Author name code: vanballegooijen
ADS astronomy entries on 2022-09-14
author:"van Ballegooijen, Adriaan A." OR author:"van Ballegooijen, Aad"
------------------------------------------------------------------------
Title: Study of High-temperature Emission in Solar Active Regions
Authors: Asgari-Targhi, M.; van Ballegooijen, A. A.; Davey, A. R.
Bibcode: 2019ApJ...881..107A
Altcode:
The high-temperature (T > 4 MK) emissions of nonflaring active
regions are investigated in the context of the coronal heating
problem. We study the role of emerging flux, nonpotential magnetic
fields, and sunspots in the heating of active-region loops. Using
extreme ultraviolet images from the Atmospheric Imaging Assembly on
the Solar Dynamic Observatory (SDO), we construct intensity maps in
Fe XVIII 94 Å for 48 active regions. We also use the corresponding
magnetograms from the Helioseismic and Magnetic Imager on SDO to measure
the total magnetic flux. The Fe XVIII 94 Å emission intensity of the
brightest loops is found to be correlated with the presence of sunspots
and emerging or canceling magnetic flux in the photosphere below. We
conclude that sunspots and emerging flux play an important role in
the process of coronal heating and the production of high-temperature
plasmas. We suggest that energy may be injected into the corona as a
result of the dynamics of magnetic fields associated with sunspots
and/or emerging flux. These processes may cause the large magnetic
disturbances (δB ⊥ ∼ 10 G) needed to produce strong
nanoflare-heating events.
Title: Magnetic Field Modeling of Hot Channels in Four Flare/Coronal
Mass Ejection Events
Authors: Liu, Tie; Su, Yingna; Cheng, Xin; van Ballegooijen, Adriaan;
Ji, Haisheng
Bibcode: 2018ApJ...868...59L
Altcode: 2018arXiv181003795L
We investigate the formation and magnetic topology of four flare/coronal
mass ejection events with filament-sigmoid systems, in which the
sigmoidal hot channels are located above the filaments and appear in
pairs before eruption. The formation of hot channels usually takes
several to dozens of hours, during which two J-shaped sheared arcades
gradually evolve into sigmoidal hot channels and then keep stable
for tens of minutes or hours and erupt, while the low-lying filaments
show no significant change. We construct a series of magnetic field
models and find that the best-fit preflare models contain magnetic
flux ropes with hyperbolic flux tubes (HFTs). The field lines above the
HFT correspond to the high-lying hot channel, whereas those below the
HFT surround the underlying filaments. In particular, the continuous
and long field lines representing the flux rope located above the
HFT match the observed hot channels well in three events. However,
for the SOL2014-04-18 event, the flux bundle that mimics the observed
hot channel is located above the flux rope. The flux rope axis lies
in a height range of 19.8 and 46 Mm above the photosphere for the four
events, among which the flux rope axis in the SOL2012-07-12 event has a
maximum height, which probably explains why it is often considered as
a double-decker structure. Our modeling suggests that the high-lying
hot channel may be formed by magnetic reconnections between sheared
field lines occurring above the filament before eruption.
Title: The Minimum Energy Principle Applied to Parker's Coronal
Braiding and Nanoflaring Scenario
Authors: Aschwanden, Markus; van Ballegooijen, A. A.
Bibcode: 2018csc..confE..52A
Altcode: 2018arXiv180805269A
Parker's coronal braiding and nanoflaring scenario predicts the
development of tangential discontinuities and highly misaligned
magnetic field lines, as a consequence of random buffeting of their
footpoints due to the action of sub-photospheric convection. The
increased stressing of magnetic field lines is thought to become
unstable above some critical misalignment angle and to result into
local magnetic reconnection events, which is generally referred to
as Parker's ``nanoflaring scenario''. In this study we show that
the minimum (magnetic) energy principle leads to a bifurcation of
force-free field solutions for helical twist angles at |phi(t)| =
pi, which prevents the build-up of arbitrary large free energies
and misalignment angles. The minimum energy principle predicts that
neighbored magnetic field lines are almost parallel (with misalignment
angles of Delta mu 1.6-1.8 deg, and do not reach a critical misalignment
angle prone to nanoflaring. Consequently, no nanoflares are expected
in the divergence-free and force-free parts of the solar corona, while
they are more likely to occur in the chromosphere and transition region.
Title: The Heating of Solar Coronal Loops by Alfvén Wave Turbulence
Authors: van Ballegooijen, A. A.; Asgari-Targhi, M.; Voss, A.
Bibcode: 2017ApJ...849...46V
Altcode: 2017arXiv171005074V
In this paper we further develop a model for the heating of coronal
loops by Alfvén wave turbulence (AWT). The Alfvén waves are assumed
to be launched from a collection of kilogauss flux tubes in the
photosphere at the two ends of the loop. Using a three-dimensional
magnetohydrodynamic model for an active-region loop, we investigate how
the waves from neighboring flux tubes interact in the chromosphere and
corona. For a particular combination of model parameters we find that
AWT can produce enough heat to maintain a peak temperature of about 2.5
MK, somewhat lower than the temperatures of 3-4 MK observed in the cores
of active regions. The heating rates vary strongly in space and time,
but the simulated heating events have durations less than 1 minute
and are unlikely to reproduce the observed broad differential emission
measure distributions of active regions. The simulated spectral line
nonthermal widths are predicted to be about 27 km s-1, which
is high compared to the observed values. Therefore, the present AWT
model does not satisfy the observational constraints. An alternative
“magnetic braiding” model is considered in which the coronal
field lines are subject to slow random footpoint motions, but we
find that such long-period motions produce much less heating than the
shorter-period waves launched within the flux tubes. We discuss several
possibilities for resolving the problem of producing sufficiently hot
loops in active regions.
Title: High-resolution Observations of Sympathetic Filament Eruptions
by NVST
Authors: Su, Yingna; Li, Shangwei; Zhou, Tuanhui; Van Ballegooijen,
Adriaan A.; Sun, Xudong; Ji, Haisheng
Bibcode: 2017SPD....4810503S
Altcode:
We investigate two sympathetic filament eruptions observed by the
New Vacuum Solar Telescope (NVST) on 2015 October 15. The full
picture of the eruptions is obtained from the corresponding SDO/AIA
observations. The two filaments start from the east border of active
region NOAA 12434 in the north and end in one large quiescent filament
channel in the south. The left filament erupts firstly, followed
by the right filament eruption about 10 minutes later. Clear twist
structure and rotating motion are observed in both filaments during
the eruption. Both eruptions are failed, since the filaments firstly
rise up, then flow towards the south and merge into the southern
large quiescent filament. We also observe repeating activations of
mini filaments below the right filament after its eruption. Using
magnetic field models constructed based on SDO/HMI magnetograms by
flux rope insertion method, we find that the left filament eruption
is likely to be triggered by kink instability, while weakening of
overlying magnetic fields due to magnetic reconnection at an X-point
between the two filament systems might play an important role in the
onset of the right filament eruption.
Title: High-resolution Observations of Sympathetic Filament Eruptions
by NVST
Authors: Li, Shangwei; Su, Yingna; Zhou, Tuanhui; van Ballegooijen,
Adriaan; Sun, Xudong; Ji, Haisheng
Bibcode: 2017ApJ...844...70L
Altcode: 2018arXiv180306088L
We investigate two sympathetic filament eruptions observed by the New
Vacuum Solar Telescope on 2015 October 15. The full picture of the
eruptions is obtained from the corresponding Solar Dynamics Observatory
(SDO)/Atmospheric Imaging Assembly (AIA) observations. The two filaments
start from active region NOAA 12434 in the north and end in one large
quiescent filament channel in the south. The left filament erupts first,
followed by the right filament eruption about 10 minutes later. Clear
twist structure and rotating motion are observed in both filaments
during the eruption. Both eruptions failed, since the filaments first
rise up, then flow toward the south and merge into the southern large
quiescent filament. We also observe repeated activations of mini
filaments below the right filament after its eruption. Using magnetic
field models constructed based on SDO/HMI magnetograms via the flux
rope insertion method, we find that the left filament eruption is
likely to be triggered by kink instability, while the weakening of
overlying magnetic fields due to magnetic reconnection at an X-point
between the two filament systems might play an important role in the
onset of the right filament eruption.
Title: Direct and Inverse Cascades in the Acceleration Region of
the Fast Solar Wind
Authors: van Ballegooijen, A. A.; Asgari-Targhi, M.
Bibcode: 2017ApJ...835...10V
Altcode: 2016arXiv161202501V
Alfvén waves are believed to play an important role in the
heating and acceleration of the fast solar wind emanating
from coronal holes. Nonlinear interactions between the
dominant {{\boldsymbol{z}}}+ waves and minority
{{\boldsymbol{z}}}- waves have the potential to transfer wave
energy either to smaller perpendicular scales (“direct cascade”)
or to larger scales (“inverse cascade”). In this paper we
use reduced magnetohydrodynamic (RMHD) simulations to investigate
how the cascade rates {ɛ }+/- depend on perpendicular
wavenumber and radial distance from the Sun center. For models with
a smooth background atmosphere, we find that an inverse cascade
({ɛ }+< 0) occurs for the dominant waves at radii
between 1.4 and 2.5 {R}⊙ and dimensionless wavenumbers
in the inertial range (15< {a}\perp < 44), and a
direct cascade ({ɛ }+> 0) occurs elsewhere. For a
model with density fluctuations, there are multiple regions with an
inverse cascade. In both cases, the cascade rate {ɛ }+
varies significantly with perpendicular wavenumber, indicating that
the cacsade is a highly nonlocal process. As a result of the inverse
cascades, the energy dissipation rates are much lower than expected
from a phenomenological model and are insufficient to maintain the
temperature of the background atmosphere. We conclude that RMHD models
are unable to reproduce the observed properties of the fast solar wind.
Title: Reflection driven wave turbulence in an open field and the
structure of solar wind
Authors: Asgari-Targhi, M.; van Ballegooijen, A. A.
Bibcode: 2016AGUFMSH51B2595A
Altcode:
We present results from an extensive study of an open magnetic field
line positioned at the center of a coronal hole. We test the hypothesis
that reflection-driven wave turbulence can provide the energy needed for
heating the coronal plasma in the acceleration region of the fast solar
wind. We use the reduced magnetohydrodynamic simulations to describe
the wave turbulence where the simulated wave dissipation rates are
compared with those needed to sustain the background atmosphere. We
consider the effects of density fluctuations, which may significantly
increase the turbulent heating rate. These density variations simulate
the effects of compressive MHD waves on the Alfvén waves. We find
that such variations significantly enhance the wave reflection and
thereby the turbulent dissipation rates, producing enough heat to
maintain the background atmosphere. We conclude that interactions
between Alfvén and compressive waves may play an important role in
the turbulent heating of the fast solar wind.
Title: Modeling the heating and acceleration of the fast solar wind
based on the Alfven wave turbulence
Authors: Asgari, Mah; van Ballegooijen, A. A.
Bibcode: 2016shin.confE..72A
Altcode:
We present numerical simulations of reduced magnetohydrodynamic ( RMHD)
turbulence in a magnetic flux tube at the center of a polar coronal
hole. The turbulence is driven by nonlinear interactions between the
counterpropagating Alfén waves. Results are presented for two models
of the background atmosphere. In the first model the plasma density
and Alfvén speed vary smoothly with height, resulting in minimal
wave reflections and low-energy dissipation rates. In the second
model we introduce additional density variations along the flux tube
with a correlation length of 0.04 Rsun and with relative amplitude of
10%. These density variations simulate the effects of compressive MHD
waves on the Alfén waves. We find that such variations significantly
enhance the wave reflection and thereby the turbulent dissipation
rates, producing enough heat to maintain the background atmosphere. We
conclude that interactions between Alfvén and compressive waves may
play an important role in the turbulent heating of the fast solar wind.
Title: Heating and Acceleration of the Fast Solar Wind by Alfvén
Wave Turbulence
Authors: van Ballegooijen, A. A.; Asgari-Targhi, M.
Bibcode: 2016ApJ...821..106V
Altcode: 2016arXiv160206883V
We present numerical simulations of reduced magnetohydrodynamic (RMHD)
turbulence in a magnetic flux tube at the center of a polar coronal
hole. The model for the background atmosphere is a solution of the
momentum equation and includes the effects of wave pressure on the
solar wind outflow. Alfvén waves are launched at the coronal base and
reflect at various heights owing to variations in Alfvén speed and
outflow velocity. The turbulence is driven by nonlinear interactions
between the counterpropagating Alfvén waves. Results are presented
for two models of the background atmosphere. In the first model the
plasma density and Alfvén speed vary smoothly with height, resulting
in minimal wave reflections and low-energy dissipation rates. We find
that the dissipation rate is insufficient to maintain the temperature
of the background atmosphere. The standard phenomenological formula
for the dissipation rate significantly overestimates the rate derived
from our RMHD simulations, and a revised formula is proposed. In
the second model we introduce additional density variations along
the flux tube with a correlation length of 0.04 R⊙ and
with relative amplitude of 10%. These density variations simulate the
effects of compressive MHD waves on the Alfvén waves. We find that
such variations significantly enhance the wave reflection and thereby
the turbulent dissipation rates, producing enough heat to maintain the
background atmosphere. We conclude that interactions between Alfvén
and compressive waves may play an important role in the turbulent
heating of the fast solar wind.
Title: Investigation of Solar Eruptive Prominences
Authors: Su, Yingna; McCauley, Patrick; van Ballegooijen, Adriaan;
Ji, Haisheng; Reeves, Katharine; DeLuca, Edward
Bibcode: 2015IAUGA..2256101S
Altcode:
At first, we will present an investigation of the polar crown
prominence that erupted on 2012 March 12. This prominence is observed
at the southeast limb by SDO/AIA (end-on view) and displays a quasi
vertical-thread structure. Bright U-shape (horn-like) structure is
observed surrounding the upper portion of the prominence (171 Angstrom)
before the eruption and becomes more prominent during the eruption. When
viewed on the disk, STEREO-B shows that this long prominence is composed
of a series of vertical threads and displays a half loop-like structure
during the eruption. We focus on the magnetic support of the prominence
by studying the structure and dynamics of the prominence before
and during the eruption using observations from SDO and STEREO. We
construct a series of magnetic field models (including sheared arcade
model, twisted flux rope model, and model with HFT), then compare
with observations. Various observational characteristics appear to
support the twisted flux rope model. Our study suggests that the flux
rope supporting the prominence enters the regime of torus instability
at the onset of the fast rise phase, and signature of reconnection
appears about one hour later. In the second part, we will present
a statistical study on the kinematics of limb eruptive prominences
observed by SDO/AIA. A brief introduction on an online catalog of
prominence eruptions observed by SDO/AIA will also be presented.
Title: Magnetic Structure and Dynamics of the Erupting Solar Polar
Crown Prominence on 2012 March 12
Authors: Su, Yingna; van Ballegooijen, Adriaan; McCauley, Patrick;
Ji, Haisheng; Reeves, Katharine K.; DeLuca, Edward E.
Bibcode: 2015ApJ...807..144S
Altcode: 2015arXiv150506826S
We present an investigation of the polar crown prominence that erupted
on 2012 March 12. This prominence is observed at the southeast limb
by the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly
(AIA; end-on view) and displays a quasi-vertical thread structure. A
bright U-shaped or horn-like structure is observed surrounding the upper
portion of the prominence at 171 Å before the eruption and becomes more
prominent during the eruption. The disk view of {STEREO}\_B shows that
this long prominence is composed of a series of vertical threads and
displays a half-loop-like structure during the eruption. We focus on
the magnetic support of the prominence vertical threads by studying the
structure and dynamics of the prominence before and during the eruption
using observations from SDO and STEREO_B. We also construct a series of
magnetic field models (sheared arcade model, twisted flux rope model,
and unstable model with hyperbolic flux tube). Various observational
characteristics appear to be in favor of the twisted flux rope model. We
find that the flux rope supporting the prominence enters the regime of
torus instability at the onset of the fast-rise phase, and signatures of
reconnection (posteruption arcade, new U-shaped structure, rising blobs)
appear about one hour later. During the eruption, AIA observes dark
ribbons seen in absorption at 171 Å corresponding to the bright ribbons
shown at 304 Å, which might be caused by the erupting filament material
falling back along the newly reconfigured magnetic fields. Brightenings
at the inner edge of the erupting prominence arcade are also observed
in all AIA EUV channels, which might be caused by the heating due to
energy released from reconnection below the rising prominence.
Title: Investigation on Eruptive Prominences Observed by SDO
Authors: Su, Yingna; McCauley, Patrick; van Ballegooijen, Adriaan;
Ji, Haisheng; Reeves, Katharine; DeLuca, Edward
Bibcode: 2015TESS....121203S
Altcode:
We will present an investigation of the polar crown prominence that
erupted on 2012 March 12. This prominence is observed at the southeast
limb by SDO/AIA (end-on view) and displays a quasi vertical-thread
structure. Bright U-shape (horn-like) structure is observed surrounding
the upper portion of the prominence (171 Angstrom) before the eruption
and becomes more prominent during the eruption. When viewed on the
disk, STEREO-B shows that this long prominence is composed of a series
of vertical threads and displays a half loop-like structure during
the eruption. We focus on the magnetic support of the prominence
by studying the structure and dynamics of the prominence before and
during the eruption using observations from SDO and STEREO. During
the eruption, AIA observes dark ribbons seen in absorption at 171
Angstrom in corresponding to the bright ribbons at 304 Angstrom. We
construct a series of magnetic field models (including sheared arcade
model, twisted flux rope model, and model with HFT), then compare
with observations. Various observational characteristics appear to
support the twisted flux rope model. Our study suggests that the flux
rope supporting the prominence enters the regime of torus instability
at the onset of the fast rise phase, and evidence of reconnection
(post-eruption arcade, new U-shape Structure, rising blobs) appears
about one hour later. We will also present a statistical study on the
kinematics of limb eruptive prominences observed by SDO/AIA. A brief
introduction on an online catalog of prominence eruptions observed by
SDO/AIA will also be presented.
Title: Nonlinear Force-free Field Modeling of the Solar Magnetic
Carpet and Comparison with SDO/HMI and Sunrise/IMaX Observations
Authors: Chitta, L. P.; Kariyappa, R.; van Ballegooijen, A. A.;
DeLuca, E. E.; Solanki, S. K.
Bibcode: 2014ApJ...793..112C
Altcode: 2014arXiv1408.0497C
In the quiet solar photosphere, the mixed polarity fields form a
magnetic carpet that continuously evolves due to dynamical interaction
between the convective motions and magnetic field. This interplay is a
viable source to heat the solar atmosphere. In this work, we used the
line-of-sight (LOS) magnetograms obtained from the Helioseismic and
Magnetic Imager on the Solar Dynamics Observatory, and the Imaging
Magnetograph eXperiment instrument on the Sunrise balloon-borne
observatory, as time-dependent lower boundary conditions, to study the
evolution of the coronal magnetic field. We use a magneto-frictional
relaxation method, including hyperdiffusion, to produce a time series
of three-dimensional nonlinear force-free fields from a sequence
of photospheric LOS magnetograms. Vertical flows are added up to a
height of 0.7 Mm in the modeling to simulate the non-force-freeness
at the photosphere-chromosphere layers. Among the derived quantities,
we study the spatial and temporal variations of the energy dissipation
rate and energy flux. Our results show that the energy deposited in
the solar atmosphere is concentrated within 2 Mm of the photosphere and
there is not sufficient energy flux at the base of the corona to cover
radiative and conductive losses. Possible reasons and implications are
discussed. Better observational constraints of the magnetic field in
the chromosphere are crucial to understand the role of the magnetic
carpet in coronal heating.
Title: Structure and Dynamics of One Polar Crown Prominence Eruption
Authors: Su, Yingna; Van Ballegooijen, Adriaan A.; McCauley, Patrick I;
Reeves, Kathy; DeLuca, Edward E.; Ji, Haisheng
Bibcode: 2014AAS...22421820S
Altcode:
We will present the recent progress on the investigation of the polar
crown prominence that erupted on 2012 March 12. This prominence is
viewed at the east limb by SDO/AIA and displays a quasi vertical-thread
structure. Bright U-shape (horn-like) structure is observed surrounding
the upper portion of the prominence before the eruption and becomes
more prominent during the eruption. When viewed on the disk, STEREO-B
shows that this prominence is composed of a series of vertical threads
and displays a loop-like structure during the eruption. We focus on
the magnetic support of the prominence by studying the structure
and dynamics of the prominence before and during the eruption
using observations from SDO, Hinode, and STEREO. We found that the
transition from slow rise to fast rise phase is associated with magnetic
reconnection below rising prominence threads. We also constructed a
series of magnetic field models (including sheared arcade model and
twisted flux rope model) of the prominence using the “flux rope
insertion method”, we will compare them with observations in order
to find the best-fit model. Our recent progress on the thermodynamics
of the erupting prominence will also be presented.
Title: Forward Modeling of Coronal Emission
Authors: Malanushenko, Anna; Schrijver, Carolus J.; Van Ballegooijen,
Adriaan A.
Bibcode: 2014AAS...22432102M
Altcode:
In this work, we present simulations of the coronal emission in
Extreme Ultraviolet wavelengths, subject to the possible physical
models of how the solar corona is heated. In order to maximize the
match of the simulations with the observations, we also use models
of coronal magnetic field which are constructed to match the observed
coronal features (see Malanushenko et al, 2014). While we utilize the 1D
quasi-steady atmosphere approach (as in Schrijver & van Ballegoijen,
2005), we take a step away from the commonly used assumption about
circular cross-sections of magnetic flux tubes, as our previous research
(Malanushenko & Schrijver, 2013) suggests that this assumption might
lead to substantial artefacts when comparing the simulations to the
observations. In this work, we explore how such treatment of magnetic
flux tubes is capable of producing realistic coronal features. Using
these two major advances, the realistic field model and the realistic
treatment of the cross-section of flux tubes, we test a wide range
of possible heating scenarios, ruling out possibilities by comparing
the simulations with data from a wide range of EUV channels onboard
SDO/AIA spacecraft.
Title: Investigating a Complex X-class Solar Flare Using Magnetic
Field Modeling
Authors: Su, Yingna; Forland, Blake; Van Ballegooijen, Adriaan A.;
Feng, Li; Ji, Haisheng
Bibcode: 2014AAS...22412329S
Altcode:
We will present the investigation of a complex X-class flare occurred
in NOAA Active Region 11283 on 2011 September 6. This flare is
associated with two filament eruptions and a coronal mass ejection
(CME) with speed of 575 km/s. AR11283 contains mainly two bipoles,
a large decaying bipole and a small “L” shape emerging bipole with
sunspots. The main eruption initiated and occurred in the small bipolar
region. Multiple flare ribbons are observed during the eruption. We
studied the structure and dynamics of the erupting filaments and
flare ribbons observed by SDO/AIA and Hinode/XRT as well as the
dynamics of the photospheric magnetic fields observed by SDO/HMI. We
also constructed magnetic field models using the flux rope insertion
method, in order to understand how the eruption is triggered and how
the flare ribbons are formed during the eruption.
Title: Comparison of Extreme Ultraviolet Imaging Spectrometer
Observations of Solar Coronal Loops with Alfvén Wave Turbulence
Models
Authors: Asgari-Targhi, M.; van Ballegooijen, A. A.; Imada, S.
Bibcode: 2014ApJ...786...28A
Altcode:
The observed non-thermal widths of coronal emission lines could be
due to Alfvén wave turbulence. To test this idea, we examine and
analyze the dynamics of an active region observed on 2012 September
7. We use spectral line profiles of Fe XII, Fe XIII, Fe XV, and Fe
XVI obtained by the Extreme-ultraviolet Imaging Spectrometer on the
it Hinode spacecraft. The observations show non-thermal velocities,
Doppler outflows, and intensities for loops in this active region. The
observed non-thermal velocities are compared with predictions from
models for Alfvén wave turbulence in the observed coronal loops. This
modeling takes into account the relationship between the width of the
coronal emission lines and the orientation of the coronal loops with
respect to the line-of-sight direction. We find that in order to produce
the observed line widths we need to introduce a random parallel-flow
component in addition to the perpendicular velocity due to Alfvén
waves. The observed widths are consistent with photospheric footpoint
velocities in the range 0.3-1.5 km s-1. We conclude that
the Alfvén wave turbulence model is a strong candidate for explaining
how the observed loops are heated.
Title: On the Relationship Between Photospheric Footpoint Motions
and Coronal Heating in Solar Active Regions
Authors: van Ballegooijen, A. A.; Asgari-Targhi, M.; Berger, M. A.
Bibcode: 2014ApJ...787...87V
Altcode:
Coronal heating theories can be classified as either direct current
(DC) or alternating current (AC) mechanisms, depending on whether
the coronal magnetic field responds quasi-statically or dynamically
to the photospheric footpoint motions. In this paper we investigate
whether photospheric footpoint motions with velocities of 1-2 km
s-1 can heat the corona in active regions, and whether the
corona responds quasi-statically or dynamically to such motions (DC
versus AC heating). We construct three-dimensional magnetohydrodynamic
models for the Alfvén waves and quasi-static perturbations generated
within a coronal loop. We find that in models where the effects of the
lower atmosphere are neglected, the corona responds quasi-statically
to the footpoint motions (DC heating), but the energy flux into
the corona is too low compared to observational requirements. In
more realistic models that include the lower atmosphere, the corona
responds more dynamically to the footpoint motions (AC heating) and the
predicted heating rates due to Alfvén wave turbulence are sufficient
to explain the observed hot loops. The higher heating rates are due
to the amplification of Alfvén waves in the lower atmosphere. We
conclude that magnetic braiding is a highly dynamic process.
Title: Structure and Dynamics of an Eruptive Prominence on the
Quiet Sun
Authors: Su, Yingna; Reeves, Katharine K.; McCauley, Patrick; van
Ballegooijen, Adriaan A.; DeLuca, Edward E.
Bibcode: 2014IAUS..300..460S
Altcode:
We present preliminary results on the investigation of one polar crown
prominence that erupted on 2012 March 11. This prominence is viewed
at the east limb by SDO/AIA and displays a simple vertical-thread
structure. A bright U-shape (double horn-like) structure is observed
surrounding the upper portion of the prominence before the eruption and
becomes more prominent during the eruption. When viewed on the disk,
STEREO_B shows that this prominence is composed of series of vertical
threads and displays a loop-like structure during the eruption. We focus
on the magnetic support of the prominence by studying the structure
and dynamics before and during the eruption using observations from
SDO and STEREO. We will also present preliminary DEM analysis of the
cavity surrounding the prominence.
Title: Column Density Measurements of a Prominence Observed by AIA
Authors: McCauley, Patrick I.; Su, Yingna; DeLuca, Edward; van
Ballegooijen, Adriaan
Bibcode: 2014IAUS..300..449M
Altcode:
We present column density measurements of a polar crown prominence
observed on March 9th, 2012 by the Atmospheric Imaging Assembly (AIA)
aboard the Solar Dynamics Observatory. The structure was viewed on
the east limb by AIA and erupted about 30 hours after the observations
shown here. We estimate column density by approximating the obscured
background emission to obtain an optical depth. This can then be
combined with the absorption cross sections of neutral hydrogen and
helium, along with the He:H abundance ratio, to calculate column
density. We perform this calculation for the 171, 193, 211, and 335
Å AIA passbands.
Title: Structure and Topology of Magnetic Fields in Solar Prominences
Authors: van Ballegooijen, Adriaan A.; Su, Yingna
Bibcode: 2014IAUS..300..127V
Altcode:
Recent observations and models of solar prominences are reviewed. The
observations suggest that prominences are located in or below magnetic
flux ropes that lie horizontally above the PIL. However, the details
of the magnetic structure are not yet fully understood. Gravity likely
plays an important role in shaping the vertical structures observed in
quiescent prominences. Preliminary results from a time-dependent model
describing the interaction of a magnetic flux rope with photospheric
magnetic elements are presented.
Title: Magnetohydrodynamic Modeling of the Solar Eruption on 2010
April 8
Authors: Kliem, B.; Su, Y. N.; van Ballegooijen, A. A.; DeLuca, E. E.
Bibcode: 2013ApJ...779..129K
Altcode: 2013arXiv1304.6981K
The structure of the coronal magnetic field prior to eruptive
processes and the conditions for the onset of eruption are important
issues that can be addressed through studying the magnetohydrodynamic
(MHD) stability and evolution of nonlinear force-free field (NLFFF)
models. This paper uses data-constrained NLFFF models of a solar active
region (AR) that erupted on 2010 April 8 as initial conditions in MHD
simulations. These models, constructed with the techniques of flux rope
insertion and magnetofrictional relaxation (MFR), include a stable,
an approximately marginally stable, and an unstable configuration. The
simulations confirm previous related results of MFR runs, particularly
that stable flux rope equilibria represent key features of the
observed pre-eruption coronal structure very well, and that there is
a limiting value of the axial flux in the rope for the existence of
stable NLFFF equilibria. The specific limiting value is located within a
tighter range, due to the sharper discrimination between stability and
instability by the MHD description. The MHD treatment of the eruptive
configuration yields a very good agreement with a number of observed
features, like the strongly inclined initial rise path and the close
temporal association between the coronal mass ejection and the onset of
flare reconnection. Minor differences occur in the velocity of flare
ribbon expansion and in the further evolution of the inclination;
these can be eliminated through refined simulations. We suggest that
the slingshot effect of horizontally bent flux in the source region
of eruptions can contribute significantly to the inclination of the
rise direction. Finally, we demonstrate that the onset criterion,
formulated in terms of a threshold value for the axial flux in the
rope, corresponds very well to the threshold of the torus instability
in the considered AR.
Title: Solar Magnetic Carpet III: Coronal Modelling of Synthetic
Magnetograms
Authors: Meyer, K. A.; Mackay, D. H.; van Ballegooijen, A. A.; Parnell,
C. E.
Bibcode: 2013SoPh..286..357M
Altcode: 2013arXiv1303.1342M
This article is the third in a series working towards the construction
of a realistic, evolving, non-linear force-free coronal-field model
for the solar magnetic carpet. Here, we present preliminary results of
3D time-dependent simulations of the small-scale coronal field of the
magnetic carpet. Four simulations are considered, each with the same
evolving photospheric boundary condition: a 48-hour time series of
synthetic magnetograms produced from the model of Meyer et al. (Solar
Phys.272, 29, 2011). Three simulations include a uniform, overlying
coronal magnetic field of differing strength, the fourth simulation
includes no overlying field. The build-up, storage, and dissipation of
magnetic energy within the simulations is studied. In particular, we
study their dependence upon the evolution of the photospheric magnetic
field and the strength of the overlying coronal field. We also consider
where energy is stored and dissipated within the coronal field. The
free magnetic energy built up is found to be more than sufficient to
power small-scale, transient phenomena such as nanoflares and X-ray
bright points, with the bulk of the free energy found to be stored low
down, between 0.5 - 0.8 Mm. The energy dissipated is currently found
to be too small to account for the heating of the entire quiet-Sun
corona. However, the form and location of energy-dissipation regions
qualitatively agree with what is observed on small scales on the
Sun. Future MHD modelling using the same synthetic magnetograms may
lead to a higher energy release.
Title: The Spatial and Temporal Dependence of Coronal Heating by
Alfvén Wave Turbulence
Authors: Asgari-Targhi, M.; van Ballegooijen, A. A.; Cranmer, S. R.;
DeLuca, E. E.
Bibcode: 2013ApJ...773..111A
Altcode: 2013arXiv1306.6038A
The solar atmosphere may be heated by Alfvén waves that propagate up
from the convection zone and dissipate their energy in the chromosphere
and corona. To further test this theory, we consider wave heating in
an active region observed on 2012 March 7. A potential field model of
the region is constructed, and 22 field lines representing observed
coronal loops are traced through the model. Using a three-dimensional
(3D) reduced magnetohydrodynamics code, we simulate the dynamics
of Alfvén waves in and near the observed loops. The results for
different loops are combined into a single formula describing the
average heating rate Q as a function of position within the observed
active region. We suggest this expression may be approximately valid
also for other active regions, and therefore may be used to construct
3D, time-dependent models of the coronal plasma. Such models are needed
to understand the role of thermal non-equilibrium in the structuring
and dynamics of the Sun's corona.
Title: Numerical simulations of the CME on 2010 April 8
Authors: Su, Yingna; Kliem, Bernhard; van Ballegooijen, Adriaan;
Deluca, Edward
Bibcode: 2013IAUS..294..575S
Altcode:
We present 3D zero-beta ideal MHD simulations of the solar flare/CME
event that occurred in Active Region 11060 on 2010 April 8. The initial
magnetic configurations of the two simulations are stable nonlinear
force-free field and unstable magnetic field models constructed by Su
et al. (2011) using the flux rope insertion method. The MHD simulations
confirm that the stable model relaxes to a stable equilibrium, while
the unstable model erupts as a CME. Comparisons between observations
and MHD simulations of the CME are also presented.
Title: A Non-Linear Force-Free Field Model for the Solar Magnetic
Carpet
Authors: Meyer, Karen; Mackay, D.; van Ballegooijen, A.; Parnell, C.
Bibcode: 2013SPD....4430201M
Altcode:
The magnetic carpet is defined to be the small-scale photospheric
magnetic field of the quiet-Sun. Recent high resolution, high cadence
observations have shown that although small-scale, the magnetic carpet
is far from 'quiet', it is continually evolving in a complex and
dynamic manner. I will present a two-component model for the dynamic
evolution of the Sun's magnetic carpet. The first component is a 2D
model for the photospheric evolution of the small-scale solar magnetic
field, that reproduces many observed parameters. The basic evolution of
magnetic elements within the model is governed by a supergranular flow
profile. In addition, magnetic elements may evolve through the processes
of emergence, cancellation, coalescence and fragmentation. The synthetic
magnetograms produced by the 2D model are then applied as photospheric
boundary data to drive the continuous evolution of a 3D non-linear
force-free coronal field. We studied the resultant complex, small-scale
coronal magnetic field, in particular the energetics of the field.
Title: Structure and Dynamics of the Polar Crown Prominence that
Erupted on 2012 March 12
Authors: Su, Yingna; Van Ballegooijen, A. A.; McCauley, P.; Reeves,
K.; DeLuca, E. E.
Bibcode: 2013SPD....4420302S
Altcode:
We will present preliminary results on the investigation of one polar
crown prominence that erupted on 2012 March 12. This prominence is
viewed at the east limb by SDO/AIA and displays a simple vertical-thread
structure. Bright U-shape (horn-like) structure is observed surrounding
the upper portion of the prominence before the eruption and becomes
more prominent during the eruption. When viewed on the disk, STEREO-B
shows that this prominence is composed of series of vertical threads
and displays a loop-like structure during the eruption. We focus on
the magnetic support of the prominence by studying the structure and
dynamics before and during the eruption using observations from SDO,
Hinode, and STEREO. We will explore magnetic field modeling of this
prominence using the flux rope insertion method. We will also present
preliminary analysis on the thermodynamics of the prominence, namely
DEM analysis of the cavity surrounding the prominence, as well as
column density measurements. This work is supported by NASA Grant
(#NNX12AB25G) and NASA Contract (#SP02H1701R) from LMSAL to SAO.
Title: An MHD Model of a Solar Eruption Starting from NLFFF Initial
Conditions
Authors: DeLuca, Edward E.; Su, Y.; Kliem, B.; Van Ballegooijen, A. A.
Bibcode: 2013SPD....4410301D
Altcode:
The structure of the coronal magnetic field prior to eruptive processes
and the conditions for the onset of eruption are important issues that
can be addressed through studying the magnetohydrodynamic stability
and evolution of nonlinear force-free field (NLFFF) models. This
talk uses data-constrained NLFFF models of a solar active region that
erupted on 2010 Apri 8 as initial conditions in MHD simulations. These
models, constructed with the techniques of flux rope insertion and
magnetofrictional relaxation, include a stable, an approximately
marginally stable, and an unstable configuration. The simulations
confirm previous related results of magnetofrictional relaxation runs,
in particular that stable flux rope equilibria represent key features
of the observed pre-eruption coronal structure very well and that there
is a limiting value of the axial flux in the rope for the existence
of stable NLFFF equilibria. The specific limiting value is located
within a tighter range, due to the sharper discrimination between
stability and instability by the MHD description. The MHD treatment of
the eruptive configuration yields very good agreement with a number of
observed features like the strongly inclined initial rise path and the
close temporal association between the coronal mass ejection and the
onset of flare reconnection. Minor differences occur in the velocity of
flare ribbon expansion and in the further evolution of the inclination;
these can be eliminated through refined simulations. We suggest that
the slingshot effect of horizontally bent flux in the source region
of eruptions can contribute significantly to the inclination of the
rise direction. Finally, we demonstrate that the onset criterion
formulated in terms of a threshold value for the axial flux in the
rope corresponds very well to the threshold of the torus instability
in the considered active region.
Title: The Spatial and Temporal Dependence of Coronal Heating by
Alfven Wave Turbulence
Authors: Asgari-Targhi, Mahboubeh; Van Ballegooijen, A. A.; Cranmer,
S. R.; DeLuca, E. E.
Bibcode: 2013SPD....4430501A
Altcode:
The solar atmosphere may be heated by Alfven waves that propagate up
from the convection zone and dissipate their energy in the chromosphere
and corona. To further test this theory, we consider wave heating in an
active region observed on 2012 March 7. A potential field model of the
region is constructed, and 22 field lines representing observed coronal
loops are traced through the model. Using a three-dimensional (3D)
reduced magneto-hydrodynamics (MHD) code, we simulate the dynamics of
Alfven waves in and near the observed loops. The results for different
loops are combined into a single formula describing the average
heating rate $Q$ as function of position within the observed active
region. We suggest this expression may be approximately valid also
for other active regions, and therefore may be used to construct 3D,
time-dependent models of the coronal plasma. Such models are needed
to understand the role of thermal non-equilibrium in the structuring
and dynamics of the Sun's corona.
Title: Observations and Modeling of Solar Coronal Structures Using
High-Resolution Eclipse Images and Space-based Telescopes with Wide
Field of View
Authors: Lu, Muzhou; Pasachoff, J. M.; Su, Y.; Van Ballegooijen,
A. A.; Seaton, D. B.; West, M.
Bibcode: 2013SPD....44...25L
Altcode:
We present a comparison of the solar corona observed during the total
solar eclipses on 2010 July 11 and on 2012 November 13. The white
light images were taken at Easter Island in 2010 and at Northeast
Queensland, Australia, in 2012; while the concurrent EUV images were
take with SDO/AIA and PROBA2/SWAP. The 2010 eclipse was observed at
the beginning of Sunspot Cycle 24 [1], which peaked near our 2012
observation. We compare a plethora of corona features in the white
light images and reveal some interesting differences in the enhanced EUV
images taken by SDO/AIA and PROBA2/SWAP. We construct potential field
models using our newly refined Coronal Modeling System (CMS2) software
with line-of-sight photospheric magnetograms from SDO/HMI. The source
surface heights derived from detailed comparison between our models
and observations are compared to the standard source-surface model. We
also compare the dynamics of the two eclipse observations. Similar to
the 2010 eclipse, a CME was observed using temporally spaced eclipse
images. We address unresolved problems in the models and observations
with the hope of correcting them for future eclipse observations, such
as the 2017 total solar eclipse across the continental U.S. References
[1] Pasachoff, J. M., Rusin, V., Druckmüllerová, H., Saniga, M.,
Lu, M., Malamut, C., Seaton, D. B., Golub, L., Engell, A. J., Hill,
S. W., Lucas, R., 2011, ApJ, 734, 114
Title: The relationship between magnetic field expansion factors
and solar wind parameters in the corona
Authors: Strachan, Leonard; van Ballegooijen, A.; SOHO/UVCS;
SOHO/LASCO; Peak, NSO/Kitt
Bibcode: 2013SPD....44...33S
Altcode:
We use the Horizontal Current - Current Sheet (HCCS) magnetic field
model to characterize the coronal hole/streamer boundaries in the
corona for the Solar Cycle 23 Minimum (1996-1998). The HCCS model
describes the interface region much more accurately than traditional
PFSS (potential field - source surface) models. Once the models are
computed we can compute expansion factors and magnetic field strengths
anywhere in a 3D grid. These data are combined with coronal outflow
velocities and electron densities from UVCS and LASCO on SOHO to show
how the coronal plasma parameters (at 2.3 solar radii) relate to the
magnetic field geometry.
Title: A Topological View at CME/flare Features with Application to
3D Reconnection
Authors: Savcheva, Antonia Stefanova; Pariat, E.; van Ballegooijen,
A.; Mckillop, S.; Hanson, E.; DeLuca, Y. Su E.
Bibcode: 2013shin.confE.143S
Altcode:
We conduct topology analysis of erupting non-linear force-free
configurations of five sigmoidal active regions observed with Hinode/XRT
and SDO/AIA. The models are computed using the flux rope insertion
method and unstable models are utilized to represent the erupting
configurations. Topology analysis shows that the quasi-separatrix layers
(QSLs) in the chromosphere match well the flare ribbons observed in
these regions. Post-flare loops are also matched well by field lines
lying under the X-line in the models. In addition, we show that
low-lying QSLs associated with the rising flux rope change shape
and extent to match the separating flare ribbons in the images. We
use this kind of topology analysis to extend the standard CME/flare
model to full 3D in observed configurations and find implications to
reconnection in 3D.
Title: The Storage and Dissipation of Magnetic Energy in the Quiet
Sun Corona Determined from SDO/HMI Magnetograms
Authors: Meyer, K. A.; Sabol, J.; Mackay, D. H.; van Ballegooijen,
A. A.
Bibcode: 2013ApJ...770L..18M
Altcode:
In recent years, higher cadence, higher resolution observations
have revealed the quiet-Sun photosphere to be complex and rapidly
evolving. Since magnetic fields anchored in the photosphere extend
up into the solar corona, it is expected that the small-scale
coronal magnetic field exhibits similar complexity. For the first
time, the quiet-Sun coronal magnetic field is continuously evolved
through a series of non-potential, quasi-static equilibria, deduced
from magnetograms observed by the Helioseismic and Magnetic Imager
on board the Solar Dynamics Observatory, where the photospheric
boundary condition which drives the coronal evolution exactly
reproduces the observed magnetograms. The build-up, storage, and
dissipation of magnetic energy within the simulations is studied. We
find that the free magnetic energy built up and stored within the
field is sufficient to explain small-scale, impulsive events such
as nanoflares. On comparing with coronal images of the same region,
the energy storage and dissipation visually reproduces many of the
observed features. The results indicate that the complex small-scale
magnetic evolution of a large number of magnetic features is a key
element in explaining the nature of the solar corona.
Title: Influence of surface stressing on stellar coronae and winds
Authors: Jardine, M.; Vidotto, A. A.; van Ballegooijen, A.; Donati,
J. -F.; Morin, J.; Fares, R.; Gombosi, T. I.
Bibcode: 2013MNRAS.431..528J
Altcode: 2013arXiv1304.0349J
The large-scale field of the Sun is well represented by its lowest
energy (or potential) state. Recent observations, by comparison,
reveal that many solar-type stars show large-scale surface magnetic
fields that are highly non-potential - that is, they have been stressed
above their lowest energy state. This non-potential component of the
surface field is neglected by current stellar wind models. The aim of
this paper is to determine its effect on the coronal structure and
wind. We use Zeeman-Doppler surface magnetograms of two stars - one
with an almost potential, one with a non-potential surface field - to
extrapolate a static model of the coronal structure for each star. We
find that the stresses are carried almost exclusively in a band of
unidirectional azimuthal field that is confined to mid-latitudes. Using
this static solution as an initial state for a magnetohydrodynamic (MHD)
wind model, we then find that the final state is determined primarily
by the potential component of the surface magnetic field. The band of
azimuthal field must be confined close to the stellar surface, as it
is not compatible with a steady-state wind. By artificially increasing
the stellar rotation rate, we demonstrate that the observed azimuthal
fields cannot be produced by the action of the wind but must be due
to processes at or below the stellar surface. We conclude that the
background winds of solar-like stars are largely unaffected by these
highly stressed surface fields. Nonetheless, the increased flare
activity and associated coronal mass ejections that may be expected
to accompany such highly stressed fields may have a significant impact
on any surrounding planets.
Title: Observations and Modeling of the Emerging Extreme-ultraviolet
Loops in the Quiet Sun as Seen with the Solar Dynamics Observatory
Authors: Chitta, L. P.; Kariyappa, R.; van Ballegooijen, A. A.;
DeLuca, E. E.; Hasan, S. S.; Hanslmeier, A.
Bibcode: 2013ApJ...768...32C
Altcode: 2013arXiv1303.3426C
We used data from the Helioseismic and Magnetic Imager (HMI) and the
Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory
(SDO) to study coronal loops at small scales, emerging in the quiet
Sun. With HMI line-of-sight magnetograms, we derive the integrated
and unsigned photospheric magnetic flux at the loop footpoints in the
photosphere. These loops are bright in the EUV channels of AIA. Using
the six AIA EUV filters, we construct the differential emission measure
(DEM) in the temperature range 5.7-6.5 in log T (K) for several hours
of observations. The observed DEMs have a peak distribution around
log T ≈ 6.3, falling rapidly at higher temperatures. For log T <
6.3, DEMs are comparable to their peak values within an order of
magnitude. The emission-weighted temperature is calculated, and its
time variations are compared with those of magnetic flux. We present
two possibilities for explaining the observed DEMs and temperatures
variations. (1) Assuming that the observed loops are composed of
a hundred thin strands with certain radius and length, we tested
three time-dependent heating models and compared the resulting DEMs
and temperatures with the observed quantities. This modeling used
enthalpy-based thermal evolution of loops (EBTEL), a zero-dimensional
(0D) hydrodynamic code. The comparisons suggest that a medium-frequency
heating model with a population of different heating amplitudes can
roughly reproduce the observations. (2) We also consider a loop model
with steady heating and non-uniform cross-section of the loop along
its length, and find that this model can also reproduce the observed
DEMs, provided the loop expansion factor γ ~ 5-10. More observational
constraints are required to better understand the nature of coronal
heating in the short emerging loops on the quiet Sun.
Title: Connecting the Sun's High-resolution Magnetic Carpet to the
Turbulent Heliosphere
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.; Woolsey,
Lauren N.
Bibcode: 2013ApJ...767..125C
Altcode: 2013arXiv1303.0563C
The solar wind is connected to the Sun's atmosphere by flux tubes
that are rooted in an ever-changing pattern of positive and negative
magnetic polarities on the surface. Observations indicate that the
magnetic field is filamentary and intermittent across a wide range
of spatial scales. However, we do not know to what extent the complex
flux-tube topology seen near the Sun survives as the wind expands into
interplanetary space. In order to study the possible long-distance
connections between the corona and the heliosphere, we developed new
models of turbulence-driven solar wind acceleration along empirically
constrained field lines. We used a potential field model of the quiet
Sun to trace field lines into the ecliptic plane with unprecedented
spatial resolution at their footpoints. For each flux tube, a
one-dimensional model was created with an existing wave/turbulence code
that solves equations of mass, momentum, and energy conservation from
the photosphere to 4 AU. To take account of stream-stream interactions
between flux tubes, we used those models as inner boundary conditions
for a time-steady magnetohydrodynamic description of radial and
longitudinal structure in the ecliptic. Corotating stream interactions
smear out much of the smallest-scale variability, making it difficult
to see how individual flux tubes on granular or supergranular scales
can survive out to 1 AU. However, our models help clarify the level of
"background" variability with which waves and turbulent eddies should
be expected to interact. Also, the modeled fluctuations in magnetic
field magnitude were seen to match measured power spectra quite well.
Title: On the Support of Solar Prominence Material by the Dips of
a Coronal Flux Tube
Authors: Hillier, Andrew; van Ballegooijen, Adriaan
Bibcode: 2013ApJ...766..126H
Altcode: 2013arXiv1303.4130H
The dense prominence material is believed to be supported against
gravity through the magnetic tension of dipped coronal magnetic
field. For quiescent prominences, which exhibit many gravity-driven
flows, hydrodynamic forces are likely to play an important role in
the determination of both the large- and small-scale magnetic field
distributions. In this study, we present the first steps toward creating
a three-dimensional magneto-hydrostatic prominence model where the
prominence is formed in the dips of a coronal flux tube. Here 2.5D
equilibria are created by adding mass to an initially force-free
magnetic field, then performing a secondary magnetohydrodynamic
relaxation. Two inverse polarity magnetic field configurations are
studied in detail, a simple o-point configuration with a ratio of the
horizontal field (Bx ) to the axial field (By
) of 1:2 and a more complex model that also has an x-point with a
ratio of 1:11. The models show that support against gravity is either
by total pressure or tension, with only tension support resembling
observed quiescent prominences. The o-point of the coronal flux tube
was pulled down by the prominence material, leading to compression of
the magnetic field at the base of the prominence. Therefore, tension
support comes from the small curvature of the compressed magnetic field
at the bottom and the larger curvature of the stretched magnetic field
at the top of the prominence. It was found that this method does not
guarantee convergence to a prominence-like equilibrium in the case
where an x-point exists below the prominence flux tube. The results
imply that a plasma β of ~0.1 is necessary to support prominences
through magnetic tension.
Title: Rotating Motions and Modeling of the Erupting Solar Polar-crown
Prominence on 2010 December 6
Authors: Su, Yingna; van Ballegooijen, Adriaan
Bibcode: 2013ApJ...764...91S
Altcode: 2012arXiv1211.6967S
A large polar-crown prominence composed of different segments spanning
nearly the entire solar disk erupted on 2010 December 6. Prior to
the eruption, the filament in the active region part split into two
layers: a lower layer and an elevated layer. The eruption occurs in
several episodes. Around 14:12 UT, the lower layer of the active region
filament breaks apart: One part ejects toward the west, while the other
part ejects toward the east, which leads to the explosive eruption of
the eastern quiescent filament. During the early rise phase, part of
the quiescent filament sheet displays strong rolling motion (observed
by STEREO-B) in the clockwise direction (viewed from east to west)
around the filament axis. This rolling motion appears to start from
the border of the active region, then propagates toward the east. The
Atmospheric Imaging Assembly (AIA) observes another type of rotating
motion: In some other parts of the erupting quiescent prominence, the
vertical threads turn horizontal, then turn upside down. The elevated
active region filament does not erupt until 18:00 UT, when the erupting
quiescent filament has already reached a very large height. We develop
two simplified three-dimensional models that qualitatively reproduce
the observed rolling and rotating motions. The prominence in the models
is assumed to consist of a collection of discrete blobs that are tied
to particular field lines of a helical flux rope. The observed rolling
motion is reproduced by continuous twist injection into the flux rope in
Model 1 from the active region side. Asymmetric reconnection induced by
the asymmetric distribution of the magnetic fields on the two sides of
the filament may cause the observed rolling motion. The rotating motion
of the prominence threads observed by AIA is consistent with the removal
of the field line dips in Model 2 from the top down during the eruption.
Title: Alfvén Waves and the Heating of Solar Coronal Loops
Authors: Asgari-Targhi, M.; van Ballegooijen, A. A.
Bibcode: 2013ASPC..470...77A
Altcode:
We construct a 3D magnetic model for the dissipation and propagation of
Alfvén waves in a coronal loop. The waves are assumed to be generated
by foot-point motions inside the kilogauss magnetic flux elements at
the two ends of the loop. Counter propagating waves are subject to
non-linear interactions that lead to turbulent decay of the waves
and heating of the chromospheric and coronal plasma. We find that
hot coronal loops typically found in active region and the underlying
chromosphere can be explained by Alfvén wave turbulence.
Title: MHD Modeling of the Solar Eruption on 2010 April 8
Authors: Kliem, B.; Su, Y.; Van Ballegooijen, A. A.; DeLuca, E.
Bibcode: 2012AGUFMSH51A2194K
Altcode:
We present a numerical MHD study of the solar eruption on 2010 April
8, extending the previous modeling of the source region in Su et
al. (2011) which had employed the flux rope insertion method and
magnetofrictional relaxation. The threshold of the rope's axial flux
for the loss of equilibrium obtained in Su et al. is confirmed. We find
that the inserted flux rope partly splits for slightly subcritical axial
flux. Starting with slightly supercritical axial flux in the rope, the
MHD simulation yields a fast and strongly inclined eruption as observed
by the STEREO and SDO instruments. The causes of the inclination will
be explored. We also model photospheric changes that may have driven
the flux rope from a stable to the unstable configuration.
Title: Relating Alfvén Wave Heating Model to Observations of a
Solar Active Region
Authors: Yoritomo, J. Y.; Van Ballegooijen, A. A.
Bibcode: 2012AGUFMSH33B2228Y
Altcode:
We compared images from the Solar Dynamics Observatory's (SDO)
Atmospheric Imaging Assembly (AIA) with simulations of propagating and
dissipating Alfvén waves from a three-dimensional magnetohydrodynamic
(MHD) model (van Ballegooijen et. al 2011; Asgari-Targhi & van
Ballegooijen 2012). The goal was to search for observational evidence of
Alfvén waves in the solar corona and understand their role in coronal
heating. We looked at one particular active region on the 5th of May
2012. Certain distinct loops in the SDO/AIA observations were selected
and expanded. Movies were created from these selections in an attempt
to discover transverse motions that may be Alfvén waves. Using a
magnetogram of that day and the corresponding synoptic map, a potential
field model was created for the active region. Three-dimensional
MHD models for several loops in different locations in the active
region were created. Each model specifies the temperature, pressure,
magnetic field strength, average heating rate, and other parameters
along the loop. We find that the heating is intermittent in the loops
and reflection occurs at the transition region. For loops at larger
and larger height, a point is reached where thermal non-equilibrium
occurs. In the center this critical height is much higher than in
the periphery of the active region. Lastly, we find that the average
heating rate and coronal pressure decrease with increasing height
in the corona. This research was supported by an NSF grant for the
Smithsonian Astrophysical Observatory (SAO) Solar REU program and a
SDO/AIA grant for the Smithsonian Astrophysical Observatory.
Title: The Spatial Dependence of Coronal Heating by Alfven Wave
Turbulence
Authors: Asgari-targhi, M.; Van Ballegooijen, A. A.; Cranmer, S. R.;
DeLuca, E. E.
Bibcode: 2012AGUFMSH31B..05A
Altcode:
We consider the wave heating in an active region observed on 7th of
March 2012 (Image). Using a potential field model we choose 22 field
lines and construct 3D MHD models of the Alfven waves along those
field lines. Based on those results we develop a heating formula
for the coronal loops observed. In our calculations, we establish
explicit relationships between the energy deposited and the loop
parameters, such as the length, and the magnetic field strength along
the loop. We also look at the variation of the heating within the loops
and predict the velocity fluctuations seen with future high-resolution
spectrographs.A potential field modeling of an active region observed
on 7th of March 2012.
Title: New Models of Solar Wind Acceleration and Stream Interactions
in the Sun's Topologically Complex Magnetic Field
Authors: Cranmer, S. R.; Van Ballegooijen, A. A.; Woolsey, L. N.
Bibcode: 2012AGUFMSH53A2266C
Altcode:
The last decade has seen significant progress toward identifying and
characterizing the processes that heat the corona and accelerate the
solar wind. It is believed that the low-speed solar wind comes from
a wide range of source regions in the corona, including streamers,
pseudostreamers, active regions, and small coronal holes. These
source regions tend to be associated with the most topologically
complex magnetic fields, and it is unclear how the coronal field lines
connect to the large-scale open heliospheric field. To learn more about
these connections, we present new models of turbulence-driven coronal
heating and solar wind acceleration along empirically constrained field
lines. To begin, we chose a time period during which the footpoints
linked to the ecliptic plane were rooted in Quiet Sun (QS) regions away
from both large coronal holes and strong-field active regions. The
weak and mixed-polarity QS field was observed at high resolution by
the VSM instrument of SOLIS, and we extrapolated this field into the
corona using the potential field source surface method. Time-steady
1D models of individual flux tubes were created with the ZEPHYR code
(Cranmer et al. 2007) that solves the one-fluid equations of mass,
momentum, and energy conservation from the photosphere to 4 AU. Then,
to take account of stream-stream interactions between the flux tubes, we
solved a 2D time-steady set of MHD conservation equations to determine
the corotating longitudinal structure in the ecliptic plane. We aim to
understand the extent to which fine-scale inter-tube plasma structures
in the corona survive to large distances. In other words, we want to
know how much of the coronal flux tube "spaghetti" is either shredded
by turbulence or smeared out by stream interactions. We also plan
to evaluate the level of high-resolution detail that is needed in
coronal flux tube modeling in order to accurately predict the space
weather consequences of various kinds of corotating structures in the
solar wind.
Title: Photospheric Flux Cancellation and the Build-up of Sigmoidal
Flux Ropes on the Sun
Authors: Savcheva, A. S.; Green, L. M.; van Ballegooijen, A. A.;
DeLuca, E. E.
Bibcode: 2012ApJ...759..105S
Altcode:
In this study we explore the scenario of photospheric flux cancellation
being the primary formation mechanism of sigmoidal flux ropes in
decaying active regions. We analyze magnetogram and X-ray observations
together with data-driven non-linear force-free field (NLFFF) models of
observed sigmoidal regions to test this idea. We measure the total and
canceled fluxes in the regions from MDI magnetograms, as well as the
axial and poloidal flux content of the modeled NLFFF flux ropes for
three sigmoids—2007 February, 2007 December, and 2010 February. We
infer that the sum of the poloidal and axial flux in the flux ropes for
most models amounts to about 60%-70% of the canceled flux and 30%-50%
of the total flux in the regions. The flux measurements and the analysis
of the magnetic field structure show that the sigmoids first develop
a strong axial field manifested as a sheared arcade and then, as flux
cancellation proceeds, form long S-shaped field lines that contribute to
the poloidal flux. In addition, the dips in the S-shaped field lines are
located at the sites of flux cancellation that have been identified from
the MDI magnetograms. We find that the line-of-sight-integrated free
energy is also concentrated at these locations for all three regions,
which can be liberated in the process of eruption. Flare-associated
brightenings and flare loops coincide with the location of the X-line
topology that develops at the site of most vigorous flux cancellation.
Title: Observations and Magnetic Field Modeling of a Solar Polar
Crown Prominence
Authors: Su, Yingna; van Ballegooijen, Adriaan
Bibcode: 2012ApJ...757..168S
Altcode: 2012arXiv1208.1524S
We present observations and magnetic field modeling of the large
polar crown prominence that erupted on 2010 December 6. Combination
of Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly
(AIA) and STEREO_Behind/EUVI allows us to see the fine structures
of this prominence both at the limb and on the disk. We focus on the
structures and dynamics of this prominence before the eruption. This
prominence contains two parts: an active region part containing mainly
horizontal threads and a quiet-Sun part containing mainly vertical
threads. On the northern side of the prominence channel, both AIA
and EUVI observe bright features which appear to be the lower legs
of loops that go above then join in the filament. Filament materials
are observed to frequently eject horizontally from the active region
part to the quiet-Sun part. This ejection results in the formation
of a dense-column structure (concentration of dark vertical threads)
near the border between the active region and the quiet Sun. Using
the flux rope insertion method, we create nonlinear force-free field
models based on SDO/Helioseismic and Magnetic Imager line-of-sight
magnetograms. A key feature of these models is that the flux rope
has connections with the surroundings photosphere, so its axial flux
varies along the filament path. The height and location of the dips
of field lines in our models roughly replicate those of the observed
prominence. Comparison between model and observations suggests
that the bright features on the northern side of the channel are
the lower legs of the field lines that turn into the flux rope. We
suggest that plasma may be injected into the prominence along these
field lines. Although the models fit the observations quiet well,
there are also some interesting differences. For example, the models
do not reproduce the observed vertical threads and cannot explain the
formation of the dense-column structure.
Title: Asymmetric Structure of Quiescent Filament Channels Observed
by Hinode/XRT and STEREO/EUVI
Authors: Su, Y.; van Ballegooijen, A.; Golub, L.
Bibcode: 2012ASPC..454..113S
Altcode: 2012arXiv1208.1529S
We present a study on the structure of quiescent filament channels
observed by Hinode/XRT and STEREO/EUVI from December 2006 to February
2009. For 10 channels identified on the solar disk, we find that the
emission on the two sides of the channel is asymmetric in both X-rays
and EUV: one side has curved bright features while the other side
has straight faint features. We interpret the results in terms of a
magnetic flux rope model. The asymmetry in the emission is due to the
variation in axial magnetic flux along the channel, which causes one
polarity to turn into the flux rope, while the field lines from the
other polarity are open or connected to very distant sources. For 70
channels identified by cavities at the limb, the asymmetry cannot be
clearly identified.
Title: Proton, Electron, and Ion Heating in the Fast Solar Wind from
Nonlinear Coupling between Alfvénic and Fast-mode Turbulence
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.
Bibcode: 2012ApJ...754...92C
Altcode: 2012arXiv1205.4613C
In the parts of the solar corona and solar wind that experience the
fewest Coulomb collisions, the component proton, electron, and heavy ion
populations are not in thermal equilibrium with one another. Observed
differences in temperatures, outflow speeds, and velocity distribution
anisotropies are useful constraints on proposed explanations for how the
plasma is heated and accelerated. This paper presents new predictions of
the rates of collisionless heating for each particle species, in which
the energy input is assumed to come from magnetohydrodynamic (MHD)
turbulence. We first created an empirical description of the radial
evolution of Alfvén, fast-mode, and slow-mode MHD waves. This model
provides the total wave power in each mode as a function of distance
along an expanding flux tube in the high-speed solar wind. Next, we
solved a set of cascade advection-diffusion equations that give the
time-steady wavenumber spectra at each distance. An approximate term
for nonlinear coupling between the Alfvén and fast-mode fluctuations is
included. For reasonable choices of the parameters, our model contains
enough energy transfer from the fast mode to the Alfvén mode to excite
the high-frequency ion cyclotron resonance. This resonance is efficient
at heating protons and other ions in the direction perpendicular to
the background magnetic field, and our model predicts heating rates
for these species that agree well with both spectroscopic and in situ
measurements. Nonetheless, the high-frequency waves comprise only a
small part of the total Alfvénic fluctuation spectrum, which remains
highly two dimensional as is observed in interplanetary space.
Title: Comparison of a Magnetohydrodynamical Simulation and a
Non-Linear Force-Free Field Model of a Sigmoidal Active Region.
Authors: Pariat, Etienne; DeLuca, Edward; Van Ballegooijen, Adriaan;
Aulanier, Guillaume; Savcheva, Antonia
Bibcode: 2012cosp...39.1448P
Altcode: 2012cosp.meet.1448P
Sigmoids are solar magnetic structures where highly non-potential
fields (strong shear/twist) are believed to be present. Thanks to
the high level of free magnetic energy, active regions with sigmoids
possess a higher eruptivity. In the present study, we will present
a comparive topological analysis between a Non-Linear Force Free
Field (NLFFF) model of sigmoid region, and a three-dimensional (3D)
magnetohydrodynamics numerical simulation of the formation and eruption
of such a structure. The MHD simulation is based on an idealized
magnetic field distribution and the sigmoidal flux rope is built by
means of shearing motions and magnetic polarity diffusion. The NLFFF
model is based on the flux rope insertion method which utilizes line of
sight magnetograms and X-ray observations of the region to constrain the
models. We compare the geometrical and topological properties of the 3D
magnetic fields given by both methods in their pre-eruptive phases. We
arrive at a consistent picture for the evolution and eruption of the
sigmoid by using the idealized MHD simulation as a context for the more
specific observationally-constrained NLFFF models and data. Although,
the two models are very different in their setups, we identify strong
similarities between the two models and understandable differences. By
computing the squashing factor in different horizontal maps at various
heights above the photosphere and in vertical cuts in the domains,
we demonstrate the existence of key Quasi-Separatrix Layers (QSL)
eventually involved in the dynamic of the structure. We also show that
there are electric current concentrations coinciding with the main
QSLs. Finally, we perform torus instability analysis and show that
a combination between reconnection at the main QSL and the resulting
expansion of the flux rope into the torus instability domain is the
cause of the CME in both models. This study finally highlights the
interest of the use of in-depth topological tools to study highly
non-potential magnetic fields.
Title: Photospheric flux cancellation and the build-up of sigmoidal
flux ropes
Authors: Savcheva, Antonia Stefanova; Green, L.; van Ballegooijen,
A.; DeLuca, E.
Bibcode: 2012shin.confE.122S
Altcode:
The magnetic structure of sigmoidal active regions is generally
associated with the presence of a twisted flux rope held down by a
potential arcade. There are competing theories of how the flux rope
develops - by flux emergence, cancellation, or footpoint motions. We
look at how flux cancellation in several sigmoidal regions, observed
with XRT, affects the buildup of the underlying flux ropes. We use
MDI magnetograms to quantify the flux cancellation, and the flux rope
insertion method to construct non-linear force free field models of the
regions. These models allow us to produce 3-D magnetic field models
and see how the fields evolve in time. The models show how the flux
ropes energy and magnetic flux changes during the different stages in
the flux cancellation. Flux cancellation events are associated with
build up of twist in the region in accordance with the accepted flux
cancellation picture. The location of flares and build-up of free
energy is well correlated with flux cancellation events.
Title: Dynamics of the Solar Magnetic Bright Points Derived from
Their Horizontal Motions
Authors: Chitta, L. P.; van Ballegooijen, A. A.; Rouppe van der Voort,
L.; DeLuca, E. E.; Kariyappa, R.
Bibcode: 2012ApJ...752...48C
Altcode: 2012arXiv1204.4362C
The subarcsecond bright points (BPs) associated with the small-scale
magnetic fields in the lower solar atmosphere are advected by
the evolution of the photospheric granules. We measure various
quantities related to the horizontal motions of the BPs observed in
two wavelengths, including the velocity autocorrelation function. A
1 hr time sequence of wideband Hα observations conducted at the
Swedish 1 m Solar Telescope (SST) and a 4 hr Hinode G-band time
sequence observed with the Solar Optical Telescope are used in this
work. We follow 97 SST and 212 Hinode BPs with 3800 and 1950 individual
velocity measurements, respectively. For its high cadence of 5 s as
compared to 30 s for Hinode data, we emphasize more the results from
SST data. The BP positional uncertainty achieved by SST is as low as 3
km. The position errors contribute 0.75 km2 s-2
to the variance of the observed velocities. The raw and corrected
velocity measurements in both directions, i.e., (vx ,
vy ), have Gaussian distributions with standard deviations
of (1.32, 1.22) and (1.00, 0.86) km s-1, respectively. The
BP motions have correlation times of about 22-30 s. We construct the
power spectrum of the horizontal motions as a function of frequency,
a quantity that is useful and relevant to the studies of generation
of Alfvén waves. Photospheric turbulent diffusion at timescales less
than 200 s is found to satisfy a power law with an index of 1.59.
Title: Solar Magnetic Carpet II: Coronal Interactions of Small-Scale
Magnetic Fields
Authors: Meyer, K. A.; Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2012SoPh..278..149M
Altcode: 2012arXiv1211.3924M
This paper is the second in a series of studies working towards
constructing a realistic, evolving, non-potential coronal model for
the solar magnetic carpet. In the present study, the interaction of
two magnetic elements is considered. Our objectives are to study
magnetic energy build-up, storage and dissipation as a result of
emergence, cancellation, and flyby of these magnetic elements. In
the future these interactions will be the basic building blocks of
more complicated simulations involving hundreds of elements. Each
interaction is simulated in the presence of an overlying uniform
magnetic field, which lies at various orientations with respect to the
evolving magnetic elements. For these three small-scale interactions,
the free energy stored in the field at the end of the simulation ranges
from 0.2 - 2.1×1026 ergs, whilst the total energy dissipated
ranges from 1.3 - 6.3×1026 ergs. For all cases, a stronger
overlying field results in higher energy storage and dissipation. For
the cancellation and emergence simulations, motion perpendicular
to the overlying field results in the highest values. For the flyby
simulations, motion parallel to the overlying field gives the highest
values. In all cases, the free energy built up is sufficient to explain
small-scale phenomena such as X-ray bright points or nanoflares. In
addition, if scaled for the correct number of magnetic elements for
the volume considered, the energy continually dissipated provides a
significant fraction of the quiet Sun coronal heating budget.
Title: Sigmoidal Active Region on the Sun: Comparison of a
Magnetohydrodynamical Simulation and a Nonlinear Force-free Field
Model
Authors: Savcheva, A.; Pariat, E.; van Ballegooijen, A.; Aulanier,
G.; DeLuca, E.
Bibcode: 2012ApJ...750...15S
Altcode:
In this paper we show that when accurate nonlinear force-free
field (NLFFF) models are analyzed together with high-resolution
magnetohydrodynamic (MHD) simulations, we can determine the physical
causes for the coronal mass ejection (CME) eruption on 2007 February
12. We compare the geometrical and topological properties of the
three-dimensional magnetic fields given by both methods in their
pre-eruptive phases. We arrive at a consistent picture for the
evolution and eruption of the sigmoid. Both the MHD simulation and
the observed magnetic field evolution show that flux cancellation
plays an important role in building the flux rope. We compute the
squashing factor, Q, in different horizontal maps in the domains. The
main shape of the quasi-separatrix layers (QSLs) is very similar
between the NLFFF and MHD models. The main QSLs lie on the edge of
the flux rope. While the QSLs in the NLFFF model are more complex due
to the intrinsic large complexity in the field, the QSLs in the MHD
model are smooth and possess lower maximum value of Q. In addition,
we demonstrate the existence of hyperbolic flux tubes (HFTs) in both
models in vertical cross sections of Q. The main HFT, located under the
twisted flux rope in both models, is identified as the most probable
site for reconnection. We also show that there are electric current
concentrations coinciding with the main QSLs. Finally, we perform torus
instability analysis and show that a combination between reconnection
at the HFT and the resulting expansion of the flux rope into the torus
instability domain is the cause of the CME in both models.
Title: Observations and Magnetic Field Modeling of a Large Polar
Crown Prominence
Authors: Su, Yingna; van Ballegooijen, A.
Bibcode: 2012AAS...22031004S
Altcode:
We focus on the structures and dynamics of a large polar crown
prominence before its eruption on 2010 December 6. Combination of
SDO/AIA and STEREO/EUVI allows us to see the fine structures of this
prominence both at the limb and on the disk. This prominence contains
two parts: active region part containing mainly horizontal threads,
and quiet Sun part containing mainly vertical threads. Both EUVI
and AIA observe bright features on the northern side of the filament
channel, while no clear counterparts are identified on the southern
side. The earlier limb observations by AIA suggest that the bright
features are the lower legs of the loops that go above then join in
the filament. Horn-like features seen by AIA/171 are located above
the prominence vertical threads. Filament materials frequently eject
horizontally from the active region part to the quiet Sun part, which
results in the formation of a dense-column structure (concentration of
dark vertical threads) near the border of the active region. Using the
flux-rope insertion method, we create two non-linear force-free field
models with highly and weakly twisted flux rope, respectively. The
models are constructed based on the line-of-sight magnetograms provided
by SDO/HMI. The height and location of field lines' dips in both
models roughly replicate those of the observed prominence. Comparison
between model and observations suggests that the bright features
on the northern side of the channel are the lower legs of the field
lines that turn into the flux rope. The observed horn-like features
suggest that the model with weakly twisted flux rope may be a better
model. However, there are also some interesting differences between
models and observations. For example, the model cannot explain the
existence of vertical threads nor the dense-column structure.
Title: Asymmetric Magnetic Reconnection in Solar Flare and Coronal
Mass Ejection Current Sheets
Authors: Murphy, N. A.; Miralles, M. P.; Pope, C. L.; Raymond, J. C.;
Winter, H. D.; Reeves, K. K.; Seaton, D. B.; van Ballegooijen, A. A.;
Lin, J.
Bibcode: 2012ApJ...751...56M
Altcode: 2012arXiv1203.5360M
We present two-dimensional resistive magnetohydrodynamic simulations
of line-tied asymmetric magnetic reconnection in the context of solar
flare and coronal mass ejection current sheets. The reconnection
process is made asymmetric along the inflow direction by allowing the
initial upstream magnetic field strengths and densities to differ, and
along the outflow direction by placing the initial perturbation near
a conducting wall boundary that represents the photosphere. When the
upstream magnetic fields are asymmetric, the post-flare loop structure
is distorted into a characteristic skewed candle flame shape. The
simulations can thus be used to provide constraints on the reconnection
asymmetry in post-flare loops. More hard X-ray emission is expected
to occur at the footpoint on the weak magnetic field side because
energetic particles are more likely to escape the magnetic mirror
there than at the strong magnetic field footpoint. The footpoint on
the weak magnetic field side is predicted to move more quickly because
of the requirement in two dimensions that equal amounts of flux must
be reconnected from each upstream region. The X-line drifts away from
the conducting wall in all simulations with asymmetric outflow and
into the strong magnetic field region during most of the simulations
with asymmetric inflow. There is net plasma flow across the X-line
for both the inflow and outflow directions. The reconnection exhaust
directed away from the obstructing wall is significantly faster than
the exhaust directed toward it. The asymmetric inflow condition allows
net vorticity in the rising outflow plasmoid which would appear as
rolling motions about the flux rope axis.
Title: Applications of Quasi-Separatrix Layer Maps in Understanding
an XRT Sigmoid
Authors: Savcheva, A.; van Ballegooijen, A.; DeLuca, E. E.
Bibcode: 2012ASPC..455..261S
Altcode:
We present our recent work on utilizing Quasi-Separatrix Layer (QSL)
maps for understanding the structure, evolution, and pre-eruption
behavior of the quiescent sigmoid observed with Hinode/XRT in February
2007. QSL maps are created at various heights in the corona and for
10 different observations during the evolution of the long-lasting
sigmoid. This is the first QSL analysis based on a non-linear force
free field of a sigmoid. We point out some major properties of the
QSL topology in the sigmoid and we explore how they change spatially
and temporally with the evolution of the sigmoid. We explore how QSL
topology and strength relate to current distributions and torsion
factors in the moments leading to the B-class flare. While the current
distribution in the region is smooth and extended, the QSLs show much
finer structure which may prove to be of help in pinpointing possible
reconnection or heating sites.
Title: Topological Tools For The Analysis Of Active Region Filament
Stability
Authors: DeLuca, Edward E.; Savcheva, A.; van Ballegooijen, A.;
Pariat, E.; Aulanier, G.; Su, Y.
Bibcode: 2012AAS...22020207D
Altcode:
The combination of accurate NLFFF models and high resolution MHD
simulations allows us to study the changes in stability of an active
region filament before a CME. Our analysis strongly supports the
following sequence of events leading up to the CME: first there is a
build up of magnetic flux in the filament through flux cancellation
beneath a developing flux rope; as the flux rope develops a hyperbolic
flux tube (HFT) forms beneath the flux rope; reconnection across
the HFT raises the flux rope while adding addition flux to it; the
eruption is triggered when the flux rope becomes torus-unstable. The
work applies topological analysis tools that have been developed over
the past decade and points the way for future work on the critical
problem of CME initiation in solar active regions. We will present
the uses of this approach, current limitations and future prospects.
Title: Structure and Dynamics of Quiescent Prominence Eruptions
Authors: Su, Y.; Lu, M.; van Ballegooijen, A.
Bibcode: 2012ASPC..456..165S
Altcode:
We present a survey on the fine structure and dynamics of quiescent
prominence eruptions observed both on the disk and at the limb. We
have identified 45 quiescent prominence eruptions by looking at the SDO
(Solar Dynamics Observatory)/AIA (Atmospheric Imaging Assembly) daily
movies from April to June in 2011. Among these events, there are 24
symmetric eruptions (coherent loop-like eruptions) and 21 asymmetric
eruptions (one footpoint lifts off) as shown by AIA and STEREO/EUVI
observations. Vertical filament threads are identified in 10 out of
the 45 events, while horizontal threads are observed in almost all
eruptions. We find 23 events with twisting/untwisting motions. For 14
selected limb events, we carry out a detailed study of the eruption
dynamics using AIA observations at 304 Å. We find that the initial
heights of these erupting prominences are located around 50-110 Mm
above the limb. The eruptions start from a speed of less than 5 km/s,
then increase to several tens km/s in the AIA field of view. The
maximum speed of these events is 50 km/s. The acceleration plots show
a positive acceleration in the range of 0 to 20 m/s2. No
significant difference is identified in the dynamics of the symmetric
and asymmetric eruptions.
Title: Dynamics of the Photospheric Bright Points Observed With SST
and Hinode
Authors: Chitta, Lakshmi Pradeep; van Ballegooijen, A.; Rouppe van
der Voort, L.; DeLuca, E.; Kariyappa, R.
Bibcode: 2012AAS...22020614C
Altcode: 2012AAS...22020614P
The horizontal motions of the solar magnetic bright points (BPs)
observed in two wavelengths (SST Halpha and Hinode/SOT G-band)
is studied in detail. With emphasis on SST results: the velocity
distribution of horizontal motions is found to be a Gaussian. The
auto-correlations of observed velocities is also obtained. An
empirical fit to the observed auto-correlation gives us a positional
uncertainty of 3 km and the error in the velocity measurements to be
0.87 km s$^{-1}$. Due to the non-Lorentzian, cusp-like nature of the
auto-correlation, the power spectrum of the BP motions shows enhanced
power at frequencies exceeding 0.02 Hz. The diffusion of magnetic field
due to granular evolution at short timescales is found to satisfy a
power law with a slope of 1.59.
Title: The Effect of Flux Cancellation on Building Sigmoidal Flux
Ropes
Authors: Savcheva, Antonia; Green, L.; van Ballegooijen, A.; DeLuca, E.
Bibcode: 2012AAS...22041105S
Altcode:
The magnetic structure of sigmoidal active regions is generally
associated with the presence of a twisted flux rope held down by a
potential arcade. There are competing theories of how the flux rope
develops - by flux emergence, cancellation, or footpoint motions. We
look at how flux cancellation in several sigmoidal regions, observed
with XRT, affects the buildup of the underlying flux ropes. We use
MDI magnetograms to quantify the flux cancellation, and the flux rope
insertion method to construct non-linear force free field models of the
regions. These models allow us to produce 3-D magnetic field models
and see how the fields evolve in time. The models show how the flux
ropes energy and magnetic flux changes during the different stages in
the flux cancellation. Flux cancellation events are associated with
build up of twist in the region in accordance with the accepted flux
cancelation picture. The location of flares and build-up of free energy
is well correlated with flux cancellation events.
Title: Topological tools for the analysis of active region filament
stability
Authors: DeLuca, Edward E.; Savcheva, A.; van Ballegooijen, A.;
Pariat, E.; Aulanier, G.; Su, Y.
Bibcode: 2012decs.confE..64D
Altcode:
The combination of accurate NLFFF models and high resolution MHD
simulations allows us to study the changes in stability of an active
region filament before a CME. Our analysis strongly supports the
following sequence of events leading up to the CME: first there is a
build up of magnetic flux in the filament through flux cancellation
beneath a developing flux rope; as the flux rope develops a hyperbolic
flux tube (HFT) forms beneath the flux rope; reconnection across
the HFT raises the flux rope while adding addition flux to it; the
eruption is triggered when the flux rope becomes torus-unstable. The
work applies topological analysis tools that have been developed over
the past decade and points the way for future work on the critical
problem of CME initiation in solar active regions. We will discuss
the uses of this approach, current limitations and future prospects.
Title: The role of magnetic braiding and MHD wave dynamics in the
heating of the Sun's outer atmosphere
Authors: van Ballegooijen, A. A.
Bibcode: 2012decs.confE..34V
Altcode:
The solar corona is thought to be heated by dissipation of magnetic
disturbances that propagate up from the Sun's convection zone. Two
types of disturbance have often been considered: (1) twisting and
braiding of coronal field lines as a result of random footpoint
motions in the photosphere, and (2) MHD waves launched by such
motions. One difficulty with the former is that coronal observations
with Hinode/XRT and other imaging instruments show little evidence
for braided fields. Furthermore, quasi-static braiding models predict
that in active regions the misalignment angles of the braided field
lines relative to the potential field should be large ( 20 degrees),
which is not consistent with coronal loop observations. We suggest
that the braiding occurs on small transverse length scales in the
corona (a few Mm or less), and must involve small misalignment angles
(at most a few degrees). We argue that the braiding is dominated by
small-scale footpoint motions occurring inside the kilogauss flux tubes
in the photosphere (size < 100 km). Results from 3D MHD simulations
of braided fields in coronal loops are presented. According to these
models the footpoint motions cause Alfven waves to be launched into
the system. The waves strongly reflect at the transition region,
which leads to counter-propagating Alfven waves and turbulence in the
chromospheric parts of the flux tube. Such turbulence has a major effect
on the properties of the Alfven waves injected into the corona: the wave
periods and transverse scales of the waves are much smaller than those
of the photospheric footpoint motions. As a result, the turbulence in
the corona proceeds very rapidly and is able to dissipate the injected
energy very quickly, leading to small misalignment angles consistent
with coronal observations. We find that most of the wave energy is
dissipated in the lower atmosphere, consistent with observations of
chromospheric and coronal heating rates in active regions. Therefore,
this new dynamic braiding model appears to be consistent with a variety
of observational constraints. We conclude there is a close relationship
between the braiding of coronal field lines and wave heating processes.
Title: Model for Alfvén Wave Turbulence in Solar Coronal Loops:
Heating Rate Profiles and Temperature Fluctuations
Authors: Asgari-Targhi, M.; van Ballegooijen, A. A.
Bibcode: 2012ApJ...746...81A
Altcode:
It has been suggested that the solar corona may be heated by dissipation
of Alfvén waves that propagate up from the solar photosphere. According
to this theory, counterpropagating Alfvén waves are subject to
nonlinear interactions that lead to turbulent decay of the waves and
heating of the chromospheric and coronal plasma. To test this theory,
better models for the dynamics of Alfvén waves in coronal loops are
required. In this paper, we consider wave heating in an active region
observed with the Solar Dynamics Observatory in 2010 May. First a
three-dimensional (3D) magnetic model of the region is constructed,
and ten magnetic field lines that match observed coronal loops are
selected. For each loop we construct a 3D magnetohydrodynamic model of
the Alfvén waves near the selected field line. The waves are assumed
to be generated by footpoint motions inside the kilogauss magnetic
flux elements at the two ends of the loop. Based on such models,
we predict the spatial and temporal profiles of the heating along
the selected loops. We also estimate the temperature fluctuations
resulting from such heating. We find that the Alfvén wave turbulence
model can reproduce the observed characteristics of the hotter loops
in the active region core, but the loops at the periphery of the region
have large expansion factors and are predicted to be thermally unstable.
Title: Field Topology Analysis of a Long-lasting Coronal Sigmoid
Authors: Savcheva, A. S.; van Ballegooijen, A. A.; DeLuca, E. E.
Bibcode: 2012ApJ...744...78S
Altcode:
We present the first field topology analysis based on nonlinear
force-free field (NLFFF) models of a long-lasting coronal sigmoid
observed in 2007 February with the X-Ray Telescope on Hinode. The
NLFFF models are built with the flux rope insertion method and
give the three-dimensional coronal magnetic field as constrained by
observed coronal loop structures and photospheric magnetograms. Based
on these models, we have computed horizontal maps of the current and
the squashing factor Q for 25 different heights in the corona for all
six days of the evolution of the region. We use the squashing factor
to quantify the degree of change of the field line linkage and to
identify prominent quasi-separatrix layers (QSLs). We discuss the major
properties of these QSL maps and devise a way to pick out important QSLs
since our calculation cannot reach high values of Q. The complexity
in the QSL maps reflects the high degree of fragmentation of the
photospheric field. We find main QSLs and current concentrations that
outline the flux rope cavity and that become characteristically S-shaped
during the evolution of the sigmoid. We note that, although intermittent
bald patches exist along the length of the sigmoid during its whole
evolution, the flux rope remains stable for several days. However,
shortly after the topology of the field exhibits hyperbolic flux tubes
(HFT) on February 7 and February 12 the sigmoid loses equilibrium
and produces two B-class flares and associated coronal mass ejections
(CMEs). The location of the most elevated part of the HFT in our model
coincides with the inferred locations of the two flares. Therefore, we
suggest that the presence of an HFT in a coronal magnetic configuration
may be an indication that the system is ready to erupt. We offer a
scenario in which magnetic reconnection at the HFT drives the system
toward the marginally stable state. Once this state is reached, loss
of equilibrium occurs via the torus instability, producing a CME.
Title: Coronal Heating of an Active Region Observed by the Solar
Dynamics Observatory
Authors: Asgari-targhi, M.; Van Ballegooijen, A. A.
Bibcode: 2011AGUFMSH43B1943A
Altcode:
A realistic model of Alfven wave turbulence for coronal loops in
an observed active region is constructed. An important question is
whether turbulent heating leads to thermally stable loops, as seems
to be required by the observations (Klimchuk et al. 2010). We use
data from the Solar Dynamics Observatory (SDO) for an active region
observed on 2010 May 5. Coronal images obtained with the Atmospheric
Imager Assembly (AIA) in several EUV passbands show the presence of
coronal loops with temperatures in the range 1-3 MK. Our goal is to
determine whether these loops may be heated by Alfven wave turbulence,
and if so, to predict the observational signatures of such waves and
turbulence. Using data from the Heliospheric and Magnetic Imager (HMI),
we construct a three-dimensional (3D) magnetic model of the region,
and select field lines that match eleven of the observed loops (image
1). This provides us with the magnetic field strength B0(s) along
each loop. Using a modified version of the reduced MHD code developed
in our previous paper (van Ballegooijen et al. 2011), we construct 3D
time-dependent MHD models for the Alfven waves in each loop and derive
estimates of the heating rate averaged over time and cross-sectional
area of the loop. Then we iteratively adjust the coronal temperature and
density for each loop in order to satisfy the requirements of energy
balance. This ensures that the rate of energy loss from the modeled
coronal loops is consistent with the volumetric heating rate predicted
by the Alfven wave turbulence model. The results of this modeling are
compared with the observed fine structures of the coronal loops. We
find that the Alfven wave turbulence model can reproduce the observed
characteristics of the hotter loops in the active region core, but
the cooler loops at the periphery of the region have large expansion
factors and are predicted to be thermally unstable.
Title: Tools for Predicting the Rates of Turbulent Heating for
Protons, Electrons, and Heavy Ions in the Solar Wind
Authors: Cranmer, S. R.; Chandran, B. D.; Van Ballegooijen, A. A.
Bibcode: 2011AGUFMSH41C..04C
Altcode:
In the parts of the solar corona and solar wind that experience
the fewest Coulomb collisions, the various particle species (i.e.,
protons, electrons, and heavy ions) are not in thermal equilibrium
with one another. The particles exhibit a range of different outflow
speeds, temperatures, and velocity distribution anisotropies, and
these differences can be used to probe the kinetic physical processes
that are responsible for depositing energy into the plasma. In this
presentation, we outline a new modeling framework for simulating the
rates of collisionless heating for each species, in which the energy
input is assumed to come from MHD turbulence. We begin by creating a
one-dimensional model of damped wave action conservation for Alfven,
fast-mode, and slow-mode MHD waves. This model provides the total wave
power in each mode as a function of radial distance along an expanding
solar wind flux tube. Next we solve a set of cascade advection-diffusion
equations that give the time-steady Fourier wavenumber spectra at each
distance. An approximate term for nonlinear mode coupling between
the Alfven and fast-mode fluctuations is included. We find that for
sufficiently high amplitudes of the fast-mode waves, there arises
enough Alfven wave energy at high frequencies to excite the proton
and ion cyclotron resonances and heat these particles in the direction
perpendicular to the background magnetic field. Although results will be
shown primarily for the plasma conditions in polar coronal holes that
give rise to high-speed solar wind streams, the tools outlined above
can be applied straightforwardly in other plasma environments as well.
Title: Solar Magnetic Carpet I: Simulation of Synthetic Magnetograms
Authors: Meyer, K. A.; Mackay, D. H.; van Ballegooijen, A. A.; Parnell,
C. E.
Bibcode: 2011SoPh..272...29M
Altcode: 2011SoPh..tmp..294M; 2011SoPh..tmp..198M; 2011SoPh..tmp..319M;
2011SoPh..tmp..267M; 2011arXiv1108.1080M
This paper describes a new 2D model for the photospheric evolution
of the magnetic carpet. It is the first in a series of papers
working towards constructing a realistic 3D non-potential model
for the interaction of small-scale solar magnetic fields. In the
model, the basic evolution of the magnetic elements is governed by a
supergranular flow profile. In addition, magnetic elements may evolve
through the processes of emergence, cancellation, coalescence and
fragmentation. Model parameters for the emergence of bipoles are based
upon the results of observational studies. Using this model, several
simulations are considered, where the range of flux with which bipoles
may emerge is varied. In all cases the model quickly reaches a steady
state where the rates of emergence and cancellation balance. Analysis
of the resulting magnetic field shows that we reproduce observed
quantities such as the flux distribution, mean field, cancellation
rates, photospheric recycle time and a magnetic network. As expected,
the simulation matches observations more closely when a larger, and
consequently more realistic, range of emerging flux values is allowed
(4×1016 - 1019 Mx). The model best reproduces
the current observed properties of the magnetic carpet when we take
the minimum absolute flux for emerging bipoles to be 4×1016
Mx. In future, this 2D model will be used as an evolving photospheric
boundary condition for 3D non-potential modeling.
Title: Heating of the Solar Chromosphere and Corona by Alfvén
Wave Turbulence
Authors: van Ballegooijen, A. A.; Asgari-Targhi, M.; Cranmer, S. R.;
DeLuca, E. E.
Bibcode: 2011ApJ...736....3V
Altcode: 2011arXiv1105.0402V
A three-dimensional magnetohydrodynamic (MHD) model for the propagation
and dissipation of Alfvén waves in a coronal loop is developed. The
model includes the lower atmospheres at the two ends of the loop. The
waves originate on small spatial scales (less than 100 km) inside
the kilogauss flux elements in the photosphere. The model describes
the nonlinear interactions between Alfvén waves using the reduced
MHD approximation. The increase of Alfvén speed with height in
the chromosphere and transition region (TR) causes strong wave
reflection, which leads to counter-propagating waves and turbulence
in the photospheric and chromospheric parts of the flux tube. Part of
the wave energy is transmitted through the TR and produces turbulence
in the corona. We find that the hot coronal loops typically found in
active regions can be explained in terms of Alfvén wave turbulence,
provided that the small-scale footpoint motions have velocities of 1-2
km s-1 and timescales of 60-200 s. The heating rate per
unit volume in the chromosphere is two to three orders of magnitude
larger than that in the corona. We construct a series of models with
different values of the model parameters, and find that the coronal
heating rate increases with coronal field strength and decreases
with loop length. We conclude that coronal loops and the underlying
chromosphere may both be heated by Alfvénic turbulence.
Title: Observations and Magnetic Field Modeling of the Flare/coronal
Mass Ejection Event on 2010 April 8
Authors: Su, Yingna; Surges, Vincent; van Ballegooijen, Adriaan;
DeLuca, Edward; Golub, Leon
Bibcode: 2011ApJ...734...53S
Altcode:
We present a study of the flare/coronal mass ejection event that
occurred in Active Region 11060 on 2010 April 8. This event also
involves a filament eruption, EIT wave, and coronal dimming. Prior to
the flare onset and filament eruption, both SDO/AIA and STEREO/EUVI
observe a nearly horizontal filament ejection along the internal
polarity inversion line, where flux cancellations frequently occur as
observed by SDO/HMI. Using the flux-rope insertion method developed
by van Ballegooijen, we construct a grid of magnetic field models
using two magneto-frictional relaxation methods. We find that the
poloidal flux is significantly reduced during the relaxation process,
though one relaxation method preserves the poloidal flux better
than the other. The best-fit pre-flare NLFFF model is constrained by
matching the coronal loops observed by SDO/AIA and Hinode/XRT. We find
that the axial flux in this model is very close to the threshold of
instability. For the model that becomes unstable due to an increase of
the axial flux, the reconnected field lines below the X-point closely
match the observed highly sheared flare loops at the event onset. The
footpoints of the erupting flux rope are located around the coronal
dimming regions. Both observational and modeling results support
the premise that this event may be initiated by catastrophic loss of
equilibrium caused by an increase of the axial flux in the flux rope,
which is driven by flux cancellations.
Title: The Coronal Physics Investigator (cpi) Experiment For Iss:
A New Vision For Understanding Solar Wind Acceleration
Authors: Raymond, John C.; Janzen, P. H.; Kohl, J. L.; Reisenfeld,
D. B.; Chandran, B. D. G.; Cranmer, S. R.; Forbes, T. G.; Isenberg,
P. A.; Panasyuk, A. V.; van Ballegooijen, A. A.
Bibcode: 2011SPD....42.2406R
Altcode: 2011BAAS..43S.2406R
We propose an Explorer Mission of Opportunity program to develop and
operate a large-aperture ultraviolet coronagraph spectrometer called
the Coronal Physics Investigator (CPI) as an attached International
Space Station (ISS) payload. The primary goal of this program is
to identify and characterize the physical processes that heat and
accelerate the primary and secondary components of the fast and slow
solar wind. Also, CPI can make key measurements needed to understand
CMEs. CPI is dedicated to high spectral resolution measurements of
the off-limb extended corona with far better stray light suppression
than can be achieved by a conventional instrument. UVCS/SOHO allowed
us to identify what additional measurements need to be made to answer
the fundamental questions about how solar wind streams are produced,
and CPI's next-generation capabilities were designed specifically to
make those measurements. Compared to previous instruments, CPI provides
unprecedented sensitivity, a wavelength range extending from 25.7 to
126 nm, higher temporal resolution, and the capability to measure line
profiles of He II, N V, Ne VII, Ne VIII, Si VIII, S IX, Ar VIII, Ca IX,
and Fe X, never before seen in coronal holes above 1.3 solar radii. CPI
will constrain the properties and effects of coronal MHD waves by (1)
observing many ions over a large range of charge and mass,(2) providing
simultaneous measurements of proton and electron temperatures to probe
turbulent dissipation mechanisms, and (3) measuring amplitudes of
low-frequency compressive fluctuations. CPI is an internally occulted
ultraviolet coronagraph that provides the required high sensitivity
without the need for a deployable boom, and with all technically mature
hardware including an ICCD detector. A highly experienced Explorer and
ISS contractor, L-3 Com Integrated Optical Systems and Com Systems East
will provide the tracking and pointing system as well as the instrument,
and the integration to the ISS.
Title: Photospheric Flux Cancellation and the Build-up of Sigmoidal
Flux Ropes
Authors: Savcheva, Antonia; Green, L.; DeLuca, E.; van Ballegooijen, A.
Bibcode: 2011SPD....42.1806S
Altcode: 2011BAAS..43S.1806S
The magnetic structure of sigmoidal active regions is generally
associated with the presence of a twisted flux rope held down by a
potential arcade. There are competing theories of how the flux rope
develops - by flux emergence, cancellation, or footpoint motions. We
look at how flux cancellation in several sigmoidal regions, observed
with XRT and AIA, affects the buildup of the underlying flux ropes. We
use MDI and HMI magnetograms to quantify the flux cancellation, and the
flux rope insertion method to construct non-linear force free field
models of the regions. We present magnetic maps and the 3D flux rope
structure. We correlate the locations of flares and build-up of free
energy and helicity with flux cancellation events. We show how the
flux ropes energy and flux budget changes with the different stages
in the flux cancellation.
Title: Simulating Coronal Emission in Six AIA Channels Using
Quasi-Static Atmosphere Models and Non-Linear Magnetic Field Models
Authors: Malanushenko, Anna; Schrijver, C.; DeRosa, M.; Aschwanden,
M.; Wheatland, M. S.; van Ballegooijen, A. A.
Bibcode: 2011SPD....42.2116M
Altcode: 2011BAAS..43S.2116M
We present the results of simulations of the EUV coronal emission in
AIA channels. We use a non-linear force-free model of magnetic field
constructed in such a way that its field lines resemble the observed
coronal loops in EUV. We then solve one-dimensional quasi-steady
atmosphere model along the magnetic field lines (Schrijver &
Ballegooijen, 2005). Using coronal abundances from CHIANTI and AIA
response functions we then simulate the emission that would be observed
in AIA EUV channels. The resulting intensities are compared against the
real observations in a manner similar to that in Aschwanden et. al.,
2011. The study is similar to those by Lindquist et. al., 2008, with a
few important differences. We use a model of the coronal magnetic field
that resembles the topology observed in EUV, we study EUV emission of
cool loops (rather than SXR) and we make use of high resolution and
cadence AIA and HMI data.
Title: The Coronal Physics Investigator (CPI) Experiment for ISS:
A New Vision for Understanding Solar Wind Acceleration
Authors: Kohl, J. L.; Cranmer, S. R.; Raymond, J. C.; Norton, T. J.;
Cucchiaro, P. J.; Reisenfeld, D. B.; Janzen, P. H.; Chandran, B. D. G.;
Forbes, T. G.; Isenberg, P. A.; Panasyuk, A. V.; van Ballegooijen,
A. A.
Bibcode: 2011arXiv1104.3817K
Altcode:
In February 2011 we proposed a NASA Explorer Mission of Opportunity
program to develop and operate a large-aperture ultraviolet
coronagraph spectrometer called the Coronal Physics Investigator
(CPI) as an attached International Space Station (ISS) payload. The
primary goal of this program is to identify and characterize the
physical processes that heat and accelerate the primary and secondary
components of the fast and slow solar wind. In addition, CPI can make
key measurements needed to understand CMEs. UVCS/SOHO allowed us to
identify what additional measurements need to be made to answer the
fundamental questions about how solar wind streams are produced, and
CPI's next-generation capabilities were designed specifically to make
those measurements. Compared to previous instruments, CPI provides
unprecedented sensitivity, a wavelength range extending from 25.7 to
126 nm, higher temporal resolution, and the capability to measure line
profiles of He II, N V, Ne VII, Ne VIII, Si VIII, S IX, Ar VIII, Ca IX,
and Fe X, never before seen in coronal holes above 1.3 solar radii. CPI
will constrain the properties and effects of coronal MHD waves by
(1) observing many ions over a large range of charge and mass, (2)
providing simultaneous measurements of proton and electron temperatures
to probe turbulent dissipation mechanisms, and (3) measuring amplitudes
of low-frequency compressive fluctuations. CPI is an internally occulted
ultraviolet coronagraph that provides the required high sensitivity
without the need for a deployable boom, and with all technically mature
hardware including an ICCD detector. A highly experienced Explorer and
ISS contractor, L-3 Com Integrated Optical Systems and Com Systems East,
will provide the tracking and pointing system as well as the instrument,
and the integration to the ISS.
Title: Modeling the Dispersal of an Active Region: Quantifying Energy
Input into the Corona
Authors: Mackay, Duncan H.; Green, L. M.; van Ballegooijen, Aad
Bibcode: 2011ApJ...729...97M
Altcode: 2011arXiv1102.5296M
In this paper, a new technique for modeling nonlinear force-free fields
directly from line-of-sight magnetogram observations is presented. The
technique uses sequences of magnetograms directly as lower boundary
conditions to drive the evolution of coronal magnetic fields between
successive force-free equilibria over long periods of time. It is
illustrated by applying it to SOHO: MDI observations of a decaying
active region, NOAA AR 8005. The active region is modeled during a
four-day period around its central meridian passage. Over this time,
the dispersal of the active region is dominated by random motions
due to small-scale convective cells. Through studying the buildup of
magnetic energy in the model, it is found that such small-scale motions
may inject anywhere from (2.5-3) × 1025 erg s-1
of free magnetic energy into the coronal field. Most of this energy
is stored within the center of the active region in the low corona,
below 30 Mm. After four days, the buildup of free energy is 10%
that of the corresponding potential field. This energy buildup is
sufficient to explain the radiative losses at coronal temperatures
within the active region. Small-scale convective motions therefore
play an integral part in the energy balance of the corona. This new
technique has wide ranging applications with the new high-resolution,
high-cadence observations from the SDO:HMI and SDO:AIA instruments.
Title: Recent Successes of Wave/Turbulence Driven Models of Solar
Wind Acceleration
Authors: Cranmer, S. R.; Hollweg, J. V.; Chandran, B. D.; van
Ballegooijen, A. A.
Bibcode: 2010AGUFMSH41B1786C
Altcode:
A key obstacle in the way of producing realistic simulations of the
Sun-heliosphere system is the lack of a first-principles understanding
of coronal heating. Also, it is still unknown whether the solar wind
is "fed" through flux tubes that remain open (and are energized by
footpoint-driven wavelike fluctuations) or if mass and energy are
input intermittently from closed loops into the open-field regions. In
this presentation, we discuss self-consistent models that assume the
energy comes from solar Alfven waves that are partially reflected,
and then dissipated, by magnetohydrodynamic turbulence. These models
have been found to reproduce many of the observed features of the fast
and slow solar wind without the need for artificial "coronal heating
functions" used by earlier models. For example, the models predict
a variation with wind speed in commonly measured ratios of charge
states and elemental abundances that agrees with observed trends. This
contradicts a commonly held assertion that these ratios can only be
produced by the injection of plasma from closed-field regions on the
Sun. This presentation also reviews two recent comparisons between the
models and empirical measurements: (1) The models successfully predict
the amplitude and radial dependence of Faraday rotation fluctuations
(FRFs) measured by the Helios probes for heliocentric distances between
2 and 15 solar radii. The FRFs are a particularly sensitive test of
turbulence models because they depend not only on the plasma density
and Alfven wave amplitude in the corona, but also on the turbulent
correlation length. (2) The models predict the correct sense and
magnitude of changes seen in the polar high-speed solar wind by Ulysses
from the previous solar minimum (1996-1997) to the more recent peculiar
minimum (2008-2009). By changing only the magnetic field along the polar
magnetic flux tube, consistent with solar and heliospheric observations
at the two epochs, the model correctly predicts that the wind speed
remains relatively unchanged, but the in-situ density and temperature
decrease by approximately 20 percent and 10 percent, respectively.
Title: Observations and Magnetic Field Modeling of the Flare/CME
Event on 2010 April 8
Authors: Su, Y.; Surges, V.; van Ballegooijen, A. A.
Bibcode: 2010AGUFMSH11A1613S
Altcode:
We present a study on the flare/CME event that occurred in Active
Region 11060 on 2010 April 8. This flare is well observed at multiple
EUV and UV channels by SDO/AIA. This flare is a typical two-ribbon
flare associated with both coronal dimming and wave, the associated
CME is observed by STEREO and SOHO/LASCO. Using the flux-rope insertion
method developed by van Ballegooijen (2004), two magnetic field models
of the active region are created: (1) best-fit nonlinear force-free
field model (NLFFF) of the region prior to the eruption; (2) unstable
state of the magnetic fields present at the onset of the event. The
boundary conditions of the magnetic fields are provided by the LOS
magnetograms taken by SDO/HMI. The best-fit NLFFF model is constrained
by the coronal loops observed by SDO/AIA and Hinode/XRT. The best-fit
model prior to the flare contains a highly sheared and weakly twisted
flux rope. The axial flux of the flux rope in this model is very close
to the threshold of instability. The location of the flare ribbons
and flare loops at the event onset closely match the reconnected field
lines from the unstable model. The footpoints of the erupting flux rope
are located near the coronal dimming region. These results suggest
that this event may be due to the loss-of-equilibrium mechanism. The
unstable model can also be used as initial condition for full 3D-MHD
simulations of the observed CME event.
Title: Review of Models for Solar Prominences and Coronal Cavities
(Invited)
Authors: van Ballegooijen, A. A.
Bibcode: 2010AGUFMSH54A..02V
Altcode:
Solar prominences are relatively cool structures embedded in the
million-degree corona. Magnetic fields are thought to play an important
role in supporting the prominence plasma against gravity, and in
insulating it from the surrounding hot corona. Quiescent prominences
(QP) have intricate fine-scale structures that continually evolve due
to plasma flows and heating and cooling processes. In this talk I review
recent theoretical models for the structure and formation of QPs. It is
argued that hedgerow prominences may be located in vertically oriented
current sheets, and may be supported by small-scale, tangled magnetic
fields. The role of Rayleigh-Taylor instabilities in the formation
of hedgerow prominences will be discussed. Many QPs are located in
coronal cavities, suggesting the presence of large-scale magnetic flux
ropes held down by overlying coronal arcades. Nonlinear force-free
field models of such flux ropes will be presented and compared with
prominence and cavity observations.
Title: A Unified Model for Chromospheric and Coronal Heating Driven
by Small-Scale Random Footpoint Motions
Authors: van Ballegooijen, A. A.; Cranmer, S. R.; Asgari-Targhi, M.;
Deluca, E. E.
Bibcode: 2010AGUFMSH31C1802V
Altcode:
The solar corona is thought to be heated by dissipation of magnetic
disturbances that propagate up from the Sun's convection zone. We
propose that a major contribution to the heating comes from disturbances
that originate on small spatial scales inside the kilogauss magnetic
flux elements in the photosphere. Interactions of convective flows with
such flux elements produce Alfven waves that travel upward along the
magnetic field lines. When they reach the chromosphere and transition
region, the waves reflect, producing counter-propagating waves in the
chromosphere. Such counter-propagating waves are subject to well-known
nonlinear wave-wave interactions that can lead to the development
of turbulence. We simulate the dynamics of Alfven waves using a 3D
MHD model of a coronal loop (including the lower atmospheres at the
two ends of the loop) and we find that strong turbulence does indeed
develop in the lower parts of the flux tube. Some of the wave energy is
transmitted into the corona and produces turbulence there. We find that
the hot coronal loops typically found in active regions can be explained
in terms of Alfven wave turbulence, provided the photospheric footpoint
motions have a velocity of 1 - 2 km/s and a correlation time of about
60 seconds. The heating rate in the chromosphere is 2 to 3 orders of
magnitude larger than that in the corona, consistent with empirical
models of facular regions. We conclude that coronal loops and the
underlying chromosphere may both be heated by Alfven wave turbulence.
Title: Ultraviolet Coronagraph Spectroscopy: A Key Capability for
Understanding the Physics of Solar Wind Acceleration
Authors: Cranmer, S. R.; Kohl, J. L.; Alexander, D.; Bhattacharjee,
A.; Breech, B. A.; Brickhouse, N. S.; Chandran, B. D. G.; Dupree,
A. K.; Esser, R.; Gary, S. P.; Hollweg, J. V.; Isenberg, P. A.; Kahler,
S. W.; Ko, Y. -K.; Laming, J. M.; Landi, E.; Matthaeus, W. H.; Murphy,
N. A.; Oughton, S.; Raymond, J. C.; Reisenfeld, D. B.; Suess, S. T.;
van Ballegooijen, A. A.; Wood, B. E.
Bibcode: 2010arXiv1011.2469C
Altcode:
Understanding the physical processes responsible for accelerating the
solar wind requires detailed measurements of the collisionless plasma
in the extended solar corona. Some key clues about these processes
have come from instruments that combine the power of an ultraviolet
(UV) spectrometer with an occulted telescope. This combination enables
measurements of ion emission lines far from the bright solar disk,
where most of the solar wind acceleration occurs. Although the UVCS
instrument on SOHO made several key discoveries, many questions remain
unanswered because its capabilities were limited. This white paper
summarizes these past achievements and also describes what can be
accomplished with next-generation instrumentation of this kind.
Title: Can the Solar Wind be Driven by Magnetic Reconnection in the
Sun's Magnetic Carpet?
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.
Bibcode: 2010ApJ...720..824C
Altcode: 2010arXiv1007.2383C
The physical processes that heat the solar corona and accelerate the
solar wind remain unknown after many years of study. Some have suggested
that the wind is driven by waves and turbulence in open magnetic flux
tubes, and others have suggested that plasma is injected into the open
tubes by magnetic reconnection with closed loops. In order to test the
latter idea, we developed Monte Carlo simulations of the photospheric
"magnetic carpet" and extrapolated the time-varying coronal field. These
models were constructed for a range of different magnetic flux imbalance
ratios. Completely balanced models represent quiet regions on the Sun
and source regions of slow solar wind streams. Highly imbalanced models
represent coronal holes and source regions of fast wind streams. The
models agree with observed emergence rates, surface flux densities,
and number distributions of magnetic elements. Despite having no
imposed supergranular motions in the models, a realistic network
of magnetic "funnels" appeared spontaneously. We computed the rate
at which closed field lines open up (i.e., recycling times for open
flux), and we estimated the energy flux released in reconnection events
involving the opening up of closed flux tubes. For quiet regions and
mixed-polarity coronal holes, these energy fluxes were found to be much
lower than that which is required to accelerate the solar wind. For the
most imbalanced coronal holes, the energy fluxes may be large enough to
power the solar wind, but the recycling times are far longer than the
time it takes the solar wind to accelerate into the low corona. Thus,
it is unlikely that either the slow or fast solar wind is driven by
reconnection and loop-opening processes in the magnetic carpet.
Title: A nonpotential model for the Sun's open magnetic flux
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.;
Constable, J. A.
Bibcode: 2010JGRA..115.9112Y
Altcode: 2010JGRA..11509112Y; 2010arXiv1006.4011Y
Measurements of the interplanetary magnetic field (IMF) over several
solar cycles do not agree with computed values of open magnetic flux
from potential field extrapolations. The discrepancy becomes greater
around solar maximum in each cycle when the IMF can be twice as strong
as predicted by the potential field model. Here we demonstrate that this
discrepancy may be resolved by allowing for electric currents in the
low corona (below 2.5R$\odot$). We present a quasi-static
numerical model of the large-scale coronal magnetic evolution, which
systematically produces these currents through flux emergence and
shearing by surface motions. The open flux is increased by 75%-85%
at solar maximum, but only 25% at solar minimum, bringing it in line
with estimates from IMF measurements. The additional open flux in the
nonpotential model arises through inflation of the magnetic field by
electric currents, with superimposed fluctuations due to coronal mass
ejections. The latter are modeled by the self-consistent ejection of
twisted magnetic flux ropes.
Title: Structure and Dynamics of Quiescent Filament Channels Observed
by Hinode/XRT and STEREO/EUVI
Authors: Su, Yingna; van Ballegooijen, Adriaan; Golub, Leon
Bibcode: 2010ApJ...721..901S
Altcode:
We present a study of the structure and dynamics of quiescent filament
channels observed by Hinode/XRT and STEREO/EUVI at the solar minimum
23/24 from 2006 November to 2008 December. For 12 channels identified on
the solar disk (Group I channels), we find that the morphology of the
structure on the two sides of the channel is asymmetric in both X-rays
and EUV: the eastern side has curved features while the western side has
straight features. We interpret the results in terms of a magnetic flux
rope model. The asymmetry in the morphology is due to the variation in
axial flux of the flux rope along the channel, which causes the field
lines from one polarity to turn into the flux rope (curved feature),
while the field lines from the other polarity are connected to very
distant sources (straight). For most of the 68 channels identified by
cavities at the east and west limbs (Group II channels), the asymmetry
cannot be clearly identified, which is likely due to the fact that the
axial flux may be relatively constant along such channels. Corresponding
cavities are identified only for 5 of the 12 Group I channels, while
Group II channels are identified for all of the 68 cavity pairs. The
studied filament channels are often observed as dark channels in
X-rays and EUV. Sheared loops within Group I channels are often seen
in X-rays, but are rarely seen in Group II channels as shown in the
X-ray Telescope daily synoptic observations. A survey of the dynamics
of studied filament channels shows that filament eruptions occur at an
average rate of 1.4 filament eruptions per channel per solar rotation.
Title: Structure and Dynamics of Quiescent Filament Channels Observed
by Hinode/XRT and STEREO/EUVI
Authors: Su, Yingna; van Ballegooijen, Adriaan; Golub, Leon
Bibcode: 2010shin.confE..83S
Altcode:
We present a study on the structure and dynamics of quiescent filament
channels observed by Hinode/XRT and STEREO/EUVI at the solar minimum
23/24 from December 2006 to December 2008. For 12 channels identified
on the solar disk (Group I channels), we find that the emission on
the two sides of the channel is asymmetric in both X-rays and EUV:
eastern side has curved features while the western side has straight
features. We interpret the results in terms of a magnetic flux rope
model. The asymmetry in the emission is due to the variation in axial
magnetic flux along the channel, which causes the field lines from one
polarity to turn into the flux rope, while the field lines from the
other polarity are connected to very distant sources. For 67 channels
identified by cavities at the east and west limbs (Group II channels),
the asymmetry cannot be clearly identified, which is likely due to the
fact that these channels are nearly parallel to the equator and the
axial flux may be relatively constant along such channels. Corresponding
cavities are identified only for 5 of the 12 Group I channels, while
Group II channels are identified for all of the 68 cavity pairs. The
studied filament channels are often observed as dark channels in
X-rays and EUV. Sheared loops within Group I channels are often seen
in X-rays, but are rarely seen in Group II channels as shown in the
XRT daily synoptic observations. A survey on the dynamics of studied
filament channels shows that filament eruptions occur at an average
rate of 1.4 filament eruptions per channel per solar rotation.
Title: Flare Energy Build-Up in a Decaying Active Region Near a
Coronal Hole
Authors: Su, Yingna; van Ballegooijen, Adriaan; Schmieder, Brigitte;
Berlicki, Arkadiusz; Guo, Yang; Golub, Leon; Huang, Guangli
Bibcode: 2010shin.confE.131S
Altcode:
A B1.7 two-ribbon flare occurred in a highly non-potential decaying
active region near a coronal hole at 10:00 UT on May 17, 2008. This
flare is large in the sense that it involves the entire region, and
it is associated with both a filament eruption and a CME. We present
multi-wavelength observations from EUV (TRACE, STEREO/EUVI), X-rays
(Hinode/XRT), and H-alpha (THEMIS, BBSO) prior to, during and after
the flare. Prior to the flare, the region contained two filaments. The
long J-shaped sheared loops corresponding to the southern filament
were evolved from two short loop systems, which happened around 22:00
UT after a filament eruption on May 16. Formation of highly sheared
loops in the south eastern part of the region was observed by STEREO
8 hours before the flare. We also perform non-linear force free field
(NLFFF) modeling for the region at two times prior to the flare, using
the flux rope insertion method. The models include the non-force-free
effect of magnetic buoyancy in the photosphere. The best-fit NLFFF
models show good fit to observations both in the corona (X-ray and EUV
loops) and chromosphere (H-alpha filament). We find that the horizontal
fields in the photosphere are relatively insensitive to the present of
flux ropes in the corona. The axial flux of the flux rope in the NLFFF
model on May 17 is twice that on May 16, and the model on May 17 is only
marginally stable. We also find that the quasi-circular flare ribbons
are associated with the separatrix between open and closed fields. This
observation and NLFFF modeling suggest that this flare may be triggered
by the reconnection at the null point on the separatrix surface.
Title: Connections Between the Magnetic Carpet and the Unbalanced
Corona: New Monte Carlo Models
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.
Bibcode: 2010shin.confE...2C
Altcode:
It is clear from observations of the solar magnetic carpet that much
of the heating in closed-field regions is driven by the interplay
between emergence, separation, merging, and cancellation of many small
flux elements. However, we do not yet know to what extent the open flux
tubes are energized by these processes. In order to begin investigating
this, we developed Monte Carlo simulations of the photospheric magnetic
carpet and extrapolated the time-varying magnetic field up into the
corona. These models were constructed for a range of different magnetic
flux imbalance ratios (i.e., for both quiet regions and coronal holes),
and they appear to be the first simulations to utilize newly observed
flux emergence rates that are at least an order of magnitude larger than
those used in earlier models. The results agree with a wide range of
observations, including surface flux densities and number distributions
of magnetic elements. Despite having no imposed supergranular motions
in the models, a realistic network of magnetic funnels appeared
spontaneously. We also computed the rate at which closed field lines
open up (i.e., the recycling times for open flux), and we estimated
the energy flux released in reconnection events involving the opening
up of closed flux tubes. For quiet regions and mixed-polarity coronal
holes, these energy fluxes were found to be much lower than required
to accelerate the solar wind. For the most imbalanced coronal holes,
the energy fluxes may be large enough to power the solar wind, but the
recycling times are far longer than the time it takes the solar wind
to accelerate into the low corona. Thus, reconnection and loop-opening
processes in the magnetic carpet may be responsible for intermittent
events in coronal holes (e.g., polar jets), but probably not for the
majority of bulk solar wind acceleration.
Title: Extended Coronal Heating and Solar Wind Acceleration over
the Solar Cycle
Authors: Cranmer, S. R.; Kohl, J. L.; Miralles, M. P.; van
Ballegooijen, A. A.
Bibcode: 2010ASPC..428..209C
Altcode: 2010arXiv1002.0297C
This paper reviews our growing understanding of the physics behind
coronal heating (in open-field regions) and the acceleration of the
solar wind. Many new insights have come from the last solar cycle's
worth of observations and theoretical work. Measurements of the
plasma properties in the extended corona, where the primary solar wind
acceleration occurs, have been key to discriminating between competing
theories. We describe how UVCS/SOHO measurements of coronal holes and
streamers over the last 14 years have provided clues about the detailed
kinetic processes that energize both fast and slow wind regions. We also
present a brief survey of current ideas involving the coronal source
regions of fast and slow wind streams, and how these change over the
solar cycle. These source regions are discussed in the context of recent
theoretical models (based on Alfvén waves and MHD turbulence) that have
begun to successfully predict both the heating and acceleration in fast
and slow wind regions with essentially no free parameters. Some new
results regarding these models—including a quantitative prediction
of the lower density and temperature at 1 AU seen during the present
solar minimum in comparison to the prior minimum—are also shown.
Title: Asymmetric Structure of Quiescent Filament Channels Observed
by Hinode/XRT and STEREO/EUVI
Authors: Su, Yingna; van Ballegooijen, A.; Golub, L.
Bibcode: 2010AAS...21640513S
Altcode: 2010BAAS...41..891S
We present a study on the structure of quiescent filament channels
observed by Hinode/XRT and STEREO/EUVI from December 2006 to February
2009. For 10 channels identified on the solar disk, we find that the
emission on the two sides of the channel is asymmetric in both X-rays
and EUV: one side has curved bright features while the other side
has straight faint features. We interpret the results in terms of a
magnetic flux rope model. The asymmetry in the emission is due to the
variation in axial magnetic flux along the channel, which causes one
polarity to turn into the flux rope, while the field lines from the
other polarity are open or connected to very distant sources. For 70
channels identified by cavities at the limb, the asymmetry cannot be
clearly identified. US members of the XRT team are supported by NASA
contract NNM07AB07C to Smithsonian Astrophysical Observatory.
Title: Application of Quasi-Separatrix Layer Maps to Understanding
the Structure and Evolution of Sigmoids
Authors: Savcheva, Antonia; DeLuca, E.; Van Ballegooijen, A.
Bibcode: 2010AAS...21640522S
Altcode:
We present some preliminary work in attempt to utilize Quasi-Separatrix
Layer (QSL) maps for understanding the structure and evolution of
sigmoids. We show sample QSL maps calculated at different heights
above the photosphere and different times over the evolution of the
quiescent sigmoid from February, 2007, observed with Hinode/XRT. The
QSL maps use already existing static MHD models of the sigmoid, based
on the flux rope insertion method. We give a short overview of the
method used to set-up these maps. By comparing current distributions
and the squashing factors at different height and cross sections over
the sigmoid location we suggest the use of QSLs as tracers of surface
and/or volumetric currents. We look at the distribution, structure,
and concentration of QSLs in combination with the size and location
of bald patches at different stages of the sigmoid development. We
attempt to use this analysis to help us discriminate between the
main scenarios for the formation and X-ray appearance of the S-like
structure - flux emergence (or cancellation) and twisting foot point
motions. This method may possibly shed some light on the pre-eruption
configuration and eruption mechanism in sigmoids as well.
Title: Tangled Magnetic Fields in Solar Prominences
Authors: van Ballegooijen, A. A.; Cranmer, S. R.
Bibcode: 2010ApJ...711..164V
Altcode: 2010arXiv1001.2757V
Solar prominences are an important tool for studying the structure and
evolution of the coronal magnetic field. Here we consider so-called
hedgerow prominences, which consist of thin vertical threads. We
explore the possibility that such prominences are supported by tangled
magnetic fields. A variety of different approaches are used. First,
the dynamics of plasma within a tangled field is considered. We find
that the contorted shape of the flux tubes significantly reduces
the flow velocity compared to the supersonic free fall that would
occur in a straight vertical tube. Second, linear force-free models
of tangled fields are developed, and the elastic response of such
fields to gravitational forces is considered. We demonstrate that the
prominence plasma can be supported by the magnetic pressure of a tangled
field that pervades not only the observed dense threads but also their
local surroundings. Tangled fields with field strengths of about 10 G
are able to support prominence threads with observed hydrogen density
of the order of 1011 cm-3. Finally, we suggest
that the observed vertical threads are the result of Rayleigh-Taylor
instability. Simulations of the density distribution within a prominence
thread indicate that the peak density is much larger than the average
density. We conclude that tangled fields provide a viable mechanism
for magnetic support of hedgerow prominences.
Title: Comparison of a Global Magnetic Evolution Model with
Observations of Coronal Mass Ejections
Authors: Yeates, A. R.; Attrill, G. D. R.; Nandy, Dibyendu; Mackay,
D. H.; Martens, P. C. H.; van Ballegooijen, A. A.
Bibcode: 2010ApJ...709.1238Y
Altcode: 2009arXiv0912.3347Y
The relative importance of different initiation mechanisms for coronal
mass ejections (CMEs) on the Sun is uncertain. One possible mechanism is
the loss of equilibrium of coronal magnetic flux ropes formed gradually
by large-scale surface motions. In this paper, the locations of flux
rope ejections in a recently developed quasi-static global evolution
model are compared with observed CME source locations over a 4.5 month
period in 1999. Using extreme ultraviolet data, the low-coronal source
locations are determined unambiguously for 98 out of 330 CMEs. An
alternative method of determining the source locations using recorded
Hα events was found to be too inaccurate. Despite the incomplete
observations, positive correlation (with coefficient up to 0.49) is
found between the distributions of observed and simulated ejections,
but only when binned into periods of 1 month or longer. This binning
timescale corresponds to the time interval at which magnetogram data are
assimilated into the coronal simulations, and the correlation arises
primarily from the large-scale surface magnetic field distribution;
only a weak dependence is found on the magnetic helicity imparted to the
emerging active regions. The simulations are limited in two main ways:
they produce fewer ejections, and they do not reproduce the strong
clustering of observed CME sources into active regions. Due to this
clustering, the horizontal gradient of radial photospheric magnetic
field is better correlated with the observed CME source distribution
(coefficient 0.67). Our results suggest that while the gradual formation
of magnetic flux ropes over weeks can account for many observed CMEs,
especially at higher latitudes, there exists a second class of CMEs (at
least half) for which dynamic active region flux emergence on shorter
timescales must be the dominant factor. Improving our understanding
of CME initiation in future will require both more comprehensive
observations of CME source regions and more detailed magnetic field
simulations.
Title: Dynamical Evolution of X-Ray Bright Points with Hinode/XRT
Authors: Kariyappa, R.; Varghese, B. A.; DeLuca, E. E.; van
Ballegooijen, A. A.
Bibcode: 2010ASSP...19..440K
Altcode: 2010mcia.conf..440K
We analyzed a 7-h long time sequence of soft X-ray images obtained
on 14 April 2007 from a quiet region using the X-Ray Telescope (XRT)
onboard Hinode. The aim was to observe intensity oscillations in
coronal XBPs of different brightness and to study differences, if
any, in the periodicity of the intensity variations and the heating
mechanism during their dynamical evolution. We have compared the XRT
images with GONG magnetograms using Coronal Modeling Software (CMS),
and found that some of the XBPs are located at magnetic bipoles. The
coronal XBPs are highly dynamic and oscillatory in nature, showing a
wide variety of time scales in their intensity variations.
Title: Modeling Twisted Coronal Loops: AR 10938
Authors: Golub, L.; Engell, A. J.; van Ballegooijen, A. A.; Korreck,
K. E.; Reeves, K. K.
Bibcode: 2009ASPC..415..268G
Altcode:
When modeling coronal loops by calculating the potential field
from magnetograms it is often found that field lines highlighted
of the potential field do not match the coronal loops observed in
the data. To rectify this situation, we construct a non-potential
field in which helical ``twisted'' currents with prescribed radii
are inserted along certain potential field lines. We then relax
the magnetic field to a non-linear force-free field (NLFFF) using
magneto-frictional relaxation. In doing so, we find that we are able
to approach a geometrical match between the field lines and the coronal
loops observed in AR 10938 on January 18, 2007.
Title: A Non-Linear Force-Free Field Model for the Evolving Magnetic
Structure of Solar Filaments
Authors: Mackay, Duncan H.; van Ballegooijen, A. A.
Bibcode: 2009SoPh..260..321M
Altcode:
In this paper the effect of a small magnetic element approaching
the main body of a solar filament is considered through non-linear
force-free field modeling. The filament is represented by a series of
magnetic dips. Once the dips are calculated, a simple hydrostatic
atmosphere model is applied to determine which structures have
sufficient column mass depth to be visible in Hα. Two orientations
of the bipole are considered, either parallel or anti-parallel to
the overlying arcade. The magnetic polarity that lies closest to the
filament is then advected towards the filament. Initially for both the
dominant and minority polarity advected elements, right/left bearing
barbs are produced for dextral/sinsitral filaments. The production
of barbs due to dominant polarity elements is a new feature. In later
stages the filament breaks into two dipped sections and takes a highly
irregular, non-symmetrical form with multiple pillars. The two sections
are connected by field lines with double dips even though the twist
of the field is less than one turn. Reconnection is not found to play
a key role in the break up of the filament. The non-linear force-free
fields produce very different results to extrapolated linear-force free
fields. For the cases considered here the linear force-free field does
not produce the break up of the filament nor the production of barbs
as a result of dominant polarity elements.
Title: Plasma Heating Rates for a Coronal Mass Ejection on 28
June 2000
Authors: Murphy, N. A.; Raymond, J. C.; Korreck, K. E.; van
Ballegooijen, A. A.
Bibcode: 2009AGUFMSH41B1667M
Altcode:
Several recent observational results suggest that the heating of coronal
mass ejections (CMEs) continues even after the ejected material leaves
the region from where it was launched. To investigate the importance
of heating in the energy budget of these events, we analyze a partial
halo CME on 28 June 2000 that was observed by the UVCS, LACSO, and
EIT instruments aboard SOHO and the MK4 coronagraph at the Mauna
Loa Solar Observatory (MLSO). Heating rates are estimated by using a
time-dependent ionization code with the densities derived from UVCS
measurements as the outer boundary condition. The energy deposited into
heat is then compared to the kinetic energy of the CME estimated from
LASCO, MK4, and UVCS observations. We assess the relevance of theories
of flux rope heating and consider connections with similar phenomena
in laboratory plasma experiments.
Title: Flare Energy Build-up in a Decaying Active Region Near a
Coronal Hole
Authors: Su, Yingna; van Ballegooijen, Adriaan; Schmieder, Brigitte;
Berlicki, Arkadiusz; Guo, Yang; Golub, Leon; Huang, Guangli
Bibcode: 2009ApJ...704..341S
Altcode:
A B1.7 two-ribbon flare occurred in a highly non-potential decaying
active region near a coronal hole at 10:00 UT on 2008 May 17. This
flare is "large" in the sense that it involves the entire region,
and it is associated with both a filament eruption and a coronal mass
ejection. We present multi-wavelength observations from EUV (TRACE,
STEREO/EUVI), X-rays (Hinode/XRT), and Hα (THEMIS, BBSO) prior to,
during and after the flare. Prior to the flare, the region contained
two filaments. The long J-shaped sheared loops corresponding to the
southern filament were evolved from two short loop systems, which
happened around 22:00 UT after a filament eruption on May 16. Formation
of highly sheared loops in the southeastern part of the region was
observed by STEREO 8 hr before the flare. We also perform nonlinear
force-free field (NLFFF) modeling for the region at two times prior
to the flare, using the flux rope insertion method. The models include
the non-force-free effect of magnetic buoyancy in the photosphere. The
best-fit NLFFF models show good fit to observations both in the corona
(X-ray and EUV loops) and chromosphere (Hα filament). We find that
the horizontal fields in the photosphere are relatively insensitive
to the present of flux ropes in the corona. The axial flux of the
flux rope in the NLFFF model on May 17 is twice that on May 16,
and the model on May 17 is only marginally stable. We also find that
the quasi-circular flare ribbons are associated with the separatrix
between open and closed fields. This observation and NLFFF modeling
suggest that this flare may be triggered by the reconnection at the
null point on the separatrix surface.
Title: Nonlinear Force-free Modeling of a Long-lasting Coronal Sigmoid
Authors: Savcheva, Antonia; van Ballegooijen, Adriaan
Bibcode: 2009ApJ...703.1766S
Altcode:
A study of the magnetic configuration and evolution of a long-lasting
quiescent coronal sigmoid is presented. The sigmoid was observed by
Hinode/XRT and Transition Region and Coronal Explorer (TRACE) between
2007 February 6 and 12 when it finally erupted. We construct nonlinear
force-free field models for several observations during this period,
using the flux-rope insertion method. The high spatial and temporal
resolution of the X-Ray Telescope (XRT) allows us to finely select
best-fit models that match the observations. The modeling shows that a
highly sheared field, consisting of a weakly twisted flux rope embedded
in a potential field, very well describes the structure of the X-ray
sigmoid. The flux rope reaches a stable equilibrium, but its axial
flux is close to the stability limit of about 5 × 1020
Mx. The relative magnetic helicity increases with time from February 8
until just prior to the eruption on February 12. We study the spatial
distribution of the torsion parameter α in the vicinity of the flux
rope, and find that it has a hollow-core distribution, i.e., electric
currents are concentrated in a current layer at the boundary between
the flux rope and its surroundings. The current layer is located near
the bald patch separatrix surface (BPSS) of the magnetic configuration,
and the X-ray emission appears to come from this current layer/BPSS,
consistent with the Titov and Démoulin model. We find that the twist
angle Φ of the magnetic field increases with time to about 2π just
prior to the eruption, but never reaches the value necessary for the
kink instability.
Title: Non-potential Enhancement of the Sun's Open Magnetic Flux
Authors: Yeates, Anthony R.; Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2009shin.confE.182Y
Altcode:
Measurements of the interplanetary magnetic field (IMF) over several
solar cycles do not agree with expected values of open magnetic flux
from potential field models. The discrepancy becomes greater around
solar maximum in each cycle, when the IMF can be twice as strong as
predicted by the models. It has previously been suggested that the
increased rate of coronal mass ejections around solar maximum could
be responsible for enhancing the open flux. We test this idea by
removing the potential field assumption in simulations of the coronal
magnetic field evolution over the solar cycle. The simulations use
magneto-frictional relaxation in response to flux emergence and surface
flux transport, and allow for the development of coronal currents
and for the ejection of twisted magnetic flux ropes. Preliminary
results show that the non-potential fields have significantly more
open flux than potential extrapolations, in agreement with IMF
observations. We find that the additional open flux arises from two
separate effects: temporary enhancement due to coronal mass ejections,
and a background enhancement due to the non-potential nature of the
force-free equilibria. (Supported by NASA/LWS grant NNG05GK32G, and
NASA contract NNM07AB07C to SAO.)
Title: Radiative Transfer in the Presence of Strong Magnetic Fields
Authors: van Ballegooijen, A.
Bibcode: 2009nrt..book..279V
Altcode:
No abstract at ADS
Title: Magnetic Nonpotentiality Accumulation Prior to a "Large"
B1.7 Two-Ribbon Flare
Authors: Su, Yingna; van Ballegooijen, A.; Schmieder, B.; Berlicki,
A.; Guo, Y.; Golub, L.
Bibcode: 2009SPD....40.2002S
Altcode:
A B1.7 two-ribbon flare occurred in a highly non-potential decaying
active region near a coronal hole at 10:00 UT on May 17, 2008. This
flare is "large" in the sense that it involves the entire region, and
it is associated with both a filament eruption and a CME. We present
multi-wavelength observations from EUV (TRACE, STEREO/EUVI), X-rays
(Hinode/XRT), and H-alpha (THEMIS, BBSO) prior to, during and after
the flare. Prior to the flare, the observed coronal loops are mainly
two sets of highly sheared loops surrounding two filaments. The sheared
loops in the southern part of the region appeared in TRACE around 22:00
UT after a filament ejection on May 16. We also performed non-linear
force free field (NLFFF) modeling for the region prior to the flare,
using the flux rope insertion method. The model at 11:42 UT on May 16 is
based on THEMIS (vector) and MDI magnetograms, and the model at 08:03 UT
on May 17 is based only on MDI magnetogram. The best-fit NLFFF models
show good fit to observations both in the corona (X-ray and EUV loops)
and chromosphere (H-alpha filament). In the photosphere, the vectors
in the best-fit NLFFF model are similar to those in the potential
model. However, in the chromosphere, the vectors in the best-fit NLFFF
model present strong non-potentiality, especially at the end of the
inserted flux rope. We also found that the axial flux of the flux rope
in the NLFFF model on May 17 is much higher than that on May 16.
Title: Observations Of Filament Channels By Hinode/XRT And STEREO/EUVI
Authors: Su, Yingna; van Ballegooijen, A.; Golub, L.
Bibcode: 2009SPD....40.1013S
Altcode:
We will present preliminary results of an investigation on filament
channels observed by Hinode/XRT and STEREO/EUVI. We selected 8 filament
channels that are located in active region remnants. Filament channels
1, 2 and 3 appear to be the same channel observed at different solar
rotations, and channels 5, 6, 7 and 8 are also the same channel at
different solar rotations. We study the X-ray and EUV structures as
well as the evolution of these channels observed by Hinode/XRT and
STEREO/EUV. The corresponding H-alpha filament information is provided
by KSO and MLSO. In order to understand the magnetic configuration of
filament channels, we will also explore non-linear force-free field
modeling of two selected filament channels. The modeling is based
on flux rope insertion method, and the magnetic field information is
provided by SOLIS.
Title: A Global Magnetic Field Evolution Model for the Solar Corona
Authors: Yeates, Anthony R.; Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2009SPD....40.3708Y
Altcode:
We have developed new simulations of the global magnetic field
evolution in the solar corona. Using a coupled surface flux transport
and magnetofrictional model, we can follow, for the first time, the
build-up of magnetic helicity and shear on a global scale over many
solar rotations. The evolution is driven by surface motions and by
flux emergence, with properties of new active regions determined from
synoptic normal-component photospheric magnetograms from NSO/Kitt
Peak. As a first application we compare the model to observations
of sheared magnetic fields in filaments (aka. prominences), over
a 6-month period. We have unprecedented success in reproducing the
chirality (axial magnetic field direction) of filaments. Depending on
the sign of helicity in newly-emerging regions, the correct chirality
is simulated for up to 96% of filaments tested. On the basis of these
simulations, an explanation for the observed hemispheric pattern of
filament chirality is put forward, including why exceptions occur
for filaments in certain locations. When too much axial magnetic flux
builds up in filament channels, magnetic flux ropes lose equilibrium
and are ejected from the simulation. Using automated techniques for
detecting flux ropes and their ejection in the global simulations,
we find that the number of ejections depends on both the magnitude
and sign of the emerging helicity. For reasonable parameter choices,
loss of equilibrium of magnetic flux ropes formed by quasi-static flux
cancellation produces about 50% of the observed CME rate.
Title: A Prominence Puzzle Explained?
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2009AIPC.1094..216Y
Altcode: 2009csss...15..216Y
Long-standing observations reveal a global organisation of the magnetic
field direction in solar prominences (aka filaments), large clouds of
cool dense plasma suspended in the Sun's hot corona. However, theorists
have thus far been unable to explain the origin of this hemispheric
pattern. In particular, simple shearing by large-scale surface motions
would appear to lead to the wrong magnetic field direction. To explain
the observations, we have developed a new model of the global magnetic
field evolution in the solar corona over six months. For the first time
our model can follow the build-up of magnetic helicity and shear on a
global scale, driven by flux emergence and surface motions. The model
is successful in predicting the correct magnetic field direction in the
vast majority of prominences tested, and has enabled us to determine
the key physical mechanisms behind the mysterious hemispheric pattern.
Title: Vector Magnetic Fields and Electric Currents From the Imaging
Vector Magnetograph
Authors: Li, Jing; van Ballegooijen, A. A.; Mickey, Don
Bibcode: 2009ApJ...692.1543L
Altcode: 2008arXiv0811.0054L
First, we describe a general procedure to produce high-quality vector
magnetograms using the Imaging Vector Magnetograph (IVM) at Mees Solar
Observatory. Two IVM effects are newly discussed and taken into account:
(1) the central wavelength of the Fabry-Pérot is found to drift with
time as a result of undiagnosed thermal or mechanical instabilities in
the instrument; (2) the Stokes V-sign convention built into the IVM is
found to be opposite to the conventional definition used in the study of
radiative transfer of polarized radiation. At the spatial resolution 2''
× 2'', the Stokes Q, U, V uncertainty reaches ~1 × 10-3
to 5 × 10-4 in time-averaged data over 1 hr in the quiet
Sun. When vector magnetic fields are inferred from the time-averaged
Stokes spectral images of FeI 6302.5 Å, the resulting uncertainties are
on the order of 10 G for the longitudinal fields (B par),
40 G for the transverse field strength (B bottom) and ~9°
for the magnetic azimuth (phi). The magnetic field inversion used in
this work is the "Triplet" code, which was developed and implemented
in the IVM software package by the late B. J. LaBonte. The inversion
code is described in detail in the Appendix. Second, we solve for the
absolute value of the vertical electric current density, |Jz
|, accounting for the above IVM problems, for two different active
regions. One is a single sunspot region (NOAA 10001 observed on
2002 June 20) while the other is a more complex, quadrupolar region
(NOAA10030 observed on 2002 July 15). We use a calculation that
does not require disambiguation of 180° in the transverse field
directions. The |Jz | uncertainty is on the order of ~7.0 mA
m-2. The vertical current density increases with increasing
vertical magnetic field. The rate of increase is about 1-2 times as
large in the quadrupolar NOAA 10030 region as in the simple NOAA 10001,
and it is more spatially variable over NOAA 10030 than over NOAA 10001.
Title: Evolution of Current Helicity in Full-Sun Simulations
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2009AIPC.1094..772Y
Altcode: 2009csss...15..772Y
The density of current helicity quantifies the location of twisted and
sheared non-potential structures in a magnetic field. We simulate the
continuous evolution over many solar rotations of the magnetic field in
the Sun's global corona, in response to flux emergence and shearing by
photospheric motions. The latitudinal distribution of current helicity
in our simulation develops a clear statistical pattern, matching the
observed hemispheric sign at active latitudes. Also in agreement
with observations there is significant scatter and intermixing of
both signs of helicity, and we find local values of current helicity
density that are much higher than those predicted by linear force-free
extrapolations. Forthcoming full-disk vector magnetograms from Solar
Dynamics Observatory will provide an ideal opportunity to test our
theoretical results.
Title: Observations and Nonlinear Force-Free Field Modeling of Active
Region 10953
Authors: Su, Yingna; van Ballegooijen, Adriaan; Lites, Bruce W.;
Deluca, Edward E.; Golub, Leon; Grigis, Paolo C.; Huang, Guangli;
Ji, Haisheng
Bibcode: 2009ApJ...691..105S
Altcode:
We present multiwavelength observations of a simple bipolar active
region (NOAA 10953), which produced several small flares (mostly B class
and one C8.5 class) and filament activations from April 30 to May 3 in
2007. We also explore nonlinear force-free field (NLFFF) modeling of
this region prior to the C8.5 flare on May 2, using magnetograph data
from SOHO/MDI and Hinode/SOT. A series of NLFFF models are constructed
using the flux-rope insertion method. By comparing the modeled field
lines with multiple X-ray loops observed by Hinode/XRT, we find that
the axial flux of the flux rope in the best-fit models is (7± 2)×
1020 Mx, while the poloidal flux has a wider range of
(0.1-10)× 1010 Mx cm-1. The axial flux in the
best-fit model is well below the upper limit (~15× 1020
Mx) for stable force-free configurations, which is consistent with the
fact that no successful full filament eruption occurred in this active
region. From multiwavelength observations of the C8.5 flare, we find
that the X-ray brightenings (in both RHESSI and XRT) appeared about 20
minutes earlier than the EUV brightenings seen in TRACE 171 Å images
and filament activations seen in MLSO Hα images. This is interpreted as
an indication that the X-ray emission may be caused by direct coronal
heating due to reconnection, and the energy transported down to the
chromosphere may be too low to produce EUV brightenings. This flare
started from nearly unsheared flare loop, unlike most two-ribbon flares
that begin with highly sheared footpoint brightenings. By comparing
with our NLFFF model, we find that the early flare loop is located
above the flux rope that has a sharp boundary. We suggest that this
flare started near the outer edge of the flux rope, not at the inner
side or at the bottom as in the standard two-ribbon flare model.
Title: Observations and analysis of the April 9, 2008 CME using
STEREO, Hinode TRACE and SoHO data
Authors: Reeves, K. K.; Patsourakos, S.; Stenborg, G.; Miralles, M.;
Deluca, E.; Forbes, T.; Golub, L.; Kasper, J.; Landi, E.; McKenzie,
D.; Narukage, N.; Raymond, J.; Savage, S.; Su, Y.; van Ballegooijen,
A.; Vourlidas, A.; Webb, D.
Bibcode: 2008AGUFMSH12A..04R
Altcode:
On April 9, 2008 a CME originating from an active region behind the limb
was well-observed by STEREO, Hinode, TRACE and SoHO. Several interesting
features connected to this eruption were observed. (1) The interaction
of the CME with open field lines from a nearby coronal hole appeared
to cause an abrupt change in the direction of the CME ejecta. (2) The
prominence material was heated, as evidenced by a change from absorption
to emission in the EUV wavelengths. (3) Because the active region was
behind the limb, the X-Ray Telescope on Hinode was able to take long
enough exposure times to observe a faint current- sheet like structure,
and it was able to monitor the dynamics of the plasma surrounding this
structure. This event is also being studied in the context of activity
that occurred during the Whole Heliosphere Interval (WHI).
Title: Hyperdiffusion as a Mechanism for Solar Coronal Heating
Authors: van Ballegooijen, A. A.; Cranmer, S. R.
Bibcode: 2008ApJ...682..644V
Altcode: 2008arXiv0802.1751V
A theory for the heating of coronal magnetic flux ropes is
developed. The dissipated magnetic energy has two distinct
contributions: (1) energy injected into the corona as a result
of granule-scale, random footpoint motions and (2) energy from the
large-scale, nonpotential magnetic field of the flux rope. The second
type of dissipation can be described in terms of hyperdiffusion, a type
of magnetic diffusion in which the helicity of the mean magnetic field
is conserved. The associated heating rate depends on the gradient of
the torsion parameter of the mean magnetic field. A simple model of
an active region containing a coronal flux rope is constructed. We
find that the temperature and density on the axis of the flux rope are
lower than in the local surroundings, consistent with observations of
coronal cavities. The model requires that the magnetic field in the
flux rope be stochastic in nature, with a perpendicular length scale
of the magnetic fluctuations of the order of 1000 km.
Title: X-Ray Spectroscopic Signatures of the Extended Corona of
FK Comae
Authors: Drake, Jeremy J.; Chung, Sun Mi; Kashyap, Vinay; Korhonen,
Heidi; Van Ballegooijen, Adriaan; Elstner, Detlef
Bibcode: 2008ApJ...679.1522D
Altcode:
High-resolution Chandra X-ray spectra and surface Doppler images
obtained from optical spectra of the rapidly rotating giant FK Com have
been analyzed in order to investigate links between coronal and surface
magnetic structures. Net redshifts were detected at the 3 σ level in
the light of Ne λ12.13 amounting to ~140 km s-1. Smaller
shifts of ~60 km s-1 at the ~2 σ level are seen in the
X-ray spectrum as a whole, while the observed position of O VIII
λ18.97, the second strongest line in the spectrum after Ne X, is
also consistent with its rest wavelength. There is no statistical
difference between redshifts during the first and second halves of the
observation. Spectral line widths are most consistent with thermal
broadening combined with rotational broadening at a scale height of
~1Rsstarf, although they are also statistically consistent
with surface rotational broadening. We interpret the results as
indicative of hot plasma predominantly residing in extended structures
centered at phase phi = 0.75 with a size similar to that of the star
itself. The contemporaneous Doppler images of the surface of FK Com
reveal active longitudes at phases phi ~ 0.6 and 0.9. We speculate
that extended coronal structures correspond to magnetic fields joining
the two active longitudes, which theoretical models predict are of
opposite magnetic polarity. Such structures are supported by coronal
potential field extrapolations of typical theoretical model surface
magnetic field distributions. This study was based on a relatively
short 40 ks Chandra observation. A much longer observation of FK Com
with the HETGS, combined with contemporaneous Zeeman-Doppler imaging,
would be of great value for constraining magnetospheric structure and
dynamo models of rapidly rotating stars.
Title: Evolution and Distribution of Current Helicity in Full-Sun
Simulations
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2008ApJ...680L.165Y
Altcode: 2008arXiv0805.1883Y
Current helicity quantifies the location of twisted and sheared
nonpotential structures in a magnetic field. We simulate the evolution
of magnetic fields in the solar atmosphere in response to flux
emergence and shearing by photospheric motions. In our global-scale
simulation over many solar rotations, the latitudinal distribution
of current helicity develops a clear statistical pattern, matching
the observed hemispheric sign at active latitudes. In agreement with
observations, there is significant scatter and intermixing of both
signs of helicity, where we find local values of current helicity
density that are much higher than those predicted by linear force-free
extrapolations. Forthcoming full-disk vector magnetograms from the
Solar Dynamics Observatory will provide an ideal opportunity to test
our theoretical results on the evolution and distribution of current
helicity, both globally and in single active regions.
Title: Dynamics of the Solar Magnetic Network. II. Heating the
Magnetized Chromosphere
Authors: Hasan, S. S.; van Ballegooijen, A. A.
Bibcode: 2008ApJ...680.1542H
Altcode: 2008arXiv0802.3509H
We consider recent observations of the chromospheric network and
argue that the bright network grains observed in the Ca II H and K
lines are heated by an as-yet-unidentified quasi-steady process. We
propose that the heating is caused by dissipation of short-period
magnetoacoustic waves in magnetic flux tubes (periods less than 100
s). Magnetohydrodynamic (MHD) models of such waves are presented. We
consider wave generation in the network due to two separate processes:
(1) transverse motions at the base of the flux tube and (2) the
absorption of acoustic waves generated in the ambient medium. We
find that the former mechanism leads to efficient heating of the
chromosphere by slow magnetoacoustic waves propagating along magnetic
field lines. This heating is produced by shock waves with a horizontal
size of a few hundred kilometers. In contrast, acoustic waves excited
in the ambient medium are converted into transverse fast modes that
travel rapidly through the flux tube and do not form shocks, unless
the acoustic sources are located within 100 km from the tube axis. We
conclude that the magnetic network may be heated by magnetoacoustic
waves that are generated in or near the flux tubes.
Title: Wave propagation in multiple flux tubes and chromospheric
heating
Authors: Hasan, S. S.; van Ballegooijen, A.; Steiner, O.
Bibcode: 2008IAUS..247...82H
Altcode: 2007IAUS..247...82H
This investigation is a continuation of earlier work on the dynamics of
the magnetic network. In a previous calculation (Hasan et al. 2005),
we examined the response of a single flux tube to transverse motions
of its footpoints. We now extend this analysis to a more realistic
model of the network consisting of multiple flux tubes. We apply a
transverse velocity perturbation uniformly along the lower boundary
located at the base of the photosphere. Our 2-D MHD simulations enable
us to study the complex wave pattern due to waves generated in the
individual tubes as well as their interaction with those emanating
from adjacent tubes. Our results show that the dominant heating of the
chromosphere occurs due to slow magnetoacoustic waves in a region that
is close to the central region of the flux tube.
Title: NLFF Model of a Coronal Sigmoid
Authors: Savcheva, A. S.; Archontis, V.; van Ballegooijen, A.
Bibcode: 2008AGUSMSP31A..05S
Altcode:
Between Feb 10 and 12, 2007, the X-ray telescope on Hinode produced
some very high-cadence and high- resolution observations of a prominent
coronal sigmoid. Here we show our results from computing a NLFF model
of the sigmoid and compare it qualitatively to the XRT and TRACE
observations. In addition we include some preliminary qualitative
and quantitative results from 2.5D flux emergence simulation. We also
discuss the future goals of this project.
Title: Inferring the chromospheric magnetic topology through waves
Authors: Hasan, S. S.; Steiner, O.; van Ballegooijen, A.
Bibcode: 2008IAUS..247...78H
Altcode: 2007IAUS..247...78H
The aim of this work is to examine the hypothesis that the wave
propagation time in the solar atmosphere can be used to infer the
magnetic topography in the chromosphere as suggested by Finsterle et
al. (2004). We do this by using an extension of our earlier 2-D MHD
work on the interaction of acoustic waves with a flux sheet. It is well
known that these waves undergo mode transformation due to the presence
of a magnetic field which is particularly effective at the surface
of equipartition between the magnetic and thermal energy density, the
β = 1 surface. This transformation depends sensitively on the angle
between the wave vector and the local field direction. At the β =
1 interface, the wave that enters the flux sheet, (essentially the
fast mode) has a higher phase speed than the incident acoustic wave. A
time correlation between wave motions in the non-magnetic and magnetic
regions could therefore provide a powerful diagnostic for mapping the
magnetic field in the chromospheric network.
Title: Observations and NLFFF Modeling of Active Region 10953
Authors: Su, Y.; van Ballegooijen, A.; Golub, L.; Deluca, E.
Bibcode: 2008AGUSMSP43C..07S
Altcode:
We explored the non-linear force free field (NLFFF) modeling of a
simple bipolar active region (NOAA 10953), which produced several small
flares (mostly B class and one C8.5 class) and filament activations
from April 30 to May 3 in 2007. These events appear to be associated
with the frequent flux cancellations (SOHO/MDI) that occurred in the
region close to the polarity inversion line. Some preliminary results
of multi-wavelength observations of a C8.5 flare that occurred on May
2 will be presented. We constructed a series of NLFFF models for this
active region at three times, using the flux-rope insertion method. The
models are constructed based on MDI magnetograms, and constrained by
Hα filaments and highly sheared loops observed by XRT aboard Hinode. We
find good NLFFF models that fit the observations before the C8.5 flare,
but not for the case after the flare. The flux rope contains highly
sheared but weakly twisted magnetic fields. Before the C8.5 flare,
this active region is close to an eruptiveon state: the axial flux in
the flux rope is close to the upper limit for instability.
Title: Nonlinear Force-Free Modeling of Coronal Magnetic
Fields. II. Modeling a Filament Arcade and Simulated Chromospheric
and Photospheric Vector Fields
Authors: Metcalf, Thomas R.; De Rosa, Marc L.; Schrijver, Carolus J.;
Barnes, Graham; van Ballegooijen, Adriaan A.; Wiegelmann, Thomas;
Wheatland, Michael S.; Valori, Gherardo; McTtiernan, James M.
Bibcode: 2008SoPh..247..269M
Altcode: 2008SoPh..tmp...17M
We compare a variety of nonlinear force-free field (NLFFF) extrapolation
algorithms, including optimization, magneto-frictional, and Grad -
Rubin-like codes, applied to a solar-like reference model. The model
used to test the algorithms includes realistic photospheric Lorentz
forces and a complex field including a weakly twisted, right helical
flux bundle. The codes were applied to both forced "photospheric" and
more force-free "chromospheric" vector magnetic field boundary data
derived from the model. When applied to the chromospheric boundary data,
the codes are able to recover the presence of the flux bundle and the
field's free energy, though some details of the field connectivity are
lost. When the codes are applied to the forced photospheric boundary
data, the reference model field is not well recovered, indicating
that the combination of Lorentz forces and small spatial scale
structure at the photosphere severely impact the extrapolation of the
field. Preprocessing of the forced photospheric boundary does improve
the extrapolations considerably for the layers above the chromosphere,
but the extrapolations are sensitive to the details of the numerical
codes and neither the field connectivity nor the free magnetic energy in
the full volume are well recovered. The magnetic virial theorem gives
a rapid measure of the total magnetic energy without extrapolation
though, like the NLFFF codes, it is sensitive to the Lorentz forces in
the coronal volume. Both the magnetic virial theorem and the Wiegelmann
extrapolation, when applied to the preprocessed photospheric boundary,
give a magnetic energy which is nearly equivalent to the value derived
from the chromospheric boundary, but both underestimate the free
energy above the photosphere by at least a factor of two. We discuss
the interpretation of the preprocessed field in this context. When
applying the NLFFF codes to solar data, the problems associated with
Lorentz forces present in the low solar atmosphere must be recognized:
the various codes will not necessarily converge to the correct, or
even the same, solution.
Title: Life and Death of Solar Active Regions
Authors: van Ballegooijen, A. A.
Bibcode: 2008ASPC..383..191V
Altcode:
Recent models for the formation and decay of active regions are
discussed. Large active regions are thought to be Ω loops that
emerge from a toroidal field located near the base of the convection
zone. After an Ω loop has fully emerged at the solar surface, it
continues to evolve under the influence of subsurface convective
flows and solar differential rotation. The magnetic helicity of
active regions originates below the photosphere, but its spatial
distribution in the corona is significantly altered by reconnection
processes. Results from 3-D flux transport models of decaying active
regions are presented. It is found that such models provide a natural
explanation for the occurrence of flares and coronal mass ejections. The
models predict that submerging magnetic fields are transported back
to the base of the convection zone, leading to the ``repair'' of the
toroidal flux ropes. Interactions between Ω loops are also considered.
Title: Modeling Nonpotential Magnetic Fields in Solar Active Regions
Authors: Bobra, M. G.; van Ballegooijen, A. A.; DeLuca, E. E.
Bibcode: 2008ApJ...672.1209B
Altcode:
Electric currents are present in the coronae above solar active regions,
producing nonpotential magnetic fields that can be approximated as
nonlinear force-free fields (NLFFFs). In this paper NLFFF models for
two active regions observed in 2002 June are presented. The models are
based on magnetograms from SOHO MDI and are constrained by nonpotential
structures seen in BBSO Hα images and TRACE EUV images. The models are
constructed using the flux rope insertion method. We find that the axial
fluxes of the flux ropes are well constrained by the observations. The
flux ropes are only weakly twisted, and electric currents flow mainly at
the interface between the flux rope and its surroundings. In one case,
the flux rope is anchored with both ends in the active region; in the
other case, the flux rope extends to the neighboring quiet Sun. We
find that the magnetic fields in these active regions are close to
an eruptive state: the axial flux in the flux ropes is close to the
upper limit for eruption. We also derive estimates for magnetic free
energy and helicity in these regions.
Title: Modelling the Global Solar Corona II: Coronal Evolution and
Filament Chirality Comparison
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2008SoPh..247..103Y
Altcode: 2007arXiv0711.2887Y
This paper considers the hemispheric pattern of solar filaments using
newly developed simulations of the real photospheric and 3D coronal
magnetic fields over a six-month period, on a global scale. The
magnetic field direction in the simulation is compared directly with
the chirality of observed filaments, at their observed locations. In
our model the coronal field evolves through a continuous sequence
of nonlinear force-free equilibria, in response to the changing
photospheric boundary conditions and the emergence of new magnetic
flux. In total 119 magnetic bipoles with properties matching observed
active regions are inserted. These bipoles emerge twisted and inject
magnetic helicity into the solar atmosphere. When we choose the sign of
this active-region helicity to match that observed in each hemisphere,
the model produces the correct chirality for up to 96% of filaments,
including exceptions to the hemispheric pattern. If the emerging
bipoles have zero helicity, or helicity of the opposite sign, then
this percentage is much reduced. In addition, the simulation produces
a higher proportion of filaments with the correct chirality after
longer times. This indicates that a key element in the evolution
of the coronal field is its long-term memory, and the build-up and
transport of helicity from low to high latitudes over many months. It
highlights the importance of continuous evolution of the coronal field,
rather than independent extrapolations at different times. This has
significant consequences for future modelling such as that related to
the origin and development of coronal mass ejections.
Title: Observations and NLFFF modeling of active region 10953
Authors: Su, Yingna; van Ballegooijen, Adriaan; Golub, Leon; Deluca,
Edward
Bibcode: 2008cosp...37.3075S
Altcode: 2008cosp.meet.3075S
We explore the non-linear force free field (NLFFF) modeling of a simple
bipolar active region (NOAA 10953), which produced several small
flares (mostly B class and one C8.5 class) and filament activations
from April 30 to May 3 in 2007. These events appear to be associated
with the frequent flux cancellations (SOHO/MDI) that occurred in the
region close to the polarity inversion line. Some preliminary results
of multi-wavelength observations of a C8.5 flare that occurred on May
2 will be presented. We constructed a series of NLFFF models for this
active region at three times, using the flux-rope insertion method. The
models are constructed based on MDI magnetograms, and constrained by
Hα filaments and highly sheared loops observed by XRT aboard Hinode. We
find good NLFFF models that fit the observations before the C8.5 flare,
but not for the case after the flare. The flux rope contains highly
sheared but weakly twisted magnetic fields. Before the C8.5 flare,
this active region is close to an eruptive state: the axial flux in
the flux rope is close to the upper limit for instability.
Title: Magnetic Evolution of X-Ray Bright Points
Authors: Deluca, Edward; van Ballegooijen, Adriaan; Harvey, Jack
Bibcode: 2008cosp...37..690D
Altcode: 2008cosp.meet..690D
The evolution of magnetic structures that form X-Ray Bright Points will
be studied using local high resolution potential field extrapolations
of GONG magnetograph observations. The relationship between the X-ray
structures seen in Hinode/XRT and the magnetic structures will be
discussed.
Title: Assessment and Validation of MHD Models for the Solar Corona
and Inner Heliosphere
Authors: Strachan, L.; Zurbuchen, T. H.; Kohl, J. L.; Panasyuk, A. V.;
Raymond, J. R.; van Ballegooijen, A.
Bibcode: 2007AGUFMSH31A0238S
Altcode:
We describe the status of a model assessment and validation project
for testing MHD codes that simulate the solar corona and inner
heliosphere. The goal of the project is to test MHD codes by applying
firm empirical constraints to their boundary conditions in the corona
and at 1 AU. The project has produced a database of coronal and solar
wind observations from SOHO, ACE, Wind, and Ulysses. In addition to the
database, software tools for comparing these data sets to the outputs
for the MHD model codes under test will be demonstrated. The first
step is to take the plasma parameters that are produced from the model
codes and apply forward modeling to simulate the coronal observations
of emission lines (H I Lyman alpha and O VI 103.2 nm). In situ solar
wind data are used not only to provide benchmarks near 1 AU but also
to provide coronal constraints for the coronal source regions of the
solar wind. Future stages will involve making more direct comparisons
of the plasma properties predicted from the model codes through the
use of empirical coronal and solar wind models. We also describe a
set of metrics that are used for making comparisons between the model
code outputs and the empirical data. This work is supported by NASA
under Grants NNX07AB98G to the Smithsonian Astrophysical Observatory
and NNX07AB99G to the University of Michigan.
Title: Evidence for Alfvén Waves in Solar X-ray Jets
Authors: Cirtain, J. W.; Golub, L.; Lundquist, L.; van Ballegooijen,
A.; Savcheva, A.; Shimojo, M.; DeLuca, E.; Tsuneta, S.; Sakao, T.;
Reeves, K.; Weber, M.; Kano, R.; Narukage, N.; Shibasaki, K.
Bibcode: 2007Sci...318.1580C
Altcode:
Coronal magnetic fields are dynamic, and field lines may misalign,
reassemble, and release energy by means of magnetic reconnection. Giant
releases may generate solar flares and coronal mass ejections and,
on a smaller scale, produce x-ray jets. Hinode observations of polar
coronal holes reveal that x-ray jets have two distinct velocities:
one near the Alfvén speed (~800 kilometers per second) and another
near the sound speed (200 kilometers per second). Many more jets were
seen than have been reported previously; we detected an average of
10 events per hour up to these speeds, whereas previous observations
documented only a handful per day with lower average speeds of 200
kilometers per second. The x-ray jets are about 2 × 103 to
2 × 104 kilometers wide and 1 × 105 kilometers
long and last from 100 to 2500 seconds. The large number of events,
coupled with the high velocities of the apparent outflows, indicates
that the jets may contribute to the high-speed solar wind.
Title: The Magnetic Structure of X-Ray Bright Points
Authors: Deluca, E. E.; van Ballegooijen, A. A.; Harvey, J. W.;
Henney, C. J.
Bibcode: 2007AGUFMSH53A1058D
Altcode:
The magnetic structure of X-Ray Bright Points (XBPs) observed by Hinode,
SOLIS and GONG are well represented by potential field extrapolation
models. SOLIS observations are used to compare the magnetic field
model with X-ray observations. GONG observation allow us to follow
the evolution of the magnetic structures that form the XBPs. Detailed
examination of the nulls in the corona around the bright points will
presented and discussed.
Title: Evolution of the Sheared Magnetic Fields of Two X-Class Flares
Observed by Hinode/XRT
Authors: Su, Yingna; Golub, Leon; van Ballegooijen, Adriaan; Deluca,
Edward E.; Reeves, Kathy K.; Sakao, Taro; Kano, Ryouhei; Narukage,
Noriyuki; Shibasaki Kiyoto
Bibcode: 2007PASJ...59S.785S
Altcode:
We present multi-wavelength observations of the evolution of the sheared
magnetic fields in NOAA Active Region 10930, where two X-class flares
occurred on 2006 December 13 and December 14, respectively. Observations
made with the X-ray Telescope (XRT) and the Solar Optical Telescope
(SOT) aboard Hinode suggest that the gradual formation of the sheared
magnetic fields in this active region is caused by the rotation and
west-to-east motion of an emerging sunspot. In the pre-flare phase
of the two flares, XRT shows several highly sheared X-ray loops in
the core field region, corresponding to a filament seen in the TRACE
EUV observations. XRT observations also show that part of the sheared
core field erupted, and another part of the sheared core field stayed
behind during the flares, which may explain why a large part of the
filament is still seen by TRACE after the flare. About 2-3 hours after
the peak of each flare, the core field becomes visible in XRT again,
and shows a highly sheared inner and less-sheared outer structure. We
also find that the post-flare core field is clearly less sheared than
the pre-flare core field, which is consistent with the idea that the
energy released during the flares is stored in the highly sheared
fields prior to the flare.
Title: Modelling the Global Solar Corona: Filament Chirality
Observations and Surface Simulations
Authors: Yeates, A. R.; Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2007SoPh..245...87Y
Altcode: 2007arXiv0707.3256Y
The hemispheric pattern of solar filaments is considered in the context
of the global magnetic field of the solar corona. In recent work
Mackay and van Ballegooijen have shown how, for a pair of interacting
magnetic bipoles, the observed chirality pattern could be explained
by the dominant range of bipole tilt angles and helicity in each
hemisphere. This study aims to test this earlier result through
a direct comparison between theory and observations, using newly
developed simulations of the actual surface and 3D coronal magnetic
fields over a 6-month period, on a global scale. We consider two key
components: (1) observations of filament chirality for the sample of
255 filaments and (2) our new simulations of the large-scale surface
magnetic field. Based on a flux-transport model, these will be used as
the lower boundary condition for the future 3D coronal simulations. Our
technique differs significantly from those of other authors, where the
coronal field is either assumed to be purely potential or has to be
reset back to potential every 27 days for the photospheric field to
remain accurate. In our case we ensure accuracy by the insertion of
newly emerging bipolar active regions, based on observed photospheric
synoptic magnetograms. The large-scale surface field is shown to
remain accurate over the 6-month period, without any resetting. This
new technique will enable future simulations to consider the long-term
buildup and transport of helicity and shear in the coronal magnetic
field over many months or years.
Title: What Determines the Intensity of Solar Flare/CME Events?
Authors: Su, Yingna; Van Ballegooijen, Adriaan; McCaughey, James;
Deluca, Edward; Reeves, Katharine K.; Golub, Leon
Bibcode: 2007ApJ...665.1448S
Altcode:
We present a comprehensive statistical study addressing the question of
what determines the intensity of a solar flare and associated coronal
mass ejection (CME). For a sample of 18 two-ribbon flares associated
with CMEs, we have examined the correlations between the GOES soft X-ray
peak flare flux (PFF), the CME speed (VCME) obtained from
SOHO LASCO observations, and six magnetic parameters of the flaring
active region. These six parameters measured from both TRACE and SOHO
MDI observations are: the average background magnetic field strength
(B), the area of the region where B is counted (S), the magnetic
flux of this region (Φ), the initial shear angle (θ1,
measured at the flare onset), the final shear angle (θ2,
measured at the time when the shear change stops), and the change of
shear angle (θ12=θ1-θ2) of the
footpoints. We have found no correlation between θ1
and the intensity of flare/CME events, while the other five
parameters are either positively or negatively correlated with
both log10(PFF) and VCME. Among these five parameters,
Φ and θ12 show the most significant correlations with
log10(PFF) and VCME. The fact that both log10(PFF) and
VCME are highly correlated with θ12 rather
than with θ1 indicates that the intensity of flare/CME
events may depend on the released magnetic free energy rather than
the total free energy stored prior to the flare. We have also found
that a linear combination of a subset of these six parameters shows
a much better correlation with the intensity of flare/CME events than
each parameter itself, and the combination of log10Φ, θ1,
and θ12 is the top-ranked combination.
Title: Self-consistent Coronal Heating and Solar Wind Acceleration
from Anisotropic Magnetohydrodynamic Turbulence
Authors: Cranmer, Steven R.; van Ballegooijen, Adriaan A.; Edgar,
Richard J.
Bibcode: 2007ApJS..171..520C
Altcode: 2007astro.ph..3333C
We present a series of models for the plasma properties along open
magnetic flux tubes rooted in solar coronal holes, streamers, and active
regions. These models represent the first self-consistent solutions
that combine (1) chromospheric heating driven by an empirically guided
acoustic wave spectrum; (2) coronal heating from Alfvén waves that have
been partially reflected, then damped by anisotropic turbulent cascade;
and (3) solar wind acceleration from gradients of gas pressure, acoustic
wave pressure, and Alfvén wave pressure. The only input parameters
are the photospheric lower boundary conditions for the waves and the
radial dependence of the background magnetic field along the flux
tube. We have not included multifluid or collisionless effects (e.g.,
preferential ion heating), which are not yet fully understood. For a
single choice for the photospheric wave properties, our models produce a
realistic range of slow and fast solar wind conditions by varying only
the coronal magnetic field. Specifically, a two-dimensional model of
coronal holes and streamers at solar minimum reproduces the latitudinal
bifurcation of slow and fast streams seen by Ulysses. The radial
gradient of the Alfvén speed affects where the waves are reflected
and damped, and thus whether energy is deposited below or above the
Parker critical point. As predicted by earlier studies, a larger
coronal ``expansion factor'' gives rise to a slower and denser wind,
higher temperature at the coronal base, less intense Alfvén waves at
1 AU, and correlative trends for commonly measured ratios of ion charge
states and FIP-sensitive abundances that are in general agreement with
observations. These models offer supporting evidence for the idea that
coronal heating and solar wind acceleration (in open magnetic flux
tubes) can occur as a result of wave dissipation and turbulent cascade.
Title: Modeling the Hemispheric Pattern of Solar Filaments
Authors: Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2007ASPC..368..343M
Altcode:
New results in modeling the hemispheric pattern of solar filaments
are presented. The simulations consider what type of chirality forms
along the Polarity Inversion Line (PIL) lying in between two magnetic
bipoles as they interact. The results demonstrate not only the origin
of the dominant hemispheric pattern, but also why exceptions to
it occur. The dominant hemispheric pattern may be attributed to the
dominant range of bipole tilt angles and helicities in each hemisphere
(\cite{dunc-1989SoPh..124...81W,dunc-1995ApJ...440L.109P}). Exceptions
to the hemispheric pattern occur in cases of no initial helicity
or for helicity of the minority type in each hemisphere, when large
positive bipole tilt angles are used. As the simulations show a clear
dependence of the chirality on observational quantities, this may be
used to check the validity of the results.
Title: Magnetic Shear in Two-ribbon Solar Flares
Authors: Su, Yingna; Golub, L.; Van Ballegooijen, A.; McCaughey, J.;
Deluca, E. E.; Reeves, K.; Gros, M.
Bibcode: 2007AAS...210.3702S
Altcode: 2007BAAS...39Q.151S
To study shear motion of the footpoints in solar flares, we selected
50 X- and M- class two-ribbon flares observed by TRACE (in 1998-2005)
as our data sample. We found that: 1) 86% (43 out of 50) of these
flares show both strong-to-weak shear change of footpoints and ribbon
separation. Shear motion of footpoints is thus a common feature
in two-ribbon flares; 2) the initial and final shear angles of the
footpoints in this type of flare are mainly in the range from 50° to
80° and 15° to 55°, respectively; 3) in 10 out of the 14 events with
both measured shear angle and corresponding hard X-ray observations,
the cessation of shear change is 0-2 minutes earlier than the end of
the impulsive phase. This may suggest that the change from impulsive to
gradual phase is related to magnetic shear change. We then selected 20
flares with measured shear angles and corresponding CMEs from our data
sample. For these flares, we found that the magnetic flux and change
of shear angle show comparably strong correlations with the peak flare
flux and CME speed, while the intial shear angle does not. This result
indicates that the intensity of flare/CME events may depend mainly on
the released magnetic free energy rather than the total magnetic free
energy stored prior to the eruption. After a successful launch last
September, Hinode (Solar-B) caught two X-class solar flares, which
occurred in AR 10930 on Dec 13 and 14, 2006. Using these new datasets
(Hinode/XRT, Hinode/SOT, TRACE, and SOHO/MDI), we carried out a study of
the evolution of the sheared magnetic fields involved in these flares,
and some preliminary results will also be presented. The TRACE analysis
was supported at Smithsonian Astrophysical Observatory by a contract
from Lockheed Martin.
Title: Modeling Non-Potential Magnetic Fields in Solar Active Regions
Authors: Bobra, Monica; van Ballegooijen, A. A.; DeLuca, E. E.
Bibcode: 2007AAS...210.9103B
Altcode: 2007BAAS...39..204B
Many models aim to reproduce the non-linear force free fields in
the solar corona; in this particular study, the magnetofrictional
relaxation method is tested. This method produces non-linear force
free fields based on line-of-sight magnetograms and ground-based Hα
images to define the location of a filament, which is represented by
a flux rope. The models are tested by comparing the results to TRACE
and Hinode X-Ray Telescope (XRT) observations of highly sheared,
non-potential loop structures in active regions. For each event, we
(1) determine the amount of flux contained in the flux ropes needed
to reproduce the observed loops, (2) estimate the current, torsion
parameter α, and quasi-separatrix layer distributions, and (3)
estimate the relative magnetic helicity and magnetic free energy in
the computational domain. This work was supported by NASA LWS grant
NNG05GK32G.
Title: Structure and Coronal Activity around Filament Channels
Observed with Hinode XRT And TRACE
Authors: Lundquist, Loraine L.; van Ballegooijen, A. A.; Reeves,
K. K.; Sakao, T.; DeLuca, E. E.; Narukage, N.; Kano, R.
Bibcode: 2007AAS...210.9427L
Altcode: 2007BAAS...39..221L
The combination of multi-wavelength, high resolution, high cadence
data from the Hinode X-Ray Telescope (XRT) and the Transition Region
And Coronal Explorer (TRACE) give an unprecedented view of solar
active region dynamics and coronal topology. We focus on examples of
filament structures observed by TRACE and XRT in December 2006 and
February 2007. Co-alignment of observations in these two instruments
yields a striking picture of the coronal structures, with loops lying
both along and above the filament. Overlying loops exhibit remarkable
dynamics while the filament lies dormant, and numerous x-point and
triple-leg structures undergo repeated brightenings. We also employ
magnetic field data from SOT and from SOLIS to compare a non-linear
force-free model of the coronal magnetic field with the observed loops.
Title: Non-linear Force-free Modeling Of Coronal Magnetic Fields
Authors: Metcalf, Thomas R.; De Rosa, M. L.; Schrijver, C. J.; Barnes,
G.; van Ballegooijen, A.; Wiegelmann, T.; Wheatland, M. S.; Valori,
G.; McTiernan, J. M.
Bibcode: 2007AAS...210.9102M
Altcode: 2007BAAS...39..204M
We compare a variety of nonlinear force-free field (NLFFF)
extrapolation algorithms, including optimization, magneto-frictional,
and Grad-Rubin-like codes, applied to a solar-like reference
model. The model used to test the algorithms includes realistic
photospheric Lorentz forces and a complex field including a weakly
twisted, right helical flux bundle. The codes were applied to both
forced "photospheric'' and more force-free "chromospheric'' vector
magnetic field boundary data derived from the model. When applied to
the chromospheric boundary data, the codes are able to recover
the presence of the flux bundle and the field's free energy, though
some details of the field connectivity are lost. When the codes are
applied to the forced photospheric boundary data, the reference
model field is not well recovered, indicating that the Lorentz
forces on the photosphere severely impact the extrapolation of the
field. Preprocessing of the photospheric boundary does improve the
extrapolations considerably, although the results depend sensitively
on the details of the numerical codes. When applying the NLFFF codes
to solar data, the problems associated with Lorentz forces present in
the low solar atmosphere must be recognized: the various codes will
not necessarily converge to the correct, or even the same, solution.
Title: Data Analysis with Imaging Vector Magnetograph (IVM)
Authors: Li, Jing; van Ballegooijen, A.; Mickey, D.
Bibcode: 2007AAS...210.2407L
Altcode: 2007BAAS...39R.129L
We discuss a practical data procedure by choosing the best quality of
data sets taken by Imaging Vector Magnetograph (IVM) at Mees Solar
Observatory. We point out that three quantities which cannot be
removed by the regular IVM data reduction, affect the quality of the
data. They are seeing condition, air mass and the wavelength shifts
probably due to the change of Fabry-Perot's transparent plate. The
first two quantities are known, and have been discussed before. The
third quantity results in the distortion of the spectral line profiles
that further affects the magnetic field derived from fitting all line
profiles in four Stokes parameters. The best quality of data sets are
chosen by avoiding the worst of all three quantities. The IVM data can
be used to construct non-linear force free field models of the solar
atmosphere. Initial results from this work will be presented.
Title: Models of the Large-Scale Corona: Formation, Evolution and
Lift-Off of Magnetic Flux Ropes
Authors: Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2007ASPC..368..251M
Altcode:
No abstract at ADS
Title: Model for the Coupled Evolution of Subsurface and Coronal
Magnetic Fields in Solar Active Regions
Authors: van Ballegooijen, A. A.; Mackay, D. H.
Bibcode: 2007ApJ...659.1713V
Altcode:
According to Babcock's theory of the solar dynamo, bipolar active
regions are Ω-shaped loops emerging from a toroidal field located near
the base of the convection zone. In this paper, a mean field model for
the evolution of a twisted Ω-loop is developed. The model describes the
coupled evolution of the magnetic field in the convection zone and the
corona after the loop has fully emerged into the solar atmosphere. Such
a coupled evolution is required to fully understand what happens to the
coronal and subsurface fields as magnetic flux cancels at polarity
inversion lines on the photosphere. The jump conditions for the
magnetic field at the photosphere are derived from the magnetic stress
balance between the convection zone and corona. The model reproduces
the observed spreading of active region magnetic flux over the solar
surface. At polarity inversion lines, magnetic flux submerges below the
photosphere, but the component of magnetic field along the inversion
line cannot submerge, because the field in the upper convection zone is
nearly radial. Therefore, magnetic shear builds up in the corona above
the inversion line, which eventually leads to a loss of equilibrium of
the coronal fields and the ``lift-off'' of a coronal flux rope. Fields
that submerge are transported back to the base of the convection zone,
leading to the repair of the toroidal flux rope. Following Martens
and Zwaan, interactions between bipoles are also considered.
Title: Modeling magnetic flux ropes in the solar atmosphere
Authors: van Ballegooijen, A. A.; Deluca, E. E.; Squires, K.; Mackay,
D. H.
Bibcode: 2007JASTP..69...24V
Altcode: 2007JATP...69...24V
Coronal flux ropes are highly sheared or twisted magnetic fields
overlying polarity inversion lines on the solar photosphere. The
formation of such flux ropes is briefly discussed. A coronal flux
rope can be stable for many days and then suddenly lose equilibrium
and erupt, producing a coronal mass ejection (CME). To understand
what causes such eruptions, we need to determine the 3D magnetic
structure of observed active regions prior to CMEs. This requires
constructing nonlinear force free field models of active regions based
on observed photospheric vector fields, Hα filaments, or coronal loop
structures. We describe a new method for constructing models containing
flux ropes.
Title: A Statistical Study of Shear Motion of the Footpoints in
Two-Ribbon Flares
Authors: Su, Yingna; Golub, Leon; Van Ballegooijen, Adriaan A.
Bibcode: 2007ApJ...655..606S
Altcode:
We present a statistical investigation of shear motion of the
ultraviolet (UV) or extreme-ultraviolet (EUV) footpoints in two-ribbon
flares, using the high spatial resolution data obtained in 1998-2005
by TRACE. To do this study, we have selected 50 well-observed X and
M class two-ribbon flares as our sample. All 50 of these flares are
classified into three types based on the motions of the footpoints
with respect to the magnetic field (SOHO MDI). The relation between our
classification scheme and the traditional classification scheme (i.e.,
``ejective'' and ``confined'' flares) is discussed. We have found that
86% (43 out of 50) of these flares show both strong-to-weak shear
change of footpoints and ribbon separation (type I flares), and 14%
of the flares show no measurable shear change of conjugate footpoints,
including two flares with very small ribbon separation (type II flares)
and five flares having no ribbon separation at all through the entire
flare process (type III flares). Shear motion of footpoints is thus a
common feature in two-ribbon flares. A detailed analysis of the type
I flares shows (1) for a subset of 20 flares, the initial and final
shear angles of the footpoints are mainly in the range 50°-80° and
15°-55°, respectively; and (2) in 10 of the 14 flares having both
measured shear angle and corresponding hard X-ray observations, the
cessation of shear change is 0-2 minutes earlier than the end of the
impulsive phase, which may suggest that the change from impulsive to
gradual phase is related to magnetic shear change.
Title: Coronal Loop Recognition: A Diagnostic Tool for Magnetic
Field Extrapolation Models
Authors: Sandell, Julia; Kashyap, V.; Weber, M.; van Ballegooijen,
A.; Deluca, E.; Bobra, M.
Bibcode: 2006AAS...209.1604S
Altcode: 2006BAAS...38..918S
Constraining the structure and extent of the coronal magnetic field is
important for theories of coronal heating. This can be accomplished
by matching the models of magnetic fields derived by extrapolating
measurements of surface magnetic flux, with highly detailed structure
present in EUV and X-ray images of the corona. Using high resolution
TRACE images we detect loops in an automated manner, and aim to
replace the currently used method of manually selecting pixels that
might comprisea loop. We apply this method to a set of TRACE images
and identify loops for further analysis. We fit force-free potential
field models of the magnetic field to these loops and derive useful
parameters that describe the geometric and physical parameters of
the loop. We find that the loops are generally of length > 1010
cm. If assumed to be in static equilibrium, the loop top temperatures
are 3-5 MK. the field lines are characterized by strength ranging
from 0.4-27 G. After carrying out numerous Monte Carlo simulations,
each time varying different parameters used in the program, we found
this new automated process to be stable and robust. We thank the
Harvard-Smithsonian Summer REU program for making this summer project
possible as well as a grant from NASA (NASA grant NNG05GM44G), and
the NSF for funding the REU program.
Title: Analysis of Magnetic Shear in An X17 Solar Flare on October
28, 2003
Authors: Su, Y. N.; Golub, L.; Van Ballegooijen, A. A.; Gros, M.
Bibcode: 2006SoPh..236..325S
Altcode:
An X17 class (GOES soft X-ray) two-ribbon solar flare on October 28,
2003 is analyzed in order to determine the relationship between the
timing of the impulsive phase of the flare and the magnetic shear change
in the flaring region. EUV observations made by the Transition Region
and Coronal Explorer (TRACE) show a clear decrease in the shear of
the flare footpoints during the flare. The shear change stopped in the
middle of the impulsive phase. The observations are interpreted in terms
of the splitting of the sheared envelope field of the greatly sheared
core rope during the early phase of the flare. We have also investigated
the temporal correlation between the EUV emission from the brightenings
observed by TRACE and the hard X-ray (HXR) emission (E > 150 keV)
observed by the anticoincidence system (ACS) of the spectrometer SPI
on board the ESA INTEGRAL satellite. The correlation between these
two emissions is very good, and the HXR sources (RHESSI) late in the
flare are located within the two EUV ribbons. These observations are
favorable to the explanation that the EUV brightenings mainly result
from direct bombardment of the atmosphere by the energetic particles
accelerated at the reconnection site, as does the HXR emission. However,
if there is a high temperature (T > 20 MK) HXR source close to the
loop top, a contribution of thermal conduction to the EUV brightenings
cannot be ruled out.
Title: The Non-Potential Structure of Solar Active Regions
Authors: DeLuca, Edward E.; van Ballegooijen, A.; Bobra, M.
Bibcode: 2006AAS...208.6508D
Altcode: 2006BAAS...38R.145D
Various methods for constructing 3D models of the coronal field are
discussed. The flux-rope insertion method is found to give a good fit to
TRACE observations of coronal loops near an H-alpha filament. Data from
the Advanced Stokes Polarimeter is used for extrapolating photospheric
vector fields into the corona. Tests indicate that models based on
chromospheric vector-field data are more reliable than those based on
photospheric data.
Title: Modeling Non-Potential Fields in Solar Active Regions
Authors: Bobra, Monica; DeLuca, E. E.; Van Ballegooijen, A. A.
Bibcode: 2006SPD....37.0112B
Altcode: 2006BAAS...38..217B
Many models aim to reproduce the non-linear force-free fields in
the solar corona; in this particular study, the magnetofrictional
relaxation method is tested. This method produces non-linear force-free
fields from line-of-sight magnetograms and uses ground-based H-alpha
images to define the location of a filament. Testing the model involves
comparing the model's results to TRACE observations of highly sheared,
non-potential magnetic field structures in several non-flaring active
regions. Preliminary results from such tests are presented. This work
is supported by NASA LWS grant NNG05GK32G.
Title: Magnetic Shear Change in Solar Flares
Authors: Su, Yingna; Golub, L.; Van Ballegooijen, A.
Bibcode: 2006SPD....37.0827S
Altcode: 2006BAAS...38..235S; 2006SPD....37.0827Y
It is well known that during a two-ribbon flare the two footpoint
ribbons, residing in opposite magnetic polarities, expand outward and
away from each other. However, the shear motion of the footpoints
along the direction parallel to the magnetic inversion line has
only been examined by several authors in some individual two-ribbon
flares. This motion, also described as shear change of the footpoints,
is interpreted by a magnetic field model with a highly sheared inner and
less sheared outer magnetic field configuration during the pre-flare
phase, and the cessation of the shear change during the impulsive
phase is interpreted as a splitting of the envelope of the highly
sheared core field. We have made a statistical investigation of the
EUV footpoint shear motion along the direction parallel to the magnetic
inversion line in two-ribbon flares, using the high spatial resolution
data obtained in 1998-2005 by Transition Region and Coronal Explorer
(TRACE). For this study we identified 52 well-observed X- and M- class
two-ribbon flares which fit our criteria: (1) most portions of the
two ribbons are inside the field of view (FOV); (2) TRACE obtained
several images during the rise and impulsive phase, from which we
can see the two ribbons and their evolution clearly; (3) limb flares,
which make the two ribbons and their evolution unable to be seen are
not considered. Our preliminary results are: a) 87% (45 out of 52
of these flares (Type I flare) show shear change of footpoints and
ribbon separation; b) in most Type I flares, the cessation of shear
change is 1-2 minutes earlier than the end of the impulsive phase; c)
there is no obvious ribbon separation in those 7 flares (Type II flare)
which have no shear change of the conjugate footpoints.
Title: Investigations of the Reconnecting Current Sheets in Solar
Eruptions
Authors: Lin, Jun; Li, J.; Forbes, T. G.; Ko, Y.; Raymond, J. C.;
van Ballegooijen, A. A.; Vourlidas, A.
Bibcode: 2006SPD....37.0826L
Altcode: 2006BAAS...38..235L
Observations and theories continuously provide evidence of formation
and development of the reconnecting current sheets during major
eruptions. Because the high electric conductivity and the force-free
environment of the solar corona, the current sheet is usually
confined in a small volume compared to the other structures involved
in the eruption in order to allow the energy conversion or magnetic
reconnection to take place at a plausible rate. The work investigating
the sizes, especially thickness, of the current sheet developed by the
eruption in reality was never conducted before since it is believed
that the current sheet is too thin to be observable. It has often been
stated that the thickness is limited either by the Lamor radius of
particles, which is about tens of meters in the coronal environment, or
by the mean-free-path of particles in the region where the interaction
between particles and ion-acoustic waves occurs. In the latter case,
the particle mean-free-path is a few kilometers. Here we use a set
of unique observations provided by the UVCS and LASCO experiments
on SOHO to determine the thickness, d, and furthermore the electric
resistivity, etae, of the current sheets for three eruptive
events. We find that d ranges from 1.0 x 104 km to 6.0
x 105 km, and etae from 1.0 x 105
ohm m to 4.0 x 106 ohm m. These values of etae
are 12-14 orders of magnitude greater than the classical collisional
resistivity, 4-6 orders of magnitude greater than estimates of anomalous
resistivity, and even 1-3 orders greater than those often used for solar
flares. The existence of such large values for d and etae
suggests that large scale turbulent processes are operating within
the current sheet. Understanding how a high level of turbulence can
develop so rapidly is a challenging goal for future research.
Title: Models of the Large-Scale Corona. II. Magnetic Connectivity
and Open Flux Variation
Authors: Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2006ApJ...642.1193M
Altcode:
In this paper the changing connectivity of the coronal magnetic
field during the formation and ejection of magnetic flux ropes
is considered. Using recent simulations of the coronal field,
it is shown that reconnection may occur both above and below the
flux ropes. Those occurring above slowly strip away coronal arcades
overlying the flux ropes and allow the flux ropes to be ejected. In
contrast, those below help to push the flux ropes out. It is found
that the reconnection occurring below each flux rope may result in
significant skew being maintained within the coronal field above the
PIL after the flux rope is ejected. In addition, after the eruption, as
the coronal field closes down, the large-scale transport of open flux
across the bipoles takes place through the process of ``interchange
reconnection.'' As a result, new photospheric domains of open flux
are created within the centers of the bipoles, where field lines were
previously closed. The net open flux in the simulation may be split
into two distinct contributions. The first contribution is due to the
nonpotential equilibrium coronal fields of the bipoles. The second
contribution is a temporary enhancement to this during the ejection of
the flux ropes, where previously closed field lines become open. It is
shown that the nonpotential equilibrium contribution to the open flux
is significantly higher than that due to a potential field deduced
from the same photospheric boundary conditions. These results suggest
that the nonpotential nature of coronal magnetic fields may affect the
variation of the Sun's open flux during periods of high solar activity
and should be considered in future simulations.
Title: Models of the Large-Scale Corona. I. Formation, Evolution,
and Liftoff of Magnetic Flux Ropes
Authors: Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2006ApJ...641..577M
Altcode:
The response of the large-scale coronal magnetic field to transport of
magnetic flux in the photosphere is investigated. In order to follow the
evolution on long timescales, the coronal plasma velocity is assumed
to be proportional to the Lorentz force (magnetofriction), causing
the coronal field to evolve through a series of nonlinear force-free
states. Magnetofrictional simulations are used to study the formation
and evolution of coronal flux ropes, highly sheared and/or twisted
fields located above polarity inversion lines on the photosphere. As in
our earlier studies, the three-dimensional numerical model includes the
effects of the solar differential rotation and small-scale convective
flows; the latter are described in terms of surface diffusion. The
model is extended to include the effects of coronal magnetic diffusion,
which limits the degree of twist of coronal flux ropes, and the solar
wind, which opens up the field at large height. The interaction of two
bipolar magnetic regions is considered. A key element in the formation
of flux ropes is the reconnection of magnetic fields associated with
photospheric flux cancellation at the polarity inversion lines. Flux
ropes are shown to form both above the external inversion line between
bipoles (representing type B filaments) and above the internal inversion
line of each bipole in a sigmoid shape. It is found that once a flux
rope has formed, the coronal field may diverge from equilibrium with
the ejection of the flux rope. After the flux rope is ejected, the
coronal field once again relaxes down to an equilibrium. This ability
to follow the evolution of the coronal fields through eruptions is
essential for future full-Sun simulations in which multiple bipoles
are evolved for many months or years.
Title: Properties of the Post-CME Current Sheets in Solar Eruptions
Authors: Lin, J.; Li, J.; Forbes, T. G.; Ko, Y. -K.; Raymond, J. C.;
van Ballegooijen, A. A.; Vourlidas, A.
Bibcode: 2006cosp...36..198L
Altcode: 2006cosp.meet..198L
Solar eruptions constitute the most violent energy release process in
the solar system They are are created when magnetic energy is suddenly
converted into heat and kinetic energy by magnetic reconnection in
a field reversal region or current sheet The effective electrical
resistivity eta e of the sheet plasma plays a crucial role in energy
conversion Here we present the results for the current sheet thickness
d and eta e determined by analyzing a set of unique data for three
eruptions observed by the UVCS and the LASCO experiments on SOHO Such a
work was never conducted before since it is believed that the current
sheet is too thin to be observable The extremely large values of d
and eta e obtained in this work suggest that the current sheet in
solar eruptions is observable in some circumstances and large-scale
turbulence is operating within the current sheet This constitutes a
challenging goal for future research on the magnetic energy conversion
occurring in both space and laboratory
Title: Wave Propagation in the Magnetic Network on the Sun
Authors: Hasan, S. S.; Vigeesh, G.; van Ballegooijen, A. A.
Bibcode: 2006IAUS..233..116H
Altcode:
Hasan et al. (2005) have recently presented 2-D dynamical calculations
on wave propagation in in the magnetic network of the Sun. The latter
is idealized as consisting of non-potential flux tubes in the quiet
solar chromosphere. It is of interest to understand how the nature of
wave propagation is influenced by the choice of initial equilibrium
configuration of the magnetic field. We examine this by comparing
the earlier calculations with those when the network is modelled as
a potential structure. Our calculations demonstrate that the nature
of the wave propagation is significantly different, particularly
the transport of energy which for the potential case, occurs more
isotropically than for the non-potential configuration.
Title: A statistical study of shear motion of EUV footpoints in
two-ribbon flares
Authors: Su, Y. N.; Golub, L.; van Ballegooijen, A. A.; Huang, G. L.
Bibcode: 2006IAUS..233..409S
Altcode:
We present a statistical investigation of shear motion of the EUV
footpoints in two-ribbon solar flares, using the high spatial resolution
data obtained in 1998-2005 by the Transition Region and Coronal
Explorer (TRACE). In this study we have selected 50 well-observed X-
and M- class two-ribbon flares. Our preliminary results are: a) 86%
(43 out of 50) of these flares show shear change of footpoints and
ribbon separation; b) there is no obvious ribbon separation in those
7 flares which have no shear change of the conjugate footpoints; c)
in most events, the cessation of shear change is 0-2 minutes earlier
than the end of the impulsive phase.
Title: A Solar Energetic Particle Mission (SEPM) for the S3C Great
Observatory
Authors: Strachan, L.; Kohl, J. L.; Cranmer, S. R.; Esser, R.; Gardner,
L. D.; Lin, J.; Raymond, J. C.; van Ballegooijen, A.; Socker, D. G.
Bibcode: 2005AGUFMSH51C1221S
Altcode:
The S3C Great Observatory concept is guided by a systems approach
to understanding the heliosphere. A Solar Energetic Particle Mission
(SEPM) can make valuable contributions to this Great Observatory in
conjunction with upcoming Living with a Star (LWS) missions e.g. Solar
Dynamics Observatory and Sentinels. SEPM can provide the remote sensing
component to a program for better understanding how, when, and where
solar energetic particles (SEPs) are produced. Such a coordinated
approach will include coronagraphic UV spectroscopy and visible light
polarimetery with SEPM, along with in situ particles and fields,
X-ray and gamma-ray measurements from spacecraft close to the Sun
(Inner Heliospheric Sentinels). While SEPM will use plasma diagnostics
that are derived from the UVCS and LASCO coronagaraphs on SOHO, it
will have an order of magnitude improvement in its capabilities.
Title: Analysis of EUV Brightenings of Solar Flares
Authors: Su, Y.; Golub, L.; van Ballegooijen, A.; Gros, M.
Bibcode: 2005AGUFMSH13A0272S
Altcode:
An X17 class (GOES soft X-ray) two-ribbon solar flare occurred on
2003 October 28. EUV observations made by the Transition Region and
Coronal Explorer (TRACE) show us that the two flare ribbons are
initially composed of several discrete bright kernels. High time
resolution hard X-ray (HXR) (E >150 KeV) data were obtained by the
anticoincidence system (ACS) of the spectrometer SPI on board the ESA
INTEGRAL satellite, and 8 spikes can be seen during the impulsive
phase of the solar flare. The comparison of the evolution of EUV
bright kernels and HXR light curves shows us that individual peaks in
the HXR bursts are associated with peaks in the EUV light curves of
individual bright kernels, which suggests that the EUV kernels result
from direct energy deposition of the energetic particles accelerated
in the flare. EUV bright kernels within the flare ribbons are tracked
and identified as conjugate footpoints using a manual method. The
co-alignment of TRACE/EUV and Hα images from Big Bear Solar Observatory
(BBSO) using EIT and MDI data from SOHO allows us to obtain information
about the ribbon's underlying magnetic inversion line. We find that
the EUV conjugate footpoints start (3 minutes before the hard X-ray
onset) at a position close to, but with a large separation along,
the magnetic inversion line and evolve into straight across and far
from the inversion line during the impulsive phase. We also made a
statistical investigation of the two-ribbon flares observed by TRACE. We
find that this progressive decrease in shear of footpoints is a common
feature, which appears to indicate that the pre-flare magnetic field
configuration of this kind of flare is highly sheared close to, and
less sheared far from the magnetic inversion line.
Title: Dynamics of the Solar Magnetic Network: Two-dimensional
MHD Simulations
Authors: Hasan, S. S.; van Ballegooijen, A. A.; Kalkofen, W.;
Steiner, O.
Bibcode: 2005ApJ...631.1270H
Altcode: 2005astro.ph..3525H
The aim of this work is to identify the physical processes that occur
in the network and contribute to its dynamics and heating. We model the
network as consisting of individual flux tubes, each with a nonpotential
field structure, that are located in intergranular lanes. With a typical
horizontal size of about 150 km at the base of the photosphere, they
expand upward and merge with their neighbors at a height of about 600
km. Above a height of approximately 1000 km the magnetic field starts
to become uniform. Waves are excited in this medium by means of motions
at the lower boundary. We focus on transverse driving, which generates
both fast and slow waves within a flux tube and acoustic waves at
the interface of the tube and the ambient medium. The acoustic waves
at the interface are due to compression of the gas on one side of
the flux tube and expansion on the other. These longitudinal waves
are guided upward along field lines at the two sides of the flux
tube, and their amplitude increases with height due to the density
stratification. Being acoustic in nature, they produce a compression
and significant shock heating of the plasma in the chromospheric part of
the flux tube. For impulsive excitation with a time constant of 120 s,
we find that a dominant feature of our simulations is the creation of
vortical motions that propagate upward. We have identified an efficient
mechanism for the generation of acoustic waves at the tube edge, which
is a consequence of the sharp interface of the flux concentration. We
examine some broad implications of our results.
Title: Is the Quiet-Sun Corona a Quasi-steady, Force-free Environment?
Authors: Schrijver, Carolus J.; van Ballegooijen, Adriaan A.
Bibcode: 2005ApJ...630..552S
Altcode:
We model a coronal volume over a quiet, mixed-polarity solar network as
an ensemble of quasi-steady loop atmospheres. These are contained by an
assumed potential field, including the associated variations in the loop
cross section through the coronal volume and the loop flows induced by
such asymmetries. The average temperature and density stratifications
are close to those of the quiet-Sun corona for a coronal heating flux
density into the corona of FH=8×1014B/L (ergs
cm-2 s-1) for loop-base field strengths B (G)
and loop half-lengths L (cm). Earlier, that heating parameterization
was shown to be consistent with the appearance and radiative losses of a
solar corona in which active regions dominated the emission. This study
thus supports the hypothesis that the same, likely braiding-driven,
heating dominates throughout the quiescent corona. The average ratio
β of gas to magnetic pressure lies close to unity throughout the
modeled coronal height range of 22 Mm, with β>1 in ~30% of the
volume and β>0.4 in ~90% of the volume, perhaps indicating that
the quiet-Sun corona is driven to near its maximum heating capacity by
the random walk of its footpoints. Our findings that the solar corona
has β close to unity, and that our model corona exhibits insufficient
fine structure and no significant spatially averaged Doppler shifts,
imply that the quiet-Sun corona is often neither quasi-steady nor
force free and thus that dynamic magnetohydrodynamics (MHD) models
are essential to furthering our understanding of the quiet solar corona.
Title: Slingshot prominences above stellar X-ray coronae
Authors: Jardine, M.; van Ballegooijen, A. A.
Bibcode: 2005MNRAS.361.1173J
Altcode: 2005MNRAS.tmp..626J; 2005astro.ph..6229J
We present a new model for the coronal structure of rapidly rotating
solar-type stars. The presence of prominences trapped in co-rotation
2-5 stellar radii above the stellar surface has been taken as evidence
that the coronae of these stars must be very extended. The observed
surface magnetic fields, however, cannot contain X-ray emitting
gas out to these distances. We present an alternative model: that
these prominences are trapped in long thin loops embedded not in
the X-ray corona, but in the wind. Above coronal helmet streamers,
oppositely directed wind-bearing field lines reconnect to form closed
loops, which then fill up with gas that was originally part of the
wind. We demonstrate that static equilibria exist for these loops at
a range of pressures and temperatures. The maximum loop height falls
as the rotation rate increases, but rises as the loop temperature
decreases. For a solar mass star with a rotation period of 0.5 d,
whose X-ray corona extends 1 R* above the surface, loops
at temperatures of 104K can extend out to 5 R*.
Title: Equilibrium and Evolution in Multipolar Magnetic Configurations
Resulting from Interactions among Active Regions
Authors: Lin, J.; van Ballegooijen, A. A.
Bibcode: 2005ApJ...629..582L
Altcode:
The evolution of magnetic configurations with a current-carrying flux
rope for modeling a prominence is investigated. The configurations
evolve as a result of the interaction between two or more magnetic
source regions. The background field of the system is produced
by either two dipoles or four monopoles located on the boundary
surface. Our purpose is to study the catastrophic behaviors of the
systems' evolutions in response to the change in either the distance
between source regions or the strength of background field, and to
find the maximum heights that the flux rope can attain before it
loses equilibrium and catastrophe occurs. We find that the domain in
parameter space over which the flux rope remains in equilibrium is
smaller than in the simpler configurations that have been previously
investigated. Consequently, the equilibrium heights of the flux rope
studied in this work are fairly low, such that the maximum values
of these heights compared to the corresponding distances between the
active regions vary from less than 3% to about 23%, with an average of
about 13.2%, all of which are below 25%. This result has an important
observational consequence, namely that a prominence appearing between
different active regions will tend to be located in the low corona
and will tend to become unstable if its height increases.
Title: Magnetic Reconnection and the Deduced Properties of Plasma
inside the CME/Flare Current Sheet
Authors: Lin, J.; Li, J.; Forbes, T. G.; Ko, Y.; Raymond, J. C.;
van Ballegooijen, A. A.
Bibcode: 2005AGUSMSH51A..04L
Altcode:
In the present work, we display our results of studying and analyzing
the observational data from UVCS and other remote sensing instruments
for three CME/flare events that obviously developed a long current
sheet during the eruptions. These results include the thickness of the
current sheets, magnetic diffusivities and electrical conductivities
(resistivities) of the plasma inside the current sheets. This is
the first time that the electrical conductivity (resistivity) within
magnetic reconnection region during the real eruptive processes has
been deduced since the theory of magnetic reconnection was applied to
the solar eruptions about 6 decades ago. The thickness of the current
sheet developed during the January 8, 2002 event varies from 7 ×
104 km to 2.2 × 105 km for altitudes between 2.5 R⊙ and 5.5 R⊙,
with the average thickness of 1.4× 105 km, and the speed of magnetic
reconnection inflow near the current sheet is about 10 km s-1. These
results suggest a magnetic diffusivity of the plasma inside the current
sheet to be 0.7 × 1012 m2 s-1 (compared to the classical value for
the quiet corona of 1 m2 s-1, and to the corresponding value for the
"turbulent plasma" of 3.4× 106 m2 s-1). For the event occurring on
November 18, 2003, the data from UVCS indicate that the upper limit
of the current sheet thickness at altitude of 1.7 R⊙ is about 2.8
× 104 km, and that the velocity of magnetic reconnection inflow near
the current sheet ranges from 10.5 km s-1 to 106 km s-1. Combining
these results yields a range of magnetic diffusivity from 1.4× 1011
m2 s-1 to 1.4× 1012 m2 s-1. During the event observed on March 23,
1998, the upper limit of the thickness of the current sheet in the
wake of a CME is about 105 km according to data from UVCS. No data
for the velocity of the magnetic reconnection near the current sheet
in this event were obtained. Considering the fact that this event was
more gradual than the other two cases, we assume the inflow speed in
this event to be 5 km s-1. So, we obtain that the magnetic diffusivity
of the plasma inside the current sheet has an upper limit of 2.5×
1011 m2 s-1. We notice that values of magnetic diffusivity deduced
for three different events are within the range of magnitude.
Title: Towards a Universal Physics-based "Coronal Heating Function"
for Electrons, Protons, and Heavy Ions in the Accelerating Solar Wind
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2005AGUSMSH51A..03C
Altcode:
The Sun is often highlighted as a benchmark for the study of other
stars, and as a stepping stone to the study of galaxies and cosmic
distances. Not to be outdone, the solar wind is rapidly becoming a key
baseline for the understanding of basic plasma phenomena such as MHD
turbulence, kinetic wave-particle interactions, and nonlinear wave-mode
coupling. In keeping with the IHY focus on these kinds of universal
processes, we present a distillation of recent modeling efforts to
understand how Alfven waves are generated, reflected, cascaded, and
damped throughout the solar wind. A physical understanding of solar
wind turbulence is crucial to the modeling of energetic particle
transport in the heliosphere and the interaction with interstellar
neutrals. The goal of this work is to derive a useful "recipe" for solar
wind modelers that, given the background zero-order plasma properties,
yields the wave amplitudes, the turbulent cascade rates, and the
kinetic partitioning of the resultant heating into electrons, protons,
and heavy ions (differentiating between parallel and perpendicular
heating as well). We also discuss preliminary ideas concerning how the
collisionless particle heating is modified if the turbulent cascade
ends with the production of small-scale reconnection current sheets.
Title: New Results in Modeling the Hemispheric Pattern of Solar
Filaments
Authors: Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2005ApJ...621L..77M
Altcode:
New results in modeling the hemispheric pattern of solar filaments
through magnetic flux transport and magnetofrictional simulations are
presented. The simulations consider for the first time what type of
chirality forms along the polarity inversion line lying in between two
magnetic bipoles as they interact. Such interactions are important for
filament formation, as observations by F. Tang show that the majority of
filaments form in between bipolar regions rather than within a single
magnetic bipole. The simulations also include additional physics of
coronal diffusion and a radial outflow velocity at the source surface,
which was not included in previous studies. The results clearly
demonstrate for the first time not only the origin of the dominant
hemispheric pattern but also why exceptions to it may occur. The
dominant hemispheric pattern may be attributed to the dominant range
of bipole tilt angles and helicities in each hemisphere. Exceptions to
the hemispheric pattern are found to only occur in cases of no initial
helicity or for helicity of the minority type in each hemisphere when
large positive bipole tilt angles (α>20deg) are used. As
the simulations show a clear dependence of the hemispheric pattern
and its exceptions on observational quantities, this may be used to
check the validity of the results. Future programs to consider this
are put forward.
Title: Inferring Coronal Structure from X-Ray Light Curves and
Doppler Shifts: A Chandra Study of AB Doradus
Authors: Hussain, G. A. J.; Brickhouse, N. S.; Dupree, A. K.; Jardine,
M. M.; van Ballegooijen, A. A.; Hoogerwerf, R.; Collier Cameron, A.;
Donati, J. -F.; Favata, F.
Bibcode: 2005ApJ...621..999H
Altcode: 2004astro.ph.11571H
The Chandra X-Ray Observatory continuously monitored the single
cool star AB Dor for a period lasting 88 ks (1.98Prot)
in 2002 December with the Low-Energy Transmission Grating HRC-S. The
X-ray light curve shows rotational modulation with three peaks that
repeat in two consecutive rotation cycles. These peaks may indicate the
presence of compact emitting regions in the quiescent corona. Centroid
shifts as a function of phase in the strongest line profile, O VIII
λ18.97, indicate Doppler rotational velocities with a semiamplitude
of 30+/-10 km s-1. By taking these diagnostics into account
along with constraints on the rotational broadening of line profiles
(provided by archival Chandra High-Energy Transmission Grating Fe
XVII and Far Ultraviolet Spectroscopic Explorer Fe XVIII profiles),
we can construct a simple model of the X-ray corona that requires two
components. One of these components is responsible for 80% of the X-ray
emission and arises from the pole and/or a homogeneously distributed
corona. The second component consists of two or three compact active
regions that cause modulation in the light curve and contribute to
the O VIII centroid shifts. These compact regions account for 16% of
the emission and are located near the stellar surface with heights of
less than 0.3R*. At least one of the compact active regions
is located in the partially obscured hemisphere of the inclined star,
while another of the active regions may be located at 40°. High-quality
X-ray data such as these can test the models of the coronal magnetic
field configuration as inferred from magnetic Zeeman Doppler imaging.
Title: Inferring coronal structure using X-ray spectra: a Chandra
study of AB Dor
Authors: Hussain, G. A. J.; Brickhouse, N. S.; Dupree, A. K.;
Jardine, M.; van Ballegooijen, A.; Collier Cameron, A.; Donati,
J. -F.; Favata, F.
Bibcode: 2005ESASP.560..665H
Altcode: 2004astro.ph.10213H; 2005csss...13..665H
The Chandra X-ray observatory monitored the single cool star, AB
Doradus, continuously for a period lasting 88ksec (1.98 Prot) in 2002
December with the LETG/HRC-S. The X-ray lightcurve shows significant
rotational modulation. It can be represented as having a flat level
of emission superimposed with bright flaring regions that appear
at the same phases in both rotation cycles. Phase-binned OVIII line
profiles show centroid shifts that also repeat in consecutive rotation
cycles. These Doppler shifts trace a roughly sinusoidal pattern with
a a semi-amplitude of 30 +/-10km/s. By taking both the lightcurve
and spectral diagnostics into account along with constraints on the
rotational broadening of line profiles (provided by archival Chandra
HETG FeXVII line profiles) we can construct a simple model of the
X-ray corona. The corona can be described as having two components,
one component is homogeneously distributed, extending less than 1.75R*;
and the other consists of at least two compact emitting regions near
the stellar surface. These compact regions account for 16% of the
X-ray emission and are likely to be located less than 0.4R* above the
stellar surface.
Title: On the Generation, Propagation, and Reflection of Alfvén
Waves from the Solar Photosphere to the Distant Heliosphere
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2005ApJS..156..265C
Altcode: 2004astro.ph.10639C
We present a comprehensive model of the global properties of Alfvén
waves in the solar atmosphere and the fast solar wind. Linear non-WKB
wave transport equations are solved from the photosphere to a distance
past the orbit of the Earth, and for wave periods ranging from 3 s
to 3 days. We derive a radially varying power spectrum of kinetic and
magnetic energy fluctuations for waves propagating in both directions
along a superradially expanding magnetic flux tube. This work differs
from previous models in three major ways. (1) In the chromosphere
and low corona, the successive merging of flux tubes on granular and
supergranular scales is described using a two-dimensional magnetostatic
model of a network element. Below a critical flux-tube merging height
the waves are modeled as thin-tube kink modes, and we assume that all
of the kink-mode wave energy is transformed into volume-filling Alfvén
waves above the merging height. (2) The frequency power spectrum of
horizontal motions is specified only at the photosphere, based on prior
analyses of G-band bright point kinematics. Everywhere else in the model
the amplitudes of outward and inward propagating waves are computed
with no free parameters. We find that the wave amplitudes in the
corona agree well with off-limb nonthermal line-width constraints. (3)
Nonlinear turbulent damping is applied to the results of the linear
model using a phenomenological energy loss term. A single choice for
the normalization of the turbulent outer-scale length produces both
the right amount of damping at large distances (to agree with in situ
measurements) and the right amount of heating in the extended corona (to
agree with empirically constrained solar wind acceleration models). In
the corona, the modeled heating rate differs by more than an order of
magnitude from a rate based on isotropic Kolmogorov turbulence.
Title: UVCS Observations of a Helical CME Structure
Authors: Suleiman, R. M.; Crooker, N. U.; Raymond, J. C.; van
Ballegooijen, A.
Bibcode: 2005IAUS..226...71S
Altcode:
A helical structure in the coronal mass ejection (CME) of 12 September
2000 was observed by the Ultraviolet Coronagraph Spectrometer (UVCS)
aboard the Solar and Heliospheric Observatory (SOHO) at heliocentric
distances of 3.5 and 6 R⊙. A difference of 300 km
sec-1 in line-of-sight velocities for two segments of the
helix obtained from Doppler measurements implies expansion and allows
one to distinguish which segment was closest to the observer. The
tilt of the segment then determines the handedness. Observed Ly α
and C III line emissions indicate that the helix was threaded with
filament plasma of varying density. While the helix constituted the
bright core of filament plasma, the helix itself was most likely not
the pre-existing filament structure.
Title: Magnetic Structure Equilibria and Evolutions due to Active
Region Interactions
Authors: Lin, J.; van Ballegooijen, A. A.
Bibcode: 2005IAUS..226..257L
Altcode:
Equilibria and evolutions in the coronal magnetic configurations
due to the interactions among active regions are investigated. The
magnetic structure includes a current-carrying flux rope that is
used to model the prominence or filament. We use either two dipoles
or four monopoles on the boundary surface to model active regions,
and the change in the boundary conditions corresponds to either
the horizontal motion of magnetic sources or decaying of the active
regions. Both cases show the catastrophic behavior in the system's
evolutions. The results have important observational consequences:
most eruptive prominences that give rise to CMEs are driven by the
interactions between two or more active regions. Such eruptions may
not necessarily take place in the growing phase of the active regions,
instead they usually occur at the decay phase.
Title: Observations and Modeling of a Filament on the Sun
Authors: van Ballegooijen, A. A.
Bibcode: 2004ApJ...612..519V
Altcode:
Hα observations of a filament were obtained at the Swedish Vacuum
Solar Telescope in 1998 June. The U-shaped filament has a prominent
barb that exhibits interesting fine structure and internal motions. A
three-dimensional magnetic model of the filament is presented. The model
is based on a National Solar Observatory (Kitt Peak) magnetogram and
is constructed by inserting a twisted flux rope into a potential field
representing the overlying coronal arcade; the flux rope has an axial
flux of 3.4×1019 Mx and poloidal flux of 3.7×109
Mx cm-1. Magnetofrictional relaxation is used to drive
the configuration to a nonlinear force-free field. The shape of the
resulting flux rope is distorted by neighboring network elements. The
dips in the helical field lines reproduce the observed filament barb,
which is caused by a local distortion of the flux rope resulting from a
weak-field extension (~4 G) of a neighboring network element. The pitch
of the helical field lines is larger than expected on the basis of a
model of flux rope formation. I suggest that this is due to magnetic
diffusion within the flux rope. A simple model of magnetic diffusion
in a cylindrical flux rope is presented.
Title: An Overview of Alfven Wave Generation, Reflection, and Damping
from the Solar Photosphere to the Distant Heliosphere
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2004AAS...204.0401C
Altcode: 2004BAAS...36..698C
The continually evolving convection below the solar photosphere gives
rise to a wide spectrum of magnetohydrodynamic (MHD) fluctuations in
the magnetic atmosphere and solar wind. The propagation of waves through
the solar atmosphere has been studied for more than a half century, and
the mainly incompressible Alfven mode has been believed to be dominant
in regions that are open to the heliosphere. As a part of an ongoing
study of various aspects of solar MHD waves and turbulence, we present
a comprehensive model of the radially evolving properties of Alfvenic
fluctuations in a representative open magnetic region. This work
differs from previous models in the following ways. (1) The background
plasma density, magnetic field, and flow velocity are constrained
empirically from below the photosphere to distances past 1 AU. The
successive merging of flux tubes on granular and supergranular scales
is described using a two-dimensional magnetostatic model of a magnetic
network element. (2) The frequency power spectrum of horizontal motions
is specified only at the photosphere, based on prior analyses of G-band
bright points. Everywhere else in the model the amplitudes of outward
and inward propagating waves are computed with no free parameters. We
compare the resulting wave properties with observed nonthermal motions
in the chromosphere and corona, radio scintillation measurements,
and in-situ fluctuation spectra. This work is supported by NASA
under grants NAG5-11913, NAG5-12865, and NAG5-10996 to the Smithsonian
Astrophysical Observatory, by Agenzia Spaziale Italiana, and by the
Swiss contribution to the ESA PRODEX program.
Title: The Role of Magnetic Reconnection in the Observable Features
of Solar Eruptions
Authors: Lin, J.; Raymond, J. C.; van Ballegooijen, A. A.
Bibcode: 2004ApJ...602..422L
Altcode:
There are two competing classes of models for coronal mass ejections
(CMEs): those that assume a preexisting magnetic flux rope and
those that can make a flux rope during the eruption by magnetic
reconnection. The present work is based on the model with a preexisting
flux rope. We investigate the evolution of morphological features of
the magnetic configuration in a CME according to a catastrophe model
of flux rope CMEs developed previously. For the parameters chosen for
the present work, roughly half of the total mass and magnetic flux
are contained in the initial flux rope, while the remaining plasma and
poloidal magnetic flux are brought by magnetic reconnection from the
corona into the current sheet and from there into the CME bubble. These
features and the corresponding physical processes are identical to
those described by the non-flux rope models. Thus, the flux rope and
non-flux rope models are less distinct than is generally assumed. The
reconnected magnetic flux can account for the rapid expansion of the
ejecta, and the plasma flowing out of the current sheet fills the
outer shell of the ejecta. We tentatively identify the outer shell,
the expanded bubble, and the flux rope with the leading edge, void,
and core of the three-component CME structure, respectively. Thus, the
final mass, speed, and magnetic energy-the quantities that determine the
geoeffectiveness of the CME-are determined not in the initial eruption
but during the CME expansion, at heights of a few solar radii. The
aspects of this explanation that need improvement are also discussed.
Title: Using Chandra/LETG Spectra to Probe Stellar Coronae
Authors: Hussain, G. A. J.; Brickhouse, N.; Dupree, A. K.; van
Ballegooijen, A. A.; Collier Cameron, A.; Jardine, M.; Donati, J. -F.
Bibcode: 2004IAUS..219..301H
Altcode: 2003IAUS..219E.169H; 2004astro.ph..3215H
We probe the relationship between surface magnetic fields and the
X-ray emitting corona in the rapidly rotating star AB Dor. Circularly
polarised spectra have been inverted to produce a surface (photospheric)
magnetic field map. This surface map has been extrapolated to model
AB Dor's coronal field topology and X-ray light curve. Chandra/LETG
light curves of AB Dor from the same epoch show intrinsic variability
at the 30% level. Period analysis indicates a fraction of this is
due to rotational modulation. We measure velocity shifts in emission
line centroids as a function of rotation period and find evidence
of rotational modulation (max. vel. ~40+/- 13km/s). This modulation
may indicate the presence of a localised X-ray emitting region at
mid-to-high latitudes.
Title: Non-WKB Alfven Wave Reflection from the Solar Photosphere to
the Distant Heliosphere
Authors: Cranmer, S. R.; van Ballegooijen, A.
Bibcode: 2003AGUFMSH21B0115C
Altcode:
Magnetohydrodynamic (MHD) turbulence has been considered for
several decades as a possibly substantial heat source for the solar
chromosphere, corona, and heliosphere. However, it is still not well
understood how the turbulent fluctuations are generated and how they
evolve in frequency and wavenumber. Although the dominant population of
Alfvén waves near the Sun must be propagating outwards, one also needs
waves propagating inwards in order to ``seed'' a turbulent cascade. As
a part of an ongoing study of various aspects of solar MHD turbulence,
we present a model of linear, non-WKB reflection of Alfvén waves that
propagate in both directions along an open magnetic flux tube. Our work
differs from previous models in the following ways. (1) The background
plasma density, magnetic field, and flow velocity are constrained
empirically from below the photosphere to distances past 1 AU. The
successive merging of flux tubes on granular and supergranular scales
is described using a two-dimensional magnetostatic model of a magnetic
network element in the stratified solar atmosphere. (2) The amplitudes
of horizontal wave motions are specified only at the photosphere, based
on previous analyses of G-band bright point motions. Everywhere else in
the model the amplitudes of outward and inward propagating waves are
computed self-consistently. We compare the resulting wave properties
with observed nonthermal motions in the chromosphere and corona, radio
scintillation measurements, and in-situ fluctuation spectra. Quantities
such as the MHD turbulent heating rate and the non-WKB wave pressure
are computed, and the need for other sources of inward waves (e.g.,
nonlinear reflection or scattering off density inhomogeneities) will
also be discussed. This work is supported by the National Aeronautics
and Space Administration under grants NAG5-11913 and NAG5-12865 to the
Smithsonian Astrophysical Observatory, by Agenzia Spaziale Italiana,
and by the Swiss contribution to the ESA PRODEX program.
Title: Magnetic Configuration in Low Solar Atmosphere Prior to
Eruptions
Authors: van Ballegooijen, A. A.
Bibcode: 2003AGUFMSH21C..01V
Altcode:
Vector magnetograph observations of active regions prior to large
flares often show strongly sheared magnetic fields, and the associated
Hα filaments show long threads parallel to the neutral line. This
suggests that the filament is embedded in a horizontal flux tube that
is basically untwisted. In contrast, eruptive prominences often show
helical structures, suggesting a flux rope with multiple twists. To
reconcile these observations, we propose a model of the pre-eruptive
state in which an untwisted horizontal flux tube is held down by an
overlying magnetic arcade. Unlike in previous models, electric currents
flow mainly at the interface between the two flux systems. The two
ends of the flux tube are anchored in the photosphere. We use 3D MHD
modeling based on NSO/KP magnetograph data to demonstrate that such
a system can be in stable force-free equilibrium, provided the arcade
field is sufficiently strong to restrain the flux tube. A weakening of
the arcade or interaction with a neighboring filament can cause loss
of magnetostatic equilibrium, resulting in the eruption of part of the
flux tube (Sturrock et al. 2001, ApJ 548, 492). Magnetic reconnection
during the early phase of the eruption causes the arcade field to be
wrapped around the filament flux, creating the unstable flux rope seen
in erupting prominences. The model is applied to Hα observations of
a filament obtained at the Swedish Vacuum Solar Telescope (La Palma)
and TRACE observation of its eruption on June 21-22, 1998.
Title: Evolution of Morphological Features of CMEs Deduced from
Catastrophe Model of Solar Eruptions
Authors: Lin, J.; Raymond, J. C.; van Ballegooijen, A. A.
Bibcode: 2003AGUFMSH22B..04L
Altcode:
We investigate the evolution of morphological features during a coronal
mass ejection (CME) occurring in a specific magnetic configuration in
the present work. The results indicate that part of the magnetic flux
and plasma ejected into the heliosphere by a CME exist in the flux rope
or prominence prior to the eruption. For the parameters we chose for
the present work, our calculations show that more than one third of the
ejected plasma is continuously brought by magnetic reconnection from the
corona during the eruption, and around a half amount of the poloidal
flux, together with the plasma, is collected by reconnection from the
coronal magnetic field and then is sent into interplanetary space via
the upper tip of the current sheet. The reconnected magnetic flux is
able to account for the fast expansion of the ejecta. The temperature
of the reconnected plasma is fairly high (up to ∼ 107 K),
and blending of this hot plasma with cold prominence material may drive
the prominence from absorption to emission in the EUV. This process
constitutes a natural and straightforward mechanism for prominence
heating during the eruption.
Title: The Effect of Proton Temperature Anisotropy on the Solar
Minimum Corona and Wind
Authors: Vásquez, Alberto M.; van Ballegooijen, Adriaan A.; Raymond,
John C.
Bibcode: 2003ApJ...598.1361V
Altcode: 2003astro.ph.10846V
A semiempirical, axisymmetric model of the solar minimum corona
is developed by solving the equations for conservation of mass and
momentum with prescribed anisotropic temperature distributions. In
the high-latitude regions, the proton temperature anisotropy is strong
and the associated mirror force plays an important role in driving the
fast solar wind; the critical point where the outflow velocity equals
the parallel sound speed (v=c∥) is reached already at 1.5
Rsolar from Sun center. The slow wind arises from a region
with open-field lines and weak anisotropy surrounding the equatorial
streamer belt. The model parameters were chosen to reproduce the
observed latitudinal extent of the equatorial streamer in the corona and
at large distance from the Sun. We find that the magnetic cusp of the
closed-field streamer core lies at about 1.95 Rsolar. The
transition from fast to slow wind is due to a decrease in temperature
anisotropy combined with the nonmonotonic behavior of the nonradial
expansion factor in flow tubes that pass near the streamer cusp. In
the slow wind, the plasma β is of order unity and the critical point
lies at about 5 Rsolar, well beyond the magnetic cusp. The
predicted outflow velocities are consistent with O5+ Doppler
dimming measurements from UVCS/SOHO. We also find good agreement with
polarized brightness (pB) measurements from LASCO/SOHO and H I Lyα
images from UVCS/SOHO.
Title: Physical Parameters of the 2000 February 11 Coronal Mass
Ejection: Ultraviolet Spectra versus White-Light Images
Authors: Ciaravella, A.; Raymond, J. C.; van Ballegooijen, A.;
Strachan, L.; Vourlidas, A.; Li, J.; Chen, J.; Panasyuk, A.
Bibcode: 2003ApJ...597.1118C
Altcode:
We present spectra of a three-part coronal mass ejection (CME) observed
by the Ultraviolet Coronagraph Spectrometer aboard SOHO on 2000
February 11. Images of the CME in different spectral lines show how
the morphology depends on the temperature, density, and outflow speed
of the ejected plasma. The H I Lyα is the line that best resembles
the white-light data, although it can be rather different where the
outflow speed severely dims its radiative component. We estimate the
ranges of temperature and density in the front, prominence core, and
void. We also estimate the outflow speed that is the true speed of the
ejecta as obtained from the Doppler dimming technique, its component
projected on the plane of the sky, and the line-of-sight speed for the
three components of the CME. The plasma in the front was denser, cooler,
and more depleted in O and Si than the ambient coronal streamer. These
characteristics indicate that it originated in the closed field core
of the pre-CME streamer. The leading edge was not the projection of
a simple spherical shell onto the plane of the sky. The line profiles
suggest a wide looplike structure, although a more complete shell that
was brighter in some areas could also match the data. The prominence has
a structure in temperature and density with the hotter top end emitting
in the Mg X and Si XII lines while the bottom end was much cooler and
visible only in the H I Lyman lines. Emission in the void was rather
faint. The outflow speed obtained from Doppler dimming of the radiative
lines, the line-of-sight speed measured from the Doppler shift of the
lines, and the plane-of-the-sky speed estimated from the comparison
of the images taken at 2.3 and 2.6 Rsolar give speeds much
lower than those estimated at greater heights (>4 Rsolar)
from LASCO and indicate a stronger acceleration at lower heights.
Title: Modeling the Corona of AB Doradus
Authors: Hussain, G. A. J.; van Ballegooijen, A. A.; Jardine, M.;
Collier Cameron, A.
Bibcode: 2003csss...12...50H
Altcode:
We present a model for the coronal topology of the active, rapidly
rotating K0 dwarf, AB Doradus. Surface magnetic field maps obtained
using a technique based on Zeeman Doppler imaging indicate the
presence of a strong non-potential component near the pole of the
star. The coronal topology is obtained by extrapolating these surface
maps. The temperature and density in the corona are evaluated using an
energy balance model. Emission measure distributions computed using
our models compare favorably with observations. However, the density
observed by EUVE, ne ≈ 1013 cm-3,
at the emission measure peak temperature of 8 × 106
K remains difficult to explain satisfactorily.
Title: Alfvénic Turbulence in the Extended Solar Corona: Kinetic
Effects and Proton Heating
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2003ApJ...594..573C
Altcode: 2003astro.ph..5134C
We present a model of magnetohydrodynamic (MHD) turbulence in the
extended solar corona that contains the effects of collisionless
dissipation and anisotropic particle heating. Recent observations have
shown that preferential heating and acceleration of positive ions occur
in the first few solar radii of the high-speed solar wind. Measurements
made by the Ultraviolet Coronagraph Spectrometer aboard SOHO have
revived interest in the idea that ions are energized by the dissipation
of ion cyclotron resonant waves, but such high-frequency (i.e.,
small-wavelength) fluctuations have not been observed. A turbulent
cascade is one possible way of generating small-scale fluctuations
from a preexisting population of low-frequency MHD waves. We
model this cascade as a combination of advection and diffusion in
wavenumber space. The dominant spectral transfer occurs in the direction
perpendicular to the background magnetic field. As expected from earlier
models, this leads to a highly anisotropic fluctuation spectrum with a
rapidly decaying tail in the parallel wavenumber direction. The wave
power that decays to high enough frequencies to become ion cyclotron
resonant depends on the relative strengths of advection and diffusion in
the cascade. For the most realistic values of these parameters, however,
there is insufficient power to heat protons and heavy ions. The dominant
oblique fluctuations (with dispersion properties of kinetic Alfvén
waves) undergo Landau damping, which implies strong parallel electron
heating. We discuss the probable nonlinear evolution of the electron
velocity distributions into parallel beams and discrete phase-space
holes (similar to those seen in the terrestrial magnetosphere), which
can possibly heat protons via stochastic interactions.
Title: Exploration of Stability Regime for Coronal Loops with
Asymmetric Footpoint Heating
Authors: Boyd, J. F.; Weber, M. A.; DeLuca, E. E.; Van Ballegooijen,
A. A.
Bibcode: 2003SPD....34.0405B
Altcode: 2003BAAS...35..811B
We use an iterative code developed by Van Ballegooijen and Hussain
to compute steady state solutions of the hydrodynamic equations
for a one-dimensional coronal loop model with asymmetric footpoint
heating. We vary the loop length and heating scale height to construct
a two-dimensional parameter space similar to that of Aschwanden et
al. (2001). This parameter space contains a boundary that divides a
regime where steady state solutions exist from the regime where there
are no steady state solutions. In this poster we vary the heating scale
height and explore the onset of instability using a state-of-the-art
adaptive mesh code called FLASH to solve the time-dependent hydrodynamic
equations for the one-dimensional coronal loop model with asymmetric
footpoint heating. TRACE is supported by contract NAS5-38099
from NASA to LMATC. The software used in this work was in part
developed by the DOE-supported ASCI/Alliance Center for Astrophysical
Thermonuclear Flashes at the University of Chicago.
Title: Motions of Isolated G-Band Bright Points in the Solar
Photosphere
Authors: Nisenson, P.; van Ballegooijen, A. A.; de Wijn, A. G.;
Sütterlin, P.
Bibcode: 2003ApJ...587..458N
Altcode: 2002astro.ph.12306N
Magnetic elements on the quiet Sun are buffeted by convective flows
that cause lateral motions on timescales of minutes. The magnetic
elements can be observed as bright points (BPs) in the G band at 4305
Å. We present observations of BPs based on a long sequence of G-band
images recorded with the Dutch Open Telescope and postprocessed using
speckle-masking techniques. From these images we measured the proper
motions of isolated BPs and derived the autocorrelation function of
their velocity relative to the solar granulation pattern. The accuracy
of BP position measurements is estimated to be less than 23 km on
the Sun. The rms velocity of BPs (corrected for measurement errors)
is about 0.89 km s-1, and the correlation time of BP motions
is about 60 s. This rms velocity is about 3 times the velocity measured
using cork tracking, almost certainly due to the fact that isolated BPs
move more rapidly than clusters of BPs. We also searched for evidence
of vorticity in the motions of G-band BPs.
Title: Kink and Longitudinal Oscillations in the Magnetic Network
on the Sun: Nonlinear Effects and Mode Transformation
Authors: Hasan, S. S.; Kalkofen, W.; van Ballegooijen, A. A.;
Ulmschneider, P.
Bibcode: 2003ApJ...585.1138H
Altcode:
We examine the propagation of kink and longitudinal waves in the solar
magnetic network. Previously, we investigated the excitation of network
oscillations in vertical magnetic flux tubes through buffeting by
granules and found that footpoint motions of the tubes can generate
sufficient wave energy for chromospheric heating. We assumed that
the kink and longitudinal waves are decoupled and linear. We overcome
these limitations by treating the nonlinear MHD equations for coupled
kink and longitudinal waves in a thin flux tube. For the parameters
we have chosen, the thin tube approximation is valid up to the layers
of formation of the emission features in the H and K lines of Ca II,
at a height of about 1 Mm. By solving the nonlinear, time-dependent MHD
equations we are able to study the onset of wave coupling, which occurs
when the Mach number of the kink waves is of the order of 0.3. We also
investigate the transfer of energy from the kink to the longitudinal
waves, which is important for the dissipation of the wave energy in
shocks. We find that kink waves excited by footpoint motions of a
flux tube generate longitudinal modes by mode coupling. For subsonic
velocities, the amplitude of a longitudinal wave increases as the square
of the amplitude of the transverse wave, and for amplitudes near Mach
number unity, the coupling saturates and becomes linear when the energy
is nearly evenly divided between the two modes.
Title: Physics of Photospheric Magnetic Field (Invited review)
Authors: van Ballegooijen, A. A.; Hasan, S. S.
Bibcode: 2003ASPC..286..155V
Altcode: 2003ctmf.conf..155V
No abstract at ADS
Title: The Advanced Spectroscopic and Coronagraphic Explorer (ASCE)
Mission Concept Study
Authors: Kohl, J.; Howard, R.; Davila, J.; Noci, G.; Gardner, L.;
Socker, D.; Romoli, M.; Strachan, L.; Floyd, L.; Cranmer, S.; Raymond,
J.; van Ballegooijen, A.
Bibcode: 2002AGUFMSH52A0463K
Altcode:
The ASCE Mission is currently in a Phase A feasibility study as a
candidate for the upcoming MIDEX selection. The ASCE science payload
provides next generation spectroscopic and polarimetric instrumentation
aimed at identifying the physical processes governing solar wind
generation and coronal mass ejections (CMEs). During the current phase,
engineering design and analyses have demonstrated the feasibility of
accomplishing the original mission objectives within the MIDEX mission
constraints. The launch is planned for early 2007 and the operations
and analyses are expected to continue for 5 years. ASCE data along with
data analysis software and calibration data will be unrestricted and
available to the scientific community via an automated web site. A
Guest Investigator program is planned with an average of 15 grants
running concurrently during 2008 to 2012. Grants would be awarded in
response to proposals submitted during the first and subsequent years
of the mission.
Title: Evolution of a semicircular flux rope with two ends anchored
in the photosphere
Authors: Lin, J.; van Ballegooijen, A. A.; Forbes, T. G.
Bibcode: 2002JGRA..107.1438L
Altcode:
We investigated a coronal magnetic configuration including a
semicircular flux rope with two ends anchored in the photosphere. The
background field is produced by two source regions on the
photosphere. We study the evolution of this configuration in response
to the gradual change in the background field, which is modeled by
varying either the strength of the source or the distance between
the source regions on the photosphere. Our results indicate that the
evolution due to the change in source strength shows the likelihood of
catastrophic loss of equilibrium, and that the evolution due to the
change in the distance is smooth and does not manifest any tendency
to lose equilibrium. In the former case, the current sheet starts
developing fairly early; it forms even before the evolution reaches
the maximum current state. We notice that the effect of the curvature
of flux rope on the evolution of the system is significant, such that
the equilibrium curve does not form a cusp-catastrophic structure but
a simple fold-catastrophic structure even if the evolution is ideal
MHD and a current sheet attached to the boundary surface occurs. The
curvature strengthens the magnetic compression between the flux rope
and the photosphere and makes the loss of equilibrium easier. However,
the question of how the system behaves after the loss of equilibrium
is still open since the flux rope is not likely to remain semicircular
at the stage of fast evolution.
Title: Modeling the corona of AB Doradus
Authors: Hussain, G. A. J.; van Ballegooijen, A. A.; Jardine, M.;
Collier Cameron, A.
Bibcode: 2002ASPC..277..409H
Altcode: 2002sccx.conf..409H; 2001astro.ph..9430H
We present a technique that combines Zeeman Doppler imaging (ZDI)
principles with a potential field mapping prescription in order
to gain more information about the surface field topology of rapid
rotators. This technique is an improvement on standard ZDI, which can
sometimes suffer from the suppression of one vector component due to the
effects of stellar inclination, poor phase coverage or lack of flux from
dark areas on the surface. Defining a relationship beween the different
vector components allows information from one component to compensate
for reduced information in another. We present simulations demonstrating
the capability of this technique and discuss its prospects.
Title: Proton Heating in the Extended Solar Corona Resulting From
Kinetic Alfven Turbulence
Authors: Cranmer, S. R.; van Ballegooijen, A. A.
Bibcode: 2002AGUFMSH12A0407C
Altcode:
Spectroscopic observations of the solar corona have made it clear that
the ``coronal heating problem'' comprises not only the local deposition
of heat immediately above the transition region, but also extended
heat deposition throughout the (collisionless) acceleration region of
the solar wind. The dissipation of magnetohydrodynamic (MHD) waves
and/or turbulence has been considered as a likely heating mechanism
in the solar wind for several decades. However, it is still not well
understood how MHD fluctuations are generated, how they evolve in
frequency and wavenumber, or how their damping leads to the observed
proton, electron, and ion properties of the fast wind. We present
a model of MHD turbulence that specifically addresses the issue of
kinetic dissipation and particle heating in the collisionless extended
corona. The nonlinear cascade is modeled as a combination of advection
and diffusion in wavenumber space, with the dominant cascade occurring
in the direction perpendicular to the background magnetic field. This
leads to a highly anisotropic fluctuation spectrum (as expected,
based on many earlier simulations and scaling models) with a rapidly
decreasing power-law tail in the parallel wavenumber direction. In
the low-plasma-beta corona, the dominant oblique fluctuations (with
dispersion properties of kinetic Alfven waves) are dissipated by
electron Landau damping, with only a tiny fraction of the energy going
to high-frequency ion cyclotron waves. This implies strong parallel
electron heating and weak proton and ion heating, which is not what is
observed. We discuss the probable nonlinear evolution of the electron
velocity distributions into parallel beams and discrete phase-space
holes (similar to those seen in the terrestrial magnetosphere) which
can possibly heat protons via stochastic interactions.
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: Catastrophic and Noncatastrophic Mechanisms for Coronal
Mass Ejections
Authors: Lin, J.; van Ballegooijen, A. A.
Bibcode: 2002ApJ...576..485L
Altcode:
It has been suggested that coronal mass ejections (CMEs) are triggered
by the loss of equilibrium of a coronal magnetic field configuration
containing a twisted flux rope. We propose that there are two types of
CMEs: fast CMEs that are triggered by a catastrophic loss of equilibrium
and slow CMEs that do not involve a true catastrophe, but nevertheless
show rapid evolution of the system. As an extension of the 1991 work
by Forbes and Isenberg, we investigated the evolution of a magnetic
configuration taking into account deviations from ideal MHD. We find
that the non ideal-MHD evolution makes it easier for the catastrophic
loss of equilibrium to occur, and the catastrophic behavior of the
system is no longer constrained by the radius of the flux rope. For
ideal-MHD evolution, we find that noncatastrophic solutions can account
for slow CMEs. We also discuss the conditions under which the ideal-MHD
approximation holds.
Title: The Coronal Topology of the Rapidly Rotating K0 Dwarf AB
Doradus. I. Using Surface Magnetic Field Maps to Model the Structure
of the Stellar Corona
Authors: Hussain, G. A. J.; van Ballegooijen, A. A.; Jardine, M.;
Collier Cameron, A.
Bibcode: 2002ApJ...575.1078H
Altcode: 2002astro.ph..7452H
We reanalyze spectropolarimetric data of AB Dor taken in 1996 December
using a surface imaging code that can model the magnetic field of the
star as a nonpotential current-carrying magnetic field. We find that a
nonpotential field needs to be introduced in order to fit the data set
at this epoch. This nonpotential component takes the form of a strong
unidirectional azimuthal field of a similar strength to the radial
field. This azimuthal field is concentrated around the boundary of
the dark polar spot recovered at the surface of the star using Doppler
imaging. As polarization signatures from the center of starspots are
suppressed, it is unclear whether or not this nonpotential component
genuinely represents electric current at the unspotted surface or
whether it results from the preferred detection of horizontal field
in starspot penumbrae. This model contains 20% more energy than the
corresponding potential field model at the surface. This amount of free
energy drops to under 1% about 1R* above the photosphere. We
use these surface maps to model the coronal structure of the star. The
mixed radial polarities at the pole in the surface maps support closed
coronal loops in the high-latitude regions, indicating that a component
of the X-ray emission may originate in this area. Assuming that the
field remains closed out to 5R*, we find stable surfaces
where prominences may form out to the observed distances using this
coronal model.
Title: Halo CMEs in the Ultraviolet
Authors: Raymond, J. C.; Ciaravella, A.; van Ballegooijen, A. A.
Bibcode: 2002AAS...200.3707R
Altcode: 2002BAAS...34R.695R
Halo CMEs are especially important for connecting remote sensing
observations with in situ measurements, as well as for Space
Weather. However, analysis of coronagraphic observations of halo CMEs
is complicated by relatively severe projection effects. We discuss
the use of Doppler shifts to constrain CME structure and evolution
along the line-of-sight, and we discuss the excitation of UV emission
lines at high velocities and large heights above the Sun. This work
was supported by NASA Grant NAG5-11420.
Title: A New Code for Simulating Dynamic Coronal Loops
Authors: Boyd, J. F.; DeLuca, E. E.; van Ballegooijen, A. A.; Arber,
T. D.
Bibcode: 2002AAS...200.0211B
Altcode: 2002BAAS...34..641B
Recent observations with TRACE suggest that apparently steady
coronal loops are inconsistent with hydrostatic loop models. We
present results from a new hydrodynamic code that will be used to
model recent observations from TRACE, SOHO and HESSI. In this poster
we describe the code and show detail comparisons between the results
from this code and a range of loop models. The validation of the code
is an ongoing process, but the simulations we have run to-date suggest
that it will be extremely useful for the testing of detailed heating
models by comparison of forward models with detailed multi-wavelength
observations. Other posters at this meeting will present applications
of this code to active region loops and flares. TRACE is supported by
contract NAS5-38099 from NASA to LMATC.
Title: Analytical Investigation of 3D Model for Coronal Mass Ejections
Authors: Lin, J.; van Ballegooijen, A. A.
Bibcode: 2002AAS...200.2906L
Altcode: 2002BAAS...34..682L
We investigate a coronal magnetic configuration including
a semi-circular flux rope with two ends anchored in the
photosphere. The background field is produced by two source regions
on the photosphere. We study the evolution of this configuration
in response to the gradual change in the background field, which is
modeled by varying either the strength of the source or the length
scale of the source region. Our results indicate that the evolution due
to the change in source strength shows the likelihood of catastrophic
loss of equilibrium, and that the evolution due to the change in the
length scale is smooth and does not manifest any intention to lose
equilibrium. In the former case, the current sheet starts developing
fairly early, it forms even before the evolution reaches the maximum
current state. We notice that the effect of the curvature of flux
rope on the evolution of the system is significant, such that the
equilibrium curve does not form a cusp-catastrophic structure but a
simple fold-catastrophic structure even if the evolution is ideal MHD
and a current sheet attached to the boundary surface develops. The
curvature strengthens the magnetic compression between the flux rope
and the photosphere, and makes the loss of equilibrium easier. But the
question of how the system behaves after loss of equilibrium is still
open since the flux rope may not be able to remain semi-circular at
the stage of fast evolution.
Title: Effects Of Turbulent Pumping In Flux-transport Dynamos
Authors: Dikpati, M.; van Ballegooijen, A. A.
Bibcode: 2002AAS...200.8907D
Altcode: 2002BAAS...34..792D
Motivated by recent findings that the interaction of a magnetic sheet
with stratified convection induces a strong tendency for pumping the
magnetic fields downward (Dorch & Nordlund 2001, A&A, 365, 562),
we explore the implications of such downward transport of magnetic field
using a kinematic, flux-transport dynamo. We formulate the pumping
effect by prescribing it as a flow field in addition to differential
rotation and meridional circulation in such a way as to conserve the
mass. The primary aim of such study is to investigate whether in the
presence of such downward pumping the Babcock-Leighton flux-transport
dynamo can withstand the parity selection issue. It has recently been
shown that the formation of an elongated equatorial dipole is necessary
for coupling N&S hemispheres through the antisymmetric magnetic
field about the equator (Dikpati & Gilman 2001, ApJ, 559, 428), as
inferred from Hale's polarity observation. Poloidal fields generated
in a Babcock-Leighton model fail to do so because they undergo large
decay during their long traversal to reach the equator at the shear
layer. Present study indicates that the inclusion of a strong downward
pumping ( ~ 10 m/s) in a Babcock-Leighton flux-transport dynamo can
indeed provide additional downward speed for the transport of flux
for forming the extended dipole at the base of the convection zone
and hence, regain the solar-like mode-symmetry. This work has been
supported by NASA grants W-19752 and S-10145-X.
Title: Heating AB Dor's corona: discriminating between different
loop models
Authors: Hussain, G. A. J.; van Ballegooijen, A. A.
Bibcode: 2002AAS...200.6104H
Altcode: 2002BAAS...34..744H
We have developed a code that allows us to reconstruct realistic
3-D coronal magnetic field models for single, rapidly rotating
ZAMS stars. These models are produced using an advanced version
of the magnetic field mapping technique called Zeeman Doppler
imaging. We compute temperatures and densities using the 3-D model
obtained for the active K0V, AB Doradus (Prot=0.5d,
Lx}/L{bol=10-3), by solving hydrostatic
equilibrium and energy balance equations along field lines. Field
lines where the gas pressure exceeds the magnetic pressure are
assumed to be open. These models can therefore be used to evaluate
the filling factor of the corona. Three different types of loop models
that can explain the observed EUVE emission measure distribution for
AB Doradus are analysed. These include: (a) very dense, compact loops
(typical heights, H=0.0004R*); (b) loops with heights between
0.1-0.4R* affected by continuum absorption in the EUV; and
(c) longer loops (H=0.7R*) with expanding cross-sectional
areas. At present none of these models reproduce EUVE, HST and Chandra
observations of AB Dor fully. We discuss what is required from future
EUV observations in order to place more constraints on these models.
Title: High-Resolution Imaging of the Solar Photosphere Using
Simultaneous G-Band and Continuum Observations
Authors: van Ballegooijen, A. A.; Nisenson, P.
Bibcode: 2002AAS...200.3801V
Altcode: 2002BAAS...34..698V
Imaging of the Sun with ground-based telescopes requires accurate
correction for the effects of turbulence in the Earth's atmosphere
(``seeing"). An observed solar image is a convolution of a true
solar image with a point spread function (PSF) describing the seeing
effects. We present a new method for image reconstruction that uses
pairs of images taken at two nearby wavelengths (G-band, 430.5 nm,
and nearby continuum, 450.8 nm). Each image pair is taken strictly
simultaneously, so that the two images are affected by the same
PSF. A burst of 21 such image pairs is taken within a few seconds;
the sun does not change much during this time, but the seeing varies
randomly from one pair to the next. We determine the true solar images
(and 21 PSFs) by iterative deconvolution. The method is applied to
measurements obtained at the Dunn Solar Telescope (NSO/Sacramento
Peak) on November 2, 2001. Preliminary results from the analysis of
these data are presented. This work is supported by a grant from the
National Science Foundation.
Title: Steady Flows Detected in Extreme-Ultraviolet Loops
Authors: Winebarger, Amy R.; Warren, Harry; van Ballegooijen, Adriaan;
DeLuca, Edward E.; Golub, Leon
Bibcode: 2002ApJ...567L..89W
Altcode:
Recent Transition Region and Coronal Explorer (TRACE) observations have
detected a class of active region loops whose physical properties are
inconsistent with previous hydrostatic loop models. In this Letter we
present the first co-aligned TRACE and the Solar Ultraviolet Measurement
of Emitted Radiation (SUMER) observations of these loops. Although these
loops appear static in the TRACE images, SUMER detects line-of-sight
flows along the loops of up to 40 km s-1. The presence
of flows could imply an asymmetric heating function; such a heating
function would be expected for heating that is proportional to
(often asymmetric) footpoint field strength. We compare a steady flow
solution resulting from an asymmetric heating function to a static
solution resulting from a uniform heating function in a hypothetical
coronal loop. We find that the characteristics associated with the
asymmetrically heated loop better compare with the characteristics of
the loops observed in the TRACE data.
Title: a Semiempirical Model for the Large-Scale Minimum Activity
Solar Corona
Authors: Vásquez, A. M.; van Ballegooijen, A. A.; Raymond, J. C.
Bibcode: 2002RMxAC..14..108V
Altcode:
No abstract at ADS
Title: The Advanced Spectroscopic and Coronagraphic Explorer Mission
Authors: Kohl, J. L.; Howard, R.; Davila, J.; Noci, G.; Esser, R.;
Ciaravella, A.; Cranmer, S.; Fineschi, S.; Gardner, L.; Raymond, J.;
Romoli, M.; Smith, P.; Socker, D.; Strachan, L.; Van Ballegooijen, A.
Bibcode: 2001AGUFMSH31B0711K
Altcode:
SOHO has provided profound insights into the physics of solar wind
acceleration and coronal mass ejections. Although significant
progress has been made, most of the dominant physical processes
controlling these phenomena are still not identified. The Advanced
Spectroscopic and Coronagraphic Explorer Mission provides next
generation spectroscopic and polarimetric instrumentation aimed at
identifying these processes. The launch is planned for March 2007 with
mission operations and data analysis continuing for 5 years. The data
will be unrestricted and available to the community. The envisioned
program includes a Guest Investigator Program with an average of 15
grants to be awarded in response to proposals submitted during the
first year of the mission. Information about the proposed scientific
goals and instrumentation will be presented.
Title: Magnetic Interface in Coronal Magnetic Field and Solar
Activities
Authors: Lin, J.; Van Ballegooijen, A. A.
Bibcode: 2001AGUFMSH12B0754L
Altcode:
Theoretical and observational works (Lin and Forbes, 2000; Forbes and
Lin, 2000; Akmal et al., 2001) indicate the existence of an extensive
current sheet below the plasmoid ejected by a major eruption, such as
two-ribbon flare, eruptive prominence, as well as coronal mass ejection
(CME). This current sheet, together with the field lines connected to
it which is known as separatrix, obviously distinguishes two magnetic
flux systems that have different topological connections. Extending
to three-dimensional configurations, they become separatrix surface
(Démoulin et al., 1993) and quasi-separatrix layers (Priest and
Démoulin, 1995). These terms imply that activities or eruptons occur
around the surface or the layer. On the other hand, both theories and
observations also show that it is not necessary for all of the surfaces
or layers which separate two different topological connection magnetic
fluxes to be the precursors of eruptions although the likelyhood of
eruptions can be very high. To generalize the concept, Wang (1998)
and Wang (1999) suggested to use the term of "magnetic interface"
to describe such a surface or layer in the coronal that separates two
or more magnetic flux systems which do not topologically connect. The
electric conductivity of the coronal plasma is fairly high although
it is not infinitely high, the dissipation of the magnetic field
is generally very slow in the region without neutral point, so the
magnetic fluxes with different topological connections can survive
for a quite while. This is the base for the concept of the magnetic
interface. However, the coronal magnetic field evolves in response
to the change in the photospheric field because the coronal field is
line-tied to the photosphere. This implies that the magnetic interface,
if exists, has to evolves simultaneously. In the present work, we
are going to specify the concept clearly on the base of current CME
models. We will show how an 3D interface evolves to a current sheet
and separatrix surface, and will also show how an interface survives
during evolution without evovling to the current sheet or separatrix. We
found that the introduction of the magnetic interface is quite useful
for identifying prospective active regions and for forecasting the
energetic activities.
Title: A Possible Solar Cycle Dependence to the Hemispheric Pattern
of Filament Magnetic Fields?
Authors: Mackay, D. H.; van Ballegooijen, A. A.
Bibcode: 2001ApJ...560..445M
Altcode:
The origin of the observed hemispheric pattern of filament magnetic
fields is considered. Using a magnetic flux transport model, we simulate
the interactions of magnetic bipoles with each other and with polar
magnetic fields in the rising and declining phases of the solar activity
cycle. In contrast to previous studies, the nonpotential character of
the initial coronal fields is taken into account, and the dependence
of the hemispheric pattern on the initial tilt and helicity of the
bipoles is considered. For the rising phase of the cycle, a range
of initial bipole twists and tilt angles can be found that reproduce
the observed hemispheric pattern. However, for the declining phase no
such range can be found: the predicted fields on the return arms at
the rear of switchbacks are consistent with filament observations,
but those on the high-latitude east-west arms are not. It is argued
that existing observations of the hemispheric pattern are weighted
toward the rising phase of the solar activity cycle and may give us a
biased view of the Sun. New observations of filament magnetic fields
are needed to determine whether there is a cycle dependence of the
observed hemispheric pattern.
Title: Analysis and Modeling of the UV Spectra of the Feb. 12,
2000 Flux Rope CME.
Authors: Ciaravella, A.; Raymond, J.; van ballegooijen, A.
Bibcode: 2001AGUSM..SH51B01C
Altcode:
UltraViolet spectra of a typical flux rope CME have been taken with
the UltraViolet Coronagraph Spectrometer aboard SOHO above the NW
limb at heliocentric distance of 2.3 Rsun. The high temporal and
spectral resolutions of the spectra provide a detailed monitoring of
the dynamical and physical properties of the CME. Beside the cool
lines of H I (1216 and 1025 A) and the intermediate lines of O VI
(1032, 1037 A) usually observed in CME this event shows emission in
the hot lines of SI XII (520 A) and Mg X (610 A). Doppler shift of the
observed spectral lines give the line of sight velocity of the bright
front and the prominence core plasma. The latter showing evidence for
helical motion. A model of the Doppler shift evolution in both front
and core of the CME is presented.
Title: Intensity Fluctuations in the Solar Chromosphere
Authors: Kalkofen, W.; Warren, H. P.; Winebarger, A. R.; van
Ballegooijen, A.; Avrett, E. H.
Bibcode: 2001AGUSM..SP41C05K
Altcode:
We examine two very different empirical models of the solar
chromosphere. The first model implies steady heating, is hot at the
top of the chromosphere and has temperature fluctuations of relatively
low amplitude; the second model is heated intermittently, is cold most
of the time and undergoes large temperature variations. Estimates of
intensity fluctuations of chromospheric radiation are very different
for the two models. We compare the model predictions with observations
made by Skylab and by SUMER.
Title: Workshop Summary
Authors: van Ballegooijen, A. A.
Bibcode: 2001ASPC..236....1V
Altcode: 2001aspt.conf....1V
No abstract at ADS
Title: Three-Dimensional Modeling of Coronal Magnetic Fields
Authors: van Ballegooijen, A. A.
Bibcode: 2001ASPC..236..235V
Altcode: 2001aspt.conf..235V
No abstract at ADS
Title: Theory of Solar Chromospheric and Coronal Magnetic Fields
Authors: van Ballegooijen, A. A.; Mackay, D. H.
Bibcode: 2001ASPC..248..105V
Altcode: 2001mfah.conf..105V
No abstract at ADS
Title: Modelling the Coronal Topology of Late-Type Stars
Authors: Hussain, G. A. J.; van Ballegooijen, A. A.; Jardine, M.
Bibcode: 2001ASPC..248..263H
Altcode: 2001mfah.conf..263H
No abstract at ADS
Title: Comparison of Theory and Observations of the Chirality of
Filaments within a Dispersing Activity Complex
Authors: Mackay, D. H.; Gaizauskas, V.; van Ballegooijen, A. A.
Bibcode: 2000ApJ...544.1122M
Altcode:
We investigate the origin of the hemispheric pattern of filaments and
filament channels by comparing theoretical predictions with observations
of the chirality of filament channels within a dispersing activity
complex. Our aim is to determine how the chirality of each specific
channel arises so that general principles underlying the hemispheric
pattern can be recognized. We simulate the field lines representing
the filaments in the activity complex by applying a model of global
flux transport to an initial magnetic configuration. The model
combines the surface effects of differential rotation, meridional
flows, and supergranular diffusion along with a magnetofrictional
relaxation method in the overlying corona. The simulations are run
with and without injecting axial magnetic fields at polarity inversion
lines in the dispersing activity complex for four successive solar
rotations. When the initial magnetic configuration, based on synoptic
magnetic maps, is set to a potential field at the beginning of each
rotation, the simulations poorly predict the chirality of the filament
channels and filaments. The cases that predict the correct chirality
correspond to an initial polarity inversion line, which is north-south
the wrong chirality arises when the initial polarity inversion lines
lie east-west. Results improve when field-line connectivities at low
latitudes are retained and allowed to propagate to higher latitudes
without resetting the field to a potential configuration between
each rotation. When axial flux emergence exceeding 1×1019
Mx day-1 is included at the location of each filament, an
excellent agreement is obtained between the theory and observations. In
additon to predicting the correct chirality in all cases, axial flux
emergence allows more readily the production of inverse-polarity
dipped field lines needed to support filamentary mass. An origin
for the hemispheric pattern as a result of the combined effects of
flux transport, axial flux emergence, and magnetic helicity is then
discussed.
Title: Mean Field Model for the Formation of Filament Channels on
the Sun
Authors: van Ballegooijen, A. A.; Priest, E. R.; Mackay, D. H.
Bibcode: 2000ApJ...539..983V
Altcode:
The coronal magnetic field is subject to random footpoint motions that
cause small-scale twisting and braiding of field lines. We present a
mean field theory describing the effects of such small-scale twists on
the large-scale coronal field. This theory assumes that the coronal
field is force free, with electric currents flowing parallel or
antiparallel to magnetic field lines. Random footpoint motions are
described in terms of diffusion of the mean magnetic field at the
photosphere. The appropriate mean field equations are derived, and
a numerical method for solving these equations in three dimensions
is presented. Preliminary results obtained with this method are
also presented. In particular the formation of filament channels is
studied. Filament channels are regions where the coronal magnetic field
is strongly aligned with the underlying polarity inversion line in
the photosphere. It is found that magnetic flux cancellation plays an
important role in the formation of such channels. Various models of the
coronal field are presented, including some in which the axial field is
assumed to originate from below the photosphere. The models reproduce
many of the observed features of filament channels, but the observed
hemisphere pattern of dextral and sinistral channels remains a mystery.
Title: Excitation of Oscillations in the Magnetic Network on the Sun
Authors: Hasan, S. S.; Kalkofen, W.; van Ballegooijen, A. A.
Bibcode: 2000ApJ...535L..67H
Altcode: 2000astro.ph..4246H
We examine the excitation of oscillations in the magnetic network of
the Sun through the footpoint motion of photospheric magnetic flux
tubes located in intergranular lanes. The motion is derived from a
time series of high-resolution G-band and continuum filtergrams using
an object-tracking technique. We model the response of the flux tube
to the footpoint motion in terms of the Klein-Gordon equation, which
is solved analytically as an initial value problem for transverse
(kink) waves. We compute the wave energy flux in upward-propagating
transverse waves. In general we find that the injection of energy into
the chromosphere occurs in short-duration pulses, which would lead
to a time variability in chromospheric emission that is incompatible
with observations. Therefore, we consider the effects of turbulent
convective flows on flux tubes in intergranular lanes. The turbulent
flows are simulated by adding high-frequency motions (periods 5-50 s)
with an amplitude of 1 km s-1. The latter are simulated by
adding random velocity fluctuations to the observationally determined
velocities. In this case, we find that the energy flux is much
less intermittent and can in principle carry adequate energy for
chromospheric heating.
Title: New Insights on CMEs from Spectroscopic observations of
UltraViolet Coronagraph Spectrometer
Authors: Ciaravella, Angela; Raymond, J. C.; van Ballegooijen, A.;
St. Cyr, O. C.; Plunkett, S. P.; UVCS Mission Operations Team
Bibcode: 2000SPD....31.0277C
Altcode: 2000BAAS...32..824C
Since the SOHO launch spectroscopic observations of Coronal Mass
Ejections have been performed with the Ultraviolet Coronagraph
Spectrometer (UVCS). The data provide new insights on the physical and
dynamical conditions of the ejected material which can in turn be used
to put constraints on the theoretical models. Detailed diagnostics
of temperatures, densities, abundances have been obtained for some
events observed by UVCS. Energetics, heating and evolution during the
propagation through the corona have been derived as well. The component
along the line of sight can be measures from the Doppler shift of the
spectral lines which together with white light observations can be
used to derive a three dimensional picture of the event. We present
the results of the spectroscopic analysis of a CME observed with UVCS
and a comparison with LASCO white light observations.
Title: Solar and Heliospheric Observatory Observations of a Helical
Coronal Mass Ejection
Authors: Ciaravella, A.; Raymond, J. C.; Thompson, B. J.; van
Ballegooijen, A.; Strachan, L.; Li, J.; Gardner, L.; O'Neal, R.;
Antonucci, E.; Kohl, J.; Noci, G.
Bibcode: 2000ApJ...529..575C
Altcode:
The EUV Imaging Telescope (EIT), Large Angle Spectrometric Coronagraph
(LASCO), and Ultraviolet Coronagraph Spectrometer (UVCS) instruments
aboard the SOHO satellite observed a prominence eruption (coronal mass
ejection) on 1997 December 12. Ejected plasma moved at about 130 km
s-1 in the plane of the sky and showed Doppler shifts between
-350 and +30 km s-1. The eruption appeared as a strongly
curved arch in EIT images low in the corona. Emission in ions ranging
from Si III to O VI in the UVCS spectra indicates a temperature range
between 104.5 and 105.5 K. The morphology of the
bright emission regions seen by all three instruments suggests several
strands of a helical structure of moderate pitch angle. A reasonable
fit to the spatial structure and the velocity evolution measured by
UVCS is provided by a left-handed helix untwisting at a rate of about
9×10-4 radians s-1.
Title: On the Comparison of Filament Chirality and Axial Magnetic
Fields Deduced from a Flux Transport Model
Authors: Mackay, D. H.; Gaizauskas, V.; van Ballegooijen, A. A.
Bibcode: 1999ESASP.448..507M
Altcode: 1999mfsp.conf..507M; 1999ESPM....9..507M
No abstract at ADS
Title: Model of solar wind flow near an equatorial coronal streamer
Authors: Vásquez, A. M.; van Ballegooijen, A. A.; Raymond, J. C.
Bibcode: 1999AIPC..471..243V
Altcode: 1999sowi.conf..243V
In a previous work (1) we developed a semiempirical axisymmetric and
magnetostatic model of the minimum activity corona, accounting for
the gas pressure gradient effects on the magnetic structure. The model
is able to reproduce the streamer belt closed region and the streamer
surrounding open field lines, predicting high plasma β values (>1)
in the closed region, low values in the surrounding open-field regions
(streamer legs) and even lower values for the polar hole region. In this
work we add to that model a solar wind solution under the assumption
of mass and momentum flux conservation. For open field lines arising
from the polar hole region we find a fast wind solution that reaches
values of about 600 km/sec at 10 Rsolar. For open field
lines arising from the surroundings of the closed region (streamer
legs), the morphology of the field lines is fast diverging, we find
that this results in the existence of two posible sonic points. The
first sonic point, located below the streamer cusp, gives a relatively
fast solar wind solution, reaching values of about 400 km/sec at 10
Rsolar. The second sonic point, located above the streamer
cusp, gives a slow solution, reaching values of up to 200 km/sec for
lines close to the streamer core, becoming supersonic only well above
the streamer cusp.
Title: SOHO Observations of a Helical Coronal Mass Ejection
Authors: Raymond, J. C.; Ciaravella, A.; van Ballegooijen, A.;
Thompson, B.
Bibcode: 1999AAS...194.1701R
Altcode: 1999BAAS...31..853R
The EIT, LASCO and UVCS instruments aboard SOHO observed a CME on
Dec. 11/12 1997. The ejected prominence material rose relatively
slowly, averaging 150 km/s, but Doppler shifts as great at -300 km/s
were seen in the O VI lines. The observed gas spans the temperature
range from about 30,000 K to 300,000 K. The observations can
be modeled as left-handed helix which untwists at about 0.0009
radians/sec. Considerable heating as the plasma moves from the
solar surface to 1.7 solar radii is required to explain the observed
temperature range.
Title: TRACE and SVST Observations of an Active-Region Filament
Authors: van Ballegooijen, A. A.; Deluca, E. E.
Bibcode: 1999AAS...194.7806V
Altcode: 1999BAAS...31..962V
In June 1998 the Transition Region and Coronal Explorer (TRACE) observed
filaments and prominences in coordination with various ground-based
solar observatories, including the Swedish Vacuum Solar Telescope (SVST)
on La Palma. Here we present results for an active-region filament
observed on June 21-22. This horse-shoe shaped filament had a "barb"
that reached down from the filament spine to the chomosphere below. We
use high-resolution images obtained at the SVST on June 21 from 18:03
to 19:04 UT to study the fine structure and dynamics of plasmas in the
barb and other parts of the filament. The data consist of narrowband
Hα images taken with the Lockheed Tunable Filtergraph operating at a
cadence of 20 s. We present Doppler maps derived from these images. The
filament erupted six hours after the SVST observations. The eruption
was observed with TRACE, which obtained images in Fe IX/X 171, Fe XII
195, Fe XV 284 and H I Lyalpha . At the start of the event, a thin
bright loop appears high above the filament at the location of the
barb. We interpret this feature as the outline of a magnetic "bubble"
which forms as a result of kink instability in the magnetic field that
supports the filament. The bright loop appears to be due to particle
acceleration and impulsive heating along certain field lines on the
periphery of this magnetic structure. A few minutes later, the dark
filament threads turn into emission and move outward, exhibiting a
helical structure. We discuss the magnetic structure of the barb and
its possible role in the filament eruption.
Title: Dynamics of Magnetic Elements in the Photosphere and the
Formation of Spicules
Authors: van Ballegooijen, A. A.; Nisenson, P.
Bibcode: 1999ASPC..183...30V
Altcode: 1999hrsp.conf...30V
We consider the proper motions of photospheric magnetic elements, and
the effects of these motions on flows at larger heights. We summarize
the results from recent analyses of high resolution G-band data obtained
at the Swedish Vacuum Solar Telescope (SVST) on La Palma. The G-band
images show small bright features which are known to correspond to
kilogauss magnetic fields. We measure the motions of these bright
points and find that the autocorrelation time of the bright point
velocity is about 100 s. From the observed continuum intensity images,
we derive a model of the granulation flow velocity as function of
time and position on the Sun. We use this flow model to simulate the
horizontal motions of photospheric magnetic elements, assuming that
the elements are passively advected by the granulation flow. We find
that this passive advection model is in reasonable agreement with the
observed spatial distribution of the G-band bright points. Finally, we
use potential-field modeling to extrapolate the magnetic and velocity
fields from the photosphere, where the flux tubes are well separated,
to a height of 1500 km in the chromosphere, where the flux tubes fill
the available volume due to the spreading out of the field lines. We
find that strong shear flows occur near separatrix surfaces where
neighboring flux tubes slide past each other in the chromosphere. We
propose that spicules are formed in these separatrix layers.
Title: UVCS Observations and Modeling of Streamers
Authors: Vásquez, A. M.; Raymond, J. C.; van Ballegooijen, A. A.
Bibcode: 1999SSRv...87..335V
Altcode:
We present results derived from the analysis of an equatorial streamer
structure as observed by the UVCS instrument aboard SOHO. From
observations of the H I Lyα and Lyβ lines we infer the density and
temperature of the plasma. We develop a preliminary axisymmetric,
magnetostatic model of the corona which includes the effects of gas
pressure gradients on the magnetic structure. We infer a coronal
plasma β > 1 in the closed field regions and near the cusp of
the streamer. We add to the model a parallel velocity field assuming
mass flux conservation along magnetic flux tubes. We then compute the
Lyα emissivity and the line-of-sight integrals to obtain images of
Lyα intensity, taking into account projection effects and Doppler
dimming. The images we obtain from this preliminary model are in good
general agreement with the UVCS observations, both qualitatively and
quantitatively.
Title: Photospheric Motions as a Source of Twist in Coronal Magnetic
Fields
Authors: van Ballegooijen, A. A.
Bibcode: 1999GMS...111..213V
Altcode:
The interaction of magnetic fields with granulation and supergranulation
flows causes the photospheric magnetic flux of an active region to be
dispersed over the solar surface on a timescale of days to months. This
photospheric ``diffusion'' process leads to magnetic flux cancellation
at the polarity inversion line separating the leading and following
polarity parts of the region. I present a magnetohydrodynamic (MHD)
model which takes into account the effects of the solar differential
rotation, photospheric diffusion, and flux cancellation on the
three-dimensional structure of the active-region magnetic field. The
model assumes that the coronal magnetic field evolves through a
series of force free equilibrium states. It is shown that magnetic
reconnection associated with flux cancellation causes the formation
of a helical flux rope overlying the polarity inversion line. For
initially twisted bipoles, the diffusion of flux to the polarity
inversion line produces an S-shaped right-helical flux rope or an
inverse-S-shaped left-helical flux rope, depending on the sign of
the initial twist. The shapes of the simulated flux ropes are similar
to the observed coronal X-ray structures. For an initially untwisted
bipole, the model predicts the formation of a left-helical flux rope
in the North or a right-helical flux rope in the South, in agreement
with the observed weak correlation between active-region helicity and
latitude. This suggests that the observed correlation may be due to
the effects of the solar differential rotation acting on active-region
magnetic fields after they emerge through the photosphere.
Title: Dynamics of Magnetic Flux Elements in the Solar Photosphere
Authors: van Ballegooijen, A. A.; Nisenson, P.; Noyes, R. W.; Löfdahl,
M. G.; Stein, R. F.; Nordlund, Å.; Krishnakumar, V.
Bibcode: 1998ApJ...509..435V
Altcode: 1998astro.ph..2359V
The interaction of magnetic fields and convection is investigated in
the context of the coronal heating problem. We study the motions of
photospheric magnetic elements using a time series of high-resolution
G-band and continuum filtergrams obtained at the Swedish Vacuum
Solar Telescope at La Palma. The G-band images show bright points
arranged in linear structures (``filigree'') located in the lanes
between neighboring granule cells. We measure the motions of these
bright points using an object tracking technique, and we determine
the autocorrelation function describing the temporal variation of
the bright point velocity. The correlation time of the velocity is
about 100 s. To understand the processes that determine the spatial
distribution of the bright points, we perform simulations of horizontal
motions of magnetic flux elements in response to solar granulation
flows. Models of the granulation flow are derived from the observed
granulation intensity images using a simple two-dimensional model
that includes both inertia and horizontal temperature gradients; the
magnetic flux elements are assumed to be passively advected by this
granulation flow. The results suggest that this passive advection model
is in reasonable agreement with the observations, indicating that on
a timescale of 1 hr the flux tubes are not strongly affected by their
anchoring at large depth. Finally, we use potential-field modeling
to extrapolate the magnetic and velocity fields to larger height. We
find that the velocity in the chromosphere can be locally enhanced at
the separatrix surfaces between neighboring flux tubes. The predicted
velocities are several km s-1, significantly larger than
those of the photospheric flux tubes. The implications of these results
for coronal heating are discussed.
Title: Magnetic Flux Transport and the Formation of Filament Channels
on the Sun
Authors: van Ballegooijen, A. A.; Cartledge, N. P.; Priest, E. R.
Bibcode: 1998ApJ...501..866V
Altcode:
Observations of filaments and filament channels on the Sun indicate
that the magnetic fields in these structures exhibit a large-scale
organization: filament channels in the northern hemisphere predominantly
have axial fields directed to the right when viewed from the positive
polarity side of the channel (dextral orientation), while those in the
south have axial fields directed to the left (sinistral orientation). In
this paper we attempt to explain this pattern in terms of the most
natural mechanism, namely, solar differential rotation acting on
already emerged magnetic fields. We develop a model of global magnetic
flux transport that includes the effects of differential rotation,
meridional flow, and magnetic diffusion on photospheric and coronal
fields. The model is applied to National Solar Observatory/Kitt
Peak data1 on the photospheric magnetic flux distribution. We also
present results from a simulation of solar activity over a period
of two solar cycles, which gives a buildup of flux at the poles of a
magnitude, in agreement with observations. We find that differential
rotation acting on initially north-south oriented polarity inversion
lines (PILs) does produce axial fields consistent with the observed
hemispheric pattern. The fields associated with switchbacks in the
PILs are predicted to have a definite orientation: the high-latitude
``lead'' arms of the switchbacks are preferentially sinistral (dextral)
in the north (south), while the lower latitude ``return'' arms are, in
agreement with observations, preferentially dextral (sinistral). The
predicted orientation of fields at the polar crown, however, appear
to be in conflict with observations. Further observational studies
are needed to determine whether the model can explain the observed
hemispheric pattern.
Title: Role of Helicity in the Formation of Intermediate Filaments
Authors: Mackay, D. H.; Priest, E. R.; Gaizauskas, V.; van
Ballegooijen, A. A.
Bibcode: 1998SoPh..180..299M
Altcode:
In the last few years new observations have shown that solar filaments
and filament channels have a surprising hemispheric pattern. To explain
this pattern, a new theory for filament channel and filament formation
is put forward. The theory describes the formation of a specific type of
filament, namely the `intermediate filament' which forms either between
active regions or at the boundary of an active region. It describes the
formation in terms of the emergence of a sheared activity complex. The
complex then interacts with remnant flux and, after convergence and
flux cancellation, the filament forms in the channel. A key feature
of the model is the net magnetic helicity of the complex. With the
correct sign a filament channel can form, but with the opposite sign
no filament channel forms after convergence. It is shown how the
hemispheric pattern of helicity in emerging flux regions produces the
observed hemispheric pattern for filaments.
Title: Study of Magnetic Structure in the Solar Photosphere and
Chromosphere
Authors: Noyes, Robert W.; Avrett, Eugene; Nisenson, Peter; Uitenbroek,
Han; van Ballegooijen, Adriaan
Bibcode: 1998nasa.reptV....N
Altcode:
This grant funded an observational and theoretical program to study the
structure and dynamics of the solar photosphere and low chromosphere,
and the spectral signatures that result. The overall goal is to learn
about mechanisms that cause heating of the overlying atmosphere, and
produce variability of solar emission in spectral regions important
for astrophysics and space physics. The program exploited two new
ground-based observational capabilities: one using the Swedish Solar
Telescope on La Palma for very high angular resolution observations
of the photospheric intensity field (granulation) and proxies of the
magnetic field (G-band images); and the other using the Near Infrared
Magnetograph at the McMath-Pierce Solar Facility to map the spatial
variation and dynamic behavior of the solar temperature minimum
region using infrared CO lines. We have interpreted these data using
a variety of theoretical and modelling approaches, some developed
especially for this project. Previous annual reports cover the work
done up to 31 May 1997. This final report summarizes our work for the
entire period, including the period of no-cost extension from 1 June
1997 through September 30 1997. In Section 2 we discuss observations
and modelling of the photospheric flowfields and their consequences
for heating of the overlying atmosphere, and in Section 3 we discuss
imaging spectroscopy of the CO lines at 4.67 mu.
Title: Dynamics of Magnetic Flux Elements in the Solar Photosphere
Authors: van Ballegooijen, A. A.
Bibcode: 1998EOSTr..79..282V
Altcode:
The magnetic field in the solar photosphere interacts with convective
flows on a variety of length and time scales. On the scale of the solar
granulation the magnetic field is pushed into the intergranular lanes,
producing magnetic flux concentrations with kilogauss field strength. In
this talk I present results from a recent study (van Ballegooijen et al
1998) of the motions of these magnetic elements and their interaction
with the granulation flow. The study is based on a time series of high
resolution G-band (4305 Angstrom) and continuum (4686 Angstrom) images
obtained at the Swedish Vacuum Solar Telescope at La Palma, Spain. The
G-band images show bright points which are known to be associated with
the magnetic flux concentrations. We use object tracking techniques to
measure the motions of these bright points over a 70 minute period, and
derive the autocorrelation function of the bright point velocity, which
is found to vary on a time scale of about 100 s. We construct models
of the granulation flow field and simulate the dynamics of magnetic
elements assuming they are passively advected by these flows. Finally,
we use potential field modeling to simulate magnetic and velocity
fields above the observed region (up to a height of 1500 km in the
chromosphere). The results indicate that the spreading of the magnetic
flux tubes with height, and their merging in the chromosphere, produce
local enhancements in velocity near the separatrix surfaces between the
flux tubes in the chromosphere. The chromospheric velocity exceeds 5
km/s, much larger than the velocity of the underlying photospheric flux
tubes. We suggest that such velocity enhancements play an important role
in the generation of MHD waves and the formation of H-alpha spicules.
Title: Why Does the Sun Have Kilogauss Magnetic Fields?
Authors: Hasan, S. S.; van Ballegooijen, A. A.
Bibcode: 1998ASPC..154..630H
Altcode: 1998csss...10..630H
Magnetic fields in the solar photosphere are concentrated in flux
tubes with kilogauss field strength surrounded by nearly field-free
plasma. Observations show that the flux tubes are located in convective
downdrafts where the temperature is lower than average. We assume that
the convective downdrafts extend to large depths in the convection
zone, and that flux tubes follow the downdrafts to these depths. We
develop a model for the magnetic field strength B(z) in the flux
tubes as a function of depth z below the surface. Our calculations
reveal that epsilon, the ratio of magnetic pressure to gas pressure,
has a large depth variation: at the base of the convection zone where
epsilon ~10^{-5} (B ~10^5 G), while at the top epsilon ~1, in broad
agreement with solar observations. Thus the model can explain why the
field strength at the photosphere is around 1 kG.
Title: Magnetic Flux Transport and Formation of Filament Channels
Authors: van Ballegooijen, A. A.; Cartledge, N. P.; Priest, E. R.
Bibcode: 1998ASPC..150..265V
Altcode: 1998npsp.conf..265V; 1998IAUCo.167..265V
No abstract at ADS
Title: Understanding the Solar Cycle
Authors: van Ballegooijen, A. A.
Bibcode: 1998ASPC..140...17V
Altcode: 1998ssp..conf...17V
No abstract at ADS
Title: Absolute Abundances in Streamers from UVCS
Authors: Raymond, J. C.; Suleiman, R. M.; van Ballegooijen, A. A.;
Kohl, J. L.
Bibcode: 1997ESASP.415..383R
Altcode: 1997cpsh.conf..383R
The Ultraviolet Coronagraph Spectrometer on SOHO covers the 940-1350
Angstrom range as well as the 470-630 Angstrom range in second
order. It has detected coronal emission lines of H, N, O, Mg, Al, Si,
S, Ar, Ca, Fe and Ni in coronal streamers. We are able to determine
the ionization state and the absolute elemental abundances in the
gas. There is a strong First Ionization Potential (FIP) dependence in
the abundances. Oxygen is depleted relative to hydrogen, by a full order
of magnitude for oxygen in the center of a quiescent streamer. Here, we
extend the earlier work by considering greater heights in the streamer
and by estimating the projection effects (the line of sight passing
through the streamer edges). We suggest that the FIP fractionation
occurs in the chromosphere, but that in addition gravitational settling
produces an overall depletion at large heights in the closed-field
region. We present numerical results of diffusion models.
Title: Sustaining the Quiet Photospheric Network: The Balance of
Flux Emergence, Fragmentation, Merging, and Cancellation
Authors: Schrijver, Carolus J.; Title, Alan M.; van Ballegooijen,
Adriaan A.; Hagenaar, Hermance J.; Shine, Richard A.
Bibcode: 1997ApJ...487..424S
Altcode:
The magnetic field in the solar photosphere evolves as flux
concentrations fragment in response to sheared flows, merge when they
collide with others of equal polarity, or (partially) cancel against
concentrations of opposite polarity. Newly emerging flux replaces the
canceled flux. We present a quantitative statistical model that is
consistent with the histogram of fluxes contained in concentrations
of magnetic flux in the quiet network for fluxes exceeding ~2 ×
1018 Mx, as well as with estimated collision frequencies
and fragmentation rates. This model holds for any region with weak
gradients in the magnetic flux density at scales of more than a few
supergranules. We discuss the role of this dynamic flux balance (i)
in the dispersal of flux in the photosphere, (ii) in sustaining the
network-like pattern and mixed-polarity character of the network, (iii)
in the formation of unipolar areas covering the polar caps, and (iv) on
the potential formation of large numbers of very small concentrations
by incomplete cancellation. Based on the model, we estimate that as
much flux is cancelled as is present in quiet-network elements with
fluxes exceeding ~2 × 1018 Mx in 1.5 to 3 days, which is
compatible with earlier observational estimates. This timescale is
close to the timescale for flux replacement by emergence in ephemeral
regions, so that this appears to be the most important source of flux
for the quiet-Sun network; based on the model, we cannot put significant
constraints on the amount of flux that is injected on scales that are
substantially smaller than that of the ephemeral regions. We establish
that ephemeral regions originate in the convection zone and are not
merely the result of the reemergence of previously cancelled network
flux. We also point out that the quiet, mixed-polarity network is
generated locally and that only any relatively small polarity excess
is the result of flux dispersal from active regions.
Title: Mean-Field Electrodynamics of the Solar Corona
Authors: van Ballegooijen, A. A.
Bibcode: 1997SPD....28.0234V
Altcode: 1997BAAS...29..899V
The magnetic field lines in the solar corona are subject to random
motions of the photospheric footpoints due to their interaction with
granulation and supergranulation flows. These random motions cause
small-scale twisting and braiding of the coronal field, which leads
to magnetic reconnection and heating of the coronal plasma. In this
poster I present a mean-field theory which describes the effects
of random footpoint motions on the evolution of the mean (spatially
averaged) coronal magnetic field. The approach is similar to that used
in kinematic dynamo theory, but unlike in dynamo theory the magnetic
pressure is assumed to be large compared to the gas pressure, so
that the magnetic field is nearly force-free. Another key assumption
is that the photospheric motions are purely horizontal, so that the
radial field at the photosphere obeys Leighton's (1964) diffusion
equation. It is shown that magnetic diffusion in the corona can be
described in terms of an anisotropic diffusion tensor which varies in
space and time. The theory provides a formalism for computing the mean
velocity in the corona, which is needed to determine the evolution of
the mean magnetic field. Using a simple model of a decaying active
region, it is shown that the mean velocity at the tops of coronal
loops is directed downward, causing the magnetic shear in the region
to be concentrated at the polarity inversion line. This may explain
observations of localized shear in solar active regions, and could
also play a role in the formation of filament channels on the quiet sun.
Title: The dynamic nature of the supergranular network
Authors: Title, A. M.; Schrijver, C. J.; van Ballegooijen, A. A.;
Hagenaar, H. J.; Shine, R. A.
Bibcode: 1997SPD....28.0242T
Altcode: 1997BAAS...29..900T
The magnetic field in the quiet solar photosphere evolves as flux
concentrations fragment in response to sheared flows, merge when they
collide with others of equal polarity, or (partially) cancel against
concentrations of opposite polarity. Newly emerging flux, mostly in
ephemeral regions, replaces the canceled flux in a matter of a few
days. We present a quantitative statistical model to describe the
resulting histogram of fluxes contained in concentrations of magnetic
flux in the quiet network. We discuss this dynamic flux balance with
respect to (i) the potential dispersal of flux in the photosphere
as a function of ephemeral-region properties, (ii) sustaining the
network--like pattern and mixed--polarity character of the network,
and (iii) the formation of unipolar areas covering the polar caps. We
establish that ephemeral regions are not the result of the re-emergence
of previously cancelled network flux. Moreover, their emergence cannot
be correlated to the emergence of active regions but must instead
be relatively homogeneous. We also point out that the bulk of the
quiet, mixed-polarity network is generated locally, and that only any
relatively small polarity excess is the result of flux dispersal from
active regions.
Title: First Results from UVCS: Dynamics of the Extended Corona
Authors: Antonucci, E.; Noci, G.; Kohl, J. L.; Tondello, G.; Huber,
M. C. E.; Giordano, S.; Benna, C.; Ciaravella, A.; Fineschi, S.;
Gardner, L. D.; Martin, R.; Michels, J.; Naletto, G.; Nicolosi, P.;
Panasyuk, A.; Raymond, C. J.; Romoli, M.; Spadaro, D.; Strachan, L.;
van Ballegooijen, A.
Bibcode: 1997ASPC..118..273A
Altcode: 1997fasp.conf..273A
The Ultraviolet Coronagraph Spectrometer (UVCS) started to observe
the Sun at the end of January 1996. Here we present a selection of
results obtained with the UVCS in the first months of operation. UV
spectral line profiles in coronal holes, and in general in regions
with open magnetic field lines, are much broader than in closed
field line regions; that is, line-of-sight velocities are much
larger in open field lines . Polar plumes have narrower profiles
than interplume regions. The O VI ratio diagnostics indicates that in
polar coronal holes the outflow velocity is progressively increasing
with heliodistance and exceeds 100 km/sec near 2--2.5 solar radii. A
coronal mass ejection observation has revealed line--of--sight plasma
motions of 100 km/sec and a complex dynamics.
Title: Dynamics of Solar Magnetic Fields: Theoretical Aspects
Authors: van Ballegooijen, A. A.
Bibcode: 1996AAS...188.3303V
Altcode: 1996BAAS...28..868V
The distribution of magnetic fields on the solar surface is determined
by processes of flux emergence, transport and cancellation. Magnetic
flux emerges in the form of bipoles of various size, strength
and orientation. Large and medium-sized active regions emerge in
two activity belts on either side of the equator, and generally
have orientations in accordance with Hale's polarity law. Smaller
ephemeral regions have a broader latitude distribution and a more
random orientation. Weak intranetwork fields are seen to emerge in
the centers of supergranules everywhere on the Sun. The emerged field
interacts with convective flows on a variety of spatial and temporal
scales, creating a continually evolving pattern of small-scale flux
concentrations and causing the magnetic elements to perform a random
walk across the solar surface. Chance encounters of opposite polarity
elements can lead to flux cancellation, i.e. disappearance of magnetic
flux from the photosphere. In this talk I review recent developments
in the theory and modelling of these physical processes. The emphasis
is on the structure and dynamics of magnetic fields in the photosphere
and below. I conclude with a list of key scientific questions to be
addressed by future research.
Title: A Model for Dextral and Sinistral Prominences
Authors: Priest, E. R.; van Ballegooijen, A. A.; Mackay, D. H.
Bibcode: 1996ApJ...460..530P
Altcode:
In a recent paper Martin and coworkers have discussed several striking
facts about the structure of solar prominences and the filament
channels in which they lie. They form two classes, called dextral and
sinistral. In a dextral (sinistral) prominence, an observer viewing a
prominence or filament channel from the positive-polarity side would
see the magnetic field point to the right (left) along the axis of the
filament channel, whereas an observer viewing from above would see the
prominence feet bear off the axis to the right (left). Furthermore,
dextral prominences dominate the northern hemisphere and sinistral
the southern hemisphere, regardless of the cycle. Fibrils in the
filament channels do not cross the prominence but usually stream from
or to plagettes parallel to the prominence axis. These pioneering
observations suggest that there is a coherent organizational principle
orchestrating the global nature of prominences, and they have led us
to reexamine the standard paradigms of contemporary prominence theory,
such as that (1) prominences form in a sheared force-free arcade, (2)
formation is by radiative instability, (3) the prominence material is
static, and (4) eruption occurs when the shear or twist is too great. We
propose a new model which accounts for the above new observational
features in a natural way, replaces many of the above paradigms, and
explains the previously puzzling feet of a prominence. It is a dynamic
model in which a prominence is maintained by the continual input of mass
and magnetic flux. The correct global dextral and sinistral patterns
for high-latitude east-west prominences (such as those in the polar
crown) are created by an organizational principle that includes the
combined effects of differential rotation on subphotospheric flux,
its subsequent emergence by magnetic buoyancy, and its rearrangement
by flux reconnection to form a filament channel with magnetic flux
oriented along its axis. Continual emergence and reconnection creates
a prominence as a flux tube along the filament channel axis and
filled with cool plasma which is lifted up from the photosphere and
chromosphere by the reconnection process. Prominences at low latitudes
are in this model formed in a similar way, except that it is a general
subphotospheric flow (rather than differential rotation) which acts and
so may produce either dextral or sinistral structures, depending on the
sense of the flow. The effect of neighboring plagettes in avoiding the
prominence and making it snake its way along the filament channel is
modeled. It is suggested that feet are short-lived structures caused
by the interaction of nearby magnetic fragments with the prominence
field and may represent either the addition or the extraction of mass
from the prominence.
Title: Ultraviolet Emission-Line Intensities and Coronal Heating by
Velocity Filtration: Collisionless Results
Authors: Anderson, Stephen W.; Raymond, John C.; van Ballegooijen,
Adriaan
Bibcode: 1996ApJ...457..939A
Altcode:
We test the velocity filtration coronal heating model by calculating
predicted UV emission-line intensities for comparison with observed
values. The essence of velocity filtration is that a non-Maxwellian
particle distribution in a gravitational well can have a temperature
that increases with height, without any local heating source. To test
this theory, we consider in turn five different non-Maxwellian particle
distributions in the lower corona and use the collisionless Vlasov
equation to estimate the distribution function fe(υ, z)
at all heights. For each height we calculate the ionization balance,
assuming coronal equilibrium, and predict the emission-line intensity
for a number of ions for comparison with Skylab data. To facilitate
comparisons with observations, we also present apparent emission
measures derived from the predicted UV emission lines. Two results
stand out: velocity filtration heating (1) can produce an emission
measure curve that decreases with temperature, as observed for lines
formed below 105 K, but (2) cannot simultaneously reproduce
the increasing emission measure observed for higher temperature
lines. This is precisely the opposite problem faced by most heating
models. We conclude that the present version of velocity filtration
does not match UV observations, but note that it neglects Coulomb
collisions, realistic geometry, and ambipolar diffusion. Our methods
will provide a useful test for more complete versions of velocity
filtration as they emerge.
Title: The Ultraviolet Coronagraph Spectrometer for the Solar and
Heliospheric Observatory
Authors: Kohl, J. L.; Esser, R.; Gardner, L. D.; Habbal, S.; Daigneau,
P. S.; Dennis, E. F.; Nystrom, G. U.; Panasyuk, A.; Raymond, J. C.;
Smith, P. L.; Strachan, L.; Van Ballegooijen, A. A.; Noci, G.;
Fineschi, S.; Romoli, M.; Ciaravella, A.; Modigliani, A.; Huber,
M. C. E.; Antonucci, E.; Benna, C.; Giordano, S.; Tondello, G.;
Nicolosi, P.; Naletto, G.; Pernechele, C.; Spadaro, D.; Poletto, G.;
Livi, S.; Von Der Lühe, O.; Geiss, J.; Timothy, J. G.; Gloeckler,
G.; Allegra, A.; Basile, G.; Brusa, R.; Wood, B.; Siegmund, O. H. W.;
Fowler, W.; Fisher, R.; Jhabvala, M.
Bibcode: 1995SoPh..162..313K
Altcode:
The SOHO Ultraviolet Coronagraph Spectrometer (UVCS/SOHO) is composed of
three reflecting telescopes with external and internal occultation and
a spectrometer assembly consisting of two toric grating spectrometers
and a visible light polarimeter. The purpose of the UVCS instrument is
to provide a body of data that can be used to address a broad range
of scientific questions regarding the nature of the solar corona and
the generation of the solar wind. The primary scientific goals are
the following: to locate and characterize the coronal source regions
of the solar wind, to identify and understand the dominant physical
processes that accelerate the solar wind, to understand how the coronal
plasma is heated in solar wind acceleration regions, and to increase the
knowledge of coronal phenomena that control the physical properties of
the solar wind as determined byin situ measurements. To progress toward
these goals, the UVCS will perform ultraviolet spectroscopy and visible
polarimetry to be combined with plasma diagnostic analysis techniques
to provide detailed empirical descriptions of the extended solar corona
from the coronal base to a heliocentric height of 12 solar radii.
Title: Ultraviolet Coronagraph Spectrometer for the Solar and
Heliospheric Observatory: instrument description and calibration
overview
Authors: Kohl, J. L.; Esser, R.; Gardner, Larry D.; Habbal, S.;
Daigneau, P. S.; Nystrom, George U.; Raymond, John C.; Strachan,
Leonard; van Ballegooijen, A. A.; Noci, G.; Fineschi, Silvano; Romoli,
Marco; Ciaravella, A.; Modigliani, A.; Huber, Martin C.; Antonucci, E.;
Benna, C.; Giordano, S.; von der Luehe, Oskar; Tondello, Giuseppe;
Nicolosi, Piergiorgio; Naletto, Giampiero; Pernechele, Claudio;
Geiss, J.; Gloeckler, G.; Poletto, G.; Spadaro, D.; Allegra, A.;
Basile, G.; Brusa, R.; Wood, B.; Siegmund, Oswald H.
Bibcode: 1995SPIE.2517...40K
Altcode:
The SOHO ultraviolet coronagraph spectrometer (UVCS/SOHO) is
composed of three reflecting telescopes with external and internal
occultation and a spectrometer assembly consisting of two toric grating
spectrometers and a visible light polarimeter. The UVCS will perform
ultraviolet spectroscopy and visible polarimetry to be combined with
plasma diagnostic analysis techniques to provide detailed empirical
descriptions of the extended solar corona from the coronal base to a
heliographic height of 12 R. In this paper, the salient features of
the design of the UVCS instrument are described. An overview of the
UVCS test and calibration activities is presented. The results from
the calibration activity have demonstrated that the UVCS can achieve
all its primary scientific observational goals.
Title: Velocity Filtration Coronal Heating and UV Observations
Authors: Anderson, S. W.; Raymond, J.; van Ballegooijen, A.
Bibcode: 1995SPD....26..911A
Altcode: 1995BAAS...27..975A
No abstract at ADS
Title: The Ultraviolet Coronagraph Spectrometer for the Solar and
Hellospheric Observatory
Authors: Raymond, J. C.; Kohl, J. L.; Esser, R.; Gardner, L. D.;
Habbal, S.; Strachan, L.; van Ballegooijen, A. A.; Noci, G.; Fineschi,
S.; Romoli, M.; Huber, M. C. E.; Antonucci, E.; Benna, C.; von der
Luhe, O.; Naletto, G.; Nicolosi, P.; Pernechele, C.; Tondello, G.;
Geiss, J.; Gloeckler, G.; Spadaro, D.; Daigneau, P. S.; Nystrom,
G. U.; Allegra, A.; Basile, G.; Brusa, R.; Wood, B.; Siegmund, O. H. W.
Bibcode: 1995SPD....26..720R
Altcode: 1995BAAS...27..970R
No abstract at ADS
Title: The Ultraviolet Coronagraph Spectrometer
Authors: Noci, G.; Kohl, J. L.; Huber, M. C. E.; Antonucci, E.;
Fineschi, S.; Gardner, L. D.; Naletto, G.; Nicolosi, P.; Raymond,
J. C.; Romoli, M.; Spadaro, D.; Strachan, L.; Tondello, G.; van
Ballegooijen, A.
Bibcode: 1995LNP...444..261N
Altcode: 1995cmer.conf..261N
The Ultraviolet Coronagraph Spectrometer (UVCS) is an instrument onboard
the Solar and Heliospheric (SOHO) spacecraft, a joint ESA/NASA mission
to be launched in 1995. The UVCS will provide ultraviolet spectroscopic
measurements to determine the primary plasma parameters of the solar
corona (temperatures, densities, velocities), from its base to as high
as 10 R⊙. We review briefly, here, its science objectives
and give an instrument description.
Title: UVCS Science from SOHO
Authors: Antonucci, E.; Kohl, J. L.; Noci, G.; Huber, M. C. E.;
Fineschi, S.; Gardner, L. D.; Naletto, G.; Nicolosi, P.; Pernechele,
C.; Raymond, J. C.; Romoli, M.; Spadaro, Daniele; Strachan, L.;
Tondello, G.; van Ballegooijen, A.
Bibcode: 1995jena.conf...80A
Altcode:
The ultraviolet Coronagraph Spectrometer (UVCS), to be launched on
board SOHO in 1995, has been designed to determine the primary plasma
parameters in order to obtain a far more complete description of the
coronal plasma than presently exists. This will be accomplished by
obtaining ultraviolet spectroscopic observations in some EUV lines (HI
Ly Alpha, O VI, Mg X, Si XII, Fe XII) and in the visible continuum,
form the base of the solar corona to as high as 12 solar radii. The
profiles and intensities of the measured UV lines are sensitive
to effective temperature of protons, minor ions and electrons; ion
densities; chemical abundances; and outflow velocities of protons and
ions into the solar wind. The electron density is determined by means
of white light observations. The UVCS data will be used to address a
broad range of scientific questions concerning the nature of the solar
corona and the generation of the solar wind. The primary scientific
objectives are in fact those of identifying the source regions of the
slow and fast solar wind, understanding the dominant processes that
accelerate the solar wind, and the mechanisms for heating the coronal
plasma in the extended corona.
Title: Magnetic fine structures in coronal loops
Authors: van Ballegooijen, A. A.
Bibcode: 1994SSRv...70...31V
Altcode:
The formation of magnetic fine structures and associated electric
currents is considered in the context of the coronal heating
problem. The penetration of field-aligned electric currents into the
lower atmosphere is discussed. It is argued that currents strong enough
to heat the corona can persist only for short periods of time. The
formation of thin current sheets is discussed. It is argued that
photospheric magnetic structures (flux tubes) play an important role
in the generation of coronal currents.
Title: Energy release in stellar magnetospheres
Authors: van Ballegooijen, A. A.
Bibcode: 1994SSRv...68..299V
Altcode:
The interaction of a stellar magnetosphere with a thin accretion
disk is considered. Specifically, I consider a model in which (1) the
accretion disk is magnetically linked to the star over a large range
of radii and (2) the magnetic diffusivity of the disk is sufficiently
small that there is little slippage of field lines within the disk on
the rotation time scale. In this case the magnetic energy built up
as a result of differential rotation between the star and the disk
is released in quasi-periodic reconnection events occuring in the
magnetosphere (Aly and Kuijpers 1990). The radial transport of magnetic
flux in such an accretion disk is considered. It is show that the
magnetic flux distribution is stationary on the accretion time scale,
provided the time average of the radial component of the field just
above the disk vanishes. A simple model of the time-dependent structure
of the magnetosphere is presented. It is shown that energy release in
the magnetosphere must take place for (differential) rotation angles
less than about 3 radians. The magnetic flux distribution in the disk
depends on the precise value of the rotation angle.
Title: The SPARTAN Ultraviolet Coronagraph
Authors: Gardner, L. D.; Esser, R.; Habbal, S. R.; Hassler, D. M.;
Raymond, J. C.; Strachan, L.; van Ballegooijen, A. A.; Kohl, J. L.;
Fineschi, S.
Bibcode: 1992AAS...180.5202G
Altcode: 1992BAAS...24..815G
An ultraviolet coronagraph (UVC) is being prepared for a series of
orbital flights on NASA's Spartan 201 which is deployed and retrieved
by Shuttle. The Spartan 201 payload consists of the UVC and a white
light coronagraph developed by the High Altitude Observatory. Spartan
is expected to provide 26 orbits of solar observations per flight. The
first flight is scheduled for May 1993 and subsequent flights
are planned to occur at each polar passage of Ulysses (1994 and
1995). The UVC measures the intensity and spectral line profile of
resonantly scattered H I Ly-alpha and the intensities of O VI lambda
1032 and lambda 1037 at heliocentric heights between 1.3 and 3.5 solar
radii. A description of the UVC instrument, its characteristics, and the
observing program for the first flight will be presented. The initial
scientific objective is to determine the random velocity distribution
and bulk outflow velocity of coronal protons and the density and outflow
velocity of O(5+) in polar coronal holes and adjoining high latitude
streamers. This work is supported by NASA under Grant No. NAG5-613 to
the Smithsonian Astrophysical Observatory.
Title: Nonpotential Models of Active Region Magnetic Fields
Authors: van Ballegooijen, A. A.
Bibcode: 1991BAAS...23.1035V
Altcode:
No abstract at ADS
Title: Magnetic Fields in Quiescent Prominences
Authors: van Ballegooijen, A. A.; Martens, P. C. H.
Bibcode: 1990ApJ...361..283V
Altcode:
The origin of the axial fields in high-latitude quiescent prominences
is considered. The fact that almost all quiescent prominences obey
the same hemisphere-dependent rule strongly suggests that the solar
differential rotation plays an important role in producing the axial
fields. However, the observations are inconsistent with the hypothesis
that the axial fields are produced by differential rotation acting on
an existing coronal magnetic field. Several possible explanations for
this discrepancy are considered. The possibility that the sign of the
axial field depends on the topology of the magnetic field in which
the prominence is embedded is examined, as is the possibility that
the neutral line is tilted with respect to the east-west direction, so
that differential rotation causes the neutral line also to rotate with
time. The possibility that the axial fields of quiescent prominences
have their origin below the solar surface is also considered.
Title: Structure and Equilibrium of Coronal Magnetic Fields
Authors: van Ballegooijen, A. A.
Bibcode: 1990IAUS..142..303V
Altcode:
In 'closed' magnetic structures (i.e., coronal loops) the random
shuffling of magnetic footpoints in the photosphere causes twisting
and braiding of field lines in the corona. If the motions are
sufficiently slow, the coronal field evolves through a sequence of
force-free equilibrium states. Numerical simulations are presented
for a simplified model in which the overall curvature of the coronal
loop is neglected. It is shown that magnetic fine structures develop
on spatial scales significantly smaller than those of the imposed
'photospheric' velocity field.
Title: Helical flux ropes in solar prominences
Authors: Martens, P. C. H.; van Ballegooijen, A. A.
Bibcode: 1990GMS....58..337M
Altcode:
The present numerical method for the computation of force-free,
cancelling magnetic structures shows that flux cancellation at the
neutral line in a sheared magnetic arcade generates helical field
lines that can support a prominence's plasma. With increasing flux
cancellation, the axis of the helical fields moves to greater heights;
this is suggestive of a prominence eruption. Two alternative scenarios
are proposed for the formation of polar crown prominences which yield
the correct axial magnetic field sign. Both models are noted to retain
the formation of helical flux tubes through flux cancellation as their
key feature.
Title: Magnetic Heating of Stellar Chromospheres and Coronae
Authors: van Ballegooijen, A. A.
Bibcode: 1990ASPC....9...15V
Altcode: 1990csss....6...15V
The theoretical discussion of magnetic heating focuses on heating by
dissipation of field-aligned electric currents. Several mechanisms
are set forth to account for the very high current densities needed
to generate the heat, but observed radiative losses do not justify the
resultant Ohmic heating rate. Tearing modes, 'turbulent resistivity',
and 'hyper-resistivity' are considered to resolve the implied
inefficiency of coronal heating. Because the mechanisms are not readily
applicable to the sun, transverse magnetic energy flows and magnetic
flare release are considered to account for the magnitude of observed
radiative loss. High-resolution observations of the sun are concluded
to be an efficient way to examine the issues of magnetic heating in
spite of the very small spatial scales of the heating processes.
Title: Formation and Eruption of Solar Prominences
Authors: van Ballegooijen, A. A.; Martens, P. C. H.
Bibcode: 1989ApJ...343..971V
Altcode:
A model for the magnetic field associated with solar prominences is
considered. It is shown that flux cancellation at the neutral line of a
sheared magnetic arcade leads to the formation of helical field lines
which are capable, in principle, of supporting prominence plasma. A
numerical method for the computation of force-free, canceling magnetic
structures is presented. Starting from an initial potential field
we prescribe the motions of magnetic footpoints at the photosphere,
with reconnection occurring only at the neutral line. As more and more
flux cancels, magnetic flux is transferred from the arcade field to
the helical field. Results for a particular model of the photospheric
motions are presented. The magnetic structure is found to be stable: the
arcade field keeps the helical field tied down at the photosphere. The
axis of the helical field moves to larger and larger height, suggestive
of prominence eruption. These results suggest that prominence eruptions
may be trigered by flux cancellation.
Title: Three-Dimensional Models of the Solar Atmosphere
Authors: van Ballegooijen, A. A.; Habbal, S. R.; Dowdy, J. F.
Bibcode: 1989BAAS...21..836V
Altcode:
No abstract at ADS
Title: Magnetic Fields in the Accretion Disks of Cataclysmic Variables
Authors: van Ballegooijen, A. A.
Bibcode: 1989ASSL..156...99V
Altcode: 1989admf.proc...99V
The radial transport of magnetic flux in CV accretion disks is
considered. Turbulent diffusion in a disk with finite outer radius Rd
leads to decay of large-scale magnetic fields, as magnetic flux can leak
out at the outer edge of the disk. Numerical computations of the decay
rate and magnetic structure are presented. It is shown that the decay
time is significantly shorter than the accretion time. Centrifugally
driven winds may be possible from the outer parts of the disk, provided
there exist efficient dynamo processes which regenerate the field.
Title: UVCS: An Ultraviolet Coronagraph Spectrometer for SOHO
Authors: Kohl, J. L.; Hartmann, L. W.; van Ballegooijen, A. A.;
Withbroe, G. L.; Rayomnd, J. C.; Weiser, H.; Noci, G.; Antonucci,
E.; Geiss, J.; Gloeckler, G.
Bibcode: 1989ESASP1104...49K
Altcode:
The UltraViolet Coronagraph Spectrometer (UVCS) of SOHO (solar
and heliospheric observatory) provides ultraviolet spectroscopic
observations of the solar corona out to ten solar radii from
suncenter. This capability is expected to greatly expand the
number of plasma parameters that can be specified by remote sensing
techniques. Plasma diagnostic techniques are expected to provide a
sufficient number of empirically derived parameters to significantly
constrain theories of solar wind acceleration, coronal heating, and
solar wind composition. The spectral line profiles and intensities
are sensitive to random velocity distributions and to the effective
temperatures of protons, minor ions, and electrons. Ion densities,
chemical abundances, and outflow velocities of coronal protons and
ions into the solar wind are also taken into consideration.
Title: Magnetic fine structure of coronal loops.
Authors: van Ballegooijen, A. A.
Bibcode: 1989ftsa.conf...49V
Altcode:
The generation of magnetic fine structures and current sheets in
coronal loops is discussed. Using a simplified model of the magnetic
field in a coronal loop, the author shows that the random motions
of the photospheric footpoints lead to a cascade of magnetic energy
towards smaller spatial scales. Both analytic and numerical models of
this cascade process are presented.
Title: UVCS: an Ultraviolet Coronagraph Spectrometer for SOHO
Authors: Kohl, J. L.; Hartmann, L. W.; van Ballegooijen, A. A.;
Withbroe, G. L.; Raymond, J. C.; Weiser, H.; Noci, G.; Antonucci,
E.; Geiss, J.; Gloeckler, G.
Bibcode: 1988sohi.rept...49K
Altcode:
The UltraViolet Coronagraph Spectrometer (UVCS) of SOHO (solar
and heliospheric observatory) provides ultraviolet spectroscopic
observations of the solar corona out to ten solar radii from
sun-center. This capability is expected to greatly expand the
number of plasma parameters that can be specified by remote sensing
techniques. Plasma diagnostic techniques are expected to provide a
sufficient number of empirically derived parameters to significantly
constrain theories of solar wind acceleration, coronal heating, and
solar wind composition. The spectral line profiles and intensities
are sensitive to random velocity distributions and to the effective
temperatures of protons, minor ions, and electrons. Ion densities,
chemical abundances, and outflow velocities of coronal protons and
ions into the solar wind are also taken into consideration.
Title: The Possible Role of Meridional Flows in Suppressing Magnetic
Buoyancy
Authors: van Ballegooijen, A. A.; Choudhuri, A. R.
Bibcode: 1988ApJ...333..965V
Altcode:
The equation of motion for a toroidal flux ring in a stellar
convective envelope is derived, and the equilibrium of such a ring
is considered. Necessary conditions for the stability of toroidal
flux rings are derived, and results of stability calculations for a
particular model of the meridional flow are presented. The motions of
the flux rings when the rings are far from their equilibrium position
or when equilibrium does not exist are considered. The results confirm
the linear stability analysis, and show that in the absence of stable
equilibrium, the rings move toward the solar surface along a trajectory
which is parallel to the rotation axis. It is expected that viscosity
will tend to reduce the rotational velocity difference between the
flux ring and its surroundings, thus reducing the Coriolis force and
altering the equilibrium. The storage time of toroidal flux rings is
estimated, and some implications for the sun are discussed.
Title: Magnetic Fields in Solar Prominences
Authors: van Ballegooijen, A. A.
Bibcode: 1988BAAS...20..978V
Altcode:
No abstract at ADS
Title: Structure and Dynamics of Cataclysmic Variable Winds
Authors: Raymond, J. C.; van Ballegooijen, A. A.; Mauche, C. W.
Bibcode: 1988BAAS...20.1020R
Altcode:
No abstract at ADS
Title: Magnetic fine structure of solar coronal loops.
Authors: van Ballegooijen, A. A.
Bibcode: 1988sscd.conf..115V
Altcode:
A numerical simulation of the effect of a random photospheric flow
on the magnetic structure of a coronal loop is presented. The author
considers an initially uniform field embedded in a perfectly conducting
plasma, extending between two flat parallel plates which represent the
solar photosphere at the two ends of the loop. The field is perturbed
by a sequence of randomly phased, sinusoidal flow patterns applied
at one of the boundary plates, and the corresponding sequence of
(nonlinear) force free fields is determined. It is found that the
electric currents generated by these flows develop a fine structure on
a (transverse) scale significantly smaller than the wavelength of the
velocity patterns. This suggests that magnetic energy is transferred
to smaller scales via a cascade process. Some implications for coronal
magnetic structure and heating are discussed.
Title: Force free fields and coronal heating part I. The formation
of current sheets
Authors: van Ballegooijen, A. A.
Bibcode: 1988GApFD..41..181V
Altcode:
We consider the formation of small-scale magnetic structures in solar
coronal loops, with the aim of understanding the possible role of these
structures in the process of coronal heating. A simplified model of a
coronal loop is discussed. Neglecting loop curvature, we consider an
initially uniform magnetic field embedded in a perfectly conducting
plasma between two flat parallel plates z=0 and z =L, which represent
the photosphere at the two ends of the loop. Slow, random motions
at these boundary plates produce twists and braids in the magnetic
field. We discuss the properties of such braided fields assuming
the field evolves through a series of force-free equilibria. Using
a Lagrangean description of the field, the equilibrium problem is
formulated as a boundary-value problem for the functions X(x0, y0,
z, t) and Y(x0, y0, z, t) which describe the shape of field lines
characterized by the initial coordinates x0 and y0. We argue that
X(x0, y0, z, t) and Y(x0, y0, z, t) are continuous functions of
x0 and y0 at time t=T, provided X and Y are continuous in x0 and
y0 at the boundary plates (z=0 and z=L) for all intermediate times
0<t<T. In particular, we show that isolated infinitesimally
thin current sheets do not arise if the field between the plates is
force free. This suggests that spatially continuous velocity fields
at the boundary plates do not produce tangential discontinuities in
the magnetic structure as first suggested by Parker (1972). It also
implies that ideal-MHD instabilities, if they occur in this model,
do not lead to tangential discontinuities. We contrast our results
with those obtained for more complicated field topologies containing
multiple flux systems. Instead of the catastrophic "non-equilibrium"
process of current-sheet formation proposed by Parker (1972), we propose
a more gradual process in which small-scale structures are produced by
the random intermixing of magnetic footpoints in the solar photosphere.
Title: Is there a weak mixed polarity background field? Theoretical
arguments
Authors: Spruit, H. C.; Title, A. M.; van Ballegooijen, A. A.
Bibcode: 1987SoPh..110..115S
Altcode:
A number of processes associated with the formation of active regions
produce `U-loops': fluxtubes having two ends at the photosphere but
otherwise still embedded in the convection zone. The mass trapped on the
field lines of such loops makes them behave in a qualitative different
way from the `omega-loops' that form active regions. It is shown that
U-loops will disperse though the convection zone and form a weak (down
to a few gauss) field that covers a significant fraction of the solar
surface. This field is tentatively identified with the inner-network
fields observed at Kitt Peak and Big Bear. The process by which these
fields escape through the surface is described; a remarkable property is
that it can make active regions fields apparently disappear in situ. The
mixed polarity moving magnetic features near sunspots are interpreted
as a locally intense form of this disappearance by escape of U-loops.
Title: Is there a weak mixed polarity background field? Theoretical
arguments.
Authors: Spruit, H. C.; Title, A. M.; van Ballegooijen, A. A.
Bibcode: 1987MPARp.271.....S
Altcode:
No abstract at ADS
Title: Radiative Transfer in the Presence of Strong Magnetic Fields
Authors: van Ballegooijen, A. A.
Bibcode: 1987nrt..book..279V
Altcode:
No abstract at ADS
Title: Cascade of Magnetic Energy as a Mechanism of Coronal Heating
Authors: van Ballegooijen, A. A.
Bibcode: 1986ApJ...311.1001V
Altcode:
A statistical model is defined for the quasi-static evolution of the
motion of photospheric structures through a cascade process. Since
the magnetic footprints move slowly, the coronal field can adapt to
changing boundary conditions as free magnetic energy is transported
over timescales significantly smaller than those of the movements of the
footprints. The energy is transported as coronal loops are shredded into
increasingly finer segments by randomly changing velocity gradients in
the photosphere, a process which is stochastic. Numerical computations
are provided which show that the magnetic energy is transferred to
larger wavenumbers by a cascade process. Application of the model to
coronal heating is discussed.
Title: Cascade model of coronal heating.
Authors: van Ballegooijen, A. A.
Bibcode: 1986NASCP2442..439V
Altcode: 1986copp.nasa..439V
It is suggested that the quasi-static evolution of coronal magnetic
structures is characterized by a cascade of magnetic energy to
smaller length scales. This cascade process takes place on a time scale
tb determined entirely by the photospheric motions. The Ohmic
heating rate EH in the statistically stationary state was
estimated using observational data on the diffusivity of photospheric
motions; EH turned out to be too small by a factor of 40
when compared with observed coronal energy losses. However, given
the fact that the theoretical estimate is based on a rather uncertain
extrapolation to the diffusive regime, current heating cannot be ruled
out as a viable mechanism of coronal heating.
Title: On the Surface Response of Solar Giant Cells
Authors: van Ballegooijen, A. A.
Bibcode: 1986ApJ...304..828V
Altcode:
The size and screening mechanism of the solar giant cells are
examined. A plane-parallel model of the inviscid flows within a
stratified rotating layer is described. The vorticity equation for the
flow is derived and stationary solutions of the equation that represent
the convective roll patterns are obtained. The density stratification
and solar rotation of the convection zones are investigated; it is
observed that the density stratification of the convection zone causes
screening of the flow pattern due to varying velocity amplitudes. The
convective fluctuations in the stratified, differentially rotating layer
with constant entropy gradient are analyzed. The analysis reveals that
the most unstable modes of solar giant cell convection are modes with
wavenumbers greater than eight and wavelengths between 100-150 Mm.
Title: Electric currents in the solar corona and the existence of
magnetostatic equilibrium
Authors: van Ballegooijen, A. A.
Bibcode: 1985ApJ...298..421V
Altcode:
The random motions of magnetic field lines induced by convective
flows below the solar surface cause braiding and twisting of the
coronal magnetic field, and may be responsible for heating the solar
corona. The suggestion by Parker (1972) that the field does in general
not attain equilibrium, and must develop current sheets in which
the braiding patterns are dissipated (topological dissipation), is
considered. Using an analogy with two-dimensional flows, it is shown
that invariance of the winding pattern in the general direction of
the field is not a necessary requirement for equilibrium, as Parker
suggested. Discontinuities in the magnetic field (current sheets)
arise only if the velocity field at the photospheric boundary is itself
a discontinuous function of position. This suggests that the corona
field can simply adjust to the slowly changing boundary conditions in
the photosphere, and that 'topological dissipation' of the winding
patterns does not take place. Some implications for coronal heating
are discussed.
Title: Cascade of Magnetic Energy as a Mechanism of Coronal Heating
Authors: van Ballegooijen, A. A.
Bibcode: 1985tphr.conf..268V
Altcode:
No abstract at ADS
Title: Transport of Polarized Light in Small Flux Tubes
Authors: van Ballegooijen, A. A.
Bibcode: 1985tphr.conf..167V
Altcode:
No abstract at ADS
Title: Contribution functions for Zeeman-split lines, and line
formation in photospheric faculae
Authors: van Ballegooijen, A. A.
Bibcode: 1985svmf.nasa..322V
Altcode:
The transfer of polarized light in an inhomogeneous stellar atmosphere,
and the formation of magnetically sensitive spectral lines, are
discussed. A new method for the solution of the transfer equations is
proposed. The method gives a natural definition of the contribution
functions for Stokes' parameters, i.e., functions describing the
contributions from different parts along the line-of-sight (LOS). The
formalism includes all magneto-optical effects, and allows for an
arbitrary variation of magnetic field, velocity field, temperature,
density, etc., along the LOS. The formation of FeI lambda 5250.2 in
photospheric faculae is described. A potential-field model of a facular
element is presented, and spectra profiles and contribution functions
are computed for the Stokes parameters I, Q, and V.
Title: An upper limit on the size of giant cells
Authors: van Ballegooijen, A. A.
Bibcode: 1985BAAS...17..643V
Altcode:
No abstract at ADS
Title: Contribution functions for Zeeman-split lines, and line
formation in photospheric faculae.
Authors: van Ballegooijen, A. A.
Bibcode: 1985NASCP2374..322V
Altcode:
The transfer of polarized light in an inhomogeneous stellar atmosphere,
and the formation of magnetically sensitive spectral lines, are
discussed. A new method for the solution of the transfer equations
is proposed. The formalism includes all magneto-optical effects, and
allows for an arbitrary variation of magnetic field, velocity field,
temperature, density, etc. along the line-of-sight. To illustrate
the method the formation of Fe I λ5250.2 in photospheric faculae is
discussed. A potential-field model of a facular element is presented,
and spectral profiles and contribution functions are computed for the
Stokes parameters I, Q, and V.
Title: Cascade of magnetic energy as a mechanism of coronal heating.
Authors: van Ballegooijen, A. A.
Bibcode: 1985MPARp.212..268V
Altcode:
The mechanism of DC coronal heating is discussed. It is shown that, due
to the large electrical conductivity of the coronal plasma, significant
heating is possible only if the electric current density is sufficiently
large. This suggests that the required current densities are produced
via a cascade process, in which "free" magnetic energy is transferred
from large to small length scales.
Title: Transport of polarized light in small flux tubes.
Authors: van Ballegooijen, A. A.
Bibcode: 1985MPARp.212..167V
Altcode:
Some radiative transport effects that are important for observations
of small magnetic flux tubes are discussed.
Title: Coronal Heating and the Quasi-static Evolution of Magnetic
Fields
Authors: van Ballegooijen, A. A.
Bibcode: 1984BAAS...16.1003V
Altcode:
No abstract at ADS
Title: On the Temperature Structure of Sunspot Umbrae
Authors: van Ballegooijen, A. A.
Bibcode: 1984SoPh...91..195V
Altcode:
From a high-resolution spectrum of a sunspot umbra (1.1 < λ <
2.3 μm) we derive models of the temperature stratification in the deep
layers of the umbra. The observed spectrum is corrected for straylight
using the HI Paschen line at gl = 1.282 μm. A method is described
for the iterative fitting of empirical temperature models to spectral
information, and the method is applied to the present data. We find that
the observed profiles of 3 high-excitation lines of SiI and the observed
continuum contrast between umbra and photosphere cannot be reproduced
with a single one-component model of the umbral atmosphere: the Si
I lines require a model that is 460 K hotter at gt0.5 = 3
than the continuum model. This indicates that hot and cool components
coexist within the umbra. A temperature model derived from the relative
intensity in the wings of 3 low-excitation lines of MgI, AlI, and SiI
is not significantly different from the continuum model.
Title: The Effect of Reynolds Stress in the Solar Convective Zone
on the Vertical Structure of Flux Tubes, and on Their Convective
Instability
Authors: van Ballegooijen, A. A.
Bibcode: 1984ssdp.conf..260V
Altcode:
A model of the fieldstrength in slender magnetic flux tubes, as
function of depth in the convective zone, is described. The tubes
are assumed to be vertical, and in thermal equilibrium with the
surrounding medium. Deviations from hydrostatic equilibrium, due to
Reynolds stresses in the convective zone, are taken into account. The
convective instability of the flux tubes is briefly discussed.
Title: On the stability of toroidal flux tubes in differentially
rotating stars
Authors: van Ballegooijen, A. A.
Bibcode: 1983A&A...118..275V
Altcode:
The stability of magnetic flux tubes, located in the equatorial
plane of a rotating star, is studied using the slender flux tube
approximation. The effects of a longitudinal mass flow in the tubes
and of a radial gradient of angular velocity in the surrounding medium
are considered. It is shown that differential rotation has a dominant
effect on the stability in the case of rapid rotation where the notation
rate is much greater than the Alfven velocity divided by two pressure
scale heights. Flux tubes are unstable (stable) if the angular velocity
decreases (increases) with distance from the rotation axis. The apparent
stability of toroidal flux tubes in the sun suggests that in the deep
convective zone the angular velocity increases radially outward. This
has important implications for stellar dynamo theories.
Title: The overshoot layer at the base of the solar convective zone
and the problem of magnetic flux storage.
Authors: van Ballegooijen, A. A.
Bibcode: 1982A&A...113...99V
Altcode:
Possibilities for storing a horizontal magnetic field in the overshoot
layer at the base of the solar convective zone, or in the radiative zone
below it, are studied using a one-dimensional model of the overshoot
layer. The convective energy flux is estimated using a linear mode
analysis. The thickness of the overshoot layer is found to be a
few tenths of the pressure scale height, with an abrupt temperature
gradient change at the base of the layer. Neutrally buoyant horizontal
flux tubes, with an equipartition field strength greater than or about
equal to 10 to the fourth Gauss, could be stabilized in the overshoot
layer, but an additional force is needed to counteract the buoyancy
force from an inflow of heat into the tubes. The Coriolis force on a
mass flow along the toroidal flux with a flow velocity of 8 m/s may
hold tubes with an equipartition field strength of about 10 to the
fourth Gauss in the overshoot layer, and such flux tubes may be stable
for solar cycle periods.
Title: Stability of toroidal flux tubes in stars
Authors: Spruit, H. C.; van Ballegooijen, A. A.
Bibcode: 1982A&A...106...58S
Altcode:
A thin tube approximation is used to study the stability of a magnetic
flux tube in the equatorial plane of a star, where (1) only adiabatic
disturbances are considered, (2) the tubes are unstable to poleward
motion and to (3) poleward motions within the equatorial plane, if
the superadiabaticity of the stratification is large enough. While the
curvature of the tube in a spherical geometry has a stabilizing effect,
it is not strong enough to stabilize flux tubes in the convective
envelopes of main sequence stars. The longer wavelengths are favored
by the instability and, for the case of the sun, modes m equals 0-4
are unstable. It is suggested in view of this instability that toroidal
fields, in a stellar dynamo, occur at the interface between convection
zone and radiative interior rather than within the convection zone.
Title: The structure of the solar magnetic field below the
photosphere. I - Adiabatic flux tube models
Authors: van Ballegooijen, A. A.
Bibcode: 1982A&A...106...43V
Altcode:
A first approximation for the structure of solar emerged flux tubes
is made possible by a model of adiabatic flux tubes rooted in the
stable layer below the convective zone. In addition to obtaining an
estimate of the horizontal flux system position from the observed
photosphere field strength, it is shown that a horizontal drift must
occur due to unbalanced tension in the horizontal part of emerged flux
tubes. An important new problem, of horizontal flux system initial
destabilization, is encountered. It is suggested that the horizontal
drift of emerged flux tubes, coupled with the Coriolis effect, may
play an essential role in the regenerative process underlying the
solar cycle, where flux tubes with following polarity move against the
direction of rotation and direct the Coriolis force towards the pole
in each hemisphere. It is concluded that the model presented may lead
to a better understanding of the solar cycle.
Title: Sunspots and the physics of magnetic flux tubes in the sun
Authors: van Ballegooijen, Adriaan Andries Aad
Bibcode: 1982PhDT........43V
Altcode:
No abstract at ADS
Title: Energy Transport in Deep Umbral Layers
Authors: van Ballegooijen, A. A.
Bibcode: 1981phss.conf..140V
Altcode:
No abstract at ADS
Title: A model for slender flux tubes and its application to sunspots
Authors: van Ballegooijen, A. A.
Bibcode: 1981phss.conf..115V
Altcode:
Two models of a flux-tube configuration for the connection between the
vertical magnetic fields in the solar photosphere and the horizontal
magnetic field within the solar mantle are discussed. In particular,
Parker's model (1979) is modified to include a bending of the flux
tubes from vertical to horizontal deep beneath a sunspot. The field
strength is obtained as a function of depth after assuming hydrostatic
equilibrium and an adiabatic stratification within the tubes. Magnetic
tension in the horizontal segment of the tubes is found to cause a
horizontal motion of the emerged flux tubes through the convective zone,
a factor which is suggested to be significant for sunspot theories.