra of the leading spot. A
detailed analysis of the daily radio and soft X-ray maps of the spot
indicates that (i) the umbral magnetic field is strongly confined,
(ii) the umbral and penumbral loops have distinct atmospheres and (iii)
temperature gradients may exist both radially and vertically over the
spot. A possible scenario for the temporal evolution of the spot is
also presented.
---------------------------------------------------------
Title: Soft X-Ray (Yohkoh) and Radio (VLA) Observations of Solar
Narrowband, Millisecond Spike Events
Authors: Krucker, Sam; Benz, Arnold O.; Aschwanden, Markus J.
1996ASPC..111..129K Altcode: 1997ASPC..111..129K
The source regions of metric spike events are investigated on Yohkoh
soft X-ray (SXR) maps. The spikes are identified by the spectrometer
Phoenix between 300 MHz and 360 MHz and are associated with groups
of type III bursts at lower frequencies reaching also the decametric
range. The Very Large Array (VLA) provides simultaneously spatial
information at 333 MHz and 1445 MHz. Similar to the previous VLA
observation of a metric spike event, the new data are consistent with a
high altitude of the spike sources of about 5×10<SUP>10</SUP>cm above
the photosphere. The additionally available SXR data for one of the
presented events give the following new information: (i) The spike
sources occur near open field lines and near regions of a slightly
enhanced SXR flux relative to the ambient plasma. (ii) Contrary to SXR
observations of type III bursts without metric spike activity, no SXR
jet is observed. (iii) At low altitude, a weak SXR enhancement occurs,
peaking about 60 s after the spike event. The SXR source and the spike
sources can be connected by potential field lines. The observations
corroborate a model in which a metric spike event is attributed to
an energy release region at high altitude, while upwards propagating
electrons produce type III bursts and downward moving electrons are
responsible for SXR emission of heated plasma.
---------------------------------------------------------
Title: Joint Radio and Soft X-Ray Imaging of an `Anemone' Active
Region
Authors: Vourlidas, A.; Bastian, T. S.; Nitta, N.; Aschwanden, M. J.
1996SoPh..163...99V Altcode:
The Very Large Array and the Soft X-ray Telescope (SXT) aboard the
Yohkoh satellite jointly observed the rapid growth and decay of a
so-called `anemone' active region on 3-6 April, 1992 (AR 7124). The
VLA obtained maps of the AR 7124 at 1.5, 4.7, and 8.4 GHz. In general,
discrete coronal loop systems are rarely resolved at 1.5 GHz wavelengths
because of limited brightness contrast due to optical depth effects and
wave scattering. Due to its unusual anemone-like morphology, however,
several discrete loops or loop systems are resolved by both the VLA
at 1.5 GHz and the SXT in AR 7124.
---------------------------------------------------------
Title: Do Electron Time-of-Flight Measurements Support Magnetic
Reconnection Models?
Authors: Aschwanden, Markus
1996ASPC..111..216A Altcode: 1997ASPC..111..216A
Energy-dependent hard X-ray (HXR) timing measurements have became an
important new tool to determine the electron time-of-flight (TOF)
distance between the acceleration site and the HXR-emitting energy
loss site during solar flares. This electron TOF distance l' was found
to have a scale-invariant ratio to the flare loop half length s for
a large range of flare loop sizes, i.e. l'/s = 1.4±0.3 (Aschwanden
et al. 1996). The author discusses the consequences of this result,
specifically whether it constrains the location of the electron
acceleration to be inside or outside flare loops. In the latter case,
it would support flare models with magnetic reconnection in the cusp
region of the loop.
---------------------------------------------------------
Title: Electron Time-of-Flight Measurements
Authors: Aschwanden, M. J.
1996mpsa.conf..209A Altcode: 1996IAUCo.153..209A
No abstract at ADS
---------------------------------------------------------
Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft
X-ray Maps
Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S.
1996mpsa.conf..441K Altcode: 1996IAUCo.153..441K
No abstract at ADS
---------------------------------------------------------
Title: Accuracy, Uncertainties, and Delay Distribution of Electron
Time-of-Flight Measurements in Solar Flares
Authors: Aschwanden, Markus J.; Schwartz, Richard A.
1995ApJ...455..699A Altcode:
A systematic time lag of ≍20 ms between the 25-50 keV and 50-100
keV hard X-ray (HXR) emission has been recently discovered in solar
flares. This was interpreted in terms of electron time-of-flight
differences (Aschwanden, Schwartz, & Alt 1995c). Here we attempt
to determine the accuracy and uncertainties of such energy-dependent
time delay measurements using burst-trigger data from DISCSC/BATSE
on the CGRO spacecraft, recorded with a time resolution of 64 ms. We
evaluate the time delays by cross-correlating entire flare time profiles
at different energies and evaluate the statistical uncertainty of a
delay measurement with a Monte Carlo method, in which random noise is
added to the raw data. We examine also uncertainties resulting from
aliasing, incomplete sampling, and pulse pileup. <P />We measure the
time delays τ = t<SUB>25keV</SUB> - t<SUB>50keV</SUB> in 622 flares,
with a statistical uncertainty of u ≤ 32 ms in 29% of the events,
or u < 64 ms in 65% of the events. The distribution f(τ) of the
time delays from all flares can be characterized with three components:
(1) a Gaussian peak at τ = 23.2 + 1.2 ms with a standard deviation
of σ<SUB>τ</SUB> = 25.5 ms, (2) a power-law tail with a slope of -
2.0 for large positive delays (τ = 0.1-4 s), and (3) a power-law
tail with a slope of - 1.3 for large negative delays ( |-τ| =
0.1-7.7 s). The percentages of flares in these three regimes are
15%, 69%, and 16%. Flares with short delays (|τ| ≤ 0.1 s) exhibit
rapid fluctuations with subsecond pulses. Such rapid fluctuations are
almost absent in flares with longer delays. We find also a systematic
trend of softer spectra in flares with large positive delays. <P />We
develop a simple physical model that combines electron time-of-flight
differences in the thin-target and thick-target model. We are able to
reproduce the observed time delay distribution in the range of |τ|
≲ 0.1 s, requiring a distribution of electron densities in the range
of n<SUB>e</SUB> ≤ 3.0 × 10<SUP>12</SUP> cm<SUP>-3</SUP> and flare
loop heights in the range of h ≤ 35,000 km. Large negative delays
(τ ≲ -0.1 s) can be produced in low-density loops with efficient
magnetic trapping. Large positive delays (τ ≳ 0.1 s) occur in flares
with a strong thermal component due to the convolution of the injection
profile with the heating and cooling function. This study demonstrates
that energy-dependent HXR time delays can be used as a diagnostic and
discriminator of flare models.
---------------------------------------------------------
Title: Active Region 7123: Its Peculiar Radio Polarization and the
Distribution of the Umbral Coronal Plasma
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.
1995AAS...18710105V Altcode: 1995BAAS...27.1426V
A comprehensive set of microwave and soft x-ray observations of solar
active region 7123 was obtained during 3--10 April, 1992. Here, we
present a detailed analysis of the polarized radio emission from the
sunspot-associated component of AR 7123. We use the VLA observations
at 1.5, 4.7 and 8.4 GHz, supported by the available x-ray and magnetic
data. We concentrate on understanding the 4.7 GHz o-mode polarization
over the umbra of the leading spot of AR 7123 and its variation
with aspect angle within the physical context provided by the x-ray
observations (SXT/Yohkoh) and past work on umbral atmospheres.
---------------------------------------------------------
Title: Solar Electron Beams Detected in Hard X-Rays and Radio Waves
Authors: Aschwanden, Markus J.; Benz, Arnold O.; Dennis, Brian R.;
Schwartz, Richard A.
1995ApJ...455..347A Altcode:
We present a statistical survey of electron beam signatures that are
detected simultaneously at hard X-ray (HXR) and radio wavelengths
during solar flares. For the identification of a simultaneous event we
require a type III (normal-drifting or reverse-slope-drifting) radio
burst that coincides (within ± 1 s) with a significant (≥ 3 σ HXR
pulse of similar duration (≥ 1 s). Our survey covers all HXRBS/SMM and
BATSE/CGRO flares that were simultaneously observed with the 0.1-1 GHz
spectrometer Ikarus or the 0.1-3 GHz spectrometer Phoenix of ETH Zurich
during 1980-1993. The major results and conclusions are as follows:
<P />1. We identified 233 HXR pulses (out of 882) to be correlated with
type III-like radio bursts: 77% with normal-drifting type III bursts,
34% with reverse-slope (RS)-drifting bursts, and 13% with oppositely
drifting (III + RS) burst pairs. The majority of these cases provide
evidence for acceleration of bidirectional electron beams. <P />2. The
detailed correlation with type III-like radio bursts suggests that most
of the subsecond fluctuations detectable in ≥ 25 keV HXR emission
are related to discrete electron injections. This is also supported
by the proportionality of the HXR pulse duration with the radio burst
duration. The distribution of HXR pulse durations W<SUB>X</SUB> is
found to have an exponential distribution, i.e., N(W<SUB>X</SUB>) ∝
exp (-W<SUB>X</SUB>/0.25 s) in the measured range of W<SUB>X</SUB> ≍
0.5-1.5 s. <P />3. From oppositely drifting radio burst pairs we infer
electron densities of n<SUB>e</SUB> = 10<SUP>9</SUP>-10<SUP>10</SUP>
cm<SUP>-3</SUP> at the acceleration site. From the absence of a
frequency gap between the simultaneous start frequencies of upward and
downward drifting radio bursts, we infer an upper limit of L ≤ 2000
km for the extent of the acceleration site and an acceleration time of
Δt ≤ 3 ms for the (≥ 5 keV) radio-emitting electrons (in the case
of parallel electric fields). <P />4. The relative timing between HXR
pulses and radio bursts is best at the start frequency (of earliest
radio detection), with a coincidence of ≲0.1 s in the statistical
average, while the radio bursts are delayed at all other frequencies
(in the statistical average). The timing is consistent with the scenario
of electron injection at a mean coronal height of h ≍ 10<SUP>4</SUP>
km. The radio-emitting electrons are found to have lower energies
(≳ 5 keV) than the ≥ 25 keV HXR-emitting electrons. <P />5. The
modulated HXR flux that correlates with electron beam signatures
in radio amounts to 2%-6% of the total HXR count rate (for BATSE
flares). The associated kinetic energy in electrons is estimated to be E
= 4 × 10<SUP>22</SUP>-10<SUP>27</SUP> ergs per beam, or N<SUB>e</SUB>
= 4 × 10<SUP>28</SUP>-10<SUP>33</SUP> electrons per beam, considering
the spread from the smallest to the largest flare detected by HXRBS. <P
/>6. The average drift rate of propagating electron beams is found
here to be [dv/dt] = 0.10ν<SUP>1.4</SUP> MHz km s<SUP>-1</SUP> in
the frequency range of ν = 200-3000 MHz, which is lower than expected
from the Alvarez & Haddock relation for frequencies ≤ 550 MHz. <P
/>7. The frequency distributions of HXR fluxes (F<SUB>x</SUB>) and radio
type III burst fluxes (F<SUB>R</SUB>), which both can be characterized
by a power law, are found to have a significantly different slope,
i.e., N(F<SUB>x</SUB>) ∝ F<SUB>x</SUB><SUP>-1.87</SUP> versus
N(F<SUB>R</SUB>) ∝ F<SUB>R</SUB><SUP>-1.28</SUP>. The difference
in the slope is attributed to the fundamental difference between
incoherent and coherent emission processes. <P />In summary, these
findings suggest a flare scenario in which bidirectional streams of
electrons are accelerated during solar flares at heights of 10 km
above the photosphere in rather compact regions (L ≲ 2000 km). The
acceleration site is likely to be located near the top of flare loops
(defined by HXR double footpoints) or in the cusp above, where electrons
have also access to open field lines or larger arches. The observed
bidirectionality of electron beams favors acceleration mechanisms with
oppositely directed electric fields or stochastic acceleration in an
X-type reconnection geometry.
---------------------------------------------------------
Title: Solar Rotation Stereoscopy in Microwaves
Authors: Aschwanden, Markus J.; Lim, Jeremy; Gary, Dale E.; Klimchuk,
James A.
1995ApJ...454..512A Altcode:
We present here the first stereoscopic altitude measurements of active
region sources observed at microwave frequencies (10-14 GHz The active
region NOAA 7128 was observed with the Owens Valley Radio Observatory
(OVRO) on 1992 April 13, 14, 15, and 16 as it passed through the central
meridian. From white-light data of the underlying sunspot we determined
the rotation rate of the active region, which was found to have a
relative motion of dL/dt = +0°.240 day<SUP>-1</SUP> with respect to the
standard photospheric differential rotation rate. Based on this rotation
rate we determine for the microwave sources stereoscopic altitudes of
3.3-11.0 Mm above the photosphere. The altitude spectrum h(v) of the
right circular polarization (RCP) main source shows a discontinuity
at 12 GHz and can be satisfactorily fitted with a dipole model with a
transition from the second to the third harmonic level at 12 GHz. The
dominance of the third harmonic for frequencies above 12 GHz occurs
because the second harmonic level drops below the transition region, at
a height of 2.6±0.6 Mm according to the microwave data. The altitude
spectrum h(v) serves also to invert the temperature profile T(h)
from the optically thick parts of the radio brightness temperature
spectrum T<SUB>B</SUB>(ν[h]). The microwave emission in both circular
polarizations can be modeled with gyroresonance emission, with x-mode
for RCP and o-mode in LCP, with the same harmonics at each frequency,
but different emission angles in both modes. The contributions from
free-free emission are negligible in both polarizations, based on the
peak emission measure of EM ≍ 6 × 10<SUP>28</SUP> cm<SUP>-5</SUP>
observed in soft X-rays by Yohkoh/SXT. <P />This study demonstrates
that the height dependence of the coronal magnetic field B(h) and
the plasma temperature T(h) in an active region can be inverted from
the stereoscopic altitude spectra h(v) and the observed brightness
temperature spectra T<SUB>B</SUB>(ν).
---------------------------------------------------------
Title: First VLA observation of a solar narrowband, millisecond
spike event.
Authors: Krucker, S.; Aschwanden, M. J.; Bastian, T. S.; Benz, A. O.
1995A&A...302..551K Altcode:
The first spatially resolved observation of solar, narrowband spikes
in two dimensions is presented. The 'metric' spikes around 333MHz
are classified by the broadband spectrometer Phoenix (ETH Zuerich),
whereas the simultaneously observing Very Large Array (VLA) provides
high angular resolution images of the solar disc. At lower frequencies,
a group of associated type III bursts is detected. The spikes occur
at high altitude (=~4.5x10^10^cm above the photosphere), and at least
3 separated locations of emission can be identified. The different
spike sources are separated by up to 130" and show different degrees of
polarization. Spikes and type III bursts have the same sense of circular
polarization, and according to the extrapolated potential field lines,
the polarization of the different spike sources is in x-mode. With
a delay of 42s, a thermal source appears on the same extrapolated
potential field lines as the spikes at the second frequency of the
VLA (1446MHz). The location of the energy release relative to the
spikes source is discussed. A scenario is proposed where the energy
is released in or near the spike source, and in which the spikes,
the type III bursts and the thermal source originate from the same
energy release. Hot electrons expanding along the field lines generate
a type III burst (upward direction) and heat the underlying dense plasma
(thermal source).
---------------------------------------------------------
Title: Location of Type I Radio Continuum and Bursts on YOHKOH Soft
X-Ray Maps
Authors: Krucker, S.; Benz, A. O.; Aschwanden, M. J.; Bastian, T. S.
1995SoPh..160..151K Altcode:
A solar type I noise storm was observed on 30 July, 1992 with the
radio spectrometer Phoenix of ETH Zürich, the Very Large Array (VLA)
and the soft X-ray (SXR) telescope on board theYohkoh satellite. The
spectrogram was used to identify the type I noise storm. In the VLA
images at 333 MHz a fully left circular polarized (100% LCP) continuum
source and several highly polarized (70% to 100% LCP) burst sources
have been located. The continuum and the bursts are spatially separated
by about 100″ and apparently lie on different loops as outlined
by the SXR. Continuum and bursts are separated in the perpendicular
direction to the magnetic field configuration. Between the periods of
strong burst activities, burst-like emissions are also superimposed
on the continuum source. There is no obvious correlation between the
flux density of the continuum and the bursts. The burst sources have
no systematic motion, whereas the the continuum source shows a small
drift of ≈ 0.2″ min<SUP>−1</SUP> along the X-ray loop in the
long-time evolution. The VLA maps at higher frequency (1446 MHz) show
no source corresponding to the type I event. The soft X-ray emission
measure and temperature were calculated. The type I continuum source
is located (in projection) in a region with enhanced SXR emission,
a loop having a mean density of «n<SUB>e</SUB>» = (1.5 ± 0.4) ×
10<SUP>9</SUP> cm<SUP>−3</SUP> and a temperature ofT = (2.1 ± 0.1)
× 10<SUP>6</SUP> K. The centroid positions of the left and right
circularly polarized components of the burst sources are separated
by 15″-50″ and seem to be on different loops. These observations
contradict the predictions of existing type I theories.
---------------------------------------------------------
Title: Electron Time-of-Flight Differences in Solar Flares
Authors: Aschwanden, Markus J.; Schwartz, Richard A.; Alt, Daniel M.
1995ApJ...447..923A Altcode:
In the thick-target model for hard X-ray (HXR) emission in solar
flares, electron acceleration is assumed to occur in flaring loops
at coronal heights, while HXR bremsstrahlung emission is produced
in the chromosphere. Under this assumption, the velocity spectrum of
the accelerated electrons causes time-of-flight differences that are
expected to result in the lower energy HXRs to be delayed with respect
to the higher energies. Here we report on the first observational
evidence for such a delay. The electron time-of-flight differences
between electrons that produce 25-50 keV and 50-100 keV HXR emission
are found to have a distribution with a mean of τ = 16.7±1.9 ms and
a standard deviation of σ<SUB>τ</SUB> = 16.8 ms. This result is based
on the statistics of 5430 HXR pulses detected during 640 solar flares,
recorded in the Discriminator Science Data (DISCSC) burst trigger
mode with a time resolution of 64 ms by the Burst and Transient
Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory
(CGRO). From the time-of-flight differences we infer a mean altitude of
the acceleration site of H = 7300±800 km (with a standard deviation of
σ<SUB>H</SUB> = 7300 km) above the level at which the electrons lose
their energy. This derived mean loop height should be considered as a
lower limit because it is based on the predominance of time-of-flight
effects over opposite delay effects caused by pitch-angle scattering or
trapping. For the electron density in the flare loops we find an upper
limit of n<SUB>e</SUB> ≤ 4 × 10<SUP>12</SUP> cm<SUP>-3</SUP>, based
on the requirement that the electron travel time has to be shorter than
the collision time. The relatively small time-of-flight differences
correspond typically to only ≍3% of the HXR pulse duration, and,
therefore, no rapid variation in the spectral slope of the observed
HXR spectrum is expected.
---------------------------------------------------------
Title: Aspect Angle Dependence of the Polarized Radio Emission from
AR 7123
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M. J.; Nitta, N.
1995SPD....26..701V Altcode: 1995BAAS...27..965V
No abstract at ADS
---------------------------------------------------------
Title: Electron Time-of-Flight Measurements in Solar Flares
Authors: Aschwanden, M. J.; Schwartz, R. A.
1995SPD....26.1208A Altcode: 1995BAAS...27..984A
No abstract at ADS
---------------------------------------------------------
Title: Sequences of Correlated Hard X-Ray and Type III Bursts during
Solar Flares
Authors: Aschwanden, Markus J.; Montello, Maria L.; Dennis, Brian R.;
Benz, Arnold O.
1995ApJ...440..394A Altcode:
Acceleration and injection of electron beams in solar flares can be
traced from radio type III (or type U) bursts and correlated hard
X-ray pulses with similar timescales and nonthermal spectra. We
perform a systematic survey of such correlated radio and hard X-ray
(HXR) pulses with timescales of less than or approximately 2 s in
flares simultaneously observed with the radio spectrometer Ikarus and
the Hard X-Ray Burst Spectrometer (HXRBS) on solar maximum mission
(SMM). We applied an epoch-folding technique to enhance correlated
time patterns in burst sequences at the two wavelengths. We present
the results from the strongest (10) flares with a HXRBS count
rate greater than or = 3000 counts/s, which have a satisfactory
signal-to-noise ratio for subsecond pulses. The major findings of
this study are presented. These observations strongly suggest that
particle acceleration in solar flares occurs in a pulsed mode where
electron beams are simultaneously injected in upward and downward
directions. Since the sequences of correlated HXR and radio bursts
show identical durations and intervals at the two wavelengths, they
are believed to reflect most directly the temporal dynamics of the
underlying common accelerator. As a consequence, thick-target models
should be reconsidered under the aspect of electron injection with
pulse durations of 0.2-2.0 s and duty cycles of approximately = 50%.
---------------------------------------------------------
Title: Chromospheric Evaporation and Decimetric Radio Emission in
Solar Flares
Authors: Aschwanden, Markus J.; Benz, Arnold O.
1995ApJ...438..997A Altcode:
We have discovered decimetric signatures of the chromospheric
evaporation process. Evidence for the radio detection of chromospheric
evaporation is based on the radio-inferred values of (1) the electron
density, (2) the propagation speed, and (3) the timing, which are
found to be in good agreement with statistical values inferred from
the blueshifted Ca XIX soft X-ray line. The physical basis of our
model is that free-free absorption of plasma emission is strongly
modified by the steep density gradient and the large temperature
increase in the upflowing flare plasma. The steplike density increase
at the chromospheric evaporation front causes a local discontinuity
in the plasma frequency, manifested as almost infinite drift rate in
decimetric type III bursts. The large temperature increase of the
upflowing plasma considerably reduces the local free-free opacity
(due to the T<SUP>-3/2</SUP> dependence) and thus enhances the
brightness of radio bursts emitted at the local plasma frequency near
the chromospheric evaporation front, while a high-frequency cutoff
is expected in the high-density regions behind the front, which can
be used to infer the velocity of the upflowing plasma. From model
calculations we find strong evidence that decimetric bursts with a
slowly drifting high-frequency cutoff are produced by fundamental plasma
emission, contrary to the widespread belief that decimetric bursts
are preferentially emitted at the harmonic plasma level. We analyze
21 flare episodes from 1991-1993 for which broadband (100-3000 MHz)
radio dynamic spectra from Pheonix, hard X-ray data from (BATSE/CGRO)
and soft X-ray data from Burst and Transient Source Experiment/Compton
Gamma Ray Observatory (GOES) were available.
---------------------------------------------------------
Title: Imaging, Stereoscopy, and Tomography of the Solar Corona in
Soft X-Rays and Radio
Authors: Aschwanden, M. J.
1995LNP...444...13A Altcode: 1995cmer.conf...13A
We review simultaneous imaging observations of the (quiet) solar corona
in soft X-rays (or EUV) and radio, and outline recent developments
that involve three-dimensional (3D) reconstruction techniques, such as
stereoscopy and tomography. The 3D reconstruction of coronal structures
involves not only accurate measurements of geometric parameters
(position, altitude, rotation rate), but also the deconvolution of
physical parameters (density, temperature, magnetic field) along the
line-of-sight, which is most feasible with simultaneous observations
in complementary wavelengths, e.g. in soft X-rays, EUV, and radio.
---------------------------------------------------------
Title: Joint Radio and Soft X-ray Imaging of an “Anemone” Active
Region
Authors: Vourlidas, A.; Bastian, T. S.; Aschwanden, M.; Nitta, N.
1994AAS...185.8609V Altcode: 1994BAAS...26Q1465V
The Very Large Array and the Soft X-ray Telescope aboard the Yohkoh
satellite observed the rapid growth and decay of a so-called “anemone”
active region on 3-6 April (AR 7124). In general, discrete coronal
loop systems are rarely resolved at radio wavelengths due to optical
depth effects and scattering. In the case of AR 7124, however, several
discrete loops or loop systems are resolved by both the VLA and the SXT,
probably due to its unusual “anemone”-like morphology. Furthermore,
the region exhibited a significant amount of variability, especially
on April 3, marked by many subflares and intensity changes as has been
reported in flare patrol observations. In the present study, physical
parameters derived from multiband radio and soft x-ray techniques for
discrete loops are compared. The temporal evolution of these parameters
is also examined.
---------------------------------------------------------
Title: Electron beams in solar flares
Authors: Aschwanden, Markus J.; Dennis, Brian R.; Benz, Arnold O.
1994umd..reptR....A Altcode:
A list of publications resulting from this program includes 'The
Timing of Electron Beam Signatures in Hard X-Ray and Radio: Solar
Flare Observations by BATSE/Compton Gamma-Ray Observatory and PHOENIX';
'Coherent-Phase or Random-Phase Acceleration of Electron Beams in Solar
Flares'; 'Particle Acceleration in Flares'; 'Chromospheric Evaporation
and Decimetric Radio Emission in Solar Flares'; 'Sequences of Correlated
Hard X-Ray and Type 3 Bursts During Solar Flares'; and 'Solar Electron
Beams Detected in Hard X-Rays and Radiowaves.' Abstracts and reprints
of each are attached to this report.
---------------------------------------------------------
Title: Coherent-Phase or Random-Phase Acceleration of Electron Beams
in Solar Flares
Authors: Aschwanden, Markus J.; Benz, Arnold O.; Montello, Maria L.
1994ApJ...431..432A Altcode:
Time structures of electron beam signatures at radio wavelengths
are investigated to probe correlated versus random behavior in solar
flares. In particular we address the issue whether acceleration and
injection of electron beams is coherently modulated by a single source,
or whether the injection is driven by a stochastic (possibly spatially
fragmented) process. We analyze a total of approximately = 6000 type III
bursts observed by Ikarus (Zurich) in the frequency range of 100-500
MHz, during 359 solar flares with simultaneous greater than or = 25
keV hard X-ray emission, in the years 1890-1983. In 155 flares we find
a total of 260 continuous type III groups, with an average number of
13 +/- 9 bursts per group, a mean duration of D = 12 +/- 14 s, a mean
period of P = 2.0 +/- 1.2 s, with the highest burst rate at a frequency
of nu = 310 +/- 120 MHz. Pulse periods have been measured between 0.5
and 10 s, and can be described by an exponential distribution, i.e.,
N(P) varies as e <SUP>-P/1.0s</SUP>. The period shows a frequency
dependence of P(nu)=46(exp-0.6)<SUB>MHz</SUB>s for different flares,
but is invariant during a particular flare. We measure the mean
period P and its standard deviation sigma <SUB>p</SUB> in each type
III group, and quantify the degree of periodicity (or phase-coherence)
by the dimensionless parameter sigma <SUB>p</SUB>P. The representative
sample of 260 type III burst groups shows a mean periodicity of sigma
<SUB>p/P</SUB> = 0.37 +/- 0.12, while Monte Carlo simulations of an
equivalent set of truly random time series show a distinctly different
value of sigma <SUB>p</SUB>P = 0.93 +/- 0.26. This result indicates
that the injection of electron beams is coherently modulated by a
particle acceleration source which is either compact or has a global
organization on a timescale of seconds, in contrast to an incoherent
acceleration source, which is stochastic either in time or space. We
discuss the constraints on the size of the acceleration region resulting
from electron beam propagation delays and from Alfvenic synchronization
during a pulse period. We discuss two periodic processes in flares,
which potentially control quasi-periodic particle acceleration: (1)
MHD oscillations, and (2) current sheets with oscillatory dynamics.
---------------------------------------------------------
Title: Particle Acceleration in Flares
Authors: Benz, A. O.; Kosugi, T.; Aschwanden, M. J.; Benka, S. G.;
Chupp, E. L.; Enome, S.; Garcia, H.; Holman, G. D.; Kurt, V. G.;
Sakao, T.; Stepanov, A. V.; Volwerk, M.
1994SoPh..153...33B Altcode:
Particle acceleration is intrinsic to the primary energy release in
the impulsive phase of solar flares, and we cannot understand flares
without understanding acceleration. New observations in soft and hard
X-rays, γ-rays and coherent radio emissions are presented, suggesting
flare fragmentation in time and space. X-ray and radio measurements
exhibit at least five different time scales in flares. In addition,
some new observations of delayed acceleration signatures are also
presented. The theory of acceleration by parallel electric fields is
used to model the spectral shape and evolution of hard X-rays. The
possibility of the appearance of double layers is further investigated.
---------------------------------------------------------
Title: VLA Stereoscopy of Solar Active Regions
Authors: Aschwanden, M. J.; Bastian, T. S.
1994kofu.symp..357A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Irradiance observations of the 1 8 Å solar soft X-ray flux
from goes
Authors: Aschwanden, Markus J.
1994SoPh..152...53A Altcode: 1994IAUCo.143...53A; 1994svs..coll...53A
The solar 0.5-8 å soft X-ray flux was monitored by the NOAA
Geostationary Operational Environmental Satellites (GOES) from
1974 to the present, providing a continuous record over two solar
activity cycles. Attempts have been made to determine a soft X-ray
(SXR) background flux by subtracting out solar flares (using the daily
lowest flux level). The SXR background flux represents the quiescent
SXR flux from heated plasma in active regions, and reflects similar
(intermediate-term) variability and periodicities (e.g. 155-day period)
as the SXR or hard X-ray (HXR) flare rate, although it is determined
in non-flaring time intervals. The SXR background flux peaks late
in Solar Cycle 21 (2-3 years after the sunspot maximum), similar to
the flare rate measured in SXR, HXR, or gamma rays, possibly due the
increasing complexity of coronal magnetic structures in the decay phase
of the solar cycle. The SXR background flux appears to be dominated
by postflare emission from the dominant active regions, while the
contributions from the quiet Sun are appreciable in the Solar Minimum
only (A1-level). Comparisons with full-disk integrated images from
YOHKOH suggest that the presence of coronal holes can decrease the
quietest SXR irradiance level by an additional order of magnitude,
but only in the rare case of absence of active regions.
---------------------------------------------------------
Title: VLA Stereoscopy of Solar Active Regions. I. Method and Tests
Authors: Aschwanden, Markus J.; Bastian, T. S.
1994ApJ...426..425A Altcode:
We develop a new technique for extracting three-dimensional information
from multiday solar Very Large Array (VLA) observations. While standard
stereoscopic methods provide a three-dimensional view of an object by
combining simultaneous observations from two different aspect angles,
we relax the condition of simultaneity and exploit solar rotation
to vary the aspect angle. The solar radio images are decomposed into
Gaussian source components, which are then cross-correlated in maps
from preceding and following days. This provides measurements of the
three-dimensional position of correlated source centroids. In this
first paper, we describe the stereoscopic method and perform tests
with simulated and real radio maps (from the VLA at 20 cm), in order
to study the accuracy of altitude measurements, and the limitations
introduced by (i) source confusion, (ii) source motion, and (iii)
the assumed differential rotation rate. The tests demonstrate that
(i) the information content of a VLA map relevant for stereoscopic
correlation can be conveniently represented in terms of a small number
of Gaussian components; (ii) the fitting of the three-dimensional
source position is stable within a numerical accuracy of less than or
approximately equal to 0.02 map pixels, (iii) the relative accuracy
of the altitude determination is uniform over the solar disk, and (iv)
source confusion does not affect the accuracy of stereoscopic position
measurements for sources with a signal-to-noise ratio of greater than
or approximately equal to 36.
---------------------------------------------------------
Title: VLA Stereoscopy of Solar Active Regions. II. Altitude, Relative
Motion, and Center-to-Limb Darkening of 20 Centimeter Emission
Authors: Aschwanden, Markus J.; Bastian, T. S.
1994ApJ...426..434A Altcode:
A newly developed method of stereoscopic correlation is applied to solar
radio maps at a wavelength of 20 cm. On the basis of stereoscopic
correlations between radio maps obtained on 6 different days we
establish the presence of 66 radio source components associated with
22 active regions. We find the following statistical results for
active region source structures at 20 cm: 1. The observed lifetime
is consistent with an exponential distribution having an e-folding
time scale of greater than 18 days. 2. The average altitude of 20 cm
sources is 25 +/- 15 Mm; 90% of the sources are found in heights less
than 40 Mm. 3. The average diameter of discrete source structures
is 48 +/- 15 Mm, implying a vertical/horizontal aspect ratio of
q<SUB>A</SUB> approx. = 0.5. 4. No significant source motion has
been found with respect to the standard differential rotation rate
of Omega = 13.45 deg - ((3 deg (sin<SUP>2</SUP>B)). 5. We find a
statistical limb darkening, which can be described by the relation
(T<SUB>B</SUB>(alpha))/T<SUB>B</SUB>(0)) = 0.4 + ((0.6(cos<SUP>2</SUP>
alpha)) for sources with T<SUB>B</SUB> greater than 0.5 MK. 6. The
degree of source polarization is 15% +/- 10% and is independent of
source location. 7. Bright sources (greater than or approximately
equal to 0.5 MK), or equivalently, long-lived sources (greater than
or approximately equal to 5 days) show a systematic variation of their
altitude as function of the center-limb distance.
---------------------------------------------------------
Title: Periodic or random acceleration in solar flares?
Authors: Aschwanden, Markus J.; Benz, Arnold O.
1994SSRv...68..193A Altcode:
The issue whether acceleration and injection of electron beams is
coherently modulated by a single quasi-periodic source, or whether the
injection is driven by a stochastic process in time or (eventually
fragmented) in space, is investigated by menas of a periodicity
analysis of metric type III bursts. We analyze 260 continuous type
III groups observed byIkarus (ETH Zurich) in the frequency range
of 100 500 MHz during 359 solar flares with simultaneous ≥25 keV
hard X-ray emission, in the years 1980 1983. Pulse periods have been
measured between 0.5 and 10 s, and can be described by an exponential
distribution, i.e.N(P) ∝e <SUP>-P/1.0s</SUP>. We measure the mean
periodP and its standard deviation σp in each type III group, and
quantify the degree of periodicity by the dimensionless parameter
σp/P. The representative sample of 260 type III burst groups shows a
mean periodicity of σp/P=0.37±0.12, while Monte-Carlo simulations
of an equivalent set of truly random time series show a distinctly
different value of σp/P=0.93±0.26. This result suggests that the
injection of electron beams is periodically modulated by a particle
acceleration source which is either compact or has a global organization
on a time scale of seconds.
---------------------------------------------------------
Title: Pulsed Acceleration in Solar Flares
Authors: Aschwanden, Markus J.; Benz, Arnold O.; Dennis, Brian R.;
Kundu, Mukul R.
1994ApJS...90..631A Altcode: 1994IAUCo.142..631A
We study the nonlinear dynamics of particle acceleration in solar flares
by analyzing the time series of various quasi-periodic radio signatures
during flares. In particular we present the radio and hard X-ray data
of three flares which suppport the following tentative conclusions:
(1) Particle acceleration and injection into magnetic structures
occurs intrinsically in a pulsed mode (with a typical period of
1-2 s), produced by a single, spatially coherent, nonlinear system,
rather than by a stochastic system with many spatially independent
components ('statistical flare' produced by a fragmented primary energy
release). (2) The nonlinear (quasi-periodic) mode of pulsed particle
acceleration and injection into a coronal loop can be stabilized by
phase locking with an MHD wave (oscillation) mode, if both periods
are close to each other. (3) Pulsed injection of electron beams into
a coronal loop may trigger nonlinear relaxational oscillations of
wave-particle interactions. This is particularly likely when the limit
cycles of both systems are similar.
---------------------------------------------------------
Title: The Timing of Electron Beam Signatures in Hard X-Ray and Radio:
Solar Flare Observations by BATSE/Compton Gamma-Ray Observatory
and PHOENIX
Authors: Aschwanden, Markus J.; Benz, Arnold O.; Schwartz, Richard A.
1993ApJ...417..790A Altcode:
We analyzed two solar flares of 1992 September 5 and 6, using the high
time resolution (64 ms) hard X-ray data from BATSE/CGRO, and 100-3000
MHz radio (100 ms) dynamic spectra from PHOENIX. The broadband radio
data reveal a separatrix frequency (at 620 and 750 MHz in the two
flares) between normal- and reverse-drifting radio bursts, indicating
a compact acceleration source where electron beams are injected in
both the upward and downward direction. We find a mean injection rate
of 1.2 bursts s<SUP>-1</SUP> in one flare and more than 0.7 bursts
s<SUP>-1</SUP> in the other. From 12 broad-band, reverse-drifting radio
bursts we find in five cases an unambiguous one-to-one correlation
between the reverse-drifting radio bursts and hard X-ray (HXR) pulses
of similar duration (400±220 ms). The high significance (15±6 σ) of
the HXR pulses and the small scatter (±150 ms) in the relative timing
strongly supports a close causal connection. The cross-correlation
between HXR and radio pulses shows that the HXR pulses are coincident
(within the instrumental time resolution) with the reverse-drifting
bursts at the injection frequency (880±50 MHz), and lead the radio
bursts by 270±150 ms at the highest observable frequency (1240±100
MHz). The average drift time of the downward propagating radio bursts
is measured to 150 ms, corresponding to a drift rate of 2350 MHz
s<SUP>-1</SUP>. <P />We examined various effects to model the observed
timing of radio and HXR pulses (propagation delays, radio wave growth
and damping, group velocity delays, radio wave scattering, radio wave
ducting, light path differences, etc.). Assuming an exciter velocity
of υ<SUB>R</SUB>/c = 0.2±0.1 for the reverse-drifting radio bursts,
we infer an altitude difference of H = 8000±3000 km between the
injection site and the HXR source. The most likely explanation for the
retarded radio emission seems to be a combination of the following two
effects: (1) HXR-emitting (>25 keV) and radio-emitting electrons have
different energies (the exciter velocity of the reverse-drifting radio
bursts is associated with ≲5 keV electrons), and (2) a low (marginal)
growth rate for plasma emission at the second harmonics. Delay effects
caused by group velocity, collisional damping, wave scattering, and
wave ducting are found to be minor (<30 ms each).
---------------------------------------------------------
Title: Quasi-periodic Particle Injection into Coronal Loops
Authors: Aschwanden, Markus J.; Benz, Arnold O.; Dennis, Brian R.;
Gaizauskas, Victor
1993ApJ...416..857A Altcode:
We present observations of the flare of 1989 June 22, 1445 UT (in active
region NOAA 5555), obtained with the Hard X-Ray Burst Spectrometer
(HXRBS) on SMM at energies >52 keV, and with the broad-band radio
spectrometer PHOENIX at ETH/Zurich in the frequency range of 100-2800
MHz. The radio emission is dominated by a ≲100% polarized decimetric
continuum at 400-1400 MHz, peaking at 750 MHz. The decimetric radio
flux is highly correlated with the 50-150 keV hard X-ray flux but
is delayed by 3.5-5.4 s with respect to the hard X-rays. The HXR
emission shows an excess of ≳10 fast (≳100 ms) spikes (according to
Poisson statistics). The radio emission exhibits weak fine structure,
consisting of ≍45 quasi-periodic pulses with a mean period of 1.6
s. The frequency-time drift pattern of this fine structure is found
to be consistent with segments of inverted-U type bursts, suggesting
quasi-periodic injection of electron beams into a loop system. The loop
system has an average height of 68,000 km and expands with a velocity of
200 km s<SUP>-1</SUP> due to flare heating. Chromospheric evaporation
enhances the electron density near the footpoints. The type III-
exciting electrons have a mean velocity of υ/c = 0.30±0.10 (22 keV)
and propagate along inverted-U burst trajectories with a mean duration
of 2.5 s. For those electrons which reach the mirror point near the
opposite footpoint of the loop system, we calculate (from the density
and loop length) a low energy cutoff of ≥ 8 keV due to collisional
deflection, yielding a propagation velocity of v/c = 0.18 and a
propagation delay of 5.1±1.0 s, which agrees well with the observed
delay of 5.16 s between the cross-correlated HXR and radio flux. The
≥ 8 keV electrons provide free energy for a loss cone instability
near the secondary footpoint, which is observed as decimetric continuum
polarized in the same sense of circular polarization as the type III
bursts. The constraints from the Hα flare position and the magnetic
potential field extrapolation indicate that the loss cone emission is
produced in the diverging field region above the umbra of the leading
sunspot, which has a photospheric field strength of -1600 G. <P />This
flare allows us to deconvolve quasi-periodic particle injection and
subsequently triggered coherent radio emission from trapped particles
in flare-associated loops. It demonstrates that quasi-periodic modes
of particle acceleration, particle dynamics in mirror loops, and
the resulting plasma instabilities can be efficiently diagnosed from
correlated hard X-ray and radio signatures.
---------------------------------------------------------
Title: Frequency distributions of solar X-ray flare parameters
Authors: Crosby, N.; Aschwanden, M.; Dennis, B.
1993AdSpR..13i.179C Altcode: 1993AdSpR..13..179C
We have determined frequency distributions of flare parameters from
the over 12,000 solar flares that were recorded with the Hard X-ray
Burst Spectrometer (HXRBS) on SMM. These parameters include the total
flare duration, peak counting rate, the peak hard X-ray flux, the
total energy in electrons, and the peak hard X-ray flux in electrons
(the latter two computed assuming a thick-target model). The energies
were computed above a threshold energy between 25 and 50 keV. All the
distributions can be represented by power-laws over a range of several
orders of magnitude, above the HXRBS sensitivity threshold. Power-laws
can be taken as evidence for self-organized criticality in solar
flares. Correlations among these parameters are determined from
linear regression fits as well as from the slopes from the frequency
distributions. Variations of the frequency distributions were also
investigated with respect to the solar cycle.
---------------------------------------------------------
Title: VLA Stereoscopy of Solar Active Region 7123
Authors: Aschwanden, M. J.; Bastian, T. S.; Nitta, N.
1993BAAS...25.1224A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Frequency distributions and correlations of solar X-ray
flare parameters
Authors: Crosby, Norma B.; Aschwanden, Markus J.; Dennis, Brian R.
1993SoPh..143..275C Altcode:
We have determined frequency distributions of flare parameters from
over 12000 solar flares recorded with the Hard X-Ray Burst Spectrometer
(HXRBS) on the Solar Maximum Mission (SMM) satellite. These parameters
include the flare duration, the peak counting rate, the peak hard
X-ray flux, the total energy in electrons, and the peak energy flux
in electrons (the latter two computed assuming a thick-target flare
model). The energies were computed above a threshold energy between
25 and 50 keV. All of the distributions can be represented by power
laws above the HXRBS sensitivity threshold. Correlations among these
parameters are determined from linear regression fits as well as
from the slopes of the frequency distributions. Variations of the
frequency distributions were investigated with respect to the solar
activity cycle.
---------------------------------------------------------
Title: The Coevolution of Decimetric Millisecond Spikes and Hard
X-Ray Emission during Solar Flares
Authors: Aschwanden, Markus J.; Guedel, Manuel
1992ApJ...401..736A Altcode:
Results are presented of an analysis of a comprehensive data set of 27
solar flares with decimetric millisecond spikes between 1980 and 1989,
simultaneously observed with the Zuerich radio spectrometers and the
Hard X-ray Burst Spectrometer on the SMM spacecraft. Two contradictory
relationships of the coevolution of hard X-ray and spiky radio emissions
during flares are found: the temporal evolution of both emissions
reveals a close functional dependence, but there is a substantial
time delay between the two emissions. Five possible scenarios for the
hard-X-ray-associated radio spike emission which may account for both
their detailed coevolution and their substantial intervening time delay
are discussed. All five scenarios are able to explain both the close
coevolution of hard X-ray and radio emission as well as their mutual
delay to some degree, but none of them can explain all observational
aspects in a simple way.
---------------------------------------------------------
Title: 3D reconstruction methods of coronal structures by radio
observations.
Authors: Aschwanden, M. J.; White, S. M.; Bastian, T. S.
1992ESASP.348..217A Altcode: 1992cscl.work..217A
The ability to carry out a three-dimensional reconstruction of
structures in the solar corona would represent a major advance
in our study of the physical properties in active regions and
in flares. The authors describe several new methods which allow
a geometric reconstruction of quasi-stationary coronal structures
(e.g. active region loops) or dynamic structures (e.g. flaring loops):
(1) steroscopy of multi-day imaging observations by the VLA. (2)
Tomography of optically thin emission (in radio or soft X-rays). (3)
Multi-frequency band imaging by the VLA. (4) Tracing of magnetic field
lines by propagating electron beams.
---------------------------------------------------------
Title: Simultaneous Observations of Coronal Bright Points in X-Ray
and Radio Wavelengths
Authors: Nitta, Nariaki; Bastian, Timothy S.; Aschwanden, Markus J.;
Harvey, Karen L.; Strong, Keith T.
1992PASJ...44L.167N Altcode:
We present a first explicit comparison of coronal bright points in
soft X-ray and radio wavelengths, using the Soft X-ray Telescope
aboard the Yohkoh spacecraft and the Very Large Array. About half of
the 33 compact sources indentified in a 20-cm full-disk map appear
as X-ray bright points in the X-ray data. The other half apparently
corresponds to unipolar regions with enhanced magnetic fields. Thus,
the identification of radio bright points alone cannot reliably serve
as a proxy for X-ray bright points. A preliminary analysis reveals that
bright points commonly observed at 20 cm and in X-rays have temperatures
of (1.4--2.9) times 10(6) K and emission measures of (0.4--2.5) times
10(45) cm(-3) . The observed brightness temperatures at 20 cm [(1--2.5)
times 10(5) K] can be explained in terms of optically thin free-free
emission from a plasma with these parameters.
---------------------------------------------------------
Title: Decimetric Solar Type U Bursts: VLA and PHOENIX Observations
Authors: Aschwanden, Markus J.; Bastian, T. S.; Benz, A. O.; Brosius,
J. W.
1992ApJ...391..380A Altcode:
Observations of type U bursts, simultaneously detected by the VLA at
1.446 GHz and by the broadband spectrometer Phoenix in the 1.1-1.7
GHz frequency band on August 13, 1989 are reported. Extrapolations
of the coronal magnetic field, assuming a potential configuration,
indicate that the VLA 20 cm source demarcates an isodensity level. The
source covers a wide angle of diverging magnetic field lines whose
footpoints originate close to a magnetic intrusion of negative polarity
into the main sunspot group of the active region with dominant positive
polarity. The centroid of the 20-cm U-burst emission, which corresponds
to the turnover frequency of the type U bursts and remains stationary
during all U bursts, coincides with the apex of extrapolated potential
field lines at a height of about 130,000 km. It is demonstrated
that the combination of radio imaging and broadband dynamic spectra,
combined with the magnetic field reconstruction from magnetograms,
can constrain all physical parameters of a magnetic loop system.
---------------------------------------------------------
Title: VLA-PHOENIX-BATSE Studies of Impulsive Bursts
Authors: Willson, R. F.; Lang, K. R.; Aschwanden, M. J.
1992AAS...180.3009W Altcode: 1992BAAS...24..776W
No abstract at ADS
---------------------------------------------------------
Title: Self-Organized Criticality in Solar Flares
Authors: Aschwanden, M. J.; Dennis, B. R.
1992AAS...180.3015A Altcode: 1992BAAS...24..777A
No abstract at ADS
---------------------------------------------------------
Title: Solar Radio Pulsation Event Observed by the VLA and OVRO
Authors: Aschwanden, M. J.; Bastian, T. S.; Gary, D. E.
1992AAS...180.4505A Altcode: 1992BAAS...24..802A
We investigate interpretational aspects of the radio pulsation
event which has been observed by the Very Large Array (VLA) and
the Owens Valley Radio Observatory (OVRO) on December 21, 1990,
1930 UT, during the second MAX'91 observing campaign. The VLA was
observing with a time resolution of 0.4 s at 0.33, 1.4 and 4.9 GHz,
while OVRO used a time resolution of 0.2 s at 1.2, 1.4, 1.6, 1.8
and 2.0 GHz. The radio pulsation event was triggered by a C3.3 class
flare in soft X-rays in active region 6412 (NOAA). The time profile
of the quasi-periodic radio emission exhibits a period of 8.8 s and
shows indications of secondary periodicities with faster periods. We
investigate whether these periodicities can be explained in terms of
(i) MHD eigen-modes in cylindrical fluxtubes (MHD surface modes and
harmonic modes), or (ii) relaxational oscillations of a nonlinear
dissipative system. We test these two options by means of Fourier
spectra and by reconstruction of the “strange attractor dimension”
of nonlinear systems. We attempt a three-dimensional reconstruction of
the pulsating source by means of magnetic field extrapolation from the
photospheric magnetogram and by using the constraints of the radio
maps at multiple frequencies. Preliminary analysis indicates that
plasma emission as well as gyrosynchrotron emission is coherently
modulated in spatially diverging magnetic structures. We investigate
whether the pulsating radio emission originates from plasma confined
in a pulsating loop structure or whether the radio emission is driven
by electron beams which are produced in a pulsating acceleration
mechanism. The latter possibility would support a model where the
primary energy release itself is governed by a nonlinear dissipative
system, e.g. by oscillatory magnetic reconnection.
---------------------------------------------------------
Title: VLA, PHOENIX and BATSE observations of an X1 flare.
Authors: Willson, Robert F.; Aschwanden, Markus J.; Benz, Arnold O.
1992NASCP3137..515W Altcode: 1992como.work..515W
The authors present observations of an X1 flare (Jul 18, 1991) detected
simultaneously with the VLA, the PHOENIX Digital Radio Spectrometer
and the Burst and Transient Source Experiment (BATSE) aboard the Gamma
Ray Observatory. The VLA was used to produce snapshot maps of the
impulsive burst emission on timescales of 1.7 sec at both 20 and 91
cm. The results indicate electron acceleration in the higher corona
several minutes before the onset of the hard X-ray burst detected
by BATSE. Comparisons with high spectral and temporal observations
by PHOENIX reveal a variety of radio bursts at 20 cm, such as type
III bursts, intermediate drift bursts, and quasi-periodic pulsations
during different stages of the X1 flare. The described X1 flare is
unique in the sense that it appeared at the east limb, providing the
most accurate information on the vertical structure of different flare
tracers visible in radio wavelengths.
---------------------------------------------------------
Title: Characteristics of the impulsive phase of flares
Authors: Benz, A. O.; Aschwanden, M. J.
1992LNP...399..106B Altcode: 1992esf..coll..106B; 1992LNP...399..106A; 1992IAUCo.133..106B
The impulsive phase of flares is an observational concept, characterized
by spiky emissions from -rays to radio waves. It is generally
agreed that during this time a large fraction of the. original flare
energy resides in energetic particles which are manifested in these
emissions. Here we concentrate on recent decimeter and microwave
observations that indicate a high level of fragmentation of this
energy release when related to hard X-ray (HXR) flux. Recent attempts
to characterize the flare and the distribution of the radio bursts in
time and frequency by statistical methods are also reviewed.
---------------------------------------------------------
Title: The association of solar millisecond radio spikes with hard
X-ray emission
Authors: Guedel, M.; Benz, A. O.; Aschwanden, M. J.
1991A&A...251..285G Altcode:
Conventional observational data regarding solar millisecond spikes
are compared with data gathered simultaneously in the hard X-ray
band by means of a statistical analysis. The analysis considers
the association rate, correlation degree, and relative time delays
between hard X-ray emissions (in the 25-438 keV range) and radio-spike
events. About 95 percent of the radio-spike bursts occur during
impulsive hard X-ray bursts, and approximately 43 percent of the
compared events are characterized by hard X-ray time profiles that
mimic the concentration of simultaneous radio spikes. The delay of
the radio emission with respect to the hard X-ray bursts puts some
constraints on the acceleration and propagation of particles. The time
delays and the quantization into discrete radio events are theorized
to be caused by the operation of the accelerator.
---------------------------------------------------------
Title: The attractor dimension of solar decimetric radio pulsations
Authors: Kurths, J.; Benz, A. O.; Aschwanden, M. J.
1991A&A...248..270K Altcode:
The temporal characteristics of decimetric pulsations and related
radio emissions during solar flares are analyzed using statistical
methods recently developed for nonlinear dynamic systems. The results
of the analysis is consistent with earlier reports on low-dimensional
attractors of such events and yield a quantitative description of their
temporal characteristics and hidden order. The estimated dimensions
of typical decimetric pulsations are generally in the range of 3.0 +
or - 0.5. Quasi-periodic oscillations and sudden reductions may have
dimensions as low as 2. Pulsations of decimetric type IV continua have
typically a dimension of about 4.
---------------------------------------------------------
Title: Solar and Stellar Radio Spikes: Limits on the Saturation of
the Electron-Cyclotron Maser
Authors: Wentzel, Donat G.; Aschwanden, Markus J.
1991ApJ...372..688W Altcode:
The solar millisecond radio 'spikes' have been explained in terms of
X-mode radiation generated by a maser near the electron gyrofrequency,
acting on fast coronal electrons with a loss cone. This maser is a
phenomenon described by quasi-linear theory. It is sensitive to the
small first-relativistic correction to the gyrofrequency. Thus, it might
be disrupted rather easily by nonlinear effects. The maximum radiation
density that can be reached before the radiation entrains (phase-locks)
the electrons and saturates the maser is discussed. If the observed
durations of solar radio spikes are a measure of the rate of scattering
into the loss-cone, then the inferred energy density is at least two
orders of magnitude less than the energy density at which entrainment
sets in. Also, maser emission from auroral kilometric radiation does not
reach wave energies critical for electron entrainment. Maser emissions
from flare stars, however, show 3-4 orders of magnitude higher radio
fluxes and brightness temperatures than for the solar case and are
likely to be saturated by entrainment.
---------------------------------------------------------
Title: Coordinated VLA-PHOENIX-SMM Observations: Microwave Type
U-Bursts
Authors: Aschwanden, M. J.; Bastian, T. S.; Benz, A. O.; Dennis, B. R.
1991BAAS...23.1065A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Temporally, Spectrally, and Spatially Resolved Observations
of a Pulsating Solar Radio Burst
Authors: Bastian, T. S.; Aschwanden, M. J.; Gary, D. E.
1991BAAS...23Q1072B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Simultaneous Radio and Hard X-ray Observations by
PHOENIX/Zurich and HXRBS/SMM during Max'91 (June 1989)
Authors: Aschwanden, M. J.; Dennis, B. R.; Benz, A. O.
1991max..conf..234A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Flare fragmentation and type III productivity in the 1980
June 27 flare
Authors: Aschwanden, M. J.; Benz, A. O.; Schwartz, R. A.; Lin, R. P.;
Pelling, R. M.; Stehling, W.
1990SoPh..130...39A Altcode:
We present observations of the solar flare on 1980 June 27, 16:14-16:33
UT, which was observed by a balloon-borne 300 cm<SUP>2</SUP> phoswich
hard X-ray detector and by the IKARUS radio spectrometer. This flare
shows intense hard X-ray (HXR) emission and an extreme productivity of
(at least 754) type III bursts at 200-400 MHz. A linear correlation
was found between the type III burst rate and the HXR fluence, with a
coefficient of ≈ 7.6 × 10<SUP>27</SUP> photons keV<SUP>−1</SUP> per
type III burst at 20 keV. The occurrence of ≈ 10 type III bursts per
second, and also the even higher rate of millisecond spikes, suggests
a high degree of fragmentation in the acceleration region. This high
quantization of injected beams, assuming the thick-target model,
shows up in a linear relationship between hard X-ray fluence and the
type III rate, but not as fine structures in the HXR time profile.
---------------------------------------------------------
Title: Relaxation of the loss-cone by quasi-linear diffusion of the
electron-cyclotron maser instability in the solar corona
Authors: Aschwanden, Markus J.
1990A&AS...85.1141A Altcode:
A self-consistent numeric two-dimensional code using kinetic-wave
particle equations has been developed and applied to the maser dynamics
of the solar corona. Time histories generated by the code reveal details
of the evolution of the linear phase, the saturation and subsequent
relaxation of the loss cone. Quantitative values are obtained for the
saturation time, the amount of the converted free energy, the critical
wave energy level for onset of maser diffusion, the velocity and pitch
angle range of the resonant particles, and the relative importance
of the different magnetoionic modes and harmonics. A wide range of
initial conditions is covered by varying the loss-cone distribution
and the ambient cold plasma parameters.
---------------------------------------------------------
Title: The saturation of the electron-cyclotron maser instability
and the interpretation of solar millisecond spikes
Authors: Aschwanden, M. J.
1990A&A...237..512A Altcode:
A self-consistent numeric two-dimensional code of the kinetic
wave-particle equations developed to investigate the maser dynamics in
the solar context is applied to solar millisecond-spike observations
in order to improve the diagnostic capabilities of the theory of
the electron-cyclotron maser instablitity. Attention is given to the
inhomogeneity of the magnetic field selecting magneto-ionic modes with
relatively short saturation lengths and suppressing mechanisms such
as collisional deflection, free-free absorption, and gyroresonance
absorption. The time scales of maser saturation in respect to time
scales of global particle changes in a magnetic loop are covered,
relevant observations of solar millisecond spikes are described,
and the interpretation in terms of physical parameters deduced from
the quasi-linear maser simulations are presented. It is demonstrated
that the quasi-linear simulations make it possible to constrain the
physical parameters from the observed time scale and frequency.
---------------------------------------------------------
Title: The Evolution of Decimetric Millisecond Spikes and Hard X-Ray
Emission During Solar Flares
Authors: Aschwanden, M. J.; Güdel, M.
1990BAAS...22..823A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Correlation of solar radio pulsations with hard X-ray emission
Authors: Aschwanden, M. J.; Benz, A. O.; Kane, S. R.
1990A&A...229..206A Altcode:
A systematic study of the correlation of quasi-periodic broad-band
decimetric pulsations with hard X-ray (HXR) emission is carried out. It
is found that, in 11 out of 56 simultaneously observed events, the
decimetric quasi-periodic pulsations in the impulsive phase of flares
are correlated. If events with concurring type III bursts are included,
19 cases of radio pulsations are correlated with HXR.
---------------------------------------------------------
Title: The Attractor Dimension of Solar Decimetric Radio Pulsations
Authors: Kurths, J.; Benz, A. O.; Aschwanden, M. J.
1990PDHO....7..196K Altcode: 1990dysu.conf..196K; 1990ESPM....6..196K
The authors have analyzed the temporal characteristics of decimetric
pulsations and related radio emissions during solar flares.
---------------------------------------------------------
Title: The Time Scale of Radio Millisecond Spikes: Theory Versus
Observations
Authors: Aschwanden, M. J.; Gudel, M.
1989BAAS...21..847A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Electron-Cyclotron Maser Instability. II. Pulsations
in the Quasi-stationary State
Authors: Aschwanden, M. J.; Benz, A. O.
1988ApJ...332..466A Altcode:
In the previous paper we have studied the electron-cyclotron maser
instability of a source in quasi-equilibrium. In a situation of weak
diffusion such a state will remain for a limited time regardless of the
particular loss-cone instability involved. Here our major interest is in
oscillations set up by small disturbances around the steady state. Both
the linear growth as well as the coupled diffusion rate are subject
to a common oscillation period, since wave growth and diffusion are
interlocked by a Lotka-Volterra type of coupled equations. Periods
in the limit of small amplitude (limit cycle) of the self-organizing
wave-particle system are investigated for the magnetoionic X- and O-mode
and for harmonic numbers s = 1, 2 of the maser instability. Pulsation
periods in the order of 1 s, as observed in the decimetric range of
solar flare emission are found exist for (1) fundamental (s = 1) O-mode
for 0.3 < ω<SUB>p</SUB>/Ω<SUB>e</SUB> ≲ 1.0, and (2) harmonic
(s = 2) X-mode (for 1.0 ≲ ω<SUB>p</SUB>/Ω<SUB>e</SUB> 1.4). The
period is sensitive to variations of the shape of the loss-cone and
therefore not stable. The proposed model provides an interpretation
of the observed quasi-periodic decimetric solar pulsations as well as
some of similar stellar phenomena.
---------------------------------------------------------
Title: On the Electron-Cyclotron Maser Instability. I. Quasi-linear
Diffusion in the Loss Cone
Authors: Aschwanden, M. J.; Benz, A. O.
1988ApJ...332..447A Altcode:
Emission and quasi-linear velocity diffusion by the electron-cyclotron
maser instability are studied under conditions of continuous
operation. For the first time, the maser-induced quasi-linear
diffusion is computed with the same accuracy as the well-known linear
growth rates, including integration along the actual resonance
space in k-space. This permits the feedback of the emission on
the particle distribution to be quantified. A novel result is the
discovery of relatively long diffusion time scales for typical solar
conditions. Oscillations set up by small disturbances around the
steady state are considered. Periods in the limit of small amplitude
of the self-organizing wave-particle system are investigated for the
magnetoionic X- and O-mode and for harmonic numbers s = 1, 2 of the
maser instability. Pulsation periods in the order of 1 s, as observed
in the decimetric range of solar flare emission, are found to exist
for fundamental O-mode and for harmonic X-mode.
---------------------------------------------------------
Title: Pulsations of the Radio Emission of the Solar Corona
Authors: Aschwanden, M. J.
1988BAAS...20..682A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Theory of Radio Pulsations in Coronal Loops
Authors: Aschwanden, M. J.
1987SoPh..111..113A Altcode:
`Pulsations' include a wide range of phenomena from strictly sinusoidal
oscillations up to quasiperiodic fine structures, observed in the
radio, microwave and X-ray frequency range. The various versions of
pulsation models are reviewed and classified in three groups according
to their driver mechanisms: (1) Magnetic flux tube oscillations
(the emissivity of trapped particles is modulated by a standing
or propagating MHD wave), (2) cyclic self-organizing systems of
plasma instabilities (wave-particle, wave-wave interactions), and (3)
modulation of acceleration (acceleration/injection of particles into
the source). Observational references illustrate the applicability
of the models. In conclusion, discrimination criteria of models are
discussed, in order to give a key for interpretation of observations.
---------------------------------------------------------
Title: Pulsations of the radio emission of the solar corona: analysis
of observations and theory of the pulsating electron-cyclotron maser
Authors: Aschwanden, Markus Josef
1987PhDT.......103A Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The frequency-time drift of pulsations.
Authors: Aschwanden, M. J.; Benz, A. O.
1986A&A...158..102A Altcode:
A set of 1270 digitally recorded type III bursts and pulsations in
the frequency range of 100 - 1000 MHz was analyzed by a structure
recognizing program. Numerical fitting methods determined the drift
rate with much higher accuracy than previous analysis of analog
data. The analysis of the drift of solar radio bursts in frequency
vs. time provides information not only about the dynamical parameters
of the source but also reveals atmospherical properties. Type III
events and pulsations are clearly distinguished in their drift
distribution. Pulsations statistically exhibit 3 times higher drift
rates than type III events, thus requiring considerably shorter
local scale heights. The source direction of the type III bursts is
preferentially upwards oriented, the pulsations show a downwards
oriented anisotropy. Evidence is given that the pulsations have
a different origin than the common type III burst, concerning the
coronal environment as well as the emission mechanism. Outlines to
construct models of decimetric pulsations are discussed.
---------------------------------------------------------
Title: The Polarization of Decimetric Pulsations
Authors: Aschwanden, M. J.
1986SoPh..104...57A Altcode:
A sample of 10 decimetric broadband pulsations were observed in
1980-1983 and analyzed in polarization. Half of the data set was 85-100%
circularly polarized, the other half showed a mild polarization of
15-55%. The polarization is constant in time and frequency for the
strongly polarized group. All the mildly polarized bursts originate
from near the limb; the lower degree of circular polarization is
likely to be caused by depolarization due to propagation effects. The
degree of polarization is constant throughout the event, but varies
in frequency for the mild polarized group. Following the leading spot
hypothesis, the magneto-ionic mode of the emission was found to be
extraordinary. The high circular polarization of the pulsations was
interpreted to be determined by the emission mechanism itself, not by
propagation effects or cut-offs (contrary to the metric type I noise
storms). Implications for pulsation models are discussed.
---------------------------------------------------------
Title: Correlation of Solar Decimetric Radio Bursts with X-Ray Flares
Authors: Aschwanden, M. J.; Wiehl, H. J.; Benz, A. O.; Kane, S. R.
1985SoPh...97..159A Altcode:
Several hundred radio bursts in the decimetric wavelength range
(300-1000 MHz) have been compared with simultaneous soft and hard X-ray
emission. Long lasting (type IV) radio events have been excluded. The
association of decimetric emission with hard X-rays has been found
to be surprisingly high (48%). The association rate increases with
bandwidth, duration, number of structural elements, and maximum
frequency. Type III-like bursts are observed up to the upper limit of
the observed band. This demonstrates that the corona is transparent up
to densities of about 10<SUP>10</SUP> cm<SUP>−3</SUP>, contrary to
previous assumptions. This can only be explained in an inhomogeneous
corona with the radio source being located in a dense structure. The
short decimetric bursts generally occur during the impulsive phase,
i.e. simultaneously with hard X-rays. The times of maximum flux are
well correlated (within 2 s). The HXR emission lasts 4 times longer then
the radio emission in the average. This work finds a close relationship
between decimetric and HXR emission with sufficient statistics offering
additional information on the flare process.
---------------------------------------------------------
Title: Different time constants of solar decimetric bursts in the
range 100 1000 MHz
Authors: Wiehl, H. J.; Benz, A. O.; Aschwanden, M. J.
1985SoPh...95..167W Altcode:
Between 1980, January 1 and 1981, December 31 a total of 664 `decimetric
pulsation' events, abbreviated DCIM, were observed with the Zürich
spectrometers in the frequency range 100 to 1000 MHz. All of these
events were recorded on film, allowing an effective resolution in
time of 0.5 s, and 5 MHz in frequency. Some of these events were also
recorded digitally with higher time and frequency resolution.
---------------------------------------------------------
Title: Decimetric Radio Emission During Solar Flares
Authors: Benz, A. O.; Aschwanden, M. J.; Wiehl, H. J.
1985spit.conf..597B Altcode:
No abstract at ADS
