galsgaard

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Author name code: galsgaard
ADS astronomy entries on 2022-09-14
author:"Galsgaard, Klaus"

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Title: Mini-filament eruption, QSL reconnection, and
reconnection-driven outflows: IRIS and AIA/HMI/SDO observations
and modelling
Authors: Madjarska, Maria S.; Mackay, Duncan H.; Galsgaard, Klaus;
Xie, Haixia; Wiegelmann, Thomas
2022cosp...44.2533M Altcode:
We will present unique observations of a mini-filament eruption
associated with cancelling magnetic fluxes of a small-scale loop system
known as a coronal bright point. The event is uniquely recorded in
both the imaging and spectroscopic data taken with the Interface
Region Imaging Spectrograph (IRIS). The study aims at providing
a better understanding of the physical processes driving these
ubiquitous small-scale eruptions. We also analysed images taken in the
extreme-ultraviolet channels of the Atmospheric Imaging Assembly (AIA)
and line-of-sight magnetic-field data from the Helioseismic Magnetic
Imager (HMI) onboard the Solar Dynamics Observatory. As the observations
can only give an inkling about the possible physical processes at play,
we also employed a non-linear force-free field (NLFFF) relaxation
approach based on the HMI magnetogram time series. Furthermore, we
computed the squashing factor, Q, in different horizontal planes of
the NLFFF model. This allowed us to further investigate the evolution
of the magnetic-field structures involved in the eruption process.

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Title: Eruptions from coronal bright points: A spectroscopic
view by IRIS of a mini-filament eruption, QSL reconnection, and
reconnection-driven outflows
Authors: Madjarska, Maria S.; Mackay, Duncan H.; Galsgaard, Klaus;
Wiegelmann, Thomas; Xie, Haixia
2022A&A...660A..45M Altcode: 2022arXiv220200370M
Context. Our study investigates a mini-filament eruption associated with
cancelling magnetic fluxes. The eruption originates from a small-scale
loop complex commonly known as a coronal bright point (CBP). The
event is uniquely recorded in both the imaging and spectroscopic data
taken with the Interface Region Imaging Spectrograph (IRIS). 
Aims: The investigation aims to gain a better understanding of the
physical processes driving these ubiquitous small-scale eruptions. Methods: We analysed IRIS spectroscopic and slit-jaw imaging
observations as well as images taken in the extreme-ultraviolet
channels of the Atmospheric Imaging Assembly (AIA) and line-of-sight
magnetic-field data from the Helioseismic Magnetic Imager (HMI) on
board the Solar Dynamics Observatory. As the observations can only
indicate the possible physical processes at play, we also employed a
non-linear force-free field (NLFFF) relaxation approach based on the
HMI magnetogram time series. This allowed us to further investigate
the evolution of the magnetic-field structures involved in the eruption
process. Results: We identified a strong small-scale brightening
as a micro-flare in a CBP, recorded in emission from chromospheric to
flaring plasmas. The mini-eruption is manifested via the ejection of hot
(CBP loops) and cool (mini-filament) plasma recorded in both the imaging
and spectroscopic data. The micro-flare is preceded by the appearance
of an elongated bright feature in the IRIS slit-jaw 1400 Å images,
located above the polarity inversion line. The micro-flare starts
with an IRIS pixel size brightening and propagates bi-directionally
along the elongated feature. We detected, in both the spectral and
imaging IRIS data and AIA data, strong flows along and at the edges of
the elongated feature; we believe that these represent reconnection
outflows. Both edges of the elongated feature that wrap around the
edges of the erupting MF evolve into a J-type shape, creating a
sigmoid appearance. A quasi-separatrix layer (QSL) is identified in
the vicinity of the polarity inversion line by computing the squashing
factor, Q, in different horizontal planes of the NLFFF model. 
Conclusions: This CBP spectro-imaging study provides further evidence
that CBPs represent downscaled active regions and, as such, they may
make a significant contribution to the mass and energy balance of
the solar atmosphere. They are the sources of all range of typical
active-region features, including magnetic reconnection along QSLs,
(mini-)filament eruptions, (micro-)flaring, reconnection outflows,
etc. The QSL reconnection site has the same spectral appearance
as the so-called explosive events identified by strong blue- and
red-shifted emission, thus providing an answer to an outstanding
question regarding the true nature of this spectral phenomenon. Movies associated to Figs. A.1 and A.2 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202142439/olm">https://www.aanda.org</A>

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Title: The chromospheric component of coronal bright points. Coronal
and chromospheric responses to magnetic-flux emergence
Authors: Madjarska, Maria S.; Chae, Jongchul; Moreno-Insertis,
Fernando; Hou, Zhenyong; Nóbrega-Siverio, Daniel; Kwak, Hannah;
Galsgaard, Klaus; Cho, Kyuhyoun
2021A&A...646A.107M Altcode: 2020arXiv201209426M
Context. We investigate the chromospheric counterpart of small-scale
coronal loops constituting a coronal bright point (CBP) and its
response to a photospheric magnetic-flux increase accompanied by
co-temporal CBP heating. Aims: The aim of this study is
to simultaneously investigate the chromospheric and coronal layers
associated with a CBP, and in so doing, provide further understanding on
the heating of plasmas confined in small-scale loops. Methods:
We used co-observations from the Atmospheric Imaging Assembly and
Helioseismic Magnetic Imager on board the Solar Dynamics Observatory,
together with data from the Fast Imaging Solar Spectrograph taken
in the Hα and Ca II 8542.1 Å lines. We also employed both linear
force-free and potential field extrapolation models to investigate
the magnetic topology of the CBP loops and the overlying corona,
respectively. We used a new multi-layer spectral inversion technique
to derive the temporal variations of the temperature of the Hα loops
(HLs). Results: We find that the counterpart of the CBP, as
seen at chromospheric temperatures, is composed of a bundle of dark
elongated features named in this work Hα loops, which constitute an
integral part of the CBP loop magnetic structure. An increase in the
photospheric magnetic flux due to flux emergence is accompanied by
a rise of the coronal emission of the CBP loops, that is a heating
episode. We also observe enhanced chromospheric activity associated
with the occurrence of new HLs and mottles. While the coronal emission
and magnetic flux increases appear to be co-temporal, the response of
the Hα counterpart of the CBP occurs with a small delay of less than
3 min. A sharp temperature increase is found in one of the HLs and
in one of the CBP footpoints estimated at 46% and 55% with respect
to the pre-event values, also starting with a delay of less than 3
min following the coronal heating episode. The low-lying CBP loop
structure remains non-potential for the entire observing period. The
magnetic topological analysis of the overlying corona reveals the
presence of a coronal null point at the beginning and towards the end
of the heating episode. Conclusions: The delay in the response
of the chromospheric counterpart of the CBP suggests that the heating
may have occurred at coronal heights. Movies are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202039329/olm">https://www.aanda.org</A>

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Title: Eruptions from coronal hole bright points: Observations and
non-potential modelling
Authors: Madjarska, Maria S.; Galsgaard, Klaus; Mackay, Duncan H.;
Koleva, Kostadinka; Dechev, Momchil
2020A&A...643A..19M Altcode: 2020arXiv200904628M
Context. We report on the third part of a series of studies on eruptions
associated with small-scale loop complexes named coronal bright points
(CBPs). Aims: A single case study of a CBP in an equatorial
coronal hole with an exceptionally large size is investigated to
expand on our understanding of the formation of mini-filaments, their
destabilisation, and the origin of the eruption triggering the formation
of jet-like features recorded in extreme ultraviolet (EUV) and X-ray
emission. We aim to explore the nature of the so-called micro-flares
in CBPs associated with jets in coronal holes and mini coronal mass
ejections in the quiet Sun. Methods: Co-observations from the
Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager
(HMI) on board the Solar Dynamics Observatory as well as GONG Hα
images are used together with a non-linear force free field (NLFFF)
relaxation approach, where the latter is based on a time series of
HMI line-of-sight magnetograms. Results: A mini-filament (MF)
that formed beneath the CBP arcade about 3-4 h before the eruption is
seen in the Hα and EUV AIA images to lift up and erupt triggering the
formation of an X-ray jet. No significant photospheric magnetic flux
concentration displacement (convergence) is observed and neither is
magnetic flux cancellation between the two main magnetic polarities
forming the CBP in the time period leading to MF lift-off. The
CBP micro-flare is associated with three flare kernels that formed
shortly after the MF lift-off. No observational signature is found
for magnetic reconnection beneath the erupting MF. The applied NLFFF
modelling successfully reproduces both the CBP loop complex as well
as the magnetic flux rope that hosts the MF during the build-up to
the eruption. Conclusions: The applied NLFFF modelling is
able to clearly show that an initial potential field can be evolved
into a non-potential magnetic field configuration that contains
free magnetic energy in the region that observationally hosts the
eruption. The comparison of the magnetic field structure shows that the
magnetic NLFFF model contains many of the features that can explain
the different observational signatures found in the evolution and
eruption of the CBP. In the future, it may eventually indicate the
location of destabilisation that results in the eruptions of flux
ropes. Movies associated to Figs. 9 and B.2 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/202038287/olm">https://www.aanda.org</A>

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Title: Eruptions from quiet Sun coronal bright
points. II. Non-potential modelling
Authors: Galsgaard, Klaus; Madjarska, Maria S.; Mackay, Duncan H.;
Mou, Chaozhou
2019A&A...623A..78G Altcode: 2019arXiv190109875G
Context. Our recent observational study shows that the majority
of coronal bright points (CBPs) in the quiet Sun are sources of
one or more eruptions during their lifetime. Aims: Here, we
investigate the non-potential time-dependent structure of the magnetic
field of the CBP regions with special emphasis on the time-evolving
magnetic structure at the spatial locations where the eruptions are
initiated. Methods: The magnetic structure is evolved in time
using a non-linear force-free field (NLFFF) relaxation approach based
on a time series of helioseismic and magnetic imager (HMI) longitudinal
magnetograms. This results in a continuous time series of NLFFFs. The
time series is initiated with a potential field extrapolation based
on a magnetogram taken well before the time of the eruptions. This
initial field is then evolved in time in response to the observed
changes in the magnetic field distribution at the photosphere. The
local and global magnetic field structures from the time series of NLFFF
field solutions are analysed in the vicinity of the eruption sites at
the approximate times of the eruptions. Results: The analysis
shows that many of the CBP eruptions reported in a recent publication
contain a twisted flux tube located at the sites of eruptions. The
presence of flux ropes at these locations provides in many cases a
direct link between the magnetic field structure, their eruption,
and the observation of mini coronal mass ejections (mini-CMEs). It is
found that all repetitive eruptions are homologous. Conclusions:
The NLFFF simulations show that twisted magnetic field structures are
created at the locations hosting eruptions in CBPs. These twisted
structures are produced by footpoint motions imposed by changes in
the photospheric magnetic field observations. The true nature of the
micro-flares remains unknown. Further 3D data-driven magnetohydrodynamic
modelling is required to show how these twisted regions become unstable
and erupt. Movies associated to Figs. 1-5 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201834329/olm">https://www.aanda.org</A>

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Title: Eruptions from quiet Sun coronal bright points: Observations
&amp; Modeling
Authors: Madjarska, Maria S.; Galsgaard, Klaus; Mou, Chauzhou
2018csc..confE.122M Altcode:
We present a two part study that aims first to observationally explore
in full detail the morphological and dynamical evolution of eruptions
from coronal bright points (CBPs) in the context of the full lifetime
evolution of 11 CBPs. Next, we employ data-driven modelling based on a
relaxation code to reproduce the time evolution of the magnetic field
of these eruptive CBPs, and provide an insight on the possible causes
for destabilisation and eruption. Observations of the full lifetime
of CBPs in data taken with the Atmospheric Imaging Assembly (AIA) on
board the Solar Dynamics Observatory in four passbands He II 304 Å,
Fe IX/X 171 Å, Fe XII 193 Å, and Fe XVIII 94 Å are investigated
for the occurrence of plasma ejections, micro-flaring, mini-filament
eruptions and mini coronal mass ejections (mini-CMEs). Data from the
Helioseismic and Magnetic Imager are analysed to study the longitudinal
photospheric magnetic field evolution associated with the CBPs and
related eruptions. The magnetic structure of each CBP is then evolved
in time using the relaxation approach, based on a time series of
HMI magnetograms. This results in a series of Non-Linear Force Free
Field Extrapolations (NLFFF). The time series is initiated with a
potential field extrapolation based on a HMI magnetogram well before
the eruptions, and evolved in time as a response to the changes in the
magnetic field distribution in the photosphere. This time series of
NLFFF field solutions is analysed for the local and global magnetic
field structure in the vicinity of the eruption sites.

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Title: Eruptions from quiet Sun coronal bright points. I. Observations
Authors: Mou, Chauzhou; Madjarska, Maria S.; Galsgaard, Klaus;
Xia, Lidong
2018A&A...619A..55M Altcode: 2018arXiv180804541M
Context. Eruptions from coronal bright points (CBPs) are investigated
in a two-part study. Aims: The present study aims to explore in
full detail the morphological and dynamical evolution of these eruptions
in the context of the full lifetime evolution of CBPs. A follow-up
study employs data-driven modelling based on a relaxation code to
reproduce the time evolution of the magnetic field of these eruptive
CBPs, and provide insight into the possible causes for destabilisation
and eruption. Methods: Observations of the full lifetime of
CBPs in data taken with the Atmospheric Imaging Assembly (AIA) on
board the Solar Dynamics Observatory in four passbands, He II 304 Å,
Fe IX/X 171 Å, Fe XII 193 Å, and Fe XVIII 94 Å are investigated
for the occurrence of plasma ejections, micro-flaring, mini-filament
eruptions, and mini coronal-mass ejections (mini-CMEs). Data from
the Helioseismic and Magnetic Imager are analysed to study the
longitudinal photospheric magnetic field evolution associated with
the CBPs and related eruptions. Results: First and foremost,
our study shows that the majority (76%) of quiet Sun CBPs (31 out of
42 CBPs) produce at least one eruption during their lifetime. From 21
eruptions in 11 CBPs, 18 of them occur, on average, ∼17 h after the
CBP formation. The average lifetime of the CBPs in AIA 193 Å is ∼21
h. The time delay in the eruption occurrence coincides in each CBP with
the convergence and cancellation phase of the CBP bipole evolution
during which the CBPs become smaller until they fully disappear. The
remaining three eruptions happen 4-6 h after the CBP formation. In
16 out of the 21 eruptions, the magnetic convergence and cancellation
involve the CBP main bipoles, while in three eruptions, one of the BP
magnetic fragments and a pre-existing fragment of opposite polarity
converge and cancel. In one BP with two eruptions, cancellation was
not observed. The CBP eruptions involve in most cases the expulsion of
chromospheric material either as an elongated filamentary structure
(mini-filament, MF) or a volume of cool material (cool plasma cloud,
CPC), together with the CBP or higher overlying hot loops. Coronal waves
were identified during three eruptions. A micro-flaring is observed
beneath all erupting MFs/CPCs. Whether the destabilised MF causes
the micro-flaring or the destabilisation and eruption of the MF is
triggered by reconnection beneath the filament remains uncertain. In
most eruptions, the cool erupting plasma either partially or fully
obscures the micro-flare until the erupting material moves away from the
CBP. From 21 eruptions, 11 are found to produce mini-CMEs. The dimming
regions associated with the CMEs are found to be occupied by both the
"dark" cool plasma and areas of weakened coronal emission caused by
the depleted plasma density. Conclusions: The present study
demonstrates that the small-scale loop structures in the quiet Sun,
the evolution of which is determined by their magnetic footpoint
motions and/or ambient field topology, evolve into an eruptive phase
that triggers the ejection of cool and hot plasma in the corona. The movies associated to Figs 1, 3, 4, 6 are available at <A
href="https://www.aanda.org/10.1051/0004-6361/201833243/olm">https://www.aanda.org</A>

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Title: Magnetic topological analysis of coronal bright points
Authors: Galsgaard, K.; Madjarska, M. S.; Moreno-Insertis, F.; Huang,
Z.; Wiegelmann, T.
2017A&A...606A..46G Altcode: 2017arXiv170704174G
Context. We report on the first of a series of studies on coronal
bright points which investigate the physical mechanism that generates
these phenomena. Aims: The aim of this paper is to understand
the magnetic-field structure that hosts the bright points. 
Methods: We use longitudinal magnetograms taken by the Solar Optical
Telescope with the Narrowband Filter Imager. For a single case,
magnetograms from the Helioseismic and Magnetic Imager were added
to the analysis. The longitudinal magnetic field component is used
to derive the potential magnetic fields of the large regions around
the bright points. A magneto-static field extrapolation method is
tested to verify the accuracy of the potential field modelling. The
three dimensional magnetic fields are investigated for the presence
of magnetic null points and their influence on the local magnetic
domain. Results: In nine out of ten cases the bright point
resides in areas where the coronal magnetic field contains an opposite
polarity intrusion defining a magnetic null point above it. We find that
X-ray bright points reside, in these nine cases, in a limited part of
the projected fan-dome area, either fully inside the dome or expanding
over a limited area below which typically a dominant flux concentration
resides. The tenth bright point is located in a bipolar loop system
without an overlying null point. Conclusions: All bright points
in coronal holes and two out of three bright points in quiet Sun regions
are seen to reside in regions containing a magnetic null point. An as
yet unidentified process(es) generates the brigh points in specific
regions of the fan-dome structure. The movies are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201731041/olm">http://www.aanda.org</A>

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Title: Observable Signatures of Energy Release in Braided Coronal
Loops
Authors: Pontin, D. I.; Janvier, M.; Tiwari, S. K.; Galsgaard, K.;
Winebarger, A. R.; Cirtain, J. W.
2017ApJ...837..108P Altcode:
We examine the turbulent relaxation of solar coronal loops containing
non-trivial field line braiding. Such field line tangling in the
corona has long been postulated in the context of coronal heating
models. We focus on the observational signatures of energy release
in such braided magnetic structures using MHD simulations and forward
modeling tools. The aim is to answer the following question: if energy
release occurs in a coronal loop containing braided magnetic flux,
should we expect a clearly observable signature in emissions? We
demonstrate that the presence of braided magnetic field lines does not
guarantee a braided appearance to the observed intensities. Observed
intensities may—but need not necessarily—reveal the underlying
braided nature of the magnetic field, depending on the degree and
pattern of the field line tangling within the loop. However, in all
cases considered, the evolution of the braided loop is accompanied
by localized heating regions as the loop relaxes. Factors that
may influence the observational signatures are discussed. Recent
high-resolution observations from Hi-C have claimed the first direct
evidence of braided magnetic fields in the corona. Here we show that
both the Hi-C data and some of our simulations give the appearance of
braiding at a range of scales.

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Title: Mini-CME eruptions in a flux emergence event in a coronal
hole environment
Authors: Galsgaard, K.; Moreno-Insertis, F.
2016usc..confE..64G Altcode:
Small scale jets are observed to take place at the interface between
the open magnetic field in coronal holes and bipolar magnetic field
concentrations. A fraction of these shows an eruptive behavior, where
a combination of cold dense and hot light plasma has been observed
to propagate out along the jet region, combining traditional jets
with what looks like the eruption of mini-CMEs. Here we discuss a
simple model scenario for the explosive energy release process that
leads to a mixture of hot and cold plasma being accelerated upwards
simultaneously. The model explains both the typical steady state
inverted-Y jet and the subsequent mini-CME eruptions found in blowout
jets. The numerical experiment consists of a buoyant unstable flux
rope that emerges into an overlying slanted coronal field, thereby
creating a bipolar magnetic field distribution in the photosphere
with coronal loops linking the polarities. Reconnection between the
emerged and preexisting magnetic systems including the launching of
a classical inverted-Y jet. The experiment shows that this simple
model provides for a very complicated dynamical behavior in its late
phases. Five independent mini-CME eruptions follow the initial near
steady-state jet phase. The first one is a direct consequence of the
reconnection of the emerged magnetic flux, is mediated by the formation
of a strongly sheared arcade followed by a tether-cutting reconnection
process, and leads to the eruption of a twisted flux rope. The final
four explosive eruptions, instead, are preceded by the formation of a
twisted torus-like flux rope near the strong magnetic concentrations
at the photosphere. As the tube center starts emerging an internal
current sheet is formed below it. This sheet experiences a tether
cutting process that provides the important upwards kick of the
newly formed mini-CME structure. As the fast rising cold and dense
tube interacts with the overlying magnetic field, it reconnects at
different spatial locations, either through a null region or through
a local strong shear region without nulls. The restructuring of the
magnetic field lines generate magneto-acoustic waves that transport
twist and cold plasma out along the less stressed parts of the newly
reconnected field lines. The emphasis of the talk will be on the
physical forces responsible for the initial flux tube rising and the
effects and reasons for the following destruction of the mini-CMEs.

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Title: Why Are Flare Ribbons Associated with the Spines of Magnetic
Null Points Generically Elongated?
Authors: Pontin, David; Galsgaard, Klaus; Démoulin, Pascal
2016SoPh..291.1739P Altcode: 2016arXiv160505704P; 2016SoPh..tmp..101P
Coronal magnetic null points exist in abundance, as demonstrated by
extrapolations of the coronal field, and have been inferred to be
important for a broad range of energetic events. These null points
and their associated separatrix and spine field lines represent
discontinuities of the field line mapping, making them preferential
locations for reconnection. This field line mapping also exhibits strong
gradients adjacent to the separatrix (fan) and spine field lines, which
can be analysed using the "squashing factor", Q . In this article we
analyse in detail the distribution of Q in the presence of magnetic
nulls. While Q is formally infinite on both the spine and fan of the
null, the decay of Q away from these structures is shown in general to
depend strongly on the null-point structure. For the generic case of a
non-radially-symmetric null, Q decays most slowly away from the spine or
fan in the direction in which |B | increases most slowly. In particular,
this demonstrates that the extended elliptical high-Q halo around the
spine footpoints observed by Masson et al. (Astrophys. J.700, 559,
2009) is a generic feature. This extension of the Q halos around the
spine or fan footpoints is important for diagnosing the regions of the
photosphere that are magnetically connected to any current layer that
forms at the null. In light of this, we discuss how our results can be
used to interpret the geometry of observed flare ribbons in circular
ribbon flares, in which typically a coronal null is implicated. We
conclude that both the physics in the vicinity of the null and how
this is related to the extension of Q away from the spine or fan can be
used in tandem to understand observational signatures of reconnection
at coronal null points.

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Title: Why are flare ribbons generically elongated in configurations
with magnetic null points?
Authors: Pontin, David Iain; Galsgaard, Klaus; Demoulin, Pascal
2016SPD....47.0625P Altcode:
Coronal magnetic null points exist in abundance as demonstrated by
extrapolations of the coronal field, and have been inferred to be
important for a broad range of energetic events. These null points
and their associated separatrix and spine field lines represent
discontinuities of the field line mapping, making them preferential
locations for reconnection in the corona. In addition, the field line
mapping in the vicinity of these null points exhibits strong gradients
as measured by the “squashing factor”, Q. We demonstrate that
the extension of the Q halos around the spine/fan footpoints is in
general important for diagnosing the regions of the photosphere that are
magnetically connected to any current layer that forms at the null. In
light of this, we discuss the extent to which our results can be used
to interpret the geometry of observed flare ribbons in events in which
a coronal null is implicated. We conclude that together the physics
in the vicinity of the null and how this is related to the extension
of Q away from the spine/fan can be used in tandem to understand
observational signatures of reconnection at coronal null points.

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Title: Active region upflows. I. Multi-instrument observations
Authors: Vanninathan, K.; Madjarska, M. S.; Galsgaard, K.; Huang,
Z.; Doyle, J. G.
2015A&A...584A..38V Altcode: 2015arXiv150905624V
Context. We study upflows at the edges of active regions, called AR
outflows, using multi-instrument observations. Aims: This study
intends to provide the first direct observational evidence of whether
chromospheric jets play an important role in furnishing mass that could
sustain coronal upflows. The evolution of the photospheric magnetic
field, associated with the footpoints of the upflow region and the
plasma properties of active region upflows is investigated with the
aim of providing information for benchmarking data-driven modelling of
this solar feature. Methods: We spatially and temporally combine
multi-instrument observations obtained with the Extreme-ultraviolet
Imaging Spectrometer on board the Hinode, the Atmospheric Imaging
Assembly and the Helioseismic Magnetic Imager instruments on board
the Solar Dynamics Observatory and the Interferometric BI-dimensional
Spectro-polarimeter installed at the National Solar Observatory, Sac
Peak, to study the plasma parameters of the upflows and the impact of
the chromosphere on active region upflows. Results: Our analysis
shows that the studied active region upflow presents similarly to
those studied previously, i.e. it displays blueshifted emission of 5-20
kms-1 in Fe xii and Fe xiii and its average electron density
is 1.8 × 109 cm-3 at 1 MK. The time variation
of the density is obtained showing no significant change (in a 3σ
error). The plasma density along a single loop is calculated revealing
a drop of 50% over a distance of ~20 000 km along the loop. We find a
second velocity component in the blue wing of the Fe xii and Fe xiii
lines at 105 kms-1 reported only once before. For the first
time we study the time evolution of this component at high cadence and
find that it is persistent during the whole observing period of 3.5 h
with variations of only ±15 kms-1. We also, for the first
time, study the evolution of the photospheric magnetic field at high
cadence and find that magnetic flux diffusion is responsible for the
formation of the upflow region. High cadence Hα observations are used
to study the chromosphere at the footpoints of the upflow region. We
find no significant jet-like (spicule/rapid blue excursion) activity
to account for several hours/days of plasma upflow. The jet-like
activity in this region is not continuous and blueward asymmetries
are a bare minimum. Using an image enhancement technique for imaging
and spectral data, we show that the coronal structures seen in the
AIA 193 Å channel are comparable to the EIS Fe xii images, while
images in the AIA 171 Å channel reveal additional loops that are a
result of contribution from cooler emission to this channel. 
Conclusions: Our results suggest that at chromospheric heights there
are no signatures that support the possible contribution of spicules
to active region upflows. We suggest that magnetic flux diffusion is
responsible for the formation of the coronal upflows. The existence of
two velocity components possibly indicates the presence of two different
flows, which are produced by two different physical mechanisms,
e.g. magnetic reconnection and pressure-driven jets. Movies
associated to Figs. A.1-A.3 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526340/olm">http://www.aanda.org</A>

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Title: Active region upflows. II. Data driven magnetohydrodynamic
modelling
Authors: Galsgaard, K.; Madjarska, M. S.; Vanninathan, K.; Huang,
Z.; Presmann, M.
2015A&A...584A..39G Altcode: 2015arXiv150905639G
Context. Observations of many active regions show a slow systematic
outflow/upflow from their edges lasting from hours to days. At present
no physical explanation has been proven, while several suggestions have
been put forward. Aims: This paper investigates one possible
method for maintaining these upflows assuming, that convective
motions drive the magnetic field to initiate them through magnetic
reconnection. Methods: We use Helioseismic and Magnetic Imager
(HMI) data to provide an initial potential 3D magnetic field of the
active region NOAA 11123 on 2010 November 13 where the characteristic
upflow velocities are observed. A simple 1D hydrostatic atmospheric
model covering the region from the photosphere to the corona is
derived. Local correlation tracking of the magnetic features in the
HMI data is used to derive a proxy for the time dependent velocity
field. The time dependent evolution of the system is solved using a
resistive 3D magnetohydrodynamic code. Results: The magnetic
field contains several null points located well above the photosphere,
with their fan planes dividing the magnetic field into independent
open and closed flux domains. The stressing of the interfaces between
the different flux domains is expected to provide locations where
magnetic reconnection can take place and drive systematic flows. In
this case, the region between the closed and open flux is identified
as the region where observations find the systematic upflows. 
Conclusions: In the present experiment, the driving only initiates
magneto-acoustic waves without driving any systematic upflows at any of
the flux interfaces. Movie is available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201526339/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: ADAHELI: exploring the fast, dynamic Sun in the x-ray, optical,
and near-infrared
Authors: Berrilli, Francesco; Soffitta, Paolo; Velli, Marco; Sabatini,
Paolo; Bigazzi, Alberto; Bellazzini, Ronaldo; Bellot Rubio, Luis
Ramon; Brez, Alessandro; Carbone, Vincenzo; Cauzzi, Gianna; Cavallini,
Fabio; Consolini, Giuseppe; Curti, Fabio; Del Moro, Dario; Di Giorgio,
Anna Maria; Ermolli, Ilaria; Fabiani, Sergio; Faurobert, Marianne;
Feller, Alex; Galsgaard, Klaus; Gburek, Szymon; Giannattasio, Fabio;
Giovannelli, Luca; Hirzberger, Johann; Jefferies, Stuart M.; Madjarska,
Maria S.; Manni, Fabio; Mazzoni, Alessandro; Muleri, Fabio; Penza,
Valentina; Peres, Giovanni; Piazzesi, Roberto; Pieralli, Francesca;
Pietropaolo, Ermanno; Martinez Pillet, Valentin; Pinchera, Michele;
Reale, Fabio; Romano, Paolo; Romoli, Andrea; Romoli, Marco; Rubini,
Alda; Rudawy, Pawel; Sandri, Paolo; Scardigli, Stefano; Spandre,
Gloria; Solanki, Sami K.; Stangalini, Marco; Vecchio, Antonio;
Zuccarello, Francesca
2015JATIS...1d4006B Altcode:
Advanced Astronomy for Heliophysics Plus (ADAHELI) is a project concept
for a small solar and space weather mission with a budget compatible
with an European Space Agency (ESA) S-class mission, including launch,
and a fast development cycle. ADAHELI was submitted to the European
Space Agency by a European-wide consortium of solar physics research
institutes in response to the "Call for a small mission opportunity
for a launch in 2017," of March 9, 2012. The ADAHELI project builds
on the heritage of the former ADAHELI mission, which had successfully
completed its phase-A study under the Italian Space Agency 2007 Small
Mission Programme, thus proving the soundness and feasibility of
its innovative low-budget design. ADAHELI is a solar space mission
with two main instruments: ISODY: an imager, based on Fabry-Pérot
interferometers, whose design is optimized to the acquisition of
highest cadence, long-duration, multiline spectropolarimetric images
in the visible/near-infrared region of the solar spectrum. XSPO: an
x-ray polarimeter for solar flares in x-rays with energies in the 15
to 35 keV range. ADAHELI is capable of performing observations that
cannot be addressed by other currently planned solar space missions,
due to their limited telemetry, or by ground-based facilities, due to
the problematic effect of the terrestrial atmosphere.

---------------------------------------------------------
Title: Explosive Events on a Subarcsecond Scale in IRIS Observations:
A Case Study
Authors: Huang, Zhenghua; Madjarska, Maria S.; Xia, Lidong; Doyle,
J. G.; Galsgaard, Klaus; Fu, Hui
2014ApJ...797...88H Altcode: 2014arXiv1409.6425H
We present a study of a typical explosive event (EE) at subarcsecond
scale witnessed by strong non-Gaussian profiles with blue- and
redshifted emission of up to 150 km s-1 seen in the
transition region Si IV 1402.8 Å, and the chromospheric Mg II
k 2796.4 Å and C II 1334.5 Å observed by the Interface Region
Imaging Spectrograph (IRIS) at unprecedented spatial and spectral
resolution. For the first time an EE is found to be associated with
very small-scale (~120 km wide) plasma ejection followed by retraction
in the chromosphere. These small-scale jets originate from a compact
bright-point-like structure of ~1.”5 size as seen in the IRIS 1330
Å images. SDO/AIA and SDO/HMI co-observations show that the EE lies
in the footpoint of a complex loop-like brightening system. The EE is
detected in the higher temperature channels of AIA 171 Å, 193 Å,
and 131 Å, suggesting that it reaches a higher temperature of log
T = 5.36 ± 0.06 (K). Brightenings observed in the AIA channels with
durations 90-120 s are probably caused by the plasma ejections seen
in the chromosphere. The wings of the C II line behave in a similar
manner to the Si IV'S, indicating close formation temperatures, while
the Mg II k wings show additional Doppler-shifted emission. Magnetic
convergence or emergence followed by cancellation at a rate of 5 ×
1014 Mx s-1 is associated with the EE region. The
combined changes of the locations and the flux of different magnetic
patches suggest that magnetic reconnection must have taken place. Our
results challenge several theories put forward in the past to explain
non-Gaussian line profiles, i.e., EEs. Our case study on its own,
however, cannot reject these theories; thus, further in-depth studies
on the phenomena producing EEs are required.

---------------------------------------------------------
Title: Explosive events in connection with small scale flux emergence
in open field regions
Authors: Galsgaard, Klaus; Moreno-Insertis, Fernando, , Prof
2014cosp...40E.930G Altcode:
In recent years observations have shown that the emergence of new
magnetic flux from the convection zone into the open field regions in
the corona may generate spectacular jet phenomena. A smaller number
of jets seem to end their near steady state phase in one or more
spectacular eruptions where material is accelerated away from the
solar surface reaching fairly high velocities. To investigate the jet
phenomena, we have conducted a number of numerical MHD experiments
that investigate the general interaction between an emerging bipolar
flux region and the open coronal magnetic field. Under the correct
conditions, this generates a well defined jet phase and the model
explains many of the general characteristics of the typical Eiffel tower
jets. Towards the end phase of the jet, the remains of the emerged flux
system may experience some violent eruptions. This talk will discuss
the general characteristics of these eruptions, aiming at providing an
explanation to why they occur, and how they develop in general. These
jets and eruptions may be what is taking place in some of the so-called
breakout models described in a number of recent observational papers.

---------------------------------------------------------
Title: Particle acceleration in complex current-sheet-populated
magnetic configurations
Authors: Galsgaard, Klaus; Nordlund, Aake
2014cosp...40E.931G Altcode:
In the MHD picture it has long been postulated that continued braiding
of an initially smooth and continuous magnetic fields will eventually
lead to the formation of localised current sheets. With a small amount
of magnetic resistivity the free magnetic energy will be released
through magnetic reconnection. Observations indicate that in such
processes up to 50% of the released energy is transported away from the
diffusion region by accelerated non-thermal particles. This physics
is not covered by the MHD approach, and therefore to investigate
the process of particle acceleration and its implications on
the dynamical evolution of current sheets, one needs to adopt an
approach that can handle this. Particle-in-cell simulations provide
one such tool. Typically one associates this type of simulations
with physics on a length scales that are very much smaller than the
characteristic length scale of the dynamical systems in the solar
atmosphere. But a different PIC approach can be adopted, where one
takes the result from a large scale MHD simulation and focuses on a
sub volume, of the experiment and exports it into a PIC simulation. We
have developed a technique to perform such mixed MHD/PIC simulations,
which allows us to investigate the dynamical evolution of the particles
in and around current sheets, showing clear differences from the MHD
picture. Information can be obtained on both the acceleration mechanism
and changes to the general particle distribution function.

---------------------------------------------------------
Title: On the Nature of Reconnection at a Solar Coronal Null Point
above a Separatrix Dome
Authors: Pontin, D. I.; Priest, E. R.; Galsgaard, K.
2013ApJ...774..154P Altcode: 2013arXiv1307.6874P
Three-dimensional magnetic null points are ubiquitous in the solar
corona and in any generic mixed-polarity magnetic field. We consider
magnetic reconnection at an isolated coronal null point whose fan
field lines form a dome structure. Using analytical and computational
models, we demonstrate several features of spine-fan reconnection
at such a null, including the fact that substantial magnetic flux
transfer from one region of field line connectivity to another can
occur. The flux transfer occurs across the current sheet that forms
around the null point during spine-fan reconnection, and there is no
separator present. Also, flipping of magnetic field lines takes place
in a manner similar to that observed in the quasi-separatrix layer or
slip-running reconnection.

---------------------------------------------------------
Title: Plasma Jets and Eruptions in Solar Coronal Holes: A
Three-dimensional Flux Emergence Experiment
Authors: Moreno-Insertis, F.; Galsgaard, K.
2013ApJ...771...20M Altcode: 2013arXiv1305.2201M
A three-dimensional (3D) numerical experiment of the launching of
a hot and fast coronal jet followed by several violent eruptions is
analyzed in detail. These events are initiated through the emergence
of a magnetic flux rope from the solar interior into a coronal
hole. We explore the evolution of the emerging magnetically dominated
plasma dome surmounted by a current sheet and the ensuing pattern
of reconnection. A hot and fast coronal jet with inverted-Y shape is
produced that shows properties comparable to those frequently observed
with EUV and X-ray detectors. We analyze its 3D shape, its inhomogeneous
internal structure, and its rise and decay phases, lasting for some
15-20 minutes each. Particular attention is devoted to the field line
connectivities and the reconnection pattern. We also study the cool
and high-density volume that appears to encircle the emerged dome. The
decay of the jet is followed by a violent phase with a total of five
eruptions. The first of them seems to follow the general pattern of
tether-cutting reconnection in a sheared arcade, although modified by
the field topology created by the preceding reconnection evolution. The
two following eruptions take place near and above the strong-field
concentrations at the surface. They show a twisted, Ω-loop-like rope
expanding in height, with twist being turned into writhe, thus hinting
at a kink instability (perhaps combined with a torus instability)
as the cause of the eruption. The succession of a main jet ejection
and a number of violent eruptions that resemble mini-CMEs and their
physical properties suggest that this experiment may provide a model
for the blowout jets recently proposed in the literature.

---------------------------------------------------------
Title: 3D Solar Null Point Reconnection MHD Simulations
Authors: Baumann, G.; Galsgaard, K.; Nordlund, Å.
2013SoPh..284..467B Altcode: 2012SoPh..tmp..291B; 2012arXiv1203.1018B; 2012SoPh..tmp..270B
Numerical MHD simulations of 3D reconnection events in the solar
corona have improved enormously over the last few years, not only
in resolution, but also in their complexity, enabling more and more
realistic modeling. Various ways to obtain the initial magnetic
field, different forms of solar atmospheric models as well as diverse
driving speeds and patterns have been employed. This study considers
differences between simulations with stratified and non-stratified solar
atmospheres, addresses the influence of the driving speed on the plasma
flow and energetics, and provides quantitative formulas for mapping
electric fields and dissipation levels obtained in numerical simulations
to the corresponding solar quantities. The simulations start out from
a potential magnetic field containing a null-point, obtained from a
Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager
(MDI) magnetogram magnetogram extrapolation approximately 8 hours
before a C-class flare was observed. The magnetic field is stressed
with a boundary motion pattern similar to - although simpler than -
horizontal motions observed by SOHO during the period preceding the
flare. The general behavior is nearly independent of the driving speed,
and is also very similar in stratified and non-stratified models,
provided only that the boundary motions are slow enough. The boundary
motions cause a build-up of current sheets, mainly in the fan-plane
of the magnetic null-point, but do not result in a flare-like energy
release. The additional free energy required for the flare could have
been partly present in non-potential form at the initial state, with
subsequent additions from magnetic flux emergence or from components
of the boundary motion that were not represented by the idealized
driving pattern.

---------------------------------------------------------
Title: MHD simulations of flux emergence in an open field region:
Jet formation and explosive events.
Authors: Galsgaard, Klaus; Moreno-Insertis, Fernando
2013enss.confE..32G Altcode:
The launch of Hinode in 2006 was the start of a new interest in the jet
phenomena in open field regions. Since then observations by the Hinode,
SDO and Stereo satellites have shown that one characteristic jet type
dominates, namely the so-called Eiffeltower or inverted-Y jet. The names
arise from the jet's appearance in X-ray, where they are seen having
two small "legs" below a long monolith structure representing the
jet. This structure is interpreted as the result of the interaction
between a newly emerged bipolar field into an unipolar magnetic
field region. This picture naturally leads to magnetic reconnection
between the two flux regions, where two the high velocity outflows
from the diffusion region forms both the long jet structure and the
underlying loop structure. To investigate this scenario in detail,
we have preformed new MHD experiments of the emergence of a magnetic
dipole region into an uniform open field region. The new experiments
represent a significant extension of both the domain size, the duration
of the experiment and the details of the analysis compared to the
one presented in Moreno-Insertis et al. 2008. We find the initial jet
phase to last on the order of 10 minutes, showing a smoothly evolving
structure which, for a part of the evolution, closely resemblance the
inverted-y structure. A number of characteristic structures arises
around the footpoint region of the loop that may be compared with
observations. Towards the end of this "steady state" inverted-y jet
phase, the amount of flux in the emerged bipolar region is being
exhausted by the reconnection process and the dynamical evolution
enters a new phase. In this phase we find five explosive eruption from
different parts of the remaining structure. These eruptions arises from
only three main areas of the emerged flux region, implying that the same
physical region can host repeated instabilities in the magnetic field.

---------------------------------------------------------
Title: Explaining fast ejections of plasma and exotic X-ray emission
from the solar corona
Authors: Roussev, Ilia I.; Galsgaard, Klaus; Downs, Cooper; Lugaz,
Noé; Sokolov, Igor V.; Moise, Elena; Lin, Jun
2012NatPh...8..845R Altcode:
Coronal mass ejections (CMEs) are the most energetic events in the solar
system and can make near-Earth space a hazardous place. However, there
is still no consensus as to what physical mechanisms are responsible
for these solar eruptions. Here we demonstrate a fundamental connection
between the emergence of magnetic flux into the solar atmosphere and
the formation of solar eruptions. We present a model of the dynamics
of the solar atmosphere and inner solar wind region using a realistic
representation of the electric field at the photosphere, calculated from
flux-emergence computer simulations, as the boundary conditions. From
this, we show how magnetic flux and helicity injection leads to the
reorganization of the solar corona. We show evidence for the in situ
formation of a CME plasmoid, which is independent of the emerging flux
tube, and we conclusively connect this process to the formation of a
hot X-ray structure.

---------------------------------------------------------
Title: Scaling of turbulent and hierarchical reconnection
Authors: Nordlund, A.; Galsgaard, K.
2012EGUGA..1412646N Altcode:
We investigate the relation between the theories and scaling formulae
for turbulent and hierarchical reconnection proposed by various authors;
Parker (1972, ApJ 174, 499; 1988, ApJ 330, 474), van Ballegooijen
(1986, ApJ 311, 1001), Galsgaard &amp; Nordlund (1996, JGR 101, 13445),
and Lazarian &amp; Vishniac (1999; ApJ 517, 700), considering also
the results of Lapenta (2008, PhRvL 100, 235001) and Bettarini &amp;
Lapenta (2010, A&amp;A 518, 57).

---------------------------------------------------------
Title: Current accumulation at an asymmetric 3D null point caused
by generic shearing motions
Authors: Galsgaard, K.; Pontin, D. I.
2011A&A...534A...2G Altcode: 2011arXiv1108.3304G
Context. Here we investigate the dynamical evolution of the reconnection
process at an initially linear 3D null point that is stressed by a
localised shear motion across the spine axis. The difference to previous
investigations is that the fan plane is not rotationally symmetric and
this allows for different behaviours depending on the alignment of the
fan plane relative to the imposed driver direction. Aims: The aim
is to show how the current accumulation and the associated reconnection
process at the non-axisymmetric null depends on the relative orientation
between the driver imposed stress across the spine axis of the null
and the main eigenvector direction in the fan plane. Methods:
The time evolution of the 3D null point is investigated solving
the 3D non-ideal MHD equations numerically in a Cartesian box. The
magnetic field is frozen to the boundaries and the boundary velocity
is only non-zero where the imposed driving for stressing the system is
applied. Results: The current accumulation is found to be along
the direction of the fan eigenvector associated with the smallest
eigenvalue until the direction of the driver is almost parallel to
this eigenvector. When the driving velocity is parallel to the weak
eigenvector and has an impulsive temporal profile the null only has
a weak collapse forming only a weak current layer. However, when the
null point is stressed continuously boundary effects dominates the
current accumulation. Conclusions: There is a clear relation
between the orientation of the current concentration and the direction
of the fan eigenvector corresponding to the small eigenvalue. This
shows that the structure of the magnetic field is the most important in
determining where current is going to accumulate when a single 3D null
point is perturbed by a simple shear motion across the spine axis. As
the angle between the driving direction and the strong eigenvector
direction increases, the current that accumulates at the null becomes
progressively weaker.

---------------------------------------------------------
Title: Generalised models for torsional spine and fan magnetic
reconnection
Authors: Pontin, D. I.; Al-Hachami, A. K.; Galsgaard, K.
2011A&A...533A..78P Altcode: 2011arXiv1105.2684P
Context. Three-dimensional (3D) null points are present in abundance
in the solar corona, and the same is likely to be true in other
astrophysical environments. Recent results from solar observations and
from simulations suggest that reconnection at such 3D nulls may play an
important role in the coronal dynamics. Aims: The properties of
the torsional spine and torsional fan modes of magnetic reconnection
at 3D nulls are investigated. New analytical models are developed,
which for the first time include a current layer that is spatially
localised around the null, extending along either the spine or the
fan of the null. The principal aim is to investigate the effect of
varying the degree of asymmetry of the null point magnetic field on
the resulting reconnection process - where previous studies always
considered a non-generic radially symmetric null. Methods:
Analytical solutions are derived for the steady kinematic equations,
and are compared with the results of numerical simulations in which
the full set of resistive MHD equations is solved. Results:
The geometry of the current layers within which torsional spine and
torsional fan reconnection occur is strongly dependent on the symmetry
of the magnetic field. Torsional spine reconnection occurs in a narrow
tube around the spine, with elliptical cross-section when the fan
eigenvalues are different. The eccentricity of the ellipse increases as
the degree of asymmetry increases, with the short axis of the ellipse
being along the strong field direction. The spatiotemporal peak current,
and the peak reconnection rate attained, are found not to depend
strongly on the degree of asymmetry. For torsional fan reconnection,
the reconnection occurs in a planar disk in the fan surface, which is
again elliptical when the symmetry of the magnetic field is broken. The
short axis of the ellipse is along the weak field direction, with the
current being peaked in these weak field regions. The peak current
and peak reconnection rate in this case are clearly dependent on the
asymmetry, with the peak current increasing but the reconnection rate
decreasing as the degree of asymmetry is increased.

---------------------------------------------------------
Title: 3D Magnetic Reconnection
Authors: Parnell, Clare E.; Maclean, Rhona C.; Haynes, Andrew L.;
Galsgaard, Klaus
2011IAUS..271..227P Altcode:
Magnetic reconnection is an important process that is prevalent in a
wide range of astrophysical bodies. It is the mechanism that permits
magnetic fields to relax to a lower energy state through the global
restructuring of the magnetic field and is thus associated with a range
of dynamic phenomena such as solar flares and CMEs. The characteristics
of three-dimensional reconnection are reviewed revealing how much
more diverse it is than reconnection in two dimensions. For instance,
three-dimensional reconnection can occur both in the vicinity of null
points, as well as in the absence of them. It occurs continuously and
continually throughout a diffusion volume, as opposed to at a single
point, as it does in two dimensions. This means that in three-dimensions
field lines do not reconnect in pairs of lines making the visualisation
and interpretation of three-dimensional reconnection difficult. By considering particular numerical 3D magnetohydrodynamic models of
reconnection, we consider how magnetic reconnection can lead to complex
magnetic topologies and current sheet formation. Indeed, it has been
found that even simple interactions, such as the emergence of a flux
tube, can naturally give rise to `turbulent-like' reconnection regions.

---------------------------------------------------------
Title: Steady state reconnection at a single 3D magnetic null point
Authors: Galsgaard, K.; Pontin, D. I.
2011A&A...529A..20G Altcode: 2011arXiv1102.2351G
 Aims: We systematically stress a rotationally symmetric 3D
magnetic null point by advecting the opposite footpoints of the spine
axis in opposite directions. This stress eventually concentrates in
the vicinity of the null point, thereby forming a local current sheet
through which magnetic reconnection takes place. The aim is to look
for a steady state evolution of the current sheet dynamics, which may
provide scaling relations for various characteristic parameters of the
system. Methods: The evolution is followed by solving numerically
the non-ideal MHD equations in a Cartesian domain. The null point is
embedded in an initially constant density and temperature plasma. Results: It is shown that a quasi-steady reconnection process can
be set up at a 3D null by continuous shear driving. It appears that
a true steady state is unlikely to be realised because the current
layer tends to grow until it is restricted by the geometry of the
computational domain and the imposed driving profile. However, ratios
between characteristic quantities clearly settle after some time to
stable values, so that the evolution is quasi-steady. The experiments
show a number of scaling relations, but they do not provide a clear
consensus for extending to lower magnetic resistivity or faster
driving velocities. More investigations are needed to fully clarify
the properties of current sheets at magnetic null points.

---------------------------------------------------------
Title: Stagger: MHD Method for Modeling Star Formation
Authors: Galsgaard, Klaus
2011ascl.soft05012G Altcode:
Stagger is an astrophysical MHD code actively used to model star
formation. It is equipped with a multi-frequency radiative transfer
module and a comprehensive equation of state module that includes a
large number of atomic and molecular species, to be able to compute
realistic 3-D models of the near-surface layers of stars. The current
version of the code allows a discretization that explicitly conserves
mass, momentum, energy, and magnetic flux. The tensor formulation
of the viscosity ensures that the viscous force is insensitive
to the coordinate system orientation, thereby avoiding artificial
grid-alignment.

---------------------------------------------------------
Title: Dynamics of braided coronal loops. II. Cascade to multiple
small-scale reconnection events
Authors: Pontin, D. I.; Wilmot-Smith, A. L.; Hornig, G.; Galsgaard, K.
2011A&A...525A..57P Altcode: 2010arXiv1003.5784P
 Aims: Our aim is to investigate the resistive relaxation of a
magnetic loop that contains braided magnetic flux but no net current
or helicity. The loop is subject to line-tied boundary conditions. We
investigate the dynamical processes that occur during this relaxation,
in particular the magnetic reconnection that occurs, and discuss the
nature of the final equilibrium. Methods: The three-dimensional
evolution of a braided magnetic field is followed in a series of
resistive MHD simulations. Results: It is found that, following
an instability within the loop, a myriad of thin current layers forms,
via a cascade-like process. This cascade becomes more developed and
continues for a longer period of time for higher magnetic Reynolds
number. During the cascade, magnetic flux is reconnected multiple times,
with the level of this “multiple reconnection” positively correlated
with the magnetic Reynolds number. Eventually the system evolves into a
state with no more small-scale current layers. This final state is found
to approximate a non-linear force-free field consisting of two flux
tubes of oppositely-signed twist embedded in a uniform background field.

---------------------------------------------------------
Title: On the Origin of Coronal Mass Ejections: How Does the Emergence
of a Magnetic Flux Rope Reorganize the Solar Corona?
Authors: Roussev, I. I.; Galsgaard, K.; Lugaz, N.; Sokolov, I.
2010AGUFMSH51C1698R Altcode:
The physical causes leading to the occurrence of Coronal Mass Ejections
(CMEs) on the Sun have been debated for almost four decades now. One
of the leading mechanisms suggests that a CME may occur as the result
of the emergence of a twisted magnetic flux rope from the convection
zone into the solar corona. This process have been investigated by a
number of researchers over the years, and it has been demonstrated that
an eruption of the coronal magnetic field can in principle occur. The
majority of these studies, however, involve some ad-hoc prescription of
the electric field at the photosphere resembling flux emergence, and
they neglect the ambient coronal magnetic field. In addition, most of
these flux-emergence simulations are performed in a Cartesian domain,
which extends only to a few dozen pressure scale-heights into the
corona. Thus, it is difficult to assess the role of boundary driving
and limited computational domain on the resulting evolution of the
erupting coronal magnetic field. In this paper, we present a new model
of CMEs that mitigates these two effects. To achieve this, we couple the
"local" magnetic-flux-emergence (MFE) model of Archontis et al. (2004)
with a global MHD model of the solar corona and solar wind. The model
coupling is performed using the Space Weather Modeling Framework. In
the coupled model, the MFE simulation provides time-dependent boundary
conditions for all MHD quantities into the global model, where the
physical coupling is done at the photospheric boundary. The physical
evolution of the system is followed using the BATS-R-US "ideal" MHD
code well beyond the complete emergence of the magnetic flux from the
convection zone. We discuss the dynamics of the flux emergence process
and the related response of the pre-existing coronal magnetic field
in the context of CME production.

---------------------------------------------------------
Title: On the Origin of Coronal Mass Ejections: How Does the Emergence
of a Magnetic Flux Rope Reorganize the Solar Corona?
Authors: Roussev, Ilia; Galsgaard, Klaus; Lugaz, Noe; Jacobs, Carla;
Sokolov, Igor
2010EGUGA..12.3724R Altcode:
The physical effects responsible for the occurrence of Coronal Mass
Ejections (CMEs) on the Sun have been debated for almost four decades
now. One of the leading mechanisms suggests that a CME may occur as
the result of the emergence of a twisted magnetic flux rope from
the convection zone into the solar corona. This process has been
investigated by a number of researchers over the years, and it has
been demonstrated that an eruption of the coronal magnetic field can in
principle occur. The majority of these studies, however, involve some
ad-hoc prescription of the electric field at the photosphere resembling
flux emergence, and they neglect the ambient coronal magnetic field. In
addition, most of these flux-emergence simulations are performed in
a Cartesian domain, which extends into the corona up to only a few
dozen pressure scale-heights. Because of this, it is difficult to
assess how strongly the ad-hoc character of the driving motions and
the limited computational domain affect the simulation results for
the evolution of the erupting coronal magnetic field. In this paper,
we present a new model of CMEs that mitigates these two effects. To
achieve this, we couple the "local" magnetic-flux-emergence (MFE) model
of Archontis et al. (2004) with a global MHD model of the solar corona
and solar wind. The model coupling is performed using the Space Weather
Modeling Framework. In the coupled model, the MFE simulation provides
time-dependent boundary conditions for all MHD quantities into the
global model, where the physical coupling is done at the photospheric
boundary. The physical evolution of the system is followed using the
BATS-R-US "ideal" MHD code well beyond the complete emergence of the
magnetic flux from the convection zone. We discuss the dynamics of the
flux emergence process and the related response of the pre-existing
coronal magnetic field in the context of CME production.

---------------------------------------------------------
Title: Test particle acceleration in a numerical MHD experiment of
an anemone jet
Authors: Rosdahl, K. J.; Galsgaard, K.
2010A&A...511A..73R Altcode: 2010arXiv1002.1983R
 Aims: To use a 3D numerical MHD experiment representing magnetic
flux emerging into an open field region as a background field for
tracing charged particles. The interaction between the two flux
systems generates a localised current sheet where MHD reconnection
takes place. We investigate how efficiently the reconnection region
accelerates charged particles and what kind of energy distribution
they acquire. Methods: The particle tracing is done numerically
using the Guiding Center Approximation on individual data sets from the
numerical MHD experiment. Results: We derive particle and implied
photon distribution functions having power law forms, and look at the
impact patterns of particles hitting the photosphere. We find that
particles reach energies far in excess of those seen in observations
of solar flares. However the structure of the impact region in the
photosphere gives a good representation of the topological structure of
the magnetic field. Three movies are only available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Structure of magnetic separators and separator reconnection
Authors: Parnell, C. E.; Haynes, A. L.; Galsgaard, K.
2010JGRA..115.2102P Altcode: 2010JGRA..11502102P
Magnetic separators are important locations of three-dimensional
magnetic reconnection. They are field lines that lie along the edges
of four flux domains and represent the intersection of two separatrix
surfaces. Since the intersection of two surfaces produces an X-type
structure, when viewed along the line of intersection, the global
three-dimensional topology of the magnetic field around a separator
is hyperbolic. It is therefore usually assumed that the projection
of the magnetic field lines themselves onto a two-dimensional plane
perpendicular to a separator is also hyperbolic in nature. In this
paper, we use the results of a three-dimensional MHD experiment
of separator reconnection to show that, in fact, the projection
of the magnetic field lines in a cut perpendicular to a separator
may be either hyperbolic or elliptic and that the structure of the
magnetic field projection may change in space, along the separator,
as well as in time, during the life of the separator. Furthermore,
in our experiment, we find that there are both spatial and temporal
variations in the parallel component of current (and electric field)
along the separator, with all high parallel current regions (which
are associated with reconnection) occurring between counterrotating
flow regions. Importantly, reconnection occurs not only at locations
where the structure of the projected perpendicular magnetic field is
hyperbolic but also where it is elliptic.

---------------------------------------------------------
Title: Is Null-Point Reconnection Important for Solar Flux Emergence?
Authors: Maclean, R. C.; Parnell, C. E.; Galsgaard, K.
2009SoPh..260..299M Altcode: 2009arXiv0910.0368M
The role of null-point reconnection in a three-dimensional
numerical magnetohydrodynamic (MHD) model of solar emerging flux
is investigated. The model consists of a twisted magnetic flux
tube rising through a stratified convection zone and atmosphere to
interact and reconnect with a horizontal overlying magnetic field
in the atmosphere. Null points appear as the reconnection begins and
persist throughout the rest of the emergence, where they can be found
mostly in the model photosphere and transition region, forming two
loose clusters on either side of the emerging flux tube. Up to 26
nulls are present at any one time, and tracking in time shows that
there is a total of 305 overall, despite the initial simplicity of
the magnetic field configuration. We find evidence for the reality
of the nulls in terms of their methods of creation and destruction,
their balance of signs, their long lifetimes, and their geometrical
stability. We then show that due to the low parallel electric fields
associated with the nulls, null-point reconnection is not the main
type of magnetic reconnection involved in the interaction of the newly
emerged flux with the overlying field. However, the large number of
nulls implies that the topological structure of the magnetic field must
be very complex and the importance of reconnection along separators
or separatrix surfaces for flux emergence cannot be ruled out.

---------------------------------------------------------
Title: X-Ray Jets in Coronal Holes: Numerical Simulation and Hinode
Observations
Authors: Moreno-Insertis, F.; Galsgaard, K.; Ugarte-Urra, I.
2009ASPC..415...51M Altcode:
We report on our recent 3D numerical models of the launching of hot,
high-speed jets in a coronal hole following the emergence of magnetized
plasma from the solar interior. As part of the same research, we have
also analyzed Hinode (EIS and XRT) and Soho-MDI observational data
of an actual process of flux emergence followed by jet launching in
a coronal hole. From the observations, we reconstruct the magnetic
topology at the emergence site and calculate velocity and further
physical properties of the observed event. The 3D model was calculated
for realistic conditions in a coronal hole, including, in particular,
a low-density (108 particles cm-3), high Alfven
speed plasma prior to the emergence. After emergence, a ribbon-like
current sheet is created at the site of collision of the emerging and
preexisting magnetic systems. Field line reconnection ensues, which
leads to the ejection of the X-Ray jet. We analyze the global magnetic
topology, and the temperature, velocity and current distribution in
the 3D experiment. The numerical results provide a good match to the
observed features of the coronal hole jets. This is meant regarding
both our own observational results as well as the ranges and average
values of the statistical study by Savcheva et al. (2007).

---------------------------------------------------------
Title: The emergence of toroidal flux tubes from beneath the solar
photosphere
Authors: Hood, A. W.; Archontis, V.; Galsgaard, K.; Moreno-Insertis, F.
2009A&A...503..999H Altcode:
Context: Models of flux emergence frequently use a twisted cylindrical
loop as the initial starting configuration and ignore the coupling
between the radiation field and plasma. In these models, the axis of
the original tube never emerges through the photosphere. Without the
axis emerging, it is very difficult to form a realistic sunspot. Aims: The aim is to use a toroidal flux loop, placed beneath the solar
photosphere and study whether the axis of the system emerges fully into
the atmosphere. The toroidal curvature means that the plasma can drain
more effectively than in a straight cylindrical tube. Methods:
Three-dimensional magnetohydrodynamic numerical simulations of an
emerging magnetic flux tube are presented for an initial toroidal loop
model. The simulations use a Lagrangian-Remap code that is particularly
suited to dealing with shocks and strong current sheets. Results:
The evolution of the toroidal loop is followed and the characteristics
of the emergence process are compared with the traditional cylindrical
loops. The flux sources seen at the photosphere are more circular,
and there are less shearing motions in the upper photosphere. When the
initial magnetic field strength is relatively weak the evolution of the
system is similar to the cylindrical loop case, with the formation of
a new flux rope above the photosphere. A striking result is that for
large values of field strength the axial field of the toroidal loop
emerges fully into the corona. This is reported for the first time in
experiments of flux emergence in a highly stratified atmosphere that do
not solve self-consistently the radiation transfer problem. In addition,
the new flux rope forms below the original axis of the toroidal tube
when the field strength is sufficiently strong.

---------------------------------------------------------
Title: Magnetic Flux Emergence and Jet Formation in Coronal Holes
Authors: Galsgaard, K.; Moreno-Insertis, F.
2008ESPM...12.3.27G Altcode:
Recent observations of coronal holes with Hinode show with unprecedented
detail the launching of fast and hot jets. Many of these jets are found
to coincide with the emergence of new magnetic flux, and it is generally
assumed that the jets are initiated by magnetic reconnection between
the new emerging flux and the existing open magnetic field. Further
to this a comparison of a larger sample of jets show that about 70%
of these are followed by the formation of plumes within minutes
to an hour. How do we understand these events from a physical
point of view? To investigate this we have carried out numerical 3D
MHD experiment modeling the emergence of magnetic flux from the upper
convection zone into an open magnetic flux region resembling a coronal
hole. The emergence process drives the formation of a strong and highly
localised current sheet. Time-dependent reconnection in the current
sheet gives rise to a high-velocity jet that eventually flows along the
previously open coronal field lines. Initially the jet has transition
region temperature, but as time progresses it eventually exceeds
the coronal temperature in the model. Investigating the development
of the structure of the magnetic field, it is found that it changes
in a very characteristic way, leading to a horizontal drift of the
jet. The experiment also shows how the reconnection speed influences the
dynamical properties of both the jet parameters and the evolution of the
underlying magnetic structure. Towards the end of the experiment the jet
speed decreases and leaves a large funnel-like region above the emerging
flux domain with an enhanced temperature and density distribution.

---------------------------------------------------------
Title: Recursive Reconnection and Magnetic Skeletons
Authors: Parnell, C. E.; Haynes, A. L.; Galsgaard, K.
2008ApJ...675.1656P Altcode:
By considering a simple driven model involving the resistive 3D MHD
interaction of magnetic sources, it is shown that it is essential
to know the magnetic skeleton to determine (1) the locations of
reconnection, (2) type of reconnection, (3) the rate of reconnection,
and (4) how much reconnection is occurring. In the model, two
opposite-polarity magnetic fragments interact in an overlying magnetic
field with reconnection, first closing and then opening the magnetic
field from the sources. There are two main reconnection phases: the
first has one reconnection site at which the flux is closed, and the
second has three sites. The latter is a hybrid case involving both
closing and reopening reconnection processes. Each reconnection site
coincides with its own separator, and hence all reconnection is via
separator reconnection. All the separators connect the same two nulls
and thus mark the intersection between the same four types of flux
domain. In the hybrid state, the two competing reconnection processes
(which open and close flux connecting the same two source pairs)
run simultaneously, leading to recursive reconnection. That is, the
same flux may be closed and then reopened not just once, but many
times. This leads to two interesting consequences: (1) the global
reconnection rate is enhanced and (2) heating occurs for a longer
period and over a wider area than in the single-separator case.

---------------------------------------------------------
Title: Jets in Coronal Holes: Hinode Observations and
Three-dimensional Computer Modeling
Authors: Moreno-Insertis, F.; Galsgaard, K.; Ugarte-Urra, I.
2008ApJ...673L.211M Altcode: 2007arXiv0712.1059M
Recent observations of coronal hole areas with the XRT and
EIS instruments on board the Hinode satellite have shown with
unprecedented detail the launching of fast, hot jets away from the
solar surface. In some cases these events coincide with episodes
of flux emergence from beneath the photosphere. In this Letter we
show results of a three-dimensional numerical experiment of flux
emergence from the solar interior into a coronal hole and compare
them with simultaneous XRT and EIS observations of a jet-launching
event that accompanied the appearance of a bipolar region in MDI
magnetograms. The magnetic skeleton and topology that result in the
experiment bear a strong resemblance to linear force-free extrapolations
of the SOHO/MDI magnetograms. A thin current sheet is formed at the
boundary of the emerging plasma. A jet is launched upward along the
open reconnected field lines with values of temperature, density,
and velocity in agreement with the XRT and EIS observations. Below the
jet, a split-vault structure results with two chambers: a shrinking one
containing the emerged field loops and a growing one with loops produced
by the reconnection. The ongoing reconnection leads to a horizontal
drift of the vault-and-jet structure. The timescales, velocities, and
other plasma properties in the experiment are consistent with recent
statistical studies of this type of event made with Hinode data.

---------------------------------------------------------
Title: The Effect of the Relative Orientation between the Coronal
Field and New Emerging Flux. I. Global Properties
Authors: Galsgaard, K.; Archontis, V.; Moreno-Insertis, F.; Hood, A. W.
2007ApJ...666..516G Altcode: 2007arXiv0705.1097G
The emergence of magnetic flux from the convection zone into the
corona is an important process for the dynamical evolution of the
coronal magnetic field. In this paper we extend our previous numerical
investigations, by looking at the process of flux interaction as an
initially twisted flux tube emerges into a plane-parallel, coronal
magnetic field. Significant differences are found in the dynamical
appearance and evolution of the emergence process depending on the
relative orientation between the rising flux system and any preexisting
coronal field. When the flux systems are nearly antiparallel, the
experiments show substantial reconnection and demonstrate clear
signatures of a high-temperature plasma located in the high-velocity
outflow regions extending from the reconnection region. However, the
cases that have a more parallel orientation of the flux systems show
very limited reconnection and none of the associated features. Despite
the very different amount of reconnection between the two flux systems,
it is found that the emerging flux that is still connected to the
original tube reaches the same height as a function of time. As a
compensation for the loss of tube flux, a clear difference is found
in the extent of the emerging loop in the direction perpendicular to
the main axis of the initial flux tube. Increasing amounts of magnetic
reconnection decrease the volume, which confines the remaining tube
flux.

---------------------------------------------------------
Title: Current sheet formation and nonideal behavior at
three-dimensional magnetic null points
Authors: Pontin, D. I.; Bhattacharjee, A.; Galsgaard, K.
2007PhPl...14e2106P Altcode: 2007astro.ph..1462P
The nature of the evolution of the magnetic field, and of current
sheet formation, at three-dimensional (3D) magnetic null points is
investigated. A kinematic example is presented that demonstrates that
for certain evolutions of a 3D null (specifically those for which the
ratios of the null point eigenvalues are time-dependent), there is no
possible choice of boundary conditions that renders the evolution of
the field at the null ideal. Resistive magnetohydrodynamics simulations
are described that demonstrate that such evolutions are generic. A
3D null is subjected to boundary driving by shearing motions, and it
is shown that a current sheet localized at the null is formed. The
qualitative and quantitative properties of the current sheet are
discussed. Accompanying the sheet development is the growth of a
localized parallel electric field, one of the signatures of magnetic
reconnection. Finally, the relevance of the results to a recent theory
of turbulent reconnection is discussed.

---------------------------------------------------------
Title: Current sheets at three-dimensional magnetic nulls: Effect
of compressibility
Authors: Pontin, D. I.; Bhattacharjee, A.; Galsgaard, K.
2007PhPl...14e2109P Altcode: 2007physics...1197P
The nature of current sheet formation in the vicinity of
three-dimensional (3D) magnetic null points is investigated. The
particular focus is upon the effect of the compressibility of the plasma
on the qualitative and quantitative properties of the current sheet. An
initially potential 3D null is subjected to shearing perturbations, as
in a previous paper [Pontin et al., Phys. Plasmas 14, 052106 (2007)]. It
is found that as the incompressible limit is approached, the collapse
of the null point is suppressed and an approximately planar current
sheet aligned to the fan plane is present instead. This is the case
regardless of whether the spine or fan of the null is sheared. Both the
peak current and peak reconnection rate are reduced. The results have a
bearing on previous analytical solutions for steady-state reconnection
in incompressible plasmas, implying that fan current sheet solutions
are dynamically accessible, while spine current sheet solutions are not.

---------------------------------------------------------
Title: Magnetohydrodynamic evolution of magnetic skeletons
Authors: Haynes, A. L.; Parnell, C. E.; Galsgaard, K.; Priest, E. R.
2007RSPSA.463.1097H Altcode: 2007astro.ph..2604H
The heating of the solar corona is probably due to reconnection of the
highly complex magnetic field that threads throughout its volume. We
have run a numerical experiment of an elementary interaction between
the magnetic field of two photospheric sources in an overlying field
that represents a fundamental building block of the coronal heating
process. The key to explaining where, how and how much energy is
released during such an interaction is to calculate the resulting
evolution of the magnetic skeleton. A skeleton is essentially the web of
magnetic flux surfaces (called separatrix surfaces) that separate the
coronal volume into topologically distinct parts. For the first time,
the skeleton of the magnetic field in a three-dimensional numerical
magnetohydrodynamic experiment is calculated and carefully analysed,
as are the ways in which it bifurcates into different topologies. A
change in topology normally changes the number of magnetic reconnection
sites. In our experiment, the magnetic field evolves through
a total of six distinct topologies. Initially, no magnetic flux
joins the two sources. Then, a new type of bifurcation, called a
global double-separator bifurcation, takes place. This bifurcation
is probably one of the main ways in which new separators are created
in the corona (separators are field lines at which three-dimensional
reconnection takes place). This is the first of five bifurcations
in which the skeleton becomes progressively more complex before
simplifying. Surprisingly, for such a simple initial state, at the peak
of complexity there are five separators and eight flux domains present.

---------------------------------------------------------
Title: Current amplification and magnetic reconnection at a
three-dimensional null point: Physical characteristics
Authors: Pontin, D. I.; Galsgaard, K.
2007JGRA..112.3103P Altcode: 2007astro.ph..1555P; 2007JGRA..11203103P
The behavior of magnetic perturbations of an initially potential
three-dimensional equilibrium magnetic null point is investigated. The
basic components which constitute a typical disturbance are taken
to be rotations and shears, in line with previous work. The spine
and fan of the null point (the field lines which asymptotically
approach or recede from the null) are subjected to such rotational
and shear perturbations, using three-dimensional magnetohydrodynamic
simulations. It is found that rotations of the fan plane and about the
spine lead to current sheets which are spatially diffuse in at least
one direction and form in the locations of the spine and fan. However,
shearing perturbations lead to 3-D-localized current sheets focused
at the null point itself. In addition, rotations are associated with a
growth of current parallel to the spine, driving rotational flows and
a type of rotational reconnection. Shears, on the other hand, cause
a current through the null which is parallel to the fan plane and are
associated with stagnation-type flows and field line reconnection across
both the spine and fan. The importance of the parallel electric field,
and its meaning as a reconnection rate, are discussed.

---------------------------------------------------------
Title: Current Sheet Formation and Magnetic Reconnection at 3D
Null Points
Authors: Pontin, D. I.; Bhattacharjee, A.; Galsgaard, K.
2006AGUFMSH33B0407P Altcode:
The evolution of the magnetic field in the vicinity of a single
isolated three-dimensional magnetic null point is discussed. While
magnetic reconnection at separator lines joining two such nulls is
thought to occur in many situations in the Earth's magnetosphere and
the Solar corona, the importance of the nulls themselves is poorly
understood. Reconnection at an isolated 3D null is also thought to be
important in some solar flares, and is involved in models of magnetic
breakout. We present numerical and analytical results on current sheet
formation at such a 3D null. Under steady boundary driving the current
sheet which forms at the null grows steadily in both intensity and
dimensions, indicating that its nature is that of a Sweet-Parker current
sheet. The qualitative and quantitative properties of the current
sheet with respect to the driving parameters and plasma parameters
are discussed. The nature of current sheet formation turns out to be
strongly dependent on the compressibility of the plasma, which is highly
relevant for comparing to earlier analytical models. Accompanying the
current growth is the development of a component of the electric field
parallel to the magnetic field, a signal of the breakdown of ideal
MHD and of magnetic reconnection. This work is supported by the NSF
and the DOE.

---------------------------------------------------------
Title: 3D simulations identifying the effects of varying the twist
and field strength of an emerging flux tube
Authors: Murray, M. J.; Hood, A. W.; Moreno-Insertis, F.; Galsgaard,
K.; Archontis, V.
2006A&A...460..909M Altcode:
Aims.We investigate the effects of varying the magnetic field strength
and the twist of a flux tube as it rises through the solar interior
and emerges into the atmosphere. Methods: .Using a 3D numerical
MHD code, we consider a simple stratified model, comprising of one
solar interior layer and three overlying atmospheric layers. We set
a horizontal, twisted flux tube in the lowest layer. The specific
balance of forces chosen results in the tube being fully buoyant and
the temperature is decreased in the ends of the tube to encourage the
formation of an Ω-shape along the tube's length. We vary the magnetic
field strength and twist independently of each other so as to give clear
results of the individual effects of each parameter. Results:
.We find a self-similar evolution in the rise and emergence of the flux
tube when the magnetic field strength of the tube is modified. During
the rise through the solar interior, the height of the crest and
axis, the velocity of the crest and axis, and the decrease in the
magnetic field strength of the axis of the tube are directly dependent
upon the initial magnetic field strength given to the tube. No such
self-similarity is evident when the twist of the flux tube is changed,
due to the complex interaction of the tension force on the rise of the
tube. For low magnetic field strength and twist values, we find that
the tube cannot fully emerge into the atmosphere once it reaches the
top of the interior since the buoyancy instability criterion cannot
be fulfilled. For those tubes that do advance into the atmosphere,
when the magnetic field strength has been modified, we find further
self-similar behaviour in the amount of tube flux transported into
the atmosphere. For the tubes that do emerge, the variation in the
twist results in the buoyancy instability, and subsequent emergence,
occurring at different locations along the tube's length.

---------------------------------------------------------
Title: Numerical modelling of 3D reconnection. II. Comparison between
rotational and spinning footpoint motions
Authors: De Moortel, I.; Galsgaard, K.
2006A&A...459..627D Altcode:
The coronal magnetic field is constantly subjected to a variety of
photospheric, footpoint motions, leading to the build up, and subsequent
release, of magnetic energy. Two different types of footpoint motions
are considered here, namely (large scale) rotating and (small scale)
spinning, using 3D numerical MHD simulations. The initial model consists
of two aligned, thin flux tubes, which are forced to interact due to
the boundary driving of the footpoints. Two variations of this setup
are studied, namely with and without an additional, constant, background
magnetic field. The nature of the boundary motions determines the shape
of the central current sheet, the driving force of the reconnection
process, as well as the efficiency of the build up of quasi-separatrix
layers (when B_bg ≠ 0). The reconnection process is more efficient for
the rotating of the flux sources and when a background magnetic field
is added. In general, heating due to large and small scale motions is
of comparable magnitude when no background field is present. However,
with an additional background magnetic field, heating due to small
scale footpoint motions seems substantially more efficient.

---------------------------------------------------------
Title: Three-dimensional Plasmoid Evolution in the Solar Atmosphere
Authors: Archontis, V.; Galsgaard, K.; Moreno-Insertis, F.; Hood, A. W.
2006ApJ...645L.161A Altcode:
We present clear evidence of the formation of three-dimensional (3D)
plasmoids in the current sheet between two magnetic flux systems in a
3D numerical experiment of flux emergence into the solar atmosphere and
study their properties and time evolution. Plasmoids are most likely
the result of resistive tearing mode instabilities. They adopt the
shape of a solenoid contained within the current sheet: the solenoid
is tightly wound when the field in the two flux systems is close to
antiparallel. The plasmoids are expelled to the sides of the sheet as
a result of a reconnection imbalance between the two x-lines on their
sides. We show the complex, 3D field line geometry in various plasmoids:
individual plasmoid field lines have external linkages to the flux
system on either side of the current sheet; we also find field lines
that go through a few plasmoids in succession, probably indicating
that the field line has resulted from multiple reconnection events.

---------------------------------------------------------
Title: Particle Acceleration in a Three-Dimensional Model of
Reconnecting Coronal Magnetic Fields
Authors: Cargill, Peter J.; Vlahos, Loukas; Turkmani, Rim; Galsgaard,
Klaus; Isliker, Heinz
2006SSRv..124..249C Altcode: 2006SSRv..tmp..111C
Particle acceleration in large-scale turbulent coronal magnetic fields
is considered. Using test particle calculations, it is shown that
both cellular automata and three dimensional MHD models lead to the
production of relativistic particles on sub-second timescales with
power law distribution functions. In distinction with the monolithic
current sheet models for solar flares, particles gain energy by multiple
interactions with many current sheets. Difficulties that need to be
addressed, such as feedback between particle acceleration and MHD,
are discussed.

---------------------------------------------------------
Title: Numerical modelling of 3D reconnection due to rotational
footpoint motions
Authors: De Moortel, I.; Galsgaard, K.
2006A&A...451.1101D Altcode:
The rapid dynamical evolution of the photospheric magnetic carpet
provides a large energy source for the solar corona. In this context,
the role of 3D magnetic reconnection is crucial in releasing the free
magnetic energy, build up due to the continuous footpoint motions. To
understand the processes by which this can take place, we have to obtain
a better understanding of the basic reconnection process that can take
place in 3D magnetic field configurations. In this paper, we investigate
magnetic reconnection, driven by rotational footpoint motions, using 3D
numerical MHD simulations. The model consists of two positive and two
negative sources, which are placed symmetrically on opposite boundaries
of the cubic domain. The initially potential fluxtubes are forced to
interact by the rotational driving of the flux concentrations on the
boundaries. We consider two variations of this setup, namely with
and without an additional, constant, background magnetic field. In
the no-background case, the magnetic connectivity is divided into
independent regions by separatrix surfaces, while the case with a
background field is represented by one global connectivity region. The
dynamical evolution is followed and found to differ significantly from
the comparable potential evolution. Strong currents are concentrated
along separatrix surfaces or rapidly developing quasi-separatrix
layers (QSLs). Investigating the reconnection rates of the systems
shows that the stronger the background field is, the more efficient
the reconnection process of the flux in the respective fluxtubes.

---------------------------------------------------------
Title: Reconnection and Non-Ideal Behaviour at 3D Magnetic Null Points
Authors: Pontin, David; Bhattacharjee, A.; Galsgaard, K.
2006SPD....37.1007P Altcode: 2006BAAS...38R.238P
The evolution of the magnetic field in the vicinity of three-dimensional
magnetic null points---thought to be present in abundance in the
complex field of the Solar corona---is discussed, with reference to
the possibility that reconnection might occur there. It is shown that
in the framework of ideal MHD, certain evolutions of the null point
are prohibited, specifically, evolutions which cause the ratios of
the null point eigenvalues to change in time. Particular analytical
kinematic examples are discussed which demonstrate that in the ideal
limit, physical quantities are not smooth at the null point spine and
fan when such an evolution occurs. Simulations of the full resistive
MHD equations are then presented. The simulations demonstrate that
typical perturbations of a 3D magnetic null point inevitably cause
the null point to evolve in the very way that is excluded under the
ideal evolution. It is demonstrated that the changing eigenvalue ratio
is linked to a growth of electic current, as well as a component of
the electric field parallel to the magnetic field, at the null. This
parallel electric field is a signal of the breakdown of ideal MHD,
and of magnetic reconnection. Implications for coronal heating will
be discussed. This work is supported by the NSF and the DOE.

---------------------------------------------------------
Title: Particle acceleration in stochastic current sheets in stressed
coronal active regions
Authors: Turkmani, R.; Cargill, P. J.; Galsgaard, K.; Vlahos, L.;
Isliker, H.
2006A&A...449..749T Altcode:
Aims.To perform numerical experiments of particle acceleration in the
complex magnetic and electric field environment of the stressed solar
corona.Methods.The magnetic and electric fields are obtained from a
3-D MHD experiment that resembles a coronal loop with photospheric
regions at both footpoints. Photospheric footpoint motion leads to
the formation of a hierarchy of stochastic current sheets. Particles
(protons and electrons) are traced within these current sheets
starting from a thermal distribution using a relativistic test particle
code.Results.In the corona the particles are subject to acceleration
as well as deceleration, and a considerable portion of them leave the
domain having received a net energy gain. Particles are accelerated to
high energies in a very short time (both species can reach energies
up to 100 GeV within 5 × 10-2 s for electrons and 5
× 10-1 s for protons). The final energy distribution
shows that while one quarter of the particles retain their thermal
distribution, the rest have been accelerated, forming a two-part power
law. Accelerated particles are either trapped within electric field
regions of opposite polarities, or escape the domain mainly through
the footpoints. The particle dynamics are followed in detail and it
is shown how this dynamic affects the time evolution of the system
and the energy distribution. The scaling of these results with time
and length scale is examined and the Bremstrahlung signature of X-ray
photons resulting from escaping particles hitting the chromosphere is
calculated and found to have a main power law part with an index γ =
- 1.8, steeper than observed. Possible resolutions of this discrepency
are discussed.

---------------------------------------------------------
Title: 3D Numerical Simulations of Coronal Tectonics
Authors: De Moortel, I.; Galsgaard, K.
2006IAUS..233..149D Altcode:
We present the results of numerical simulations of 3D magnetic
reconnection driven by photospheric footpoint motions. The model
consists of two positive and two negative sources, which are placed
on opposite boundaries of the cubic domain. Two different types
of photospheric motions are then considered, namely rotating and
twisting of the sources. These different footpoint motions result in a
difference in the evolution of the magnetic skeleton and the location
and efficiency of the energy build up. Both the dynamical evolution and
the corresponding potential evolution of each system is investigated
and a comparison is made between the energy storage and release that
occurs at separators and separatrix surfaces.

---------------------------------------------------------
Title: Flux emergence and interaction with a coronal field: 3D
MHD simulations
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A. W.
2006IAUS..233...53A Altcode:
The dynamic process of magnetic flux emergence from the solar interior
to the outer atmosphere may well be related with eruptive phenomena
and intense events of the Solar activity. However, the physics of the
emergence is not still well understood. Thus, we have performed 3D MHD
simulations to study the rising motion of a twisted flux tube from the
convection zone of the Sun and its interaction with a preexisting
coronal magnetic field. The results show that the reconnection
process depends criticaly on the initial relative orientation between
the two magnetic flux systems into contact. On the other hand, the
overal process of emergence depends mostly on the dynamics of the
sub-photospheric plasma.

---------------------------------------------------------
Title: Coronal heating Numerical experiments
Authors: Galsgaard, K.
2006AdSpR..37.1323G Altcode:
The heating of the corona has been considered a general problem in
solar physics for many years. Consensus today is that the convective
motions in the photosphere stress the magnetic field that reaches into
the corona. As the field is stressed, it somehow releases the free
magnetic energy which then heats the plasma. Here, we discuss various
numerical approaches that attempt to uncover the dynamical energy
release process. This leads to the “complete” model experiments,
where attempts to simulate the photosphere to coronal environment have
been conducted. These show that present days numerical models are
capable of capturing the basic process of the complicated magnetic
interaction between the photosphere and corona. The model provides
result that compares favourably with observational data from this
domain of the Sun. The concept of the coronal heating is therefore
proven, while many details still have to be improved before getting
a perfect match between observations and model.

---------------------------------------------------------
Title: The Three-dimensional Interaction between Emerging Magnetic
Flux and a Large-Scale Coronal Field: Reconnection, Current Sheets,
and Jets
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A. W.
2005ApJ...635.1299A Altcode:
Using MHD numerical experiments in three dimensions, we study the
emergence of a bipolar magnetic region from the solar interior
into a model corona containing a large-scale, horizontal magnetic
field. An arch-shaped concentrated current sheet is formed at the
interface between the rising magnetized plasma and the ambient
coronal field. Three-dimensional reconnection takes place along
the current sheet, so that the corona and the photosphere become
magnetically connected, a process repeatedly observed in recent
satellite missions. We show the structure and evolution of the
current sheet and how it changes in time from a simple tangential
discontinuity to a rotational discontinuity with no null surface. We
find clear indications that individual reconnection events in this
three-dimensional environment in the advanced stage are not one-off
events, but instead take place in a continuous fashion, with each
field line changing connectivity during a finite time interval. We
also show that many individual field lines of the rising tube undergo
multiple processes of reconnection at different points in the corona,
thus creating photospheric pockets for the coronal field. We calculate
global measures for the amount of subphotospheric flux that becomes
linked to the corona during the experiment and find that most of
the original subphotospheric flux becomes connected to coronal field
lines. The ejection of plasma from the reconnection site gives rise to
high-speed and high-temperature jets. The acceleration mechanism for
those jets is akin to that found in previous two-dimensional models,
but the geometry of the jets bears a clear three-dimensional imprint,
having a curved-sheet appearance with a sharp interface to the overlying
coronal magnetic field system. Temperatures and velocities of the jets
in the simulations are commensurate with those measured in soft X-rays
by the Yohkoh satellite.

---------------------------------------------------------
Title: D Numerical Simulations of Magnetic Reconnection Driven by
Rotational Footpoint Motions
Authors: De Moortel, I.; Galsgaard, K.
2005ESASP.600E..22D Altcode: 2005dysu.confE..22D; 2005ESPM...11...22D
No abstract at ADS

---------------------------------------------------------
Title: Numerical Simulations of 3d Magnetic Reconnection due to
Rotational Driving
Authors: De Moortel, I.; Galsgaard, K.
2005ESASP.596E..31D Altcode: 2005ccmf.confE..31D
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Flux Emergence and its Interaction with AN Existing
Coronal Field
Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A.
2005ESASP.596E..27G Altcode: 2005ccmf.confE..27G
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Flux Emergence and its Interaction with AN Existing
Coronal Field
Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A.
2005ESASP.596E..55G Altcode: 2005ccmf.confE..55G
No abstract at ADS

---------------------------------------------------------
Title: Elementary heating events - magnetic interactions between
two flux sources. III. Energy considerations
Authors: Galsgaard, K.; Parnell, C. E.
2005A&A...439..335G Altcode: 2005astro.ph..1602G
The magnetic field plays a crucial role in heating the solar corona
- this has been known for many years - but the exact energy release
mechanism(s) is(are) still unknown. Here, we investigate in detail,
using resistive, non-ideal, MHD models, the process of magnetic energy
release in a situation where two initially independent flux systems
are forced into each other. Work done by the foot point motions goes
into building a current sheet in which magnetic reconnection releases
some of the free magnetic energy leading to magnetic connectivity
changes. The scaling relations of the energy input and output are
determined as functions of the driving velocity and the strength of
fluxes in the independent flux systems. In particular, it is found that
the energy injected into the system is proportional to the distance
travelled. Similarly, the rate of Joule dissipation is related to the
distance travelled. Hence, rapidly driven foot points lead to bright,
intense, but short-lived events, whilst slowly driven foot points
produce weaker, but longer-lived brightenings. Integrated over the
lifetime of the events both would produce the same heating if all
other factors were the same. A strong overlying field has the effect
of creating compact flux lobes from the sources. These appear to lead
to a more rapid injection of energy, as well as a more rapid release
of energy. Thus, the stronger the overlying field the more compact
and more intense the heating. This means observers need to know not
only the flux of the magnetic fragments involved in an event, but
also their rate and direction of movement, as well as the strength
and orientation of the surrounding field to be able to predict the
energy dissipated. Furthermore, it is found that rough estimates of
the available energy can be obtained from simple models, starting
from initial potential situations, but that the time scale for the
energy release and, therefore its impact on the coronal plasma, can
only be determined from more detailed investigations of the non-ideal
behaviour of the plasma.

---------------------------------------------------------
Title: Forced magnetic reconnection
Authors: Birn, J.; Galsgaard, K.; Hesse, M.; Hoshino, M.; Huba, J.;
Lapenta, G.; Pritchett, P. L.; Schindler, K.; Yin, L.; Büchner, J.;
Neukirch, T.; Priest, E. R.
2005GeoRL..32.6105B Altcode: 2005GeoRL..3206105B
Using a multi-code approach, we investigate current sheet thinning and
the onset and progress of fast magnetic reconnection, initiated by
temporally limited, spatially varying, inflow of magnetic flux. The
present study extends an earlier collaborative effort into the
transition regime from thick to thin current sheets. Again we find
that full particle, hybrid, and Hall-MHD simulations lead to the same
fast reconnection rates, apparently independent of the dissipation
mechanism. The reconnection rate in MHD simulations is considerably
larger than in the earlier study, although still somewhat smaller than
in the particle simulations. All simulations lead to surprisingly
similar final states, despite differences in energy transfer and
dissipation. These states are contrasted with equilibrium models derived
for the same boundary perturbations. The similarity of the final states
indicates that entropy conservation is satisfied similarly in fluid
and kinetic approaches and that Joule dissipation plays only a minor
role in the energy transfer.

---------------------------------------------------------
Title: Numerical Simulations of the Flux Tube Tectonics Model for
Coronal Heating
Authors: Mellor, C.; Gerrard, C. L.; Galsgaard, K.; Hood, A. W.;
Priest, E. R.
2005SoPh..227...39M Altcode:
In this paper we present results from 3D MHD numerical simulations
based on the flux tube tectonics method of coronal heating proposed by
Priest, Heyvaerts, and Title (2002). They suggested that individual
coronal loops connect to the photosphere in many different magnetic
flux fragments and that separatrix surfaces exist between the
fingers connecting a loop to the photosphere and between individual
loops. Simple lateral motions of the flux fragments could then cause
currents to concentrate along the separatrices which may then drive
reconnection contributing to coronal heating. Here we have taken a
simple configuration with four flux patches on the top and bottom
of the numerical domain and a small background axial field. Then we
move two of the flux patches on the base between the other two using
periodic boundary conditions such that when they leave the box they
re-enter it at the other end. This simple motion soon causes current
sheets to build up along the quasi-separatrix layers and subsequently
magnetic diffusion/reconnection occurs.

---------------------------------------------------------
Title: Particle Acceleration in Stressed Coronal Magnetic Fields
Authors: Turkmani, R.; Vlahos, L.; Galsgaard, K.; Cargill, P. J.;
Isliker, H.
2005ApJ...620L..59T Altcode:
This Letter presents an analysis of particle acceleration in a
model of the complex magnetic field environment in the flaring solar
corona. A slender flux tube, initially in hydrodynamic equilibrium,
is stressed by random photospheric motions. A three-dimensional
MHD code is used to follow the stochastic development of transient
current sheets. These processes generate a highly fragmented electric
field, through which particles are tracked using a relativistic test
particle code. It is shown that both ions and electrons are accelerated
readily to relativistic energies in times of order 10-2
s for electrons and 10-1 s for protons forming power-law
distributions in energy.

---------------------------------------------------------
Title: A Three-dimensional Study of Reconnection, Current Sheets,
and Jets Resulting from Magnetic Flux Emergence in the Sun
Authors: Galsgaard, K.; Moreno-Insertis, F.; Archontis, V.; Hood, A.
2005ApJ...618L.153G Altcode: 2004astro.ph.10057G
We present the results of a set of three-dimensional numerical
simulations of magnetic flux emergence from below the photosphere
and into the corona. The corona includes a uniform and horizontal
magnetic field as a model for a preexisting large-scale coronal
magnetic system. Cases with different relative orientations of the
upcoming and coronal fields are studied. Upon contact, a concentrated
current sheet with the shape of an arch is formed at the interface
that marks the positions of maximum jump in the field vector between
the two systems. Relative angles above 90° yield abundant magnetic
reconnection and plasma heating. The reconnection is seen to be
intrinsically three-dimensional in nature and to be accompanied by
marked local heating. It generates collimated high-speed outflows
only a short distance from the reconnection site, and these propagate
along the ambient magnetic field lines as jets. As a result of the
reconnection, magnetic field lines from the magnetized plasma below
the surface end up connecting to coronal field lines, thus causing a
profound change in the connectivity of the magnetic regions in the
corona. The experiments presented here yield a number of features
repeatedly observed with the TRACE and Yohkoh satellites, such as the
establishment of connectivity between emergent and preexisting active
regions, local heating, and high-velocity outflows.

---------------------------------------------------------
Title: Flux Emergence from the Solar Interior Into a Uniformly
Magnetized Corona
Authors: Moreno-Insertis, F.; Galsgaard, K.; Archontis, V.; Hood, A.
2004ESASP.575..216M Altcode: 2004soho...15..216M
No abstract at ADS

---------------------------------------------------------
Title: Fragment Driven Magnetic Reconnection
Authors: Galsgaard, K.; Parnell, C.
2004ESASP.575..351G Altcode: 2004soho...15..351G; 2004astro.ph..9562G
In this paper, we investigate a simple model where two, initially
unconnected, flux systems are forced to interact in response to the
imposed boundary driving by solving the non-ideal 3D MHD equations
numerically. The reconnection rate of the dynamical process
is determined and compared with the corresponding rate for the
potential evolution of the magnetic field. This shows that the dynamic
reconnection rate is about a factor of two smaller than the potential
(perfect, instantaneous) rate for realistic solar driving velocities
demonstrating that this three-dimensional magnetic reconnection process
is fast. The energy input for a fixed advection distance is found to be
independent of the driving velocity. The Joule dissipation associated
with the reconnection process is also found to be basically dependent
on the advection distance rather than driving velocity. This implies
that the timescale for the event determines the effect the heating
has on the temperature increase. Finally, the numerical experiments
indicate that the observational structure of the reconnection site
changes dramatically depending on the phase of the evolution of
the passage of the two flux sources. In the initial phase, where
the sources become connected, the heating is confined to a compact
region. For the disconnecting phase the energy gets distributed over
a larger area due to the reconnected field line connectivity.

---------------------------------------------------------
Title: Elementary heating events - magnetic interactions between
two flux sources. II. Rates of flux reconnection
Authors: Parnell, C. E.; Galsgaard, K.
2004A&A...428..595P Altcode:
Magnetic fragments in the photosphere are in continuous motion and, due
to the complex nature of the magnetic field in the solar atmosphere,
these motions are likely to drive a lucrative coronal energy source:
the passing of initially-unconnected opposite-polarity fragments
that release energy through both closing and then re-opening the
same fieldlines. Three-dimensional, time-dependent MHD and potential
models are used to investigate the passing of fragments in an overlying
field. The processes of closing and opening the field generally occur
through separator and separatrix reconnection, respectively. The
rates of flux reconnection in these processes are determined. They
are found to be dependent on the direction of the surrounding magnetic
field relative to the motion of the fragments and the velocity of the
sources. In particular, separator reconnection rates (closing) and
separatrix-surface reconnection rates (opening) are directly related
to the rate of flux transport perpendicular to the current sheet
(overlying field). The results suggest that both types of reconnection
are fast with the peak rates of separator and separatrix reconnection
occurring at 58% and 29% of the peak potential reconnection rate,
respectively, when the sources are driven at a hundredth of the
peak Alfvén velocity in the box. Moreover, the slower the system is
driven the closer the flux reconnection rates are to the instantaneous
potential rates. Furthermore, there is a maximum reconnection rate
for both types of reconnection as the driving speed tends to the
Alfvén speed with the separatrix reconnection rate typically half
that of separator reconnection. These results suggest that, on the
Sun, reconnection driven by the passing of small-scale network and
intranetwork fragments is a highly efficient process that is very likely
to contribute significantly to the heating of the background solar
corona. The three-dimensional reconnection processes are efficient
because, unlike in two-dimensions, there are many places within the
current sheets where reconnection can take place simultaneously giving
rise to fine-scale structure along the boundaries between the open,
closed and re-opened flux. Furthermore, due to the complexity of
the magnetic field above the photosphere the reconnection all takes
place low down at less than a quarter of the separation of the initial
fragments above the photosphere.

---------------------------------------------------------
Title: Simple Numerical Simulations of the Flux Tube Tectonics Model
for Coronal Heating
Authors: Mellor, C.; Gerrard, C. L.; Galsgaard, K.; Hood, A. W.;
Priest, E. R.
2004ESASP.575...29M Altcode: 2004soho...15...29M
No abstract at ADS

---------------------------------------------------------
Title: 3D MHD Simulations on Magnetic Flux Emergence
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood, A.
2004ESASP.575..342A Altcode: 2004soho...15..342A
No abstract at ADS

---------------------------------------------------------
Title: Emergence of magnetic flux from the convection zone into
the corona
Authors: Archontis, V.; Moreno-Insertis, F.; Galsgaard, K.; Hood,
A.; O'Shea, E.
2004A&A...426.1047A Altcode:
Numerical experiments of the emergence of magnetic flux from the
uppermost layers of the solar interior to the photosphere and its
further eruption into the low atmosphere and corona are carried out. We
use idealized models for the initial stratification and magnetic
field distribution below the photosphere similar to those used for
multidimensional flux emergence experiments in the literature. The
energy equation is adiabatic except for the inclusion of ohmic and
viscous dissipation terms, which, however, become important only at
interfaces and reconnection sites. Three-dimensional experiments for the
eruption of magnetic flux both into an unmagnetized corona and into a
corona with a preexisting ambient horizontal field are presented. The
shocks preceding the rising plasma present the classical structure of
nonlinear Lamb waves. The expansion of the matter when rising into the
atmosphere takes place preferentially in the horizontal directions: a
flattened (or oval) low plasma-β ball ensues, in which the field lines
describe loops in the corona with increasing inclination away from the
vertical as one goes toward the sides of the structure. Magnetograms
and velocity field distributions on horizontal planes are presented
simultaneously for the solar interior and various levels in the
atmosphere. Since the background pressure and density drop over many
orders of magnitude with increasing height, the adiabatic expansion
of the rising plasma yields very low temperatures. To avoid this, the
entropy of the rising fluid elements should be increased to the high
values of the original atmosphere via heating mechanisms not included in
the present numerical experiments. The eruption of magnetic flux into
a corona with a preexisting magnetic field pointing in the horizontal
direction yields a clear case of essentially three-dimensional
reconnection when the upcoming and ambient field systems come into
contact. The coronal ambient field is chosen at time t=0 perpendicular
to the direction of the tube axis and thus, given the twist of the
magnetic tube, almost anti-parallel to the field lines at the upper
boundary of the rising plasma ball. A thin, dome-shaped current layer
is formed at the interface between the two flux systems, in which ohmic
dissipation and heating are taking place. The reconnection proceeds
by merging successive layers on both sides of the reconnection site;
however, this occurs not only at the cusp of the interface, but, also,
gradually along its sides in the direction transverse to the ambient
magnetic field. The topology of the magnetic field in the atmosphere
is thereby modified: the reconnected field lines typically are part of
the flanks of the tube below the photosphere but then join the ambient
field system in the corona and reach the boundaries of the domain as
horizontal field lines.

---------------------------------------------------------
Title: Coronal Heating - Simulations
Authors: Galsgaard, K.
2004cosp...35.4393G Altcode: 2004cosp.meet.4393G
The continued advances in computing power has made it possible
to increase the physical complexity of numerical models. We have
reached a state where it is possible to model quit realistically the
dynamics of smaller active regions. Data from such experiments provide
the first real possibility for direct comparison between numerical
models and observations. The disadvantage is the high complexity of
the dynamical evolution which makes it difficult to determine the
mechanism/s responsible for heating the corona. As a supplement to
such models, specialised experiments providing insight to the basic
mechanisms are still required. Only by adopting both approaches is
it possible to optain a more detailed understanding of the physical
process that heats the corona. This talk is going to discuss recent
advances in numerical modeling of coronal heating.

---------------------------------------------------------
Title: A Numerical Investigation of a Simple 3D Magnetic Flux
Interaction Event
Authors: Galsgaard, K.; Walsh, R.
2003csss...12..230G Altcode:
The discovery of the magnetic carpet on the sun, the continuously
changing of small scale magnetic flux elements in the solar atmosphere,
indicates that the life time of individual flux elements is less than
one day. During this time the elements interact with each other and
eventually become impossible to identify individually. The complexity of
the coronal magnetic field relating to this source distribution provides
a complex pattern of overlying field line connectivity, with one flux
element connecting possibly to several other flux elements. Stressing
such a complex magnetic field by movements of the flux sources in the
photosphere, must lead undoubtfully to the formation of many localised
current concentrations that can provide local heating for the transition
region and lower coronal plasma. In this paper we investigate a simple
flux interaction event between two unbalanced magnetic sources. Using
a numerical MHD approach we examine when and how the free magnetic
energy may be released when the flux patches are rotated relative
to one another. It is found that this topological simple magnetic
configuration does not reach easily a state where a measure of the
imposed stress is released on a short dynamical time scale.

---------------------------------------------------------
Title: Magnetic Pinching of Hyperbolic Flux Tubes. II. Dynamic
Numerical Model
Authors: Galsgaard, K.; Titov, V. S.; Neukirch, T.
2003ApJ...595..506G Altcode:
In this paper we present the results of a series of numerical
experiments that extend and supplement the recent analytical
investigations by Titov et al. of the formation of strong current
layers in coronal magnetic fields containing hyperbolic flux tubes
(HFTs). The term “hyperbolic” refers to the special geometrical
properties of the magnetic field, whereas the topology of the field is
simple; i.e., there are no magnetic null points and separatrix lines
or surfaces associated with them inside the coronal volume. However,
the field lines passing through a hyperbolic flux tube show a large
variation in the mapping between their photospheric endpoints. On
the basis of analytical estimates, it has been suggested by Titov et
al. that HFTs are preferred locations for the formation of strong
current layers in coronal magnetic fields with trivial topologies,
provided the driving motions on the photospheric boundary are of
a special type. Such motions must have shearing components that
are applied across narrow HFT feet as if trying to twist it. This
system of motions is then capable of causing a pinching deformation
of the HFT by a sustained stagnation point flow inside the HFT. The
numerical experiments presented in this paper clearly confirm this
suggestion. HFTs are generic features of geometrically complex but
topologically trivial magnetic fields, and therefore our results are
very important for understanding magnetic reconnection in such fields,
since reconnection is occurring preferentially at locations with strong
current densities.

---------------------------------------------------------
Title: Quasistatic Magnetic Pinching of Force-free Hyperbolic
Flux Tubes
Authors: Neukirch, Thomas; Titov, Vyatcheslav S.; Galsgaard, Klaus
2003ANS...324R..14N Altcode: 2003ANS...324..B12N
No abstract at ADS

---------------------------------------------------------
Title: Numerical experiments on wave propagation towards a 3D null
point due to rotational motions
Authors: Galsgaard, K.; Priest, E. R.; Titov, V. S.
2003JGRA..108.1042G Altcode:
We describe 3D resistive magnetohydrodynamic (MHD) numerical experiments
at a null point driven by the rotation of magnetic field lines near
the spine of the null. When field lines around the spine are rotated,
a twist wave propagates toward the null along field lines, satisfying
a Klein-Gordon equation. While the helical Alfvén wave spreads out as
the null is approached, a fast-mode wave focuses on the null and wraps
around it. Only a weak diffusion of the twisted field line structure
is found to take place.

---------------------------------------------------------
Title: Magnetic Pinching of Hyperbolic Flux Tubes. I. Basic
Estimations
Authors: Titov, V. S.; Galsgaard, K.; Neukirch, T.
2003ApJ...582.1172T Altcode: 2002astro.ph..8112T
The concept of hyperbolic flux tubes (HFTs) is a generalization of the
concept of separator field lines for coronal magnetic fields with a
trivial magnetic topology. An effective mechanism of a current layer
formation in HFTs is proposed. This mechanism is called magnetic
pinching, and it is caused by large-scale shearing motions applied
to the photospheric feet of HFTs in a way as if trying to twist the
HFT. It is shown that in the middle of an HFT such motions produce
a hyperbolic flow that causes an exponentially fast growth of the
current density in a thin force-free current layer. The magnetic energy
associated with the current layer that is built up over a few hours
is sufficient for a large flare. Other implications of HFT pinching
for solar flares are discussed as well.

---------------------------------------------------------
Title: Magnetic reconnection throughout the solar atmosphere
Authors: Hood, A. W.; Galsgaard, K.; Parnell, C. E.
2002ESASP.505..285H Altcode: 2002solm.conf..285H; 2002IAUCo.188..285H
Magnetic reconnection is responsible for many different solar phenomena
and it is the release of magnetic energy through reconnection that
is believed to (i) drive flares, (ii) generate CMEs, (III) heat the
corona and (iv) generate MHD waves. In basic models of 2D magnetic
reconnection, the particular choise of boundary conditions influences
the form of reconnection obtained. Reconnection in 3D can occur
with and without null points. Numerical experiments have attempted to
investigate different types of reconnection but a basic understanding of
reconnection at 3D magnetic null points is essential in understanding
these fumdamental processes. The structure of magnetic regions depends
on features such as the magnetic skeleton, the mull points, the spine
and fan plane. Numerical simulations are important but, at present,
are unable to fully resolve the reconnection region. Recent analytical
and numerical results of 3D reconnection will be presented. Applications
of reconnection in the solar atmosphere will be discussed also.

---------------------------------------------------------
Title: Flux braiding in a stratified atmosphere
Authors: Galsgaard, K.
2002ESASP.505..269G Altcode: 2002solm.conf..269G; 2002IAUCo.188..269G
In recent years both Yohkoh and TRACE observations have been used
to determine temperature profiles along magnetic loop structures in
the solar corona. These have then been used to estimate the energy
deposition along the loop by solving the static one dimensional energy
equation including heat conduction and optical thin radiation. The
various results claim to support energy deposition along the loop that
are either uniform and strongly localised toward the foot-points. How
can these variations occur? and which heating mechanisms can produce
such different heating profiles? This paper investigates the effect
of flux braiding in a stratified atmosphere with a narrow transition
region. The results show a clear distinction between the energy
deposition below and above the transition region, with a fast decay
of the energy deposition below the transition region and a uniform
deposition in the coronal part of the loop.

---------------------------------------------------------
Title: Evolution of a density enhancement in a stratified atmosphere
with uniform vertical magnetic field
Authors: Mackay, D. H.; Galsgaard, K.
2002ESASP.505..485M Altcode: 2002solm.conf..485M; 2002IAUCo.188..485M
The evolution of a density enhancement under the effect of gravity in
a stratified atmosphere is considered. The atmosphere is threaded with
an initially uniform vertical magnetic field. The magnetic field plays
an important role in the evolution of the density enhancement and if
strong enough results in the density enhancement rebounding a number
of times. Both upward and downward velocities of the enhancement
are obtained with speeds much less than the free fall speed. The
enhancement can remain in the corona at least 11 times longer than
a free-fall particle. The relevance of the simulations to the solar
atmosphere is then discussed.

---------------------------------------------------------
Title: Magnetic reconnection in 2D stratified
atmospheres. I. Dynamical consequences
Authors: Galsgaard, K.; Roussev, I.
2002A&A...383..685G Altcode:
We explore the dynamical consequences of magnetic reconnection in a
2D stratified physical configuration representing a “quiet” solar
environment. By including gravity, an initial magneto-hydrostatic
solution is found that allows the magnetic field to expand with
height. The change in kinetic gas pressure with height leads to
the formation of a cold current “sheet” in the case of strong
stratification, in contrast to a hot current “sheet” in the case of
negligible stratification. Here the “sheet” temperature is measured
relative to the temperature in the ambient background plasma. The
dynamics of magnetic reconnection in a stratified atmosphere evolves
through a new initial stage, with a more complex velocity structure
than the quadrupolar velocity pattern present in traditional 2D
X-point reconnection. As time progresses, the new initial phase is
suppressed and the driven reconnection evolves into the traditional 2D
reconnection pattern. The transition time between the two regimes is
found to depend on the imposed stratification, and through this, on the
degree of expansion of the initial magnetic field with height. The new
reconnection regime undergoes a more complicated physical evolution and
seems to have a lower reconnection rate than the classical 2D X-point
reconnection. The faster the magnetic field expands with height, the
slower and more complex are the dynamics of the magnetic reconnection
at the early stages of its evolution.

---------------------------------------------------------
Title: Physical consequences of the inclusion of anomalous resistivity
in the dynamics of 2D magnetic reconnection
Authors: Roussev, I.; Galsgaard, K.; Judge, P. G.
2002A&A...382..639R Altcode:
The aim of the present paper is to explore the role of anomalous
resistivity on the dynamics of magnetic reconnection in a 2D
environment of relevance to the solar transition region. We adopt an
ad hoc but explicit form of the anomalous resistivity, motivated by a
streaming instability, in which the resistivity jumps suddenly as the
electron drift velocity exceeds some fraction of the mean electron
thermal speed. Experiments have been conducted to investigate the
impact of various critical speeds and arbitrary scaling constants
of the resistivity level on the time-dependent evolution of the
magnetic reconnection process. The specific threshold value is found
to influence the dynamics of the reconnection, with higher values
providing a localised on-off effect of patchy diffusion. For a given
normalised value of the anomalous resistivity, the amount of Joule
heating released scales inversely with the threshold value. The total
energy release is found to be above the lower limit of “quiet”
Sun nano-flares required to maintain a hot corona. The reconnection
events discussed here may be important to the energy balance of the
solar transition region and overlying corona, as already suggested in
earlier work based on SUMER observations.

---------------------------------------------------------
Title: Magnetic reconnection in 2D stratified
atmospheres. II. Observational consequences
Authors: Roussev, I.; Galsgaard, K.
2002A&A...383..697R Altcode:
We synthesise three resonance transitions of Li-like ions, C IV 1548.2
Å, O VI 1031.9 Å, and Ne VIII 770.4 Å, based on the MHD experiments
presented in Part I of this study. Part I involved 2D MHD modelling
of magnetic reconnection events in stratified magneto-hydrostatic
atmospheres selected to represent a “quiet” Sun transition region
environment. Here in Part II, we discuss some observable consequences of
these simulations by including effects of non-equilibrium ionization
in a 1D approximation. All spectral line syntheses are done along
a particular ray, oriented vertically along the initial current
structure. Computed time-series, in the three spectral lines, reveal
both blue- and red-shifted Doppler components. It is found to be easier
to detect the reconnection event in the various emission lines as
strongly Doppler-shifted components as the stratification increases. We
predict what the observational consequences of these reconnection
events would be, if various spatial resolutions under 1 arcsec become
available in future observations. This work is a continuation of
previous numerical studies related to solar explosive events.

---------------------------------------------------------
Title: Modelling of solar explosive events in 2D
environments. III. Observable consequences
Authors: Roussev, I.; Doyle, J. G.; Galsgaard, K.; Erdélyi, R.
2001A&A...380..719R Altcode:
We examine the response via line synthesis of two representative
transition region lines, namely C IV 1548.2 Å and O VI 1031.9 Å,
in various physical environments representing the “quiet” Sun to
magnetic reconnection events. Our calculations of ion populations allow
for departures from equilibrium ionization (EI), which is critical
in studies of transient events. Both lines reveal highly blue- and
red-shifted Doppler components, and some of the results are examined in
the context of solar explosive events. The observable consequences of
magnetic reconnection are subtle in the various physical circumstances
examined here and differ from one emission line to another, because
of the difference in their formation temperatures.

---------------------------------------------------------
Title: Diagnosing dynamic coronal heating: lessons from YOHKOH,
SOHO and TRACE
Authors: Walsh, R. W.; Galsgaard, K.
2001ESASP.493..427W Altcode: 2001sefs.work..427W
No abstract at ADS

---------------------------------------------------------
Title: Modelling of explosive events in the solar transition region
in a 2D environment. II. Various MHD experiments
Authors: Roussev, I.; Galsgaard, K.; Erdélyi, R.; Doyle, J. G.
2001A&A...375..228R Altcode:
We examine the response of various physical environments representing
the solar atmosphere to a magnetic reconnection event. The reconnection
is driven by a localized increase of the magnetic diffusivity in the
current concentration formed between two magnetic fluxes of opposite
polarity. The time dependent evolution is then followed by numerically
solving the 2-dimensional (2D) dissipative magnetohydrodynamic (MHD)
equations, including also effects of thermal conduction, radiative
losses, and volumetric heating.\ This work continues a previous related
study (Roussev et al. 2001a), and compares results obtained from
exploring different initial states. The choice of the initial states
is found to be crucial to the dynamics of the reconnection jets. The
numerical experiments are aimed at modelling transient events on the
quiet Sun, with an emphasis on explosive events. The 2D reconnection
experiments presented in this paper are the basis for a detailed
analysis on the line synthesis in transition region resonant lines,
presented by Roussev et al. (\cite{rou01b}).

---------------------------------------------------------
Title: Modelling of explosive events in the solar transition region
in a 2D environment. I. General reconnection jet dynamics
Authors: Roussev, I.; Galsgaard, K.; Erdélyi, R.; Doyle, J. G.
2001A&A...370..298R Altcode:
The aim of the present study is to investigate the reconnection
jets formed during the process of magnetic flux cancellation
in the physical environment of the solar transition region. The
emission properties of these jets are then computed for two resonance
transition region lines, C iv 1548.2 Å and O vi 1031.9 Å, under the
assumption of non-equilibrium ionization. The numerical modelling
involves 2-dimensional (2D) dissipative, radiative, nonlinear
magnetohydrodynamics. The nonlinear anisotropic thermal conduction,
radiative losses, and volumetric heating are taken into account in order
to assess their role in the physical situation examined. This work is a
continuation of previous related simulations where small-scale energy
depositions were modelled in 1D radiative hydrodynamics. Having an
X-point reconnection in the mid-transition region gives blue-shifts
of the order of ~ 100 km s-1, however, the red-shift can
be up to one order of magnitude less.

---------------------------------------------------------
Title: Evolution of a Density Enhancement in a Stratified Atmosphere
With Uniform Vertical Magnetic Field
Authors: Mackay, D. H.; Galsgaard, K.
2001SoPh..198..289M Altcode:
In this paper the evolution of a density enhancement under the effect of
gravity in a stratified atmosphere is considered in a 2D simulation. The
stratified atmosphere is chosen with a high-density photosphere,
transition region and low-density corona where the enhancement is added
in non-equilibrium to the corona. The atmosphere is also threaded with
an initially uniform vertical magnetic field. If sufficiently strong,
the magnetic field plays an important role in the evolution of the
density enhancement as it tries to gain equilibrium. It not only
enables the density enhancement to maintain its shape as it falls,
but if strong enough results in the density enhancement rebounding a
number of times. Therefore both upward and downward velocities of the
enhancement are obtained. In all cases the density enhancement is found
to fall with speeds much less than the free-fall speed and can remain
in the corona at least 11 times longer than a free-fall particle. The
relevance of the simulations to the solar atmosphere is then discussed.

---------------------------------------------------------
Title: Elementary heating events - Magnetic interactions between
two flux sources
Authors: Galsgaard, K.; Parnell, C. E.; Blaizot, J.
2000A&A...362..395G Altcode:
Observations taken by the SoHO MDI instrument have revealed that the
quiet photospheric magnetic flux is, on average, recycled within a few
days. As new flux emerges from the convection zone into the photosphere
it is moved around by horizontal motions resulting from overshoots of
convection cells. These motions cause the magnetic fields extending
from flux fragments to tangle, forcing different magnetic flux systems
to interact. Only the process of magnetic reconnection limits the
complexity of magnetic field line connectivity. The energy liberated
by these detangling or destressing processes act as a natural energy
source which may heat the solar coronal plasma. In this paper, we use
a numerical approach to solve the MHD equations in a three-dimensional
domain to examine the dynamical behaviour of one simple magnetic flux
interaction. The model consists of a uniform magnetic field overlying
two flux sources of opposite polarity that are initially unconnected and
are forced to interact as they are driven passed each other. We find
that the development from initially unconnected sources to connected
sources proceeds quite quickly and simply. This change takes place
through driven separator reconnection in a systematically twisted
current sheet. The out flow velocity from the reconnection is highly
asymmetric with much higher velocities in the region defined by the
field lines connected to both sources. However, the change back to two
independent sources after the nearest approach has past takes place on
a much longer time scale even though the distance between the sources
increases significantly. This is because the opening of the field has
to take place through separatrix reconnection and at this phase of
the development there are no forcing of the fluxes to drive a fast
opening of the magnetic field.

---------------------------------------------------------
Title: Modelling the Sun's Magnetic Carpet: Energy Release in the
Corona due to the Relative Motion of Magnetic Fragments
Authors: Walsh, R. W.; Galsgaard, K.
2000SPD....31.1305W Altcode: 2000BAAS...32R.845W
The three-dimensional nature of the coronal magnetic field is
very important in understanding the complex structures that we
observe. Recent high cadence observations of the photospheric magnetic
field reveal a continually changing environment where the magnetic
field fragments are in continual motion relative to one another. This
motion of what has been termed the Sun's Magnetic Carpet could be a
possible source of the large amount of energy that is heating the quiet
corona. A three-dimensional MHD resistive numerical code is introduced
to model the above dynamic scenerio. The situation is idealised by
considering the case of two unbalanced magnetic patches that are rotated
relative to one other. Results for the evolving magnetic structure,
simplified plasma response and calculated total energy release will
be presented. Implications of these results upon solar observations
will be discussed.

---------------------------------------------------------
Title: Three-dimensional Separator Reconnection - How Does It Occur?
Authors: Galsgaard, K.; Priest, E. R.; Nordlund, Å.
2000SoPh..193....1G Altcode:
In two dimensions magnetic energy release takes place at locations
where the magnetic field strength becomes zero and has an x-point
topology. The x-point topology can collapse into two y-points connected
by a current sheet when the advection of magnetic flux into the x-point
is larger than the dissipation of magnetic flux at the x-point. In
three dimensions magnetic fields may also contain singularities in the
form of three-dimensional null points. Three-dimensional nulls are
created in pairs and are therefore, at least in the initial stages,
always connected by at least one field line - the separator. The
separator line is defined by the intersection of the fan planes of the
two nulls. In the plane perpendicular to a single separator the field
line topology locally has a two dimensional x-point structure. Using
a numerical approach we find that the collapse of the separator can be
initiated at the two nulls by a velocity shear across the fan plane. It
is found that for a current concentration to connect the two nulls
along the separator, the current sheet can only obtain two different
orientations relative to the field line structure of the nulls. The
sheet has to have an orientation midway between the fan plane and the
spine axis of each null. As part of this process the spine axes are
found to lose their identity by transforming into an integrated part
of the separator surfaces that divide space into four magnetically
independent regions around the current sheet.

---------------------------------------------------------
Title: How Accurately Can We Determine the Coronal Heating Mechanism
in the Large-Scale Solar Corona?
Authors: Mackay, D. H.; Galsgaard, K.; Priest, E. R.; Foley, C. R.
2000SoPh..193...93M Altcode:
In recent papers by Priest et al., the nature of the coronal heating
mechanism in the large-scale solar corona was considered. The authors
compared observations of the temperature profile along large coronal
loops with simple theoretical models and found that uniform heating
along the loop gave the best fit to the observed data. This then led
them to speculate that turbulent reconnection is a likely method
to heat the large-scale solar corona. Here we reconsider their
data and their suggestion about the nature of the coronal heating
mechanism. Two distinct models are compared with the observations
of temperature profiles. This is done to determine the most likely
form of heating under different theoretical constraints. From this,
more accurate judgments on the nature of the coronal heating mechanism
are made. It is found that, due to the size of the error estimates in
the observed temperatures, it is extremely difficult to distinguish
between some of the different heat forms. In the initial comparison
the limited range of observed temperatures (T&gt;1.5 MK) in the
data sets suggests that heat deposited in the upper portions of the
loop, fits the data more accurately than heat deposited in the lower
portions. However if a fuller model temperature range (T&lt;1.0 MK)
is used results in contridiction to this are found. In light of this
several improvements are required from the observations in order to
produce theoretically meaningful results. This gives serious bounds
on the accuracy of the observations of the large-scale solar corona
in future satellite missions such a Solar-B or Stereo.

---------------------------------------------------------
Title: Dynamical investigation of three-dimensional reconnection in
quasi-separatrix layers in a boundary-driven magnetic field
Authors: Galsgaard, Klaus
2000JGR...105.5119G Altcode:
Quasi-separatrix layers are regions in space where the mapping of
field line connectivity changes especially rapidly. These layers have
been suggested as special locations in three-dimensional magnetic
fields that may host magnetic reconnection. Previous investigations
have been analytical and have taken different simplifying assumptions
to investigate the problem. This paper takes a numerical approach to
investigate the dynamical properties of quasi-separatrix layers. The
magnetic topology is stressed using drivers suggested by the analytical
investigations but modified to fit the adopted boundary conditions. The
experiments show that current does accumulate at specific locations in
the numerical domain. The current magnitude and location depend strongly
on the profile of the imposed driver, and they are found to be generated
by the changes in field line parts imposed by the driving. They are
therefore the manifestation of free magnetic energy in the perturbed
magnetic field. After the stressing of the field has stopped, it is
found that the plasma pressure is able to balance the Lorentz force
of the stressed magnetic field and prevent a continued growth of the
current amplitude in the current layers. Field-line changes are produced
in the experiments that include magnetic resistivity. The reconnection
takes place at locations where the electric field component along
the magnetic field is large. The changes in field-line connectivity
initiate flow velocities across the magnetic field lines at only a
small fraction of the local Alfvén velocity.

---------------------------------------------------------
Title: Modeling the evolution of solar magnetic fields
Authors: Galsgaard, K.
2000ASIC..558..149G Altcode: 2000asre.conf..149G
The solar atmosphere is a laboratory for studying complicated physical
processes. The Magneto-Hydro-Dynamic (MHD) equations is the simplest
theoretical approach that captures the complicated dynamical interaction
between plasma and magnetic fields. These equations are therefore
widely used for investigating the dynamical processes that takes place
in the solar atmosphere. The MHD equations can also be used to provide
information about the field line structure of the coronal magnetic
field. Knowledge of the magnetic field topology is required before
we can understand where the magnetic field tends to liberate it's
free energy. To fully understand the energy release process, magnetic
reconnection and wave dissipation processes have to be understood in
detail. The MHD equations can also provide information about temperature
profile along loops that are heated by, both static and time depend,
heating profiles. This paper gives a review of magnetic reconnection,
the basic idea behind magnetic field extrapolation, and the problems
related to using temperature profiles for determining the underlying
heating profile. A full understanding and utilisation of all of these
disciplines within solar physics MHD is required to significantly
advance our knowledge of the dynamical solar corona.

---------------------------------------------------------
Title: Modelling of Explosive Events in the Solar Transition Region:
Importance of Radiative Losses and Thermal Conduction
Authors: Roussev, I.; Erdélyi, R.; Doyle, J. G.; Galsgaard, K.
1999ESASP.448..641R Altcode: 1999mfsp.conf..641R; 1999ESPM....9..641R
No abstract at ADS

---------------------------------------------------------
Title: SunBlock '99: Young Scientists Investigate the Sun
Authors: Walsh, R. W.; Pike, C. D.; Mason, H.; Young, P.; Ireland,
J.; Galsgaard, K.
1999ESASP.446..693W Altcode: 1999soho....8..693W
SunBlock `99 is a Web-based Public Understanding of Science and
educational project which seeks to present the very latest solar
research as seen through the eyes of young British scientists. These
“solar guides” discuss not only their scientific interests, but also
their extra-curricular activities and the reasons they chose scientific
careers; in other words the human face of scientific research. The
SunBlock '99 pages gather a range of solar images and movies from
current solar space observatories and discuss the underlying physics
and its relationship to the school curriculum. The instructional level
is pitched at UK secondary school children (aged 13-16 years). It is
intended that the material should not only provide a visually appealing
introduction to the study of the Sun, but that it should help bridge
the often wide gap between classroom science lessons and the research
scientist `out in the field'. SunBlock '99 is managed by a team from
the Rutherford Appleton Laboratory and the Universities of St Andrews
and Cambridge, together with educational consultants. The production
has, in part, been sponsored by PPARC and the Millennium Mathematics
Project. Web site addresss: http://www.sunblock99.org.uk

---------------------------------------------------------
Title: On the location of energy release and temperature profiles
along coronal loops
Authors: Galsgaard, K.; Mackay, D. H.; Priest, E. R.; Nordlund, Å.
1999SoPh..189...95G Altcode:
Several mechanisms have been suggested to contribute to the heating
of the solar corona, each of which deposits energy along coronal
loops in a characteristic way. To compare the theoretical models
with observations one has to derive observable quantities from the
models. One such parameter is the temperature profile along a loop. Here
numerical experiments of flux braiding are used to provide the spatial
distribution of energy deposition along a loop. It is found that
braiding produces a heat distribution along the loop which has slight
peaks near the footpoints and summit and whose magnitude depends on
the driving time. Using different examples of the heat deposition,
the temperature profiles along the loop are determined assuming a
steady state. Along with this, different methods for providing average
temperature profiles from the time-series have been investigated. These
give summit temperatures within approximately 10% of each other. The
distribution of the heating has a significant impact on both the summit
temperature and the temperature distribution along the loop. In each
case the ratio between the heat deposited and radiation provides a
scaling for the summit temperature.

---------------------------------------------------------
Title: Formation of Solar Prominences by Flux Convergence
Authors: Galsgaard, K.; Longbottom, A. W.
1999ApJ...510..444G Altcode:
Observations have found three main conditions that have to be fulfilled
before a prominence will form. These are the presence of a magnetic
arcade, a transport of flux toward the polarity inversion line under the
arcade, and cancellation of magnetic flux that approaches the polarity
inversion. We have set up a three-dimensional model that initially
contains two bipolar regions, representing new and old regions
of flux on the Sun. The regions are forced together by an imposed
velocity flow on the lower boundary. As they approach one another,
the magnetic field reaches a state where a thin current distribution
is created above the polarity inversion line. When the current becomes
strong enough, magnetic reconnection starts. In the right parameter
regime it is found that the reconnected field lines are able to lift
plasma several pressure scale heights against gravity. The lifted plasma
forms a region with enhanced density above the current sheet, along the
polarity inversion line and with a length that is longer than the length
of the current sheet. Different types of field line connectivity are
found that provide the force to lift mass as the reconnection proceeds.

---------------------------------------------------------
Title: Modelling of Explosive Events in the Solar Transition Region
Authors: Roussev, I.; Erdélyi, S.; Doyle, J. G.; Galsgaard, K.
1999RoAJ....9S..57R Altcode:
We present preliminary results of modelling 2D magnetic reconnection
in the solar transition region environment. Compressible
magnetohydrodynamic (MHD) simulations are performed by using a 2D
MHD code based on staggered meshes. The present work is an attempt
to numerically simulate magnetic reconnection that occurs between
newly emerging magnetic flux and the pre-existing network field,
which is widely believed to cause explosive events observed in the
solar atmosphere. The formation of asymmetric reconnection jets is
demonstrated in this study. Nonlinear anisotropic thermal conduction,
radiative losses, and volumetric heating are included in the energy
equation in order to investigate the formation of reconnection jets
more precisely, as these processes are of fundamental importance at
that region. The role of both nonlinear anisotropic thermal conduction
and radiative losses is found to be significant in the solar transition
region.

---------------------------------------------------------
Title: Realisation of 3-dimensional data sets.
Authors: Brown, D.; Galsgaard, K.; Ireland, J.; Verwichte, E.;
Walsh, R.
1999joso.proc..211B Altcode:
The visualisation of three-dimensional objects on two dimensions is a
very common problem, but is a tricky one to solve. Every discipline
has its way of solving it. The artist uses light-shade interaction,
perspective, special colour coding. The architect produces projections
of the object. The cartographer uses both colour-coding and shading to
represent height elevations. There have been many attempts in the last
century by the entertainment industry to produce a three-dimensional
illusion, in the fifties it was fashionable to have 3d movies which
utilize the anaglyph method. Nowadays one can buy "Magic Eye" postcards
which show a hidden three dimensional picture if you stare at it half
cross-eyed. This poster attempts to demonstrate how some of these
techniques can be applied to three-dimensional data sets that can
occur in solar physics.

---------------------------------------------------------
Title: On the visualization of three-dimensional datasets
Authors: Verwichte, Erwin; Galsgaard, Klaus
1998SoPh..183..445V Altcode:
The effective visualization of three-dimensional (3D) datasets,
both observationally and computationally sourced, is becoming
common in solar physics. We present example plots of data from
a 3D magnetohydrodynamical simulation, where depth perception is
simulated using chromo-stereoscopy. The depth information is coded
into the images using colours. When such images are viewed with double
prism refraction ChromaDepthTM 3D glasses, a pronounced 3D effect is
achieved. This visualization method is especially suited for working
with and presenting computationally derived 3D datasets.

---------------------------------------------------------
Title: Visualization of three-dimensional datasets
Authors: Ireland, Jack; Walsh, Robert W.; Galsgaard, Klaus
1998SoPh..181...87I Altcode:
The effective visualization of three-dimensional (3d) datasets, both
observationally and computationally derived, is an increasing problem
in solar physics. We present here plots of computational data derived
from the 3d reconstruction of the magnetic field of a loop system,
rendered as anaglyphs. By combining images of the same 3d object
from two slightly different angles a realistic and useful 3d effect
is obtained, aiding data visualization. The application of the same
technique to real solar data (such as from the Coronal Diagnostic
Spectrometer (CDS) on board the Solar and Heliospheric Observatory
(SOHO)) is discussed.

---------------------------------------------------------
Title: Preliminary Results for Coronal Magnetic Fields as Suggested
by MDI Magnetograms
Authors: Walsh, R. W.; Ireland, J.; Mackay, D. H.; Galsgaard, K.;
Longbottom, A. W.
1998ASPC..155..371W Altcode: 1998sasp.conf..371W
No abstract at ADS

---------------------------------------------------------
Title: Are Prominences Formed by Flux Convergence?
Authors: Galsgaard, K.; Longbottom, A. W.
1998ASPC..150..282G Altcode: 1998IAUCo.167..282G; 1998npsp.conf..282G
No abstract at ADS

---------------------------------------------------------
Title: Energy Release Sites in Magnetic Fields Containing Single Or
Multiple Nulls
Authors: Galsgaard, K.; Reddy, R. V.; Rickard, G. J.
1997SoPh..176..299G Altcode:
An ongoing debate is how magnetic energy is released in solar flares,
which type of magnetic instabilities are responsible for triggering the
energy release, and which magnetic topologies are most likely to host
the instabilities. In this connection magnetic reconnection has been
a general ingredient, with most of the previous work focussing on 2D
reconnection. A natural extension to this is to investigate reconnection
in 3D topologies, in particular the behaviour of magnetic nulls and the
magnetic topology associated with them. This paper investigates the
difference in dynamical behaviour of a numerical domain that either
contains a double null-point pair connected by a separator or only a
fraction of the separator defined by the null-points. The experiments
show that nulls can either accumulate current individually, or act
together to produce a singular current collapse along the separator. The
implication of these results for the interpretation of coronal data
is discussed.

---------------------------------------------------------
Title: Coronal heating by shufling of footpoints.
Authors: Galsgaard, K.
1997smf..conf...71G Altcode:
This paper discusses numerical 3D experiments investigating the
possibility of the formation of small scale current sheets by
stressing magnetic field line topologies, by either shear or vorticity
motions. The results show that both of these perturbations drive
the magnetic field into a state where current sheets on different
length scales are formed within the numerical domain. The evolution
is highly dynamic and life times depend on local conditions. Scaling
relations between the experiment's initial parameters and the time
average dissipation are obtained. It is implied that the photospheric
motions can supply sufficient energy to maintain a hot corona.

---------------------------------------------------------
Title: Heating and activity of the solar corona. 3. Dynamics of a
low beta plasma with three-dimensional null points
Authors: Galsgaard, Klaus; Nordlund, Åke
1997JGR...102..231G Altcode:
We investigate the self-consistent nonlinear evolution of an initially
force-free three-dimensional magnetic field subjected to stress on
two boundaries. The results illustrate how complicated magnetic field
structures, such as those found in the solar corona, evolve dynamically
when forced by stress from boundaries and how the energy which is
temporarily stored in the magnetic field may be converted into other
forms of energy such as heat, flow energy, and fast particles. The
initial model state is triple periodic and contains eight magnetic null
points. During the time evolution, the current density concentrates
near particular locations in space that can be identified with the
singular field lines connecting pairs of null points of the initial
state. Current sheets are found to grow out of the singular lines formed
by the intersection of surfaces across which the magnetic connectivity
is discontinuous. Jets of plasma shoot out from the edges of the
currents sheets, driven by the “sling-shot” Lorentz force created
by reconnecting magnetic field lines. As a result of the reconnection,
most of the magnetic connectivity between the two boundaries is lost,
and the remaining magnetic field develops arcade-like structures along
the boundaries. These arcade structures are long-lived, and the system
enters a quasi-stationary state, where small-scale current sheets are
continually appearing and disappearing. The distribution of size of
these current sheets is limited only by the numerical resolution. The
current sheets dissipate the energy supplied by the boundary driving
and also slowly deplete the magnetic energy from the initial constant
alpha magnetic field. The dissipation occurs in an increasing number
of current sheets of decreasing size at higher numerical resolution,
which keeps the overall reconnection rate nearly independent of the
numerical resolution. This suggests that “fast reconnection” may occur
through the collaborative effort of a large number of many small-scale
current sheets, rather than in the very large magnetic Reynolds number
limit of single current sheets that has been so extensively discussed
in the literature. This has important applications to both the problem
of understanding coronal heating and the search for efficient flare
energy release mechanisms.

---------------------------------------------------------
Title: Topologically Forced Reconnection
Authors: Nordlund, A.; Galsgaard, K.
1997LNP...489..179N Altcode: 1997shpp.conf..179N
A magnetically dominated plasma driven by braiding motions on boundaries
at which magnetic field lines are anchored is forced to dissipate the
work being done upon it, no matter how small the electrical resistivity
may be. Recent numerical experiments have clarified the mechanisms
through which balance between the boundary work and the dissipation in
the interior is achieved. The results largely confirm Parker's (1972)
idea of "topological dissipation"; dissipation is achieved through the
formation of a hierarchy of electrical current sheets. Current sheets
form as a result of the topological interlocking of individual strands
of magnetic field. The average level of dissipation is well described
by a scaling law that is independent of the electrical resistivity.

---------------------------------------------------------
Title: Heating and activity of the solar corona. 2. Kink instability
in a flux tube
Authors: Galsgaard, Klaus; Nordlund, Åke
1997JGR...102..219G Altcode:
The development of kink instability in a flux tube is investigated
numerically, by solving the resistive MHD equations in three dimensions
for a setup where a flux tube is stressed by rotating both ends in
opposite directions. Two cases are investigated: one where the tube is
initially isolated and in pressure equilibrium with surrounding plasma
(external kink) and another with an initially uniform magnetic field,
where only a smaller part of the boundaries are used to twist the field
(internal kink). The twist angle at the onset of the kink instability
depends on several parameters, such as rotation velocity, tube diameter,
field strength, and magnetic resistivity, but is generally in the
range 4π-8π. Both sets of experiments are followed beyond the
point where they become kink unstable into the regime of nonlinear
evolution. Of particular interest is the topological evolution. As
magnetic dissipation becomes significant, the connectivity between
the two boundaries changes from ordered to chaotic, and small-scale
current sheets develop. Even though the gross features of the external
kink appear to saturate, the total magnetic energy continues to grow,
by a steady increase of the free energy in the chaotic region that
develops as a result of the kink and by a secular spreading of the
magnetic field into the initially field-free region. The internal
kink is confined to the cylinder defined by the boundary driving and
has only limited influence on the external magnetic field. After the
kink, the twist of the magnetic field is reduced, and the internal
kink settles into a quasi-steady state where the dissipation on the
average balances the Poynting flux input. The average Poynting flux
is similar in the external and internal kinks, with a magnitude that
corresponds to local winding numbers of the order of unity. Scaling of
these results to values characteristic of the solar corona indicate
that systematic rotation or shear of the endpoints could be a source
of quasi-steady heating in coronal loops.

---------------------------------------------------------
Title: Double null points and magnetic reconnection
Authors: Galsgaard, K.; Rickard, G. J.; Reddy, R. V.; Nordlund, Å.
1997AdSpR..19.1785G Altcode:
2D reconnection is possible only in connection with the existence
of a singularity in the magnetic field line topology, associated
with a magnetic null point or a current sheet. Both of these provide
an X-type structure of the magnetic field where fields of opposite
polarity meet and reconnect. In 3D a similar topology is found in a
null point pair, when the null points are connected by a separator
line. The separator is defined as the intersection line of the two
null-point fan planes. This paper reports on the topological evolution
of this configuration with respect to different perturbations emerging
from imposed boundary velocities, using a nonlinear numerical approach.

---------------------------------------------------------
Title: Investigations of numerical avalanches in a 3D vector field.
Authors: Galsgaard, K.
1996A&A...315..312G Altcode:
A numerical investigation of models of self-organised criticality in a
3D vector field is presented. The experiments show that the frequency
distributions of peak and total energy release depend sensitively
on the redistribution formula for the instabilities and on the
boundary conditions. Two criteria are found to be necessary to obtain
power law distributions for the energy release; 1) the field must be
systematically driven, so that large scale regions with coherent tension
are obtained, and 2) only a fraction of the field quantity triggering
the instability may be removed by the local redistribution procedure. If
these criteria are fulfilled, the resulting energy release distribution
functions closely resemble the ones observed for solar flares.

---------------------------------------------------------
Title: Heating and activity of the solar corona 1. Boundary shearing
of an initially homogeneous magnetic field
Authors: Galsgaard, Klaus; Nordlund, Åke
1996JGR...10113445G Altcode:
To contribute to the understanding of heating and dynamic activity
in boundary-driven, low-beta plasmas such as the solar corona, we
investigate how an initially homogeneous magnetic field responds
to random large-scale shearing motions on two boundaries, by
numerically solving the dissipative MHD equations, with resolutions
ranging from 243 to 1363. We find that even a
single application of large-scale shear, in the form of orthogonal
sinusoidal shear on two boundaries, leads to the formation of
tangential discontinuities (current sheets). The formation time
scales logarithmically with the resistivity and is of the order of a
few times the inverse shearing rate for any reasonable resistivity,
even though no mathematical discontinuity would form in a finite
time in the limit of vanishing resistivity. The reason for the
formation of the current sheets is the interlocking of two magnetic
flux systems. Reconnection in the current sheets is necessary for the
field lines to straighten out. The formation of current sheets causes
a transition to a very dynamic plasma state, where reconnection drives
supersonic and super-Alfvénic jet flows and where these, in turn,
cause the formation of smaller-scale current sheets. A statistically
steady state level for the average Poynting flux and the average Joule
dissipation is reached after a few correlation times, but both boundary
work and Joule dissipation are highly fluctuating in time and space
and are only weakly correlated. Strong and bursty Joule dissipation
events are favored when the volume has a large length/diameter ratio
and is systematically driven for periods longer than the Alfvèn
crossing time. The understanding of the reason for the current sheet
formation allows a simple scaling law to be constructed for the average
boundary work. Numerical experiments over a range of parameter values,
covering over 3 orders of magnitude in average dissipation, obey the
scaling law to within a factor of 2. The heating rate depends on the
boundary velocity amplitude and correlation time, the Alfvén speed,
and the initial magnetic field strength but appears to be independent
of the resistivity because of the formation of a hierarchy of current
sheets. Estimates of the photospheric boundary work on the solar coronal
magnetic field using the scaling law are consistent with estimates of
the required coronal heating rates. We therefore conclude that the
work supplied to the solar corona as a consequence of the motion of
the magnetic foot points in the solar photosphere and the emergence of
new flux is a significant contributor to coronal heating and flaring
and that it quite plausibly is the dominant one.

---------------------------------------------------------
Title: Dynamical Properties of Single and Double 3D Null Points
Authors: Galsgaard, K.; Rickard, G. J.; Reddy, R. V.; Nordlund, A.
1996ASPC..111...82G Altcode: 1997ASPC..111...82G
The dynamical reconnection properties of three-dimensional single and
double nulls are investigated using nonlinear simulations. The authors
confirm the importance of the three-dimensional topological structures
- the spine, fan, and separator - in the reconnection process. In
particular, they highlight the accumulated current structures in
relation to the underlying magnetic field topology as reconnection
proceeds.

---------------------------------------------------------
Title: Coronal Heating by Flux Braiding
Authors: Galsgaard, K.; Nordlund, Å.
1996ApL&C..34..175G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The challenge of numerical non-ideal MHD and investigations
of the coronal heating problem
Authors: Galsgaard, Klaus
1995PhDT.......191G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamic behavior and topology of 3D magnetic fields
Authors: Galsgaard, K.; Nordlund, Å.
1994SSRv...68...75G Altcode:
We investigate numerically the dynamical evolution of a boundary
driven, topologically complex low β plasma. The initial state is
a simple, but topologically nontrivial 3D magnetic field, and the
evolution is driven by forced motions on two opposite boundaries of
the computational domain. A large X-type reconnection event with a
supersonic one-sided jet occurs as part of a process that brakes down
the large scale topology of the initial field. An energetically steady
state is reached, with a double arcade overall topology, in which the
driving causes continuous creation of small scale thin current sheets
at various locations in the arcade structures.

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Title: Magnetoconvection and magnetoturbulence
Authors: Nordlund, Å.; Galsgaard, K.; Stein, R. F.
1994ASIC..433..471N Altcode:
No abstract at ADS

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Title: Numerical Simulations of Magnetic Reconnection in 3-D
Authors: Stein, Robert; Galsgaard, Klaus; Nordlund, Aake
1994ASPC...68..210S Altcode: 1994sare.conf..210S
No abstract at ADS

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Title: Deformation of Magnetic Null Points
Authors: Galsgaard, K.; Nordlund, A.
1992LNP...399..343G Altcode: 1992esf..coll..343G; 1992IAUCo.133..343G
No abstract at ADS

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Title: Large scale simulations
Authors: Nordlund, Ake; Galsgaard, Klaus
1992AIPC..267...13N Altcode: 1992ecsa.work...13N
We discuss large scale numerical simulations as a tool for obtaining
qualitative understanding of the processes directly and indirectly
responsible for coronal heating. The actual heating process in the
low beta coronal plasma is most likely driven by transfer of magnetic
energy from the subsurface high beta region, where magnetic energy is
created as an energetically insignificant byproduct of solar convection
and rotation. Based on the results of recent numerical experiments,
we discuss some of the processes involved.

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Title: Vector Potential Magnetic Null Points
Authors: Galsgaard, Klaus; Nordlund, Åke
1991LNP...380...89G Altcode: 1991IAUCo.130...89G; 1991sacs.coll...89G
No abstract at ADS

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Title: The Significance of Magnetic Null Points (With 1 Figure)
Authors: Galsgaard, K.; Nordlund, Å.
1991mcch.conf..541G Altcode:
No abstract at ADS