Author name code: longcope
ADS astronomy entries on 2022-09-14
author:"Longcope, Dana W."
------------------------------------------------------------------------
Title: Connecting Chromospheric Condensation Signatures to
Reconnection-driven Heating Rates in an Observed Flare
Authors: Ashfield, William H., IV; Longcope, Dana W.; Zhu, Chunming;
Qiu, Jiong
Bibcode: 2022ApJ...926..164A
Altcode: 2021arXiv211202150A
Observations of solar flare reconnection at very high spatial
and temporal resolution can be made indirectly at the footpoints
of reconnected loops into which flare energy is deposited. The
response of the lower atmosphere to this energy input includes a
downward-propagating shock called chromospheric condensation, which can
be observed in the UV and visible. In order to characterize reconnection
using high-resolution observations of this response, one must develop a
quantitative relationship between the two. Such a relation was recently
developed, and here we test it on observations of chromospheric
condensation in a single footpoint from a flare ribbon of the X1.0
flare on 2014 October 25 (SOL2014-10-25T16:56:36). Measurements taken
of Si IV 1402.77 Å emission spectra using the Interface Region Imaging
Spectrograph (IRIS) in a single pixel show the redshifted component
undergoing characteristic condensation evolution. We apply the technique
called the Ultraviolet Footpoint Calorimeter to infer energy deposition
into one footpoint. This energy profile, persisting much longer than the
observed condensation, is input into a one-dimensional, hydrodynamic
simulation to compute the chromospheric response, which contains a
very brief condensation episode. From this simulation, we synthesize
Si IV spectra and compute the time-evolving Doppler velocity. The
synthetic velocity evolution is found to compare reasonably well with
the IRIS observation, thus corroborating our reconnection-condensation
relationship. The exercise reveals that the chromospheric condensation
characterizes a particular portion of the reconnection energy release
rather than its entirety, and that the timescale of condensation does
not necessarily reflect the timescale of energy input.
Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE). II. Flares and Eruptions
Authors: Cheung, Mark C. M.; Martínez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham S.; Reeves, Katharine K.; Fletcher, Lyndsay; Jin,
Meng; Nóbrega-Siverio, Daniel; Danilovic, Sanja; Antolin, Patrick;
Allred, Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward;
Longcope, Dana; Takasao, Shinsuke; DeRosa, Marc L.; Boerner, Paul;
Jaeggli, Sarah; Nitta, Nariaki V.; Daw, Adrian; Carlsson, Mats; Golub,
Leon; The
Bibcode: 2022ApJ...926...53C
Altcode: 2021arXiv210615591C
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (subarcseconds) and temporal (less than a few tens of
seconds) scales of the coronal dynamics of solar flares and eruptive
phenomena. The highest-resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by the Interface Region Imaging
Spectrograph for the low solar atmosphere, we need high-resolution
spectroscopic measurements with simultaneous imaging to understand the
dominant processes. In this paper: (1) we introduce the Multi-slit Solar
Explorer (MUSE), a spaceborne observatory to fill this observational
gap by providing high-cadence (<20 s), subarcsecond-resolution
spectroscopic rasters over an active region size of the solar transition
region and corona; (2) using advanced numerical models, we demonstrate
the unique diagnostic capabilities of MUSE for exploring solar coronal
dynamics and for constraining and discriminating models of solar flares
and eruptions; (3) we discuss the key contributions MUSE would make
in addressing the science objectives of the Next Generation Solar
Physics Mission (NGSPM), and how MUSE, the high-throughput Extreme
Ultraviolet Solar Telescope, and the Daniel K Inouye Solar Telescope
(and other ground-based observatories) can operate as a distributed
implementation of the NGSPM. This is a companion paper to De Pontieu
et al., which focuses on investigating coronal heating with MUSE.
Title: Probing the Physics of the Solar Atmosphere with the Multi-slit
Solar Explorer (MUSE): II. Flares and Eruptions
Authors: Cheung, Chun Ming Mark; Martinez-Sykora, Juan; Testa, Paola;
De Pontieu, Bart; Chintzoglou, Georgios; Rempel, Matthias; Polito,
Vanessa; Kerr, Graham; Reeves, Katharine; Fletcher, Lyndsay; Jin,
Meng; Nobrega, Daniel; Danilovic, Sanja; Antolin, Patrick; Allred,
Joel; Hansteen, Viggo; Ugarte-Urra, Ignacio; DeLuca, Edward; Longcope,
Dana; Takasao, Shinsuke; DeRosa, Marc; Boerner, Paul; Jaeggli, Sarah;
Nitta, Nariaki; Daw, Adrian; Carlsson, Mats; Golub, Leon
Bibcode: 2021AGUFMSH51A..08C
Altcode:
Current state-of-the-art spectrographs cannot resolve the fundamental
spatial (sub-arcseconds) and temporal scales (less than a few tens
of seconds) of the coronal dynamics of solar flares and eruptive
phenomena. The highest resolution coronal data to date are based on
imaging, which is blind to many of the processes that drive coronal
energetics and dynamics. As shown by IRIS for the low solar atmosphere,
we need high-resolution spectroscopic measurements with simultaneous
imaging to understand the dominant processes. In this paper: (1)
we introduce the Multi-slit Solar Explorer (MUSE), a spaceborne
observatory to fill this observational gap by providing high-cadence
(<20 s), sub-arcsecond resolution spectroscopic rasters over an
active region size of the solar transition region and corona; (2)
using advanced numerical models, we demonstrate the unique diagnostic
capabilities of MUSE for exploring solar coronal dynamics, and for
constraining and discriminating models of solar flares and eruptions;
(3) we discuss the key contributions MUSE would make in addressing the
science objectives of the Next Generation Solar Physics Mission (NGSPM),
and how MUSE, the high-throughput EUV Solar Telescope (EUVST) and the
Daniel K Inouye Solar Telescope (and other ground-based observatories)
can operate as a distributed implementation of the NGSPM. This is a
companion paper to De Pontieu et al. (2021, also submitted to SH-17),
which focuses on investigating coronal heating with MUSE.
Title: Characteristics and possible causes of high-density, hot
loop-top ridges in two-ribbon flares
Authors: Longcope, Dana; Qiu, Jiong; McCreery, Patrick
Bibcode: 2021AGUFMSH25E2135L
Altcode:
Some two-ribbon solar flares, notably including the Bastille flare
of 2000-Jul-14, show an extended ridge of plasma running along the
looptops of the post-reconnection arcade. Common examples are visible
in 193A EUV images due to emission in Fe XXIV at roughly 20 MK. The
high, steadily increasing emission measure suggests the ridge is
composed of an expanding column of extremely dense plasma. Several
past investigations have proposed that these structures result from
the collision of evaporation flows from opposite footpoints. We
use observations of two such events, including the Bastille event,
to characterize the ridge plasma. These characteristics are used to
compare the evaporation-collision hypothesis to an alternative: slow
magnetosonic shocks (SMS) in the reconnection outflow. We use a thin
flux tube model of the flux retracting following its reconnection to
assess the viability of the SMS hypothesis.
Title: Examining Flux Tube Interactions as a Cause of Sub-alfvénic
Outflow
Authors: Unverferth, John; Longcope, Dana
Bibcode: 2021ApJ...923..248U
Altcode:
In accepted models, magnetic tension drives reconnected magnetic flux
away from the reconnection site at the local Alfvén speed. Numerous
observational signatures of these outflows have been identified in solar
flares, notable among them being supra-arcade downflows (SADs), almost
none move at the Alfvén speed as predicted by models. Well-studied
examples of SADs or SAD loops found in the flare of 2017 September
10 (SOL2017-09-10T15:35:00) move at a quarter or less of the
expected Alfvén speed. Among those reasons posited to explain such
discrepancies is the possibility that reconnected flux experiences a
drag force during its outflow. Drag has not been included in previous
reconnection models. Here, we develop the first such model in order to
test the possibility that drag can explain sub-alfveńic reconnection
outflows. Our model uses thin flux tube dynamics, previously shown
to match features of flare observations other than outflow speed,
including for the 2017 September 10 flare. We supplement the dynamics
with a drag force representing the tube's interaction with surrounding
plasma through the formation of a wake. The wake's width appears
as a parameter in the force. We perform simulations, varying the
drag parameter and synthesizing EUV observations, to test whether a
drag force can produce a reasonable fit to observed features of the
September 10 flare. We find that that slower retraction increases the
brightness of emission and lowers the temperature of the synthetic
plasma sheet. With proper choice of parameters the drag enables the
simulation to agree reasonably with the observations.
Title: Thin Flux Tube Retraction Following Reconnection as a Model
for the Observed Hot, Loop-top Ridge Structure in a Two-ribbon Flare
Authors: McCreery, Patrick; Longcope, Dana
Bibcode: 2021AGUFMSH25E2136M
Altcode:
Shocks are a fundamental component of many self-consistent models of
solar flares. As an example, the slow magnetosonic shocks proposed in
the Petschek model heat and compress surrounding plasma. However,
there is a lack of direct evidence for the existence of these
shocks, perhaps as a result of unclear predictions of the shocks
observational manifestations. We use a thin flux tube dynamics,
including a drag force, to model the retracting flux tube to perform
numerical simulations. With these we aim to reproduce observations of
a solar flare on 18 April 2014. The solar flare of interest displays a
20MK plasma ridge with a density of about 1.0d11 per cc. Our numerical
simulations suggest that slow magnetosonic shocks may not be able to
produce such high densities unless drag is absent. The absence of drag,
however, poses problems matching typical retraction velocities.
Title: Connecting chromospheric condensation signatures to
reconnection driven heating rates in an X1.0 flare
Authors: Ashfield, William; Longcope, Dana; Zhu, Chunming; Qiu, Jiong
Bibcode: 2021AGUFMSH24B..03A
Altcode:
Observations of solar flare reconnection at very high resolution can
be indirectly made at the footpoints of reconnected loops into which
flare energy is deposited. The response of the lower atmosphere to this
energy input includes a downward-propagating shock called chromospheric
condensation, which can be observed at wavelengths including UV and
visible. In order to characterize reconnection using high resolution
observations of this shock, one must develop a quantitative relationship
between the two. Such a relation was recently developed in previous
work and here we test it on observations of chromospheric condensation
in a single footpoint in the flare ribbon of the X1.0 flare SOL2014-
10-25T16:56:36. Measurements taken of Si iv 1402.77Å emission spectra
with the Interface Region Imaging Spectrograph (IRIS) using a 5 s
cadence show a red-shifted component undergoing typical condensation
evolution, with a peak downward velocity of 35 km s-1 and a half-life
of 16 s. Simultaneous observations taken with the Atmospheric Imaging
Assembly (AIA) reveal a temporally and spatially correlated increase
in UV emission in the 1600 Å band. We apply a technique called the
Ultraviolet Footpoint Calorimeter (UFC) to the 1600 Å lightcurve
to infer the energy deposition into the footpoint. We then input
this energy into a one-dimensional, hydrodynamic simulation to
compute the chromospheric response, including condensation. From this
simulation we synthesize Si iv spectra and compute the time-evolving
Doppler velocity. This is found to compare reasonably well with the
IRIS observation, thus corroborating our reconnection-condensation
relationship.
Title: Multi-spacecraft Observations Of Coronal Loops To Verify A
Force-free Field Reconstruction And Infer Loop Cross Sections
Authors: McCarthy, M.; Longcope, D.; Malanushenko, A.
Bibcode: 2021AAS...23820501M
Altcode:
Active region EUV loops are believed to trace a subset of magnetic
field lines through the corona. Malanushenko et al. (2009) proposed a
method, using loop images and line-of-sight photospheric magnetograms,
to infer the three-dimensional shape and field strength along each
loop. McCarthy et al. (2019) used this novel method to compute the
total magnetic flux interconnecting a pair of active regions observed
by SDO/AIA. They adopted the common assumption that each loop had a
circular cross section. The accuracy of inferred shape and circularity
of cross sections can both be tested using observations of the same
loops from additional vantage points as provided by STEREO/EUVI. Here,
we use multiple viewing angles to confirm the three-dimensional
structure of loops. Of 151 viable cases, 105 (69.5%) matched some
form of visible coronal structure when viewed approximately in
quadrature. A loop with a circular cross-section should appear of
a similar width in different perspectives. In contradiction to this,
we find a puzzling lack of correlation between loop diameters seen from
different perspectives, even an anti-correlation in some cases. Features
identified as monolithic loops in AIA may, in fact, be more complex
density enhancements. The 30.5% of reconstructions from AIA which did
not match any feature in EUVI might be such enhancements. Others may be
genuine loop structures, but with elliptical cross sections. We observe
an anti-correlation between diameter and brightness, lending support
to the latter hypothesis. Four loops are consistent with non-circular
cross sections, where we find anti-correlation in both comparisons.
Title: Connecting Chromospheric Condensation Signatures To
Reconnection Driven Heating Rates In An X1.0 Flare
Authors: Ashfield, W. H.; Longcope, D.; Qiu, J.; Zhu, C.
Bibcode: 2021AAS...23812708A
Altcode:
Observations of solar flare reconnection at very high resolution
can be in-directly made at the footpoints of reconnected loops into
which flare energy is deposited. The response of the lower atmosphere
to this energy input includes a downward-propagating shock called
chromospheric condensation, which can be observed at wavelengths
including UV and visible. In order to characterize connection
using high-resolution observations of this shock, one must develop a
quantitative relationship between the two. Such a relation was recently
developed in previous work and here we test it on observations of
chromospheric condensation in a single footpoint in the flare ribbon of
the X1.0 SOL2014-10-25T16:56:36. Measurements taken of Si iv 1402.77
*A emission spectra with the Interface Region Imaging Spectrograph
(IRIS) using a 5 s cadence show a red-shifted component undergoing
typical condensation evolution, with a peak downward velocity of 35
km s*1 and a half-life of 16 s. Simultaneous
observations taken with the Atmospheric Imaging Assembly (AIA) reveal
a temporally and spatially correlated increase in UV emission in the
1600 *A band. We apply a technique called the Ultraviolet Footpoint
Calorimeter (UFC) to the 1600 *A light curve to infer the energy
deposition into the footpoint. We then input this energy into a
one-dimensional, hydrodynamic simulation to compute the chromospheric
response, including condensation. From this simulation, we synthesize
Si iv spectra and compute the time-evolving Doppler velocity. This
is found to compare reasonably well with the IRIS observation, thus
corroborating our reconnection-condensation relationship.
Title: Temperature Evolution Of Coronal Flux Observed Through Multiple
Extreme Ultraviolet Wavelengths
Authors: McCarthy, M.; Longcope, D.; Walters, J.
Bibcode: 2021AAS...23831317M
Altcode:
Coronal loops are the manifestation of magnetic fields in the solar
corona. In the environment of two adjacent active regions (ARs) any
flux linkage between the two should be observed as interconnecting
loops, if we are to assume each AR is itself its own magnetic
system. Therefore, by looking at loops between AR pairs we can quantify
magnetic reconnection, the process believed to be behind those loops'
formation. Further, these loops are believed to be impulsively heated
during their formation and should therefore be visible at progressively
lower temperatures as they cool. This cooling behavior is well known
in flares, but would be present in quiescent loops if they are heated
impulsively, for example by nanoflares. Here we test this hypothesis
by looking for each loop in a quiescent active region to cool through
multiple temperatures. In previous work, we created a catalog of coronal
loops interconnecting two active regions using AIA images in 171A. We
perform our search by looking extending the previous catalog of coronal
loops into other EUV wavelengths. Comparisons are presented between
the loops in this interconnecting magnetic domain observed at varying
wavelengths. We find evidence that some loops have been impulsively
heated and appear at progressively cooler temperatures. There are,
however, many counter-examples which may challenge the hypothesis of
impulsive heating followed by free cooling.
Title: Relating the Properties of Chromospheric Condensation to
Flare Energy Transported by Thermal Conduction
Authors: Ashfield, W. H.; Longcope, D. W.
Bibcode: 2021ApJ...912...25A
Altcode:
Chromospheric condensation is a brief episode of downflow often
accompanying energy release and evaporation in a solar flare. While
this component of a flare reflects the energy release process only
indirectly, it can be observed at high spatial and temporal resolution,
even from the ground. It appears in spectroscopic observations of
cooler lines, formed below ∼105 K, as a redshift that
peaks and decays after less than 1 minute. In order to use this
signature to infer characteristics of solar flare energy release, it
is important to establish quantitative relationships with properties of
the condensation. The initial investigation reported here does so after
restricting consideration to energy transport via thermal conduction
into a simplified, stratified chromosphere. We develop an analytical
expression for the decay of a condensation propagating into a stratified
atmosphere. This model provides a relationship between shock velocity
and preshock density structure. We also use one-dimensional gasdynamic
simulations to explore the dynamics of these shocks as they penetrate
into the stratified chromosphere. We find that the peak downflow speed
primarily reflects the energy flux into the chromosphere, while the
product of this velocity and the redshift duration is proportional to
the preshock density scale height as H ≃ 0.6u0τ.
Title: Multispacecraft Observations of Coronal Loops to Verify a
Force-free Field Reconstruction and Infer Loop Cross Sections
Authors: McCarthy, Marika I.; Longcope, Dana W.; Malanushenko, Anna
Bibcode: 2021ApJ...913...56M
Altcode: 2021arXiv210402722M
Active region EUV loops are believed to trace a subset of magnetic
field lines through the corona. Malanushenko et al. proposed a method,
using loop images and line-of-sight photospheric magnetograms, to infer
the 3D shape and field strength along each loop. McCarthy et al. used
this novel method to compute the total magnetic flux interconnecting
a pair of active regions observed by SDO/AIA. They adopted the common
assumption that each loop had a circular cross section. The accuracy
of inferred shape and circularity of cross sections can both be tested
using observations of the same loops from additional vantage points
as provided by STEREO/EUVI. Here we use multiple viewing angles to
confirm the 3D structure of loops. Of 151 viable cases, 105 (69.5%)
matched some form of visible coronal structure when viewed approximately
in quadrature. A loop with a circular cross section should appear of
a similar width in different perspectives. In contradiction to this,
we find a puzzling lack of correlation between loop diameters seen from
different perspectives, even an anticorrelation in some cases. Features
identified as monolithic loops in AIA may, in fact, be more complex
density enhancements. The 30.5% of reconstructions from AIA that did
not match any feature in EUVI might be such enhancements. Others may
be genuine loop structures, but with elliptical cross sections. We
observe an anticorrelation between diameter and brightness, lending
support to the latter hypothesis. Of 13 loops suitable for width
analysis, 4 are consistent with noncircular cross sections, where we
find anticorrelation in both comparisons.
Title: Critical Science Plan for the Daniel K. Inouye Solar Telescope
(DKIST)
Authors: Rast, Mark P.; Bello González, Nazaret; Bellot Rubio,
Luis; Cao, Wenda; Cauzzi, Gianna; Deluca, Edward; de Pontieu, Bart;
Fletcher, Lyndsay; Gibson, Sarah E.; Judge, Philip G.; Katsukawa,
Yukio; Kazachenko, Maria D.; Khomenko, Elena; Landi, Enrico; Martínez
Pillet, Valentín; Petrie, Gordon J. D.; Qiu, Jiong; Rachmeler,
Laurel A.; Rempel, Matthias; Schmidt, Wolfgang; Scullion, Eamon; Sun,
Xudong; Welsch, Brian T.; Andretta, Vincenzo; Antolin, Patrick; Ayres,
Thomas R.; Balasubramaniam, K. S.; Ballai, Istvan; Berger, Thomas E.;
Bradshaw, Stephen J.; Campbell, Ryan J.; Carlsson, Mats; Casini,
Roberto; Centeno, Rebecca; Cranmer, Steven R.; Criscuoli, Serena;
Deforest, Craig; Deng, Yuanyong; Erdélyi, Robertus; Fedun, Viktor;
Fischer, Catherine E.; González Manrique, Sergio J.; Hahn, Michael;
Harra, Louise; Henriques, Vasco M. J.; Hurlburt, Neal E.; Jaeggli,
Sarah; Jafarzadeh, Shahin; Jain, Rekha; Jefferies, Stuart M.; Keys,
Peter H.; Kowalski, Adam F.; Kuckein, Christoph; Kuhn, Jeffrey R.;
Kuridze, David; Liu, Jiajia; Liu, Wei; Longcope, Dana; Mathioudakis,
Mihalis; McAteer, R. T. James; McIntosh, Scott W.; McKenzie, David
E.; Miralles, Mari Paz; Morton, Richard J.; Muglach, Karin; Nelson,
Chris J.; Panesar, Navdeep K.; Parenti, Susanna; Parnell, Clare E.;
Poduval, Bala; Reardon, Kevin P.; Reep, Jeffrey W.; Schad, Thomas A.;
Schmit, Donald; Sharma, Rahul; Socas-Navarro, Hector; Srivastava,
Abhishek K.; Sterling, Alphonse C.; Suematsu, Yoshinori; Tarr, Lucas
A.; Tiwari, Sanjiv; Tritschler, Alexandra; Verth, Gary; Vourlidas,
Angelos; Wang, Haimin; Wang, Yi-Ming; NSO and DKIST Project; DKIST
Instrument Scientists; DKIST Science Working Group; DKIST Critical
Science Plan Community
Bibcode: 2021SoPh..296...70R
Altcode: 2020arXiv200808203R
The National Science Foundation's Daniel K. Inouye Solar Telescope
(DKIST) will revolutionize our ability to measure, understand,
and model the basic physical processes that control the structure
and dynamics of the Sun and its atmosphere. The first-light DKIST
images, released publicly on 29 January 2020, only hint at the
extraordinary capabilities that will accompany full commissioning of
the five facility instruments. With this Critical Science Plan (CSP)
we attempt to anticipate some of what those capabilities will enable,
providing a snapshot of some of the scientific pursuits that the DKIST
hopes to engage as start-of-operations nears. The work builds on the
combined contributions of the DKIST Science Working Group (SWG) and
CSP Community members, who generously shared their experiences, plans,
knowledge, and dreams. Discussion is primarily focused on those issues
to which DKIST will uniquely contribute.
Title: A statstical study of plasma energetics in flare-CME events
Authors: Zhu, C.; Qiu, J.; Longcope, D.
Bibcode: 2020AGUFMSH0500005Z
Altcode:
To study the flare energetic and energy partition in flare-CME events,
we investigate the scaling relationships between several physical
parameters during ~40 flare-CME events. We measured the total
reconnection fluxes by summing up the photospheric magnetic fluxes
swept by spreading flare ribbons. The peak of GOES X-ray radiance is
found to be strongly related to the reconnection flux, ribbon distance,
and ribbon length. The plasma energetic during each flare is quantified
by comparing the 0D EBTEL model with the multi-wavelength observations
of the solar corona in EUV and soft X-ray. Based on such measurements,
we provide a comprehensive evaluation of various energies in these
solar eruptions, including flare radiation, magnetic energy, and CME
mechanical energy, and determine the scalings among them.
Title: Characterizing Chromospheric Condensation from Shocks Driven
by Thermal Conduction
Authors: Ashfield, W.; Longcope, D.
Bibcode: 2020AGUFMSH057..07A
Altcode:
Chromospheric condensation is a downflow accompanied by a hydrodynamic
shock which accompanies the energy release by magnetic reconnection
in a solar flare. While this component of a flare reflects the energy
release process only indirectly, it can be observed at the highest
spatial and temporal resolution, even from the ground. It is therefore
important to establish a quantitative relationship between energy
release and condensation properties. In this initial investigation
we consider only energy transport via thermal conduction. We develop
an analytical solution to the decay of the condensation under the
influence of gravitational density stratification. This provides a
relationship between shock velocity and pre-shock density. We also
use one-dimensional gas-dynamic numerical simulations to explore
the dynamics of these shocks as they penetrate into the stratified
chromosphere. These suggest that the shock decay can be de-composed
into two different phases, whose properties vary with gravitational
scale height and the flare energy flux.
Title: Quantifying Chromospheric Condensation Characteristics through
IRIS Observations in Si IV
Authors: Reed, W.; Longcope, D.; Zhu, C.
Bibcode: 2020AGUFMSH004..06R
Altcode:
Chromospheric condensation is downflow that occurs, in some solar
flares, as a back reaction from chromospheric evaporation. Both effects
are the results of magnetic energy release initiated by magnetic
reconnection high in the corona. Chromospheric ribbons consist of all
the footpoints of the reconnect field lines, and therefore provide
a projection onto the chromosphere of the reconnection occurring
in the corona. Spectroscopic observations of these flare ribbons
provide information about the response of the transition region and
chromosphere, and thus provide indirect information about the localized
magnetic reconnection process initiating the flare. We use sit-and-stare
observations of the Si IV spectral line made by the Interface
Region Imaging Spectrograph (IRIS) to characterize the dynamics of
chromospheric condensation. Looking at ribbons from approximately thirty
different flares showing chromospheric condensation, we fit the Si IV
lines with single or multiple Gaussian components. We study the time
evolutions of the Doppler velocity, line width, and intensity to infer
the energy flux and duration of the release process. We use the spatial
correlation of the condensation flows to extrapolate the size of the
coronal reconnection sites. These characteristics bring us closer to
understanding coronal reconnection energy release occurring in flares.
Title: Implications of the correlation between Doppler shifts
and line widths in Si IV spectral lines from the active region
transition region
Authors: Longcope, D.; Zhu, C.
Bibcode: 2020AGUFMSH0010011L
Altcode:
Emission lines from the 100,000 K solar transition region are generally
red-shifted and broadened beyond what can be attributed to their
temperature. The reason for either remains unclear at this time. We
use observations of the Si IV1402.77A line in quiescent active regions,
made by IRIS, to show that the Doppler shift and non-thermal broadening
are strongly correlated. The correlation is approximately linear for
red-shifts between 5 and 20 km/s. Subtracting this linear trend from
the measured non-thermal broadening leaves a quantity we call the
Doppler-compensated broadening, which is uncorrelated with the Doppler
velocity. It turns out to be very nearly statistically independent
of it, suggesting that different, physically independent mechanisms
are responsible for the persistent red-shift and for a part of the
ubiquitous non-thermal broadening in the transition region. The fact
that the red-shift contributes proportionately to non-thermal broadening
offers an important clue to the nature of the mechanism responsible
for it. The other mechanism independently produces broadening which is
smaller than the full broadening (16 vs. 21 km/s) and far less variable
within one region or between different regions (varying by roughly
25%). Their different properties suggest that Doppler-compensated
broadening is generated by a mechanism operating below the transition
region, perhaps in the photosphere, while Doppler shifts are generated
above in the corona. Of several candidates for the latter mechanism we
find two to be most consistent with a broadening linearly proportional
to red-shift. These are downward propagating acoustic waves or downflows
in a coronal loop quasi-statically cooling following impulsive heating.
Title: Flare Models of Magnetic Energy Release into Plasma Heating
and Particle Acceleration
Authors: Guidoni, S. E.; Karpen, J. T.; DeVore, C. R.; Longcope, D.
Bibcode: 2020AGUFMSH045..02G
Altcode:
Understanding how flare magnetic energy can be released at rates of
the order of 1027-32 ergs/s has been a long-sought goal
in Heliophysics. Indirect observations of the lower solar corona
point to magnetic reconnection as the fundamental process that
converts free magnetic energy mainly into flows, heat, and particle
acceleration. The partitioning among these three energies is usually
inferred indirectly from subsequent radiation emitted by heated plasma
and energetic particles, but the uncertainties are large. T he energy
conversion to bulk motion and heat can be reasonably well described
with magnetohydrodynamic (MHD) models and simulations, while kinetic
models are better suited to study particle energization. However,
the scale separation between MHD and kinetic regimes in flares is
approximately 10 orders of magnitude. Therefore, it is currently
impossible to self-consistently unify flare models over all relevant
scales . We present results of our analytical 1D model of the
super-A lfvé nic shortening of reconnected field lines (reconnection
outflow) and the consequent plasma heating by strong gas-dynamic
shocks formed by this fast retraction. We also describe our efforts
to bridge the theoretical gap between MHD and kinetic regimes by
combining global flare simulations and analytical kinetic theory to
produce power-law- like particle energy spectra. This model explains
key characteristics of observed flare hard X-ray spectra, as well as
the underlying accelerated-electron properties .
Title: Modeling Sub-Alfvénic Outflow in Flare Reconnection via
Interactions with the Surrounding Plasma Sheet
Authors: Unverferth, J., IV; Longcope, D.
Bibcode: 2020AGUFMSH057..06U
Altcode:
Observations of reconnection outflows of the September 10th 2017
X 8.2(SOL2017-09-10T15:35:00) flare place them at 300 km/s to 700
km/s. This is well below the Alfvén speed, which is the speed at which
most reconnection models predict outflows. In this work, we attempt
to reconcile model with observation by modeling retraction through
the current sheet as a thin flux tube subject to a kind of aerodynamic
drag force proportional to the square of the tube's velocity. This drag
force is intended to represent the interaction with, and energy lost
to, the plasma surrounding the current sheet. We use a simulation of a
retracting flux tube to create a synthetic plasma sheet. In this sheet
we find that slower retractions result in a plasma sheet with increased
emission and lower temperatures. Conversely, we find that increasing
the strength of the field outside the current raises the temperature
lowers the emission. There appear to be value of both drag and magnetic
field strength that are consistent with observation and result in both
retraction speeds and plasma properties consistent with observation.
Title: Localized Reconnection Heating Inferred from the
Three-dimensional Locations of Bright Active Region Coronal Loops
Authors: Longcope, Dana; McCarthy, Marika; Malanushenko, Anna
Bibcode: 2020ApJ...901..147L
Altcode:
Coronal loops observed in soft X-rays and extreme ultraviolet
imaging data offer direct evidence that coronal plasma is heated
by some mechanism. That mechanism appears to energize a particular
bundle of field lines somehow selected from the magnetized coronal
volume. Magnetic reconnection localized to a patch within a coronal
current sheet is one mechanism that would select a flux bundle at
the same time it energized it. Since magnetic reconnection occurs
preferentially at topological boundaries, we would expect to find
coronal loops concentrated there if it were at work. We explore this
hypothesis using a data set, previously compiled by McCarthy et al.,
consisting of 301 coronal loops interconnecting a pair of active
regions over a 48 hr period. That work computed the three-dimensional
geometries and magnetic field strengths for most of the loops. This
revealed many bright loops lying at the periphery of the interconnecting
flux domain, possibly created and energized by the reconnection that
created the interconnecting flux. There were, however, many loops
well inside the domain which would be difficult to attribute to that
mode of reconnection. Here we use detailed magnetic models of the
interconnecting domain to show that these internal loops tend to occur
along internal boundaries: separatrices. This offers a novel form of
evidence that coronal loops are the products of patchy reconnection
even under quiescent conditions.
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena in Solar and
Heliospheric Plasmas
Authors: Ji, H.; Karpen, J.; Alt, A.; Antiochos, S.; Baalrud, S.;
Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Bhattacharjee,
A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.;
Cassak, P.; Chen, B.; Chen, L. -J.; Chen, Y.; Chien, A.; Comisso,
L.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.;
Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink,
G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto,
K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hare,
J.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le,
A.; Lebedev, S.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.;
Liu, W.; Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus,
W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sironi, L.; Sitnov, M.; Stanier, A.; Swisdak, M.; TenBarge,
J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.;
Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.;
Zenitani, S.; Zweibel, E.
Bibcode: 2020arXiv200908779J
Altcode:
Magnetic reconnection underlies many explosive phenomena in the
heliosphere and in laboratory plasmas. The new research capabilities in
theory/simulations, observations, and laboratory experiments provide the
opportunity to solve the grand scientific challenges summarized in this
whitepaper. Success will require enhanced and sustained investments
from relevant funding agencies, increased interagency/international
partnerships, and close collaborations of the solar, heliospheric,
and laboratory plasma communities. These investments will deliver
transformative progress in understanding magnetic reconnection and
related explosive phenomena including space weather events.
Title: Multi-spacecraft Observation of Coronal loops to Verify Loop
Reconstruction and Infer Cross Sections
Authors: McCarthy, M.; Longcope, D.
Bibcode: 2020SPD....5121006M
Altcode:
The corona of an active region is believed to be a low-beta plasma in
which a subset of field lines are energized and appear as coronal
loops in soft X-ray and EUV images. Malanushenko et al. (2009)
proposed a method, alpha-h-fitting, of using these loops, along with
a line-of-sight photospheric magnetogram, to infer the non-linear
force free field of the corona and the three-dimensional shape of
each loop. McCarthy et al. (2019) observed an active region pair
(NOAA AR11149/11147) over 48 hours and used the alpha-h-fitting
method on 199 of the loops interconnecting the pair observed in
SDO/AIA 171A. They used these to compute the total magnetic flux
interconnecting the regions under the assumption that each loop had
a circular cross section. The fitting and the assumption of circular
cross-sections can both be tested using observations of these regions
from multiple vantage points provided by the STEREO/EUVI images. This
observation occurred at a fortunate time since both STEREO spacecraft
are nearly in quadrature with AIA and from the twin spacecraft the
loops from the McCarthy et al. (2019) are viewed above the limb from
both vantage points. Here, we use the multiple viewing angles to
confirm the three-dimensional reconstruction. We are also able to use
the reconstruction to unambiguously map a point as viewed from AIA to
the corresponding location in the image taken with EUVI, and then the
measured diameters of that flux tube from these multiple vantage points.
Title: Modeling Observable Differences in Flare Loop Evolution due
to Reconnection Location and Current Sheet Structure
Authors: Unverferth, John; Longcope, Dana
Bibcode: 2020ApJ...894..148U
Altcode:
Flare reconnection is expected to occur at some point within a
large-scale coronal current sheet. The structure of the magnetic field
outside this sheet is almost certain to affect the flare, especially its
energy release. Different models for reconnection have invoked different
structures for the current sheet's magnetic field and different
locations for the reconnection electric field within it. Models
invoking Petschek-type reconnection often use a uniform field. Others
invoke a field bounded by two Y-points with a field strength maximum
between them and propose this maximum as the site of the reconnection
electric field. Still other models, such as the collapsing trap model,
require that the field strength peak at or near the edge of the current
sheet and propose that reconnection occurs above this peak. At present
there is no agreement as to where reconnection might occur within a
global current sheet. We study the post-reconnection dynamics under
all these scenarios, seeking potentially observable differences between
them. We find that reconnection occurring above the point of strongest
field leads to the highest density and the highest emission measure
of the hottest material. This scenario offers a possible explanation
of superhot coronal sources seen in some flares.
Title: The Creation of Twist by Reconnection of Flux Tubes
Authors: Priest, E. R.; Longcope, D. W.
Bibcode: 2020SoPh..295...48P
Altcode:
A fundamental process in a plasma is the magnetic reconnection of
one pair of flux tubes (such as solar coronal loops) to produce a new
pair. During this process magnetic helicity is conserved, but mutual
helicity can be transformed to self-helicity, so that the new tubes
acquire twist. However, until recently, when Wright (Astrophys. J.878,
102, 2019) supplied a solution, the partition of self-helicity between
the two tubes was an outstanding puzzle. Here we examine Wright's
result in detail and apply it to a variety of cases. The simplest case,
which Wright himself used to illustrate the result, is that of thin
ribbons or flux sheets. We first explicitly apply his method to the
usually expected standard case (when the tubes approach one another
without twisting before reconnection) and confirm his result is valid
for flux sheaths and tubes as well as sheets.
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena throughout
the Universe
Authors: Ji, H.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo,
F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.;
Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.;
Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.;
Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.;
Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.;
Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
Bibcode: 2020arXiv200400079J
Altcode:
This white paper summarizes major scientific challenges and
opportunities in understanding magnetic reconnection and related
explosive phenomena as a fundamental plasma process.
Title: Using Kepler's laws and Rutherford scattering to chart the
seven gravity assists in the epic sunward journey of the Parker
Solar Probe
Authors: Longcope, Dana
Bibcode: 2020AmJPh..88...11L
Altcode:
On August 12, 2018, NASA launched the Parker Solar Probe (PSP)
to explore regions very near the Sun. Losing enough energy and
angular momentum to approach the Sun requires either an impractical
amount of fuel or a maneuver called a gravity assist. A gravity
assist is essentially an elastic collision with a massive, moving
target—Rutherford scattering from a planet. Gravity assists are
often used to gain energy in missions destined for the outer solar
system, but they can also be used to lose energy. Reaching an orbit
sufficiently close to the Sun requires that PSP undergoes not one but
seven successive gravity assists off the planet Venus. This simple
description poses several conceptual challenges to the curious physics
student. Why is it so much more challenging to get to the Sun than
to leave the Solar System? Why does it take more than one gravity
assist to achieve this, and why does it require seven? Would it be
more effective to use Mercury instead of Venus? These questions can
be answered using the basic physics principles of Kepler's laws
and Rutherford scattering. The reasoning can be presented in an
illuminating graphical format to show that these and other seemingly
arcane aspects of interplanetary exploration can be understood at the
undergraduate level.
Title: Modelling Observable Differences in Flare Loop Evolution Due
to Current Sheet Structure
Authors: Unverferth, J., IV; Longcope, D. W.
Bibcode: 2019AGUFMSH13D3420U
Altcode:
Solar flares are the most energetic event in the solar system. These
events are triggered when magnetic reconnection occurs in a current
sheet in the corona above a magnetic active region. Energy release
in a solar flare is dictated by the retraction of magnetic flux
post-reconnection. The characteristics of the magnetic field are
expected to affect this process. Different models for reconnection
suggest different structures for the magnetic field. Models invoking
Petschek-type reconnection use a uniform field. Others invoke a field
bounded by two Y-points, such that the field strength is maximum
in the middle. Still other models, such as a collapsing trap model,
require that the field strength peak near or at the edge of the current
sheet. We use a thin flux tube (TFT) framework to study the effects of
the field structure on a bundle of retracting flux. Using a constant
magnetic field, this approach returns Petschek-type results. Here, we
have modified this framework to use a non-uniform magnetic field. With
this, we can now compare the expected evolution of flux bundles using
different magnetic field configurations. We find that different magnetic
field structures lead to a distinct difference in the evolution of a
flux bundle.
Title: Sub-Alfvènic Spreading of 3D Collisionless Magnetic
Reconnection and Application to Two-Ribbon Solar Flares
Authors: Arencibia, M.; Cassak, P.; Liang, H.; Priest, E. R.; Qiu,
J.; Longcope, D. W.
Bibcode: 2019AGUFMSH13D3421A
Altcode:
Ribbons in two-ribbon solar flares are observed to elongate in time
along the polarity inversion line in addition to their well-known
apparent motion away from it. This has been attributed to the spreading
of initially localized magnetic reconnection in time. Similar spreading
of reconnection has been observed in other settings, including
laboratory experiments, Earth's magnetotail, and Earth's dayside
magnetopause. Understanding how reconnection spreads is an important
aspect of understanding flare evolution including plasma energization
and particle acceleration, since these phenomena are associated with
the properties of the magnetic reconnection including its physical
size. An interesting observational result, both in solar flares and
Earth's magnetosphere, is that the reconnection tends to spread slower
than the Alfven speed. We investigate the effect of the thickness of
the current sheet being non-uniform on the speed of 3D spreading of
magnetic reconnection. We perform a parametric study using 3D two-fluid
numerical simulations of initially localized anti-parallel magnetic
reconnection and present a theory for the out-of-plane spreading
velocity as a function of the initial current sheet thicknesses of the
fluted current sheet. We find the spreading in fluted current sheets
is slower than spreading in uniform sheets with the same thickness in
as its broader region. This result provides a potential explanation
of why 3D reconnection can spread at sub-Alfvenic speeds.
Title: Measuring and Modeling the Rate of Separator Reconnection
between an Emerging and an Existing Active Region
Authors: McCarthy, Marika I.; Longcope, Dana W.; Malanushenko, Anna;
McKenzie, David E.
Bibcode: 2019ApJ...887..140M
Altcode: 2019arXiv191106340M
Magnetic reconnection occurs when new flux emerges into the corona
and becomes incorporated into the existing coronal field. A new
active region (AR) emerging in the vicinity of an existing AR
provides a convenient laboratory in which reconnection of this kind
can be quantified. We use high time-cadence 171 Å data from Solar
Dynamics Observatory (SDO)/AIA, focused on new/old active region
pair 11147/11149, to quantify reconnection. We identify new loops
as brightenings within a strip of pixels between the regions. This
strategy is premised on the assumption that the energy responsible for
brightening a loop originates in magnetic reconnection. We catalog 301
loops observed in the 48 hr time period beginning with the emergence of
AR 11149. The rate at which these loops appear between the two ARs is
used to calculate the reconnection rate between them. We then fit these
loops with magnetic field, solving for each loop’s field strength,
geometry, and twist (via its proxy, coronal α). We find the rate of
newly brightened flux overestimates the flux that could be undergoing
reconnection. This excess can be explained by our finding that the
interconnecting region is not at its lowest energy (constant-α)
state; the extrapolations exhibit loop-to-loop variation in α. This
flux overestimate may result from the slow emergence of AR 11149,
which allows time for Taylor relaxation internal to the domain of the
reconnected flux to bring the α distribution toward a single value,
providing another mechanism for brightening loops after they are
first created.
Title: Examination of Separator Reconnection Rates in a Series of
Adjacent Emerging/Existing Active Region Pairs
Authors: McCarthy, M.; Longcope, D. W.; Malanushenko, A. V.; McKenzie,
D. E.
Bibcode: 2019AGUFMSH11D3387M
Altcode:
Changes in magnetic field line topology must be occurring when new flux
emerges in the solar corona in order for it to become incorporated
in the existing coronal field. Magnetic reconnection is the process
responsible for this incorporation. We have previously quantified
this process using a system in which a new active region (AR) emerges
in the vicinity of an existing one by cataloging the loops formed
between them. We make a spatial/temporal stack plot of the region
between the ARs by extracting the pixels along a virtual slit. A
persistent, bright streak in such a plot indicates a coronal loop
connecting the newly emerging flux to the existing AR. We assert that
loops formed post-emergence between the ARs are initially the result of
reconnection. This work presents an extension of our data set to include
12 new emergence events using high time-cadence data from SDO/AIA. Our
data set includes several events in which no reconnected loops seem
to form between the two ARs. We present an improved method of our loop
cataloging procedure. Previous studies of such events have both under
and over-sampled the expected reconnected flux formed between the two
regions, and included a variety of delays between the emergence of the
new AR and the onset of reconnection. To elucidate such discrepancies,
we fit loops using a linear force-free field (LFFF) model which we
then use to construct a full, non-linear force-free field (NLFFF)
model of the system. This work was supported by NASA's HGI program.
Title: Principles Of Heliophysics: a textbook on the universal
processes behind planetary habitability
Authors: Schrijver, Karel; Bagenal, Fran; Bastian, Tim; Beer,
Juerg; Bisi, Mario; Bogdan, Tom; Bougher, Steve; Boteler, David;
Brain, Dave; Brasseur, Guy; Brownlee, Don; Charbonneau, Paul; Cohen,
Ofer; Christensen, Uli; Crowley, Tom; Fischer, Debrah; Forbes, Terry;
Fuller-Rowell, Tim; Galand, Marina; Giacalone, Joe; Gloeckler, George;
Gosling, Jack; Green, Janet; Guetersloh, Steve; Hansteen, Viggo;
Hartmann, Lee; Horanyi, Mihaly; Hudson, Hugh; Jakowski, Norbert;
Jokipii, Randy; Kivelson, Margaret; Krauss-Varban, Dietmar; Krupp,
Norbert; Lean, Judith; Linsky, Jeff; Longcope, Dana; Marsh, Daniel;
Miesch, Mark; Moldwin, Mark; Moore, Luke; Odenwald, Sten; Opher, Merav;
Osten, Rachel; Rempel, Matthias; Schmidt, Hauke; Siscoe, George;
Siskind, Dave; Smith, Chuck; Solomon, Stan; Stallard, Tom; Stanley,
Sabine; Sojka, Jan; Tobiska, Kent; Toffoletto, Frank; Tribble, Alan;
Vasyliunas, Vytenis; Walterscheid, Richard; Wang, Ji; Wood, Brian;
Woods, Tom; Zapp, Neal
Bibcode: 2019arXiv191014022S
Altcode:
This textbook gives a perspective of heliophysics in a way that
emphasizes universal processes from a perspective that draws attention
to what provides Earth (and similar (exo-)planets) with a relatively
stable setting in which life as we know it can thrive. The book is
intended for students in physical sciences in later years of their
university training and for beginning graduate students in fields of
solar, stellar, (exo-)planetary, and planetary-system sciences.
Title: STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales
from Microseconds to Years
Authors: Ray, Paul; Arzoumanian, Zaven; Ballantyne, David; Bozzo,
Enrico; Brandt, Soren; Brenneman, Laura; Chakrabarty, Deepto;
Christophersen, Marc; DeRosa, Alessandra; Feroci, Marco; Gendreau,
Keith; Goldstein, Adam; Hartmann, Dieter; Hernanz, Margarita; Jenke,
Peter; Kara, Erin; Maccarone, Tom; McDonald, Michael; Martindale,
Adrian; Nowak, Michael; Phlips, Bernard; Remillard, Ron; Schanne,
Stephane; Stevens, Abigail; Tomsick, John; Watts, Anna; Wilson-Hodge,
Colleen; Wolff, Michael; Wood, Kent; Zane, Silvia; Ajello, Marco;
Alston, Will; Altamirano, Diego; Antoniou, Vallia; Arur, Kavitha;
Ashton, Dominic; Auchettl, Katie; Ayres, Tom; Bachetti, Matteo;
Balokovic, Mislav; Baring, Matthew; Baykal, Altan; Begelman, Mitch;
Bhat, Narayana; Bogdanov, Slavko; Briggs, Michael; Bulbul, Esra;
Bult, Petrus; Burns, Eric; Cackett, Ed; Campana, Riccardo; Caspi,
Amir; Cavecchi, Yuri; Chenevez, Jerome; Cherry, Mike; Corbet, Robin;
Corcoran, Michael; Corsi, Alessandra; Degenaar, Nathalie; Drake,
Jeremy; Eikenberry, Steve; Enoto, Teruaki; Fragile, Chris; Fuerst,
Felix; Gandhi, Poshak; Garcia, Javier; Goldstein, Adam; Gonzalez,
Anthony; Grefenstette, Brian; Grinberg, Victoria; Grossan, Bruce;
Guillot, Sebastien; Guver, Tolga; Haggard, Daryl; Heinke, Craig;
Heinz, Sebastian; Hemphill, Paul; Homan, Jeroen; Hui, Michelle;
Huppenkothen, Daniela; Ingram, Adam; Irwin, Jimmy; Jaisawal, Gaurava;
Jaodand, Amruta; Kalemci, Emrah; Kaplan, David; Keek, Laurens; Kennea,
Jamie; Kerr, Matthew; van der Klis, Michiel; Kocevski, Daniel; Koss,
Mike; Kowalski, Adam; Lai, Dong; Lamb, Fred; Laycock, Silas; Lazio,
Joseph; Lazzati, Davide; Longcope, Dana; Loewenstein, Michael; Maitra,
Dipankair; Majid, Walid; Maksym, W. Peter; Malacaria, Christian;
Margutti, Raffaella; Martindale, Adrian; McHardy, Ian; Meyer, Manuel;
Middleton, Matt; Miller, Jon; Miller, Cole; Motta, Sara; Neilsen, Joey;
Nelson, Tommy; Noble, Scott; O'Brien, Paul; Osborne, Julian; Osten,
Rachel; Ozel, Feryal; Palliyaguru, Nipuni; Pasham, Dheeraj; Patruno,
Alessandro; Pelassa, Vero; Petropoulou, Maria; Pilia, Maura; Pohl,
Martin; Pooley, David; Prescod-Weinstein, Chanda; Psaltis, Dimitrios;
Raaijmakers, Geert; Reynolds, Chris; Riley, Thomas E.; Salvesen, Greg;
Santangelo, Andrea; Scaringi, Simone; Schanne, Stephane; Schnittman,
Jeremy; Smith, David; Smith, Krista Lynne; Snios, Bradford; Steiner,
Andrew; Steiner, Jack; Stella, Luigi; Strohmayer, Tod; Sun, Ming;
Tauris, Thomas; Taylor, Corbin; Tohuvavohu, Aaron; Vacchi, Andrea;
Vasilopoulos, Georgios; Veledina, Alexandra; Walsh, Jonelle; Weinberg,
Nevin; Wilkins, Dan; Willingale, Richard; Wilms, Joern; Winter,
Lisa; Wolff, Michael; in 't Zand, Jean; Zezas, Andreas; Zhang, Bing;
Zoghbi, Abdu
Bibcode: 2019BAAS...51g.231R
Altcode: 2019astro2020U.231R
STROBE-X is a probe-class mission concept, selected for study by NASA,
for X-ray spectral timing of compact objects across the mass scale. It
combines huge collecting area, high throughput, broad energy coverage,
and excellent spectral and temporal resolution in a single facility,
enabling a broad portfolio of high-priority astrophysics.
Title: Evidence for downflows in the narrow plasma sheet of 10 Sep
2017, and their significance for flare reconnection
Authors: Longcope, Dana; Unverferth, John E.; Klein, Courtney;
McCarthy, Marika; Priest, Eric R.
Bibcode: 2019AAS...23421604L
Altcode:
Current sheets are believed to form in the wakes of erupting flux ropes
and to enable the magnetic reconnection responsible for an associated
flare. Multi-wavelength observations of an eruption on 10 Sep 2017
show a long, linear feature widely taken as evidence of a current
sheet viewed edge-on. The relation between the high-temperature,
high-density plasma thus observed and any current sheet is not yet
entirely clear. We estimate the magnetic field strength surrounding
the sheet, and from that conclude that approximately one-third of all
flux in the active region was opened by the eruption. Subsequently
decreasing field strength suggests that the open flux closed down
over the next several hours through reconnection at a rate dΦ/dt =
5 × 1017 Mx/s. We find in AIA observations evidence of
downward moving, dark structures analogous to either supra-arcade
downflows more typically observed above flare arcades viewed face-on,
or to supra-arcade downflowing loops, previously reported in flares
viewed in this perspective. This suggests that the plasma sheet is
composed of the magnetic flux retracting after being reconnected high
above the arcade. We use a model of flux tube retraction following
reconnection to show that this process can generate high densities
and temperatures as observed in the plasma sheet. The retracting flux
tubes reach their highest temperatures at the end of their retraction,
well below the site of reconnection. Previous analysis of AIA and EIS
data had revealed a peak in the plasma temperature very near the base
of this particular sheet. This is consistent with our hypothesis that
downflows descend from a higher reconnection point. This work
supported by grants from NASA/HSR and NSF/REU
Title: The Influence of Current Sheet Structure on Flare Loop Dynamics
Authors: Unverferth, John E.; Longcope, Dana
Bibcode: 2019AAS...23421603U
Altcode:
Solar flares are believed to result from magnetic reconnection
occurring within a coronal current sheet. Flare energy is released
as flux retracts through the sheet following its reconnection. This
process is dictated by the magnetic field strength along the sheet
through which the flux retracts. Some theoretical models, including the
standard Petschek model, assume the field strength to be uniform along
the sheet. Others invoke a sheet terminating in field-free Y-points,
so the field strength decreases steadily during retraction. Still
others, including models of a collapsing trap, assume the field strength
increases as flux retracts. Each of these assumed configurations will
lead to an observably different solar flare. We study the effects of
these different configurations using the thin flux tube (TFT) model of
magnetic flux retraction following reconnection. This model has been
shown to yield Petschek-like results when a uniform current sheet is
assumed. We modify the model to study the effects of different current
sheet configurations. This allows us to compare flares set in current
sheets with both increasing and decreasing field strength profiles. This work was supported by NASA's HSR program.
Title: A Modified Kirkpatrick-Baez Design for a Practical Astronomical
X-ray Telescope
Authors: Longcope, Dana; Acton, Loren W.; Kankelborg, Charles
Bibcode: 2019AAS...23430101L
Altcode:
Kirkpatrick-Baez (K-B) optics offer a means of imaging soft x-rays
with modest resolution and a multi-arc-minute field of view at a cost
far below the conventional Wolter design. Such a low-cost system could
be useful for dedicated, long time-line observation of astronomical
x-ray sources from orbit. A K-B telescope consists of crossed arrays
of parabolic mirrors at grazing incidence. The classic K-B design is
subject to significant aberration, arising from interplay between
the focusing of the fore and aft mirror arrays. We demonstrate
here a modified K-B design with aberrations reduced by an order of
magnitude. We show, furthermore, that it is possible to construct
such a system by constraining flat "slats" of commercially-available
glass in precision machined grooves. The slats deform into shapes which
adequately approximate the optimal figures, thereby yielding focusing
better than the best version of the classic K-B design. The result is a
new approach that greatly simplifies the task of achieving both useful
resolution and high effective area for x-ray astronomy applications.
Title: Investigating Reconnection Evolution using Si IV Doppler
Velocity Measurments
Authors: Derks, Alysa; Longcope, Dana
Bibcode: 2019AAS...23420601D
Altcode:
Spectroscopic measurements of flare ribbons can provide information
about the magnetic reconnection process driving the flare. Part
of the response of chromospheric material to the energy input
from magnetic reconnection, is a redshift known as chromospheric
condensation. The evolution of this redshift is related to how long the
energy was deposited for, and the spatial scale of where the energy
was deposited in the corona. In this study, we use Interface Region
Imaging Spectrograph (IRIS) FUV observations of a flare on October 25,
2014 to measure Doppler shifts of the Si IV 1402 and 1393 Å lines
and the time evolution of each line. Both Si IV lines are well fit
using a sum of two Gaussian components. Here, we have developed a
fitting procedure which will directly infer the Doppler velocity
of the red and blue shifted component at each non-saturated ribbon
pixel. We see that these red and blue shifted components peak to a
maximum velocity and then decay away to its normal velocity. This
is the behavior predicted by models of chromospheric condensation
(Fisher et al. 1985, Longcope 2014). Using results from this study,
we seek evidence for the reconnection length scale and the reconnection
spatial dependence. This work is supported by a grant from NSF/AST.
Title: Measuring and modeling the rate of separator reconnection
between an emerging and existing active region
Authors: McCarthy, Marika; Longcope, Dana; Malanushenko, Anna;
McKenzie, David Eugene
Bibcode: 2019AAS...23411705M
Altcode:
Magnetic reconnection must occur when new flux emerges into the corona
and becomes incorporated into the existing coronal field. A new active
region (AR) emerging in the vicinity of an existing AR provides
a convenient laboratory in which reconnection of this kind can be
quantified. We perform such a measurement using high time-cadence
171 Å data from SDO/AIA of active region NOAA AR11149 which emerged
in the vicinity of AR11147 beginning on 20 January 2011. We make a
spatial/temporal stack plot of the region between the ARs by extracting
the pixels along a virtual slit. A persistent, bright streak in such
a plot indicates a bright coronal loop connecting the newly emerging
flux to the existing AR. This loop must have been formed through a
process of coronal reconnection across the separator separating the
four topologically distinct flux systems. We assume further that energy
released during that reconnection is responsible for its brightening. We
catalog 205 loops observed in the a 48-hour time period beginning
with the emergence of AR 11149. The rate at which new magnetic flux
appears is used to calculate the rate of separator reconnection. We
can further fit these cataloged field lines using a linear force-free
field (LFFF) extrapolation, solving for an individual loop's field
strength and twist. Ultimately, we find the rate of newly-brightened
flux overestimates the flux which could be undergoing reconnection. This
excess can be explained by our finding that the interconnecting region
is not at its lowest energy (constant-α) state; the LFFF modeling
shows a variation in values of α. This overestimate might be the result
of the region's unusually slow emergence, providing time for internal
Taylor-relaxation reconnection of the interconnecting flux following
its initial formation by reconnection. We support this hypothesis by
computing the rates of brightening within the plane of the virtual
slit. This work was supported by NASA's HGI program.
Title: COHERENT: Studying the corona as a holistic environment
Authors: Caspi, Amir; Seaton, Daniel B.; Case, Traci; Cheung, Mark;
Cranmer, Steven; DeForest, Craig E.; de Toma, Giuliana; Downs, Cooper;
Elliott, Heather; Gold, Anne U.; Longcope, Dana; Savage, Sabrina L.;
Sullivan, Susan; Viall, Nicholeen; Vourlidas, Angelos; West, Matthew J.
Bibcode: 2019shin.confE.241C
Altcode:
The solar corona and the heliosphere must be part of a single
physical system, but because the dominant physical processes change
dramatically from the magnetically-dominated low corona, through the
sparsely-observed middle corona, and into the plasma flow-dominated
outer corona and heliospheric interface, unified frameworks to study
the corona as a whole are essentially nonexistent. Understanding how
physical processes shape and drive the dynamics of the corona as a
global system, on all spatiotemporal scales, is critical for solving
many fundamental problems in solar and heliospheric physics. However,
the lack of unifying observations and models has led to a fragmentation
of the community into distinct regimes of plasma parameter space,
largely clustering around regions where existing instrumentation has
made observations widely available and where models can be sufficiently
self-contained to be tractable. We describe COHERENT, the 'Corona as a
Holistic Environment' Research Network, a focused effort to facilitate
interdisciplinary collaborative research to develop frameworks for
unifying existing and upcoming observations, theory, models, and
analytical tools to study the corona as a holistic system.
Title: Historical astronomical data: urgent need for preservation,
digitization enabling scientific exploration
Authors: Pevtsov, Alexei; Griffin, Elizabeth; Grindlay, Jonathan;
Kafka, Stella; Bartlett, Jennifer; Usoskin, Ilya; Mursula, Kalevi;
Gibson, Sarah; Pillet, Valentín; Burkepile, Joan; Webb, David; Clette,
Frédéric; Hesser, James; Stetson, Peter; Muñoz-Jaramillo, Andres;
Hill, Frank; Bogart, Rick; Osborn, Wayne; Longcope, Dana
Bibcode: 2019BAAS...51c.190P
Altcode: 2019arXiv190304839P; 2019astro2020T.190P
This white paper emphasizes critical importance of preservation,
digitization and scientific exploration of historical astronomical
data. It outlines the rationale, provides examples of new science
with such data, and reviews the potential losses to science if nothing
it done.
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena throughout
the Universe
Authors: Ji, Hantao; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.;
Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte,
J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian,
A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu,
W.; Longcope, D.; Louriero, N.; Lu, Q. -M.; Ma, Z. -W.; Matthaeus,
W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
Bibcode: 2019BAAS...51c...5J
Altcode: 2019astro2020T...5J
This is a group white paper of 100 authors (each with explicit
permission via email) from 51 institutions on the topic of magnetic
reconnection which is relevant to 6 thematic areas. Grand challenges
and research opportunities are described in observations, numerical
modeling and laboratory experiments in the upcoming decade.
Title: STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales
from Microseconds to Years
Authors: Ray, Paul S.; Arzoumanian, Zaven; Ballantyne, David;
Bozzo, Enrico; Brandt, Soren; Brenneman, Laura; Chakrabarty, Deepto;
Christophersen, Marc; DeRosa, Alessandra; Feroci, Marco; Gendreau,
Keith; Goldstein, Adam; Hartmann, Dieter; Hernanz, Margarita;
Jenke, Peter; Kara, Erin; Maccarone, Tom; McDonald, Michael;
Nowak, Michael; Phlips, Bernard; Remillard, Ron; Stevens, Abigail;
Tomsick, John; Watts, Anna; Wilson-Hodge, Colleen; Wood, Kent; Zane,
Silvia; Ajello, Marco; Alston, Will; Altamirano, Diego; Antoniou,
Vallia; Arur, Kavitha; Ashton, Dominic; Auchettl, Katie; Ayres, Tom;
Bachetti, Matteo; Balokovic, Mislav; Baring, Matthew; Baykal, Altan;
Begelman, Mitch; Bhat, Narayana; Bogdanov, Slavko; Briggs, Michael;
Bulbul, Esra; Bult, Petrus; Burns, Eric; Cackett, Ed; Campana,
Riccardo; Caspi, Amir; Cavecchi, Yuri; Chenevez, Jerome; Cherry,
Mike; Corbet, Robin; Corcoran, Michael; Corsi, Alessandra; Degenaar,
Nathalie; Drake, Jeremy; Eikenberry, Steve; Enoto, Teruaki; Fragile,
Chris; Fuerst, Felix; Gandhi, Poshak; Garcia, Javier; Goldstein,
Adam; Gonzalez, Anthony; Grefenstette, Brian; Grinberg, Victoria;
Grossan, Bruce; Guillot, Sebastien; Guver, Tolga; Haggard, Daryl;
Heinke, Craig; Heinz, Sebastian; Hemphill, Paul; Homan, Jeroen;
Hui, Michelle; Huppenkothen, Daniela; Ingram, Adam; Irwin, Jimmy;
Jaisawal, Gaurava; Jaodand, Amruta; Kalemci, Emrah; Kaplan, David;
Keek, Laurens; Kennea, Jamie; Kerr, Matthew; van der Klis, Michiel;
Kocevski, Daniel; Koss, Mike; Kowalski, Adam; Lai, Dong; Lamb, Fred;
Laycock, Silas; Lazio, Joseph; Lazzati, Davide; Longcope, Dana;
Loewenstein, Michael; Maitra, Dipankair; Majid, Walid; Maksym,
W. Peter; Malacaria, Christian; Margutti, Raffaella; Martindale,
Adrian; McHardy, Ian; Meyer, Manuel; Middleton, Matt; Miller, Jon;
Miller, Cole; Motta, Sara; Neilsen, Joey; Nelson, Tommy; Noble,
Scott; O'Brien, Paul; Osborne, Julian; Osten, Rachel; Ozel, Feryal;
Palliyaguru, Nipuni; Pasham, Dheeraj; Patruno, Alessandro; Pelassa,
Vero; Petropoulou, Maria; Pilia, Maura; Pohl, Martin; Pooley, David;
Prescod-Weinstein, Chanda; Psaltis, Dimitrios; Raaijmakers, Geert;
Reynolds, Chris; Riley, Thomas E.; Salvesen, Greg; Santangelo, Andrea;
Scaringi, Simone; Schanne, Stephane; Schnittman, Jeremy; Smith, David;
Smith, Krista Lynne; Snios, Bradford; Steiner, Andrew; Steiner, Jack;
Stella, Luigi; Strohmayer, Tod; Sun, Ming; Tauris, Thomas; Taylor,
Corbin; Tohuvavohu, Aaron; Vacchi, Andrea; Vasilopoulos, Georgios;
Veledina, Alexandra; Walsh, Jonelle; Weinberg, Nevin; Wilkins, Dan;
Willingale, Richard; Wilms, Joern; Winter, Lisa; Wolff, Michael; in
't Zand, Jean; Zezas, Andreas; Zhang, Bing; Zoghbi, Abdu
Bibcode: 2019arXiv190303035R
Altcode:
We present the Spectroscopic Time-Resolving Observatory for Broadband
Energy X-rays (STROBE-X), a probe-class mission concept selected for
study by NASA. It combines huge collecting area, high throughput, broad
energy coverage, and excellent spectral and temporal resolution in a
single facility. STROBE-X offers an enormous increase in sensitivity
for X-ray spectral timing, extending these techniques to extragalactic
targets for the first time. It is also an agile mission capable of
rapid response to transient events, making it an essential X-ray
partner facility in the era of time-domain, multi-wavelength, and
multi-messenger astronomy. Optimized for study of the most extreme
conditions found in the Universe, its key science objectives include:
(1) Robustly measuring mass and spin and mapping inner accretion
flows across the black hole mass spectrum, from compact stars to
intermediate-mass objects to active galactic nuclei. (2) Mapping out
the full mass-radius relation of neutron stars using an ensemble of
nearly two dozen rotation-powered pulsars and accreting neutron stars,
and hence measuring the equation of state for ultradense matter
over a much wider range of densities than explored by NICER. (3)
Identifying and studying X-ray counterparts (in the post-Swift era)
for multiwavelength and multi-messenger transients in the dynamic sky
through cross-correlation with gravitational wave interferometers,
neutrino observatories, and high-cadence time-domain surveys in other
electromagnetic bands. (4) Continuously surveying the dynamic X-ray
sky with a large duty cycle and high time resolution to characterize
the behavior of X-ray sources over an unprecedentedly vast range of
time scales. STROBE-X's formidable capabilities will also enable a
broad portfolio of additional science.
Title: Evidence for Downflows in the Narrow Plasma Sheet of 2017
September 10 and Their Significance for Flare Reconnection
Authors: Longcope, Dana; Unverferth, John; Klein, Courtney; McCarthy,
Marika; Priest, Eric
Bibcode: 2018ApJ...868..148L
Altcode:
Current sheets are believed to form in the wakes of erupting flux ropes
and to enable the magnetic reconnection responsible for an associated
flare. Multiwavelength observations of an eruption on 2017 September
10 show a long, linear feature widely taken as evidence of a current
sheet viewed edge-on. The relation between the high-temperature,
high-density plasma thus observed and any current sheet is not yet
entirely clear. We estimate the magnetic field strength surrounding
the sheet and conclude that approximately one-third of all flux in
the active region was opened by the eruption. Subsequently decreasing
field strength suggests that the open flux closed down over the next
several hours through reconnection at a rate \dot{{{Φ }}}≃ 5×
{10}17 Mx s-1. We find in AIA observations
evidence of downward-moving, dark structures analogous to either
supra-arcade downflows, more typically observed above flare arcades
viewed face-on, or supra-arcade downflowing loops, previously reported
in flares viewed in this perspective. These features suggest that the
plasma sheet is composed of the magnetic flux retracting after being
reconnected high above the arcade. We model flux tube retraction
following reconnection to show that this process can generate high
densities and temperatures as observed in the plasma sheet. The
retracting flux tubes reach their highest temperatures at the end of
their retraction, well below the site of reconnection, consistent with
previous analysis of AIA and EIS data showing a peak in the plasma
temperature near the base of this particular sheet.
Title: The UV Neupert Effect: Heating and Cooling of Solar Flare
Plasmas
Authors: Qiu, Jiong; Longcope, Dana; Zhu, Chunming; Bralts-Kelly,
Lilly; Cheng, Jianxia
Bibcode: 2018cosp...42E2751Q
Altcode:
Explosive energy release in solar flares is believed to begoverned
by magnetic reconnection usually occuring in the corona.Yet at the
onset of a flare, the immediate and more significant responseto
reconnection energy release is often observed in the lower
atmosphere.The chromosphere being the lower boundary of the corona,
its radiationsignatures in UV or optical wavelengths provide clues to
understanding corona heating.The lower atmosphere is also the only
place where reliable measurements of magneticfield are currently
available. Therefore, reconnection events in the Sun's corona can be
mapped,tracked, and measured with observations of the lower atmosphere
during the flare.In the past decades, high-resolution observations
reveal thatflare heating takes place in ``quantas", or a cluster of
flare loops and their foot-points. We have recently developed a method
to construct the history of heating of numerous flare loops fromthe
UV emission signatures at their foot-points, which can be used in
hydrodynamic models to study flare plasma evolution and compare with
coronal observations. The method helps improve the estimate of the
amount of energy used in heating these loops, and explore the nature
of heating. Our recent analysis and multi-loop modeling suggest that
heating of a flare loop likely proceeds in two phases,an intense
impulsive heating followed by a low-rate gradual heating.We discuss
properties of the different phases of heating, and the distributionof
heating events in a flare with their relation to magnetic properties
in the active region.
Title: Effects of the Canopy and Flux Tube Anchoring on Evaporation
Flow of a Solar Flare
Authors: Unverferth, John; Longcope, Dana
Bibcode: 2018ApJ...859..170U
Altcode:
Spectroscopic observations of flare ribbons typically show chromospheric
evaporation flows, which are subsonic for their high temperatures. This
contrasts with many numerical simulations where evaporation is typically
supersonic. These simulations typically assume flow along a flux tube
with a uniform cross-sectional area. A simple model of the magnetic
canopy, however, includes many regions of low magnetic field strength,
where flux tubes achieve local maxima in their cross-sectional
area. These are analgous to a chamber in a flow tube. We find that
one-third of all field lines in a model have some form of chamber
through which evaporation flow must pass. Using a one-dimensional
isothermal hydrodynamic code, we simulated supersonic flow through an
assortment of chambers and found that a subset of solutions exhibit a
stationary standing shock within the chamber. These shocked solutions
have slower and denser upflows than a flow through a uniform tube
would. We use our solution to construct synthetic spectral lines and
find that the shocked solutions show higher emission and lower Doppler
shifts. When these synthetic lines are combined into an ensemble
representing a single canopy cell, the composite line appears slower,
even subsonic, than expected due to the outsized contribution from
shocked solutions.
Title: Including both super-hot (thermal) and non-thermal electrons
in a single model of a reconnection-driven solar flare
Authors: Longcope, Dana
Bibcode: 2018tess.conf10527L
Altcode:
We have recently developed a one-dimensional flare loop model in
which magnetic energy release occurs via loop retraction following
reconnection. The plasma in our retracting flux tube evolves several
propagating shock including a kind of slow magnetosonic shock at which
the plasma is heated to flare temperatures. The model has proven able to
reproduce, quantitatively, several aspects of actual flares including
their observed thermal X-ray spectra. Our model, like that original
proposed by Petschek, is framed in terms of fluid equations (MHD),
and therefore lacks the population of non-thermal electrons. While
missing from fluid models, non-thermal electrons are one of the most
important characteristics observed in flares. A separate line of flare
modeling has focused on the generation of non-thermal electrons by,
for example, MHD turbulence. These model have not generally included
the reconnection process believed to drive that turbulence. We have
generalized our original model so that some of the energy released by
post-reconnection retraction feeds turbulence which then generates a
non-thermal electron population. The non-thermal electrons thermalize,
raising temperatures throughout the loop, especially within the dense
chromosphere. This composite model predicts the mixture of thermal and
non-thermal signatures which will appear in a flare. In particular,
non-thermal signatures will not appear alone.
Title: Modeling the Effect of Non-constant Cross-section on the EUV
Brightness of a Flaring Coronal Loop
Authors: Unverferth, John, IV; Longcope, Dana
Bibcode: 2018tess.conf30496U
Altcode:
Chromospheric evaporation is the result of energy release in the
corona during a solar flare. The speed of the upflowing material in
chromospheric material is observed to be subsonic, however, simulations
show that the upflow should be supersonic for large energy fluxes. Past
simulations tend to assume that the coronal loops have a constant cross
section. Using a model for the low corona, we find that while most
field lines expand as they rise though the corona, one third contain a
maximum area and narrow as the rise higher in the corona. Previous work
has showed that this chamber has the potential to drastically change
the properties of the chromospheric evaporation. Here we investigate the
impact that this has on both the initial and subsequent evolution of the
flaring loop, including its cooling phase. We focus in particular on the
evolution of the loop in EUV brightness over this extended evolution. We
include the effects of non-equilibrium ionization on this emission.
Title: Measuring separator reconnection between emerging and existing
active regions using extreme ultraviolet imaging observations
Authors: McCarthy, Marika; Longcope, Dana; McKenzie, David E.;
Malanushenko, Anna V.
Bibcode: 2018tess.conf20545M
Altcode:
Magnetic reconnection must be occurring when new flux emerges into the
corona, in order that the flux become incorporated into the existing
corona. The most evident, and easily quantified, example of this
occurs when a new active region (AR) emerges in the vicinity of an
existing AR. In a study of such emergence observed by TRACE, Longcope et
al. (2005) found a delay of approximately 24 hours between the new AR
emerging and its reconnection with the field of the existing AR. This
turned out to be the only suitable event in the TRACE archive. Here
we apply the same method to events observed by SDO/AIA. Using high
time-cadence images in one EUV wavelength, such as 171 A, we make
a spatial/temporal stack plot of the region between the ARs by
extracting the pixel in a virtual slit. A persistent, bright streak
indicates a bright coronal loop connecting the newly emerging flux
to the existing AR. This loop must have been formed through a process
of coronal reconnection across the separator separating the two flux
systems. The rate at which new loops appear is used to compute the
rate of separator reconnection. The continuous, high-cadence data from
AIA permits flux transfer to be quantified for intervals exceeding 48
hours. This work was supported by NASA's HGI program.
Title: Two-phase Heating in Flaring Loops
Authors: Zhu, Chunming; Qiu, Jiong; Longcope, Dana W.
Bibcode: 2018ApJ...856...27Z
Altcode: 2018arXiv180200871Z
We analyze and model a C5.7 two-ribbon solar flare observed by
the Solar Dynamics Observatory, Hinode, and GOES on 2011 December
26. The flare is made of many loops formed and heated successively
over one and half hours, and their footpoints are brightened in the
UV 1600 Å before enhanced soft X-ray and EUV missions are observed
in flare loops. Assuming that anchored at each brightened UV pixel is
a half flaring loop, we identify more than 6700 half flaring loops,
and infer the heating rate of each loop from the UV light curve at
the footpoint. In each half loop, the heating rate consists of two
phases: intense impulsive heating followed by a low-rate heating
that is persistent for more than 20 minutes. Using these heating
rates, we simulate the evolution of their coronal temperatures and
densities with the model of the “enthalpy-based thermal evolution
of loops.” In the model, suppression of thermal conduction is also
considered. This model successfully reproduces total soft X-ray and EUV
light curves observed in 15 passbands by four instruments GOES, AIA,
XRT, and EVE. In this flare, a total energy of 4.9 × 1030
erg is required to heat the corona, around 40% of this energy is in
the slow-heating phase. About two-fifths of the total energy used
to heat the corona is radiated by the coronal plasmas, and the other
three fifth transported to the lower atmosphere by thermal conduction.
Title: Observationally quantified reconnection providing a viable
mechanism for active region coronal heating
Authors: Yang, Kai E.; Longcope, Dana W.; Ding, M. D.; Guo, Yang
Bibcode: 2018NatCo...9..692Y
Altcode: 2018arXiv180206206Y
The heating of the Sun's corona has been explained by several different
mechanisms including wave dissipation and magnetic reconnection. While
both have been shown capable of supplying the requisite power, neither
has been used in a quantitative model of observations fed by measured
inputs. Here we show that impulsive reconnection is capable of producing
an active region corona agreeing both qualitatively and quantitatively
with extreme-ultraviolet observations. We calculate the heating power
proportional to the velocity difference between magnetic footpoints
and the photospheric plasma, called the non-ideal velocity. The
length scale of flux elements reconnected in the corona is found to be
around 160 km. The differential emission measure of the model corona
agrees with that derived using multi-wavelength images. Synthesized
extreme-ultraviolet images resemble observations both in their
loop-dominated appearance and their intensity histograms. This work
provides compelling evidence that impulsive reconnection events are
a viable mechanism for heating the corona.
Title: Modeling a Propagating Sawtooth Flare Ribbon Structure as a
Tearing Mode in the Presence of Velocity Shear
Authors: Parker, Jacob; Longcope, Dana
Bibcode: 2017ApJ...847...30P
Altcode: 2017arXiv170904534P
On 2014 April 18 (SOL2014-04-18T13:03), an M-class flare was observed
by IRIS. The associated flare ribbon contained a quasi-periodic
sawtooth pattern that was observed to propagate along the ribbon,
perpendicular to the IRIS spectral slit, with a phase velocity of
∼15 km s-1. This motion resulted in periodicities in both
intensity and Doppler velocity along the slit. These periodicities were
reported by Brannon et al. to be approximately ±0.″5 in position and
±20 km s-1 in velocity and were measured to be ∼180°
out of phase with one another. This quasi-periodic behavior has been
attributed by others to bursty or patchy reconnection and slipping
occurring during three-dimensional magnetic reconnection. Though able
to account for periodicities in both intensity and Doppler velocity,
these suggestions do not explicitly account for the phase velocity
of the entire sawtooth structure or the relative phasing of the
oscillations. Here we propose that the observations can be explained
by a tearing mode (TM) instability occurring at a current sheet across
which there is also a velocity shear. Using a linear model of this
instability, we reproduce the relative phase of the oscillations,
as well as the phase velocity of the sawtooth structure. We suggest a
geometry and local plasma parameters for the April 18 flare that would
support our hypothesis. Under this proposal, the combined spectral and
spatial IRIS observations of this flare may provide the most compelling
evidence to date of a TM occurring in the solar magnetic field.
Title: Modeling a Propagating Sawtooth Flare Ribbon Structure as a
Tearing Mode in the Presence of Velocity Shear
Authors: Parker, Jacob; Longcope, Dana
Bibcode: 2017SPD....4840602P
Altcode:
On April 18, 2014 (SOL2014-04-18T13:03) an M-class flare was observed
by IRIS. The associated flare ribbon contained a quasi-periodic
sawtooth pattern that was observed to propagate perpendicular the
the IRIS spectral slit with a phase velocity of approximately 15 km/s
(Brannon et al. 2015). This motion resulted in periodicities in both
intensity and Doppler velocity along the slit. These periodicities
were reported by Brannon et al. (2015) to be approximately plus-minus
.5 arcseconds in position and plus-minus 20 km/s in velocity and
were measured to be approximately 180 degrees out of phase with one
another. This quasi-periodic behavior has been attributed by others
to bursty or patchy reconnection (Brosius & Daw 2015; Brosius et
al. 2016) and slipping occurring during three-dimensional magnetic
reconnection (Li & Zhang 2015; Li et al. 2016). While able to
account for periodicities in both intensity and Doppler velocity
these suggestions do not explicitly account for the phase velocity
of the entire sawtooth structure, or for the relative phasing of the
oscillations. Here we propose that the observations can be explained
by a tearing mode instability occurring at a current sheet across
which there is also a velocity shear. We suggest a geometry and local
plasma parameters for the April 18 flare which would support our
hypothesis. Under this proposal the IRIS observations of this flare
may provide the most compelling evidence to date of a tearing mode
occurring in the solar magnetic field.
Title: Using observations of slipping velocities to test the
hypothesis that reconnection heats the active region corona
Authors: Yang, Kai; Longcope, Dana; Guo, Yang; Ding, Mingde
Bibcode: 2017SPD....4830301Y
Altcode:
Numerous proposed coronal heating mechanisms have invoked magnetic
reconnection in some role. Testing such a mechanism requires a
method of measuring magnetic reconnection coupled with a prediction
of the heat delivered by reconnection at the observed rate. In the
absence of coronal reconnection, field line footpoints move at the
same velocity as the plasma they find themselves in. The rate of
coronal reconnection is therefore related to any discrepancy observed
between footpoint motion and that of the local plasma — so-called
slipping motion. We propose a novel method to measure this velocity
discrepancy by combining a sequence of non-linear force-free field
extrapolations with maps of photospheric velocity. We obtain both
from a sequence of vector magnetograms of an active region (AR). We
then propose a method of computing the coronal heating produced under
the assumption the observed slipping velocity was due entirely to
coronal reconnection. This heating rate is used to predict density
and temperature at points along an equilibrium loop. This, in turn,
is used to synthesize emission in EUV and SXR bands. We perform this
analysis using a sequence of HMI vector magnetograms of a particular
AR and compare synthesized images to observations of the same AR made
by SDO. We also compare differential emission measure inferred from
those observations to that of the modeled corona.
Title: Update on a Solar Magnetic Catalog Spanning Four Solar Cycles
Authors: Vargas-Acosta, Juan Pablo; Munoz-Jaramillo, Andres; Vargas
Dominguez, Santiago; Werginz, Zachary; DeLuca, Michael D.; Longcope,
Dana; Harvey, J. W.; Windmueller, John; Zhang, Jie; Martens, Petrus C.
Bibcode: 2017SPD....4811202V
Altcode:
Bipolar magnetic regions (BMRs) are the cornerstone of solar
cycle propagation, the building blocks that give structure to the
solar atmosphere, and the origin of the majority of space weather
events. However, in spite of their importance, there is no homogeneous
BMR catalog spanning the era of systematic solar magnetic field
measurements. Here we present the results of an ongoing project to
address this deficiency applying the Bipolar Active Region Detection
(BARD) code to magnetograms from the 512 Channel of the Kitt Peak
Vaccum Telescope, SOHO/MDI, and SDO/HMI.The BARD code automatically
identifies BMRs and tracks them as they are rotated by differential
rotation. The output of the automatic detection is supervised by a human
observer to correct possible mistakes made by the automatic algorithm
(like incorrect pairings and tracking mislabels). Extra passes are made
to integrate fragmented regions as well as to balance the flux between
BMR polarities. At the moment, our BMR database includes nearly 10,000
unique objects (detected and tracked) belonging to four separate solar
cycles (21-24).
Title: A one-dimensional loop model invoking reconnection-driven
turbulence for electron acceleration
Authors: Longcope, Dana
Bibcode: 2017SPD....4810810L
Altcode:
We have recently developed a one-dimensional flare loop model in
which magnetic energy release occurs via loop retraction following
reconnection. The plasma in our retracting flux tube evolves several
propagating shock including a kind of slow magnetosonic shock at which
the plasma is heated to flare temperatures. The model has proven
able to reproduce several features observed in flares. Our model,
like that original proposed by Petschek, is framed in terms of fluid
equations (MHD), and therefore lacks the population of non-thermal
electrons. While missing from fluid models, non-thermal electrons
are one of the most important characteristics observed in flares. A
separate line of flare modeling has focused on the generation of
non-thermal electrons by, for example, MHD turbulence. These model
have not generally included the reconnection process believed to drive
that turbulence. Here we describe a model in which flux retracting from
reconnection generates turbulence, which then generates a non-thermal
electron population. While not entirely self-consistent, this model
combines into a single chain those elements by which magnetic energy
is converted into different forms observed in flares.
Title: Effects of the canopy and anchoring on evaporation flow from
a solar flare
Authors: Unverferth, John E.; Longcope, Dana
Bibcode: 2017SPD....4810305U
Altcode:
Spectroscopic observations of flare ribbons typically show chromospheric
evaporation flows which are subsonic for their high temperatures. This
contrasts with many numerical simulation where evaporation is typically
supersonic. These simulations typically assume flow along a flux tube
with uniform cross-sectional area. A simple model of the chromospheric
canopy, however, includes many regions of low magnetic field strength,
where flux tubes achieve local maxima in area: effectively chambers
in the flux tubes. We find that one third of all field lines in a
model have some form of chamber through which evaporation flow must
pass. Using a one dimensional isothermal hydrodynamic code we simulated
supersonic flow through an assortment of chambers. We find that there
is a subset of solutions that allow for a stationary standing shock in
the chamber. These solutions result in a slower and denser upflow into
the corona. We also constructed simple synthetic lines and found that
shocked solutions showed brighter and slower emission. When taken as
an ensemble over a cell of the model canopy, the lines appear slower,
even subsonic, than expected due to the outsized contribution from
shocked solutions.
Title: Elongation of Flare Ribbons
Authors: Qiu, Jiong; Longcope, Dana W.; Cassak, Paul A.; Priest,
Eric R.
Bibcode: 2017ApJ...838...17Q
Altcode: 2017arXiv170702478Q
We present an analysis of the apparent elongation motion of flare
ribbons along the polarity inversion line (PIL), as well as the shear
of flare loops in several two-ribbon flares. Flare ribbons and loops
spread along the PIL at a speed ranging from a few to a hundred
km s-1. The shear measured from conjugate footpoints
is consistent with the measurement from flare loops, and both show
the decrease of shear toward a potential field as a flare evolves
and ribbons and loops spread along the PIL. Flares exhibiting fast
bidirectional elongation appear to have a strong shear, which may
indicate a large magnetic guide field relative to the reconnection field
in the coronal current sheet. We discuss how the analysis of ribbon
motion could help infer properties in the corona where reconnection
takes place.
Title: Exploring impulsive solar magnetic energy release and particle
acceleration with focused hard X-ray imaging spectroscopy
Authors: Christe, Steven; Krucker, Samuel; Glesener, Lindsay; Shih,
Albert; Saint-Hilaire, Pascal; Caspi, Amir; Allred, Joel; Battaglia,
Marina; Chen, Bin; Drake, James; Dennis, Brian; Gary, Dale; Gburek,
Szymon; Goetz, Keith; Grefenstette, Brian; Gubarev, Mikhail; Hannah,
Iain; Holman, Gordon; Hudson, Hugh; Inglis, Andrew; Ireland, Jack;
Ishikawa, Shinosuke; Klimchuk, James; Kontar, Eduard; Kowalski, Adam;
Longcope, Dana; Massone, Anna-Maria; Musset, Sophie; Piana, Michele;
Ramsey, Brian; Ryan, Daniel; Schwartz, Richard; Stęślicki, Marek;
Turin, Paul; Warmuth, Alexander; Wilson-Hodge, Colleen; White, Stephen;
Veronig, Astrid; Vilmer, Nicole; Woods, Tom
Bibcode: 2017arXiv170100792C
Altcode:
How impulsive magnetic energy release leads to solar eruptions and how
those eruptions are energized and evolve are vital unsolved problems
in Heliophysics. The standard model for solar eruptions summarizes
our current understanding of these events. Magnetic energy in the
corona is released through drastic restructuring of the magnetic
field via reconnection. Electrons and ions are then accelerated by
poorly understood processes. Theories include contracting loops,
merging magnetic islands, stochastic acceleration, and turbulence at
shocks, among others. Although this basic model is well established,
the fundamental physics is poorly understood. HXR observations
using grazing-incidence focusing optics can now probe all of the key
regions of the standard model. These include two above-the-looptop
(ALT) sources which bookend the reconnection region and are likely
the sites of particle acceleration and direct heating. The science
achievable by a direct HXR imaging instrument can be summarized by the
following science questions and objectives which are some of the most
outstanding issues in solar physics (1) How are particles accelerated
at the Sun? (1a) Where are electrons accelerated and on what time
scales? (1b) What fraction of electrons is accelerated out of the
ambient medium? (2) How does magnetic energy release on the Sun lead
to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI)
instrument, which can be built now using proven technology and at modest
cost, would enable revolutionary advancements in our understanding of
impulsive magnetic energy release and particle acceleration, a process
which is known to occur at the Sun but also throughout the Universe.
Title: Flux-Rope Twist in Eruptive Flares and CMEs: Due to Zipper
and Main-Phase Reconnection
Authors: Priest, E. R.; Longcope, D. W.
Bibcode: 2017SoPh..292...25P
Altcode: 2017arXiv170100147P
The nature of three-dimensional reconnection when a twisted flux
tube erupts during an eruptive flare or coronal mass ejection is
considered. The reconnection has two phases: first of all, 3D "zipper
reconnection" propagates along the initial coronal arcade, parallel to
the polarity inversion line (PIL); then subsequent quasi-2D "main-phase
reconnection" in the low corona around a flux rope during its eruption
produces coronal loops and chromospheric ribbons that propagate away
from the PIL in a direction normal to it. One scenario starts with a
sheared arcade: the zipper reconnection creates a twisted flux rope
of roughly one turn (2 π radians of twist), and then main-phase
reconnection builds up the bulk of the erupting flux rope with a
relatively uniform twist of a few turns. A second scenario starts
with a pre-existing flux rope under the arcade. Here the zipper phase
can create a core with many turns that depend on the ratio of the
magnetic fluxes in the newly formed flare ribbons and the new flux
rope. Main phase reconnection then adds a layer of roughly uniform
twist to the twisted central core. Both phases and scenarios are
modeled in a simple way that assumes the initial magnetic flux is
fragmented along the PIL. The model uses conservation of magnetic
helicity and flux, together with equipartition of magnetic helicity,
to deduce the twist of the erupting flux rope in terms the geometry of
the initial configuration. Interplanetary observations show some flux
ropes have a fairly uniform twist, which could be produced when the
zipper phase and any pre-existing flux rope possess small or moderate
twist (up to one or two turns). Other interplanetary flux ropes have
highly twisted cores (up to five turns), which could be produced when
there is a pre-existing flux rope and an active zipper phase that
creates substantial extra twist.
Title: Focusing Solar Hard X-rays: Expected Results from a FOXSI
Spacecraft
Authors: Glesener, L.; Christe, S.; Shih, A. Y.; Dennis, B. R.;
Krucker, S.; Saint-Hilaire, P.; Hudson, H. S.; Ryan, D.; Inglis,
A. R.; Hannah, I. G.; Caspi, A.; Klimchuk, J. A.; Drake, J. F.;
Kontar, E.; Holman, G.; White, S. M.; Alaoui, M.; Battaglia, M.;
Vilmer, N.; Allred, J. C.; Longcope, D. W.; Gary, D. E.; Jeffrey,
N. L. S.; Musset, S.; Swisdak, M.
Bibcode: 2016AGUFMSH13A2282G
Altcode:
Over the course of two solar cycles, RHESSI has examined high-energy
processes in flares via high-resolution spectroscopy and imaging of
soft and hard X-rays (HXRs). The detected X-rays are the thermal
and nonthermal bremsstrahlung from heated coronal plasma and from
accelerated electrons, respectively, making them uniquely suited to
explore the highest-energy processes that occur in the corona. RHESSI
produces images using an indirect, Fourier-based method and has made
giant strides in our understanding of these processes, but it has also
uncovered intriguing new mysteries regarding energy release location,
acceleration mechanisms, and energy propagation in flares. Focusing
optics are now available for the HXR regime and stand poised to perform
another revolution in the field of high-energy solar physics. With
two successful sounding rocket flights completed, the Focusing Optics
X-ray Solar Imager (FOXSI) program has demonstrated the feasibility and
power of direct solar HXR imaging with its vastly superior sensitivity
and dynamic range. Placing this mature technology aboard a spacecraft
will offer a systematic way to explore high-energy aspects of the
solar corona and to address scientific questions left unanswered by
RHESSI. Here we present examples of such questions and show simulations
of expected results from a FOXSI spaceborne instrument to demonstrate
how these questions can be addressed with the focusing of hard X-rays.
Title: Development of a Homogenous Database of Bipolar Active Regions
Spanning Four Cycles
Authors: Munoz-Jaramillo, A.; Werginz, Z. A.; Vargas-Acosta, J. P.;
DeLuca, M. D.; Vargas-Dominguez, S.; Lamb, D. A.; DeForest, C. E.;
Longcope, D. W.; Martens, P.
Bibcode: 2016AGUFMSH11A2219M
Altcode:
The solar cycle can be understood as a process that alternates the
large-scale magnetic field of the Sun between poloidal and toroidal
configurations. Although the process that transitions the solar cycle
between toroidal and poloidal phases is still not fully understood,
theoretical studies, and observational evidence, suggest that this
process is driven by the emergence and decay of bipolar magnetic
regions (BMRs) at the photosphere. Furthermore, the emergence of
BMRs at the photosphere is the main driver behind solar variability
and solar activity in general; making the study of their properties
doubly important for heliospheric physics. However, in spite of their
critical role, there is still no unified catalog of BMRs spanning
multiple instruments and covering the entire period of systematic
measurement of the solar magnetic field (i.e. 1975 to present).In
this presentation we discuss an ongoing project to address this
deficiency by applying our Bipolar Active Region Detection (BARD)
code on full disk magnetograms measured by the 512 (1975-1993) and
SPMG (1992-2003) instruments at the Kitt Peak Vacuum Telescope (KPVT),
SOHO/MDI (1996-2011) and SDO/HMI (2010-present). First we will discuss
the results of our revitalization of 512 and SPMG KPVT data, then
we will discuss how our BARD code operates, and finally report the
results of our cross-callibration across instruments.The corrected
and improved KPVT magnetograms will be made available through the
National Solar Observatory (NSO) and Virtual Solar Observatory (VSO),
including updated synoptic maps produced by running the corrected KPVT
magnetograms though the SOLIS pipeline. The homogeneous active region
database will be made public by the end of 2017 once it has reached
a satisfactory level of quality and maturity. The Figure shows all
bipolar active regions present in our database (as of Aug 2016) colored
according to the instrument where they were detected. The image also
includes the names of the NSF-REU students in charge of the supervision
of the detection algorithm and the year in which they worked on the
catalog. Marker size is indicative of the total active region flux.
Title: A Reconnection-driven Model of the Hard X-Ray Loop-top Source
from Flare 2004-Feb-26
Authors: Longcope, Dana; Qiu, Jiong; Brewer, Jasmine
Bibcode: 2016ApJ...833..211L
Altcode: 2016arXiv161007953L
A compact X-class flare on 2004 February 26 showed a concentrated source
of hard X-rays at the tops of the flare’s loops. This was analyzed
in previous work and interpreted as plasma heated and compressed by
slow magnetosonic shocks (SMSs) generated during post-reconnection
retraction of the flux. That work used analytic expressions from a
thin flux tube (TFT) model, which neglected many potentially important
factors such as thermal conduction and chromospheric evaporation. Here
we use a numerical solution of the TFT equations to produce a more
comprehensive and accurate model of the same flare, including those
effects previously omitted. These simulations corroborate the prior
hypothesis that slow-mode shocks persist well after the retraction
has ended, thus producing a compact, loop-top source instead of
an elongated jet, as steady reconnection models predict. Thermal
conduction leads to densities higher than analytic estimates had
predicted, and evaporation enhances the density still higher, but at
lower temperatures. X-ray light curves and spectra are synthesized by
convolving the results from a single TFT simulation with the rate at
which flux is reconnected, as measured through motion of flare ribbons,
for example. These agree well with light curves observed by RHESSI and
GOES and spectra from RHESSI. An image created from a superposition of
TFT model runs resembles one produced from RHESSI observations. This
suggests that the HXR loop-top source, at least the one observed in
this flare, could be the result of SMSs produced in fast reconnection
models like Petschek’s.
Title: The best of both worlds: Using automatic detection and limited
human supervision to create a homogenous magnetic catalog spanning
four solar cycles
Authors: Muñoz-Jaramillo, Andres; Werginz, Zachary; Vargas-Acosta,
Juan Pablo; DeLuca, Michael; Windmueller, J. C.; Zhang, Jie; Longcope,
Dana; Lamb, Derek; DeForest, Craig; Vargas-Domínguez, Santiago;
Harvey, Jack; Martens, Piet
Bibcode: 2016bida.conf.3194M
Altcode: 2022arXiv220311908M
Bipolar magnetic regions (BMRs) are the cornerstone of solar
variability. They are tracers of the large-scale magnetic processes
that give rise to the solar cycle, shapers of the solar corona,
building blocks of the large-scale solar magnetic field, and significant
contributors to the free-energetic budget that gives rise to flares and
coronal mass ejections. Surprisingly, no homogeneous catalog of BMRs
exists today, in spite of the existence of systematic measurements of
the magnetic field since the early 1970's. The purpose of this work is
to address this deficiency by creating a homogenous catalog of BMRs
from the 1970's until the present. For this purpose, in this paper
we discuss the strengths and weaknesses of the automatic and manual
detection of BMRs and how both methods can be combined to form the basis
of our Bipolar Active Region Detection (BARD) code and its supporting
human supervision module. At present, the BARD catalog contains more
than 10,000 unique BMRs tracked and characterized during every day
of their observation. Here we also discuss our future plans for the
creation of an extended multi-scale magnetic catalog combining the
SWAMIS and BARD catalogs.
Title: Numerical Simulations of Plasma Dynamics in the Vicinity of
a Retracting Flux Tube
Authors: Scott, Roger B.; Longcope, Dana W.; McKenzie, David E.
Bibcode: 2016ApJ...831...94S
Altcode:
In a previous paper, we presented an analytical, zero-β model for
supra-arcade downflows in which a retracting flux tube deforms the
surrounding magnetic field, constricting the flow of plasma along
affected field lines and, in some cases, forcing the plasma to exhibit
collimated shocks. Here we present a numerical simulation based on
the same model construction—a retracting flux tube is treated as
a rigid boundary around which the plasma is forced to flow and the
magnetic field and plasma evolve according to the governing equations
of magnetohydrodynamics. We find that the collimated shocks described
in our previous study are recovered for plasma β in the range of
0 ≤ β ≲ 1, while for 1 ≲ β the behavior is similar to the
simpler hydrodynamic case, with classical bow shocks forming when
the acoustic Mach number approaches or exceeds unity. Furthermore, we
find that while the plasma β is important for identifying the various
types of behaviors, more important still is the Alfvén Mach number,
which, if large, implies that the bulk kinetic energy of the fluid
exceeds the internal energy of the magnetic field, thereby leading to
the formation of unconfined, fast-mode magnetosonic shocks, even in
the limit of small β.
Title: Direct imaging of a classical solar eruptive flare
Authors: Li, Y.; Sun, X. D.; Ding, M. D.; Qiu, J.; Priest, E. R.;
Longcope, D. W.
Bibcode: 2016usc..confE..21L
Altcode:
Solar flares are the most energetic events in the solar system that
have a potential hazard on Earth. Although a standard model for
the eruptive flare accompanied by a coronal mass ejection has been
outlined and elaborated for decades, some key aspects are still under
debate, such as what drives the eruption, what is the role of magnetic
reconnection, and how the flare loops evolve. Here we present an
excellent event exhibiting nearly all the key elements involved in the
standard flare model. Using extreme-ultraviolet imaging observations,
we detect the unambiguous rise and eruption of a magnetic flux rope,
solid evidence for magnetic reconnection, and evident slipping and
rising motions in flare loops. Modeled coronal magnetic field supports
the interpretation of a pre-existing flux rope that persists after
the eruption with reduced twist. This flare, from the observational
view, shows a clear and comprehensive picture of how a classical solar
eruptive flare occurs and evolves, and helps to clarify some of the
controversial topics in the standard flare model.
Title: Evolution of Magnetic Helicity During Eruptive Flares and
Coronal Mass Ejections
Authors: Priest, E. R.; Longcope, D. W.; Janvier, M.
Bibcode: 2016SoPh..291.2017P
Altcode: 2016arXiv160703874P; 2016SoPh..tmp..130P
During eruptive solar flares and coronal mass ejections, a non-potential
magnetic arcade with much excess magnetic energy goes unstable and
reconnects. It produces a twisted erupting flux rope and leaves behind
a sheared arcade of hot coronal loops. We suggest that the twist of the
erupting flux rope can be determined from conservation of magnetic flux
and magnetic helicity and equipartition of magnetic helicity. It depends
on the geometry of the initial pre-eruptive structure. Two cases are
considered, in the first of which a flux rope is not present initially
but is created during the eruption by the reconnection. In the second
case, a flux rope is present under the arcade in the pre-eruptive
state, and the effect of the eruption and reconnection is to add an
amount of magnetic helicity that depends on the fluxes of the rope
and arcade and the geometry.
Title: Developing a Solar Magnetic Catalog Spanning Four Cycles
Authors: Werginz, Zachary; Munoz-Jaramillo, Andres; DeLuca, Michael
D.; Vargas Acosta, Juan Pablo; Vargas Dominguez, Santiago; Zhang,
Jie; Longcope, Dana; Martens, Petrus C.
Bibcode: 2016SPD....4740502W
Altcode:
Bipolar magnetic regions (BMRs) are the cornerstone of solar
cycle propagation, the building blocks that give structure to the
solar atmosphere, and the origin of the majority of space weather
events. However, in spite of their importance, there is no homogeneous
BMR catalog spanning the era of systematic solar magnetic field
measurements. Here we present the results of an ongoing project to
address this deficiency applying the Bipolar Active Region Detection
(BARD) code to magnetograms from the 512 Channel of the Kitt Peak Vaccum
Telescope, SOHO/MDI, and SDO/HMI.The BARD code automatically identifies
BMRs and tracks them as they are rotated by differential rotation. The
output of the automatic detection is supervised by a human observer
to correct possible mistakes made by the automatic algorithm (like
incorrect pairings and tracking mislabels). Extra passes are made to
integrate fragmented regions as well as to balance the flux between
BMR polarities. At the moment, our BMR database includes 6,885 unique
objects (detected and tracked) belonging to four separate solar cycles
(21-24).
Title: Science Objectives of the FOXSI Small Explorer Mission Concept
Authors: Shih, Albert Y.; Christe, Steven; Alaoui, Meriem; Allred,
Joel C.; Antiochos, Spiro K.; Battaglia, Marina; Buitrago-Casas,
Juan Camilo; Caspi, Amir; Dennis, Brian R.; Drake, James; Fleishman,
Gregory D.; Gary, Dale E.; Glesener, Lindsay; Grefenstette, Brian;
Hannah, Iain; Holman, Gordon D.; Hudson, Hugh S.; Inglis, Andrew R.;
Ireland, Jack; Ishikawa, Shin-Nosuke; Jeffrey, Natasha; Klimchuk, James
A.; Kontar, Eduard; Krucker, Sam; Longcope, Dana; Musset, Sophie; Nita,
Gelu M.; Ramsey, Brian; Ryan, Daniel; Saint-Hilaire, Pascal; Schwartz,
Richard A.; Vilmer, Nicole; White, Stephen M.; Wilson-Hodge, Colleen
Bibcode: 2016SPD....47.0814S
Altcode:
Impulsive particle acceleration and plasma heating at the Sun, from the
largest solar eruptive events to the smallest flares, are related to
fundamental processes throughout the Universe. While there have been
significant advances in our understanding of impulsive energy release
since the advent of RHESSI observations, there is a clear need for
new X-ray observations that can capture the full range of emission
in flares (e.g., faint coronal sources near bright chromospheric
sources), follow the intricate evolution of energy release and changes
in morphology, and search for the signatures of impulsive energy
release in even the quiescent Sun. The FOXSI Small Explorer (SMEX)
mission concept combines state-of-the-art grazing-incidence focusing
optics with pixelated solid-state detectors to provide direct imaging
of hard X-rays for the first time on a solar observatory. We present
the science objectives of FOXSI and how its capabilities will address
and resolve open questions regarding impulsive energy release at the
Sun. These questions include: What are the time scales of the processes
that accelerate electrons? How do flare-accelerated electrons escape
into the heliosphere? What is the energy input of accelerated electrons
into the chromosphere, and how is super-heated coronal plasma produced?
Title: Using a reconnection-powered loop to model a real flare
Authors: Longcope, Dana; Qiu, Jiong; Brewer, Jasmine
Bibcode: 2016SPD....4730203L
Altcode:
Magnetic reconnection has long been invoked to explain the supply
of energy for solar flares. In spite of this, few models have been
able to capture reconnection-driven energy release at the same time
they reproduce other flare-related phenomena, such as chromospheric
evaporation. We present a one-dimensional numerical model of flux
tube retraction following reconnection. Unlike traditional flare
loop models, the energy supply here is not a free parameter but comes
self-consistently from the post-reconnection retraction. This model
depends on 5 free parameters, two of which can be constrained using
pre-flare observations. The remaining three parameters can be varied to
fit observations of the actual flare. In this case, they are used to the
fit the RHESSI hard X-ray spectrum from a flare on 26 Feb 2004. Once
done, the model can be used to reproduce observations from other
wavelengths, including their time-evolution. We show that our model
agrees with the two different soft X-ray light curves observed by GOES,
and the time evolutions observed in various hard X-ray bands observed
by RHESSI. The model also predicts hard X-ray emission from the top of
the flaring loops, in agreement with hard X-ray images made of the same
flare - and many others. While such loop-top sources are well known,
their theoretical explanation is still debated. The loop-top source
in this model arises from a plug of plasma compressed and heated by
slow magnetosonic shocks. This plug persists longer than slow shocks
are expected to, giving rise to concentrated source, rather than an
elongated jet, and producing considerably more emission than previous
models predicted.
Title: Observations of an X-shaped Ribbon Flare in the Sun and Its
Three-dimensional Magnetic Reconnection
Authors: Li, Y.; Qiu, J.; Longcope, D. W.; Ding, M. D.; Yang, K.
Bibcode: 2016ApJ...823L..13L
Altcode: 2016arXiv160501833L
We report evolution of an atypical X-shaped flare ribbon that
provides novel observational evidence of three-dimensional (3D)
magnetic reconnection at a separator. The flare occurred on 2014
November 9. High-resolution slit-jaw 1330 Å images from the Interface
Region Imaging Spectrograph reveal four chromospheric flare ribbons
that converge and form an X-shape. Flare brightening in the upper
chromosphere spreads along the ribbons toward the center of the
“X” (the X-point), and then spreads outward in a direction more
perpendicular to the ribbons. These four ribbons are located in a
quadrupolar magnetic field. Reconstruction of magnetic topology in the
active region suggests the presence of a separator connecting to the
X-point outlined by the ribbons. The inward motion of flare ribbons in
the early stage therefore indicates 3D magnetic reconnection between two
sets of non-coplanar loops that approach laterally, and reconnection
proceeds downward along a section of vertical current sheet. Coronal
loops are also observed by the Atmospheric Imaging Assembly on board
the Solar Dynamics Observatory confirming the reconnection morphology
illustrated by ribbon evolution.
Title: Observations of an X-shaped Ribbon Flare and Its
Three-dimensional Magnetic Reconnection with IRIS and SDO
Authors: Li, Ying; Qiu, Jiong; Longcope, Dana; Ding, Mingde
Bibcode: 2016SPD....4730206L
Altcode:
We report evolution of an atypical X-shaped flare ribbon which provides
novel observational evidence of three-dimensional (3D) magnetic
reconnection at a separator. The flare occurred on 2014 November 9, and
high-resolution slit-jaw 1330 images from IRIS reveal four chromospheric
flare ribbons that converge and form an X-shape. These four ribbons are
located in a quadrupolar magnetic field. Reconstruction of magnetic
topology in the active region suggests the presence of a separator
connecting to the X-point outlined by the ribbons. The inward motion
of flare ribbons, as well as coronal loops observed by the SDO/AIA,
indicates 3D magnetic reconnection between two sets of non-coplanar
loops that approach laterally, and the reconnection proceeds downward
to a very low height. We also study spectra of Si IV, C II, and Mg II
observed with the IRIS slit, which cuts across the flare ribbons near
the X-point. We have found two distinct types of line profiles. At the
flare ribbon, all the lines show evident redshifts with a velocity up to
50 km/s, and the redshifts are well correlated with the line intensity
and width. These redshifts suggest chromospheric condensation caused
by impulsive energy deposition from the separator reconnection. While
right outside the flare ribbon, the lines exhibit unshifted, symmetric,
yet broadened profiles; in particular, the Si IV line is significantly
broadened at the far wing. The line broadening persists for 20 minutes
till after the end of the flare. The distinct spectral features near
the X-point indicate different dynamics associated with the separator
reconnection.
Title: Modeling Evaporative Upflows Through a Flux Tube of Nonconstant
Area
Authors: Unverferth, John E.; Longcope, Dana
Bibcode: 2016SPD....47.0629U
Altcode:
Chromospheric evaporation is a long studied part of solar
flares. Spectroscopic observations of flares typically show subsonic
upflows. This contrasts with simulations which consistently predict
supersonic evaporation flows. One possible explanation is that the
actual flows occur though flux tubes which expand from confined
photospheric sources to volume-filling coronal field. Very few
flare simulations to date have accounted for this geometry, and run
instead with flare loops of uniform cross section. It is well known
that transonic flows are dramatically affected by their geoemetry,
and can exhibit shocks under certain circumstances.To investigate
this we created a simple model of the canopy of magnetic field. This
exhibited the expected expansion but also showed some cases of
over-expansion followed by constriction. The flow through those flux
tubes will encounter a kind of chamber. We then used a one-dimensional
isothermal hydrodynamics to model the flow of plasma through such a
chamber. According to this simulation, there exists a set of inflow
parameters that will generate a standing shock inside the chamber. This
solution results in a sonic outflow from a supersonic inflow.
Title: Inferring the Magnetohydrodynamic Structure of Solar Flare
Supra-Arcade Plasmas from a Data-assimilated Field Transport Model
Authors: Scott, Roger B.; McKenzie, David E.; Longcope, Dana W.
Bibcode: 2016ApJ...819...56S
Altcode:
Supra-arcade fans are highly dynamic structures that form in the
region above post-reconnection flare arcades. In these features the
plasma density and temperature evolve on the scale of a few seconds,
despite the much slower dynamics of the underlying arcade. Further,
the motion of supra-arcade plasma plumes appears to be inconsistent
with the low-beta conditions that are often assumed to exist in the
solar corona. In order to understand the nature of these highly debated
structures, it is, therefore, important to investigate the interplay
of the magnetic field with the plasma. Here we present a technique for
inferring the underlying magnetohydrodynamic processes that might lead
to the types of motions seen in supra-arcade structures. Taking as a
case study the 2011 October 22 event, we begin with extreme-ultraviolet
observations and develop a time-dependent velocity field that is
consistent with both continuity and local correlation tracking. We then
assimilate this velocity field into a simplified magnetohydrodynamic
simulation, which deals simultaneously with regions of high and low
signal-to-noise ratio, thereby allowing the magnetic field to evolve
self-consistently with the fluid. Ultimately, we extract the missing
contributions from the momentum equation in order to estimate the
relative strength of the various forcing terms. In this way we are
able to make estimates of the plasma beta, as well as predict the
spectral character and total power of Alfvén waves radiated from the
supra-arcade region.
Title: Long Duration Flare Emission: Impulsive Heating or Gradual
Heating?
Authors: Qiu, Jiong; Longcope, Dana W.
Bibcode: 2016ApJ...820...14Q
Altcode: 2016arXiv160405342Q
Flare emissions in X-ray and EUV wavelengths have previously been
modeled as the plasma response to impulsive heating from magnetic
reconnection. Some flares exhibit gradually evolving X-ray and EUV
light curves, which are believed to result from superposition of an
extended sequence of impulsive heating events occurring in different
adjacent loops or even unresolved threads within each loop. In this
paper, we apply this approach to a long duration two-ribbon flare
SOL2011-09-13T22 observed by the Atmosphere Imaging Assembly (AIA). We
find that to reconcile with observed signatures of flare emission in
multiple EUV wavelengths, each thread should be heated in two phases,
an intense impulsive heating followed by a gradual, low-rate heating
tail that is attenuated over 20-30 minutes. Each AIA resolved single
loop may be composed of several such threads. The two-phase heating
scenario is supported by modeling with both a zero-dimensional and a
1D hydrodynamic code. We discuss viable physical mechanisms for the
two-phase heating in a post-reconnection thread.
Title: Analysis of Flows inside Quiescent Prominences as Captured
by Hinode/Solar Optical Telescope
Authors: Freed, M. S.; McKenzie, D. E.; Longcope, D. W.; Wilburn, M.
Bibcode: 2016ApJ...818...57F
Altcode: 2016arXiv160203821F
Developing an understanding of how magnetic fields can become entangled
in a prominence is important for predicting a possible eruption. This
work investigates the kinetic energy and vorticity associated with
plasma motion residing inside quiescent prominences (QPs). These plasma
flow characteristics can be utilized to improve our understanding of how
the prominence maintains a stable magnetic field configuration. Three
different contrast-enhanced solar prominence observations from
Hinode/Solar Optical Telescope were used to construct velocity
maps—in the plane of the sky—via a Fourier local correlation
tracking program. The resulting velocities were then used to perform the
first-ever analysis of the two-dimensional kinetic energy and enstrophy
spectra of a prominence. Enstrophy is introduced here as a means of
quantifying the vorticity that has been observed in many QPs. The
kinetic energy power spectral density (PSD) produced indices ranging
from -1.00 to -1.60. There was a consistent anisotropy in the kinetic
energy spectrum of all three prominences examined. Examination of the
intensity PSD reveals that a different scaling relationship exists
between the observed prominence structure and velocity maps. All of
the prominences exhibited an inertial range of at least 0.8≤slant
k≤slant 2.0 {rads} {{Mm}}-1. Quasi-periodic oscillations
were also detected in the centroid of the velocity distributions for one
prominence. Additionally, a lower limit was placed on the kinetic energy
density (ɛ ∼ 0.22-7.04 {{km}}2 {{{s}}}-2)
and enstrophy density (ω ∼ 1.43-13.69 × \quad {10}-16
{{{s}}}-2) associated with each prominence.
Title: Contextualizing Solar Cycle 24: Report on the Development of
a Homogenous Database of Bipolar Active Regions Spanning Four Cycles
Authors: Munoz-Jaramillo, A.; Werginz, Z. A.; DeLuca, M. D.;
Vargas-Acosta, J. P.; Longcope, D. W.; Harvey, J. W.; Martens, P.;
Zhang, J.; Vargas-Dominguez, S.; DeForest, C. E.; Lamb, D. A.
Bibcode: 2015AGUFMSH33D..06M
Altcode:
The solar cycle can be understood as a process that alternates the
large-scale magnetic field of the Sun between poloidal and toroidal
configurations. Although the process that transitions the solar cycle
between toroidal and poloidal phases is still not fully understood,
theoretical studies, and observational evidence, suggest that this
process is driven by the emergence and decay of bipolar magnetic
regions (BMRs) at the photosphere. Furthermore, the emergence of
BMRs at the photosphere is the main driver behind solar variability
and solar activity in general; making the study of their properties
doubly important for heliospheric physics. However, in spite of their
critical role, there is still no unified catalog of BMRs spanning
multiple instruments and covering the entire period of systematic
measurement of the solar magnetic field (i.e. 1975 to present).In
this presentation we discuss an ongoing project to address this
deficiency by applying our Bipolar Active Region Detection (BARD)
code on full disk magnetograms measured by the 512 (1975-1993) and
SPMG (1992-2003) instruments at the Kitt Peak Vacuum Telescope (KPVT),
SOHO/MDI (1996-2011) and SDO/HMI (2010-present). First we will discuss
the results of our revitalization of 512 and SPMG KPVT data, then we
will discuss how our BARD code operates, and finally report the results
of our cross-callibration.The corrected and improved KPVT magnetograms
will be made available through the National Solar Observatory (NSO)
and Virtual Solar Observatory (VSO), including updated synoptic maps
produced by running the corrected KPVT magnetograms though the SOLIS
pipeline. The homogeneous active region database will be made public
by the end of 2017 once it has reached a satisfactory level of quality
and maturity. The Figure shows all bipolar active regions present in
our database (as of Aug 2015) colored according to the sign of their
leading polarity. Marker size is indicative of the total active region
flux. Anti-Hale regions are shown using solid markers.
Title: How Gas-dynamic Flare Models Powered by Petschek Reconnection
Differ from Those with Ad Hoc Energy Sources
Authors: Longcope, D. W.; Klimchuk, J. A.
Bibcode: 2015ApJ...813..131L
Altcode: 2015arXiv151005985L
Aspects of solar flare dynamics, such as chromospheric evaporation
and flare light curves, have long been studied using one-dimensional
models of plasma dynamics inside a static flare loop, subjected
to some energy input. While extremely successful at explaining
the observed characteristics of flares, all such models so
far have specified energy input ad hoc, rather than deriving it
self-consistently. There is broad consensus that flares are powered
by magnetic energy released through reconnection. Recent work has
generalized Petschek’s basic reconnection scenario, topological
change followed by field line retraction and shock heating, to permit
its inclusion in a one-dimensional flare loop model. Here we compare
the gas dynamics driven by retraction and shocking to those from more
conventional static loop models energized by ad hoc source terms. We
find significant differences during the first minute, when retraction
leads to larger kinetic energies and produces higher densities at
the loop top, while ad hoc heating tends to rarify the loop top. The
loop-top density concentration is related to the slow magnetosonic
shock, characteristic of Petschek’s model, but persists beyond
the retraction phase occurring in the outflow jet. This offers an
explanation for observed loop-top sources of X-ray and EUV emission,
with advantages over that provided by ad hoc heating scenarios. The
cooling phases of the two models are, however, notably similar to one
another, suggesting that observations at that stage will yield little
information on the nature of energy input.
Title: Spectroscopic Observations of an Evolving Flare Ribbon
Substructure Suggesting Origin in Current Sheet Waves
Authors: Brannon, S. R.; Longcope, D. W.; Qiu, J.
Bibcode: 2015ApJ...810....4B
Altcode: 2015arXiv150701554B
We present imaging and spectroscopic observations from the Interface
Region Imaging Spectrograph of the evolution of the flare ribbon in the
SOL2014-04-18T13:03 M-class flare event, at high spatial resolution
and time cadence. These observations reveal small-scale substructure
within the ribbon, which manifests as coherent quasi-periodic
oscillations in both position and Doppler velocities. We consider
various alternative explanations for these oscillations, including
modulation of chromospheric evaporation flows. Among these, we find
the best support for some form of wave localized to the coronal current
sheet, such as a tearing mode or Kelvin-Helmholtz instability.
Title: Topological energy estimates of an AR: the MCC
Authors: Longcope, Dana
Bibcode: 2015shin.confE..29L
Altcode:
A single closed coronal field line interconnects photospheric of
opposing polarities. The full coronal field footpoints of opposing
polarities. The full coronal field thus defines a mapping between
all points on the photosphere. A stable, non-linear force-free field
(NLFFF) is that coronal field having the least possible magnetic energy
for a specified mapping. Barring coronal reconnection, field-line
connections will be preserved as photospheric points move. Such motion
will generally complicate the mapping and thereby raising the energy
minimum characterizing the NLFFF. The mapping between all pairs of
photospheric points constitutes an infinite number of constraints
and could never be completely measured. One practical approach is to
group photospheric points into a finite number of unipolar regions
and impose constraints only on interconnections between region
pairs. These region-wise constraints are a subset of the point-wise
mapping constraints, and subjecting the energy minimization to only that
subset provides a lower bound on the energy of the actual NLFFF. This
prescription, known as the minimum current corona (MCC), has been used
to estimate energy build-up in active region coronae over several
days. I review these applications and discuss the unique advantages
of the MCC. Among these is its ability to estimate energy release by
reconnection in particular portions of the coronal field. Reconnection
changes footpoint connections, alters the mapping constraints, and thus
permits access to a still lower energy minimum. Relaxing one or more
constraints in the MCC provides an estimate of the energy available
for release by reconnection affecting those constraints.
Title: How gas-dynamic flare models powered by Petschek reconnection
differ from those with ad hoc energy sources
Authors: Longcope, Dana; Klimchuk, Jim
Bibcode: 2015shin.confE...9L
Altcode:
Aspects of solar flare dynamics, such as chromospheric evaporation
and flare light-curves, have long been studied using one-dimensional
models of plasma dynamics inside a static flare loop, subjected to some
energy input. While extremely successful at explaining the observed
characteristics of flares, all such models so far have specified energy
input ad hoc, rather than deriving it self-consistently. There is broad
consensus that flares are powered by magnetic energy released through
reconnection. Recent work has generalized Petschek's basic reconnection
scenario, topological change followed by field line retraction and
shock heating, to permit its inclusion into a one-dimensional flare
loop model. Here we compare the gas dynamics driven by retraction and
shocking to those from more conventional static loop models energized
by ad hoc source terms. We find significant differences during the
first minute, when retraction leads to larger kinetic energies and
produces higher densities at the loop top, while ad hoc heating tends
to rarify the loop top. The loop-top density concentration is related
to the slow magnetosonic shock, characteristic of Petschek's model,
but persists beyond the retraction phase occurring in the outflow
jet. This offers an explanation of observed loop-top sources of X-ray
and EUV emission, with advantages over that provided by ad hoc heating
scenarios. The cooling phases of the two models are, however, notably
similar to one another, suggesting observations at that stage will
yield little information on the nature of energy input.
Title: Using the Minimum Current Corona model to estimate free
magnetic energy for 3 major eruptions in Active Region 11158
Authors: Tarr, Lucas A.; Longcope, Dana; Millhouse, Margaret
Bibcode: 2015shin.confE..30T
Altcode:
It is well known that photospheric flux emergence is an important
process for stressing coronal fields and storing magnetic free energy,
which may then be released during a flare. The Helioseismic and
Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO)
captured the entire emergence of NOAA AR 11158. This region emerged as
two distinct bipoles, possibly connected underneath the photosphere,
yet characterized by different photospheric field evolutions and
fluxes. The combined active region complex produced 15 GOES C-class,
2 M-class, and the X2.2 Valentine's Day Flare during the four days
after initial emergence on 2011 February 12. The M and X class flares
are of particular interest because they are nonhomologous, involving
different subregions of the active region. We use a Magnetic Charge
Topology together with the Minimum Current Corona model of the coronal
field to model field evolution of the complex. Combining this with
observations of flare ribbons in the 1600Å channel of the Atmospheric
Imaging Assembly on board SDO, we propose a minimization algorithm
for estimating the amount of reconnected flux and resulting drop in
magnetic free energy during a flare. For the M6.6, M2.2, and X2.2
flares, we find a flux exchange of 4.2e20 Mx, 2.0e20 Mx, and 21.0e20
Mx, respectively, resulting in free energy drops of 3.89e30 erg,
2.62e30 erg, and 1.68e32 erg.
Title: The Minimum of Solar Cycle 23: As Deep as It Could Be?
Authors: Muñoz-Jaramillo, Andrés; Senkpeil, Ryan R.; Longcope,
Dana W.; Tlatov, Andrey G.; Pevtsov, Alexei A.; Balmaceda, Laura A.;
DeLuca, Edward E.; Martens, Petrus C. H.
Bibcode: 2015ApJ...804...68M
Altcode: 2015arXiv150801222M
In this work we introduce a new way of binning sunspot group data
with the purpose of better understanding the impact of the solar
cycle on sunspot properties and how this defined the characteristics
of the extended minimum of cycle 23. Our approach assumes that
the statistical properties of sunspots are completely determined
by the strength of the underlying large-scale field and have no
additional time dependencies. We use the amplitude of the cycle
at any given moment (something we refer to as activity level) as a
proxy for the strength of this deep-seated magnetic field. We find
that the sunspot size distribution is composed of two populations:
one population of groups and active regions and a second population
of pores and ephemeral regions. When fits are performed at periods
of different activity level, only the statistical properties of the
former population, the active regions, are found to vary. Finally,
we study the relative contribution of each component (small-scale
versus large-scale) to solar magnetism. We find that when hemispheres
are treated separately, almost every one of the past 12 solar minima
reaches a point where the main contribution to magnetism comes from
the small-scale component. However, due to asymmetries in cycle phase,
this state is very rarely reached by both hemispheres at the same
time. From this we infer that even though each hemisphere did reach
the magnetic baseline, from a heliospheric point of view the minimum
of cycle 23 was not as deep as it could have been.
Title: Spectroscopic observations of evolving flare ribbon
substructure suggesting origin in current sheet waves
Authors: Brannon, Sean R.; Longcope, Dana; Qiu, Jiong
Bibcode: 2015TESS....110705B
Altcode:
A flare ribbon is the chromospheric image of reconnection at a
coronal current sheet. The dynamics and structure of the ribbon can
thus reveal properties of the current sheet, including motion of
the reconnecting flare loops. We present imaging and spectroscopic
observations from the Interface Region Imaging Spectrograph (IRIS) of
the evolution of a flare ribbon at high spatial resolution and time
cadence. These reveal small-scale substructure in the ribbon, which
manifest as oscillations in both position and Doppler velocities. We
consider various alternative explanations for these oscillations,
including modulation of chromospheric evaporation flows. Among these
we find the best support for some form of elliptical wave localized to
the coronal current sheet, such as a tearing mode or Kelvin-Helmholtz
instability.IRIS is a NASA Small Explorer mission developed and operated
by Lockheed Martin Solar and Astrophysics Laboratory. This work is
supported by contract 8100002702 from Lockheed Martin to Montana State
University, a Montana Space Grant Consortium fellowship, and by NASA
through HSR.
Title: The Myth of Long Duration Flare Emission: Slow Heating or
Slow Cooling?
Authors: Qiu, Jiong; Longcope, Dana; Klimchuk, James A.
Bibcode: 2015TESS....130214Q
Altcode:
Long duration flare emissions lasting for a few hours are likely
governed by magnetic reconnection that continuously heats flare plasmas
in continuously formed flare loops. In this study, we confirm that
this process leads to the long-duration total emission for up to four
hours in a C2.9 flare on 2011 September 13. Observed by AIA, the flare
exhibits an ordered spread of flare UV ribbons along the polarity
inversion line, followed by the sequential formation of post-flare
loops in EUV emissions. We infer heating rates of thousands of flare
loops from the UV light curves at the flare foot-points, and model
the flare total emission with the 0d EBTEL model, which reproduces the
global evolution pattern of the long-duration flare EUV emissions as
the result of superposition of continuously formed and heated flare
loops. However, observations at single loop pixels also show long
duration EUV emission at 10 MK, long cooling time from 10 MK to 3 MK,
and later on very short duration of EUV emission at 1-2 MK. All of these
signatures cannot be produced by superposition of multiple impulsive
heating events. Our experiments, with both the 0d EBTEL model and a
1d hydrodynamic model, have demonstrated that a heating profile in
a single loop consisting of two parts, an intense impulsive heating
followed by a low-rate heating 1-2 orders of magnitude smaller that
is attenuated over 20-30 minutes, is required to produce the observed
time evolution signatures in a single loop. The total energy in the
gradual heating phase is comparable with that in the impulsive heating
phase in a flare loop. We discuss viable physical mechanisms for such
two-phase heating in a post-reconnection flare loop.
Title: How gas-dynamic flare models powered by Petschek reconnection
differ from thosewith ad hoc energy sources
Authors: Longcope, Dana; Klimchuk, James A.
Bibcode: 2015TESS....130212L
Altcode:
Many aspects of solar flare dynamics including chromospheric evaporation
have been, for more than thirty years, studied using one-dimensional
models of static flaring loops. These models solve one-dimensional
gas-dynamic equations for the dynamics of plasma inside a static
loop, subjected to energy input through either non-thermal particles
or heating. While they have been extremely successful at explaining
the characteristics of emission observed in flares, none so far have
been developed in which the energy input is derived self-consistently
from the loop's dynamics. Instead the energy input is specified ad
hoc. According to another line of theoretical investigation, flares
derive their energy from magnetic energy released through fast magnetic
reconnection. In the model due originally to Petschek reconnection
occurring in a small diffusion region produces a bent flux tube whose
retraction generates fast flows (an outflow jet) and shocks where flow
energy is thermalized. In a recent line of work this scenario has been
generalized so it may be incorporated into a one-dimensional loop model
of the kind used so successfully in flare modeling. In this new model
the flaring loop itself undergoes the retraction and shock formation,
and thereby introduces the flare energy self-consistently. Here we
compare the gas dynamics driven by retraction and shocking to those from
more conventional static loop models. We find significant differences
during the first minute, when retraction produces high densities at
the loop top, while ad hoc heating tends to rarify the loop top.
Title: The Minimum of Solar Cycle 23: As Deep as It Could Be?
Authors: Munoz-Jaramillo, Andres; Senkpeil, Ryan; Longcope, Dana;
Tlatov, Andrey; Pevtsov, Alexei A.; Balmaceda, Laura; DeLuca, Edward
E.; Martens, Petrus C.
Bibcode: 2015TESS....130803M
Altcode:
After a lull lasting more than 60 years of seemly uniform solar minima,
the solar minimum of solar cycle 23 came as a great surprise due to its
depth, duration, and record lows in a wide variety of solar activity
indices and solar wind properties. One of the consequence of such an
event is the revival of the interest in extreme minima, grand minima,
and the identification of a solar basal state of minimum magnetic
activity.In this presentation we will discuss a new way of binning
sunspot group data, with the purpose of better understanding the impact
of the solar cycle on sunspot properties, and how this defined the
characteristics of the extended minimum of cycle 23. Our main result
is centered around the fact that the sunspot size distribution is
composed of two populations, a population of groups and active regions,
and second of pores and ephemeral regions. We find that only the
properties of the former population, the active regions, is found to
vary with the solar cycle, while the propeties of pores and ephemeral
regions does not.Taking advantage of our statistical characterization
we probe the question of the solar baseline magnetism. We find that,
when hemispheres are treated separately, almost every one of the past
12 solar minima reaches such a point. However, due to asymmetries in
cycle phase, the basal state is very rarely reached by both hemispheres
at the same time. From this we infer that, even though each hemisphere
did reach the magnetic baseline, from a heliospheric point of view
the minimum of cycle 23 was not as deep as it could have been.
Title: Bright EUV knots on post-flare loops: Are we seeing slow
shocks?
Authors: Unverferth, John; Longcope, Dana; Reeves, Katharine
Bibcode: 2015TESS....130213U
Altcode:
Post flare loops imaged in the EUV sometimes show bright knots of
emission at their apices. Knots from the loops in an arcade often line
up to form a bar of coronal emission parallel to the polarity inversion
line. These features have been variously interpreted as the results
of colliding evaporation flows or volume enhancement at the point
of weakest magnetic field. Here we consider the possibility that the
features are produced through density enhancement resulting from shock
compression during the reconnection process. We present simulations
of thin flux tube dynamics following reconnection, which capture the
essential physics of Petschek's fast reconnection model. The slow
shock present during reconnection and subsequent retraction produce a
high density region which persists even after the loop has achieved
its ultimate equilibrium configuration. This high density produces
enhanced emission at the bottom of the current sheet. Evaporation flows
impinge on the high density region resulting in further enhancement to
the density and the emission at the same position: the bottom of the
current sheet. We compare these results to those from more conventional
simulation where ad hoc heating drives evaporation from both feet.
Title: Connecting Coronal Holes and Open Magnetic Field via Numerical
Modeling and Observations
Authors: Lowder, Chris; Qui, Jiong; sLeamon, Robert J.; Longcope, Dana
Bibcode: 2015TESS....140904L
Altcode:
Coronal holes are regions of the Sun's surface that map the footprints
of open magnetic field lines traced down from the corona and heliosphere
beyond. Without the ability to directly and easily observe coronal
magnetic field line structure, mapping their footprint 'dance'
throughout the solar cycle is crucial for understanding this open
field contribution to space weather. Coronal holes provide just this
proxy.Using a combination of SOHO:EIT, SDO:AIA, and STEREO:EUVI A/B
extreme ultraviolet (EUV) observations from 1996-2014, coronal holes
are automatically detected and characterized throughout this span,
enabling long-term solar-cycle-timescale study. In particular,
the combination of SDO:AIA and STEREO:EUVI A/B data provides a
new viewpoint on understanding coronal hole evolution. As the two
STEREO spacecraft drift ahead and behind of the Earth in their orbit,
respectively, they are able to peek around the corner and closer to
the poles, providing the ability to image nearly the entire solar
surface in EUV wavelengths, using SDO data in conjunction. A flux
transport model driven by observed bipole data allows for the study
and comparison of far-side magnetic field evolution. By combining our
numerical models of solar open magnetic field evolution with coronal
hole observations, comparison of far-side and polar dynamics becomes
possible. Model constraints and boundary conditions are more easily
fine-tuned with these global observations. Understanding the dynamics
of boundary changes and distribution throughout the solar cycle yields
important insight into connecting models of open magnetic field.
Title: On the Magnetohydrodynamics of Supra-Arcade Fan Structures
Authors: Scott, Roger B.; McKenzie, David; Longcope, Dana
Bibcode: 2015TESS....140705S
Altcode:
Supra-Arcade Fan Structures are highly dynamic regions that form above
post-reconnection arcades. In these regions, the plasma density and
thermal structure evolve on the scale of a few seconds, despite the
much slower dynamics of the underlying arcade. Further, the motion
of supra-arcade plasma plumes appears to be inconsistent with the
low-beta conditions that many authors assume to exist in the solar
corona. In order to understand the nature of these highly debated
structures it is, therefore, important to investigate the interplay
of the magnetic field with the plasma. Here we present a technique
for inferring the underlying MagnetoHydroDynamic processes that might
lead to the types of motions seen in supra-arcade structures. We begin
with EUV observations and develop a time dependent velocity field
that is consistent with both mass conservation and local correlation
tracking. We then assimilate this velocity field into a simplified
MHD code, which deals simultaneously with regions of high and low SNR,
thereby allowing the magnetic field to evolve self-consistently with
the fluid. Ultimately, we extract the missing contributions from the
underlying momentum equation in order to estimate the relative strength
of forcing terms. In this way we are able to make estimates of the
plasma beta as well as predicting the spectral character of radiated
Alfvén waves. It is our hope that this work will help to improve
our understanding of the energy balance in these complex regions
and, thereby, contribute to our knowledge of the solar corona as a
whole. This work is supported by NASA under contract NNM07AB07C with
the Smithsonian Astrophysical Observatory, and by grant NNX14AD43G.
Title: Relating magnetic reconnection to coronal heating
Authors: Longcope, D. W.; Tarr, L. A.
Bibcode: 2015RSPTA.37340263L
Altcode: 2015arXiv150106546L
It is clear that the solar corona is being heated and that coronal
magnetic fields undergo reconnection all the time. Here we attempt
to show that these two facts are related - i.e. coronal reconnection
generates heat. This attempt must address the fact that topological
change of field lines does not automatically generate heat. We
present one case of flux emergence where we have measured the rate
of coronal magnetic reconnection and the rate of energy dissipation
in the corona. The ratio of these two, Embedded Image, is a current
comparable to the amount of current expected to flow along the boundary
separating the emerged flux from the pre-existing flux overlying it. We
can generalize this relation to the overall corona in quiet Sun or
in active regions. Doing so yields estimates for the contribution
to coronal heating from magnetic reconnection. These estimated rates
are comparable to the amount required to maintain the corona at its
observed temperature.
Title: Three-Year Global Survey of Coronal Null Points from
Potential-Field-Source-Surface (PFSS) Modeling and Solar Dynamics
Observatory (SDO) Observations
Authors: Freed, M. S.; Longcope, D. W.; McKenzie, D. E.
Bibcode: 2015SoPh..290..467F
Altcode: 2014SoPh..tmp..136F; 2014arXiv1410.4493F
This article compiles and examines a comprehensive coronal
magnetic-null-point survey created by potential-field-source-surface
(PFSS) modeling and Solar Dynamics Observatory/Atmospheric Imaging
Assembly (SDO/AIA) observations. The locations of 582 potential magnetic
null points in the corona were predicted from the PFSS model between
Carrington Rotations (CR) 2098 (June 2010) and 2139 (July 2013). These
locations were manually inspected, using contrast-enhanced SDO/AIA
images in 171 Å at the East and West solar limb, for structures
associated with nulls. A Kolmogorov-Smirnov (K-S) test showed a
statistically significant difference between observed and predicted
latitudinal distributions of null points. This finding is explored
further to show that the observability of null points could be affected
by the Sun's asymmetric hemisphere activity. Additional K-S tests show
no effect on observability related to eigenvalues associated with the
fan and spine structure surrounding null points or to the orientation
of the spine. We find that approximately 31 % of nulls obtained from
the PFSS model were observed in SDO/AIA images at one of the solar
limbs. An observed null on the East solar limb had a 51.6 % chance
of being observed on the West solar limb. Predicted null points going
back to CR 1893 (March 1995) were also used for comparing radial and
latitudinal distributions of nulls to previous work and to test for
correlation of solar activity to the number of predicted nulls.
Title: Small-scale and Global Dynamos and the Area and Flux
Distributions of Active Regions, Sunspot Groups, and Sunspots:
A Multi-database Study
Authors: Muñoz-Jaramillo, Andrés; Senkpeil, Ryan R.; Windmueller,
John C.; Amouzou, Ernest C.; Longcope, Dana W.; Tlatov, Andrey G.;
Nagovitsyn, Yury A.; Pevtsov, Alexei A.; Chapman, Gary A.; Cookson,
Angela M.; Yeates, Anthony R.; Watson, Fraser T.; Balmaceda, Laura A.;
DeLuca, Edward E.; Martens, Petrus C. H.
Bibcode: 2015ApJ...800...48M
Altcode: 2014arXiv1410.6281M
In this work, we take advantage of 11 different sunspot group,
sunspot, and active region databases to characterize the area
and flux distributions of photospheric magnetic structures. We
find that, when taken separately, different databases are better
fitted by different distributions (as has been reported previously
in the literature). However, we find that all our databases can be
reconciled by the simple application of a proportionality constant,
and that, in reality, different databases are sampling different
parts of a composite distribution. This composite distribution
is made up by linear combination of Weibull and log-normal
distributions—where a pure Weibull (log-normal) characterizes the
distribution of structures with fluxes below (above) 1021Mx
(1022Mx). Additionally, we demonstrate that the Weibull
distribution shows the expected linear behavior of a power-law
distribution (when extended to smaller fluxes), making our results
compatible with the results of Parnell et al. We propose that this is
evidence of two separate mechanisms giving rise to visible structures
on the photosphere: one directly connected to the global component of
the dynamo (and the generation of bipolar active regions), and the other
with the small-scale component of the dynamo (and the fragmentation of
magnetic structures due to their interaction with turbulent convection).
Title: Temperature and Electron Density Diagnostics of a
Candle-flame-shaped Flare
Authors: Guidoni, S. E.; McKenzie, D. E.; Longcope, D. W.; Plowman,
J. E.; Yoshimura, K.
Bibcode: 2015ApJ...800...54G
Altcode:
Candle-flame-shaped flares are archetypical structures that provide
indirect evidence of magnetic reconnection. A flare resembling Tsuneta's
famous 1992 candle-flame flare occurred on 2011 January 28; we present
its temperature and electron density diagnostics. This flare was
observed with Solar Dynamics Observatory/Atmospheric Imaging Assembly
(SDO/AIA), Hinode/X-Ray Telescope (XRT), and Solar Terrestrial Relations
Observatory Ahead (STEREO-A)/Extreme Ultraviolet Imager, resulting
in high-resolution, broad temperature coverage, and stereoscopic
views of this iconic structure. The high-temperature images reveal
a brightening that grows in size to form a tower-like structure at
the top of the posteruption flare arcade, a feature that has been
observed in other long-duration events. Despite the extensive work on
the standard reconnection scenario, there is no complete agreement
among models regarding the nature of this high-intensity elongated
structure. Electron density maps reveal that reconnected loops that are
successively connected at their tops to the tower develop a density
asymmetry of about a factor of two between the two legs, giving the
appearance of "half-loops." We calculate average temperatures with a
new fast differential emission measure (DEM) method that uses SDO/AIA
data and analyze the heating and cooling of salient features of the
flare. Using STEREO observations, we show that the tower and the
half-loop brightenings are not a line-of-sight projection effect of
the type studied by Forbes & Acton. This conclusion opens the door
for physics-based explanations of these puzzling, recurrent solar flare
features, previously attributed to projection effects. We corroborate
the results of our DEM analysis by comparing them with temperature
analyses from Hinode/XRT.
Title: Automatic vs. Human Detection of Bipolar Magnetic Regions:
Using the Best of Both Worlds
Authors: Munoz-Jaramillo, A.; DeLuca, M. D.; Windmueller, J. C.;
Longcope, D. W.
Bibcode: 2014AGUFMSH34A..04M
Altcode:
The solar cycle can be understood as a process that alternates the
large-scale magnetic field of the Sun between poloidal and toroidal
configurations. Although the process that transitions the solar cycle
between toroidal and poloidal phases is still not fully understood,
theoretical studies, and observational evidence, suggest that this
process is driven by the emergence and decay of bipolar magnetic
regions (BMRs) at the photosphere. Furthermore, the emergence of
BMRs at the photosphere is the main driver behind solar variability
and solar activity in general; making the study of their properties
doubly important for heliospheric physics. However, in spite of their
critical role, there is still no unified catalog of BMRs spanning
multiple instruments and covering the entire period of systematic
measurement of the solar magnetic field (i.e. 1975 to present).One
of the interesting aspects of the detection of BMRs is that, due to
the time and spatial scales of interest, it is tractable for both
human observers and automatic detection algorithms. This makes it
ideal for comparative studies of the advantages and failing of both
approaches. In this presentation we will compare three different BMR
catalogs, reduced from magnetograms taken by SOHO/MDI, using human,
automatic, and hybrid methods of detection. The focus will be the
comparative performance between the three methods, their merits, and
disadvantages, and the lessons that can be applied to other imaging
data sets.
Title: Solar Coronal Holes and Open Magnetic Flux
Authors: Lowder, C.; Qiu, J.; Leamon, R. J.; Longcope, D. W.
Bibcode: 2014AGUFMSH13A4081L
Altcode:
Using SDO/AIA and STEREO/EUVI EUV data in conjunction with an
instrument-specific adaptive intensity thresholding algorithm, we are
able to track coronal hole boundaries across the entire solar surface at
a cadence of 12 hours. SOHO/EIT provides earlier era data, allowing the
building EUV coronal hole maps over the course of a solar rotation. We
find that for solar cycle 23 the unsigned magnetic flux enclosed
by coronal hole boundaries ranges from (2-5)x10^{22} Mx, covering
5%-17% of the solar surface. For solar cycle 24 this flux ranges from
(2-4)x10^{22} Mx, covering 5%-10% of the solar surface. Using a surface
flux transport model, we compare observational coronal hole boundaries
and computed potential open field for solar cycles 23 and 24. From both
our observed coronal holes and modeled open magnetic field, we find that
low-latitude regions are significant in area, contributing to the total
open magnetic flux, and should be considered in more significant detail.
Title: Two-Ribbon Flares Spreading in the Third Dimension
Authors: Qiu, J.; Longcope, D. W.
Bibcode: 2014AGUFMSH22A..07Q
Altcode:
Two-ribbon flares, often associated with eruptive filaments or
CMEs, aretextbook demonstration of the standard flare model that
describessimultaneous reconnection of an arcade of anti-parallel
magnetic field linescrossing at the 2-dimensional macroscopic current
sheet in the corona.However, flare ribbons are often observed to
brighten sequentially alongtheir length with an apparent speed ranging
from 10 to 200 km/s, indicativeof a slow (sub-Alfvenic) and organized
pattern of reconnection spreadingalong the assumed current sheet. This
can be hardly explained by themechanism of driven reconnection due
to ideal MHD instability thatexplosively rushes open coronal field
lines leading to subsequentreconnection. We present observations of
flare ribbon spreadingeither uni-directionally or bi-directionally
along the magnetic polarityinversion line, or the assumed direction of
the overlying current sheet andexplore diagnostics of 3-dimensional
(for example with a guidefield) physics that may govern the observed
spread ofreconnection (e.g. Shepherd and Cassak, 2012).
Title: A Simple Model of Chromospheric Evaporation and Condensation
Driven Conductively in a Solar Flare
Authors: Longcope, D. W.
Bibcode: 2014ApJ...795...10L
Altcode: 2014arXiv1409.1886L
Magnetic energy released in the corona by solar flares reaches the
chromosphere where it drives characteristic upflows and downflows
known as evaporation and condensation. These flows are studied here
for the case where energy is transported to the chromosphere by
thermal conduction. An analytic model is used to develop relations
by which the density and velocity of each flow can be predicted
from coronal parameters including the flare's energy flux F. These
relations are explored and refined using a series of numerical
investigations in which the transition region (TR) is represented
by a simplified density jump. The maximum evaporation velocity, for
example, is well approximated by ve ~= 0.38(F/ρco,
0)1/3, where ρco, 0 is the mass density
of the pre-flare corona. This and the other relations are found to
fit simulations using more realistic models of the TR both performed
in this work, and taken from a variety of previously published
investigations. These relations offer a novel and efficient means
of simulating coronal reconnection without neglecting entirely the
effects of evaporation.
Title: Modeling Properties of Chromospheric Evaporation Driven by
Thermal Conduction Fronts from Reconnection Shocks
Authors: Brannon, Sean; Longcope, Dana
Bibcode: 2014ApJ...792...50B
Altcode: 2014arXiv1408.1705B
Magnetic reconnection in the corona results in contracting flare loops,
releasing energy into plasma heating and shocks. The hydrodynamic
shocks produced in this manner drive thermal conduction fronts (TCFs)
which transport energy into the chromosphere and drive upflows
(evaporation) and downflows (condensation) in the cooler, denser
footpoint plasma. Observations have revealed that certain properties of
the transition point between evaporation and condensation (the "flow
reversal point" or FRP), such as temperature and velocity-temperature
derivative at the FRP, vary between different flares. These properties
may provide a diagnostic tool to determine parameters of the coronal
energy release mechanism and the loop atmosphere. In this study,
we develop a one-dimensional hydrodynamical flare loop model with
a simplified three-region atmosphere (chromosphere/transition
region/corona), with TCFs initiated by shocks introduced in the
corona. We investigate the effect of two different flare loop parameters
(post-shock temperature and transition region temperature ratio) on the
FRP properties. We find that both of the evaporation characteristics
have scaling-law relationships to the varied flare parameters, and we
report the scaling exponents for our model. This provides a means of
using spectroscopic observations of the chromosphere as quantitative
diagnostics of flare energy release in the corona.
Title: Quiescent Reconnection Rate Between Emerging Active Regions
and Preexisting Field, with Associated Heating: NOAA AR 11112
Authors: Tarr, Lucas A.; Longcope, Dana W.; McKenzie, David E.;
Yoshimura, Keiji
Bibcode: 2014SoPh..289.3331T
Altcode: 2013arXiv1311.3705T; 2014SoPh..tmp....1T
When magnetic flux emerges from beneath the photosphere, it displaces
the preexisting field in the corona, and a current sheet generally forms
at the boundary between the old and new magnetic domains. Reconnection
in the current sheet relaxes this highly stressed configuration to
a lower energy state. This scenario is most familiar and most often
studied in flares, where the flux transfer is rapid. We present here
a study of steady, quiescent flux transfer occurring at a rate three
orders of magnitude lower than that in a large flare. In particular,
we quantify the reconnection rate and the related energy release that
occurred as the new polarity emerged to form NOAA Active Region 11112
(SOL16 October 2010T00:00:00L205C117) within a region of preexisting
flux. A bright, low-lying kernel of coronal loops above the emerging
polarity, observed with the Atmospheric Imaging Assembly onboard the
Solar Dynamics Observatory and the X-ray Telescope onboard Hinode,
originally showed magnetic connectivity only between regions of newly
emerged flux when overlaid on magnetograms from the Helioseismic and
Magnetic Imager. Over the course of several days, this bright kernel
advanced into the preexisting flux. The advancement of an easily
visible boundary into the old flux regions allows measuring the rate
of reconnection between old and new magnetic domains. We compare the
reconnection rate with the inferred heating of the coronal plasma. To
our knowledge, this is the first measurement of steady, quiescent
heating related to reconnection. We determined that the newly emerged
flux reconnects at a fairly steady rate of 0.38×1016 Mx
s−1 over two days, while the radiated power varies between
(2 - 8)×1025 erg s−1 over the same time. We
found that as much as 40 % of the total emerged flux at any given
time may have reconnected. The total amounts of transferred flux (∼
1×1021 Mx) and radiated energy (∼ 7.2×1030
ergs) are comparable to that of a large M- or small X-class flare,
but are stretched out over 45 hours.
Title: Challenges Posed by Invoking Reconnection to Explain Magnetic
Energy Release from a Global Field
Authors: Longcope, Dana
Bibcode: 2014shin.confE..33L
Altcode:
One question to which magnetic reconnection may hold the answer is
how stored magnetic energy can be converted into other forms such as
heat. This is the role it has been assigned in explaining solar flares,
for example. Theoretical studies have shown that fast reconnection
occurs on small scales, probably within a thin current sheet. Current
sheets serve as the natural location for small-scale processes to
effect significant topological change to magnetic field lines. It
is far less clear, however, that a small-scale process occurring
within a thin current sheet can significantly change the over-all
magnetic energy. In a field with a current sheet, magnetic energy is
stored throughout the volume, not within the sheet itself. In order
to tap this globally-stored energy, localized reconnection must
initiate some effect capable of reaching all parts of the global
field. The reconnection itself may therefore serve as the initiator
of energy release, but probably not as the actual energy conversion
mechanism. Some insight into this cross-scale coupling can be gained
from simplified, semi-analytic models of transient reconnection in a
finite-length current sheet.
Title: Solar Coronal Holes and Open Magnetic Flux
Authors: Lowder, Chris; Qiu, Jiong; Leamon, Robert; Longcope, Dana;
Liu, Yang
Bibcode: 2014shin.confE..27L
Altcode:
Coronal holes are regions on the Sun"s surface that map the footprints
of open magnetic field lines. Using SDO/AIA and STEREO/EUVI EUV data
coupled with an adaptive thresholding routine we are able to track the
boundaries of coronal holes across the entire solar surface at a cadence
of 12 hours. Notably, the combination of AIA and EUVI data allows
for the continuous tracking of coronal hole boundary evolution on the
far-side of the sun. Incorporating SOHO/EIT data allows access to these
boundaries spanning the previous solar cycle. We find that for solar
cycle 23 the unsigned magnetic flux enclosed by coronal hole boundaries
ranges from (2-5)x10^22 Mx, covering 5%-17% of the solar surface. For
solar cycle 24 this flux ranges from (2-4)x10^22 Mx, covering 5%-10%
of the solar surface. Notably, from both observational coronal hole
boundaries and modeled open magnetic field regions the low-latitude open
field contributes significantly to the total open magnetic flux. Using
a flux transport model in conjunction with a potential field model,
we compare observational coronal holes and computed open field for
solar cycles 23 and 24, paying particular attention to the latitudinal
distribution of open magnetic field. Carrington rotations 2099 and
2106 are additionally explored in more detail.
Title: Two-ribbon Flares Spreading in the Third Dimension
Authors: Qiu, Jiong; Longcope, Dana
Bibcode: 2014shin.confE..34Q
Altcode:
Two-ribbon flares, often associated with eruptive filaments or CMEs,
are textbook demonstration of the standard flare model that describes
simultaneous reconnection of an arcade of anti-parallel magnetic
field lines crossing at the 2-dimensional macroscopic current sheet
in the corona. However, flare ribbons are often observed to brighten
sequentially along their length with an apparent speed ranging from 10
to 200 km/s, indicative of a slow (sub-Alfvenic) and organized pattern
of reconnection spreading along the assumed current sheet. This can be
hardly explained by the mechanism of driven reconnection due to ideal
MHD instability that explosively rushes open coronal field lines leading
to subsequent reconnection. We present a few well-observed examples of
flare ribbon spreading either uni-directionally or bi-directionally
along the magnetic polarity inversion line, or the assumed direction
of the overlying current sheet, analyze the coronal magnetic field
configuration, and explore diagnostics of 3-dimensional (for example
with a guide field) current sheet dynamics that may govern the observed
spread of reconnection (e.g. Shepherd and Cassak, 2012).
Title: Modeling the response of the lower atmosphere to flare
reconnection
Authors: Longcope, Dana; Qiu, Jiong; Klimchuk, James A.
Bibcode: 2014AAS...22412324L
Altcode:
It has long been recognized that energy release in a solar flare gives
rise to ablation of material from the chromosphere (more commonly
called evaporation). The prevailing view is that energy is initially
transformed from stored magnetic energy by the process of magnetic
reconnection. In some models reconnection accelerates electrons, either
directly or indirectly, and these non-thermal electrons carry energy
to the chromospheric footpoints. In others the reconnection converts
magnetic energy into heat in the corona and thermal conduction carries
that heat to the chromosphere. While no comprehensive, self-consistent
model yet exists for the conversion of magnetic energy to non-thermal
electron energy, models of the conversion to heat, via slow magnetosonic
shocks, have been available since Petschek's 1964 paper. We present
a numerical model encompassing the conversion of magnetic energy to
shocks, to heat, and then to conduction-driven evaporation. We compare
its results to those of more traditional conduction-driven models where
reconnection is replaced by an ad hoc plasma heating. We consider, in
particular, observable signatures such as doppler shifts and formation
of flare ribbons.This work was supported by the NASA SR&T program.
Title: A Comparison of EUV Coronal Hole Measurements and Modeled
Open Magnetic Field
Authors: Lowder, Chris; Qiu, Jiong; Leamon, Robert; Longcope, Dana;
Liu, Yang
Bibcode: 2014AAS...22432338L
Altcode:
Coronal holes are regions on the Sun's surface that map the footprints
of open magnetic field lines. We have developed an automated routine
to detect and track boundaries of long-lived coronal holes using
full-disk extreme-ultraviolet (EUV) images obtained by SOHO/EIT,
SDO/AIA, and STEREO/EUVI. Using these observations in conjunction
with the potential field source surface (PFSS) model, we find that
from 1996 through 2010, coronal holes extend between 5% and 17% of
the solar surface area, with total unsigned open flux varying between
(2-5)x1022 Mx. AIA/EUVI measurements spanning 2010 through
2013 mark coronal hole coverage areas of 5% to 10% of total solar
surface area, with total unsigned open magnetic flux ranging from
(2-4)x1022 Mx. A detailed comparison indicates that coronal
holes in low latitudes significantly contribute to the total open
magnetic flux. Previous studies using the He I 10830 line or EIT EUV
images do not always accurately measure these low latitude coronal
holes. Enhanced observations from AIA/EUVI in conjunction with an
observation-driven flux transport model allow a more accurate measure
of these low latitude coronal holes and their resulting contribution
to solar open magnetic flux.
Title: A one-dimensional solar flare model capturing reconnection
energy release, evaporation, and gradually cooling post-flare loops
Authors: Longcope, Dana; Qiu, Jiong; Klimchuk, Jim
Bibcode: 2014shin.confE..32L
Altcode:
The most obvious signature of energy release in a solar flare is the
large amount of chromospheric material heated to coronal temperatures
through a process known (inaccurately) as chromospheric evaporation. The
thousand-fold increase in X-ray luminosity we associate with a
flare is due entirely to evaporation. The most successful models of
flare evaporation to date have come from one-dimensional flux tube
simulations. These have provided the best means of resolving the
very thin pre-flare transition region, and of easily accommodating
the perfect field-alignment of the energy transport by either
non-thermal electrons or thermal conduction. In traditional flux
tube models magnetic reconnection is represented by an ad hoc heating
term. This adds energy but no momentum, and represents only crudely
known models of magnetic reconnection. Here we present a new approach
which captures the physics of fast Petschek reconnection in a flux tube
simulation. Following its creation by localized reconnection within a
current sheet, the flux tube retracts under magnetic tension, converting
magnetic energy into bulk flows; this is the outflow jet. These flows
form a slow magnetosonic shock which heats the coronal plasma and
drives a conduction front into the chromosphere. Our one-dimensional
model captures the energy release, thermalization, and the evaporation
it drives. We find observable signatures of the interplay between
reconnection energy release and evaporation; signatures different
from those found in conventional flux tube models with ad hoc heating,
but similar to actual flare observations.
Title: Advection of Magnetic Field Lines in Supra-Arcade Fan
Structures
Authors: Scott, Roger B.; Freed, Michael; McKenzie, David Eugene;
Longcope, Dana
Bibcode: 2014AAS...22432344S
Altcode:
Recent attempts to characterize the apparent motion of supra-arcade
fan structures have revealed bulk velocity and displacement spectra
that may give insights into the energy distribution in supra-arcade
plasma sheets. In order to form a more complete picture of the energy
balance in these structures it is important to understand the magnetic
field on a similar scale. In this work we used velocity maps found
through local correlation tracking (LCT) as source functions for a 2D,
time-dependent, ideal induction equation. We began with an assumed
initial configuration and then evolved the magnetic field in order
to maintain the frozen-in condition. We then characterized the energy
deposition into the field as well as the field strength spectrum and
several other quantities of interest. It is our hope that this study
will serve to improve our understanding of the interplay between the
plasma and the magnetic field in the supra-arcade region.
Title: Heating Rate in Reconnection Formed Flare Loops
Authors: Liu, Wenjuan; Qiu, Jiong; Longcope, Dana; Caspi, Amir
Bibcode: 2014AAS...22412313L
Altcode:
High-resolution ultraviolet (UV) and extreme ultraviolet (EUV) images
of solar flares have revealed that flare loops are formed by magnetic
reconnection events successively and heated separately. Our recent work
(Qiu et al. 2012) suggests that the heating rate in individual flare
loops could be inferred from the rapid rise of UV brightness at the
foot-points of these loops. The heating rate is further restricted
by comparing the observed coronal radiation and the synthetic one
from plasma in all these loops computed by the Enthalpy-Based Thermal
Evolution of Loops (EBTEL, Klimchuk et al. 2008, Cargrill et al. 2012)
model. Therefore, the method uses observations to constrain the heating
rates from both the input and output of the loop heating model. In this
study, we apply this method to three flares of different magnitude,
respectively. Comparison of the results from the three events show
that the synthetic coronal radiation compares reasonable well with
observations from plasma with temperature in the range of 3-10MK. This
experiment provides another independent constraint to determination of
the heating rates. Furthermore, using RHESSI hard X-ray observations, we
also infer the fraction of non-thermal beam heating in the total heating
rate of flare loops, and discuss its effect on plasma evolution. For
the 2005 May 13 M8.0 flare that exhibits significant thick-target hard
X-ray emissions, the lower limit of the total energy used to heat the
flare loops is 1.2e31 ergs, out of which, less than 20% is carried by
beam-driven upflows during the impulsive phase.
Title: Breakout and Tether-Cutting Eruption Models Are Both
Catastrophic (Sometimes)
Authors: Longcope, D. W.; Forbes, T. G.
Bibcode: 2014SoPh..289.2091L
Altcode: 2013arXiv1312.4435L; 2014SoPh..tmp....3L
We present a simplified analytic model of a quadrupolar magnetic field
and flux rope to model coronal mass ejections. The model magnetic field
is two-dimensional, force-free and has current only on the axis of
the flux rope and within two current sheets. It is a generalization of
previous models containing a single current sheet anchored to a bipolar
flux distribution. Our new model can undergo quasi-static evolution
either due to changes at the boundary or due to magnetic reconnection at
either current sheet. We find that all three kinds of evolution can lead
to a catastrophe, known as loss of equilibrium. Some equilibria can be
driven to catastrophic instability either through reconnection at the
lower current sheet, known as tether cutting, or through reconnection
at the upper current sheet, known as breakout. Other equilibria can be
destabilized through only one and not the other. Still others undergo
no instability, but they evolve increasingly rapidly in response
to slow steady driving (ideal or reconnective). One key feature of
every case is a response to reconnection different from that found
in simpler systems. In our two-current-sheet model a reconnection
electric field in one current sheet causes the current in that sheet
to increase rather than decrease. This suggests the possibility for
the microscopic reconnection mechanism to run away.
Title: Modeling Observable Effects Of Canopy Structure On
Conduction-driven Evaporation From Impulsive Energy Release
Authors: Brannon, Sean; Longcope, Dana
Bibcode: 2014AAS...22411102B
Altcode:
It is well established that the solar corona displays a highly
structured appearance. The magnetic field is partially responsible
for this structuring (e.g. coronal loops), but the complex density
and temperature distribution of plasma along the field also determines
appearance. This distribution of plasma may be dictated in part by the
coronal heating mechanism, which might be constant (steady-heating)
or impulsive (nanoflares). In the impulsive picture, reconnection
and loop contraction results in magnetosonic shocks which compress
and heat the coronal plasma. Thermal conduction then transports and
deposits heat into the chromosphere, resulting in an overpressure that
drives plasma up into the loop (a process referred to as chromospheric
evaporation). It is expected, however, that the evaporation process will
be sensitive to variations of the cross-sectional area of the flux tube,
due to the structure of the magnetic canopy above the footpoint. The
presence of a magnetic canopy may therefore have a substantial
effect on the supply of mass to the corona in response to impulsive
heating. In this work, we simulate chromospheric evaporation using a
1-D hydrodynamic code that models a flare loop as a piston shocktube,
neglecting gravity and loop curvature. We include a simplified model
of the chromosphere and transition region, and use a nozzle-like area
profile as a proxy for the canopy variation of the flux tube. We use
this code to explore observable effects of the canopy on evaporative
flow velocities, loop emission measure, and coronal mass supply during
impulsive evaporation.
Title: Long Duration Flare Emission by Sequential Reconnection
and Heating
Authors: Qiu, Jiong; Longcope, Dana; Klimchuk, James A.
Bibcode: 2014AAS...22412325Q
Altcode:
Long duration flare emissions lasting for a few hours are likely
produced by magnetic reconnection that continuously forms flare loops
and heats plasma inside. In this study, we demonstrate that this
process leads to the long duration emission in a C2.9 flare on 2011
September 13. Observed by AIA, the flare exhibits an organized pattern
of evolution with UV brightenings in flare ribbons spreading along the
polarity inversion line, followed by sequential formation of post-flare
loops seen in EUV emissions. The spatially resolved observation of
flare ribbons can be used to infer heating rates insequentially formed
and heated flare loops, with which we synthesize flare emission in
these loops with hydrodynamic models. The 0d EBTEL model (Klimchuk
et al. 2008) efficiently computes meanproperties of thousands of
flare loops identified from flare ribbon signatures, and the synthetic
tempo-spatial evolution of the total emission is in reasonable agreement
with EUV observations. The 1d model applied on a few selected loops
reveals physics of the heating mechanism and along-the-loop dynamics,
particularly during the impulsive heating phase. During the four hours
of this event, the estimated total energy in the heating amounts to
2e30 erg, with the total reconnection flux about 1e21 Mx.
Title: Findings from a Three Year Survey of Coronal Null Points
Authors: Freed, Michael; Longcope, Dana; McKenzie, David Eugene
Bibcode: 2014AAS...22432329F
Altcode:
We report the findings from a comprehensive coronal magnetic null point
survey created by Potential Field Source Surface (PFSS) modeling &
Solar Dynamic Observatory/Atmospheric Imaging Assembly (SDO/AIA)
observations. Locations of magnetic null points in the corona were
predicted from the PFSS model from Carrington Rotation 2098 to 2139
and manually compared to contrast enhanced SDO/AIA images in 171
angstroms. Statistical results will be presented that illustrate
the characteristics associated with the observed and predicted null
points. These characteristics include the radial & latitudinal
distribution; eigenvalues associated with null point structure; and
the effect spine orientation has on observability.
Title: Peristaltic Pumping near Post-coronal Mass Ejection
Supra-arcade Current Sheets
Authors: Scott, Roger B.; Longcope, Dana W.; McKenzie, David E.
Bibcode: 2013ApJ...776...54S
Altcode: 2013arXiv1308.5026S
Temperature and density measurements near supra-arcade current sheets
suggest that plasma on unreconnected field lines may experience
some degree of "pre-heating" and "pre-densification" prior to
reconnection. Models of patchy reconnection allow for heating and
acceleration of plasma along reconnected field lines but do not offer
a mechanism for transport of thermal energy across field lines. Here,
we present a model in which a reconnected flux tube retracts, deforming
the surrounding layer of unreconnected field. The deformation creates
constrictions that act as peristaltic pumps, driving plasma flow along
affected field lines. Under certain circumstances, these flows lead to
shocks that can extend far out into the unreconnected field, altering
the plasma properties in the affected region. These findings have
direct implications for observations in the solar corona, particularly
in regard to such phenomena as high temperatures near current sheets
in eruptive solar flares and wakes seen in the form of descending
regions of density depletion or supra-arcade downflows.
Title: Preface
Authors: Mansour, Nagi N.; Kosovichev, Alexander G.; Komm, Rudolf;
Longcope, Dana; Leibacher, John W.
Bibcode: 2013SoPh..287....1M
Altcode:
No abstract at ADS
Title: Ultraviolet and Extreme-ultraviolet Emissions at the Flare
Footpoints Observed by Atmosphere Imaging Assembly
Authors: Qiu, Jiong; Sturrock, Zoe; Longcope, Dana W.; Klimchuk,
James A.; Liu, Wen-Juan
Bibcode: 2013ApJ...774...14Q
Altcode: 2013arXiv1305.6899Q
A solar flare is composed of impulsive energy release events by magnetic
reconnection, which forms and heats flare loops. Recent studies have
revealed a two-phase evolution pattern of UV 1600 Å emission at the
feet of these loops: a rapid pulse lasting for a few seconds to a few
minutes, followed by a gradual decay on timescales of a few tens of
minutes. Multiple band EUV observations by the Atmosphere Imaging
Assembly further reveal very similar signatures. These two phases
represent different but related signatures of an impulsive energy
release in the corona. The rapid pulse is an immediate response of
the lower atmosphere to an intense thermal conduction flux resulting
from the sudden heating of the corona to high temperatures (we rule
out energetic particles due to a lack of significant hard X-ray
emission). The gradual phase is associated with the cooling of hot
plasma that has been evaporated into the corona. The observed footpoint
emission is again powered by thermal conduction (and enthalpy), but now
during a period when approximate steady-state conditions are established
in the loop. UV and EUV light curves of individual pixels may therefore
be separated into contributions from two distinct physical mechanisms to
shed light on the nature of energy transport in a flare. We demonstrate
this technique using coordinated, spatially resolved observations of
UV and EUV emissions from the footpoints of a C3.2 thermal flare.
Title: Multi--Instrument Estimation Of The Non--Flaring Heating And
Reconnection Rates Of Emerging Active Region NOAA AR11112
Authors: Tarr, Lucas; Longcope, D.; McKenzie, D. E.; Yoshimura, K.
Bibcode: 2013SPD....4430202T
Altcode:
In NOAA Active Region 11112, a small bipole emerges into an area of
preexisting, unipolar flux. The bright, low lying kernel of coronal
loops above the emerging field, observed with AIA and XRT, originally
show magnetic connectivity only between regions of newly emerged flux
when overlaid on HMI magnetograms. Over the course of several days,
this bright kernel advances into the preexisting flux. The advancement
of this easily visible boundary into the old flux regions over time
provides a quantifiable rate of reconnection between old and new
magnetic domains. We compare the reconnection rate to the inferred
heating of the coronal plasma. To our knowledge, this is the first
measurement of steady, quiescent heating related to reconnection. While
AR11112 does produce an M3.0 flare on Oct 16th, 2010, the implied
reconnection we focus on here predates the flare by several days,
and does not result in any observable flaring active of its own, such
as increases in the GOES light curve, chromospheric flare ribbons, or
post--flare loops. We determine that the newly emerged flux reconnects
at a fairly steady average rate of 3.5e16 Mx/s over two days, while
the radiated power varies between 2-8e25erg/s over the same time.
Title: Determining Heating Rates in Reconnection Formed Flare Loops
Authors: Liu, Wenjuan; Qiu, J.; Longcope, D.; Caspi, A.; Courtney,
C.; O'Hara, J.
Bibcode: 2013SPD....4420003L
Altcode:
High-resolution UV and EUV observations have revealed that flare
loops are formed and heated by reconnection events taking place
successively. Our recent work (Qiu et al. 2012) suggests that the rapid
rise of UV brightness at the foot-points of individual flare loops could
be used to infer the impulsive heating rate in these loops. Using these
heating rates and the Enthalpy-Based Thermal Evolution of Loops (EBTEL,
Klimchuk et al. 2008, Cargrill et al. 2012) model, we can compute plasma
evolution in thousands of flare loops anchored at the UV foot-points,
and calculate the synthetic coronal radiation by these loops to
compare with observations. Therefore, the method uses observations
to constrain the heating rates from both the input and output of the
loop heating model. In this study, we apply this method to two M-class
flares occurred on 2005 May 13 and 2011 March 07, respectively, and show
that the synthetic soft X-ray and EUV spectra and light curves compare
favorably with the observations by RHESSI and EVE. With a steady-state
assumption, we also compute the transition-region DEM at the base of
each flare loop during its decay phase, and compare the predicted UV and
EUV emission at the foot-points with AIA observations. This experiment
provides another independent constraint to determination of the heating
rates. Furthermore, using RHESSI hard X-ray observations, we also
infer the fraction of non-thermal beam heating in the total heating
rate of flare loops, and discuss its effect on plasma evolution. For
the 2005 May 13 M8.0 flare that exhibits significant thick-target hard
X-ray emissions, the lower limit of the total energy used to heat the
flare loops is $1.2 \times 10^31$ ergs, out of which, less than 20%
is carried by beam-driven upflows during the impulsive phase.
Title: Breakout and tether-cutting eruption models are both
catastrophic (sometimes)
Authors: Longcope, Dana; Forbes, T. G.
Bibcode: 2013SPD....44...68L
Altcode:
We present a simplified analytic model of a quadrupolar magnetic field
and flux rope to model coronal mass ejections. The model magnetic field
is two-dimensional, force-free and has current only on the axis of the
flux rope and within two currents sheets. It is a generalization of
previous models containing a single current sheet anchored to a bipolar
flux distribution. Our new model can undergo quasi-static evolution
due either to changes at the boundary or to magnetic reconnection at
either current sheet. We find that all three kinds of evolution can lead
to a catastrophe known as loss of equilibrium. Some equilibria can be
driven to catastrophic instability either through reconnection at the
lower current sheet, known as tether cutting, or through reconnection
at the upper current sheet, known as breakout. Other equilibria can
be destabilized through only one and not the other. Still others
undergo no instability, but evolve increasingly rapidly in response
to slow steady driving (ideal or reconnective). One key feature of
every case is a response to reconnection different from that found
in simpler systems. In our two-current sheet model a reconnection
electric field in one current sheet causes the current in that sheet
to increase rather than decrease. This suggests the possibility for
the microscopic reconnection mechanism to run away.
Title: The Effects Of Canopy Expansion On Chromospheric Evaporation
Driven By Thermal Conduction Fronts
Authors: Brannon, Sean; Longcope, D.; Rozpedek, F. D.
Bibcode: 2013SPD....44...61B
Altcode:
The solar corona is well known for its highly structured
appearance. This structuring is partly due to its magnetic field, and
partly due to the complex distribution of mass within the field. Coronal
mass density is set by coronal heating which might be constant (the
steady-heating picture) or might be sporadic (the so-called nanoflare
picture). In the latter scenario, a mass flux occurs through a process
referred to as chromospheric evaporation. Reconnection and subsequent
loop contraction generate shocks in the corona which result in thermal
conduction fronts. These fronts impulsively deposit heat into the
cooler chromosphere and drive supersonic upward flows which is the
evaporation. This process has been extensively studied in the past,
but generally using models with uniform magnetic field connecting
the corona and chromosphere. Transonic flows are known, in general,
to be highly sensitive to variation in the cross-section though which
they are driven. It is therefore expected that the complex structure
of the magnetic canopy could have a dramatic effect on the supply of
mass into the corona. We explore this possibility using a simplified
1-D hydrodynamic model of evaporation occurring through a varying
magnetic field canopy.
Title: UV and EUV Emissions at the Flare Foot-points Observed by AIA
Authors: Qiu, Jiong; Sturrock, Z.; Longcope, D.; Klimchuk, J. A.;
Liu, W.
Bibcode: 2013SPD....44...53Q
Altcode:
A solar flare is composed of impulsive energy release events by magnetic
reconnection, which forms and heats flare loops. Recent studies have
revealed a two-phase evolution pattern of UV 1600A emission at the
feet of these loops: a rapid pulse lasting for a few seconds to a
few minutes, followed by a gradual decay on timescales of a few tens
of minutes. Multiple band EUV observations by AIA further reveal
very similar signatures. These two phases represent different but
related signatures of an impulsive energy release in the corona. The
rapid pulse is an immediate response of the lower atmosphere to an
intense thermal conduction flux resulting from the sudden heating of
the corona to high temperatures (we rule out energetic particles due
to a lack of significant hard X-ray emission). The gradual phase is
associated with the cooling of hot plasma that has been evaporated
into the corona. The observed footpoint emission is again powered
by thermal conduction (and enthalpy), but now during a period when
approximate steady state conditions are established in the loop. UV
and EUV light curves of individual pixels may therefore be separated
into contributions from two distinct physical mechanisms to shed
light on the nature of energy transport in a flare. We demonstrate
this technique using coordinated, spatially resolved observations of
UV and EUV emission from the footpoints of a C3.2 thermal flare.
Title: How reconnection within a current sheet can release energy
stored over the global corona - insights from a toy model
Authors: Longcope, Dana
Bibcode: 2013SPD....44...60L
Altcode:
According to current understanding, solar flares occur when magnetic
reconnection releases magnetic energy stored in the corona. Current
sheets are essential elements in models of fast magnetic reconnection
which demand large electric fields on small scales. While current
sheets are also associated with magnetic energy storage, they are not
the actual site at which energy is stored: free magnetic energy is
stored throughout the coronal volume. This means that reconnection
on very small scales must initiate the release of energy stored on
much larger scales. Some insight into this cross-scale coupling can be
gained from simplified, semi-analytic models of transient reconnection
in a finite-length current sheet. In one such model, presented here,
the localized reconnection electric field launches a fast magnetosonic
pulse carrying the sheet's current at its front. Magnetic energy
is converted in place, by the pulse, into bulk kinetic energy of
reconnection inflow and outflow. The model predicts, for example,
the fraction of stored energy directly thermalized, or converted to
other forms such as magnetosonic waves and bulk flows. This work was
supported by a joint NSF/DOE grant.
Title: Peristaltic Pumping near Post-CME Supra-Arcade Current Sheets
Authors: Scott, Roger B.; Longcope, D.; McKenzie, D. E.
Bibcode: 2013SPD....4430402S
Altcode:
Measurements of temperature and density near supra-arcade current
sheets suggest that plasma on unreconnected field lines may experience
some degree of “pre-heating” and “pre-densification” prior to
their reconnection. Models of patchy reconnection allow for heating and
acceleration of plasma along reconnected field lines but do not offer a
mechanism for transport of energy and momentum across field lines. Here
we present a model in which a reconnected flux tube retracts, deforming
the surrounding layer of unreconnected field. The deformation creates
constrictions that act as peristaltic pumps, driving plasma flow along
affected field lines. Under certain circumstances these flows lead to
shocks that can extend far out into the unreconnected field, altering
the plasma properties in the affected region. These findings have
direct implications for observations in the solar corona, particularly
in regard to such phenomena as wakes seen behind supra-arcade downflows
and high temperatures near current sheets in eruptive solar flares. This
work was supported by NASA, the NSF and the DOE.
Title: Survey of Coronal Null Points with SDO/AIA & WSO
Authors: Freed, Michael; McKenzie, D. E.; Longcope, D.
Bibcode: 2013SPD....44...21F
Altcode:
Magnetic fields in the corona can be approximated by using PFSS
(Potential Field Source Surface) model in conjunction with magnetogram
measurements of the photosphere. This approach is incorporated here to
find locations of magnetic null points in the solar corona. Observations
from WSO (Wilcox Solar Observatory) provide the necessary harmonic
coefficients for a PFSS model. We located all magnetic null points in
the PFSS model going back to Carrington Rotation 2098. The time and
location where they cross the West limb is compared to high resolution
observations made by SDO/AIA. Variations in predicted and observed
null point locations, and estimates of the duration of each null,
will be examined. This work will provide a catalog of coronal nulls
observed by SDO that can be examined further for interesting dynamical
behavior or variations in neighboring plasma.
Title: Modeling Properties Of Chromospheric Evaporation Driven By
Thermal Conduction Fronts From Reconnection Shocks
Authors: Brannon, Sean; Longcope, D.
Bibcode: 2013SPD....4430403B
Altcode:
Magnetic reconnection in the corona results in contracting flare loops,
releasing energy into plasma heating and shocks. These hydrodynamic
shocks drive thermal conduction fronts (TCFs), which deposit energy
into the chromosphere, driving upflows (evaporation) and downflows
(condensation) across a range of temperatures. Observations have
revealed that the transition between evaporation and condensation, the
"velocity reversal point" (VRP), occurs at a characteristic temperature
and with a characteristic slope, which vary between different flares. In
this study, we develop a 1-D hydrodynamical flare loop model with a
simplified three-region atmosphere (chromosphere / transition region
(TR) / corona), with TCFs initiated by piston shocks introduced
in the corona. We investigate the effect of three different flare
loop parameters (post-shock temperature, TR temperature ratio, and
TR thickness) on the temperature and slope of the VRP. We find that
both of the evaporation characteristics have power-law relationships
to the varied flare parameters, and we report the scaling exponents
for our model. Finally, we develop a method to determine the best-fit
post-shock temperature and TR temperature ratio based on the observed
quantities, and discuss the results for two sets of published data.
Title: Photospheric Magnetic Diffusion by Measuring Moments of
Active Regions
Authors: Engell, Alexander; Longcope, D.
Bibcode: 2013SPD....44..119E
Altcode:
Photospheric magnetic surface diffusion is an important constraint
for the solar dynamo. The HMI Active Region Patches (HARPs) program
automatically identify all magnetic regions above a certain flux. In our
study we measure the moments of ARs that are no longer actively emerging
and can thereby give us good statistical constraints on photospheric
diffusion. We also present the diffusion properties as a function of
latitude, flux density, and single polarity (leading or following)
within each HARP.
Title: Determining Heating Rates in Reconnection Formed Flare Loops
of the M8.0 Flare on 2005 May 13
Authors: Liu, Wen-Juan; Qiu, Jiong; Longcope, Dana W.; Caspi, Amir
Bibcode: 2013ApJ...770..111L
Altcode: 2013arXiv1304.4521L
We analyze and model an M8.0 flare on 2005 May 13 observed by the
Transition Region and Coronal Explorer and the Reuven Ramaty High Energy
Solar Spectroscopic Imager (RHESSI) to determine the energy release
rate from magnetic reconnection that forms and heats numerous flare
loops. The flare exhibits two ribbons in UV 1600 Å emission. Analysis
shows that the UV light curve at each flaring pixel rises impulsively
within a few minutes, and decays slowly with a timescale longer than
10 minutes. Since the lower atmosphere (the transition region and
chromosphere) responds to energy deposit nearly instantaneously,
the rapid UV brightening is thought to reflect the energy release
process in the newly formed flare loop rooted at the footpoint. In
this paper, we utilize the spatially resolved (down to 1'') UV light
curves and the thick-target hard X-ray emission to construct heating
functions of a few thousand flare loops anchored at the UV footpoints,
and compute plasma evolution in these loops using the enthalpy-based
thermal evolution of loops model. The modeled coronal temperatures
and densities of these flare loops are then used to calculate coronal
radiation. The computed soft X-ray spectra and light curves compare
favorably with those observed by RHESSI and by the Geostationary
Operational Environmental Satellite X-ray Sensor. The time-dependent
transition region differential emission measure for each loop during
its decay phase is also computed with a simplified model and used to
calculate the optically thin C IV line emission, which dominates the
UV 1600 Å bandpass during the flare. The computed C IV line emission
decays at the same rate as observed. This study presents a method to
constrain heating of reconnection-formed flare loops using all available
observables independently, and provides insight into the physics of
energy release and plasma heating during the flare. With this method,
the lower limit of the total energy used to heat the flare loops in
this event is estimated to be 1.22 × 1031 erg, of which
only 1.9 × 1030 erg is carried by beam-driven upflows
during the impulsive phase, suggesting that the coronal plasmas are
predominantly heated in situ.
Title: Calculating Separate Magnetic Free Energy Estimates for Active
Regions Producing Multiple Flares: NOAA AR11158
Authors: Tarr, Lucas; Longcope, Dana; Millhouse, Margaret
Bibcode: 2013ApJ...770....4T
Altcode: 2013arXiv1302.1787T
It is well known that photospheric flux emergence is an important
process for stressing coronal fields and storing magnetic free energy,
which may then be released during a flare. The Helioseismic and
Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO)
captured the entire emergence of NOAA AR 11158. This region emerged as
two distinct bipoles, possibly connected underneath the photosphere,
yet characterized by different photospheric field evolutions and
fluxes. The combined active region complex produced 15 GOES C-class,
two M-class, and the X2.2 Valentine's Day Flare during the four
days after initial emergence on 2011 February 12. The M and X class
flares are of particular interest because they are nonhomologous,
involving different subregions of the active region. We use a Magnetic
Charge Topology together with the Minimum Current Corona model of
the coronal field to model field evolution of the complex. Combining
this with observations of flare ribbons in the 1600 Å channel of the
Atmospheric Imaging Assembly on board SDO, we propose a minimization
algorithm for estimating the amount of reconnected flux and resulting
drop in magnetic free energy during a flare. For the M6.6, M2.2, and
X2.2 flares, we find a flux exchange of 4.2 × 1020 Mx, 2.0
× 1020 Mx, and 21.0 × 1020 Mx, respectively,
resulting in free energy drops of 3.89 × 1030 erg, 2.62
× 1030 erg, and 1.68 × 1032 erg.
Title: Heating of Flare Loops During a Two-ribbon Flare on 2011
March 07 Observed by AIA and EVE
Authors: Liu, W. -J.; O'Hara, J.; Peck, C.; Qiu, J.; Longcope, D. W.
Bibcode: 2013enss.confE.109L
Altcode:
Recent high-resolution EUV observations have revealed that flare
loops are formed and heated by reconnection events taking place
successively. Our recent work shows that the rapidly rising phase of
the UV emission at the foot-points of the flare loops could be used to
infer the time profile of the impulsive heating rate. In this study,
we analyze an M-class flare observed by AIA and EVE. We utilize the
spatially resolved UV brightness time profiles to infer heating rates of
a few thousand flux tubes anchored at the UV foot-points, and compute
plasma evolution in each flux tube using the EBTEL model (Klimchuk
et al. 2008, Cargrill et al. 2012). The coronal radiation is then
calculated and compared with soft X-ray and EUV light curves observed
by GOES and AIA. With a steady-state assumption, we also compute the
transition-region DEM for each flux tube during its decay phase, and
compare the predicted optically-thin transition-region emission in
UV and EUV with AIA foot-points observations. The EUV emissions from
both loops and foot points are also compared with irradiance observed
by EVE. This study presents a method to infer heating functions of
reconnection formed flare loops and how they affect evolution of the
overlying corona as well as the lower-atmosphere dynamics coherently.
Title: Temperature and Electron Density Diagnostics of a Candle-Flame
Shaped Flare. Asymmetric Reconnection Evidence
Authors: Guidoni, Silvina E.; McKenzie, David E.; Longcope, Dana W.;
Plowman, Joseph E.; Yoshimura, Keiji
Bibcode: 2013enss.confE..62G
Altcode:
Candle-flame shaped flares are archetypical structures that represent
indirect evidence of magnetic reconnection. For long-lived events, most
of their observed features can be explained with the classic magnetic
reconnection model of solar flares, the CSHKP model. A flare resembling
1992 Tsuneta's famous candle-flame flare occurred on January 28 2011;
we present its temperature and electron density diagnostics. This flare
was observed with Hinode/XRT, SDO/AIA, and STEREO (A)/EUVI, resulting
in high resolution, broad temperature coverage, and stereoscopic
views of this iconic structure. Our XRT filter-ratio temperature and
density maps corroborate the general reconnection scenario. The high
temperature images reveal a brightening that grows in size to form a
tower-like structure at the top of the post-flare arcade, a feature
that has been observed in other long duration events. This tower is
a localized density increase, as shown by our XRT electron density
maps. Despite the extensive work on the standard reconnection scenario,
there is no complete agreement among models regarding the nature of
this tower-like structure. The XRT maps also reveal that reconnected
loops that are successively connected at their tops to this tower
develop a density increase in one of their legs that can reach over
2 times the density value of the other leg, giving the appearance of
``half-loops''. Their density is nevertheless still lower than at
the tower. These jumps in density last longer than several acoustic
transit times along the loops. We use STEREO images to show that the
half-loop brightening is not a line-of- sight projection effect of the
type suggested by Forbes and Acton (1996). This would indicate that
asymmetric reconnection took place between loops originally belonging
to systems with different magnetic field strengths, densities, and
temperatures. We hypothesize that the heat generated by reconnection's
slow shocks is then transferred to each leg of the loop at different
rates. Therefore, the increase in electron density due to chromospheric
evaporation is different in each leg. Thermal pressure balance between
the legs is prevented by shocked plasma at the top of the loops. We also
present preliminary results comparing a new fast DEM method that uses
SDO/AIA data with the XRT filter ratio method. Both methods complement
each other, they agree at the overlap between their instruments'
temperature response functions (3-12 MK) while the SDO/AIA method
works well at lower temperatures and the XRT one at higher temperatures.
Title: Multi-instrument estimation of the non-flaring heating and
reconnection rates of an emerging active region: NOAA AR11112
Authors: Tarr, Lucas A.; Longcope, Dana W.; Yoshimura, Keiji
Bibcode: 2013enss.confE..37T
Altcode:
In NOAA Active Region 11112, a small bipole emerges into an area of
preexisting, unipolar flux. When overlaid on HMI magnetograms, the
bright, low lying kernel of loops above the emerging field, observed
with AIA and XRT, originally show magnetic connectivity only between
regions of newly emerged flux. Over the course of several days, this
bright kernel advances into the preexisting flux. The advancement
of this easily visible boundary into the old flux regions over time
provides a quantifiable rate of reconnection between old and new
magnetic domains. We compare the reconnection rate to the inferred
heating of the coronal plasma. To our knowledge, this is the first
measurement of steady, quiescent heating related to reconnection. While
AR11112 does produce an M3.0 flare on Oct 16th, 2010, the implied
reconnection we focus on here predates the flare by several days,
and does not result in any observable flaring active of its own, such
as increases in the GOES light curve, chromospheric flare ribbons,
or post-flare loops.
Title: The Role of Fast Magnetosonic Waves in the Release and
Conversion via Reconnection of Energy Stored by a Current Sheet
Authors: Longcope, D. W.; Tarr, L.
Bibcode: 2012ApJ...756..192L
Altcode: 2012arXiv1207.4818L
Using a simple two-dimensional, zero-β model, we explore the manner
by which reconnection at a current sheet releases and dissipates free
magnetic energy. We find that only a small fraction (3%-11% depending on
current-sheet size) of the energy is stored close enough to the current
sheet to be dissipated abruptly by the reconnection process. The
remaining energy, stored in the larger-scale field, is converted
to kinetic energy in a fast magnetosonic disturbance propagating
away from the reconnection site, carrying the initial current and
generating reconnection-associated flows (inflow and outflow). Some of
this reflects from the lower boundary (the photosphere) and refracts
back to the X-point reconnection site. Most of this inward wave energy
is reflected back again and continues to bounce between X-point and
photosphere until it is gradually dissipated, over many transits. This
phase of the energy dissipation process is thus global and lasts
far longer than the initial purely local phase. In the process, a
significant fraction of the energy (25%-60%) remains as undissipated
fast magnetosonic waves propagating away from the reconnection site,
primarily upward. This flare-generated wave is initiated by unbalanced
Lorentz forces in the reconnection-disrupted current sheet, rather
than by dissipation-generated pressure, as some previous models have
assumed. Depending on the orientation of the initial current sheet,
the wave front is either a rarefaction, with backward-directed flow,
or a compression, with forward-directed flow.
Title: Heating of Flare Loops with Observationally Constrained
Heating Functions
Authors: Qiu, Jiong; Liu, Wen-Juan; Longcope, Dana W.
Bibcode: 2012ApJ...752..124Q
Altcode: 2012arXiv1201.0973Q
We analyze high-cadence high-resolution observations of a C3.2 flare
obtained by AIA/SDO on 2010 August 1. The flare is a long-duration event
with soft X-ray and EUV radiation lasting for over 4 hr. Analysis
suggests that magnetic reconnection and formation of new loops
continue for more than 2 hr. Furthermore, the UV 1600 Å observations
show that each of the individual pixels at the feet of flare loops
is brightened instantaneously with a timescale of a few minutes,
and decays over a much longer timescale of more than 30 minutes. We
use these spatially resolved UV light curves during the rise phase
to construct empirical heating functions for individual flare loops,
and model heating of coronal plasmas in these loops. The total coronal
radiation of these flare loops are compared with soft X-ray and EUV
radiation fluxes measured by GOES and AIA. This study presents a method
to observationally infer heating functions in numerous flare loops that
are formed and heated sequentially by reconnection throughout the flare,
and provides a very useful constraint to coronal heating models.
Title: Understanding the Differences in Three Consecutive Large Flares
Authors: Tarr, L.; Longcope, D.
Bibcode: 2012ASPC..456...61T
Altcode:
The active region NOAA 11158 produced three large flares over a 31
hour interval centered on 14 Feb. 2011. The flare ribbons observed
in 1600Å AIA images reveal very different characters between the
three. We use a model of the coronal magnetic field topology and its
evolution to explain the differences in structures and energies of
these three flares.
Title: Nozzle Driven Shocks in Post-CME Plasma
Authors: Scott, Roger B.; Longcope, D. W.; McKenzie, D. E.
Bibcode: 2012AAS...22020407S
Altcode:
Models of patchy reconnection allow for heating and acceleration of
plasma along reconnected field lines but do not offer a mechanism for
transport of energy and momentum across field lines. Here we present a
simple 2D model in which a localized region of reconnected flux creates
an apparent constriction in the surrounding layer of unreconnected
field. The moving constriction acts as a de Laval nozzle and ultimately
leads to shocks which can extend out to several times the diameter of
the flux tube, altering the density and temperature of the plasma in
that region. These findings have direct implications for observations in
the solar corona, particularly in regard to such phenomena as wakes seen
behind supra-arcade downflows and high temperatures in post-CME current
sheets. This work was supported by a joint grant from the NSF and DOE.
Title: Heating of Flare Foops During a Two-ribbon Flare on 2011
March 07
Authors: Liu, Wenjuan; O'Hara, J.; Peck, C.; Qiu, J.; Longcope, D. W.
Bibcode: 2012AAS...22020427L
Altcode:
Recent observations have revealed that flare loops are formed and
heated by reconnection events taking place successively. This is
evident in high-resolution EUV observations of post-flare loops, as
well as the apparent "motion" of UV bright kernels outlining the feet
of these loops. Our recent study shows that the spatially resolved UV
brightness at the foot-points of individual loops grows rapidly on
timescales of a few minutes, followed by a long decay on timescales
of tens of minutes. This distinctive two-phase pattern of the UV light
curve represents the impulsive energy deposition during the rapid rise
and the subsequent hydrodynamic plasma evolution in the coronal loop
during the gradual decay. In this study, we analyze an M-class flare
observed by AIA. We utilize the spatially resolved UV brightness time
profiles to infer impulsive heating functions of a few thousand flux
tubes anchored at the UV foot-points, and compute plasma evolution
in each flux tube using the EBTEL model (Klimchuk et al. 2008). The
coronal radiation is then calculated and compared with soft X-ray
and EUV light curves observed by GOES and AIA. With a steady-state
assumption, we also compute the transition-region DEM for each flux
tube during its decay phase, and compare the predicted optically-thin
transition-region line (C IV) emission with the observation. This study
presents a method to infer heating functions of reconnection formed
flare loops and how they affect evolution of the overlying corona as
well as the lower-atmosphere dynamics coherently.
Title: UV Signatures of Flare Heating and Cooling
Authors: Qiu, Jiong; Longcope, D. W.
Bibcode: 2012AAS...22020307Q
Altcode:
A solar flare is comprised of impulsive energy release events by
magnetic reconnection, which forms and heats flux tubes, the elementary structure of flare loops. Recent studies have revealed
a two-phase evolution pattern of UV 1600Å emission at the feet of
these flux tubes: a rapid pulse lasting for a few minutes, followed
by a gradual decay on timescales of a few tens of minutes. These
signatures are indicative of instantaneous lower-atmosphere response
to impulsive energy deposition, and the subsequent plasma evolution in
overlying coronal loops. We present analysis of the UV 1600Å emission
at the foot-points of numerous reconnection-formed flux tubes observed
by SDO/AIA, and discuss diagnostics of the lower-atmosphere and corona
dynamics, which are governed by different but coherent physics during
the heating and decay phases.
Title: Modeling Observed Characteristics of Chromospheric Evaporation
Driven by Thermal Conduction Fronts from Reconnection
Authors: Brannon, Sean; Longcope, D.
Bibcode: 2012AAS...22020312B
Altcode:
Recent observations of flaring loop footpoints have revealed the
presence of both blueshifted and redshifted components to flare-heated
chromospheric plasma, implying that the chromospheric anchors of
the loop experience both upflows ("evaporation") and downflows
("condensation"). These observations also point to a transition
temperature at which the flows switch from downward (redshift) to upward
(blueshift), and show that this temperature is supercoronal. We present
a 1D hydrodynamic model of a half-loop with a simplified three-region
structure (chromosphere / transition region / corona) to study the
characteristics of chromospheric evaporation and condensation. We use
a hydrodynamic shock to create a thermal conduction front, which then
propagates down the loop and heats the chromosphere. We use the results
to calculate velocity-temperature profiles and differential emission
measures, for several values of shock speed and chromosphere/corona
temperature ratio. We also investigate the appearance of a large
pressure peak that forms at the base of the transition region,
and discuss the evaporation evolution in terms of this pressure
peak. Finally, we discuss the physical mechanisms underlying the
position and magnitude of the evaporation.
Title: Determine the Heating Rate in Reconnection Formed Flare Loops
of the M8.0 flare on 2005 May 13
Authors: Liu, Wenjuan; Qiu, J.; Longcope, D. W.; Caspi, A.
Bibcode: 2012AAS...22051601L
Altcode:
Many eruptive flares exhibit two extended ribbons in the
lower-atmosphere outlining the feet of the post-flare coronal
arcade. High-cadence and high-resolution UV observations by TRACE
reveal that the flare ribbon consists of small patches sequentially
brightened along the ribbon, suggesting that reconnection takes place
sequentially forming individual post-flare loops along the arcade,
as often seen in coronal observations in the EUV wavelengths. These
reconnection events and formation of new loops continue well into
the decay phase. Our recent study further shows that the spatially
resolved UV brightness at the foot-points of individual loops grows
rapidly on timescales of a few minutes, followed by a long decay on
timescales of more than 10 minutes. The rapid rise of UV radiation is
correlated with the hard X-ray light curve during the impulsive phase,
hence is most likely a direct response of instantaneous heating in the
reconnection formed flux tubes. In this study, we utilize the spatially
resolved UV brightness time profiles to reconstruct instantaneous
heating functions of a few thousand flux tubes anchored at the UV
foot-points, and compute plasma evolution in each flux tube using the
EBTEL model (Klimchuk et al. 2008). The temperature and density of
these flux tubes are then used to calculate coronal radiation. The
computed soft X-ray spectra and light curves compare favorably with
those observed by RHESSI and GOES. The time-dependent transition region
DEM for each tube during its decay phase is also computed and used to
calculate optically-thin transition region line emissions, which are
compared with UV observations at the decay phase. This study presents
a method to constrain heating functions of reconnection formed flare
loops using all available observables, and provides a powerful way to
examine physics of heating discrete flux tubes.
Title: Calculating Separate Magnetic Free Energy Estimates for Active
Regions Producing Multiple Flares: NOAA AR11158
Authors: Tarr, Lucas; Longcope, D. W.
Bibcode: 2012AAS...22020432T
Altcode:
It is well known that photospheric flux emergence is an important
process for stressing coronal fields and generating magnetic free
energy, which may then be released during a flare. The Helioseismic
and Magnetic Imager(HMI) onboard the Solar Dynamics Observatory (SDO)
captured the entire emergence of NOAA AR 11158. This region emerged as
two distinct bipoles, possibly connected underneath the photosphere,
yet characterized by different photospheric field evolutions and
fluxes. The combined active region complex produced 15 GOES C-class,
2 M-class, and the X2.2 Valentine's Day Flare during the four days
after initial emergence on February 12th, 2011. The M and X class
flares are of particular interest because they are nonhomologous,
involving different subregions of the active region. We use a Magnetic
Charge Topology together with the Minimum Current Corona model of
the coronal field to model field evolution of the complex. Combining
this with observations of flare ribbons in the 1600 Angstrom channel
of the Atmospheric Imaging Assembly (AIA) onboard SDO, we generate a
separate energy estimate for each major flare using their respective
unique subsets of stressed magnetic domains. This work is supported
under contract SP02H3901R from Lockheed-Martin to MSU.
Title: Efficiency Of Energy Dissipation At A Magnetic X-point
Authors: Longcope, Dana; Tarr, L.
Bibcode: 2012AAS...22020423L
Altcode:
Magnetic reconnection at a current sheet is believed to release
stored magnetic energy by decreasing the net current carried by the
sheet. The current change will affect magnetic field throughout the
coronal volume, not just on field lines transferred across the current
sheet. This global effect results from a fast magnetosonic rarefaction
wave launched by the reconnection, which carries away the excess
current and converts free magnetic energy to kinetic energy through
the volume. The present work demonstrates, in a simplified model, how
reflection of this wave from the photospheric boundary determines the
total energy dissipation possible. Previous investigations by Craig
and McClymont (1991) and Hassam (1992) assumed one-hundred percent of
the reflected wave returned to the dissipation region (the X-point),
and thereby concluded that all of the stored energy could be dissipated
eventually. The present investigation uses a more realistic geometry
to show that only a fraction of the stored magnetic energy could
be directly dissipated, at least within the dissipation region. The
remaining energy continues to propagate as a fast magnetosonic wave.
Title: Measuring Solar Photospheric Diffusion By The Second Moment
of Active Region Magnetograms
Authors: Engell, Alexander; Longcope, D.
Bibcode: 2012AAS...22020711E
Altcode:
The process at which magnetic flux is transported on the solar
photosphere is known to be dominated by diffusion, meridional flow,
and differential rotation. In order to determine the diffusion constant
we examine the diffusion term in the flux transport model and solve
it. We study three relatively inactive active regions from HMI 12
minute magnetograms. We relate our solved diffusion equation to the
second moment of our extracted magnetograms. For our active regions
studied so far we find a mean diffusion constant of 256 km2s-1 with
a standard deviation of 3.6 km2s-1.
Title: Flare Half-Loops: What Are They?
Authors: McKenzie, David Eugene; Guidoni, S. E.; Longcope, D. W.;
Yoshimura, K.
Bibcode: 2012AAS...22032201M
Altcode:
The M1.4 flare of 28 January 2011 has a remarkable resemblance to
the famous "Tsuneta candle-flame" flare of 1992. It was observed
with Hinode/XRT, SDO/AIA, and STEREO (A)/EUVI, resulting in higher
resolution, greater temperature coverage, and stereoscopic views of
this iconic structure. The high temperature images reveal a brightening
that grows in size to form a tower-like structure at the top of the
arcade. They also show that loops which are successively connected to
this tower develop a density increase in one of their legs that can
exceed twice the density of the other leg, giving the appearance of
"half loops". These jumps in density last for an extended period of
time. On the other hand, XRT filter ratios suggest that temperature
is approximately uniform along the entire loop. XRT filter-ratio
density maps corroborate that the brighter legs have higher density
than the fainter halves. The tower is associated with a localized
density increase, with even higher densities than either leg of the
loop. This spatial variation of density may correspond to a shock
at the top of the loops. We use STEREO images to show that the half
loop brightening is not a line-of-sight projection effect of the type
suggested by Forbes & Acton. This work is supported under contract
SP02H3901R from Lockheed-Martin to MSU, and under contract NNM07AB07C
with the Harvard-Smithsonian Astrophysical Observatory.
Title: Calculating Energy Storage Due to Topological Changes in
Emerging Active Region NOAA AR 11112
Authors: Tarr, Lucas; Longcope, Dana
Bibcode: 2012ApJ...749...64T
Altcode: 2012arXiv1202.0075T
The minimum current corona model provides a way to estimate stored
coronal energy using the number of field lines connecting regions
of positive and negative photospheric flux. This information is
quantified by the net flux connecting pairs of opposing regions in
a connectivity matrix. Changes in the coronal magnetic field, due to
processes such as magnetic reconnection, manifest themselves as changes
in the connectivity matrix. However, the connectivity matrix will also
change when flux sources emerge or submerge through the photosphere,
as often happens in active regions. We have developed an algorithm
to estimate the changes in flux due to emergence and submergence
of magnetic flux sources. These estimated changes must be accounted
for in order to quantify storage and release of magnetic energy in
the corona. To perform this calculation over extended periods of
time, we must additionally have a consistently labeled connectivity
matrix over the entire observational time span. We have therefore
developed an automated tracking algorithm to generate a consistent
connectivity matrix as the photospheric source regions evolve over
time. We have applied this method to NOAA Active Region 11112, which
underwent a GOES M2.9 class flare around 19:00 on 2010 October 16th,
and calculated a lower bound on the free magnetic energy buildup of
~8.25 × 1030 erg over 3 days.
Title: Predictions of Energy and Helicity in Four Major Eruptive
Solar Flares
Authors: Kazachenko, Maria D.; Canfield, Richard C.; Longcope, Dana
W.; Qiu, Jiong
Bibcode: 2012SoPh..277..165K
Altcode: 2011arXiv1104.3593K
In order to better understand the solar genesis of interplanetary
magnetic clouds (MCs), we model the magnetic and topological properties
of four large eruptive solar flares and relate them to observations. We
use the three-dimensional Minimum Current Corona model (Longcope, 1996,
Solar Phys.169, 91) and observations of pre-flare photospheric magnetic
field and flare ribbons to derive values of reconnected magnetic flux,
flare energy, flux rope helicity, and orientation of the flux-rope
poloidal field. We compare model predictions of those quantities to
flare and MC observations, and within the estimated uncertainties of
the methods used find the following: The predicted model reconnection
fluxes are equal to or lower than the reconnection fluxes inferred
from the observed ribbon motions. Both observed and model reconnection
fluxes match the MC poloidal fluxes. The predicted flux-rope helicities
match the MC helicities. The predicted free energies lie between the
observed energies and the estimated total flare luminosities. The
direction of the leading edge of the MC's poloidal field is aligned
with the poloidal field of the flux rope in the AR rather than the
global dipole field. These findings compel us to believe that magnetic
clouds associated with these four solar flares are formed by low-corona
magnetic reconnection during the eruption, rather than eruption of
pre-existing structures in the corona or formation in the upper corona
with participation of the global magnetic field. We also note that since
all four flares occurred in active regions without significant pre-flare
flux emergence and cancelation, the energy and helicity that we find
are stored by shearing and rotating motions, which are sufficient to
account for the observed radiative flare energy and MC helicity.
Title: Post-Flare Half-Loops: What are They?
Authors: Guidoni, Silvina E.; McKenzie, David E.; Longcope, Dana W.
Bibcode: 2012decs.confE..12G
Altcode:
The M1.4 flare of Jan 28, 2011 has a remarkable resemblance to the
famous Tsuneta flare of 1992. It was observed with Hinode/XRT, SDO/AIA,
and STEREO (A)/EUVI, giving us higher resolution, greater temperature
coverage, and stereoscopic views of this iconic structure. The high
temperature images reveal a brightening that grows in size to form
a tower-like structure at the top of the post-flare arcade. They
also show that loops that are successively connected to this tower
develop a density increase in one of their legs that can reach 4
times the density value of the other leg, giving the appearance of
"half loops". These jumps in density last for an extended period of
time. On the other hand, XRT filter ratios suggest that temperature
is approximately uniform along the entire loop. XRT filter-ratio
density maps corroborate that the brighter legs have higher density
than the rest of the loop. The tower is a localized density increase
at even higher densities than the brighter side of the loop. This may
correspond to a shock at the top of the loops. We use STEREO images to
show that the half loop brightening is not a line-of-sight projection
effect of the type suggested by Forbes & Acton.
Title: Consequences of spontaneous reconnection at a two-dimensional
non-force-free current layer
Authors: Fuentes-Fernández, J.; Parnell, C. E.; Hood, A. W.; Priest,
E. R.; Longcope, D. W.
Bibcode: 2012PhPl...19b2901F
Altcode: 2012arXiv1202.0161F
Magnetic neutral points, where the magnitude of the magnetic field
vanishes locally, are potential locations for energy conversion in the
solar corona. The fact that the magnetic field is identically zero at
these points suggests that for the study of current sheet formation and
of any subsequent resistive dissipation phase, a finite beta plasma
should be considered, rather than neglecting the plasma pressure as
has often been the case in the past. The rapid dissipation of a finite
current layer in non-force-free equilibrium is investigated numerically,
after the sudden onset of an anomalous resistivity. The aim of this
study is to determine how the energy is redistributed during the initial
diffusion phase, and what is the nature of the outward transmission of
information and energy. The resistivity rapidly diffuses the current
at the null point. The presence of a plasma pressure allows the vast
majority of the free energy to be transferred into internal energy. Most
of the converted energy is used in direct heating of the surrounding
plasma, and only about 3% is converted into kinetic energy, causing a
perturbation in the magnetic field and the plasma which propagates away
from the null at the local fast magnetoacoustic speed. The propagating
pulses show a complex structure due to the highly non-uniform initial
state. It is shown that this perturbation carries no net current as it
propagates away from the null. The fact that, under the assumptions
taken in this paper, most of the magnetic energy released in the
reconnection converts internal energy of the plasma, may be highly
important for the chromospheric and coronal heating problem.
Title: Heliophysics
Authors: Austin, M.; Guhathakurta, M.; Bhattacharjee, A.; Longcope,
D. W.; Sojka, J. J.; Schrijver, C. J.; Siscoe, G. L.
Bibcode: 2011AGUFMSH13B1968A
Altcode:
Heliophysics is a developing scientific discipline integrating
studies of the Sun's variability, the surrounding heliopsphere, and
climate environments. Over the past few centuries, our understanding
of how the Sun drives space weather and climate on the Earth and
other planets has advanced at an ever-increasing rate. NASA Living
With a Star and the UCAR Visiting Scientist Programs, sponsor the
annual Heliophysics Summer Schools to build the next generation of
scientists in this emerging field. The highly successful series of
summer schools (commencing 2007) trains a select group of graduate
students, postdoctoral fellows and university faculty to learn and
develop the science of heliophysics as a broad, coherent discipline
that reaches in space from the Earth's troposphere to the depths
of the Sun, and in time from the formation of the solar system to
the distant future. The first three years of the school resulted in
the publication of three textbooks now being used at universities
worldwide. Subsequent years have also developed the complementary
materials that support teaching of heliophysics at both graduate and
undergraduate levels. The textbooks are edited by Carolus J. Schrijver,
Lockheed Martin, and George L. Siscoe, Boston University. The books
provide a foundational reference for researchers in heliophysics,
astrophysics, plasma physics, space physics, solar physics, aeronomy,
space weather, planetary science and climate science. The Jack Eddy
Postdoctoral Fellowship program matches newly graduated postdoctorates
with hosting mentors for the purpose of training the next generation of
researchers needed in heliophysics. The fellowships are for two years,
and any U.S university or research lab may apply to host a fellow. Two
major topics of focus for the program are the science of space weather
and of the Sun-climate connection. Since the goal of this fellowship
program is to train Sun-Earth system researchers, preference is also
given to research projects that cross the traditional heliophysics
subdomains of the Sun, heliosphere, magnetosphere, and ionosphere/upper
atmosphere, as well as Sun-climate investigations. Host mentors play
critical roles. Potential hosts may enter information about their
research on a central database. Application deadline: January 11, 2012
Title: Theories of magnetic energy release and conversion in solar
flares: possible roles for magnetic reconnection
Authors: Longcope, Dana
Bibcode: 2011sdmi.confE..13L
Altcode:
Solar flares are now generally believed to occur through the rapid
release of magnetic energy through reconnection. The basic scenario for
this process was outlined more than 50 years ago, but only in the past
decade have theoretical models been able to explain the generation of
large parallel electric fields previously pre-supposed. The upshot of
these theories is that reconnection electric fields must be localized
within a current sheet in order to change magnetic field line topology
at observed rates. The apparent resolution of this long-standing
puzzle (Petschek mode vs. Sweet-Parker mode) raises a host of new
questions concerning how reconnection can result in the conversion of
magnetic field energy to other forms. If it is in fact localized, the
reconnection electric field cannot be directly responsible for plasma
heating or particle acceleration. The magnetic energy is stored over a
large coronal volume whose plasma which will never be exposed to the
reconnection electric field. Energy conversion must therefore occur
away from the reconnection site, as an indirect consequence. In spite
of its secondary place in the chain of events, this energy conversion
is the main effect we typically ascribe to solar flares. I will review
the current theoretical understanding of how magnetic reconnection
might lead to a release and the conversion of magnetic energy. This
work supported by a joint grant from NSF and DOE
Title: A Model for the Origin of High Density in Looptop X-Ray Sources
Authors: Longcope, D. W.; Guidoni, S. E.
Bibcode: 2011ApJ...740...73L
Altcode: 2011arXiv1107.2441L
Super-hot (SH) looptop sources, detected in some large solar flares, are
compact sources of HXR emission with spectra matching thermal electron
populations exceeding 30 MK. High observed emission measure (EM) and
inference of electron thermalization within the small source region
both provide evidence of high densities at the looptop, typically more
than an order of magnitude above ambient. Where some investigators have
suggested such density enhancement results from a rapid enhancement in
the magnetic field strength, we propose an alternative model, based on
Petschek reconnection, whereby looptop plasma is heated and compressed
by slow magnetosonic shocks generated self-consistently through flux
retraction following reconnection. Under steady conditions such shocks
can enhance density by no more than a factor of four. These steady shock
relations (Rankine-Hugoniot relations) turn out to be inapplicable to
Petschek's model owing to transient effects of thermal conduction. The
actual density enhancement can in fact exceed a factor of 10 over
the entire reconnection outflow. An ensemble of flux tubes retracting
following reconnection at an ensemble of distinct sites will have a
collective EM proportional to the rate of flux tube production. This
rate, distinct from the local reconnection rate within a single tube,
can be measured separately through flare ribbon motion. Typical flux
transfer rates and loop parameters yield EMs comparable to those
observed in SH sources.
Title: Direct Measurements of Magnetic Twist in the Solar Corona
Authors: Malanushenko, A.; Yusuf, M. H.; Longcope, D. W.
Bibcode: 2011ApJ...736...97M
Altcode: 2012arXiv1202.5421M
In the present work, we study the evolution of magnetic helicity in the
solar corona. We compare the rate of change of a quantity related to
the magnetic helicity in the corona to the flux of magnetic helicity
through the photosphere and find that the two rates are similar. This
gives observational evidence that helicity flux across the photosphere
is indeed what drives helicity changes in the solar corona during
emergence. For the purposes of estimating coronal helicity, we neither
assume a strictly linear force-free field nor attempt to construct a
nonlinear force-free field. For each coronal loop evident in extreme
ultraviolet, we find a best-matching line of a linear force-free field
and allow the twist parameter α to be different for each line. This
method was introduced and its applicability discussed in Malanushenko
et al. The object of this study is emerging and rapidly rotating AR
9004 over about 80 hr. As a proxy for coronal helicity, we use the
quantity langα i Li /2rang averaged over many
reconstructed lines of magnetic field. We argue that it is approximately
proportional to the "flux-normalized" helicity H/Φ2,
where H is the helicity and Φ is the total enclosed magnetic flux
of the active region. The time rate of change of such a quantity in
the corona is found to be about 0.021 rad hr-1, which is
comparable with the estimates for the same region obtained using other
methods, which estimated the flux of normalized helicity to be about
0.016 rad hr-1.
Title: Predictions of Energy and Helicity in Four Major Eruptive
Solar Flares
Authors: Kazachenko, Maria; Canfield, Richard C.; Longcope, Dana W.;
Qiu, Jiong
Bibcode: 2011shin.confE...4K
Altcode:
In order to better understand the solar genesis of interplanetary
magnetic clouds (MCs), we model the magnetic and topological properties
of four large eruptive solar flares and relate them to observations. We
use the three-dimensional Minimum Current Corona model (Longcope in
Solar Phys. 169, 91, 1996) and observations of pre-flare photospheric
magnetic field and flare ribbons to derive values of reconnected
magnetic flux, flare energy, flux rope helicity, and orientation of
the flux rope poloidal field. We compare model predictions of those
quantities to flare and MC observations and within the estimated
uncertainties of the methods used find the following. The predicted
model reconnection fluxes are equal to or lower than the reconnection
fluxes inferred from the observed ribbon motions. Both observed and
model reconnection fluxes match the MC poloidal fluxes. The predicted
flux-rope helicities match the MC helicities. The predicted free
energies lie between the observed energies and the estimated total
flare luminosities. The direction of the leading edge of the MC's
poloidal field is aligned with the poloidal field of the flux rope in
the AR rather than the global dipole field. These findings compel us to
believe that magnetic clouds associated with these four solar flares are
formed by low-corona magnetic reconnection during the eruption, rather
than eruption of pre-existing structures in the corona or formation in
the upper corona with participation of the global magnetic field. We
also note that since all four flares occurred in active regions without
significant pre-flare flux emergence and cancelation, the energy and
helicity that we find are stored by shearing and rotating motions,
which are sufficient to account for the observed radiative flare energy
and MC helicity.
Title: Predictions Of Energy And Helicity In Four Major Eruptive
Solar Flares
Authors: Kazachenko, Maria; Canfield, R. C.; Longcope, D. W.; Qiu, J.
Bibcode: 2011SPD....42.2218K
Altcode: 2011BAAS..43S.2218K
In order to better understand the solar genesis of interplanetary
magnetic clouds (MCs) we model the magnetic and topological properties
of four large eruptive solar flares and relate them to observations. We
use the three-dimensional Minimum Current Corona model and observations
of pre-flare photospheric magnetic field and flare ribbons to derive
values of reconnected magnetic flux, flare energy, flux rope helicity
and orientation of the flux rope poloidal field. We compare model
predictions of those quantities to flare and MC observations and within
the estimated uncertainties of the methods used find the following. The
predicted model reconnection fluxes are equal to or lower than the
observed reconnection fluxes from the ribbon motions. Both observed and
model reconnection fluxes match the MC poloidal fluxes. The predicted
flux rope helicities match the MC helicities. The predicted free
energies lie between the observed energies and the estimated total
flare luminosities. The direction of the leading edge of the MC's
poloidal field is aligned with the poloidal field of the flux rope in
the AR rather than the global dipole field. These findings compel us to
believe that magnetic clouds associated with these four solar flares are
formed by low-corona magnetic reconnection during the eruption, rather
than eruption of pre-existing structures in the corona or formation in
the upper corona with participation of the global magnetic field. We
also note that since all four flares occurred in active regions without
significant pre-flare flux emergence and cancellation, the energy and
helicity we find are stored by shearing and rotating motions, which
are sufficient to account for the observed radiative flare energy and
MC helicity.
Title: Heating of Flare Loops During a Two-ribbon Flare on 2005 May 13
Authors: Qiu, Jiong; Liu, W.; Longcope, D. W.
Bibcode: 2011SPD....42.1205Q
Altcode: 2011BAAS..43S.1205Q
Many eruptive flares exhibit two extended ribbons in the
lower-atmosphere outlining the feet of the post-flare coronal
arcade. High-cadence high-resolution UV observations by TRACE reveal
that a flare ribbon consists of small patches sequentially brightened
along the ribbon, suggesting that reconnection takes place sequentially
forming individual post-flare loops along the arcade, as often seen in
coronal observations in the EUV wavelengths. These reconnection events
and formation of new loops continue into the decay phase. Our recent
study (Qiu et al. 2010) further shows that the spatially resolved UV
brightness at the foot-points of individual loops grows rapidly on
timescales of 1 minute, followed by a long decay on timescales of more
than 10 minutes. The rapid rise of UV radiation is correlated with
the hard X-ray light curve during the impulsive phase, hence is most
likely a direct response of instantaneous heating in the reconnection
formed flux tubes. In this study, we utilize the spatially resolved UV
brightness time profiles to reconstruct instantaneous heating functions
of individual flux tubes, and compute evolution of each flux tube using
the EBTEL model (Klimchuk et al. 2008). To build the heating function,
we take into account the scaling between the total UV peak count rate,
the hard X-ray energy flux derived from RHESSI spectral analysis during
the impulsive phase, and as well the reconnection rate that persists
from the pre-impulsive phase to the decay phase. The sum of the computed
coronal radiation in all the flux tubes compares favorably with the
gross coronal radiation observed by GOES. This study presents the
first effort to constrain heating functions of flare loops directly
using all available observables, and provides a method to examine
physics of heating discrete flux tubes formed by reconnection events
throughout the flare. The work is supported by NSF grant ATM-0748428.
Title: Calculating Energy Storage Due to Topological Changes in
Emerging Active Region NOAA AR11112
Authors: Tarr, Lucas; Longcope, D.
Bibcode: 2011SPD....42.0502T
Altcode: 2011BAAS..43S.0502T
The Minimum Current Corona (MCC) model provides a way to estimate stored
coronal energy using the number of field lines connecting regions of
positive and negative photospheric flux. MCC assumes that the amount
of flux connecting pairs of regions is fixed, even as the photospheric
field evolves. As the fixed flux in each domain becomes increasingly
different from a potential field configuration the system builds up
magnetic free energy. We have developed a method for quantifying the
field evolution by tracking photospheric magnetic sources measured
with SDO/HMI, and therefore energy storage. In particular, we present
an algorithm quantifying the flux evolution of each pair of regions
due to submergence and emergence through the photosphere. We have
applied this method to NOAA Active Region 11112, which underwent a
GOES M--2.9 class flare around 19:00 on Oct. 16, 2010, and calculated a
free magnetic energy buildup of 8x1030 ergs over 3 days. This work was supported NASA LWS.
Title: Response of a Model Chromosphere to Shock-generated Conduction
Fronts
Authors: Brannon, Sean; Longcope, D.
Bibcode: 2011SPD....42.1741B
Altcode: 2011BAAS..43S.1741B
It is currently believed that solar flares result from magnetic
reconnection leading to rapidly contracting loops. Plasma trapped on
the contracting field lines is compressed and heated at two shocks
propagating downward to the footpoints. These shocks, and heat
fronts preceding them, drive hard X-ray emission and chromospheric
evaporation. We study the response of the chromosphere using both
time-dependent non-linear simulations as well as linear analysis. The
linear model considers thermally diffusing acoustic pulses reflecting
from a region of non-uniform sound speed. We compare results from
these two models in the case of small-amplitude linear pulses. This work is supported by grants from the NSF and DOE.
Title: Computing Magnetic Energy From Aia Images And Hmi Line-of-sight
Magnetograms
Authors: Longcope, Dana; Malanushenko, A.; Tarr, L.
Bibcode: 2011SPD....42.2118L
Altcode: 2011BAAS..43S.2118L
The state of the art for computing the magnetic energy in an active
region's corona is to extrapolate a non-linear force-free field from
vector magnetic field data. This method infers coronal properties from
photospheric data without direct use of any coronal information. We
present here an alternative which uses the shapes of loops visible
in EUV or soft X-ray images to infer coronal currents. The method of
Malanushenko et al. (2009) is used to infer magnetic field strength
along each coronal loop. This sparse sampling of magnetic information is
used in a Monte Carlo integral to compute the total magnetic energy. We
also present a method for computing the free energy (the difference
between the energy of the actual field and the corresponding potential
field) directly as a single Monte Carlo integral. Both integrals are
estimates with known statistical uncertainties which are reasonably
small for samples as small as 25 loops. We demonstrate the method using
a test field and then apply it to observations of an active region.
Title: Testing the Thin Flux Tube Model with Fully Three-dimensional
Magnetohydrodynamic Simulations
Authors: Guidoni, Silvina; Longcope, D. W.; Linton, M. G.
Bibcode: 2011SPD....42.2202G
Altcode: 2011BAAS..43S.2202G
Observations of supra-arcade downflows suggest that some flare
reconnection may occur in patches within the current sheet above
the arcade. The energy release following such reconnection may be
modeled using the thin flux tube formalism. The patch of reconnection
creates two bent flux tubes which retract rapidly due to magnetic
tension. In the model, the supersonic collision of plasma generates
shocks inside the retracting tube. We test the validity of the thin
flux tube formalism by comparing results of the model with those from
fully three-dimensional magnetohydrodynamic (MHD) simulations (using
ARMS code). Patchy reconnection is produced in the MHD simulation by
temporarily enhancing resistivity within a small region straddling
an equilibrium current sheet. The subsequent dynamics are compared
to the predictions of the the thin flux tube model initialized with
the same bent flux tube. The MHD simulations show similar flux tube
retraction. There are also MHD shocks within the retracting flux tubes
whose magnitude and locations compare favorably to those from the thin
flux tube model. This work was supported by NASA, NSF, DOD, and DOE.
Title: The Origin Of High Density In Loop-top X-ray Sources
Authors: Longcope, Dana; Guidoni, S.
Bibcode: 2011SPD....42.1105L
Altcode: 2011BAAS..43S.1105L
Super-hot looptop sources, detected in some large solar flares,
are compact sources of HXR emission with spectra matching thermal
electron populations exceeding 30 megakelvins. High observed emission
measure, as well as inference of electron thermalization within the
small source region, both provide evidence of high densities at the
looptop; typically more than an order of magnitude above ambient. Some
investigators have suggested such density enhancement results from a
rapid enhancement in the magnetic field strength. It seems unlikely,
however, that the spontaneous decrease in magnetic energy powering
the flare would increase the field strength by more than a factor of
ten. We propose an alternative model, based on Petschek reconnection,
whereby looptop plasma is heated and compressed by slow magnetosonic
shocks generated self-consistently through flux retraction following
reconnection. Under steady conditions such shocks can enhance
density by no more than a factor of four. These steady shock relations
(Rankine-Hugoniot relations) turn out to be inapplicable to Petschek's
model owing to transient thermal conduction. The actual density
enhancement can in fact exceed a factor of ten over the entire length
of the reconnection outflow. An ensemble of flux tubes retracting
following reconnection at an ensemble of distinct sites will have
a collective emission measure proportional to the rate of flux tube
production. This rate, distinct from the local reconnection rate within
a single tube, can be measured separately through observations of flare
ribbon motion. Typical flux transfer rates and loop parameters yield
emission measures comparable to those observed in super-hot sources.
Title: Heating of Flare Loops During a Two-ribbon Flare
Authors: Qiu, Jiong; Liu, W.; Longcope, D. W.
Bibcode: 2011SPD....42.2221Q
Altcode: 2011BAAS..43S.2221Q
Many eruptive flares exhibit two extended ribbons in the
lower-atmosphere outlining the feet of the post-flare coronal
arcade. High-cadence high-resolution UV observations by TRACE reveal
that the flare ribbon consists of small patches sequentially brightened
along the ribbon, suggesting that reconnection takes place sequentially
forming individual post-flare loops along the arcade, as often seen in
coronal observations in the EUV wavelengths. These reconnection events
and formation of new loops continue well into the decay phase. Our
recent study (Qiu et al. 2010) further shows that the spatially resolved
UV brightness at the foot-points of individual loops grows rapidly
on timescales of 1 minutes, followed by a long decay on timescales of
more than 10 minutes. The rapid rise of UV radiation is correlated with
the hard X-ray light curve during the impulsive phase, hence is most
likely a direct response of instantaneous heating in the reconnection
formed flux tubes. In this study, we utilize the spatially resolved UV
brightness time profiles to reconstruct instantaneous heating functions
of individual flux tubes, and compute evolution of each flux tube using
the EBTEL model (Klimchuk et al. 2008). To build the heating function,
we take into account the scaling between the total UV peak count rate,
the hard X-ray energy flux derived from RHESSI spectral analysis during
the impulsive phase, and as well the reconnection rate that persists
from the pre-impulsive phase to the decay phase. The sum of the computed
coronal radiation in all the flux tubes compares favorably with the
gross coronal radiation observed by GOES. This study presents the
first effort to constrain heating functions of flare loops directly
using all available observables, and provides a method to examine
physics of heating discrete flux tubes formed by reconnection events
throughout the flare. This work is supported by NSF grant ATM-0748428.
Title: Density Enhancements and Voids Following Patchy Reconnection
Authors: Guidoni, S. E.; Longcope, D. W.
Bibcode: 2011ApJ...730...90G
Altcode: 2011arXiv1102.0709G
We show, through a simple patchy reconnection model, that retracting
reconnected flux tubes may present elongated regions relatively
devoid of plasma, as well as long lasting, dense central hot
regions. Reconnection is assumed to happen in a small patch across a
Syrovatskiiˇ (non-uniform) current sheet (CS) with skewed magnetic
fields. The background magnetic pressure has its maximum at the center
of the CS plane and decreases toward its edges. The reconnection patch
creates two V-shaped reconnected tubes that shorten as they retract
in opposite directions, due to magnetic tension. One of them moves
upward toward the top edge of the CS, and the other one moves downward
toward the top of the underlying arcade. Rotational discontinuities
(RDs) propagate along the legs of the tubes and generate parallel
supersonic flows that collide at the center of the tube. There,
gas-dynamic shocks that compress and heat the plasma are launched
outwardly. The descending tube moves through the bottom part of the
CS where it expands laterally in response to the decreasing background
magnetic pressure. This effect may decrease plasma density by 30%-50%
of background levels. This tube will arrive at the top of the arcade
that will slow it to a stop. Here, the perpendicular dynamics is
halted, but the parallel dynamics continues along its legs; the RDs
are shut down, and the gas is rarified to even lower densities. The
hot post-shock regions continue evolving, determining a long lasting
hot region on top of the arcade. We provide an observational method
based on total emission measure and mean temperature that indicates
where in the CS the tube has been reconnected.
Title: Heliophysics
Authors: Austin, M.; Guhathakurta, M.; Bhattacharjee, A.; Longcope,
D. W.; Sojka, J. J.
Bibcode: 2010AGUFMSH11B1667A
Altcode:
Heliophysics Summer Schools. NASA Living With a Star and the
University Corporation for Atmospheric Research, Visiting Scientist
Programs sponsor the Heliophysics Summer Schools to build the next
generation of scientists in this new field. The series of summer schools
(commencing 2007) trains graduate students, postdoctoral fellows and
university faculty to learn and develop the science of heliophysics as
a broad, coherent discipline that reaches in space from the Earth’s
troposphere to the depths of the Sun, and in time from the formation
of the solar system to the distant future. The first three years of
the school resulted in the publication of three textbooks for use at
universities worldwide. Subsequent years will both teach generations
of students and faculty and develop the complementary materials that
support teaching of heliophysics at both graduate and undergraduate
levels. Heliophysics is a developing scientific discipline integrating
studies of the Sun’s variability, the surrounding heliopsphere, and
climate environments. Over the past few centuries, our understanding
of how the Sun drives space weather and climate on the Earth and
other planets has advanced at an ever-increasing rate. The three
volumes, “Plasma Physics of the Local Cosmos”, “Space Storms and
Radiation: Causes and Effects” and “Evolving Solar Activity and
the Climates of Space and Earth”, edited by Carolus J. Schrijver,
Lockheed Martin, and George L. Siscoe, Boston University, integrate
such diverse topics for the first time as a coherent intellectual
discipline. The books may be ordered through Cambridge University Press,
and provide a foundational reference for researchers in heliophysics,
astrophysics, plasma physics, space physics, solar physics, aeronomy,
space weather, planetary science and climate science. Heliophysics
Postdoctoral Program. Hosting/mentoring scientists and postdoctoral
fellows are invited to apply to this new program designed to train the
next generation of researchers in heliophysics. Two major topics of
focus for LWS are the science of space weather and of the Sun-climate
connection. Preference is given to applicants whose proposed research
addresses one of these two foci; but any research program relevant
to LWS is considered. Since the goal of this fellowship program is
to train Sun-Earth system researchers, preference is also given to
research projects that cross the traditional heliophysics subdomains of
the Sun, heliosphere, magnetosphere, and ionosphere/upper atmosphere,
as well as Sun-climate investigations. Host institutions and mentoring
scientists will play critical roles. Interested hosts may submit
information about their research on a central database for this program:
http://www.vsp.ucar.edu/Heliophysics/
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
Bibcode: 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.
Bibcode: 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: A Quantitative Model of Energy Release and Heating by
Time-dependent, Localized Reconnection in a Flare with Thermal
Loop-top X-ray Source
Authors: Longcope, D. W.; Des Jardins, A. C.; Carranza-Fulmer, T.;
Qiu, J.
Bibcode: 2010SoPh..267..107L
Altcode: 2011arXiv1106.3572L; 2010SoPh..tmp..172L
We present a quantitative model of the magnetic energy stored and then
released through magnetic reconnection for a flare on 26 February
2004. This flare, well observed by RHESSI and TRACE, shows evidence
of non-thermal electrons for only a brief, early phase. Throughout
the main period of energy release there is a super-hot (T≳30 MK)
plasma emitting thermal bremsstrahlung atop the flare loops. Our
model describes the heating and compression of such a source by
localized, transient magnetic reconnection. It is a three-dimensional
generalization of the Petschek model, whereby Alfvén-speed retraction
following reconnection drives supersonic inflows parallel to the
field lines, which form shocks: heating, compressing, and confining a
loop-top plasma plug. The confining inflows provide longer life than
a freely expanding or conductively cooling plasma of similar size and
temperature. Superposition of successive transient episodes of localized
reconnection across a current sheet produces an apparently persistent,
localized source of high-temperature emission. The temperature of the
source decreases smoothly on a time scale consistent with observations,
far longer than the cooling time of a single plug. Built from a
disordered collection of small plugs, the source need not have the
coherent jet-like structure predicted by steady-state reconnection
models. This new model predicts temperatures and emission measure
consistent with the observations of 26 February 2004. Furthermore, the
total energy released by the flare is found to be roughly consistent
with that predicted by the model. Only a small fraction of the energy
released appears in the super-hot source at any one time, but roughly a
quarter of the flare energy is thermalized by the reconnection shocks
over the course of the flare. All energy is presumed to ultimately
appear in the lower-temperature (T≲20 MK) post-flare loops. The
number, size, and early appearance of these loops in TRACE's 171 Å
band are consistent with the type of transient reconnection assumed
in the model.
Title: Reconnection Outflows and Current Sheet Observed with
Hinode/XRT in the 2008 April 9 "Cartwheel CME" Flare
Authors: Savage, Sabrina L.; McKenzie, David E.; Reeves, Katharine K.;
Forbes, Terry G.; Longcope, Dana W.
Bibcode: 2010ApJ...722..329S
Altcode: 2010arXiv1003.4758S
Supra-arcade downflows (SADs) have been observed with Yohkoh/SXT (soft
X-rays (SXR)), TRACE (extreme ultraviolet (EUV)), SOHO/LASCO (white
light), SOHO/SUMER (EUV spectra), and Hinode/XRT (SXR). Characteristics
such as low emissivity and trajectories, which slow as they reach the
top of the arcade, are consistent with post-reconnection magnetic flux
tubes retracting from a reconnection site high in the corona until they
reach a lower-energy magnetic configuration. Viewed from a perpendicular
angle, SADs should appear as shrinking loops rather than downflowing
voids. We present X-ray Telescope (XRT) observations of supra-arcade
downflowing loops (SADLs) following a coronal mass ejection (CME) on
2008 April 9 and show that their speeds and decelerations are consistent
with those determined for SADs. We also present evidence for a possible
current sheet observed during this flare that extends between the
flare arcade and the CME. Additionally, we show a correlation between
reconnection outflows observed with XRT and outgoing flows observed
with LASCO.
Title: Sunspot Rotation, Flare Energetics, and Flux Rope Helicity:
The Halloween Flare on 2003 October 28
Authors: Kazachenko, Maria D.; Canfield, Richard C.; Longcope, Dana
W.; Qiu, Jiong
Bibcode: 2010ApJ...722.1539K
Altcode:
We study the X17 eruptive flare on 2003 October 28 using Michelson
Doppler Imager observations of photospheric magnetic and velocity
fields and TRACE 1600 Å images of the flare in a three-dimensional
model of energy buildup and release in NOAA 10486. The most dramatic
feature of this active region is the 123° rotation of a large positive
sunspot over 46 hr prior to the event. We apply a method for including
such rotation in the framework of the minimum current corona model of
the buildup of energy and helicity due to the observed motions. We
distinguish between helicity and energy stored in the whole active
region and that released in the flare itself. We find that while the
rotation of a sunspot contributes significantly to the energy and
helicity budgets of the whole active region, it makes only a minor
contribution to that part of the region that flares. We conclude
that in spite of the fast rotation, shearing motions alone store
sufficient energy and helicity to account for the flare energetics
and interplanetary coronal mass ejection helicity content within their
observational uncertainties. Our analysis demonstrates that the relative
importance of shearing and rotation in this flare depends critically
on their location within the parent active region topology.
Title: Shocks and Thermal Conduction Fronts in Retracting Reconnected
Flux Tubes
Authors: Guidoni, S. E.; Longcope, D. W.
Bibcode: 2010ApJ...718.1476G
Altcode: 2010arXiv1006.4398G
We present a model for plasma heating produced by time-dependent,
spatially localized reconnection within a flare current sheet separating
skewed magnetic fields. The reconnection creates flux tubes of new
connectivity which subsequently retract at Alfvénic speeds from the
reconnection site. Heating occurs in gas-dynamic shocks (GDSs) which
develop inside these tubes. Here we present generalized thin flux
tube equations for the dynamics of reconnected flux tubes, including
pressure-driven parallel dynamics as well as temperature-dependent,
anisotropic viscosity and thermal conductivity. The evolution
of tubes embedded in a uniform, skewed magnetic field, following
reconnection in a patch, is studied through numerical solutions of
these equations, for solar coronal conditions. Even though viscosity
and thermal conductivity are negligible in the quiet solar corona,
the strong GDSs generated by compressing plasma inside reconnected
flux tubes generate large velocity and temperature gradients along
the tube, rendering the diffusive processes dominant. They determine
the thickness of the shock that evolves up to a steady state value,
although this condition may not be reached in the short times involved
in a flare. For realistic solar coronal parameters, this steady
state shock thickness might be as long as the entire flux tube. For
strong shocks at low Prandtl numbers, typical of the solar corona,
the GDS consists of an isothermal sub-shock where all the compression
and cooling occur, preceded by a thermal front where the temperature
increases and most of the heating occurs. We estimate the length of
each of these sub-regions and the speed of their propagation.
Title: Slow Shocks and Conduction Fronts from Petschek Reconnection
of Skewed Magnetic Fields: Two-fluid Effects
Authors: Longcope, D. W.; Bradshaw, S. J.
Bibcode: 2010ApJ...718.1491L
Altcode: 2010arXiv1006.0441L
In models of fast magnetic reconnection, flux transfer occurs within
a small portion of a current sheet triggering stored magnetic energy
to be thermalized by shocks. When the initial current sheet separates
magnetic fields which are not perfectly anti-parallel, i.e., they are
skewed, magnetic energy is first converted to bulk kinetic energy
and then thermalized in slow magnetosonic shocks. We show that the
latter resemble parallel shocks or hydrodynamic shocks for all skew
angles except those very near the anti-parallel limit. As for parallel
shocks, the structures of reconnection-driven slow shocks are best
studied using two-fluid equations in which ions and electrons have
independent temperature. Time-dependent solutions of these equations
can be used to predict and understand the shocks from reconnection of
skewed magnetic fields. The results differ from those found using a
single-fluid model such as magnetohydrodynamics. In the two-fluid model,
electrons are heated indirectly and thus carry a heat flux always well
below the free-streaming limit. The viscous stress of the ions is,
however, typically near the fluid-treatable limit. We find that for a
wide range of skew angles and small plasma β an electron conduction
front extends ahead of the slow shock but remains within the outflow
jet. In such cases, conduction will play a more limited role in driving
chromospheric evaporation than has been predicted based on single-fluid,
anti-parallel models.
Title: Sunspot Rotation, Eruptive Flare Energetics And Flux Rope
Helicity: Topology Matters.
Authors: Kazachenko, Maria; Canfield, R.; Longcope, D.; Qiu, J.
Bibcode: 2010AAS...21632003K
Altcode: 2010BAAS...41..911K
We study the role of rotation in the flare energy and helicity budget
of two active regions: NOAA 10486 and NOAA 10759. Using MDI and TRACE
observations of photospheric magnetic and velocity fields in those
active regions we build a topological model of their three-dimensional
coronal magnetic field. In both active regions a fast rotating sunspot
is observed. We apply a method for including such rotation in the
framework of the minimum current corona model (MCC, Longcope 1996) to
the buildup of energy and helicity associated with the X17.2 eruptive
flare on 2003 October 28 (the Halloween flare) and M8.0 eruptive flare
on 2005 May 13. We find that including the sunspot rotation in the model
changes the total flare thermal energy and flux rope helicity by only
10% for the Halloween flare, but by more than 200% for the 2005 May
13 flare. While for the Halloween flare shearing motions alone store
sufficient energy and helicity to account for the flare energetics and
ICME helicity content within their observational uncertainties, for the
2005 May 13 flare it is the rotation that dominates. We demonstrate
that the relative importance of shearing and rotation in those two
flares depends critically on their location within the parent active
region topology.
Title: Reconnection Outflows and Current Sheet Observed with
Hinode/XRT in the April 9 2008 "Cartwheel CME" Flare
Authors: Savage, Sabrina; McKenzie, D. E.; Reeves, K. K.; Forbes,
T. G.; Longcope, D. W.
Bibcode: 2010AAS...21640423S
Altcode: 2010BAAS...41R.903S
Supra-arcade downflows (SADs) have been observed with Yohkoh/SXT (soft
X-rays (SXR)), TRACE (extreme ultra-violet (EUV)), SoHO/LASCO (white
light), SoHO/SUMER (EUV spectra), and Hinode/XRT (SXR). Characteristics
such as low emissivity and trajectories which slow as they reach the
top of the arcade are consistent with post-reconnection magnetic flux
tubes. The magnetic flux within the tubes provides pressure against
filling with plasma. As with the standard model of reconnection,
the tubes then retract from a reconnection site high in the corona
until they reach a more potential magnetic configuration. Viewed from
a perpendicular angle, SADs should appear as shrinking loops rather
than downflowing voids. We will present observations of supra-arcade
downflowing loops (SADLs) following a CME on April 9, 2008 with XRT
and show that their speeds and decelerations are consistent with those
determined for SADs. We will also present evidence for a possible
current sheet observed during this flare that extends between the CME
and the flare arcade. Additionally, we will show a correlation between
reconnection outflows observed with XRT and outgoing flows observed
with LASCO.
Title: Quantifying Separator Reconnection Between Emerging and
Existing Active Regions
Authors: Malanushenko, Anna; Longcope, D. W.; McKenzie, D. E.; Yusuf,
M. H.
Bibcode: 2010AAS...21640507M
Altcode: 2010BAAS...41..890M
When one active region emerges close to an older active region,
coronal loops connecting the two regions are often observed in
extreme ultraviolet (EUV). This signifies the occurrence of magnetic
reconnection, an important mechanism in other contexts, such as solar
flares and coronal mass ejections. We measure the rate of the magnetic
reconnection by identifying each coronal loop observed in EUV that
connects the existing and the emerging active regions. For each loop we
estimate amount of magnetic flux it carries by measuring its apparent
width in EUV and the magnetic field strength in the same location using
(non)-linear force-free field reconstruction (Malanushenko et. al.,
2009). We find that the amount of reconnected flux apparent on EUV
is smaller than the flux through the separator surface in the point
charge magnetic model (Longcope, 2005). This discrepancy provides a
means to estimate the fraction of reconnecting loops that would appear
in the EUV bandpass. We measure the delay time between the noticeable
beginning of the emergence and the reconnection apparent in EUV. We
apply this analysis to five emergence events. This work expands and
improves the method described by Longcope et. al. (2005).
Title: Energy Release and Heating by Reconnection in a Flare with
a Thermal Looptop X-ray Source
Authors: Longcope, Dana; Des Jardins, A.; Carranza-Fulmer, T.; Qiu, J.
Bibcode: 2010AAS...21640424L
Altcode: 2010BAAS...41S.903L
The flare on 2004-Feb-26, well observed by RHESSI and TRACE,
showed evidence of non-thermal electrons only for a brief, early
phase. The main period of energy release was characterized by a
super-hot (T>30 MK) plasma emitting thermal brehmsstralung atop
the flare loops. We present a model for the heating and compression
of such a source by shocks generated following localized, transient
magnetic reconnection. The model is an unsteady, three-dimensional
generalization of the Petschek model whereby Alfven-speed retraction
following reconnection drives supersonic inflows parallel to the field
lines. These inflows form shocks heating, compressing and confining a
looptop plasma plug. The confining inflows provide longer life than
a freely expanding or conductively cooling plasma of similar size
and temperature. Superposition of successive transient episodes of
localized reconnection across a current sheet produces an apparently
persistent, localized source of high-temperature emission. This new
model predicts temperatures and emission measure consistent with the
observations of 2004-Feb-26. Furthermore, the total energy released
by the flare is found to be roughly consistent with that predicted by
the model. Only a small fraction of the energy released appears in the
super-hot source at any one time, but roughly a quarter of the flare
energy is thermalized by the reconnection shocks over the course of
the flare. This work was supported by NASA and NSF.
Title: Estimating Changes in Coronal Connection Fluxes Due to
Emergence and Submergence
Authors: Tarr, Lucas; Longcope, D.
Bibcode: 2010AAS...21640506T
Altcode: 2010BAAS...41..890T
The Minimum Current Corona (MCC) model provides a way to estimate
stored coronal energy using the number of field lines connecting
regions of positive and negative photospheric flux. This information
is quantified by the net flux connecting pairs of opposing regions
in a connectivity matrix. Changes in the coronal magnetic field, due
processes such as magnetic reconnection, manifest themselves as changes
in the connectivity matrix. However, the connectivity matrix will also
change when sources emerge or submerge through the photosphere, as often
happens in active regions. We have developed an algorithm to estimate
the changes in flux due to emergence and submergence of magnetic flux
sources. These estimated changes must be removed in order to quantify
storage and release of magnetic energy in the corona. To perform this
calculation over extended periods of time, we must additionally have a
consistently labeled connectivity matrix over the entire observational
timespan. We have therefore developed an automated tracking algorithm
to generate a consistent connectivity matrix as the photospheric source
regions evolve over time. This work was supported NASA LWS.
Title: Plasma Heating and Thermal Fronts Following Localized and
Impulsive Reconnection in the Solar Corona
Authors: Guidoni, Silvina; Longcope, D.
Bibcode: 2010AAS...21632004G
Altcode: 2010BAAS...41Q.911G
We present a new model of post-reconnection flux tube
dynamics. Reconnection is assumed to occur across a short-lived
and localized region in a Green-Syrovatskii current sheet with a
guide field. The reconnected field lines form two V-shaped flux
tubes whose sharp initial angle decomposes into two rotational
discontinuities (RDs) that move along the tube's legs at the local
Alfven speed, rotating and super-sonically accelerating plasma toward
the center. These two colliding flows generate gas dynamic shocks
(GDSs) that move outwardly from the center of the tube, heating the
plasma. The energy conversion in this model occurs in two steps,
converting magnetic energy first to kinetic energy at the RDs. The
kinetic energy is then partially converted to thermal energy at the
GDSs. This post-reconnection evolution is independent of reconnection
mechanism. We have included the effect of viscosity and thermal
conduction, including strong temperature dependence and field-aligned
anisotropy. We developed a computer program, called DEFT, that simulates
the dynamics of the reconnected flux tubes, including these transport
effects thereby resolving the inner structure of the GDSs. We predict
for the first time the length of the thermal fronts that extend in
front of the GDSs, capable of driving chromospheric evaporation. We also
determine the spatial and temporal variations of density and temperature
along the reconnected flux tubes, and generate synthetic differential
emission measure and emission measure profiles that can be compared to
observations. We simulate the phase of evolution where the retraction
ceases and the unconfined, high-temperature plug disassembles itself
under its own pressure. This work was supported by the NSF.
Title: Evaporation from Reconnection-driven Heat Fronts
Authors: Brannon, Sean; Longcope, D.
Bibcode: 2010AAS...21640509B
Altcode: 2010BAAS...41..890B
During the post-reconnection retraction of a flaring loop, material
confined by the magnetic field is rapidly compressed, forming a
shock wave that propagates down the loop. This shock wave, which is
similar to a one-dimensional piston shock, eventually encounters the
higher-density chromosphere at the footpoints of the loop. Due to the
large field-aligned thermal conductivity, a heat front progresses ahead
of the shock, reaching the chromosphere first and driving evaporative
backflow. This research presented here investigates the effects of
this backflow on the main shock, and under what conditions the shock
front is significantly affected by the evaporation. This work is
supported by the NSF and DOE.
Title: Evolution of Coronal Helicity in a Twisted Emerging Active
Region
Authors: Ravindra, B.; Longcope, D. W.
Bibcode: 2010ASSP...19..448R
Altcode: 2010mcia.conf..448R
Active-region magnetic fields are believed to be generated near the
shear layer of the convection zone by dynamo processes. These magnetic
fields are concentrated into fluxtubes, which rise, due to buoyancy,
through the convection zone to appear in the form of bipoles at the
photosphere. Thin-fluxtube simulations suggest that active regions
require twist to emerge. All regions are observed to emerge with some
twist; some of them show larger twist than others. A theoretical model
[Longcope andWelsch 2000, ApJ, 545, 1089] predicts that an emerging
fluxtube injects helicity into the corona for one or two days after
its initial emergence through rotation of its footpoints driven
by magnetic torque. There have been very few observational studies
of helicity injection into the corona by emerging flux. This paper
presents a study of helicity during the emergence of active region
NOAA 8578. The time history of spin helicity injection, related to
footpoint rotation, suggests that an Alfvén wave packet crossed the
apex of the emerging fluxtube.
Title: Reconnection outflows and current sheet observed with
Hinode/XRT in the April 9 2008 "Cartwheel CME" flare
Authors: McKenzie, David; Savage, Sabrina; Reeves, Kathy; Forbes,
Terry; Longcope, Dana
Bibcode: 2010cosp...38.1952M
Altcode: 2010cosp.meet.1952M
The "Cartwheel CME" event of April 9, 2008, presents an exemplary
view of motions and structures consistent with the commonly held view
of magnetic reconnection. Following the eruption of the coronal mass
ejection from the southwest limb of the Sun, an X-ray-bright arcade
was observed to form. Hinode/XRT observed coronal loops shrinking into
the top of the arcade, consistent with the reconnection picture. We
present measurements of the speeds and decelerations of these
shrinking loops, and demonstrate that the measurements are consistent
with those determined for supra-arcade downflows in other eruptive
flares. Moreover, we present evidence for a possible current sheet
observed during this flare, extending between the CME and the flare
arcade. The shrinking loops, and also outflows moving radially away
from the Sun, were observed to move along this current sheet. Finally,
we show a correlation between the anti-Sunward reconnection outflows
observed with XRT and identifiable features in the CME observed
with LASCO. This work is supported by NASA contracts NNM07AB07C and
NNX08AG44G, and NSF con-tract ATM-0837841.
Title: Reconstructing the Local Twist of Coronal Magnetic Fields and
the Three-Dimensional Shape of the Field Lines from Coronal Loops
in Extreme-Ultraviolet and X-Ray Images
Authors: Malanushenko, A.; Longcope, D. W.; McKenzie, D. E.
Bibcode: 2009ApJ...707.1044M
Altcode: 2009arXiv0909.5141M
Nonlinear force-free fields are the most general case of force-free
fields, but the hardest to model as well. There are numerous methods
of computing such fields by extrapolating vector magnetograms from the
photosphere, but very few attempts have so far made quantitative use of
coronal morphology. We present a method to make such quantitative use
of X-ray and EUV images of coronal loops. Each individual loop is fit
to a field line of a linear force-free field, allowing the estimation
of the field line's twist, three-dimensional geometry, and the field
strength along it. We assess the validity of such a reconstruction
since the actual corona is probably not a linear force-free field,
and that the superposition of linear force-free fields is generally
not itself a force-free field. To do so, we perform a series of tests
on nonlinear force-free fields, described in Low & Lou. For model
loops we project field lines onto the photosphere. We compare several
results of the method with the original field, in particular the
three-dimensional loop shapes, local twist (coronal α), distribution
of twist in the model photosphere, and strength of the magnetic
field. We find that (1) for these trial fields, the method reconstructs
twist with a mean absolute deviation of at most 15% of the range of
photospheric twist, (2) heights of the loops are reconstructed with a
mean absolute deviation of at most 5% of the range of trial heights,
and (3) the magnitude of non-potential contribution to a photospheric
field is reconstructed with a mean absolute deviation of at most 10%
of the maximal value.
Title: The Density of Coronal Null Points from Hinode and MDI
Authors: Longcope, D.; Parnell, C.; DeForest, C.
Bibcode: 2009ASPC..415..178L
Altcode: 2009arXiv0901.0865L
Magnetic null points can be located numerically in a potential field
extrapolation or their average density can be estimated from the
Fourier spectrum of a magnetogram. We use both methods to compute the
null point density from a quiet Sun magnetogram made with Hinode's NFI
and from magnetograms from SOHO's MDI in both its high-resolution and
low-resolution modes. All estimates of the super-chromospheric column
density (z>1.5 Mm) agree with one another and with the previous
measurements: 3×10-3 null points per square Mm of solar
surface.
Title: Heliophysics Science
Authors: Austin, M.; Schrjver, K.; Siscoe, G. L.; Bhattacharjee, A.;
Longcope, D. W.; Sojka, J. J.; Guhathakurta, M.
Bibcode: 2009AGUFMSH13B1511A
Altcode:
HELIOPHYSICS SUMMER SCHOOLS: NASA Living With a Star and the University
Corporation for Atmospheric Research, Visiting Scientist Programs
sponsor the Heliophysics Summer Schools to build this new field
of science. The series of summer schools, started in 2007, help
graduate students and scientists learn and develop the science of
heliophysics as a broad, coherent discipline that reaches in space
from the Earth's troposphere to the depths of the Sun, and in time
from the formation of the solar system to the distant future. The
first three years of the school will result in three textbooks for
use at universities worldwide. Subsequent years will both teach other
generations of students and develop the complementary materials that
support teaching of heliophysics at both graduate and undergraduate
levels. Heliophysics is a developing scientific discipline integrating
studies of the Sun’s variability, the surrounding heliosphere, and
climatic environments. Over the past few centuries, our understanding
of how the Sun drives space weather and climate on the Earth and other
planets has advanced at an ever-increasing rate. The first textbook,
(Heliophysics I Plasma Physics of the Local Cosmos, edited by Carolus
J. Schrijver, Lockheed Martin and George L. Siscoe, Boston University)
integrates such diverse topics for the first time as a coherent
intellectual discipline. It emphasizes the physical processes coupling
the Sun and Earth, allowing insights into the interaction of the solar
wind and radiation with the Earth’s magnetic field, atmosphere and
climate system. It provides a core resource for advanced undergraduates
and graduates, and also constitutes a foundational reference for
researchers in heliophysics, astrophysics, plasma physics, space
physics, solar physics, aeronomy, space weather, planetary science and
climate science. HELIOPHYSICS POSTDOCTORAL PROGRAM: Hosts and applicants
are invited to apply to a new postdoctoral fellowship program designed
to train the next generation of researchers in the emerging field of
Heliophysics. Two major topics of focus for LWS are the science of space
weather and of the Sun-climate connection. Preference will be given to
applicants whose proposed research addresses one of these two foci;
but any research program relevant to LWS will be considered. Since
the goal of this fellowship program is to train Sun-Earth system
researchers, preference will also be given to research projects that
cross the traditional Heliophysics subdomains of the Sun, heliosphere,
magnetosphere, and ionosphere/upper atmosphere, as well as Sun-climate
investigations. Host institutions and mentoring scientists will play
critical roles. Hosts may post information about their research on a
central database for this program: www.vsp.ucar.edu/HeliophysicsScience.
Title: Measuring Coronal Magnetic Twist Injected by Photospheric
Rotation
Authors: Malanushenko, A.; Yusuf, M.; Longcope, D. W.
Bibcode: 2009AGUFMSH23B1537M
Altcode:
Measuring the twist of the coronal magnetic field is important for
understanding and predicting solar flares. The studies of instabilities
in the past decades suggest a relation between solar flares and
instabilities, such as the external kink mode, driven by excessive
twist. We study the buildup of twist in an emerging and rapidly
rotating active region (AR 9002) using the technique developed by
Malanushenko et al. (2009). This uses EUV coronal images, from TRACE,
and line-of-sight magnetograms, from MDI, to infer properties of the
coronal magnetic field, including its local twist parameter alpha. We
find that the twist of AR 9002 does not change with time, while twist
of emerging AR 9004 starts left handed and becomes, after 80 hours,
right handed. We compare the change rate of twist for AR 9004 to the
predicted rate given the simple model of braiding and spinning flux tube
and demonstrate the general agreement of the two. We also characterize
the coronal twist of the flux interconnecting the two regions which is
produced through reconnection. This work was supported by NASA and NSF.
Title: Coronal Loop Expansion Properties Explained Using Separators
Authors: Plowman, Joseph E.; Kankelborg, Charles C.; Longcope, Dana W.
Bibcode: 2009ApJ...706..108P
Altcode: 2009arXiv0903.3430P
One puzzling observed property of coronal loops is that they are of
roughly constant thickness along their length. Various studies have
found no consistent pattern of width variation along the length of
loops observed by TRACE and SOHO. This is at odds with expectations
of magnetic flux tube expansion properties, which suggests that
loops are widest at their tops, and significantly narrower at their
footpoints. Coronal loops correspond to areas of the solar corona
which have been preferentially heated by some process, so this
observed property might be connected to the mechanisms that heat
the corona. One means of energy deposition is magnetic reconnection,
which occurs along field lines called separators. These field lines
begin and end on magnetic null points, and loops forming near them can
therefore be relatively wide at their bases. Thus, coronal energization
by magnetic reconnection may replicate the puzzling expansion properties
observed in coronal loops. We present results of a Monte Carlo survey
of separator field line expansion properties, comparing them to the
observed properties of coronal loops.
Title: Sunspot Rotation, Flare Energetics, and Flux Rope Helicity:
The Eruptive Flare on 2005 May 13
Authors: Kazachenko, Maria D.; Canfield, Richard C.; Longcope, Dana
W.; Qiu, Jiong; Des Jardins, Angela; Nightingale, Richard W.
Bibcode: 2009ApJ...704.1146K
Altcode:
We use the Michelson Doppler Imager and TRACE observations of
photospheric magnetic and velocity fields in NOAA 10759 to build a
three-dimensional coronal magnetic field model. The most dramatic
feature of this active region is the 34° rotation of its leading
polarity sunspot over 40 hr. We describe a method for including such
rotation in the framework of the Minimum Current Corona model. We
apply this method to the buildup of energy and helicity associated
with the eruptive flare of 2005 May 13. We find that including
the sunspot rotation almost triples the modeled flare energy
(1.0 × 1031 erg) and flux rope self-helicity (-7.1 ×
1042 Mx2). This makes the results consistent with
observations: the energy derived from GOES is 1.0 × 1031
erg, the magnetic cloud helicity from WIND is -5 × 1042
Mx2. Our combined analysis yields the first quantitative
picture of the helicity and energy content processed through a flare
in an active region with an obviously rotating sunspot and shows that
rotation dominates the energy and helicity budget of this event.
Title: Additive Self-helicity as a Kink Mode Threshold
Authors: Malanushenko, A.; Longcope, D. W.; Fan, Y.; Gibson, S. E.
Bibcode: 2009ApJ...702..580M
Altcode: 2009arXiv0909.4959M
In this paper, we propose that additive self-helicity, introduced
by Longcope and Malanushenko, plays a role in the kink instability
for complex equilibria, similar to twist helicity for thin flux
tubes. We support this hypothesis by a calculation of additive
self-helicity of a twisted flux tube from the simulation of Fan and
Gibson. As more twist gets introduced, the additive self-helicity
increases, and the kink instability of the tube coincides with the
drop of additive self-helicity, after the latter reaches the value
of HA /Φ2 ≈ 1.5 (where Φ is the flux of the
tube and HA is the additive self-helicity). We compare the
additive self-helicity to twist for a thin subportion of the tube
to illustrate that HA /Φ2 is equal to the
twist number, studied by Berger and Field, when the thin flux tube
approximation is applicable. We suggest that the quantity HA
/Φ2 could be treated as a generalization of a twist number,
when the thin flux tube approximation is not applicable. A threshold on
a generalized twist number might prove extremely useful studying complex
equilibria, just as the twist number itself has proven useful studying
idealized thin flux tubes. We explicitly describe a numerical method
for calculating additive self-helicity, which includes an algorithm
for identifying a domain occupied by a flux bundle and a method of
calculating potential magnetic field confined to this domain. We also
describe a numerical method to calculate twist of a thin flux tube,
using a frame parallelly transported along the axis of the tube.
Title: Sunspot Rotation, Flare Energetics and Flux Rope Helicity:
The Eruptive Flare on 2005 May 13
Authors: Kazachenko, Maria D.; Canfield, Richard C.; Longcope, Dana
W.; Qiu, Jiong; DesJardins, Angela; Nightingale, Richard W.
Bibcode: 2009shin.confE..53K
Altcode:
We use MDI and TRACE observations of photospheric magnetic and elocity
fields in NOAA 10759 to build a three-dimensional coronal magnetic
field model. The most dramatic feature of this active region is the
34 degree rotation of its leading polarity sunspot over 40 hours. We
describe a method for including such rotation in the framework of
the minimum current corona (MCC) model. We apply this method to the
buildup of energy and helicity associated with the eruptive flare
of 2005 May 13. We find that including the sunspot rotation almost
triples the modeled flare energy (-1.0 ×10^{31}ergs) and flux rope self
helicity (-7.1 ×10^{42}, Mx^2). This makes the results consistent with
observations: the energy derived from GOES is -1.0×10^{31} ergs, the
magnetic cloud helicity from WIND is -5 ×10^{42}, Mx^2. Our combined
analysis yields the first quantitative picture of the helicity and
energy content processed through a flare in an active region with
an obviously rotating sunspot and shows that rotation dominates the
energy and helicity budget of this event.
Title: Magnetic field topology
Authors: Longcope, Dana W.
Bibcode: 2009hppl.book...77L
Altcode:
No abstract at ADS
Title: Patchy reconnection in a Y-type current sheet
Authors: Linton, M. G.; DeVore, C. R.; Longcope, D. W.
Bibcode: 2009EP&S...61..573L
Altcode: 2007arXiv0712.1235L; 2009EP&S...61L.573L
We study the evolution of the magnetic field in a Y-type current
sheet subject to a brief, localized magnetic reconnection event. The
reconnection produces up- and down-flowing reconnected flux tubes
which rapidly decelerate when they hit the Y-lines and underlying
magnetic arcade loops at the ends of the current sheet. This localized
reconnection outflow followed by a rapid deceleration reproduces
the observed behavior of post-CME downflowing coronal voids. These
simulations support the hypothesis that these observed coronal downflows
are the retraction of magnetic fields reconnected in localized patches
in the high corona.
Title: Analysis of Magnetic Reconnection Sequence: from 2d to 3d
Authors: Qiu, Jiong; Longcope, D. W.
Bibcode: 2009SPD....40.2003Q
Altcode:
Magnetic reconnection, which governs explosive energy release in solar
flares, is 3-dimensional by nature. One major challenge in the field has
always been to find a useful quantitative description of observations
that can relate to theoretic models of magnetic reconnection. In this
study, we analyze the temporal and spatial evolution of UV ribbons
of an X2.0 flare observed on 2004 November 7 to infer 3D evolution
of magnetic reconnection in the corona. First, our analysis reveals
macroscopically two distinctive stages of magnetic reconnection
(e.g. Moore et al. 2001), namely parallel elongation and perpendicular
expansion of flare ribbons with respect to the polarity inversion line
(PIL). Elongation of flare ribbons along the PIL during the first stage
proceeds at apparent maximum speeds comparable with the Alfven speed in
the active region chromosphere, and the apparent perpendicular expansion
speed is a fraction of the local Alfven speed. The two stages are also
marked by a clear division in reconnection rate and energy release
rate. Furthermore, we employ a new method, the reconnection sequence
analysis, to determine the connectivity and reconnection flux during
the flare between a dozen magnetic sources defined from partitioning
the photospheric magnetogram. The method can pick up pairs of magnetic
cells that are reconnecting in a sequential manner. The observationally
derived reconnection sequence and cell-wise reconnection fluxes
are compared with computations by a topological model of magnetic
reconnection, yielding reasonable agreement. Such analysis produces
physical quantities directly comparable with topological models,
thus is promising to provide observational constraints to justify
subsequent calculation of helicity transfer and energy release from
the model. This work is supported by NSF grants ATM-0603789 and
ATM-0748428 to Montana State University.
Title: Inferring Local Twist of the Coronal Magnetic Field from
Coronal Loops in EUV and X-ray Images
Authors: Malanushenko, Anna; Longcope, D. W.; McKenzie, D. E.
Bibcode: 2009SPD....40.2902M
Altcode:
Non-linear force-free fields are the most general case of force-free
fields, but the hardest to model as well. There are numerous methods
of computing such fields by extrapolating vector magnetograms from
the photosphere, but very few attempts have so far made quantitative
use of coronal morphology. We present an improved method which infers
properties of the force-free field from X-Ray and EUV images of active
region coronal loops. Each loop evident in an image is fit to field
lines from constant-alpha fields. Our algorithm thereby estimates
the three-dimensional geometry of each loop as well its local twist
(alpha) and the magnetic field strength over its length. We assess
the performance of this method by applying it to known examples of
3D non-linear force free fields. We demonstrate that at least some
features of the local twist distribution could be reconstructed using
this method.
Title: Estimating Changes in Connection Fluxes Due to Emergence
or Submergence
Authors: Tarr, Lucas; Longcope, D.
Bibcode: 2009SPD....40.0901T
Altcode:
The Minimum Current Corona (MCC) model provides a way to estimate
stored coronal energy using the number of field lines connecting
regions of positive and negative photospheric flux. This information
is quantified by the net flux connecting pairs of opposing regions
in a connectivity matrix. Changes in the coronal magnetic field, due
processes such as magnetic reconnection, manifest themselves as changes
in the connectivity matrix. However, the connectivity matrix will
also change when sources emerge or submerge through the photosphere,
as often happens in active regions. We have developed an algorithm
to estimate the changes in flux due to emergence and submergence of
magnetic flux sources. These estimated changes must be removed in order
to quantify storage and release of magnetic energy in the corona. The work was supported NASA LWS.
Title: Plasma Heating by Gas-Dynamic Shocks in Thin Post-reconnection
Flux Tubes
Authors: Guidoni, Silvina; Longcope, D. W.
Bibcode: 2009SPD....40.2001G
Altcode:
Magnetic Reconnection initiates solar eruptions like flares and Coronal
Mass Ejections. In models of spatially and temporally localized
reconnection, called "Patchy Reconnection", magnetic energy is
converted into kinetic energy, as rapidly as observations suggest. In
this process, straight field lines forming an angle on opposite sides
of a current sheet, reconnect across a patch. Once reconnected, this
bundle of field lines forms two V-shaped thin flux tubes, and magnetic
tension at their cusps causes them to retract. For the first time,
we demonstrate the development of gas-dynamic shocks, GDSs, in these
post-reconnection flux tubes. We introduce modified thin flux tube
equations that account for dynamics parallel to the magnetic field,
where the only relevant force is thermal pressure gradient. The
shortening of the retracting tubes leads to compressive supersonic
parallel flows that develop into GDSs that can heat the plasma up to
observed temperatures ( 20 MK on top of post-flare arcades). In
the solar corona, viscosity and thermal conductivity are large along
the magnetic field. We developed a code, called DEFT, that simulates
the retraction of the two thin reconnected tubes, and includes
these transport coefficients, as well as their strong dependence on
temperature ( T5/2). Simulations are carried out using real
coronal parameters. As the flux tubes retract, they follow a time
dependent evolution until they reach the theoretical steady state
jump conditions. For high Mach numbers and low Prandtl numbers, the
internal structure of the GDSs includes an isothermal sub-shock with
thickness governed by viscosity, and a second region where temperature
increases and the entropy of the plasma achieves a maximum value. This work was supported by NASA grant LWS05-0032, and NSF.
Title: Energetics of Reconnection: A Comparison of Steady and
Transient Models in 1, 2 and 3 Dimensions
Authors: Longcope, Dana; Guidoni, S. E.; Linton, M. G.
Bibcode: 2009SPD....40.3704L
Altcode:
In Petschek's original steady-state model, anti-parallel magnetic fields
are reconnected along an infinite line of enhanced electric field. Away
from the reconnection line, a set of standing slow mode shocks reduce
the magnetic field strength, converting magnetic energy directly
into thermal and kinetic energies in comparable measure. This picture
changes when the reconnecting fields are not perfectly anti-parallel
but meet at some angle. In this case the magnetic energy release can
be attributed to shortening field lines instead of, or in addition to,
reducing field strength. The shortening occurs at intermediate shocks
where energy is converted entirely into kinetic energy of bulk flows
directed partly toward the center of the shortened portion of the field
line. These inward flows collide in slow mode shocks or gas-dynamic
shocks where their kinetic energy is partially thermalized. When
the initial beta is very low, as it would be in the corona, the
heated plasma occupies a small fraction of the shortened section. As
a consequence, only a small fraction of released magnetic energy
is ultimately thermalized, except in cases where the initial field
nearly is nearly anti-parallel. We show this energetic scenario to be
qualitatively and quantitatively consistent in models of steady and
unsteady reconnection in one, two and three dimensions, provided the
reconnecting field is not perfectly anti-parallel. Among these models,
unsteady, three-dimensional reconnection results in retracting flux
tubes similar to the ones proposed to explain supra-arcade down flows
following CMEs. This work supported by NSF and NASA
Title: Modeling the Evolving Magnetic Field in a Coronal Sigmoid
Authors: McKenzie, David Eugene; Malanushenko, A.; Longcope, D.
Bibcode: 2009SPD....40.1203M
Altcode:
The importance of coronal sigmoids as progenitors of eruptions and
mass ejections is well established. However, the storage of magnetic
energy prior to a sigmoid's eruption is difficult to quantify. While
the non-potentiality of the coronal force-free fields is clearly
responsible for the free energy, models of the field are difficult to
verify. We utilize a method, developed at Montana State University and
described at this meeting by A. Malanushenko, to model the force-free
field within a coronal sigmoid observed by TRACE and Hinode/XRT. By
modeling the twist in the sigmoid's field over the span of a few days
leading up to its eruption, it is hoped that such a model can yield
insight to the buildup of energy. This work is supported by NASA
contract NNX07AI01G, and by SAO contract SV7-77003.
Title: Sunspot Rotation, Flare Energetics and Flux Rope Helicity:
The Eruptive Flare on 2005 May 13
Authors: Kazachenko, Maria; Canfield, R. C.; Longcope, D. W.; Qiu,
J.; DesJardins, A.; Nightingale, R. W.
Bibcode: 2009SPD....40.2013K
Altcode:
We use MDI and TRACE observations of photospheric magnetic and velocity
fields in NOAA 10759 to build a three-dimensional coronal magnetic
field model. The most dramatic feature of this active region is the
34 degree rotation of its leading polarity sunspot over 40 hours. We
describe a method for including such rotation in the framework of
braiding and spinning in a magnetic charge topology (MCT) model. We
apply this method to the buildup of energy and helicity associated with
the eruptive flare of 2005 May 13. We find that adding rotation almost
triples the modeled flare energy (-1.0×1031ergs) and flux
rope self helicity (-7.1×1042 Mx2). This makes
the results consistent with observations: the energy derived from GOES
is -1.0×1031ergs, the magnetic cloud helicity from WIND is
-5×1042 Mx2. Our combined analysis yields the
first quantitative picture of the helicity and energy content processed
through a flare in an active region with an obviously rotating sunspot
and shows that rotation dominates the energy and helicity budget of
this event.
Title: Reconnection in Three Dimensions: The Role of Spines in Three
Eruptive Flares
Authors: Des Jardins, Angela; Canfield, Richard; Longcope, Dana;
Fordyce, Crystal; Waitukaitis, Scott
Bibcode: 2009ApJ...693.1628D
Altcode: 2009ApJ...693.1628J
In order to better understand magnetic reconnection and particle
acceleration in solar flares, we compare the RHESSI hard X-ray
(HXR) footpoint motions of three flares with a detailed study of the
corresponding topology given by a Magnetic Charge Topology model. We
analyze the relationship between the footpoint motions and topological
spine lines and find that the examined footpoint sources move along
spine lines. We present a three-dimensional topological model in
which this movement can be understood. As reconnection proceeds,
flux is transferred between the reconnecting domains, causing the
separator to move. The movement of the separator's chromospheric ends,
identified with the HXR footpoints, is along those spine lines on
which the separator ends.
Title: Signatures of Magnetic Stress Prior to Three Solar Flares
Observed by RHESSI
Authors: des Jardins, Angela; Canfield, Richard; Longcope, Dana;
McLinden, Emily; Dillman, Amanda
Bibcode: 2009ApJ...693..886D
Altcode: 2009ApJ...693..886J
We examine the hard X-ray (HXR) footpoint sources of three flares,
as observed by RHESSI, in combination with the topology given by the
extrapolation of line-of-sight magnetograms into the corona. Assuming
the HXR footpoint sources are chromospheric consequences of magnetic
reconnection that takes place on separators, we further assume a
relationship between the buildup of energy in stressed coronal magnetic
fields and the measurement of the change in separator flux per unit
length. We find that the value of this quantity is larger for the
separators that connect the HXR footpoint sources than the quantity for
the separators that do not. Therefore, we conclude that we are able to
understand the location of HXR sources observed in flares in terms of
a physical and mathematical model of the topology of the active region.
Title: Effects of Partitioning and Extrapolation on the Connectivity
of Potential Magnetic Fields
Authors: Longcope, D. W.; Barnes, G.; Beveridge, C.
Bibcode: 2009ApJ...693...97L
Altcode: 2008arXiv0811.1241L
Coronal magnetic field may be characterized by how its field
lines interconnect regions of opposing photospheric flux—its
connectivity. Connectivity can be quantified as the net flux connecting
pairs of opposing regions, once such regions are identified. One
existing algorithm will partition a typical active region into a
number of unipolar regions ranging from a few dozen to a few hundred,
depending on algorithmic parameters. This work explores how the
properties of the partitions depend on some algorithmic parameters,
and how connectivity depends on the coarseness of partitioning for
one particular active region magnetogram. We find the number of
connections among them scales with the number of regions even as
the number of possible connections scales with its square. There
are several methods of generating a coronal field, even a potential
field. The field may be computed inside conducting boundaries or over
an infinite half-space. For computation of connectivity, the unipolar
regions may be replaced by point sources or the exact magnetogram may
be used as a lower boundary condition. Our investigation shows that the
connectivities from these various fields differ only slightly—no more
than 15%. The greatest difference is between fields within conducting
walls and those in the half-space. Their connectivities grow more
different as finer partitioning creates more source regions. This also
gives a quantitative means of establishing how far away conducting
boundaries must be placed in order not to significantly affect
the extrapolation. For identical outer boundaries, the use of point
sources instead of the exact magnetogram makes a smaller difference in
connectivity: typically 6% independent of the number of source regions.
Title: Gas-dynamic Shock Heating of Post-flare Loops Due to Retraction
Following Localized, Impulsive Reconnection
Authors: Longcope, D. W.; Guidoni, S. E.; Linton, M. G.
Bibcode: 2009ApJ...690L..18L
Altcode: 2008arXiv0810.3661L
We present a novel model in which field lines shortening after
localized, three-dimensional reconnection heat the plasma as they
compress it. The shortening progresses away from the reconnection
site at the Alfvén speed, releasing magnetic energy and generating
parallel, compressive flows. These flows, which are highly supersonic
when β Lt 1, collide in a pair of strong gas-dynamic shocks at which
both the mass density and temperature are raised. Reconnecting field
lines initially differing by more that 100° can produce a concentrated
knot of plasma hotter that 20 MK at the loop's apex, consistent with
observations. In spite of these high temperatures, the shocks convert
less than 10% of the liberated magnetic energy into heat—the rest
remains as kinetic energy of bulk motion. These gas-dynamic shocks
arise only when the reconnection is impulsive and localized in all
three dimensions; they are distinct from the slow magnetosonic shocks
of the Petschek steady-state reconnection model.
Title: The Number of Magnetic Null Points in the Quiet Sun Corona
Authors: Longcope, D. W.; Parnell, C. E.
Bibcode: 2009SoPh..254...51L
Altcode: 2008SoPh..tmp..185L; 2008arXiv0811.0097L
The coronal magnetic field above a particular photospheric region
will vanish at a certain number of points, called null points. These
points can be found directly in a potential field extrapolation
or their density can be estimated from the Fourier spectrum of the
magnetogram. The spectral estimate, in which the extrapolated field
is assumed to be random and homogeneous with Gaussian statistics,
is found here to be relatively accurate for quiet Sun magnetograms
from SOHO's MDI. The majority of null points occur at low altitudes,
and their distribution is dictated by high wavenumbers in the Fourier
spectrum. This portion of the spectrum is affected by Poisson noise,
and as many as five-sixths of null points identified from a direct
extrapolation can be attributed to noise. The null distribution above
1500 km is found to depend on wavelengths that are reliably measured
by MDI in either its low-resolution or high-resolution mode. After
correcting the spectrum to remove white noise and compensate for
the modulation transfer function we find that a potential field
extrapolation contains, on average, one magnetic null point, with
altitude greater than 1.5 Mm, above every 322 Mm2 patch of
quiet Sun. Analysis of 562 quiet Sun magnetograms spanning the two
latest solar minima shows that the null point density is relatively
constant with roughly 10% day-to-day variation. At heights above 1.5 Mm,
the null point density decreases approximately as the inverse cube of
height. The photospheric field in the quiet Sun is well approximated as
that from discrete elements with mean flux «|φ|»=1.0×1019
Mx distributed randomly with density n=0.007 Mm−2.
Title: Erratum: "Tests and Comparisons of Velocity-Inversion
Techniques" (ApJ, 670, 1434 [2007])
Authors: Welsch, B. T.; Abbett, W. P.; DeRosa, M. L.; Fisher, G. H.;
Georgoulis, M. K.; Kusano, K.; Longcope, D. W.; Ravindra, B.; Schuck,
P. W.
Bibcode: 2008ApJ...680..827W
Altcode:
No abstract at ADS
Title: Quantitative modeling of magnetic reconnection creating a
twisted flux rope
Authors: Longcope, D.
Bibcode: 2008AGUSMSH31C..02L
Altcode:
Coronal mass ejections (CMEs) are one of the key elements coupling
solar activity throughout the heliosphere. A two-dimensional model,
called CSHKP has been used to explain the observed sequence of
events in a typical CME. They are launched from the solar corona
above reversals in photospheric magnetic polarity. Chromospheric
emission during the accompanying two-ribbon flare provides evidence of
magnetic reconnection. Magnetic clouds, structures observed in situ,
are believed to be twisted magnetic flux ropes launched into space
by the CME. Here I demonstrate a three-dimensional generalization of
the CSHKP model. This model provides a quantitative picture of how
reconnection in a two-ribbon flare can produce a twisted flux rope
from an arcade of slightly sheared coronal field lines. It quantifies
relationships between the initial shear, the amount of flux reconnected
and the total axial flux in the twisted rope. The model predicts
reconnection occurring in a sequence which progresses upward even if
the reconnection sites themselves do not move. This work was supported
by NSF through grant ATM-0416340.
Title: Modeling and measuring the flux and helicity ejected by the
two-ribbon flare on 2005-05- 13
Authors: Kazachenko, M.; Canfield, R. C.; Longcope, D. W.; Qiu, J.
Bibcode: 2008AGUSMSP43A..08K
Altcode:
In this work we study an eruptive flare event in order to quantitatively
understand flux and helicity transfer through reconnection in the
associated coronal flux rope formation. Quantitative observational
studies of solar flare reconnection have been made by a number of
research groups (Poletto and Kopp, 1986, Fletcher and Hudson 2001,
Qiu and Yurchyshyn, 2005). However, their work does not allow one
to predict, from observations, both the flux and twist that coronal
reconnection contributes to interplanetary flux ropes. Our hypothesis
is that the flux and helicity associated with eruptive solar flares
is created through a sequence of magnetic reconnections. Longcope
et al. (2007) introduced a topological method of studying flux rope
formation in 3D. In our study we use MDI, SOHO and TRACE data. We
apply two methods to derive the reconnection sequence: (1) the flaring
sequence from the TRACE observations of ribbon brightening (Qiu et al,
2007) and (2) the reconnection sequence from the topological model of
the coronal field based on the Minimum Current Corona model (Longcope,
2001). From the topological model we calculate the toroidal flux and
total helicity of the flux rope. We study in detail the M8.0 flare
in active region NOAA 10759, 13-May-2005 16:13UT. The total magnetic
flux of the active region is 1.7 ·10 22 Mx. From the photospheric
magnetogram evolution we determine that 1.9 · 1042 Mx2 of magnetic
helicity has been injected into the active region during the 40-hour
build-up prior to the flare. From the domain flux evolution we show that
reconnection in the model (2) must occur in a specific sequence which
would produce a twisted flux rope containing significantly less flux
and helicity than the whole active region. The reconnection sequence
from the model (2) compares favorably with the one inferred from the
observation sequence (1). However, the topological analysis might be
sensitive to the beginning of the buildup time, so its choice will
be discussed.
Title: Inferring Photospheric Velocity Fields Using a Combination of
Minimum Energy Fit, Local Correlation Tracking, and Doppler Velocity
Authors: Ravindra, B.; Longcope, D. W.; Abbett, W. P.
Bibcode: 2008ApJ...677..751R
Altcode:
The minimum energy fit (MEF), a velocity inversion technique, infers
all components of the photospheric velocity that are consistent with
the induction equation. From the set of consistent velocity fields,
it selects the smallest overall flow speed by minimizing a kinetic
energy functional. If partial velocity information is available from
other measurements, it can be incorporated into the MEF methodology
by minimizing the squared difference from that data. We incorporate
the partial velocity information provided by local correlation
tracking (LCT) technique and Doppler velocity measurements. We
test the incorporation of these auxiliary velocity fields using the
simulated magnetograms and velocitygrams. To the known velocity field
we compare the results obtained from the MEF alone, the MEF with LCT
constraints, and the MEF with LCT and Doppler information. We find
that the combination of MEF with LCT and vertical velocity yields
the best agreement. We also apply these three methods to actual vector
magnetograms of AR 8210 obtained by the Imaging Vector Magnetograph. The
results suggest that in this active region the helicity and energy
fluxes are dominated by the horizontal rather than the vertical
components of the velocity.
Title: Inductive magnetic footpoint tracking by combining the minimum
energy fit with the local correlation tracking and doppler velocity
Authors: Ravindra, B.; Longcope, D. W.
Bibcode: 2008JApA...29...63R
Altcode:
No abstract at ADS
Title: Defining and Calculating Self-Helicity in Coronal Magnetic
Fields
Authors: Longcope, D. W.; Malanushenko, A.
Bibcode: 2008ApJ...674.1130L
Altcode:
We introduce two different generalizations of relative helicity which
may be applied to a portion of the coronal volume. Such a quantity is
generally referred to as the self-helicity of the field occupying the
subvolume. Each definition is a natural application of the traditional
relative helicity but relative to a different reference field. One
of the generalizations, which we term additive self-helicity, can be
considered a generalization of twist helicity to volumes which are
neither closed nor thin. It shares with twist the property of being
identically zero for any portion of a potential magnetic field. The
other helicity, unconfined self-helicity, is independent of the shape
of the volume occupied by the field portion and is therefore akin to
the sum of twist and writhe helicity. We demonstrate how each kind
of self-helicity may be evaluated in practice. The set of additive
self-helicities may be used as a constraint in the minimization of
magnetic energy to produce a piecewise constant-α equilibrium. This
class of fields falls into a hierarchy, along with the flux-constrained
equilibria and potential fields, of fields with monotonically decreasing
magnetic energies. Piecewise constant-α fields generally have fewer
unphysical properties than genuinely constant-α fields, whose twist
α is uniform throughout the entire corona.
Title: Signatures of Magnetic Stress Prior to Three Solar Flares
Observed by RHESSI
Authors: Des Jardins, A. C.; Canfield, R. C.; Longcope, D. W.
Bibcode: 2007AGUFMSH13A1098D
Altcode:
In order to better understand the location and evolution of magnetic
reconnection, which is thought to be the energy release mechanism in
solar flares, we combine the analysis of hard X-ray (HXR) sources
observed by RHESSI with a three-dimensional, quantitative magnetic
charge topology (MCT) model. We examine the location of reconnection
by assuming a relationship between the build-up of energy in stressed
coronal magnetic fields and the measurement of the change in separator
flux per unit length. We find that the value of this quantity is
larger on the separators that connect the HXR footpoint sources than
the value on the separators that do not. Therefore, we conclude the
MCT model gives useful insight into the relationship between sites of
HXR emission and the topology of flare productive active regions.
Title: Tests and Comparisons of Velocity-Inversion Techniques
Authors: Welsch, B. T.; Abbett, W. P.; De Rosa, M. L.; Fisher, G. H.;
Georgoulis, M. K.; Kusano, K.; Longcope, D. W.; Ravindra, B.; Schuck,
P. W.
Bibcode: 2007ApJ...670.1434W
Altcode:
Recently, several methods that measure the velocity of magnetized
plasma from time series of photospheric vector magnetograms have been
developed. Velocity fields derived using such techniques can be used
both to determine the fluxes of magnetic energy and helicity into the
corona, which have important consequences for understanding solar
flares, coronal mass ejections, and the solar dynamo, and to drive
time-dependent numerical models of coronal magnetic fields. To date,
these methods have not been rigorously tested against realistic,
simulated data sets, in which the magnetic field evolution and
velocities are known. Here we present the results of such tests
using several velocity-inversion techniques applied to synthetic
magnetogram data sets, generated from anelastic MHD simulations of
the upper convection zone with the ANMHD code, in which the velocity
field is fully known. Broadly speaking, the MEF, DAVE, FLCT, IM, and
ILCT algorithms performed comparably in many categories. While DAVE
estimated the magnitude and direction of velocities slightly more
accurately than the other methods, MEF's estimates of the fluxes of
magnetic energy and helicity were far more accurate than any other
method's. Overall, therefore, the MEF algorithm performed best in
tests using the ANMHD data set. We note that ANMHD data simulate
fully relaxed convection in a high-β plasma, and therefore do not
realistically model photospheric evolution.
Title: Three Dimensional Structure and Time Evolution of a Transition
Region Explosive Event Observed in He II
Authors: Fox, J. L.; Kankelborg, C. C.; Thomas, R. J.; Longcope, D.
Bibcode: 2007AGUFMSH22A0840F
Altcode:
Transition Region Explosive Events (TREEs) have been observed with
slit spectrographs since at least 1975, most commonly in lines of C IV
(1548A,1550A) and Si IV (1393A, 1402A). We report what we believe to be
the first observation of a TREE in He II 304A. With the MOSES sounding
rocket, a novel type of imaging spectrograph, we are able to see the
spatial and spectral structure of the event. It consists of a bright
core expelling two jets, oppositely directed but not collinear, which
curve away from the axis of the core. The jets have both line-of-sight
and sky-plane motion. The core is a region of high non-thermal doppler
broadening, characteristic of TREEs. It is possible to resolve the
core broadening into red and blue line-of-sight components. MOSES
captured approximately 150 sec of time evolution before the rocket
flight ended. We see the beginning (core activation) and middle (jet
ejection), but not the end. It is clear from our data-set that TREEs
in He II 304A are much less common than observed in other wavelengths.
Title: A Quantitative, Topological Model of Reconnection and Flux
Rope Formation in a Two-Ribbon Flare
Authors: Longcope, D. W.; Beveridge, C.
Bibcode: 2007ApJ...669..621L
Altcode:
We present a topological model for energy storage and subsequent
release in a sheared arcade of either infinite or finite extent. This
provides a quantitative picture of a twisted flux rope produced through
reconnection in a two-ribbon flare. It quantifies relationships
between the initial shear, the amount of flux reconnected, and the
total axial flux in the twisted rope. The model predicts reconnection
occurring in a sequence that progresses upward even if the reconnection
sites themselves do not move. While some of the field lines created
through reconnection are shorter, and less sheared across the
polarity inversion line, reconnection also produces a significant
number of field lines with shear even greater than that imposed by the
photospheric motion. The most highly sheared of these is the overlying
flux rope. Since it is produced by a sequence of reconnections, the
flux rope has twist far in excess of that introduced into the arcade
through shear motions. The energy storage agrees well with previous
calculations using the full equations of magnetohydrodynamics, and
the agreement improves as the topology is defined using increasingly
finer detail. This is the first comparative study of the application
of a topological model to a continuous flux distribution. As such,
it demonstrates how the coarseness with which the photospheric flux
distribution is partitioned affects the accuracy of prediction in
topological models.
Title: Determining the Source of Coronal Helicity through Measurements
of Braiding and Spin Helicity Fluxes in Active Regions
Authors: Longcope, D. W.; Ravindra, B.; Barnes, G.
Bibcode: 2007ApJ...668..571L
Altcode:
Magnetic helicity has become a valuable tool for understanding the
energetics and dynamics of coronal magnetic fields. Recently, long
time sequences of magnetograms have been used to measure the flux of
helicity into active region coronae. We demonstrate how this helicity
flux can be usefully decomposed into contributions of differing
origin, called ``spin'' helicity and ``braiding'' helicity. These
contributions could be envisioned to come at the expense of twist
and writhe helicity, respectively, of a subphotospheric flux tube
anchored to the regions. In order to effect this decomposition, each
magnetogram is partitioned into a set of unipolar regions. We present
a method of defining such regions so that they persist through the
sequences and track the photospheric flow. The spin helicity of a
given region quantifies the mean rotation rate of motions internal
to that region, while braiding helicity is injected by the motions
of whole regions about one another. Applying the method to six active
regions shows cases where either spin or braiding dominates, and where
they have the same signs and opposite signs. Thus, it would seem that
no general statement can be made regarding the dominance of twist or
writhe in supplying helicity to the corona. In one particular case,
spin and braiding helicity follow different time histories but inject
equal and opposite net helicities. This suggests that the spinning and
braiding are driven by a kink instability in the submerged flux tube.
Title: Helicity as the Ultimate Test to the Surface Dynamo Problem
Authors: Pevtsov, A. A.; Longcope, D. W.
Bibcode: 2007ASPC..369...99P
Altcode:
It has become widely accepted that large-scale magnetic structures
on the Sun, such as active regions, are the product of a dynamo of
periodicity approximately 22 years situated at or near the base of
the convection zone. There has been speculation that the intermixed,
small-scale photospheric magnetic field is generated by a second
dynamo operating at or near the solar surface. Numerical simulations
have shown that such dynamo could work, although it would not be as
effective in generating flux as the more conventional deep-seated
dynamo. Since they are driven by flows of different sizes operating
on different time scales, the magnetic fields generated by these two
dynamos should be quantitatively different. In particular, there are
well-studied helical trends in the large-scale magnetic which could be
imprinted on them by the deep, slow flows of the dynamo which generates
them; these helical trends would be absent from a field generated by
a surface dynamo. We propose that observations of magnetic/current
helicity at very small scales can be used to establish the role of
the second, surface dynamo on the Sun.
Title: Modeling and Measuring the Flux Reconnected and Ejected by
the Two-Ribbon Flare/CME Event on 7 November 2004
Authors: Longcope, Dana; Beveridge, Colin; Qiu, Jiong; Ravindra, B.;
Barnes, Graham; Dasso, Sergio
Bibcode: 2007SoPh..244...45L
Altcode:
Observations of the large two-ribbon flare on 7 November 2004
made using SOHO and TRACE data are interpreted in terms of a
three-dimensional magnetic field model. Photospheric flux evolution
indicates that −1.4×1043 Mx2 of magnetic
helicity was injected into the active region during the 40-hour
buildup prior to the flare. The magnetic model places a lower bound
of 8×1031 ergs on the energy stored by this motion. It
predicts that 5×1021 Mx of flux would need to be reconnected
during the flare to release the stored energy. This total reconnection
compares favorably with the flux swept up by the flare ribbons, which we
measure using high-time-cadence TRACE images in 1 600 Å. Reconnection
in the model must occur in a specific sequence that would produce a
twisted flux rope containing significantly less flux and helicity
(1021 Mx and −3×1042 Mx2,
respectively) than the active region as a whole. The predicted flux
compares favorably with values inferred from the magnetic cloud observed
by Wind. This combined analysis yields the first quantitative picture of
the flux processed through a two-ribbon flare and coronal mass ejection.
Title: Active Region Magnetic Field Line Twist and Source of Coronal
Magnetic Helicity.
Authors: Belur, Ravindra; Longcope, D.; Barnes, G.; Nandy, D.
Bibcode: 2007AAS...210.2401B
Altcode: 2007BAAS...39..128B
Magnetic helicity is an important quantity which measures how the
magnetic field lines are twisted and sheared. Recently it has become
possible to measure the flux of magnetic helicity in active regions
using the observational data. These observed helicity fluxes may
arise due to the twist in the emerging active region flux tubes or it
may come from the photospheric shearing motion. Here, we decompose
the helicity flux into two different contributions called spin
and braiding. These components typically come from twist and writhe
helicity of a sub-photospheric flux tube anchored to the regions. The
spin helicity of a given region quantifies the mean rotation rate of
motion internal to that region and braiding helicity is injected by the
motions of whole regions about one another. The injected helicity
flux due to spin and braiding motion leads to the coronal magnetic
field line twist. The twist determined from vector magnetograms can be
used to estimate the total helicity content of the coronal field at one
time. The rate of change of this helicity estimate can be compared to
the total helicity flux as well as its spin and braiding component. We
make such a comparison for several active regions.
Title: Three Dimensional Structure Of A Complex Bipolar Jet:
Transition Region Imaging-spectroscopy With MOSES
Authors: Fox, Lewis; Kankelborg, C. C.; Longcope, D.
Bibcode: 2007AAS...210.9501F
Altcode: 2007BAAS...39..223F
We report on an interesting transient brightening observed in He II 304
Å above a flux-canceling magnetic bipole during the MOSES (Multi-Order
Solar EUV Spectrograph) sounding rocket flight, Feb. 8, 2006. The event
resembles a bipolar jet and has characteristics of a transition region
explosive event, but it is not a simple, linear structure at some angle
to the line-of-sight. The jets curve away from the axis of a compact,
bright core, and are not collinear. We observe transverse sky-plane
motions and infer line-of-sight doppler shifts along the length of
the jets, as well as non-thermal line broadening in the bright core,
demonstrating the utility of simultaneous imaging-spectroscopy provided
by MOSES. This work is supported by NASA LCAS and the Montana Space
Grant Consortium.
Title: Post-CME Reconnection and the Generation of Descending Solar
Coronal Voids
Authors: Linton, Mark; Longcope, D.; Warren, H.
Bibcode: 2007AAS...210.2903L
Altcode: 2007BAAS...39R.137L
Observations of solar coronal flares occurring behind coronal mass
ejections (CME's) have shown downflowing voids in the corona, which are
believed to be the signatures of descending magnetic flux tubes. We are
studying the hypothesis that these flux tubes have reconnected in the
current sheet which forms behind the CME in the high corona. We will
present three dimensional MHD simulations of a localized reconnection
event in a Y-type post-CME current sheet. The reconnected field
creates a downflow which rapidly decelerates as it hits the Y-line
and the magnetic loops below it. We will compare this deceleration
with the observed deceleration of coronal voids when they hit coronal
arcades. We will also present studies of the 3D tearing mode in this
current sheet. This tearing generates numerous localized reconnection
patches, and a cascade of reconnected fluxtubes. We will compare this
cascade with the the cascades of descending voids and coronal loops
which are seen following a CME event, providing further evidence that
the observed voids are reconnected flux tubes. This research was
supported by grants from NASA and ONR.
Title: A Quantitative, Topological Model Of Reconnection And Flux
Rope FormationIn A Two-ribbon Flare
Authors: Longcope, Dana; Beveridge, C.
Bibcode: 2007AAS...210.2924L
Altcode: 2007BAAS...39..141L
We present a topological model for energy storage and subsequent
release in a sheared arcade of either infinite or finite extent. This
provides a quantitative picture of a twisted flux rope produced through
reconnection in a two-ribbon flare. It quantifies relationships
between the initial shear, the amount of flux reconnected and
the total toroidal flux in the twisted rope. The model predicts
reconnection occurring in a sequence which progresses upward even if
the reconnection sites themselves do not move. While some of the field
lines created through reconnection are shorter, and less sheared across
the polarity inversion line, reconnection also produces a significant
number of field lines with shear even greater than that imposed by the
photospheric motion. The most highly sheared of these is the overlying
flux rope. Since it is produced by a sequence of econnections, the
flux rope has twist far in excess of that introduced into the arcade
through shear motions. The energy storage agrees well with previous
calculations using the full equations of magnetohydrodynamics, and
the agreement improves as the topology is defined using increasingly
finer detail. This is the first comparative study of the application
of a topological model to a continuous flux distribution. As such
it demonstrates how the coarseness with which the photospheric flux
distribution is partitioned affects the accuracy of prediction in
topological models. This work was supported by the National
Science Foundation
Title: Dynamics Of Post-reconnected Thin Flux Tubes
Authors: Guidoni, Silvina; Longcope, D.
Bibcode: 2007AAS...210.9327G
Altcode: 2007BAAS...39R.215G
Reconnection is a process that modifies magnetic field topologies,
releasing a large amount of magnetic energy that is converted to
particle kinetic energy. The accelerated particles are directed outward
from the reconnection region, which gives origin to Flares or Coronal
Mass Ejections. A new model, called patchy reconnection, describing
reconnection occurring within a small region of a pre-existing current
sheet of uniform strength, has been recently proposed by Linton and
Longcope (2006). We generalize this approach using current sheets of
spatially varying surface current densities (among others, Y-type and
double-Y-type current sheets are studied). We present modified equations
modeling the dynamics of thin flux tubes after being reconnected
across these different geometries. Analytical equilibrium solutions
are found, and compared to numerical simulations of the time-dependent
equations. New interesting features appear, only observable in
pure 3D geometries, like super-alfvenic motion, and density enhancement.
Title: Quantifying The Self-helicity Of A Flux Tubes
Authors: Malanushenko, Anna; Longcope, D.
Bibcode: 2007AAS...210.9110M
Altcode: 2007BAAS...39..205M
Magnetic helicity has proven to be a powerful tool for understanding
energetics of the solar corona. As it is usually defined, relative
helicity is an integral over the entire coronal volume. In this work
we consider two different generalizations by which relative helicity
of a portion of the entire coronal volume may be calculated. Such a
quantity is generally called the self helicity of the sub-volume. Each
definition is a natural application of the traditional helicity formula
but relative to different fields. One of the generalizations, which we
term "additive self-helicity", has particularly desirable properties,
such as being identically zero for any portion of a potential magnetic
field. During a flare it is believed that the total helicity of
the volume is conserved, but as reconnection transfers flux between
domains, this will change the self-helicity of those. We demonstrate
how "additive self-helicity" may be evaluated in practice to find the
self-helicities for flux systems, or domains, composed of all field
lines connecting a designated pair of photospheric source regions. It
is then possible to quantify the transfer of self-helicity which would
occur when reconnection transfers flux between flux systems.
Title: Relating RHESSI Footpoints to Reconnection: the Importance
of Spines and Separators
Authors: Des Jardins, A.; Canfield, R.; Longcope, D.; McLinden, E.;
Fordyce, C.; Waitukaitis, S.
Bibcode: 2006AGUFMSH23A0349D
Altcode:
In order to improve the understanding of both flare initiation and
evolution, we take advantage of powerful new topological methods
and the high spatial resolution of RHESSI to examine where magnetic
reconnection takes place in flare-producing solar active regions. We
use the MPOLE (http://solar.physics.montana.edu/dana/mpole/) software to
extrapolate the observed line of sight photospheric magnetic field into
the corona. MPOLE is a suite of IDL programs that implements the Minimum
Current Corona Model (Longcope 1996). Recently it has been improved to
use a hierarchy of topological features (Beveridge 2006). The coronal
extrapolation gives the location of topological features such as poles,
nulls, separatricies, separators, and spine lines. For several flares
well observed by RHESSI and MDI, we examine the locations of flare HXR
emission in the context of these topological features. Two noteworthy
relationships are found. First, when footpoints move, they move along
spine lines. Second, when separators significantly change over the
course of a flare, only those associated with the flare footpoints do
so. In this poster, we present observations supporting the relationship
between spine lines and footpoint tracks, demonstrate the importance
of separator analysis in the study of flares, explore uncertainties
in the MPOLE and RHESSI analyses, and survey possible interpretations
of the reported results. This work is supported by NASA.
Title: Topological Estimates of Free-Energy Build-up in Active Regions
Authors: Longcope, D. W.; Barnes, G.; Ravindra, B.; Beveridge, C.
Bibcode: 2006AGUFMSH31B..02L
Altcode:
There is a growing consensus that slow evolution of an active region's
photospheric flux leads to a build-up in the energy of its coronal
field. The anchoring of coronal field lines to the photosphere
defines a connectivity between photospheric footpoints of opposing
polarities. Due to the corona's extremely high electrical conductivity
these connections remain unchanged even as the footpoints move. To
estimate the energy stored this way, we group photospheric footpoints
into unipolar source regions and reduce the pointwise connectivity
map to a matrix of connections between regions. The flux in each such
connection must remain fixed even as the source regions evolve. One
coronal magnetic field, called the flux constrained equilibrium, has the
minimum possible energy for a specified connectivity. The free energy
in this equilibrium provides a lower bound on the free energy in the
actual field. We obtain such a free-energy lower bounds for several
observed active regions. Source regions are defined in a magnetogram
(MDI) time sequence of an active region. As the sources evolve the
connectivity in a potential field will change, however, the actual
connectivity will not. The growing disparity between the two is used to
estimate the free energy stored in the coronal field. Flare reconnection
will release some portion of this stored energy by changing some of
the connectivities. We compare these estimates with observational
signatures of energy release. This work supported by NASA's Living
with a Star Program and by AFOSR.
Title: Estimating Active Region Free Energy and Helicity from the
Minimum Current Corona Model
Authors: Barnes, G.; Longcope, D. W.; Beveridge, C.; Ravindra, B.;
Leka, K. D.
Bibcode: 2006IAUJD...3E..80B
Altcode:
We employ the Minimum Current Corona (MCC) model to estimate the amount
of magnetic free energy and helicity injected into the coronal magnetic
field of an active region. In the MCC model, each concentration of
photospheric magnetic flux is represented by a point source, greatly
simplifying the magnetic topology. Advecting an initial partitioning
of the flux through a long time series of magnetograms results in
a persistent set of sources. We show that the centroid velocity of
a partition compares well with the flux-weighted average over the
partition of the local correlation tracking velocities. Flux domains,
bundles of field lines interconnecting pairs of sources, are surrounded
by separatrix surfaces. The intersection of two separatrices is
a separator field line, which is the site of reconnection in this
model. The evolution of the photospheric field causes the sources
to also evolve, which would lead to changes in the domain fluxes
to maintain a potential field configuration if reconnection could
proceed rapidly. However, in the absence of reconnection, currents
begin to flow to maintain the initial distribution of domain fluxes. The
minimum energy state occurs when currents flow along the separators. The
magnitude of the separator currents can be estimated and combined with
geometrical properties of the separators to give a lower bound to
the magnetic free energy of the system. The motion of sources about
one another adds braiding helicity to the system, while the internal
rotation of a partition adds spin helicity. Starting from an initial
potential field configuration, changes in the free energy are presented
for a time series of data for NOAA AR 8210 on 1 May 1998. This work
was supported by AFOSR, NSF and NASA.
Title: Thin Flux Approximation For Reconnection In A Y-type Current
Authors: Silvina, Guidoni E.; Longcope, D.
Bibcode: 2006SPD....37.0805S
Altcode: 2006BAAS...38..231S
We study the retraction of thin flux tubes after reconnection. During
reconnection, magnetic field lines, initially oriented in different
directions and separated by a current sheet, are broken and
reconnected. This process releases a large amount of magnetic
energy that accelerates the fluid particles in, for example,
Coronal Mass Ejections. A new model, describing the effect on the
global field, of reconnection occurring within a small region of a
pre-existing current sheet of uniform strength, has been recently
proposed by Linton and Longcope (2006). This model is called patchy
reconnection. Their prototype calculation used a flat current sheet
with uniform-surface-current density. We generalize this approach
using a Y-type current sheet whose current density increases away
from the Y-point. We solve analytically and numerically thin flux
tube equations for dynamics of thin post-reconnection flux tubes. Two
types of retracting flux tubes are possible in this geometry: outward
(U-shaped) tubes moving away from the tip of the sheet at increasing
speed, and inward (Omega-shaped) tubes moving toward the tip at
decreasing speed. We present, theoretically and via simulations,
time-dependent solutions for both types of flux tubes.This work is
supported by the National Science Foundation under Grant ATM 04-16340.
Title: 3D Reconnection Simulations of Descending Coronal Voids
Authors: Linton, Mark; Longcope, D.; Warren, H.; McKenzie, D.
Bibcode: 2006SPD....37.0123L
Altcode: 2006BAAS...38R.219L
We will present simulations of a highly localized, finite durationburst
of 3D reconnection in a post-CME current sheet. Suchreconnection forms
a pair of 3D reconnected flux tubes piercing thecurrent sheet. These
tubes retract from the reconnection region,pushing their way through
the surrounding magnetic field to form apost-reconnection arcade below
the reconnection region. We willdiscuss how the evolution of these
reconnected flux tubes can formthe descending, post-eruption voids
which have been seen in thehigh corona by the Yohkoh, TRACE and LASCO
instruments. Wewill compare the velocities and deceleration profiles of
theobserved voids with those of the simulated reconnected flux tubes.We
will also show how the presence of multiple reconnectionregions in a
single current sheet affects the dynamics of thereconnected tubes.
Title: Modeling And Measuring The Flux Reconnected By The Two-ribbon
Flare On 2004-11-07
Authors: Longcope, Dana; Beveridge, C.; Qiu, J.; Belur, R.; Barnes, G.
Bibcode: 2006SPD....37.0803L
Altcode: 2006BAAS...38R.230L
Observations of the large two-ribbon flare on 2004-Nov-7 made using SOHO
and TRACE data are interpreted in terms of a three-dimensional magnetic
field model. This model predicts the amount of flux reconnected during
the flare and the energy it would release. These values are compared to
the flux swept up by the flare ribbons observed by TRACE in 1600 A and
the energy release inferred by the GOES light curves. The helicity of
the model field may be independently compared to the helicty injected
by photospheric motions during the buildup to the flare. The model also
predicts the sequence in which the reconnections should occur. This
in turn provide insight into the conversion of mutual helicity into
self-helicity during the production of a twisted flux rope.This work
is supported by NASA Grant NAG5-10489 and DoD MURI grant.
Title: The Hi-C Sounding Rocket Experiment
Authors: Golub, Leon; Cirtain, J.; DeLuca, E.; Nystrom, G.; Kankelborg,
C.; Klumpar, D.; Longcope, D.; Martens, P.
Bibcode: 2006SPD....37.0605G
Altcode: 2006BAAS...38R.226G
The High-resolution Coronal Imager, Hi-C, is a pathfinder
mission designed to place significant new limits on theories of
coronal heating and dynamics by measuring the structures at size
scales relevant to reconnection physics. The Hi-C instrument uses
normal-incidence EUV multilayer technology, as developed in the
NIXT and TRACE programs. A dual-channel long focal-length telescope
and large format back-illuminated CCD camera provide spectroscopic
imaging of the corona at 0.1 arcsec resolution.The main objective of
the Hi-C investigation is to determine the geometric configuration and
topology of the structures making up the inner corona. The secondary
objective is to examine the dynamics of those structures, within the
constraints of the 300-seconds of observing time available from a
sounding rocket. The mission is designed to study the mechanisms for
growth, diffusion and reconnection of magnetic fields, and to help
understand the coupling of small-scale dynamic and eruptive processes
to large-scale dynamics.Hi-C will benefit from a unique coordinated
observation opportunity with investigations such as AIA on SDO, XRT on
Solar-B, and STEREO. Hi-C will address basic plasma physics science
goals of the SSSC by observing the small-scale processes that are
ubiquitous in hot magnetized coronal plasma. The scientific objectives
of Hi-C are central to the SSSC goal of understanding the Sun's activity
and its effects on the terrestrial environment, by providing unique
and unprecedented views of the dynamic activity in the solar atmosphere.
Title: Magnetic Footpoint Velocities: A Combination Of Minimum Energy
Fit AndLocal Correlation Tracking
Authors: Belur, Ravindra; Longcope, D.
Bibcode: 2006SPD....37.0705B
Altcode: 2006BAAS...38..228B
Many numerical and time dependent MHD simulations of the solar
atmosphererequire the underlying velocity fields which should be
consistent with theinduction equation. Recently, Longcope (2004)
introduced a new techniqueto infer the photospheric velocity field
from sequence of vector magnetogramswhich are in agreement with the
induction equation. The method, the Minimum Energy Fit (MEF), determines
a set of velocities and selects the velocity which is smallest overall
flow speed by minimizing an energy functional. The inferred velocity
can be further constrained by information aboutthe velocity inferred
from other techniques. With this adopted techniquewe would expect that
the inferred velocity will be close to the photospheric velocity of
magnetic footpoints. Here, we demonstrate that the inferred horizontal
velocities from LCT can be used to constrain the MEFvelocities. We
also apply this technique to actual vector magnetogramsequences and
compare these velocities with velocities from LCT alone.This work is
supported by DoD MURI and NSF SHINE programs.
Title: Quantifying The Relationship Between Reconnection Rate And
Energy Release In A Survey Of Coronal Bright Points
Authors: Malanushenko, Anna V.; Longcope, D.; Aver, E.; Kankelborg, C.
Bibcode: 2006SPD....37.1001M
Altcode: 2006BAAS...38Q.237M
This is an observational study of coronal bright points aimed at
quantifying the relationship between reconnection rate and dissipated
power. We assemble surveys of 733 bright from archival SOHO data. Bright
points are found in two channels of EIT (EUV Imaging Telescope)
data. We match these features to magnetic bipoles found in photospheric
magnetic field observations of MDI. From the MDI magnetograms we extract
measurements of each quantity relevant to simple three-dimensional
reconnection model including the relative velocities of the magnetic
poles. The study reveals temporal and spatial properties of X-ray bright
points and compares them to the simple models of spatial distribution
over the disk. The temporal evolution of the poles is used to test the
hypothesis that coronal heating is due to magnetic reconnection and
furthermore to quantify the relationship between reconnection rate and
heating power.This work was supported by NASA under grant NAG5-10489.
Title: Magnetic Topology of the 29 October 2003 X10 flare
Authors: Des Jardins, Angela C.; Canfield, R.; Longcope, D.
Bibcode: 2006SPD....37.1311D
Altcode: 2006BAAS...38..242D
In order to improve the understanding of both flare initiation and
evolution, we take advantage of powerful new topological methods
and the high spatial resolution of RHESSI to examine where magnetic
reconnection takes place in flare-producing solar active regions. Up
to this time, such studies have been carried out on a very small
number of active regions. According to present ideas, reconnection
is expected to occur at either a separatrix or separator topological
feature. We use the powerful X10 flare on 29 October 2003 (peak: 20:49
UT, location: (80'', 275'')) as a test of the ability to interpret the
topological location of reconnection. The 29 October 2003 flare was well
observed by RHESSI and MDI, occurred near the sun's central meridian,
and thus is thus a prime candidate for a study on the topological
location of magnetic reconnection. In this flare study, we use the
MPOLE (http://solar.physics.montana.edu/dana/mpole/) software to
extrapolate from the photospheric magnetic field, as observed by MDI,
to a coronal field. MPOLE is a suite of IDL programs implementing the
Minimum Current Corona Model (Longcope 1996) and currently includes a
new method that uses a hierarchy of topological features (Beveridge
2006). The extrapolation gives the location of topological features
such as poles, nulls, separatricies, separators, and spine lines. We
examine the flare emission observed by RHESSI in the context of these
topological features. In the case of the 29 October 2003 flare, we
find a relationship between the spine lines and the temporal evolution
of the HXR flare footpoints. In this poster, we present observations
supporting the relationship, explore uncertainties in the consistency
between MPOLE and RHESSI data, and survey possible results.This work
is supported by NASA.
Title: A Model for Patchy Reconnection in Three Dimensions
Authors: Linton, M. G.; Longcope, D. W.
Bibcode: 2006ApJ...642.1177L
Altcode: 2005astro.ph..9348L
We show, theoretically and via MHD simulations, how a short burst of
localized reconnection on a current sheet creates a pair of reconnected
flux tubes. We focus on the post-reconnection evolution of these flux
tubes, studying their velocities and shapes. We find that slow-mode
shocks propagate along these reconnected flux tubes, releasing magnetic
energy as in steady state Petschek reconnection. The geometry of these
three-dimensional shocks, however, differs significantly from the
classical two-dimensional geometry. They propagate along the flux tube
legs in four isolated fronts, whereas in the two-dimensional Petschek
model, they form a continuous, stationary pair of V-shaped fronts. We
find that the cross sections of these reconnected flux tubes appear as
teardrop-shaped bundles of flux propagating away from the reconnection
site. Based on this, we argue that the descending coronal voids seen
by Yohkoh SXT, LASCO, and TRACE are reconnected flux tubes descending
from a flare site in the high corona, for example after a coronal
mass ejection. In this model, these flux tubes would then settle
into equilibrium in the low corona, forming an arcade of postflare
coronal loops.
Title: A Hierarchical Application of the Minimum Current Corona
Authors: Beveridge, C.; Longcope, D. W.
Bibcode: 2006ApJ...636..453B
Altcode:
We study the energy and helicity injected into the corona by the
slow motion of photospheric source regions. A previous study compared
these quantities in a simple quadrupolar configuration modeled by a
quasi-static, line-tied MHD simulation and a minimum current corona
(MCC). The MCC provides a lower bound for the coronal magnetic free
energy by quantifying the coronal linkages (flux domains) between
discrete photospheric source regions; the chosen configuration contains
four flux domains and one separator. The MCC analysis can be extended
by decomposing each source region into smaller ones, increasing
the number of flux domains and separators. This creates a hierarchy
of topological features that asymptotically approaches a line-tied
model. We demonstrate the hierarchical approach using two octopolar
decompositions of the previously studied quadrupole. One of these has
helicity and free energy significantly closer to those of the line-tied
experiment; this is primarily due to the interweaving of lower level
flux domains approximating the self-helicity of a rotating region. The
other decomposition does not allow such interweaving and has helicity
and free energy comparable to the quadrupolar MCC configuration.
Title: Measuring Braiding and Spin Helicity Fluxes in Active Regions
Authors: Belur, R.; Longcope, D. W.; Barnes, G.
Bibcode: 2005AGUFMSH11A0248B
Altcode:
Magnetic helicity has become a valuable theoretical tool for
understanding the dynamics of the solar corona. The free energy
stored in the coronal magnetic field can be estimated based on its
helicity content. Furthermore, rapid release of stored energy must be
accomplished while preserving the total magnetic helicity. Recently
long time-sequences of magnetograms have been used to measure the flux
of helicity into active regions. We demonstrate how this helicity flux
can be usefully decomposed into a sum of spin helicity terms and an
overall mutual helicity term. Each magnetogram is partitioned into
a set of unipolar regions. These must persist through the sequence
and track the photospheric flow. The spin helicity of a given region
quantifies the effects of motions internal to that region, while
braiding helicity is injected by the motions of whole regions about
one another. Since the terms themselves can be of different signs
it is possible to re-distribute the coronal helicity by reconnection
without changing the overall helicity content. This decomposition is
demonstrated on active region observations.
Title: Reconnection in the solar corona: probing the fundamental
scales
Authors: Longcope, D. W.
Bibcode: 2005AGUFMSH51D..05L
Altcode:
Magnetic reconnection is generally believed to play a crucial
role in solar coronal activity. A central paradox is that magnetic
reconnection must occur at very small scales in order to be fast enough
but must directly affect the largest scales in order to matter. In
a natural scenario for this cross-scale coupling, the large-scale
coronal field spontaneously develops thin current concentrations
(current sheets) where magnetic reconnection then occurs. It has
recently been hypothesized that in order to proceed at Alfvenic
speeds reconnection must be somehow localized to a small portion
of that sheet. Global properties of the current sheet, such as its
net current, are set by the global magnetic geometry independent
of the small scales. Small scale structure, such as the sheet's
thickness, depends on how the microphysics adapts to this imposed
global structure. An example is presented of how global properties
can be established using present solar observations and models. In
this example it is possible to quantify both the net flux transfer
and energy release from reconnection and to observe their respective
effects on the large scales. One puzzling conclusion is that current
sheets seem to persist for extended periods before reconnection begins
within them. To understand this apparent latency it is necessary to
directly observe the microscales on which the reconnection occurs ---
scales that can be estimated by modeling the response to the global
context. Observations at these scales, resolvable by the proposed
Reconnection and Microscale (RAM) Probe, would reveal the nature of
the reconnection process, and thereby help explain the localization,
latency and sudden initiation of current sheet reconnection.
Title: Topological Methods for the Analysis of Solar Magnetic Fields
Authors: Longcope, Dana W.
Bibcode: 2005LRSP....2....7L
Altcode:
The solar coronal magnetic field is anchored to a complex
distribution of photospheric flux consisting of sunspots and magnetic
elements. Coronal activity such as flares, eruptions and general
heating is often attributed to the manner in which the coronal field
responds to photospheric motions. A number of powerful techniques have
been developed to characterize the response of the coronal field by
describing its topology. According to such analyses, activity will
be concentrated around topological features in the coronal field
such as separatrices, null points or bald patches. Such topological
properties are insensitive to the detailed geometry of the magnetic
field and thereby create an analytic tool powerful and robust enough
to be useful on complex observations with limited resolution. This
article reviews those topological techniques, their developments and
applications to observations.
Title: Observations of Separator Reconnection to an Emerging Active
Region
Authors: Longcope, D. W.; McKenzie, D. E.; Cirtain, J.; Scott, J.
Bibcode: 2005ApJ...630..596L
Altcode:
Extreme-ultraviolet (EUV) observations of an emerging active region are
used to study separator reconnection in the corona. We identify each EUV
loop connecting the emerging polarity to a nearby existing active region
over the 41 hr period beginning at emergence onset. Their geometrical
resemblance to post-reconnection field lines from a magnetic model
of the active region pair implicates separator reconnection in their
production. While some reconnection is evident within 7 hr of emergence
onset, the most intense period occurs after a 1 day delay. The sum of
cross sections of all observed loops accounts for only one-fifth of the
transferred magnetic flux predicted by the model. We suggest that the
remaining loops remain at temperatures too high, or at densities too
low, to be detected in our EUV data. The most intense reconnection
requires as much as 109 V along the coronal separator;
however, the observed loops suggests that the flux is transferred as
discrete bundles of ~4×1018 Mx each. The reconnection
appears to directly dissipate only a small fraction of the energy
released, while the rest is dissipated within the post-reconnection
flux over the ensuing 6 or more hours the loops remain visible. The
net energy released, and ultimately dissipated, is consistent with
the amount that could be stored magnetically during the 24 hr delay
between emergence and reconnection.
Title: Coronal Flux Recycling Times
Authors: Close, R. M.; Parnell, C. E.; Longcope, D. W.; Priest, E. R.
Bibcode: 2005SoPh..231...45C
Altcode:
High-cadence, high-resolution magnetograms have shown that the quiet-Sun
photosphere is very dynamic in nature. It is comprised of discrete
magnetic fragments which are characterized by four key processes -
emergence, coalescence, fragmentation and cancellation. All of this
will have consequences for the magnetic field in the corona above.
Title: Implementing a Magnetic Charge Topology Model for Solar
Active Regions
Authors: Barnes, G.; Longcope, D. W.; Leka, K. D.
Bibcode: 2005ApJ...629..561B
Altcode:
Information about the magnetic topology of the solar corona is
crucial to the understanding of solar energetic events. One approach to
characterizing the topology that has had some success is magnetic charge
topology, in which the topology is defined by partitioning the observed
photospheric field into a set of discrete sources and determining which
pairs are interlinked by coronal field lines. The level of topological
activity is then quantified through the transfer of flux between
regions of differing field line connectivity. We discuss in detail how
to implement such a model for a time series of vector magnetograms,
paying particular attention to distinguishing real evolution of the
photospheric magnetic flux from changes due to variations in atmospheric
seeing, as well as uncorrelated noise. We determine the reliability
of our method and estimate the uncertainties in its results. We then
demonstrate it through an application to NOAA active region 8210,
which has been the subject of extensive previous study.
Title: A topological analysis of the magnetic breakout model for an
eruptive solar flare
Authors: Maclean, Rhona; Beveridge, Colin; Longcope, Dana; Brown,
D. S.; Priest, E. R.
Bibcode: 2005RSPSA.461.2099M
Altcode:
The magnetic breakout model gives an elegant explanation for the onset
of an eruptive solar flare, involving magnetic reconnection at a coronal
null point which leads to the initially enclosed flux ‘breaking out’
to large distances. In this paper we take a topological approach to
the study of the conditions required for this breakout phenomenon to
occur. The evolution of a simple delta sunspot model, up to the point of
breakout, is analysed through several sequences of potential and linear
force-free quasi-static equilibria. We show that any new class of field
lines, such as those connecting to large distances, must be created
through a global topological bifurcation and derive rules to predict
the topological reconfiguration due to various types of bifurcation.
Title: Coronal Heating at Separators and Separatrices
Authors: Priest, E. R.; Longcope, D. W.; Heyvaerts, J.
Bibcode: 2005ApJ...624.1057P
Altcode:
Several ways have been proposed for heating the solar corona by magnetic
reconnection in current sheets, depending on the nature of both the
coronal magnetic field and the photospheric driving. Two ways that
have recently been considered involve the formation of such current
sheets either along separatrices (surfaces that separate topologically
distinct regions) or along separators (intersections of separatrices
linking one null point to another). The effect of slow photospheric
motions on complex coronal magnetic configurations will in general
be to generate three forms of electric current, namely, nonsingular
distributed currents, singular currents on separatrices and singular
currents on separators. These currents are not mutually exclusive
but will in general coexist in the same configuration. The aim of
this paper is to compare energy storage and heating that occurs at
separatrices and separators. We use reduced MHD to model coronal loops
that are much longer than they are wide, and we construct a series of
examples for the formation of current sheets along separatrices and
separators. We deduce that coronal heating is of comparable importance
at separatrices and separators. Separatrices are twice as effective
for observed small footpoint motions, while separators are twice as
effective in the initial build-up of a new flux domain.
Title: Quantifying Magnetic Reconnection in the Solar Corona
Authors: Longcope, D. W.
Bibcode: 2005AGUSMSM32A..03L
Altcode:
Magnetic reconnection is believed to play a role in many aspects of
solar activity including flares, CMEs and quiet sun brightenings. The
process itself is fundamentally a change field line topology
resulting from some non-ideal term in the generalized Ohm's law such
as collisional resistivity or electron inertia. Such non-ideal effects
may or may not dissipate energy directly but do produce topological
field line changes at a rate proportional to the non-ideal electric
field. The rate of magnetic reconnection can be measured by quantifying
the number of field lines topologically changed over time. Chromospheric
flare ribbons are believed to reflect the footpoints of topological
boundaries; ribbon motion across photopsheric flux is therefore used
to infer reconnection rates. Topology of individual X-ray or EUV
coronal loops can be unambiguously defined when the photopsheric
field is composed of distinct source regions to which footpoints
may be assigned. Reconnection occurs as flux is transfered between
these topological regions, and the rate is found by quantifying this
change. Several measurements of this type have been made, quantifying
reconnection rates in the quiet sun and non-flaring active region
evolution. This work was funded by NASA and NSF.
Title: Patchy Reconnection in a Solar Post-CME Current Sheet
Authors: Linton, M.; Longcope, D.
Bibcode: 2005AGUSMSM32A..04L
Altcode:
We study the dynamics of multiple, highly localized reconnection
events in a post coronal mass ejection type current sheet. We impose
the reconnection in an MHD simulation by enhancing the resistivity in
small regions for a short time, thus allowing a finite amount of flux to
reconnect. This forms a pair of 3D reconnected flux tubes piercing the
current sheet. These tubes then retract from the reconnection region,
pushing their way through the surrounding magnetic field. We will study
how these tubes react when they collide with each other, and when
they reach the edge of the current sheet and collide with the field
there. We will use this model to study the theory that the descending
post-eruption voids seen by TRACE and LASCO (see e.g. Sheeley et al
2004) are in fact reconnected flux tubes from patchy reconnection. This
work has been supported by NASA and ONR.
Title: Connectivity of Quiet Sun Magnetic Features
Authors: Holt, A. W.; Longcope, D. W.
Bibcode: 2005AGUSMSP41A..10H
Altcode:
We have examined quiet sun photospheric magnetic elements to assess
their interelationships as part of a study of the connection between
photospheric bipoles and transient coronal brightenings. The spatial
relationship of elements in quiet sun have been studied in order to
compare observed properties to a hypothetical uniform well-mixed
distribution of sources. Quiet sun MDI magnetograms were used to
identify photospheric magnetic elements, and the overall distribution
of these sources on the sun was tested for uniform density. From the
sources we generated distributions of nearest neighbor distances
and signs. The distributions were then compared to those expected
for a uniform well-mixed distribution. The nearest neighbors of
opposite sign then serve as a population of bipoles defined spatially
(spatial bipoles). Another set of bipoles was then selected using
Magnetic Charge Topology (MCT) to predict connectivity among sources
(MCT bipoles). These two sets of bipoles, spatial and MCT, were then
compared to cotemporal EIT images to search for evidence of actual
connectivity between sources predicted to be bipolar. This work was
supported by NASA grant NAG5-10489.
Title: A Hierarchical Application of the Minimum Current Corona
Authors: Beveridge, C.; Longcope, D. W.
Bibcode: 2005AGUSMSP22A..04B
Altcode:
We study the energy and helicity injected into the corona of a
photospheric quadrupole by quasi-static motion. The Minimum Current
Corona (MCC) method provides a lower bound on coronal free energy by
quantifying the coronal linkages between discrete photospheric source
regions. A quadrupolar distribution can be modeled naturally with
4 photospheric sources. The coronal field connects the sources in
4 different ways (4 domains) and includes one separator. The energy
and helicity injection into this system were explored by by Longcope
and Magara (2004). The 4-source configuration can be considered the
most significant element of an MCC hierarchy whereby the continuous
photospheric field is represented by an increasing number of discrete
sources. The numbers of linkages and separators increase with the number
of sources. This hierarchy asymptotically approaches a quasi-static MHD
evolution line-tied to a continuous photospheric field. We demonstrate
this hierarchy using a quadrupolar example which can be compared to
the results of numerical simulations of quasi-static evolution. This
work is supported by NSF.
Title: On Three-Dimensional Magnetic Skeleton Elements Due to Discrete
Flux Sources
Authors: Beveridge, C.; Longcope, D. W.
Bibcode: 2005SoPh..227..193B
Altcode:
The magnetic field in the solar corona plays an important role in
coronal heating, flaring activity and many other phenomena studied
on the Sun. Magnetic topology is frequently used to understand
complicated coronal magnetic fields. By calculating the skeleton of a
field, it is possible to build up a sophisticated representation of
the key elements of a field's configuration. This paper determines
a simple relation between the numbers of separators (X), coronal
null points (Nc), flux domains (D) and flux sources (S)
in such a configuration: D=X+S−Nc−1. This equation is
used to explain the behaviour of some of the bifurcations found in
Magnetic Charge Topology, and to show that a one-to-one relationship
exists between the number of circuits in the domain graph and the
augmented null graph. Finally, it is shown that in quiet-Sun regions,
the number of separators is approximately proportional to the number
of flux sources.
Title: Distribution of the Magnetic Flux in Elements of the Magnetic
Field in Active Regions
Authors: Abramenko, V. I.; Longcope, D. W.
Bibcode: 2005ApJ...619.1160A
Altcode:
The unsigned magnetic flux content in the flux concentrations of two
active regions is calculated by using a set of 248 high-resolution Solar
and Heliospheric Observatory Michelson Doppler Imager magnetograms
for each active region. Data for flaring active region NOAA 9077
(2000 July 14) and nonflaring active region NOAA 0061 (2002 August 9)
were analyzed. We present an algorithm to automatically select and
quantify magnetic flux concentrations above a threshold p. Each
active region is analyzed using four different values of the
threshold p (p=25, 50, 75, and 100 G). Probability distribution
functions and cumulative distribution functions of the magnetic
flux were calculated and approximated by the lognormal, exponential,
and power-law functions in the range of flux Φ>1019
Mx. The Kolmogorov-Smirnov test, applied to each of the approximations,
showed that the observed distributions are consistent with the lognormal
approximation only. Neither exponential nor power-law functions can
satisfactorily approximate the observed distributions. The parameters
of the lognormal distribution do not depend on the threshold value;
however, they are different for the two active regions. For flaring
active region 9077, the expectation value of the magnetic flux content
is μ=28.1×1018 Mx, and the standard deviation of the
lognormal distribution is σ=79.0×1018 Mx. For nonflaring
active region NOAA 0061, these values are μ=23.8×1018 and
σ=29.6×1018 Mx. The lognormal character of the observed
distribution functions suggests that the process of fragmentation
dominates over the process of concentration in the formation of the
magnetic structure in an active region.
Title: Magnetic Helicity Propagation from Inside the Sun
Authors: Longcope, Dana
Bibcode: 2005HiA....13...97L
Altcode:
Magnetic helicity may be inferred from several types of observation
including filament morphology and vector magnetograms of the
photospheric magnetic fields. The latter of these which are the most
quantitative clearly reveal an anti-correlation between solar latitude
and active-region twist; field is preferentially left-handed in the
North. A key feature of this hemispheric trend is that one-quarter to
one-third of all regions violate it. Separate observations suggest
that the helicity of an active region reflects to some degree the
twist in the magnetic field below it. One mechanism by which rising
magnetic flux tubes can become twisted the Sigma-effect predicts
handedness amplitudes and levels of statistical variation consistent
with observation. This mechanism does not generate helicity rather
it produces twist and writhe of opposite signs of which only the
twist is reflected in the coronal field. A separate model calculation
predicts that during the emergence of an active region coronal twist
will increase from zero over several days as helicity propagates along
the flux tube. Recent observations by Pevtsov et al. corroborate this
predicted time history lending support to the hypothesis that coronal
helicity originates below the solar surface.
Title: A Topological Analysis of the Magnetic Breakout Model for an
Eruptive Solar Flare
Authors: Maclean, R.; Beveridge, C.; Longcope, D.; Brown, D.;
Priest, E.
Bibcode: 2004ESASP.575..485M
Altcode: 2004soho...15..485M
No abstract at ADS
Title: What We Can Learn From Solar Flare Statistics
Authors: Longcope, D.
Bibcode: 2004AGUFMNG21A..01L
Altcode:
Solar flares are caused by the rapid release of magnetic energy. Related
events corresponding to the release of smaller energies such as
microflares and transient brightenings occur more frequently. It has
been found that the frequency of events is related to their energy by
an inverse power law. This and several other statistical relationships
have been used by several investigators as clues to the fundamental
nature of solar flares. This talk will review some of the recent models,
including cellular automata, relating the nature of flares to their
statistics. This work was supported by NSF grant ATM 97227
Title: Observations of Separator Reconnection to an Emerging Active
Region
Authors: Longcope, D. W.; Cirtain, J.; McKenzie, D.; Scott, J.
Bibcode: 2004AGUFMSH13A1140L
Altcode:
Extreme ultraviolet (EUV) observations of an emerging active region
are used to study separator reconnection in the corona. We follow each
EUV loop connecting the emerging polarity to a nearby existing active
region. Their geometrical resemblance to post-reconnection field lines
from a magnetic model of the active region pair implicates separator
reconnection in their production. While some reconnection is evident
within 7 hours of emergence onset, the most intense period occurs
after a one-day delay. The sum of cross sections of all observed
loops accounts for only one-fifth of the magnetic flux whose transfer
the model predicts. We suggest that the remaining loops remain at
temperatures too high, or at densities too low, to be detected in
our EUV data. The most intense reconnection requires as much as
260 MV along the coronal separator, however, the observed loops
suggests that the flux is transfered as discrete bundles of 1.0e18
Mx each. The reconnection appears to directly dissipate only a small
fraction of the energy released, while the rest is dissipated within
the post-reconnection flux over the ensuing 6 or more hours, during
which the flux remains visible. The net energy released, and ultimately
disiipated, is consistent with the amount which could be stored during
the 24-hour delay between emergence and reconnection. This work was
supported by NASA grant NAG5-10489
Title: Three Dimensional, Patchy Reconnection in a One Dimensional
Current Sheet
Authors: Linton, M. G.; Longcope, D. W.
Bibcode: 2004AGUFMSH13A1160L
Altcode:
Due to the highly complex nature of the 3D solar coronal magnetic field,
it is likely that reconnection will often be patchy, consisting of
many short-duration, highly localized events. To investigate this
possibility and its implications, we have studied the dynamics of
localized 3D reconnection in a large scale 1D current sheet. We impose
the reconnection in an MHD simulation by enhancing the resistivity in a
small region for a short time, thus allowing a finite amount of flux to
reconnect. This forms a pair of 3D reconnected flux tubes piercing the
current sheet. These tubes then retract from the reconnection region,
pushing their way through the surrounding magnetic field. We will
discuss the dynamics of these flux tubes, their possible relation to
descending `tadpole'-like features sometimes observed during flares, and
the evolution of the shocks which form during the reconnection and then
propagate along the tubes. This work has been supported by NASA and ONR.
Title: Quantifying Magnetic Reconnection and the Heat it Generates
Authors: Longcope, D.
Bibcode: 2004ESASP.575..198L
Altcode: 2004soho...15..198L
No abstract at ADS
Title: Inferring a Photospheric Velocity Field from a Sequence of
Vector Magnetograms: The Minimum Energy Fit
Authors: Longcope, D. W.
Bibcode: 2004ApJ...612.1181L
Altcode:
We introduce a technique for inferring a photospheric velocity
from a sequence of vector magnetograms. The technique, called the
minimum energy fit, demands that the photospheric flow agree with
the observed photospheric field evolution according to the magnetic
induction equation. It selects from all consistent flows the one
with the smallest overall flow speed by demanding that it minimize an
energy functional. Partial or imperfect velocity information, obtained
independently, can be incorporated by demanding a velocity consistent
with the induction equation that minimizes the squared difference
with flow components otherwise known. The combination of low velocity
and consistency with the induction equation are desirable when using
the magnetogram data and associated flow as boundary conditions of a
numerical simulation. The technique is tested on synthetic magnetograms
generated by specified flow fields and is shown to yield reasonable
agreement. It also yields believable flows from magnetograms of NOAA
Active Region 8210 made with the Imaging Vector Magnetogram at the
Mees Solar Observatory.
Title: Recycling of the Solar Corona's Magnetic Field
Authors: Close, R. M.; Parnell, C. E.; Longcope, D. W.; Priest, E. R.
Bibcode: 2004ApJ...612L..81C
Altcode:
Magnetic fields play a dominant role in the atmospheres of the Sun
and other Sun-like stars. Outside sunspot regions, the photosphere
of the so-called quiet Sun contains myriads of small-scale magnetic
concentrations, with strengths ranging from the detection limit of
~1016 Mx up to ~3×1020 Mx. The tireless motion
of these magnetic flux concentrations, along with the continual
appearance and disappearance of opposite-polarity pairs of fluxes,
releases a substantial amount of energy that may be associated with
a whole host of physical processes in the solar corona, not least
the enigma of coronal heating. We find here that the timescale for
magnetic flux to be remapped in the quiet-Sun corona is, surprisingly,
only 1.4 hr (around 1/10 of the photospheric flux recycling time),
implying that the quiet-Sun corona is far more dynamic than previously
thought. Besides leading to a fuller understanding of the origins of
magnetically driven phenomena in our Sun's corona, such a process may
also be crucial for the understanding of stellar atmospheres in general.
Title: A Comparison of the Minimum Current Corona to a
Magnetohydrodynamic Simulation of Quasi-Static Coronal Evolution
Authors: Longcope, D. W.; Magara, T.
Bibcode: 2004ApJ...608.1106L
Altcode:
We use two different models to study the evolution of the coronal
magnetic field that results from a simple photospheric field
evolution. The first, the minimum current corona (MCC), is a
self-consistent model for quasi-static evolution that yields an
analytic expression approximating the net coronal currents and the
free magnetic energy stored by them. For the second model calculation,
the nonlinear, time-dependent equations of ideal magnetohydrodynamics
are solved numerically subject to line-tied photospheric boundary
conditions. In both models high current density concentrations form
vertical sheets along the magnetic separator. The time history of the
net current carried by these concentrations is quantitatively similar
in each of the models. The magnetic energy of the line-tied simulation
is significantly greater than that of the MCC, in accordance with the
fact that the MCC is a lower bound on energies of all ideal models. The
difference in energies can be partially explained from the different
magnetic helicity injection in the two models. This study demonstrates
that the analytic MCC model accurately predicts the locations of
significant equilibrium current accumulations. The study also provides
one example in which the energetic contributions of two different
MHD constraints, line-tying constraints and flux constraints, may be
quantitatively compared. In this example line-tying constraints store
at least an order of magnitude more energy than do flux constraints.
Title: The Dynamics of Reconnection in a Three Dimensional Current
Sheet
Authors: Linton, M. G.; Priest, E. R.; Longcope, D. W.
Bibcode: 2004AAS...204.9509L
Altcode: 2004BAAS...36R.827L
Many models for solar flares and coronal heating rely on magnetic
reconnection in three dimensional current sheets. Yet the topology and
evolution of reconnection in such current sheets is not well known. We
will present a numerical MHD study of such reconnection. We will show
how the tearing mode evolves in a finite sized, 3D current sheet, and
how this affects the dynamics of the magnetic field reconnecting in the
sheet. We will show how the flux tubes formed in isolated reconnection
regions slingshot away from the the reconnection site, how they interact
with the unreconnected field surrounding them, and how this differs
from the 2D reconnection limit. Finally we will discuss the dynamics
of flux tubes reconnecting in a patchy reconnection scenario, where
many isolated reconnection regions occur simultaneously in a current
sheet. We will show how this causes reconnected flux tubes to become
topologically entangled with each other, and how this limits the level
of energy release which can be achieved in reconnection. This
work has been supported by NASA, ONR, and PPARC.
Title: Inferring a Photospheric Velocity Field from a Sequence of
Vector Magnetograms: The Minimum Energy Fit
Authors: Longcope, D.; Leka, K. D.
Bibcode: 2004AAS...204.3704L
Altcode: 2004BAAS...36..709L
We introduce a technique for inferring a photospheric velocity
from a sequence of vector magnetograms. The technique, called The
Minimum Energy Fit, demands that the photospheric flow agree with
the observed photospheric field evolution according to the magnetic
induction equation. It selects, from all consistent flows, that with
the smallest overall flow speed by demanding that it minimize an
energy functional. Partial or imperfect velocity information may be
incorporated by demanding a velocity consistent with the induction
equation which minimizes the squared difference with flow components
otherwise known. The combination of low velocity and consistency with
the induction equation are desirable when using the magnetogram data and
associated flow as boundary conditions of a numerical simulation. The
technique is tested on synthetic magnetograms generated by specified
flow fields and shown to yield reasonable agreement. It also yields
believable flows from magnetograms of AR8210 made with the Imaging
Vector Magnetogram at the Mees Solar Observatory. This work was
supported by AFOSR under a DoD Multi-Universities Research Initiative
(MURI) grant, ``Understanding Solar Eruptions and their Interplanetary
Consequences''.
Title: Magnetic Topology, Flux Emergence/Reconnection and Velocities
from a Magnetic Charge Topology Model for Solar Active Regions
Authors: Barnes, G.; Longcope, D. W.; Leka, K. D.
Bibcode: 2004AAS...204.3906B
Altcode: 2004BAAS...36..715B
Magnetic Charge Topology (MCT) models represent the field in the solar
corona as being due to collection of point magnetic charges located at
or below the photosphere. These models have the advantage of providing a
simple quantitative description of the field topology. We apply MCT to
time series of magnetograms from the U. Hawai`i/Mees Solar Observatory
Imaging Vector Magnetograph (IVM). We first describe the evolution of
the magnetic topology of the region, by calculating such quantities
as the magnetic flux connecting each pair of point sources, and the
number and locations of magnetic separators, which are likely to be
the location of reconnection in the solar corona. Using the changes in
the magnitudes of the point sources, and in the connectivity matrix,
we estimate the rate at which flux is emerging and submerging through
the photosphere, and the rate at which reconnection is happening in
the corona. By tracking the changes in the locations of the sources,
we are also able to estimate the horizontal velocities. This work
was performed under Air Force Office of Scientific Research contracts
F49620-03-C-0019 and F49620-02-C-0191.
Title: Detection of a Taylor-like Plasma Relaxation Process in the
Sun and its Implication for Coronal Heating
Authors: Nandy, Dibyendu; Hahn, Michael; Canfield, Richard C.;
Longcope, Dana W.
Bibcode: 2004IAUS..223..473N
Altcode: 2005IAUS..223..473N
The relaxation dynamics of a magnetized plasma system is a subject
of fundamental importance in MHD - with applications ranging from
laboratory plasma devices like the Toroidal Field Pinch and Spheromaks
to astrophysical plasmas, stellar flaring activity and coronal
heating. Taylor in 1974 proposed that the magnetic field in a plasma
(of small but finite resistivity) relaxes to a minimum energy state,
subject to the constraint that its total magnetic helicity is conserved
(Woltjer 1958), such that the final magnetic field configuration is a
constant alpha (linear) force-free field - where alpha is a quantity
describing the twist in magnetic field lines. However, a clear signature
of this mechanism in astrophysical plasmas remained undetected. Here
we report observational detection of a relaxation process, similar
to what Taylor (1974, 1986) envisaged, in the magnetic fields of
flare-productive solar active regions. The implications of this result
for magnetic reconnection and the coronal heating problem are discussed.
Title: The Relationship Between X-Ray Radiance and Magnetic Flux
Authors: Pevtsov, Alexei A.; Fisher, George H.; Acton, Loren W.;
Longcope, Dana W.; Johns-Krull, Christopher M.; Kankelborg, Charles
C.; Metcalf, Thomas R.
Bibcode: 2003ApJ...598.1387P
Altcode:
We use soft X-ray and magnetic field observations of the Sun (quiet
Sun, X-ray bright points, active regions, and integrated solar disk)
and active stars (dwarf and pre-main-sequence) to study the relationship
between total unsigned magnetic flux, Φ, and X-ray spectral radiance,
LX. We find that Φ and LX exhibit a very nearly
linear relationship over 12 orders of magnitude, albeit with significant
levels of scatter. This suggests a universal relationship between
magnetic flux and the power dissipated through coronal heating. If the
relationship can be assumed linear, it is consistent with an average
volumetric heating rate Q~B/L, where B is the average field strength
along a closed field line and L is its length between footpoints. The
Φ-LX relationship also indicates that X-rays provide a
useful proxy for the magnetic flux on stars when magnetic measurements
are unavailable.
Title: Observational consequences of a magnetic flux rope topology
Authors: Gibson, S.; Barnes, G.; Demoulin, P.; Fan, Y.; Fisher, G.;
Leka, K.; Longcope, D.; Mandrini, C.; Metcalf, T.
Bibcode: 2003AGUFMSH42B0516G
Altcode:
We consider the implications of a magnetic flux rope topology for
the interpretation of observations of sigmoidal active regions. A
region of tangential magnetic discontinuities can be identified
using techniques that determine a bald patch (BP) and corresponding
separatrices or a quasi-separatrix layer (QSL) -- for a flux rope this
region can be S-shaped, or sigmoidal. If such a region is physically
driven, current sheets can form yielding conditions appropriate for
reconnective heating. Using a numerical simulation of an emerging
flux rope driven by the kink instability, Fan and Gibson (ApJL, 2003)
showed that current sheets indeed formed a sigmoidal surface. In this
poster we will demonstrate that the current sheets formed on the BP and
BP separatrices. Moreover, we will use the results of the numerical
simulation as proxies for observations: specifically the simulated
field at the photosphere as proxy for the magnetic boundary condition,
the sigmoidal current sheets as proxy for the X-ray active region
emission, and the location of dipped magnetic field lines as proxy
for a filament. We will then consider to what extent such observations
might be used to understand and constrain the basic properties of the
coronal field.
Title: A Viscoelastic Theory of Turbulent Fluid Permeated with Fibril
Magnetic Fields
Authors: Longcope, D. W.; McLeish, T. C. B.; Fisher, G. H.
Bibcode: 2003ApJ...599..661L
Altcode:
The solar convection zone is a turbulent plasma interacting with a
magnetic field. Its magnetic field is often described as fibrillar
since it consists of slender flux tubes occupying a small fraction
of the total volume. It is well known that plasma flow will exert a
force on these magnetic fibrils, but few models have accounted for
the back-reaction of the fibrils on the flow. We present a model
in which the back-reaction of the fibrils on the flow is manifest
as viscoelastic properties. On short timescales the fibrils react
elastically with a shear modulus proportional to their overall magnetic
energy density. On longer timescales they produce an effective viscosity
resulting from collective aerodynamic drag. The viscosity due to flux
tubes in the solar convection zone can be comparable to that attributed
to turbulence there. These forces might have observable effects on
the convection zone flows.
Title: Binary Reconnection and the Heating of the Solar Corona
Authors: Priest, E. R.; Longcope, D. W.; Titov, V. S.
Bibcode: 2003ApJ...598..667P
Altcode:
The relative motions of myriads of magnetic fragments in the solar
surface are likely to drive magnetic reconnection and therefore heating
among the magnetic field lines that spread from these fragments into
the solar corona. We suggest that the fundamental mechanism is one of
``binary reconnection'' due to the motion of a given magnetic source
relative to its nearest neighbor. The heating is due to several effects:
(1) the three-dimensional reconnection of field lines that start out
joining the two sources and end up joining the largest source to other
more distant sources (or vice versa), so that the field line footpoints
are exchanged; (2) the viscous or resistive damping of the waves that
are emitted by the sources as their relative orientation rotates; and
(3) the relaxation of the nonlinear force-free fields that join the
two sources and that are built up by the relative motion of the sources.
Title: Detection of a Taylor-like Plasma Relaxation Process in the Sun
Authors: Nandy, Dibyendu; Hahn, Michael; Canfield, Richard C.;
Longcope, Dana W.
Bibcode: 2003ApJ...597L..73N
Altcode:
The relaxation dynamics of a magnetized plasma system is a subject of
fundamental importance in magnetohydrodynamics-with applications ranging
from laboratory plasma devices such as the toroidal-field pinch and
spheromaks to astrophysical plasmas, stellar flaring activity, and
coronal heating. Taylor in 1974 proposed that the magnetic field in
a plasma, subject to certain constraints, relaxes to a minimum energy
state such that the final magnetic field configuration is a constant α
(linear) force-free field-where α is a quantity describing the twist in
magnetic field lines. While Taylor's theory was remarkably successful in
explaining some intriguing results from laboratory plasma experiments,
a clear signature of this mechanism in astrophysical plasmas remained
undetected. Here we report observational detection of a relaxation
process, similar to what Taylor envisaged, in the magnetic fields of
flare-productive solar active regions. The implications of this result
for magnetic reconnection and the coronal heating problem are discussed.
Title: TRACE and Yohkoh Observations of a White-Light Flare
Authors: Metcalf, Thomas R.; Alexander, David; Hudson, Hugh S.;
Longcope, Dana W.
Bibcode: 2003ApJ...595..483M
Altcode:
We present observations of a large solar white-light flare observed
on 2001 August 25, using data from the Transition Region and Coronal
Explorer (TRACE) white-light channel and Yohkoh/HXT. These emissions are
consistent with the classic type I white-light flare mechanism, and we
find that the enhanced white-light emission observed by TRACE originates
in the chromosphere and temperature minimum regions via nonequilibrium
hydrogen ionization induced by direct collisions with the electron beam
and by back-warming of the lower atmosphere. The three flare kernels
observed in hard X-rays and white light are spatially associated with
magnetic separatrices, and one of the kernels is observed to move along
a magnetic separatrix at 400 km s-1. This is evidence in
favor of particle acceleration models, which energize the electrons
via magnetic reconnection at magnetic separators.
Title: A model for elemental coronal flux loops
Authors: Beveridge, C.; Longcope, D. W.; Priest, E. R.
Bibcode: 2003SoPh..216...27B
Altcode:
The photosphere possesses many small, intense patches of magnetic
flux. Each of these patches (or sources) is connected magnetically
through the corona to several sources of opposite polarity. An
elemental flux loop consists of all of the flux joining one such
source to another. We find that each source is connected to twenty
other sources, on average, and that the typical flux and diameter
of elemental loops in the corona are 1016 Mx and 200 km;
there are approximately 17 separators for each source. We also model a
typical large-scale coronal loop consisting of many elemental loops and
determine its complex internal topology. Each upright null lies at the
end of about 22 separatrices, which tend to be clustered together in
trunk-like structures, analogous to river-valleys in a geographical
contour map. Prone nulls correspond to saddle points, while their
spines are analogous to watersheds.
Title: On the distribution of magnetic null points above the solar
photosphere
Authors: Longcope, D. W.; Brown, D. S.; Priest, E. R.
Bibcode: 2003PhPl...10.3321L
Altcode:
Many theories predict magnetic energy dissipation at locations,
called null points, where the magnetic field vanishes. In several
astrophysical contexts, most notably the solar corona, energy is
released within a low-β magnetic field anchored to a lower boundary,
the photosphere. A general expression is derived for the distribution of
magnetic null points in potential magnetic fields anchored to a random,
homogeneous distribution of field on the lower boundary. For all such
fields the null point density decreases with height and scales with the
inverse cube of the field's characteristic length. For photospheric
fields which appear unipolar at the largest scales the nulls are
confined to a narrow layer. The results are applied to models of the
quiet Sun whose photospheric field consists of discrete sources of
mixed polarity. The number of coronal nulls depends on the degree of
imbalance between positive and negative sources. Numerical experiments
reveal that the greatest column density of null points occurs when
~20% of the sources are of the minority sign. Were the coronal energy
dissipation to occur at magnetic null points this result predicts an
observable relationship between flux imbalance and the amplitude and
distribution of dissipation.
Title: Helicity Evolution in Emerging Active Regions
Authors: Pevtsov, Alexei A.; Maleev, Vasily M.; Longcope, Dana W.
Bibcode: 2003ApJ...593.1217P
Altcode:
We study the evolution of twist and magnetic helicity in the coronal
fields of active regions as they emerge. We use multiday sequences of
Solar and Heliospheric Observatory Michelson Doppler Interferometer
magnetograms to characterize the region's emergence. We quantify
the overall twist in the coronal field, α, by matching a linear
force-free field to bright coronal structures in EUV images. At the
beginning of emergence, all regions studied have α~=0. As the active
region grows, α increases and reaches a plateau within approximately
1 day of emergence. The inferred helicity transport rate is larger
than differential rotation could produce. Following the 2000 work of
Longcope & Welsch, we develop a model for the injection of helicity
into the corona by the emergence of a twisted flux tube. This model
predicts a ramp-up period of approximately 1 day. The observed time
history α(t) is fitted by this model assuming reasonable values for
the subphotospheric Alfvén speed. The implication is that helicity
is carried by twisted flux tubes rising from the convection zone and
transported across the photosphere by spinning of the poles driven by
magnetic torque.
Title: Magnetic Helicity Injection by Horizontal Flows in the Quiet
Sun. I. Mutual-Helicity Flux
Authors: Welsch, B. T.; Longcope, D. W.
Bibcode: 2003ApJ...588..620W
Altcode:
The flux of magnetic helicity through the solar photosphere has
implications in diverse areas of current solar research, including solar
dynamo modeling and coronal heating. In this work, we focus on the
flux of magnetic helicity from quiet-Sun magnetic fields. We express
the total helicity flux in terms of mutual and self-helicities, which
arise from relative motions of separate flux elements and from internal
motions within individual magnetic flux elements, respectively. Using
a novel labeling algorithm and a tracking algorithm applied to
high-cadence, high-resolution Solar and Heliospheric Observatory
Michelson Doppler Imager magnetograms, we determine the observed
mutual-helicity flux density in the quiet Sun to be ~5×1012
Mx2 cm-2 s-1 and compare this value
with a simple theoretical prediction. The observed rate corresponds to
a whole-cycle, hemispheric mutual-helicity flux of ~1043
Mx2 from the quiet Sun, meaning that helicity injection
by surface motions in quiet-Sun fields is negligible compared to the
active region helicity flux rate.
Title: Magnetic Separators: Fault Lines in the Coronal Field
Authors: Longcope, D. W.
Bibcode: 2003SPD....34.0101L
Altcode: 2003BAAS...35Q.805L
Theories have long implicated the process of magnetic reconnection in
many aspects of coronal activity. It is, however, difficult to reconcile
the simple X-point geometry of classic, two-dimensional reconnection
models with the complex appearance of the coronal plasma observed by
recent spacecraft such as Yohkoh, TRACE or SOHO. Nor is it surprising
that the real corona is so richly structured, given the complexity
of the photospheric magnetic field to which it is connected. To
understand where and why reconnection occurs in a realistic field,
let us assume that coronal field lines interconnect a complex set of
distinct, photospheric flux concentrations. A model of quasi-static
evolution follows from the assumption that the coronal field remains
in a state of minimum magnetic energy while maintaining a given set of
connections. This minimum energy field naturally contains a network of
current sheets lying on surfaces called magnetic separators, each of
which forms the interface of four different interconnections (making
it the analog of an X-point current sheet in two dimensions). In this
quasi-static model, magnetic reconnection amounts to changing the
interconnections of a set of field lines, thereby eliminating one
constraint and decreasing the overall minimum energy. The energy
difference found from this hypothetical de-constraint provides a
lower bound on the free energy available from reconnection in fields
of arbitrary complexity. I will present applications of this technique
to observed fields ranging in complexity from small X-ray bright points
to flaring active regions.
Title: Numerical simulation of a quadrupolar magnetic field in
the corona
Authors: Magara, T.; Longcope, D. W.
Bibcode: 2003SPD....34.0417M
Altcode: 2003BAAS...35Q.813M
We carried out ideal, compressible MHD numerical simulations of a
coronal magnetic field that evolves in response to the motion imposed
at the photospheric boundary. The magnetic field is formed above a
quadrupolar region in the photosphere which is composed of two dipoles
each centered at the origin. Starting from a potential field that forms
four magnetic flux domains in the corona, the magnetic field develops
when a rotational motion is imposed to the inner dipole (the outer
dipole is fixed during the simulation). We discuss the evolution of
each magnetic flux domain as well as the temporal deveopment of current
flowing in those flux domains. We also show the time variaiton of the
free magnetic energy and magnetic helicity accumulated in the corona,
both of which are used to diagnose the coronal field.
Title: Photospheric Magnetic Field Properties of Flaring
vs. Flare-Quiet Active Regions II: A Magnetic Charge Topology Model
and Statistical Results
Authors: Barnes, G.; Leka, K. D.; Longcope, D. W.
Bibcode: 2003SPD....34.1616B
Altcode: 2003BAAS...35..835B
The complexity of the coronal magnetic field extrapolated from a
Magnetic Charge Topology (MCT) model, is examined for pre-event
signatures unique to solar energetic phenomena. Although extensive
use has been made of quantities measured at the photosphere, it is
important to consider the magnetic field in the corona, where (for
example) the hard X-ray signatures of energy release in solar flares
are observed. By quantifying the inferred coronal magnetic topology we
are no longer limited to considering solely the magnetic state of the
photosphere. MCT is applied to temporally sampled photospheric
magnetic data from the U. Hawai`i Imaging Vector Magnetograph, for
24 flare-event and flare-quiet epochs from seven active regions. We
outline the methodology employed for automating the application of MCT
to large data sets of complex active regions: partitioning the observed
Bz at the photosphere, assigning a charge to each partition,
and using this charge distribution to extrapolate the field in the
corona. From the resulting field we compute the connectivity matrix
ψ ij, the location of null points and the intersection
of separatrix surfaces, i.e. separator field lines. Parameters are
constructed to describe, for example, the magnetic connectivities, the
magnetic flux in those connections, and the number of separators. Examining particular events results in no obvious trends in the
magnitude and temporal evolution of the parameters just prior to
flare events. Thus, we employ the same quantitative statistical
approach outlined in Leka and Barnes [this session], i.e. applying
discriminant analysis and Hotelling's T2-test, and ranking
all four-variable discriminant functions as a proxy for a single
all-variable discriminant function. We present those parameters which
consistently appear in the best combinations, indicating that they
may play an important role in defining a pre-event coronal state. This work was performed under Air Force Office of Scientific Research
contracts F49620-00-C-0004, F49620-03-C-0019 and F49620-02-C-0191.
Title: Injection of Magnetic Energy and Magnetic Helicity into the
Solar Atmosphere by an Emerging Magnetic Flux Tube
Authors: Magara, T.; Longcope, D. W.
Bibcode: 2003ApJ...586..630M
Altcode:
We present a detailed investigation of the dynamical behavior
of emerging magnetic flux using three-dimensional MHD numerical
simulation. A magnetic flux tube with a left-handed twist, initially
placed below the photosphere, emerges into the solar atmosphere. This
leads to a dynamical expansion of emerging field lines as well
as an injection of magnetic energy and magnetic helicity into the
atmosphere. The field-aligned distributions of forces and plasma flows
show that emerging field lines can be classified as either expanding
field lines or undulating field lines. A key parameter determining the
type of emerging field line is the aspect ratio of its shape (the ratio
of height to footpoint distance). The emergence generates not only
vertical but also horizontal flows in the photosphere, both of which
contribute to injecting magnetic energy and magnetic helicity. The
contributions of vertical flows are dominant at the early phase of
flux emergence, while horizontal flows become a dominant contributor
later. The emergence starts with a simple dipole structure formed
in the photosphere, which is subsequently deformed and fragmented,
leading to a quadrupolar magnetic structure.
Title: A Framework for Understanding the Topology of Complex Coronal
Structures
Authors: Pontin, D. I.; Priest, E. R.; Longcope, D. W.
Bibcode: 2003SoPh..212..319P
Altcode:
The Sun's coronal magnetic field is highly complex and provides the
driving force for many dynamical processes. The topology of this
complex field is made up mainly of discrete topological building
blocks produced by small numbers of magnetic fragments. In this work
we develop a method for predicting the possible topologies due to a
potential field produced by three photospheric sources, and describe
how this model accurately predicts the results of Brown and Priest
(1999). We then sketch how this idea may be extended to more general
non-symmetric configurations. It is found that, for the case of positive
total flux, a local separator bifurcation may take place with three
positive sources or with one positive and two negative sources, but
not for two positive sources and one negative.
Title: Magnetic Helicity Generation Inside the Sun
Authors: Longcope, Dana
Bibcode: 2003IAUJD...3E..26L
Altcode:
Magnetic helicity may be inferred from several types of observation
including filament morphology and vector magnetograms of the
photospheric magnetic fields. The latter of these which are the most
quantitative clearly reveal an anti-correlation between solar latitude
and active-region twist; field is preferentially left-handed in the
North. A key feature of this hemispheric trend is that one-quarter to
one-third of all regions violate it. Separate observations suggest
that the helicity of an active region reflects to some degree the
twist in the magnetic field below it. One mechanism by which rising
magnetic flux tubes can become twisted the Sigma-effect predicts
handedness amplitudes and levels of statistical variation consistent
with observation. This mechanism does not generate helicity rather
it produces twist and writhe of opposite signs of which only the
twist is reflected in the coronal field. A separate model calculation
predicts that during the emergence of an active region coronal twist
will increase from zero over several days as helicity propagates along
the flux tube. Recent observations by Pevtsov et al. corroborate this
predicted time history lending support to the hypothesis that coronal
helicity originates below the solar surface.
Title: Helicity transport and generation in the solar convection zone
Authors: Longcope, D. W.; Pevtsov, A. A.
Bibcode: 2003AdSpR..32.1845L
Altcode:
Magnetic helicity provides a theoretical tool for characterizing the
solar dynamo and the evolution of the coronal field. The magnetic
helicity may be inferred from several types of observation including
vector magnetograms of the photospheric magnetic fields. The
helicity of an active region reflects, to some degree, the twist
in the magnetic field below it. Photospheric observations reveal
a tendency for left-handed chirality in the Northern hemisphere,
although one-quarter to one-third of the active regions twist in the
opposite sense. This means that coronal magnetic field has negative
helicity in the North. Sub-photospheric fields will have left-handed
twist in the North, although the net helicity also depends on the
writhe of the flux tube axes. We show that buffeting by turbulence; the
so-called Σ-effect, can explain the handedness and level of intrinsic
variation of observed twist. This mechanism does not generate helicity,
rather it produces twist and writhe of opposite signs. In this scenario,
helicity of one sign propagates into the corona, while opposing helicity
propagates downward in the form of torsional Alfvén waves.
Title: Magnetic Helicity Injection by Horizontal Flows in the Quiet
Sun: II. Self Helicity Flux
Authors: Welsch, B. T.; Longcope, D. W.
Bibcode: 2002AGUFMSH52A0455W
Altcode:
The helicity flux from winding motions in isolated, quiet-sun magnetic
flux elements can be expressed as a sum of mutual-helicity flux (from
the braiding of field lines from distinct elements), and a self-helicity
flux (from the braiding of field lines in individual elements). In a
previous paper, we used a tracking algorithm applied to five sequences
of high-cadence, high-resolution SoHO MDI magnetograms (each eight hours
or longer) to quantify the quiet-sun mutual-helicity flux. Here, we
use the same data sets and tracking routines to quantify the quiet-sun
self-helicity flux, from the time evolution of the quadrupole moments
of individual magnetic flux elements. No systematic injection of
self-helicity from the quiet sun is obvious in our results, leading
us to conclude that there is essentially no mean self-helicity flux
from winding motions in quiet sun fields.
Title: 3-DIMENSIONAL Evolution of a Magnetic Flux Tube Emerging Into
the Solar Atmosphere
Authors: Magara, T.; Longcope, D. W.
Bibcode: 2002AGUFMSH52A0441M
Altcode:
We present results on the emergence of a magnetic flux tube into
the solar atmosphere, obtained by 3-dimensional MHD numerical
simulation. The simulation shows that emerging field lines can
be classified as either expanding field lines or undulating field
lines according to their evolution. Field lines that emerge with
a large aspect ratio (the ratio of height to footpoint distance)
simply continue to expand into the outer atmosphere, while field lines
emerging with a small aspect ratio show an undulating behavior in the
lower atmosphere. Those undulating field lines subsequently either
expand into the outer atmosphere or sink toward the photosphere; in
the latter case a dipped structure develops in the middle of field
lines. For the field lines composing a twisted magnetic flux tube, the
outer field lines are expanding field lines and the inner field lines
are undulating field lines. We analyze the injection of magnetic energy
and magnetic helicity into the atmosphere during the simulated flux
emergence. Each of the injection rates can be divided into contributions
from horizontal shearing flows and vertical emergence flows at the
base of the atmosphere. We find that the emergence contributions are
the dominant ones at the early phase of flux emergence and that later
that role is played by the shearing contributions. The emergence starts
with a simple dipole structure formed in the photosphere, which is
subsequently deformed and fragmented, leading to a quadrapole structure.
Title: A General Theory of Connectivity and Current Sheets in Coronal
Magnetic Fields Anchored to Discrete Sources
Authors: Longcope, D. W.; Klapper, I.
Bibcode: 2002ApJ...579..468L
Altcode:
A scheme is presented for mapping the connectivity of a potential
magnetic field arising from an arbitrary distribution of discrete
sources. The field lines interconnecting the sources are classified
into Nd domains, defining the field's connectivity. The
number of domains is shown to depend on the number of sources and on
the numbers of nulls and separators according to a simple relation. A
class of nonpotential equilibria are then generated by minimizing
magnetic energy subject to constraints on the flux of each domain. The
resulting equilibria are current-free within each domain and contain
singular currents along each of the field's separators.
Title: Quiet sun magnetic helicity transport: I. Mutual helicity flux
Authors: Welsch, B. T.; Longcope, D. W.
Bibcode: 2002ESASP.505..611W
Altcode: 2002solm.conf..611W; 2002IAUCo.188..611W
The flux of magnetic helicity through the solar photosphere has
implications in diverse areas of current solar research, including
solar dynamo modelling and coronal heating. Other researchers have
investigated the flux of magnetic helicity from active regions; here,
we do the same for quiet-sun magnetic fields. We derive a theoretical
expression for the total helicity flux in terms of "mutual" and "self"
helicities, which arise from the relative motions of separate flux
elements and the time evolution of the quadrupole moments of individual
magnetic flux elements, respectively. Using a tracking algorithm applied
to high cadence, high resolution SOHO/MDI magnetograms, we determine
the observed rate of mutual helicity flux in the quiet sun and compare
these measurements with our theoretical predictions. The quiet-sun
mutual helicity flux rate we find, ~1012 Mx2
s-1 cm-2, is negligible compared to published
estimates of active region helicity flux rates.
Title: A Three-dimensional Dynamical Model of Current Sheet Formation
in a Coronal Loop
Authors: Longcope, D. W.; Van Ballegooijen, A. A.
Bibcode: 2002ApJ...578..573L
Altcode:
We develop a three-dimensional model for the time evolution of a
slender coronal loop anchored in multiple isolated photospheric flux
elements. As a result of the composite photospheric boundaries, the
coronal field comprises multiple flux domains. The model shows that
motion at the footpoints results in current singularities developing
along separators between domains. Motion at one end of the loop creates
a nonsingular Alfvénic pulse. Repeated reflections from the complex
photospheric boundaries change the pulse's current into a surface
singularity traveling along the separator ribbon. Final relaxation
leads to an equilibrium that is current-free within all of the coronal
domains and contains a separator current sheet. The relation of the
equilibrium current to the footpoint displacements confirms previous
quasi-static models of three-dimensional separator current sheets.
Title: The Importance of Plasma Viscosity in Narrow Band Bright
Point Observations
Authors: McMullen, R. A.; Longcope, D. W.; Kankelborg, C. C.
Bibcode: 2002AAS...200.0201M
Altcode: 2002BAAS...34R.639M
We explore the importance of compressional viscosity in models and
observations of an X-ray bright point. Comparison of hydrodynamic
models with and without compressive viscosity allow us to separate its
physical effects during small scale heating events that differ only in
the presence or absence of viscous effects. Heating models are designed
to emulate TRACE and SOHO/MDI observations of a June 17, 1998 bright
point transient brightening through variations in the temporal heat
distribution. Analysis methods of data and model accuracy are explored.
Title: Quiet Sun Magnetic Helicity Transport: I. Mutual Helicity Flux
Authors: Welsch, B.; Longcope, D.
Bibcode: 2002AAS...200.0303W
Altcode: 2002BAAS...34..642W
The flux of magnetic helicity through the solar photosphere has
implications in diverse areas of current solar research, including
solar dynamo modelling and coronal heating. Other researchers have
investigated the flux of magnetic helicity from active regions; here,
we do the same for quiet-sun magnetic fields. We derive a theoretical
expression for the total helicity flux in terms of ``mutual''
and ``self'' helicities, which arise from the relative motions of
separate flux elements and the time evolution of the quadrupole
moments of individual magnetic flux elements, respectively. Using a
tracking algorithm applied to high cadence, high resolution SOHO/MDI
magnetograms, we determine the observed rate of MUTUAL helicity flux
in the quiet sun and compare these measurements with our theoretical
predictions. This work has been funded by the National Science
Foundation (grants ATM-9733424 and PHY99-07949) and the National
Aeronautics and Space Administration (grant NAG5-6110).
Title: Photospheric Magnetic Fields Complexity Variations and
Solar Flares
Authors: Barnes, G.; Leka, K. D.; Longcope, D. W.
Bibcode: 2002AAS...200.6808B
Altcode: 2002BAAS...34..756B
Do photospheric magnetic fields show systematic changes which precede
energetic events such as solar flares? The answer has proved elusive. We
address this question by examining vector magnetic flux maps from
the U. Hawai`i Imaging Vector Magnetograph (Mickey et al. 1996),
which obtain full Stokes spectra over entire active regions every
4 minutes on average. We compare numerous parameters derived from
the vector magnetograms of flaring active regions to those from
comparable non-flaring active regions. In addition, we determine
quantitative measurements of the complexity of the field topology
using the Minimum-Current Corona analysis (Longcope 1996). The goal
is to determine quantitative measurements of the complexity of the
field topology, and determine whether variations in those measures
correlate with or precede flare events. This project was funded by
AFOSR contract F49620-00-C-0004.
Title: 3-dimensional Evolution of an Emerging Flux Tube in the Sun
Authors: Magara, T.; Longcope, D. W.
Bibcode: 2002AAS...200.3601M
Altcode: 2002BAAS...34R.692M
The objective of this study is to investigate the dynamical behavior
of emerging magnetic field in the solar atmosphere by means of
3-dimensional MHD numerical simulation of a buoyant magnetic flux
tube. From our recent simulations, it is found that there are two kinds
of evolutionary phases of emerging field lines: an expansion phase
and a gradual phase. The outer field line of flux tube, which emerges
earlier than the inner field line, shows a simple expansion after
it comes to the solar atmosphere. On the other hand, the inner field
line has a gradual phase at first, in which the field line undulates
and rises gradually, and then it enters an expansion phase. We try to
understand this result by focusing on the physical process working on
the individual field lines. We think that the distance between the
footpoints of emerging field line plays an important role. When the
outer field line emerges, its footpoint distance is almost comparable
to the local value of the critical wavelength of Rayleigh-Taylor
instability (λ RT), although the inner field line has a
much larger footpoint distance than this value when emerging. These
facts cause the outer field line (emerging early) to make a simple
expansion, however they undulate the inner field line (emerging late)
and prevent this field line from expanding smoothly. As the height of
inner field line increases, the density of the gas supported by the
field line decreases because of the continuous drain of gas toward
the footpoints. This leads to the increase of the local value of λ
RT. When this value becomes comparable to the footpoint
distance, then the inner field line starts to expand rapidly. This
work is supported by AFOSR grant F49620-00-1-0128.
Title: Hard X-ray and White Light Observations of the August 25,
2001 X Flare
Authors: Metcalf, T. R.; Alexander, D.; Hudson, H. S.; Longcope, D.;
Myers, D.
Bibcode: 2002AAS...200.6803M
Altcode: 2002BAAS...34..755M
An X5.3 flare occurred about 16:31 UT on 2001 August 25 and was well
observed by the Yohkoh and TRACE spacecraft. The flare showed gamma-ray
emission, nuclear lines, and was a dramatic white light flare seen in
TRACE data. A preliminary analysis of the hard X-ray images from the
Yohkoh/HXT instrument shows two clear footpoints and a moving HXR
source in this very energetic flare. The moving hard X-ray source
appears to move along a magnetic separatrix at 400 km/sec. We will
discuss the hard X-ray and white light structure of this flare and
discuss the energetics and possible mechanisms for the formation of
the TRACE white light emission.
Title: Connectivity and current sheets in general coronal magnetic
fields through constrained variational methods
Authors: Longcope, D.
Bibcode: 2002AAS...200.0301L
Altcode: 2002BAAS...34Q.641L
A scheme is presented for mapping the connectivity of a potential
magnetic field arising from an arbitrary distribution of isolated
sources. The field lines interconnecting the sources are classified into
Nd domains, defining the field's connectivity. The number of domains
is shown to depend on the number of sources and on the numbers of null
and separators according to a simple relation. A class of non-potential
equilibria are then generated by minimizing magnetic energy subject
to constraints on the flux of each domain. The resulting equilibria
are current-free within each domain, and contain singular currents
along each of the field's separators. The general scheme is applied
to observed magnetic configurations. This work was supported in part
by AFOSR grant F49620-00-1-0128
Title: Topological Defects in Coronal Magnetic Fields: A Theory for
Equilibrium Current Sheets in Complex Geometries
Authors: Longcope, Dana
Bibcode: 2002kbls.confE..20L
Altcode:
No abstract at ADS
Title: What role does magnetic field play in heating the solar
corona? The present status of models and observations.
Authors: Longcope, Dana
Bibcode: 2002bhty.confE..17L
Altcode:
No abstract at ADS
Title: Solar Magnetism and Related Astrophysics
Authors: Fisher, George H.; Longcope, Dana W.
Bibcode: 2002smra.progE....F
Altcode:
The Institute will host a program on Solar Physics that is devoted
to the evolution of magnetic fields on the Sun, the energetic
consequences of magnetic field evolution, and crossover applications
to astrophysics, such as accretion disks and active stars. Areas of
focus during the workshop will include:(1) The origin and evolution
of magnetic fields in the solar interior, with a particular emphasis
on relating the observed fields at the surface of the Sun to their
subsurface properties,(2) The role that changing magnetic fields in
the corona play in the explosive release of energy,(with an emphasis
on reconnection and particle acceleration during solar flares),(3)
how magnetic field evolution at the photospheric level drives coronal
mass ejections and solar wind flows, and(4) the application of the
lessons learned from the Sun to other astrophysical
Title: Modeling the coronal loop of an X-ray bright point
Authors: McMullen, R.; Longcope, D.; McKenzie, D.; Kankelborg, C.;
Klimchuk, J.
Bibcode: 2002ocnd.confE..28M
Altcode:
No abstract at ADS
Title: The Orientational Relaxation of Bipolar Active Regions
Authors: Longcope, Dana; Choudhuri, Arnab Rai
Bibcode: 2002SoPh..205...63L
Altcode:
In the mean, bipolar active regions are oriented nearly toroidally,
according to Hale's polarity law, with a latitude-dependent tilt known
as Joy's Law. The tilt angles of individual active regions deviate
from this mean behavior and change over time. It has been found that on
average the change is toward the mean angle at a rate characteristic of
4.37 days (Howard, 1996). We show that this orientational relaxation
is consistent with the standard model of flux tube emergence from
a deep dynamo layer. Under this scenario Joy's law results from the
Coriolis effect on the rising flux tube (D'Silva and Choudhuri, 1993),
and departures from it result from turbulent buffeting of the tubes
(Longcope and Fisher, 1996). We show that relaxation toward Joy's
angle occurs because the turbulent perturbations relax on shorter
time scales than the perturbations from the Coriolis force. The
turbulent perturbations relax more rapidly because they are localized
to the topmost portion of the convection zone while the Coriolis
perturbations are more widely distributed. If a fully-developed active
region remains connected to the strong toroidal magnetic field at
the base of the convection zone, its tilt will eventually disappear,
leaving it aligned perfectly toroidally. On the other hand, if the flux
becomes disconnected from the toroidal field the bipole will assume a
tilt indicative of the location of disconnection. We compare models
which are connected and disconnected from the toroidal field. Only
those disconnected at points very deep in the convection zone are
consistent with observed time scale of orientational relaxation.
Title: Three-Dimensional MHD Simulation of an Emerging Flux Tube in
the Sun
Authors: Magara, T.; Longcope, D. W.
Bibcode: 2002mwoc.conf..195M
Altcode:
We present the results of three-dimensional magnetohydrodynamic
(MHD) simulations of a magnetic flux tube emerging through the solar
photosphere. The simulation is initialized with a straight tube of
twisted magnetic field located in the upper convection zone. Buoyancy
effects drive an arched segment of the tube upward through the
photospheric layer and into the corona. Matter drains from the coronal
field which thereafter undergoes a rapid dynamical expansion. The
coronal magnetic field formed in this manner exhibits outer poloidal
field lines resembling a potential arcade, and inner toroidal field
lines, which emerge after the tube axis, forming sigmoid structure. The
simulations suggest that neutral-line shear and sigmoidal field arise
as a natural by-product of flux emergence. We discuss several basic
properties of sigmoidal emerging flux tubes, such as i) flattening
of the cross section of tube at a photospheric boundary, ii) strong
downflows along emerged magnetic loops, iii) sigmoid structure of
emerged field lines, iv) footpoint heating, and v) alpha distribution
in the lower atmosphere.
Title: Nanoflare Modeling of an X-Ray Bright Point Coronal Loop
Authors: McMullen, R. A.; Longcope, D. W.; Kankelborg, C. C.
Bibcode: 2002mwoc.conf...95M
Altcode:
We study the spatial structure and temporal evolution of an X-ray
bright point loop in order to understand the role of magnetic energy
dissipation. We use a time-dependent gasdynamic model to simulate
the corona and transition region in the x-ray bright point's coronal
loop. For this work we model a bright point observed by TRACE and SOHO
on June 17, 1998, where the magnetic field geometry is derived from an
extrapolation of magnetograms. We study the effects of various spatial
and temporal distributions of heat deposition within the loop. The
quantity of energy deposited and the location of the energy release
is constrained by a model equilibrium magnetic field. We model the
observed transient brightening of the bright point as a series of
nanoflare events.
Title: Helicity transport and creation in the solar convection zone
Authors: Longcope, D.; Pevtsov, A.
Bibcode: 2002cosp...34E3177L
Altcode: 2002cosp.meetE3177L
Magnetic helicity provides a theoretical tool for characterizing the
solar dynamo and the evolution of the coronal field. The magnetic
helicity may be inferred from several types of observation including
vector magnetograms of the photospehric magnetic fields. The helicty
of an active region reflects, to some degree, that produced by the
solar cycle dyanmo which is believed to be operating at the base of
the convection zone, where the Rossby number is small. The helicty of
the active region is affected by the turbulence through which it rises,
and this process must be taken into account when interpreting helicity
observations. The subsequent dispersal of the active region magnetic
field will further affect the observed helicty of the photospheric
field. This transport process suggests an observational method of
identifying, through helicty measurements, the source of quiet Sun
field from either a surface (non-helical) dynamo or the fragmentation
of helical active region fields.
Title: Observational Challenges for the Next Decade of Solar
Magnetohydrodynamics
Authors: Fisher, George H.; Longcope, Dana W.
Bibcode: 2002ocnd.confE....F
Altcode:
No abstract at ADS
Title: Magnetic Helicity Transport in the Quiet Sun: Theory &
Observations
Authors: Welsch, B. T.; Longcope, D. W.
Bibcode: 2001AGUFMSH11C0721W
Altcode:
The flux of magnetic helicity through the solar photosphere has
implications in diverse areas of current solar research, including
solar dynamo and solar wind modeling. Various authors have considered
the flux of magnetic helicity from active regions (e.g., DeVore, 2000;
Chae, 2001); here, we do the same for magnetic fields in the quiet
sun. We express the helicity flux in terms of the relative motions
and deformations of isolated magnetic flux elements, summing both
``mutual helicity" and ``self-helicity" contributions to get the total
helicity flux. Using a tracking algorithm applied to high cadence,
high resolution SOHO/MDI magnetograms, we determine the observed rate
of helicity flux in the quiet sun and compare these measurements with
our theoretical predictions.
Title: Simulations of an X-Ray Bright Point's Evolution
Authors: McMullen, R. A.; Longcope, D. W.; McKenzie, D. E.; Kankelborg,
C. K.
Bibcode: 2001AGUFMSH11C0726M
Altcode:
We study the spatial structure and temporal evolution of an X-ray
bright point loop in order to understand the role of magnetic energy
dissipation. We use a time-dependent gasdynamic model to simulate
the corona and transition region in the x-ray bright point's coronal
loop. For this work we model a bright point observed by TRACE, Yohkoh
and SOHO on June 17, 1998, where the magnetic field geometry is derived
from an extrapolation of magnetograms. We study the effects of various
spatial and temporal distributions of heat deposition within the
loop. The quantity of energy deposited and the location of the energy
release is constrained by a model equilibrium magnetic field. We model
the observed transient brightening of the bright point as a series of
nanoflare events.
Title: Separator current sheets: Generic features in minimum-energy
magnetic fields subject to flux constraints
Authors: Longcope, D. W.
Bibcode: 2001PhPl....8.5277L
Altcode:
Equations are found for force-free magnetic equilibria in the
``coronal half-space'' z>0, subject to boundary conditions on
the normal magnetic field at z=0. The distribution of normal field
is assumed to be composed of NS isolated unipolar source
regions of arbitrary shape, arranged arbitrarily on the plane. The
equilibria are found by minimizing the magnetic energy subject
to constraints on the total flux interconnecting pairs of source
regions. For NS source regions interconnected in ND ways,
there are Nc=ND-NS+1 distinct constraints on
the field. Minimization subject to these constraints leads to an
Nc-dimensional space of equilibria, for given boundary
data. All field-lines connecting source regions are current-free, but
the equilibrium contains Nc current-sheets lying at certain
interfaces. In a two-dimensional magnetic field current sheets occur
at points topologically equivalent to X-type neutral points in the
potential field. In a three-dimensional field current sheets occur
at points topologically equivalent to separators in the potential
field. The free magnetic energy is a function of the Nc
fluxes used to constrain the variation.
Title: Sigmoid Structure of an Emerging Flux Tube
Authors: Magara, T.; Longcope, D. W.
Bibcode: 2001ApJ...559L..55M
Altcode:
We present the results from three-dimensional MHD simulations of
a magnetic flux tube emerging through the solar photosphere. The
simulation is initialized with a straight tube of twisted magnetic field
located in the upper convection zone. Buoyancy effects drive an arched
segment of the tube upward through the photospheric layer and into the
corona. Matter drains from the coronal field, which thereafter undergoes
a rapid expansion. The coronal magnetic field formed in this manner
exhibits outer poloidal field lines that resemble a potential arcade
and inner toroidal field lines that emerge after the tube axis, forming
sigmoid structure. The simulations suggest that the neutral-line shear
and sigmoidal field arise as a natural by-product of flux emergence.
Title: Topology is destiny: Reconnection energetics in the corona
Authors: Longcope, D. W.; Kankelborg, C. C.
Bibcode: 2001EP&S...53..571L
Altcode:
Magnetic reconnection is clearly at work in the solar corona
reorganizing and simplifying the magnetic field. It has also been
hypothesized that this reorganization process somehow supplies
the energy heating the corona. We propose a quantitative model
relating the topological role (simplification) and the energetic role
(heating) of magnetic reconnection. This model is used to analyze
multi-wavelength observations of an X-ray bright point. In the model,
motion of photospheric sources drives reconnection of coronal flux. If
reconnection occurs only sporadically then energy is stored in the
coronal field, and released by topological reconnection. We simulate
the dynamical response of the plasma to such an energy release, and
translate this into predicted observational signatures. The resulting
predictions are difficult to reconcile with the observations. This
suggests that while reconnection is important in the corona, energy
dissipation is governed by other factors, not all of which relate to
the topology of the field.
Title: 3-dimensional MHD Simulation of Emerging Flux Tubes
Authors: Magara, T.; Longcope, D. W.
Bibcode: 2001AGUSM..SH41A09M
Altcode:
We study the evolution of a magnetic flux tube that rises from the
upper convection zone to the solar atmosphere by means of 3-dimensional
MHD simulation. A Gold-Hoyle flux tube placed horizontally in the
convection zone starts rising by convective motion. As the flux tube
emerges through the photosphere, it provides noticeable flow patterns
on the surface. First, when the outermost magnetic field lines of
flux tube, which are almost transverse to the tube axis, reach the
atmosphere, we find that a photospheric plasma flows in the direction
perpendicular to the axis. Then as the inner field lines, which have a
strong axial component of magnetic field, rise across the photosphere,
the photospheric flow changes its direction, that is, a plasma motion
turns parallel to the tube axis (neutral line). This result supports the
assumption used in a lot of previous studies that shear flows (parallel
flow along neutral line) play an important role in forming energetic
magnetic structure in the solar atmosphere. As for the energetics
of magnetic arcade formed in the atmosphere, we compare the magnetic
energy of emerging field with the energy of potential field that has
the same distribution of vertical magnetic field on the surface as
the emerging field. Since the difference between these two energies
is related to the energy source of explosive phenomena, we focus on
the time variation of those energies as the emergence of flux tube
proceeds. We also discuss the configuration of emerging field lines,
which has been a hot topic in the solar activity researches since the
discovery of 'sigmoid' structures in the corona.
Title: Solar physics, Title TBA
Authors: Longcope, Dana W.
Bibcode: 2001APS..NWS.A1007L
Altcode:
No abstract at ADS
Title: Fluxon Modeling of Force Free Magnetic Fields: Voronoi Method
Authors: DeForest, C. E.; Kankelborg, C. C.; Longcope, D. W.
Bibcode: 2001AGUSM..SH41A18D
Altcode:
We present a promising new form of quasi-Lagrangian magnetic model for
the corona, a ``fluxon model''. Fluxons are discrete representations of
field lines. Fluxon-based models are similar to traditional Lagrangian
field models in that they have no numerical resistivity because
field topology is explicitly preserved. They differ from traditional
Lagrangian models in two ways: there is no fixed set of proximity
relationships between the discrete elements of the model, preventing
numerial runaway in evolving systems; and div B is explicitly maintained
at 0. In a force-free field, the magnetic pressure and tension forces
are in equilibrium. The fluxon formulation reduces determination
of the field to a relaxation process. Calculation of the magnetic
pressure and tension forces at each step in the relaxation is reduced
to a variant of known, solved problems in computational geometry. In
this presentation, we demonstrate a computationally efficient method
of calculating these forces, using an approximation to the ``Voronoi
foam'' of spatial neighborhoods defined by a particular collection of
fluxons. Magnetic field modeling with fluxons offers several advantages:
(1) it facilitates detailed investigation of field topology; (2)
reconnection is completely controllable, because there is no numerical
resistivity; (4) the modeled structures may be critically sampled,
raising the prospect of unprecedented computational efficiency. These
advantages together will ultimately allow explicit modeling of stability
and reconnection in complex, slowly evolving coronal features such as
polar plumes, prominences, and active regions.
Title: Where the Quiet Sun Magnetic Field Comes From?
Authors: Pevtsov, A. A.; Longcope, D. W.
Bibcode: 2001AGUSM..SP41C06P
Altcode:
It has been recently suggested that there is two separate dynamo
operating on the Sun. A subphotospheric (e.g. overshoot region) dynamo
is responsible for strong magnetic fields of active regions, while the
quiet Sun field is generated by the local (surface) dynamo driven by
granular flows (Cattaneo 1999). Compelling observational evidence in
support of the surface dynamo is still lacking. Because of the small
characteristic size and lifetime of granular flows, the Coriolis force
has no significant effect on them. Consequently the kinetic helicity of
granules will not depend of hemisphere or vary with latitude; it will
almost certainly average to zero. Magnetic field generated by such
a (non-helical) dynamo should exhibit no hemispheric helicity rule
either. In contrast, the sub-photospheric dynamo flows have non-zero
kinetic helicity that changes sign across the solar equator. This
dynamo will introduce hemispheric asymmetry in magnetic field's twist:
positive helicity in southern hemisphere and negative in northern
hemisphere. An observed hemispheric helicity rule for active region
magnetic fields is well documented (e.g. Pevtsov et al 1995). Thus, the
helicity approach can be used to distinguish between sub-photospheric
(helical) and surface (non-helical) solar dynamos. Using vector
magnetograms from the Advanced Stokes Polarimeter we measure current
helicity density α z = Jz / Bz
of photospheric field in the quiet Sun at few fixed latitudes. Our
results indicate a weak hemispheric asymmetry in distribution of α
z with a tendency for averaged helicity to be negative in
the northern hemisphere and positive in the southern hemisphere. We
interpret this asymmetry in a framework of the sub-photospheric origin
of the photospheric field in the quiet Sun.
Title: Evidence of Separator Reconnection in a Survey of X-Ray
Bright Points
Authors: Longcope, D. W.; Kankelborg, C. C.; Nelson, J. L.; Pevtsov,
A. A.
Bibcode: 2001ApJ...553..429L
Altcode:
X-ray bright points are among the simplest coronal structures
hypothesized to be powered by magnetic reconnection. Their
magnetic field appears to consist of a simple loop of field lines
connecting positive to negative photospheric sources. Quantitative
three-dimensional models of reconnection in this geometry are therefore
expected to apply directly to X-ray bright points. We assemble a survey
from archival Solar and Heliospheric Observatory data of 764 X-ray
bright points (EUV Imaging Telescope) along with their associated
photospheric magnetic fields (Solar Oscillation Imager/Michelson
Doppler Imager). Measurements are made of each quantity relevant to
the simple three-dimensional reconnection model. These data support
several predictions of a magnetic reconnection model providing further
evidence in favor of the hypothesis that magnetic reconnection supplies
heating power to the quiet solar corona.
Title: Modeling the Coronal Loop of an X-ray Bright Point
Authors: McMullen, R. A.; Kankelborg, C. C.; Longcope, D. W.
Bibcode: 2001AGUSM..SH41A16M
Altcode:
We use a time-dependent gasdynamic model to simulate the corona
and transition region in an x-ray bright point loop. For this
work we model a bright point observed by TRACE and SOHO on June 17,
1998. The magnetic field geometry is derived from an extrapolation of
magnetograms. We study the effects of various hypothesized spatial and
temporal distributions of heat deposition within the loop, including
Joule heating at the footpoints or decay of standing Alfvén waves.
Title: Model Solar Active Regions: Predictions of Observables
Authors: Welsch, B. T.; Longcope, D. W.
Bibcode: 2001AGUSM..SH41A15W
Altcode:
To predict observables in active regions -- loop lengths &
orientations, soft X-ray (SXR) emission distributions, and heating rates
-- from first principles, we investigate the interaction of a point
source of magnetic flux, or ``test flux,'' and a model bipolar active
region, composed of two oppositely-signed, Gaussian distributions of
``field flux.'' We vary the placement of the test flux, the separation
of the field flux distributions, and the ratio of test flux to field
flux. Employing statistical techniques, we calculate average separations
between the test flux and field fluxes for each configuration, which
we use to derive model-dependent loop orientations and lengths. We
also determine the average power generated by magnetic reconnection
between the test flux and field fluxes. We then combine these results
to generate a model-dependent prediction of the spatial distribution
of SXR emission. Finally, via whole-active-region averaging, we find
heating rates of the order of magnitude of the coronal demand.
Title: Lagrangian Modeling of Force Free Fields and Current Sheets:
Fluxon representation and the Kernel Method
Authors: Kankelborg, C. C.; Longcope, D. W.; DeForest, C. E.
Bibcode: 2001AGUSM..SH41A17K
Altcode:
In force free magnetic fields, the magnetic pressure and tension forces
are balanced. These forces may be represented in terms of arrangements
of field lines (``fluxons''). We demonstrate a novel Lagrangian
technique for modeling of force-free configurations in 2D and 3D with
and without current sheets. The fundamental computational element
in our model is the fluxon, a field-line-like entity that represents
the configuration of a finite quantity of magnetic flux. The magnetic
field in a volume is represented as a collection of fluxons, each with a
geometry defined by a connected series of points. The magnetic curvature
force is easily determined by finite differencing along a fluxon. A
smoothing kernel is used to evaluate magnetic field strength and its
gradient. By these means, the Lorentz force is determined. Relaxation
to a force free state is accomplished by displacing the fluxons in the
direction indicated by the Lorentz force. Further calculations with
the smoothing kernel allow the evaluation of stored magnetic energy
and mapping of current distributions in the volume. Magnetic field
modeling in the fluxon representation offers several advantages:
(1) it facilitates detailed investigation of field topology; (2)
reconnection is completely controllable --- it cannot occur unless
it is explicitly inserted into the model; (3) there is no numerical
resistivity; (4) current sheets are critically sampled, raising the
prospect of unprecedented computational efficiency.
Title: What We Can Learn From Flaring Statistics?
Authors: Longcope, D. W.
Bibcode: 2001AGUSM..SP51C02L
Altcode:
Solar flares are caused by the rapid release of magnetic energy. Related
events corresponding to the release of smaller energies such as
microflares and transient brightenings occur more frequently. It has
been found that the frequency of events is related to their energy by an
inverse power law. This and several other statistical relationships have
been used by several investigators to uncover the fundamental nature
of solar flares. This talk will review some of the recent models,
including cellular automata, relating the nature of flares to their
statistics. This material is based upon work supported by the National
Science Foundation under Grant No. ATM-9733424.
Title: Origin of Helicity in the Quiet Sun
Authors: Pevtsov, A. A.; Longcope, D. W.
Bibcode: 2001ASPC..236..423P
Altcode: 2001aspt.conf..423P
No abstract at ADS
Title: Multi-mode kink instability as a mechanism for δ-spot
formation
Authors: Linton, M. G.; Fisher, G. H.; Dahlburg, R. B.; Fan, Y.;
Longcope, D. W.
Bibcode: 2001AdSpR..26.1781L
Altcode:
We investigate the current driven kink instability of twisted magnetic
flux tubes in the solar convection zone. The possibility that kinking
flux tubes are responsible for the formation of some δ-spot active
regions provides the motivation for this work. We simulate the evolution
of a twisted flux tube with a highly parallelized three dimensional
MHD spectral code run on a 128 cubed grid. We find that highly twisted
flux tubes, when perturbed with a single wavenumber mode, develop large
kinks which lead to δ-spot tilt angles as large as 60°. We find that
when tubes are perturbed with multiple wavenumber modes, the modes can
interact to create a localized kink tilted by as much as 80° with
respect to the unkinked portion of the tube. We show that this kind
of kinked flux tube can create a δ-spot configuration with opposite
polarity spots emerging and remaining in close proximity to each other,
with shear developing along the neutral line as the region develops, and
with the opposite polarity regions rapidly rotating about each other.
Title: A Model for the Emergence of a Twisted Magnetic Flux Tube
Authors: Longcope, D. W.; Welsch, B. T.
Bibcode: 2000ApJ...545.1089L
Altcode:
Observations have shown that active region flux tubes often emerge in
a twisted state and that the active region formed has magnetic helicity
of the same sense as the flux tube that forms it. Separate theoretical
models have been developed for coronal magnetic fields with helicity
and for flux tubes with twist. Here we present a dynamical model that
connects a twisted subphotospheric flux tube to a force-free coronal
field. With this model it is possible to explore the emergence of a
flux tube into the corona and its effect on both the coronal field
and the subphotospheric flux tube. In particular, the model shows
that only a fraction of the current carried by the twisted flux tube
will pass into the corona. As a consequence of this ``mismatch,''
a torsional Alfvén wave is launched downward along the flux tube
at the instant of emergence. As the flux tube continues to emerge,
the helicity of the coronal field increases owing to rotation of the
footpoints. Our model predicts that the level of rotation will depend
upon the rapidity of flux emergence. After this transient period the
helicity of the active region will reflect the twist in its parent tube.
Title: Using X-ray Bright Points to Infer the Large-Scale Magnetic
Field of the Quiet Sun
Authors: Nelson, J. L.; Longcope, D. W.; Pevtsov, A. A.
Bibcode: 2000SPD....31.0143N
Altcode: 2000BAAS...32.1289N
X-ray bright points (XBPs) form above magnetic bipoles in the quiet
Sun, often at the site of convergence. According to models, the power
radiated by the XBP is supplied by magnetic reconnection as flux
is transfered from some overlying field into the bipole itself. It
follows that the morphology of an XBP depends on both the bipole and
on the large-scale overlying field. We demonstrate a novel technique
which exploits this fact to map the horizontal component of the Sun's
large-scale field using the morphology of observed XBPs. We test this
technique using data from SOHO's Michaelson Doppler Interferometer (MDI)
and EUV Imaging Telescope (EIT). The resulting measurements are compared
to masurements made using standard polarimetric methods, and to models
of the Sun's diffuse field. This material is based upon work supported
by the National Science Foundation under Grant No. ATM-9733424.
Title: Self-organized Criticality from Separator Reconnection in
Solar Flares
Authors: Longcope, D. W.; Noonan, E. J.
Bibcode: 2000ApJ...542.1088L
Altcode:
A new cellular automaton model for solar flares is presented in which a
complex coronal magnetic field is stressed by photospheric shear. The
minimum current corona model is used to describe the slow buildup
and sudden release of stress in the field. Stress takes the form of
currents flowing along the field's network of magnetic separators;
it is released by magnetic reconnection. By this model we show how
magnetic reconnection can occur as an avalanche, releasing stress
along an extended region of the separator network. The model exhibits
self-organized criticality; thus the sizes of reconnection avalanches
are distributed according to a power law. Flare durations and peak
emission levels are also distributed according to power laws, with
different exponents. Because the model is derived from the application
of magnetohydrodynamics to a coronal field, it is possible to assign
physical sizes to its avalanches. These agree well with the sizes and
frequencies of observed flares. The total power from flares of all
sizes agrees with observation and is therefore well below the inferred
coronal heating power.
Title: Statistical Properties of Magnetic Separators in Model
Active Regions
Authors: Welsch, B. T.; Longcope, D. W.
Bibcode: 2000IAUS..195..443W
Altcode:
``Transient brightenings'' (or ``microflares'') regularly deposit
1027 ergs of energy in the solar corona, and account
for perhaps 20% of the active corona's power. We assume these
events correspond to episodes of magnetic reconnection along
magnetic separators in the solar corona. Using the techniques of
magnetic charge topology, we model active region fields as arising
from normally distributed collections of ``magnetic charges'',
point-like sources/sinks of flux (or field lines). Here, we present
statistically determined separator (X-ray loop) lengths, derived from
first principles. We are in the process of statistical calculations
of heating rates due to reconnection events along many separators.
Title: A Scaling Law for Magnetic Flux Tubes on an AGN Accretion Disk
Authors: Leroux, A. M. K.; Longcope, D. W.; Tsuruta, S.
Bibcode: 2000IAUS..195..409L
Altcode:
An ASCA observation of Seyfert galaxy NGC 3227 showed flares with
a linear increase and exponential decrease similar to that of solar
flares. We derive a scaling law relating the loop length of a magnetic
flux tube to rise and decay times of the flare using cooling mechanisms
suitable for the central engine of a Seyfert galaxy. The predicted
loop lengths are consistent with physical constraints on the plasma
dynamics, suggesting that the same mechanism which explains solar
flares may explain variability in Seyfert galaxies.
Title: Having Our Cake and Eating it, Too: Fast Imaging Spectroscopy
With a Multi-Order Slitless Spectrograph
Authors: Kankelborg, C. C.; Longcope, D. W.; Martens, P. C. H.
Bibcode: 2000SPD....3102101K
Altcode: 2000BAAS...32..829K
We describe a new type of EUV imaging spectrograph that combines high
spectral, spatial and temporal resolution. The instrument consists of
a slitless spectrograph with cameras placed at several diffraction
orders. The unique information derived from simultaneous imaging
at multiple orders allows the deconvolution of spectral and spatial
information, thus overcoming the limitations of a traditional slitless
spectrograph.
Title: X-ray bright points: A case study in solar reconnection
Authors: Longcope, D.; Kankelborg, C.
Bibcode: 2000SPD....31.1304L
Altcode: 2000BAAS...32..845L
Magnetic reconnection is believed to play an important role in the
energetics of the solar corona including flaring and quiescent heating
in active regions. It is also implicated as the energy source for
X-ray bright points which occur in coronal holes and in the quiet
Sun. X-ray bright points are the ideal feature in which to study
magnetic reconnection since they have relatively simple geometry:
two isolated sources of photospheric flux approaching one another. By
assuming that all power comes from the process of forging new field
lines to connect the approaching poles we are lead to a simple
quantitative model for an X-ray bright point. To test the model the
predicted energy release is used in a dynamical simulation of loop
plasma evolution. The results of this simulation are used to sythesize
images in the EUV for direct comparison to a TRACE observation. A
second test of the model is provided by a statistical study of X-ray
bright points and bipoles in archival SOHO data. The results of this
survey support several predictions of the model. Finally, the model
is applied to a theoretical distribution of flux elements to yield a
model for heating of the quiet Sun. This produces expressions for the
density of X-ray bright points and total heat flux.
Title: Current Sheets
Authors: Longcope, Dana
Bibcode: 2000astu.confE..19L
Altcode:
No abstract at ADS
Title: Solar Public Outreach on a Shoestring Budget: A Community
Approach
Authors: Larson, M. B.; Kankelborg, C. K.; Longcope, D. W.
Bibcode: 2000SPD....3102123L
Altcode: 2000BAAS...32R.833L
Multi-thousand (or even multi-million) dollar Education and Public
Outreach (E/PO) efforts have increased science knowledge and awareness
within the public school system and amongst the general public. In
addition to such large scale outreach programs, there is a niche to
be filled by low budget, widespread outreach efforts like the one
we suggest here. We propose a low budget (approx. \$1000/yr) Public
Outreach effort which utilizes the strong network of amateur astronomy
clubs that exist in most areas. Through cooperation with local contacts,
this public outreach effort works at the community level, and involves
the delivery of scientifically interesting and visually engaging public
lectures by solar research scientists to underserved regions in their
home state.
Title: Magnetic flux tubes inside the sun
Authors: Fisher, G. H.; Fan, Y.; Longcope, D. W.; Linton, M. G.;
Abbett, W. P.
Bibcode: 2000PhPl....7.2173F
Altcode:
Bipolar magnetic active regions are the largest concentrations of
magnetic flux on the Sun. In this paper, the properties of active
regions are investigated in terms of the dynamics of magnetic flux
tubes which emerge from the base of the solar convection zone, where
the solar cycle dynamo is believed to operate, to the photosphere. Flux
tube dynamics are computed with the ``thin flux tube'' approximation,
and by using magnetohydrodynamics simulation. Simulations of active
region emergence and evolution, when compared with the known observed
properties of active regions, have yielded the following results: (1)
The magnetic field at the base of the convection zone is confined to
an approximately toroidal geometry with a field strength in the range
3-10×104 G. The latitude distribution of the toroidal
field at the base of the convection zone is more or less mirrored by
the observed active latitudes; there is not a large poleward drift of
active regions as they emerge. The time scale for emergence of an active
region from the base of the convection zone to the surface is typically
2-4 months. (2) The tilt of active regions is due primarily to the
Coriolis force acting to twist the diverging flows of the rising flux
loops. The dispersion in tilts is caused primarily by the buffeting of
flux tubes by convective motions as they rise through the interior. (3)
Coriolis forces also bend active region flux tube shapes toward the
following (i.e., antirotational) direction, resulting in a steeper
leg on the following side as compared to the leading side of an active
region. When the active region emerges through the photosphere, this
results in a more rapid separation of the leading spots away from the
magnetic neutral line as compared to the following spots. This bending
motion also results in the neutral line being closer to the following
magnetic polarity. (4) The properties of the strongly sheared, flare
productive δ-spot active regions can be accounted for by the dynamics
of highly twisted Ω loops that succumb to the helical kink instability
as they emerge through the solar interior.
Title: A model for the emergence of a twisted magnetic flux tube
Authors: Longcope, D.; Welsch, B.
Bibcode: 2000SPD....31.0401L
Altcode: 2000BAAS...32..834L
Observations have shown that active region flux tubes often emerge in a
twisted state. They also indicate that the active region formed by such
a flux tube has helicity of the same sense as the flux tube which forms
it. Separate theoretical models have been developed for coronal magnetic
fields with helicity and for flux tubes with twist. Here we present
a model matching a twisted sub-photospheric flux tube to a force-free
coronal field. With this model it is possible to explore the emergence
of a flux tube into the corona. In particular, only a fraction of the
tube's current will pass into the coronal field. As a consequence of
this "mismatch" a torsional Alfven wave is launched downward along the
flux tube at the instant of its emergence. As the flux tube continues
to emerge the helicity of the coronal field increases. This would give
the appearance of rotation applied at footpoints.
Title: The Solar Dynamo and Emerging Flux - (Invited Review)
Authors: Fisher, G. H.; Fan, Y.; Longcope, D. W.; Linton, M. G.;
Pevtsov, A. A.
Bibcode: 2000SoPh..192..119F
Altcode:
The largest concentrations of magnetic flux on the Sun occur in
active regions. In this paper, the properties of active regions are
investigated in terms of the dynamics of magnetic flux tubes which
emerge from the base of the solar convection zone, where the solar cycle
dynamo is believed to operate, to the photosphere. Flux tube dynamics
are computed using the `thin flux tube' approximation, and by using
MHD simulation. Simulations of active region emergence and evolution,
when compared with the known observed properties of active regions,
have yielded the following results: (1) The magnetic field at the
base of the convection zone is confined to an approximately toroidal
geometry with a field strength in the range (3-10)×104
G. The latitude distribution of the toroidal field at the base of
the convection zone is more or less mirrored by the observed active
latitudes; there is not a large poleward drift of active regions as
they emerge. The time scale for emergence of an active region from the
base of the convection zone to the surface is typically 2-4 months. The
equatorial gap in the distribution of active regions has two possible
origins; if the toroidal field strength is close to 105 G,
it is due to the lack of equilibrium solutions at low latitude; if it
is closer to 3×104 G, it may be due to modest poleward drift
during emergence. (2) The tilt of active regions is due primarily to the
Coriolis force acting to twist the diverging flows of the rising flux
loops. The dispersion in tilts is caused primarily by the buffeting of
flux tubes by convective motions as they rise through the interior. (3)
The Coriolis force also bends the active region flux tube shape toward
the following (i.e., anti-rotational) direction, resulting in a steeper
leg on the following side as compared to the leading side of an active
region. When the active region emerges through the photosphere, this
results in a more rapid separation of the leading spots away from the
magnetic neutral line as compared to the following spots. This bending
motion also results in the neutral line being closer to the following
magnetic polarity. (4) Active regions behave kinematically after they
emerge because of `dynamic disconnection', which occurs because of the
lack of a solution to the hydrostatic equilibrium equation once the flux
loop has emerged. This could explain why active regions decay once they
have emerged, and why the advection-diffusion description of active
regions works well after emergence. Smaller flux tubes may undergo
`flux tube explosion', a similar process, and provide a source for the
constant emergence of small-scale magnetic fields. (5) The slight trend
of most active regions to have a negative magnetic twist in the northern
hemisphere and positive twist in the south can be accounted for by the
action of Coriolis forces on convective eddies, which ultimately writhes
active region flux tubes to produce a magnetic twist of the correct
sign and amplitude to explain the observations. (6) The properties of
the strongly sheared, flare productive δ-spot active regions can be
accounted for by the dynamics of highly twisted Ω loops that succumb to
the helical kink instability as they emerge through the solar interior.
Title: Forward modeling of the coronal response to reconnection in
an X-ray bright point
Authors: Kankelborg, Charles; Longcope, Dana
Bibcode: 1999SoPh..190...59K
Altcode:
We use MDI magnetic field observations and the theory of reconnection
through a separator to constrain a numerical simulation of an X-ray
bright point observed in EUV by TRACE. A gasdynamic model is employed
to describe the corona and transition region in the bright point
loop. Nonlocal effects such as opacity and ambipolar diffusion are
important to the transition region; these effects are approximated
locally by modification of the radiative loss and thermal conduction. A
straightforward comparison of measured light curves versus those
generated by the simulation shows that the reconnection model is unable
to account for the observations.
Title: Coronal Heating by Collision and Cancellation of Magnetic
Elements
Authors: Longcope, D. W.; Kankelborg, C. C.
Bibcode: 1999ApJ...524..483L
Altcode:
A model is proposed for the coronal response to the interaction between
randomly moving photospheric magnetic flux elements. In this model the
collision between two elements of opposing signs results in reconnection
and the appearance of an X-ray bright point. A section of quiet Sun on
which elements are distributed and moving randomly will contain a number
of X-ray bright points. The model combines a distribution of element
sizes, random velocities of the elements, and a model for pair-wise
collisions. This results in quantitative predictions for surface
density of X-ray bright points, the distribution of their luminosities,
and their contribution to the total heat flux in the quiet Sun. The
predictions depend principally on the densities of flux elements of
each sign B¯+ and B¯-, the average element size
Φ¯, and the random velocity v0. The predicted heat flux,
FXBP=0.1B¯+B¯-v0, is in
rough agreement with published observational studies of X-ray bright
points but well below the flux required to supply heat to the quiet
Sun corona. Other predictions of the model are similarly consistent
with published studies.
Title: Statistical Properties of Magnetic Separators in Model
Active Regions
Authors: Welsch, Brian T.; Longcope, Dana W.
Bibcode: 1999ApJ...522.1117W
Altcode:
Previous theoretical work suggests that magnetic reconnection in the
solar corona should occur along particular topological boundaries in the
coronal magnetic field known as separators. Thus, a field's topological
structure predicts the locations of X-ray/EUV loops, assuming enhanced
emission is related to reconnection. We use this topological model in
a theoretical study of the statistical properties of active-region
loops. We model the interaction of a single ``test'' element of
photospheric magnetic flux with a much larger distribution of flux of
the opposite polarity. We first model the large-scale distribution
of flux in an active region using a mean-field approach and develop
a procedure to determine separator lengths. We then perform Monte
Carlo simulations to check the accuracy of this approximation. The
results of both methods are similar and are well described by
simple scaling laws for separator lengths. Separator lengths scale
as ~exp(αr)/N1/2, where N parameterizes the flux in the
large-scale distribution and r is the distance of the test element from
the distribution's center. This scaling law is a theoretical prediction
of X-ray loop lengths, which can be compared with observations.
Title: The Current Driven Kink Instability and Its Relationship to
delta - SPOT Active Regions
Authors: Linton, M. G.; Fisher, G. H.; Longcope, D. W.; Dahlburg,
R. B.; Fan, Y.
Bibcode: 1999AAS...194.5902L
Altcode: 1999BAAS...31..918L
The current driven kink instability may be the cause of both the unusual
morphology of solar delta -spot active regions and the tendency of
these regions to be significantly more flare active than most active
regions. We investigate the current driven kink instability of flux
tubes in the solar interior both with a linear stability analysis and
with nonlinear MHD simulations. The linear analysis shows that there is
a critical twist, which depends on the axial magnetic field profile,
that a flux tube needs to become kink unstable. This critical twist
decreases as the tube expands, so twisted flux tubes will become
increasingly unstable as they rise through the convection zone. The
nonlinear simulations show that a twisted tube excited by a single
unstable kink mode will evolve to a helical equilibrium state. The
emergence through the photosphere of such a kinked tube would create
an active region which was tilted with respect to Hale's law and
which would rotate as it evolved, as delta -spots are observed to
do. We then find that, when excited by multiple unstable kink modes,
highly twisted flux tubes develop concentrated kinks. These concentrated
kinks would produce more of the observed characteristics of delta -spot
active regions. They would create active regions which, in addition to
emerging tilted and then rotating, would remain compact as they evolved,
and develop strong shear along their magnetic neutral line. Finally,
we find that a strong concentrated kink develops a current sheet at
which the magnetic field reconnects, which may be the cause of the
high flare activity of delta -spots.
Title: A Survey of X-ray Bright Points: Implications for a
Reconnection Model
Authors: Kankelborg, C. C.; Nelson, J.; Longcope, D. W.; Pevtsov, A. A.
Bibcode: 1999AAS...194.1601K
Altcode: 1999BAAS...31..849K
We present a survey of over 350 bright points from archival SOHO
data. Extreme ultraviolet images were measured to determine orientation,
length, and brightness in the EIT 171 angstrom (Fe X, 1 MK) and 195
angstrom (Fe XII, 1.5 MK) passbands. MDI data were analyzed to obtain
the size, orientation, and magnetic flux of the corresponding magnetic
bipoles. The three-dimensional reconnection theory of Longcope (1998)
makes several predictions that may be tested with these data. For this,
the first phase of the study, we concentrate on the scaling of EUV
brightness with magnetic flux and the distribution of displacement
angles between EUV bright points and their magnetic counterparts. We
also verify the assumption of Longcope & Kankelborg (1999) that
the distribution of magnetic orientations is random and independent
of latitude.
Title: A New Self-Organized Criticality Model of Solar Flaring Using
Reconnection at Magnetic Separators
Authors: Noonan, E. J.; Longcope, D. W.
Bibcode: 1999AAS...194.5403N
Altcode: 1999BAAS...31..909N
Several authors have proposed solar flare models in the form of cellular
automota, sometimes called sandpile models (Lu and Hamilton 1991, Vlahos
et al. 1995). Such models are chiefly motivated by the observation
that flare-frequency is related to flare-amplitude by a power-law. We
propose an alternative derivation for a cellular automoton model, based
on reconnection along separators in a complex active-region magnetic
field. We present an example in which a two-dimensional array of
photospheric flux elements is sheared. Stresses develop along magnetic
separators, and are sporadically relieved by reconnection episodes
(flares) of varying sizes. A larger event occurs when reconnection on
one separator triggers reconnection on neighboring separators through
mutual inductance. This reconnection-propagation can be reduced to
a simple set of rules (a cellular automoton), which differ from
those derived by previous authors. In addition, we show that the
implimentation of our model results in a power-law size/frequency
distribution in agreement with observations. The power-law index
also differs from those found in previous two-dimensional sandpile
models. This material is based upon work supported by the National
Science Foundation under Grant No. ATM-9733424.
Title: Heating from X-ray Bright Points in the Quiet Sun Corona:
A Quantitative Model
Authors: Longcope, D. W.; Kankelborg, C. C.
Bibcode: 1999AAS...194.1602L
Altcode: 1999BAAS...31..849L
It has proven difficult to quantify, even approximately, the theoretical
heat flux due to magnetic reconnection in the solar corona. Perhaps
the simplest example of coronal reconnection is an X-ray bright
point, where two isolated concentrations of photospheric flux are
swept together. A theory has been recently proposed providing a
theoretical estimate of the heat released by reconnection between the
two flux concentrations. This energy release depends on the flux of
each element, and the strength of the overlying field. The quiet Sun
contains a dense intermixture of photospheric flux concentrations of
each sign, spanning a wide range of fluxes. We calculate the rate at
which these elements collide to produce X-ray bright points, and the
energy released by each collision. Combining these ingredients provides
quantitative estimates for several properties of the quiet Sun corona,
including the heat flux from magnetic reconnection, the surface density
of X-ray bright points and their distribution in luminosity. Each of
these predictions compares favorably with published observations.
Title: Statistical Properties of Magnetic Separators in Model
Active Regions
Authors: Welsch, B. T.; Longcope, D. W.
Bibcode: 1999AAS...194.5505W
Altcode: 1999BAAS...31..910W
In the tenuous solar corona, the magnetic pressure greatly exceeds
the gas pressure (beta << 1). Hence, the entire coronal volume
above an active region is permeated by magentic flux. Observations,
however, reveal enhanced X-ray/EUV emission along only a small subset
of field lines. Theoretical considerations suggest that these bright
loops might run along particular topological boundaries in the magnetic
field, known as separators. It is along these field lines that magnetic
flux is exchanged from one topological domain to another, and, as a
result of this reconnection process, that energy is released as the
field relaxes to a less complex state. Consequently, knowledge of a
field's topological structure allows one to make predictions about
the X-ray/EUV loops in that field's configuration. Using simple active
region models, theoretical predictions of the statistical properties of
magnetic separators have been calculated. Coronal heating rates can be
computed from these results. This material is based upon work supported
by the National Science Foundation under Grant No. ATM-9733424.
Title: Evolution Equations for Thin Twisted Flux Tubes
Authors: Klapper, I.; Longcope, D.
Bibcode: 1999ASPC..178...79K
Altcode: 1999sdnc.conf...79K
No abstract at ADS
Title: The Origin and Role of Twist in Active Regions
Authors: Fisher, G. H.; Longcope, D. W.; Linton, M. G.; Fan, Y.;
Pevtsov, A. A.
Bibcode: 1999soho....9E..56F
Altcode:
The implications of twist in active region magnetic fields is considered
in this paper. The latitudinal distribution of twist that has been
derived from recent vector magnetogram observations may be explained by
the effects of convective turbulence with a non-zero kinetic helicity
acting on active region scale magnetic flux tubes as they rise through
the convection zone. Highly twisted, kink unstable flux tubes are then
discussed as a possible explanation for many of the observed properties
of flare productive, "d-spot'' active regions.
Title: Twisted Flux Tubes and How They Get That Way
Authors: Longcope, Dana; Linton, Mark; Pevtsov, Alexei; Fisher,
George; Klapper, Isaac
Bibcode: 1999GMS...111...93L
Altcode:
According to present theories, the Sun's magnetic field rises through
the convection zone in the form of slender strands known as flux
tubes, traditionally studied using "thin flux tube" models. While
these models have been remarkably successful they have only recently
begun to account for tubes with twisted magnetic flux, in spite of
observational evidence for such twist. In this work we review the
recent developments pertaining to twisted magnetic flux tubes and
compare quantitative predictions to observations. Hydrodynamic theory
predicts a role for twist in preventing fragmentation. Excessive twist
can also lead to magnetohydrodynamic instability affecting the dynamics
of the tube's axis. A thin tube model for a twisted tube suggests
several possibilities for the origin of twist. The most successful
of these is the Sigma-effect whereby twist arises from deformation of
the tube's axis by turbulence. Simulations show that the Sigma-effect
agrees with observations in magnitude as well as latitudinal dependence.
Title: The Origin and Role of Twist in Active Regions
Authors: Fisher, G. H.; Longcope, D. W.; Linton, M. G.; Fan, Y.;
Pevtsov, A. A.
Bibcode: 1999ASPC..178...35F
Altcode: 1999sdnc.conf...35F
No abstract at ADS
Title: The Solar Dynamo and Emerging Flux
Authors: Fisher, G. H.; Fan, Y.; Longcope, D. W.; Linton, M. G.;
Pevtsov, A. A.
Bibcode: 1999soho....9E..18F
Altcode:
Much has been learned about the dynamics of magnetic flux tubes in the
solar interior over the past decade. By using theoretical models for
the dynamics of active region flux ropes, it is possible to estimate
observable properties of active regions, such as their orientation,
position on the disk, and morphology, and then compare these properties
with active region observations. By varying conditions of the magnetic
flux ropes as the base of the convection zone until observed properties
are matched, one can deduce properties of the magnetic field in the
dynamo layer, such as the magnetic field strength. Observed properties
such as the active region tilt angle, the dispersion of the tilt angle,
and magnetic helicity in active regions will be discussed in terms
of the dynamics of flux tubes rising through the convection zone and
their interaction with convective motions. Properties of Delta spot
active regions will be discussed in terms of the kink instability of
magnetic flux ropes.
Title: Coronal Heating in Active Regions as a Function of Global
Magnetic Variables
Authors: Fisher, George H.; Longcope, Dana W.; Metcalf, Thomas R.;
Pevtsov, Alexei A.
Bibcode: 1998ApJ...508..885F
Altcode:
A comparison of X-ray images of the Sun and full disk magnetograms
shows a correlation between the locations of the brightest X-ray
emission and the locations of bipolar magnetic active regions. This
correspondence has led to the generally accepted idea that magnetic
fields play an essential role in heating the solar corona. To
quantify the relationship between magnetic fields and coronal
heating, the X-ray luminosity of many different active regions
is compared with several global (integrated over entire active
region) magnetic quantities. The X-ray measurements were made with
the SXT Telescope on the Yohkoh spacecraft; magnetic measurements
were made with the Haleakala Stokes Polarimeter at the University
of Hawaii's Mees Solar Observatory. The combined data set
consists of 333 vector magnetograms of active regions taken between
1991 and 1995; X-ray luminosities are derived from time averages
of SXT full-frame desaturated (SFD) images of the given active
region taken within +/-4 hours of each magnetogram. Global magnetic
quantities include the total unsigned magnetic flux Φtot
≡ \smallint dA|Bz|, B2z,tot≡
dAB2z, Jtot ≡ \smallint
dA|Jz|, and B2⊥,tot≡
dAB2⊥, where Jz is the vertical
current density and Bz and B⊥ are the vertical
and horizontal magnetic field amplitudes, respectively. The
X-ray luminosity LX is highly correlated with all of the
global magnetic variables, but it is best correlated with the total
unsigned magnetic flux Φtot. The correlation observed
between LX and the other global magnetic variables
can be explained entirely by the observed relationship between
those variables and Φtot. In particular, no evidence
is found that coronal heating is affected by the current variable
Jtot once the observed relationship between LX
and Φtot is accounted for. A fit between LX
and Φtot yields the relationship LX ~= 1.2 ×
1026 ergs s-1(Φtot/1022
Mx)1.19. The observed X-ray luminosities are compared
with the behavior predicted by several different coronal heating
theories. The Alfvén wave heating model predicts a best relationship
between LX and Φtot, similar to what is found,
but the observed relationship implies a heating rate greater than the
model can accommodate. The ``Nanoflare Model'' of Parker predicts a best
relationship between LX and B2z,tot
rather than Φtot, but the level of heating predicted by the
model can still be compared to the observed data. The result is that
for a widely used choice of the model parameters, the nanoflare model
predicts 1.5 orders of magnitude more heating than is observed. The
``Minimum Current Corona'' model of Longcope predicts a qualitative
variation of LX with Φtot that agrees with
what is observed, but the model makes no quantitative prediction
that can be tested with the data. A comparison between LX
and the magnetic energy Emag in each active region leads
to a timescale that is typically 1 month, or about the lifetime of
an active region, placing an important observational constraint on
coronal heating models. Comparing the behavior of solar active
regions with nearby active stars suggests that the relationship observed
between LX and Φtot may be a fundamental one that
applies over a much wider range of conditions than is seen on the Sun.
Title: NOAA 7926: A Kinked Ω-Loop?
Authors: Pevtsov, Alexei A.; Longcope, Dana W.
Bibcode: 1998ApJ...508..908P
Altcode:
Using vector magnetograms and X-ray images, we study the evolution
of the decaying active region NOAA AR 7926. The active region had
bipolar structure with a leading sunspot of positive (northern [N])
polarity--non-Hale polarity of cycle 22. Observations suggest that the
following (southern [S]) polarity of this active region was in fact the
leading (S) polarity of Active Region 7918 (AR 7918) of the previous
solar rotation. Analyzing the rotation rate of both active regions
and their magnetic field topology, we conclude that they form a single
magnetic system resembling a kinked Ω-loop. During the first rotation,
the upper part of the loop was exposed, forming a bipolar active region
of normal (Hale) polarity. The rest of the Ω-loop had emerged by the
time of the second rotation, giving the appearance of non-Hale polarity.
Title: Nonlinear Evolution of Kink-unstable Magnetic Flux Tubes and
Solar δ-Spot Active Regions
Authors: Linton, M. G.; Dahlburg, R. B.; Fisher, G. H.; Longcope, D. W.
Bibcode: 1998ApJ...507..404L
Altcode:
The motivation for the work described in this paper is to understand
kink-unstable magnetic flux tubes and their role in the formation of
δ-spot active regions. It has been proposed that, during their rise
to the photosphere, a certain fraction of convection zone flux tubes
become twisted to the point where they are unstable to the current
driven kink instability. These kink-unstable flux tubes then evolve
toward a new, kinked equilibrium as they continue to rise to the
photosphere, appearing as δ-spot groups upon emergence. Because of
their kinked nature, these flux tubes could be highly susceptible to
flaring, explaining the very active nature of δ-spot groups. We
study the kinking flux tube problem with a three-dimensional numerical
model containing only the most basic features of a kink-unstable flux
tube. We build on our earlier work describing the linear phase of the
kink instability, and follow the evolution into the nonlinear regime:
(1) We perform numerical simulations of constant-twist, kink-unstable
flux tubes in an initially cylindrical equilibrium configuration in
three dimensions, in a high-β pressure-confined environment. We
consider many different initial configurations, including the
Gold-Hoyle flux tube. (2) These numerical calculations confirm the
growth-rate predictions of our earlier work, when viscous dissipation
is included. They also confirm our velocity profile predictions. (3)
The flux tubes evolve toward new helically symmetric equilibrium
configurations. (4) The timescale for saturation to the kinked
equilibrium configuration is τsat ~ 10/ω0, where
ω0 is the linear growth rate calculated as in the earlier
paper. (5) The cylindrically symmetric part of the kinked equilibrium
is well described by the m = 0 Chandrasekhar-Kendall functions (i.e.,
the Lundquist field). The m = 1 helically symmetric part, however, is
not well described by the m = 1 Chandrasekhar-Kendall functions. (6)
The equilibrium kink amplitudes are not large, at less than one-third
of the tube radius. (7) The peak kinetic energy of the instability can
be predicted from the initial excess perpendicular magnetic energy. (8)
The amplitudes of the kinked tubes are large enough to give a δ-spot
region tilt angle of up to 30° away from that of an unkinked tube.
Title: Flux-Tube Twist Resulting from Helical Turbulence: The
Σ-Effect
Authors: Longcope, D. W.; Fisher, G. H.; Pevtsov, A. A.
Bibcode: 1998ApJ...507..417L
Altcode:
Recent observational studies suggest that active region magnetic
flux emerges in a twisted state and that the sense of twist depends
weakly on solar hemisphere. We propose that this twist is imparted
to the flux through its interaction with turbulent velocities in the
convection zone. This process, designated the Σ-effect, operates on
isolated magnetic flux tubes subjected to buffeting by turbulence with
a nonvanishing kinetic helicity <u \b.dot \b.nabla × u>. The
Σ-effect leads to twist of the same sense inferred from observation
and opposite to that predicted by the α-effect. A series of numerical
calculations are performed to estimate the magnitude of the Σ-effect
in the solar convective zone. The results compare favorably with
observations in both mean value and statistical dispersion. We find
a further relationship with total magnetic flux that can be tested in
future observations. The model also predicts that twist is uncorrelated
with the tilt angle of the active region.
Title: A Model for Current Sheets and Reconnection in X-Ray Bright
Points
Authors: Longcope, D. W.
Bibcode: 1998ApJ...507..433L
Altcode:
X-ray bright points are believed to result from the interaction of
two small magnetic features of opposite polarity. As these features
move apart, flux interconnecting them can become disconnected and
joined to the overlying magnetic field. For features moving toward
one another this process will occur in reverse. In either case this
magnetic reconnection occurs at the separator field line. Assuming that
reconnection can occur only after it is ``triggered,'' the process will
heat the plasma nearby the separator, thereby giving rise to the X-ray
brightening. This can be quantified using a recent model of current
sheet formation and reconnection along separators. Application of this
model predicts the heating from reconnection based on the observable
magnetic quantities of flux, field strength, and polar separation. In
addition, the model predicts morphological aspects of the bright points
such as the apparent angle between the axes of the X-ray loop and the
magnetic bipole.
Title: A current ribbon model for energy storage and release with
application to the flare of 7 January 1992
Authors: Longcope, D. W.; Silva, A. V. R.
Bibcode: 1998SoPh..179..349L
Altcode:
Observations of the flare on 7 January 1992 are interpreted using a
topological model of the magnetic field. The model, developed here,
applies a theory of three-dimensional reconnection to the inferred
magnetic field configuration for 7 January. In the model field
a new bipole (∼ 1021 Mx) emerges amidst pre-existing
active region flux. This emergence gives rise to two current ribbons
along the boundaries (separators) separating the distinct, new and
old, flux systems. Sudden reconnection across these boundary curves
transfers ∼ 3 ×1020 Mx of flux from the bipole into the
surrounding flux. The model also predicts the simultaneous (sympathetic)
flaring of the two current ribbons. This explains the complex two-loop
structure noted in previous observations of this flare. We subject the
model predictions to comparisons with observations of the flare. The
locations of current ribbons in the model correspond closely with
those of observed soft X-ray loops. In addition the footpoints and
apexes of the ribbons correspond with observed sources of microwave
and hard X-ray emission. The magnitude of energy stored by the current
ribbons compares favorably to the inferred energy content of accelerated
electrons in the flare.
Title: Current Sheet Formation and Reconnection on Separator Field
Lines
Authors: Longcope, D. W.
Bibcode: 1998ASSL..229..179L
Altcode: 1998opaf.conf..179L
No abstract at ADS
Title: Dynamics of a Thin Twisted Flux Tube
Authors: Longcope, D. W.; Klapper, I.
Bibcode: 1997ApJ...488..443L
Altcode:
A set of dynamical equations are derived for a slender tube of isolated
magnetic flux generalizing a model due to Spruit. The tube is assumed
to consist of field lines that twist about the tube's axis at some
rate q. The equations describe the evolution of the axis and the
evolution of the twist. They include the interaction between twist and
motions of the axis described as writhing. Through this interaction,
the motion of the axis can introduce twist into a previously untwisted
tube. The twist so introduced will have a sign opposite to the local
handedness of the axial curve. This may be important for the generation
of current in emerging active regions. Tubes with sufficiently large
twist are subject to an instability that distorts the axis into a helix
of pitch similar to the tubes' field lines. Such an instability might
be responsible for the observed morphology in δ-spots on the Sun.
Title: Nonlinear Evolution of Kink Unstable Magnetic Flux Tubes
Authors: Linton, M. G.; Dahlburg, R. B.; Longcope, D. W.; Fisher, G. H.
Bibcode: 1997SPD....28.0241L
Altcode: 1997BAAS...29..900L
We investigate the kink instability of twisted magnetic flux tubes in
the solar convection zone. The possibility that kinking flux tubes are
responsible for the formation of some flare productive active regions
provides the motivation for this work. We simulate the evolution of
a twisted flux tube with a highly parallelized three dimensional MHD
spectral code run on a 128 cubed grid. This code is run on the Naval
Research Laboratory's CM500e. Our earlier work has shown that twisted
magnetic flux tubes in the solar convection zone are linearly unstable
to the kink mode for a wide range of conditions. These nonlinear
simulations support the conclusions of our earlier linear work,
showing that the tubes are unstable in the predicted regions and
with the predicted growth rates. The simulations also allow us to
follow the instability into the nonlinear regime, where it saturates
and the tube settles into a new equilibrium configuration. We will
describe the nonlinear evolution of the tube, it's eventual equilibrium
configuration, and the implications this has for convection zone flux
tubes. This work was supported by NASA GSRP training grant NGT-51377,
the NASA High Performance Computing and Communications Program, NSF
grant AST-9528474 and NASA grant NAGW 3429. The numerical simulations
were performed under a grant of time from the DoD HPC program.
Title: Statistics of Separators in a Model Bipolar Active Region Field
Authors: Welsch, B.; Longcope, D. W.
Bibcode: 1997SPD....28.0256W
Altcode: 1997BAAS...29..903W
In the ideal MHD description of the highly-conductive solar corona,
magnetic flux is "frozen in" to the plasma. This fixes the field's
topology, which constrains its evolution. Theoretical considerations
(Longcope, 1996, Sol. Phys., v. 169, 91) support the belief that,
in sufficiently complex fields, the equations of ideal MHD fail at
well-defined topological boundaries, termed separators. This breakdown
permits the exchange of magnetic flux across these boundaries, i.e.,
reconnection, and the subsequent release of energy previously stored
in the more complicated field. Consequently, the topological structure
of a given field can yield information about the locations and lengths
of coronal x-ray loops. Accordingly, a bipolar sunspot distribution is
modeled, both as a continuous magnetic field and as a superposition
of individual flux elements. Separators are located, and statistical
distributions of their lengths are found. Finally, coronal heating
rates due to reconnection along individual separators are calculated.
Title: Coordinated SOHO, Yohkoh, and Magnetogram Observations Of
Transient Loop Brightenings
Authors: Zarro, D. M.; Metcalf, T. R.; Fisher, G. H.; Siegmund, O.;
Longcope, D. W.; Kucera, T.; Griffiths, N. W.
Bibcode: 1997SPD....28.0503Z
Altcode: 1997BAAS...29..909Z
Transient soft X-ray brightenings occur frequently in solar active
regions, with typical durations of 2-10 minutes. They have been
observed with the Yohkoh Soft X-ray Telescope (SXT) and appear to be
associated primarily with interactions of multiple loops that brighten
initially near their footpoints (Shimuzu et al. 1994, Ap.J., 422,
906). Suggested mechanisms for the production of soft X-ray emission
include: conduction-driven chromospheric evaporation; Alfvenic outflows
from reconnection of colliding field lines; and expulsion of untwisting
loop material in emerging flux tubes (Uchida and Shibata 1988, Solar
Phys., 116, 291). To further study the dynamics of transient soft X-ray
brightenings and their relationship to the lower atmospheric magnetic
field, we have conducted a coordinated SOHO/Yohkoh campaign to observe
soft X-ray brightenings in a small active region at disk center on
1996 June 6. The region was observed simultaneously by Yohkoh SXT, the
Coronal Diagnostic Spectrometer (CDS) on SOHO, and the Imaging Vector
Magnetograph (IVM) at Mees Observatory, Hawaii. In particular, the CDS
instrument obtained Mg X (609 Angstroms) line spectra with 2-3 arcsec
spatial resolution in a 2x2 arcmin field with approximately 8 minute
cadence. The Mg X line is formed in the low corona at approximately
10(6) K. Individual Mg X spectra were obtained with 5 second exposures
per slit position. The SXT and CDS observations show evidence of soft
X-ray brightness variations on timescales of 5-10 minutes. The CDS Mg
X spectra show a mixture of red and blue Doppler shifts (< 100 km
s(-1) ) that are spatially associated with loop footpoints indicated
by the IVM. Based on the observed temporal and spatial variations of
the implied plasma upflows and downflows, we investigate the validity
of different proposed models of transient soft X-ray brightenings.
Title: A Reconnection Model for Observed Transient Loop Brightenings
Authors: Longcope, D. W.; Fisher, G. H.; Metcalf, T. R.; Lemen, J.;
Zarro, D. M.; Kucera, T.; Griffiths, N.; Siegmund, O. H. W.
Bibcode: 1997SPD....28.0128L
Altcode: 1997BAAS...29..884L
Several recent theoretical models explain coronal activity in terms of
magnetic reconnection at ``separator'' field lines. These are field
lines lying at the boundary between domains of coronal flux with
distinct photospheric origin. Transient brightenings of X-ray loops
(Shimizu et al. 1992) may be the manifestations of such localized events
(Longcope 1996). Their relative simplicity, compared to large events
such as flares, makes them ideal objects for exploring reconnection
models. Toward this end, a campaign of coordinated observations of
loop brightenings in a small active region was undertaken on June 6,
1996. High time cadence observations were made of the transition region
and low corona in EUV (SUMER and CDS) and of the high corona in soft
X-rays (Yohkoh), while high cadence, high resolution vector magnetograms
were obtained with the Imaging Vector Magnetograph at the University of
Hawaii. This series of magnetograms is used to calculate the magnetic
topology of the coronal field, and to locate the separator field
lines. The high time cadence of the magnetograms allows the estimation
of reconnection rates: the rate at which flux must be exchanged between
domains. The "minimum current corona" model (Longcope 1996) is then
used to provide quantitative predictions of energy released on each
separator due to this reconnection. The observational results of the
campaign are described in a companion paper by Zarro et al at this
meeting; we discuss our predictions in the context of their results.
Title: Modeling coronal reconnection using field line topology
Authors: Longcope, Dana
Bibcode: 1997SPD....28.1601L
Altcode: 1997BAAS...29Q.920L
It is common to model the Sun's corona as a force-free magnetic
field undergoing slow changes due to motions of the photosphere. The
occurrence of sudden energetic events, such as X-ray bright points,
transient loop brightenings or compact flares, suggests that there
are occasional departures from this quasi-static evolution which might
involve magnetic reconnection. To understand these phenomena using full
magnetohydrodynamics (MHD) it is typically necessary to consider simple
magnetic configurations and employ numerical simulations. Recently,
however, a type of model has been developed which treats a simplified
physical system in much more complex geometries (Demoulin et al. 1993,
Astron. Astrophys. 271, 292). These models simplify the physical
model by attributing the coronal field to discrete photospheric flux
concentrations (point charges) and considering primarily the field's
topology. While inherently less accurate than full MHD treatments,
topological field models offer several advantages both as theoretical
frameworks and as tools for interpreting observations. Discrete magnetic
sources endow the field with topology and define sharp topological
boundaries called separatrices and separators. Study of such a model
shows that motion of the photospheric sources will induce current
along the separators. It is possible to estimate the current carried
and energy stored as a function of flux displacement (Longcope 1996,
Sol. Phys. 169, 91). This leads to quantitative theoretical estimates
for energies and frequencies of flaring or loop brightening and for
average coronal heating rates. To model a specific active region,
rather than generic field structures, a magnetogram is approximated
using discrete sources. Preliminary results reveal the power of this
technique as tool for interpreting observed coronal activity in complex
active regions.
Title: Topology and Current Ribbons: A Model for Current, Reconnection
and Flaring in a Complex, Evolving Corona
Authors: Longcope, D. W.
Bibcode: 1996SoPh..169...91L
Altcode:
Magnetic field enters the corona from the interior of the Sun
through isolated magnetic features on the solar surface. These
features correspond to the tops of submerged magnetic flux tubes, and
coronal field lines often connect one flux tube to another, defining a
pattern of inter-linkage. Using a model field, in which flux tubes are
represented as point magnetic charges, it is possible to quantify this
inter-linkage. If the coronal field were current-free then motions of
the magnetic features would change the inter-linkage through implicit
(vacuum) magnetic reconnection. Without reconnection the conductive
corona develops currents to avoid changing the flux linkage. This
current forms singular layers (ribbons) flowing along topologically
significant field lines called separators. Current ribbons store
magnetic energy as internal stress in the field: the amount of energy
stored is a function of the flux tube displacement. To explore this
process we develop a model called the minimum-current corona (MCC)
which approximates the current arising on a separator in response
to displacement of photospheric flux. This permits a model of the
quasi-static evolution of the corona above a complex active region. We
also introduce flaring to rapidly change the flux inter-linkage between
magnetic features when the internal stress on a separator becomes too
large. This eliminates the separator current and releases the energy
stored by it. Implementation of the MCC in two examples reveals repeated
flaring during the evolution of simple active regions, releasing
anywhere from 1027-1029 ergs, at intervals of
hours. Combining the energy and frequency gives a general expression
for heat deposition due to flaring (i.e., reconnection).
Title: The Helical Kink Instability of Isolated, Twisted Magnetic
Flux Tubes
Authors: Linton, M. G.; Longcope, D. W.; Fisher, G. H.
Bibcode: 1996ApJ...469..954L
Altcode:
To understand the dynamics of twisted active region flux tubes below
the solar photosphere, we investigate the linear kink stability of
isolated, twisted tubes of magnetic flux. We apply linearized equations
of MHD to a cylindrical magnetic equilibrium (screw pinch), but with
significant differences from earlier work. The magnetic field vanishes
outside a radius r = R where it is confined by the higher pressure of
the unmagnetized plasma. The outside boundary of the tube is free to
move, displacing the unmagnetized plasma as it does so. We concentrate
on equilibria where all field lines have the same helical pitch:
Bθ/rBζ = q = const. The main results
are as follows. 1. These equilibria are stable, provided that
the field line pitch does not exceed a threshold; q ≤qcr
for stability. The threshold is qcr=(α)½,
where α is the r2 coefficient in the series expansion of the
equilibrium axial magnetic field (Bζ) about the tube axis (r
= 0): Bζ(r) = BO(1 - αr2 + ⋯). When
this criterion is violated, there are unstable eigenmodes, ξ ∝
e1(θ+kz). The most unstable of these have a helical pitch
k which is near (but not equal to) the field line pitch q. 2. For
weakly twisted tubes (qR ≪ 1) we derive growth rates and unstable
eigenfunctions analytically. For strongly twisted tubes (qR ≤1), we
find growth rates and unstable eigenfunctions numerically. 3. The
maximum growth rate and range of unstable wavenumbers for a strongly
twisted tube can be predicted qualitatively by using the analytical
results from the weakly twisted case. The maximum growth rate in that
case is given by ωmax = υAR(q2
- q2cr)/3.83, where υA is the
axial Alfvén speed. The range of unstable wavenumbers is (- q -
Δk/2) < k <(- q + Δk/2), where Δk = 4qR(q2
-q2cr)½/3.83. 4. The kink
instability we find consists mainly of internal motions. Helical
translations of the entire tube are stable. 5. We argue that an
emerging, twisted magnetic flux loop will tend to have a uniform q along
its length. The increase in the tube radius R as it rises results in
a decreasing value of qcr. This means that the apex of the
flux loop will become kink unstable before the rest of the tube. 6. Our results suggest that most twisted flux tubes rising through
the convection zone will be stable to kinking. Those few tubes which
are kink unstable, and which presumably become knotted or kinked active
regions upon emergence, only become kink unstable some time after they
have begun rising through the convection zone.
Title: The Evolution and Fragmentation of Rising Magnetic Flux Tubes
Authors: Longcope, D. W.; Fisher, G. H.; Arendt, S.
Bibcode: 1996ApJ...464..999L
Altcode:
From its source at the base of the convection zone magnetic flux is
believed to rise buoyantly to the solar surface in the form of isolated
tubes. As it rises, the cross section of such a tube will be distorted
through its interaction with the surrounding unmagnetized medium. This
distortion greatly affects its rate of rise and can ultimately lead
to its fragmentation into two parallel tubes. We derive a set of
Boussinesq fluid equations for studying the perpendicular dynamics
of a rising flux tube. Integrating these numerically shows the tube
distorting and then separating into two fragments with opposing senses
of fluid circulation. The same behavior was observed in numerical
simulations by Schüssler (1979). These counter-rotating elements move
apart horizontally from each other and eventually stop rising. A simple
picture of isolated buoyant material explains this result and confirms
that the rise of the flux is prevented by its fragmentation. This has
important consequences for theories of magnetic flux tube emergence.
Title: Numerical Investigation of Kink Unstable Magnetic Flux Tubes
Authors: Linton, M. G.; Dahlburg, R. B.; Longcope, D. W.; Fisher, G. H.
Bibcode: 1996AAS...188.3610L
Altcode: 1996BAAS...28..874L
We investigate the kink instability of twisted magnetic flux tubes in
the solar convection zone. The possibility that kinking flux tubes
are responsible for the formation of some flare productive active
regions provides the motivation for this work. Our earlier work has
shown that twisted magnetic flux tubes in the solar convection zone are
linearly unstable to the kink mode for a wide range of conditions. We
report on the preliminary results of our numerical simulations of
the nonlinear evolution of these tubes. We simulate the evolution
of a twisted flux tube with a highly parallelized three dimensional
MHD spectral code run on a 128 cubed grid. This code is run on the
Naval Research Laboratory's CM5. We will discuss the results of these
simulations and their implications. This work was supported by NASA
GSRP training grant NGT-51377, the NASA High Performance Computing
and Communications Program, NSF grant AST-9218085 and NASA grant NAGW
3429. The numerical simulations were performed under a grant of time
from the DoD HPC program.
Title: Coronal Heating in Active Regions as a Function of Global
Magnetic Variables
Authors: Fisher, G. H.; Longcope, D. W.; Metcalf, T. R.; Pevtsov, A. A.
Bibcode: 1996AAS...188.3304F
Altcode: 1996BAAS...28..868F
A comparison of X-ray images of the Sun and full disk magnetograms shows
a correlation between the locations of the brightest X-ray emission
and the locations of bipolar magnetic regions. This correspondence
has led to the generally accepted idea that magnetic fields play an
essential role in heating the Solar corona. To quantify the relationship
between magnetic fields and coronal heating, we have compared the
X-ray luminosity of many different Active Regions with several global
(integrated over entire active region) magnetic quantities. The X-ray
measurements were made with the SXT Telescope on the Yohkoh spacecraft;
magnetic measurements were made with the Haleakala Stokes Polarimeter
at the University of Hawaii's Mees Solar Observatory. Our combined
dataset consists of 333 vector magnetograms of active regions taken
between 1991 and 1995; SXT luminosities consist of time averages of
SFD images of the given active region taken within +/- 4 hours of each
magnetogram. Global magnetic quantities include the total unsigned
magnetic flux, area integrals of B(2) , J_z(2) (J_z is the vertical
component of the electric current density), and the best-fit alpha
of the linear force-free field for the entire active region (nabla x
B = alphaB ). Our results show clear and unmistakable relationships
between the X-ray luminosity and most of these magnetic variables. The
relationship between total unsigned magnetic flux and X-ray luminosity
is especially compelling, holding over 2 orders magnitude in both
quantities. These measurements provide important contraints on coronal
heating mechanisms. This work was supported in part by NASA grant
NAGW-3429, NSF grant AST-9218085, and Cal Space grant CS-17-95.
Title: Coronal Quakes at Magnetic Fault Lines: Current sheet formation
and magnetic reconnection along separator field lines
Authors: Longcope, D. W.
Bibcode: 1996AAS...188.3305L
Altcode: 1996BAAS...28..868L
X-ray observations of the Sun's corona show it to be a hot-bed of
energetic activity. Regions of strong magnetic field, in particular,
are subject to sporadic, localized bursts of energy: flares and
microflares. Using a simple three-dimensional model of the coronal
magnetic field it is possible to demonstrate the occurrence of
spontaneous magnetic singularities called current sheets. These
singularities occur at boundaries defined by the topological
inter-linkage of discrete flux tubes through the corona. Stresses
applied to the corona through its photosphere boundary are focused at
current sheet. This stress can be released through fast reconnection,
which we hypothesize to occur when an instability threshold is
crossed. Along these lines accumulation and release of stress at
current sheets serves as a prototype of flares and microflares. Using
this novel model it is possible to explain and quantify the local
and sporadic nature of energy release in the corona. This work was
supported in part by NASA grant NAGW-3429, NSF grant AST-9218085,
and Cal Space grant CS-17-95.
Title: The Effects of Convection Zone Turbulence on the Tilt Angles
of Magnetic Bipoles
Authors: Longcope, D. W.; Fisher, G. H.
Bibcode: 1996ApJ...458..380L
Altcode:
Bipolar magnetic regions are believed to form when flux originating
below the solar convection zone rises to the surface in the form of
long thin loops. Numerical models of rising flux tubes have been able to
explain many observed features of these bipoles, including their angle
of tilt relative to the east-west direction. Observations reveal that
the mean tilt angle, α, varies with both latitude of emergence and
with flux, in agreement with simulations. However, observed bipoles
also exhibit a considerable fluctuation, Δα, about the mean tilt
angle. Here we show that tilt angle fluctuations can arise in model
calculations from interactions with hydrodynamic turbulence during the
tube's rise. Numerical simulations indicate that both the magnitude
of these fluctuations, and their scaling with footpoint separation
(Δα ∼d-1), are consistent with observations. Best
agreement with observations occurs for flux tubes whose magnetic field
strength is similar to those used in other numerical investigations,
B0 ∼ 30 kG. Furthermore, the agreement with observation
suggests that turbulent velocities throughout the convection zone are
consistent with those derived from mixing-length convection models.
Title: The Dynamics of Magnetic Flux Tubes in the Solar Convection
Zone
Authors: Fisher, G. H.; Fan, Y.; Longcope, D. W.; Linton, M. C.
Bibcode: 1996mpsa.conf..329F
Altcode: 1996IAUCo.153..329F
No abstract at ADS
Title: The Kink Instability of Isolated, Twisted Magnetic Flux Tubes
Authors: Linton, M. G.; Longcope, D. W.; Fisher, G. H.
Bibcode: 1995SPD....26.1005L
Altcode: 1995BAAS...27..977L
No abstract at ADS
Title: The Effects of Convection Zone Turbulence on the Tilt Angles
of Magnetic Bipoles
Authors: Fisher, G. H.; Longcope, D. W.
Bibcode: 1995SPD....26..306F
Altcode: 1995BAAS...27..953F
No abstract at ADS
Title: The Form of Ideal Current Layers in Line-tied Magnetic Fields
Authors: Longcope, D. W.; Strauss, H. R.
Bibcode: 1994ApJ...437..851L
Altcode:
It has been argued that a magnetic field which is initially continuous
and is line-tied to rigid boundaries in a continuous manner cannot
develop tangential discontinuities or current sheets. This would appear
to have many consequences in those theories of reconnection and coronal
heating which are based on the existence of such current sheets. It
is shown here that while the nonexistence of current sheet may hold
in a strict sense, it is possible for simple magnetic geometries
to spontaneously develop current layers of nonzero thickness which
are indistinguishable, in a practical sense, from genuine current
sheets. The thickness of these layers can easily be more than six
orders of magnitude smaller than the apparent length scale of the
initial equilibrium. We suggest that numerical magnetohydrodynamics
simulations have encountered such features, but lacked sufficient
resolution to distinguish them from current sheets. Turbulent motion
of photospheric footpoints will generate this type of current layer
in about one eddy turnover.
Title: Evolution and Statistics of Current Sheets in Coronal
Magnetic Loops
Authors: Longcope, D. W.; Sudan, R. N.
Bibcode: 1994ApJ...437..491L
Altcode:
A resistive magnetohydrodynamic model is proposed for a straightened
coronal loop subject to continuous slow fluctuating random footpoint
driving. The characteristic timescale of this driving motion is much
longer than the Alfven transit time along the loop. The governing
equations for this model are integrated numerically until a statistical
steady state is attained. In steady state the spatial structure of
the magnetic field is dominated by thin regions of intense current
density indicative of current sheets. Using a simple model of resistive
reconnection the statistical steady state can be understood as a random
superposition of current sheets. This model predicts the scaling of
the sheet parameters and the global heating with resistivity. The
scaling is verified over the small range of values achievable in these
numerical experiments.
Title: Spontaneous Reconnection of Line-tied Flux Tubes
Authors: Longcope, D. W.; Strauss, H. R.
Bibcode: 1994ApJ...426..742L
Altcode:
A model is presented, for spontaneous reconnection of coronal magnetic
fields subject to axial line-tying. An initial equilibrium representing
multiple flux tubes of alternating helicity is shown to be linearly
unstable to a line-tied generalization of island coalescence. Numerical
time-dependent simulations of the nonlinear evolution of this system
reveal the rapid ideal development of intense current layers between
current tubes with the same sign of helicity. The flow which arises
from this stage leads to equally rapid dissipation of the current
layer. This is accompanied by reconnection as it can be defined
for three-dimensional line-tied fields. The current layers form at
the 'top' of the flux tubes, away from the line-tied ends. In this
region the reconnection very much resembles the resistive phase of
two-dimensional coalescence. The structure of the current layer near
the line-tied ends hints at a generalization of current-sheet theories
to three dimensions. Reconnection can liberate more than 20% of the
free magnetic energy in the initial equilibrium.
Title: Gravitational Ballooning Instability of Prominences
Authors: Strauss, H. R.; Longcope, D. W.
Bibcode: 1994SoPh..149...63S
Altcode:
Prominences can be unstable to a gravitational ballooning instability
of the Rayleigh-Taylor type. A two-dimensional generalized
Kippenhahn-Schlüter prominence equilibrium is constructed. Its
stability to ideal, three-dimensional, short-wavelength line-tied
perturbations is analyzed. The instability requires a critical vertical
density gradient. For a given magnetic field strength, the instability
is sensitive to the angle at which the magnetic field lines cross
the prominence. An approximate, sufficient, threshold condition is
consistent with typical prominence parameters.
Title: Theoretical Studies of Magnetohydrodynamic Equilibria and
Dynamics of a Solar Coronal Loop
Authors: Longcope, Dana Warfield
Bibcode: 1993PhDT.........1L
Altcode:
This work is concerned with the time evolution, both quasi-static
and dynamical, of the magnetic fields in the solar corona, and
its implications for the problem of solar coronal heating. A
theoretical study is made of a collection of closed magnetic flux
tubes whose footpoints are subject to slow, complicated photospheric
displacements. A simple model is developed in which the effects of
gravitation and curvature are ignored and the flux tubes are assumed
to be long and thin. The governing equations for this model are those
of nonideal reduced magnetohydrodynamics. A formalism is developed
for isolating the purely quasi-static component of the system's
evolution. Analytic calculations are carried out using this formalism to
investigate the development of small-scale magnetic structure resulting
from the footpoint motions, called a magnetic cascade. Furthermore
the formalism admits a scenario whereby quasi-static evolution is
interrupted by spontaneous dynamical behavior. This occurs when
the present equilibrium is neutrally stable to a current driven MHD
instability. The nonideal equations are solved under conditions of slow,
continuous, random driving using a time dependent three-dimensional
computer code. The system is found to achieve a statistical steady
state in which the electro -mechanical work done by the driving is
balanced by ohmic and viscous dissipation. The most evident spatial
features of these solutions are three-dimensional current sheets
which develop spontaneously in the interior. These are analyzed
using Sweet-Parker reconnection theory. The global properties of the
statistical steady state can be explained in terms of these current
sheets. It is proposed that the observed current sheets develop
when the present MHD equilibrium becomes unstable to current driven
instabilities. At such a point the system would relax abruptly to a
new equilibrium with lower magnetic energy, thereby liberating its
excess energy as heat. A picture of this process is presented in
terms of a greatly simplified low-dimensional analog of the reduced
MHD system. This low-dimensional system is evolved numerically for
long times and found to exhibit relaxations of varying amplitudes
at random intervals. The statistical distribution of these events is
compared to similar distributions observed in the solar corona.
Title: Spontaneous Development of Current Sheets in Two and Three
Dimensional MHD
Authors: Longcope, D. W.; Strauss, H. R.
Bibcode: 1993BAAS...25.1207L
Altcode:
No abstract at ADS
Title: 3D Reconnection in a Flux Tube
Authors: Strauss, H. R.; Longcope, D. W.
Bibcode: 1993BAAS...25.1207S
Altcode:
No abstract at ADS
Title: Dynamical evolution and structure of solar coronal magnetic
fields
Authors: Longcope, D. W.; Sudan, R. N.
Bibcode: 1992PhFlB...4.2277L
Altcode:
A simple dynamical system is presented which is analogous in many
ways to a solar coronal loop. When this system is subject to slow,
external driving, representing photospheric motion, it undergoes
occasional impulsive relaxations. These relaxations release some
fraction of the magnetic energy as Alfven waves in the loop. When the
model system is integrated for long times it reaches a statistical
steady state in which relaxation events occur at random times with
random amplitudes. The distributions of these quantities is compared
to similar distributions for microflares occurring in the solar corona.
Title: Impulsive events in the evolution of a forced nonlinear system
Authors: Longcope, D. W.; Sudan, R. N.
Bibcode: 1992PhRvL..68.1706L
Altcode:
Long-time numerical solutions of a low-dimensional model of the reduced
MHD equations show that, when this system is driven quasi-statically,
the response is punctuated by impulsive events. The statistics of
these events indicate a Poisson process; the frequency of these events
scales as Delta E(M) exp -1, where Delta E(M) is the energy released
in one event.
Title: Quasi-static Evolution of Coronal Magnetic Fields
Authors: Longcope, D. W.; Sudan, R. N.
Bibcode: 1992ApJ...384..305L
Altcode:
A formalism is developed to describe the purely quasi-static part
of the evolution of a coronal loop driven by its footpoints. This
is accomplished under assumptions of a long, thin loop. The
quasi-static equations reveal the possibility for sudden 'loss of
equilibrium' at which time the system evolves dynamically rather than
quasi-statically. Such quasi-static crises produce high-frequency
Alfven waves and, in conjunction with Alfven wave dissipation models,
form a viable coronal heating mechanism. Furthermore, an approximate
solution to the quasi-static equations by perturbation method verifies
the development of small-scale spatial current structure.
Title: Alternative coronal heating mechanisms
Authors: Sudan, R. N.; Longcope, D. W.
Bibcode: 1992AIPC..267..100S
Altcode: 1992ecsa.work..100S
A unified treatment of the dynamics of the random twisting and
untwisting of a solar magnetic loop by photospheric motion is
presented. For motions fast compared to Alfvén transit time,
the shear Alfvén waves damp rapidly on the stochastic field lines
of the loop. For slow motions, the loop passes through a sequence
of quasi-static equilibria but the response is also punctuated by
impulsive events identified as nanoflares. The statistics of these
events indicate a Poisson process; the frequency of these events scales
as ΔE-1M where ΔEM is the energy in each event. Their contribution
to heating is estimated.