Author name code: galsgaard ADS astronomy entries on 2022-09-14 author:"Galsgaard, Klaus" ------------------------------------------------------------------------ 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 Bibcode: 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. 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 Bibcode: 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 https://www.aanda.org 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 Bibcode: 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 https://www.aanda.org 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 Bibcode: 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 https://www.aanda.org Title: Eruptions from quiet Sun coronal bright points. II. Non-potential modelling Authors: Galsgaard, Klaus; Madjarska, Maria S.; Mackay, Duncan H.; Mou, Chaozhou Bibcode: 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 https://www.aanda.org Title: Eruptions from quiet Sun coronal bright points: Observations & Modeling Authors: Madjarska, Maria S.; Galsgaard, Klaus; Mou, Chauzhou Bibcode: 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. Title: Eruptions from quiet Sun coronal bright points. I. Observations Authors: Mou, Chauzhou; Madjarska, Maria S.; Galsgaard, Klaus; Xia, Lidong Bibcode: 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 https://www.aanda.org Title: Magnetic topological analysis of coronal bright points Authors: Galsgaard, K.; Madjarska, M. S.; Moreno-Insertis, F.; Huang, Z.; Wiegelmann, T. Bibcode: 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 http://www.aanda.org 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. Bibcode: 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. Title: Mini-CME eruptions in a flux emergence event in a coronal hole environment Authors: Galsgaard, K.; Moreno-Insertis, F. Bibcode: 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. Title: Why Are Flare Ribbons Associated with the Spines of Magnetic Null Points Generically Elongated? Authors: Pontin, David; Galsgaard, Klaus; Démoulin, Pascal Bibcode: 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. Title: Why are flare ribbons generically elongated in configurations with magnetic null points? Authors: Pontin, David Iain; Galsgaard, Klaus; Demoulin, Pascal Bibcode: 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. Title: Active region upflows. I. Multi-instrument observations Authors: Vanninathan, K.; Madjarska, M. S.; Galsgaard, K.; Huang, Z.; Doyle, J. G. Bibcode: 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 http://www.aanda.org Title: Active region upflows. II. Data driven magnetohydrodynamic modelling Authors: Galsgaard, K.; Madjarska, M. S.; Vanninathan, K.; Huang, Z.; Presmann, M. Bibcode: 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 http://www.aanda.org 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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: On the Origin of Coronal Mass Ejections: How Does the Emergence of a Magnetic Flux Rope Reorganize the Solar Corona? Authors: Roussev, Ilia Iankov; Galsgaard, Klaus; Downs, Cooper; Lugaz, Noe; Sokolov, Igor Bibcode: 2012shin.confE..38R 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 Title: Scaling of turbulent and hierarchical reconnection Authors: Nordlund, A.; Galsgaard, K. Bibcode: 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 & Nordlund (1996, JGR 101, 13445), and Lazarian & Vishniac (1999; ApJ 517, 700), considering also the results of Lapenta (2008, PhRvL 100, 235001) and Bettarini & Lapenta (2010, A&A 518, 57). Title: Current accumulation at an asymmetric 3D null point caused by generic shearing motions Authors: Galsgaard, K.; Pontin, D. I. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 http://www.aanda.org Title: Structure of magnetic separators and separator reconnection Authors: Parnell, C. E.; Haynes, A. L.; Galsgaard, K. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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: Particle Acceleration in a Three-Dimensional Model of Reconnecting Coronal Magnetic Fields Authors: Cargill, Peter J.; Vlahos, Loukas; Turkmani, Rim; Galsgaard, Klaus; Isliker, Heinz Bibcode: 2007sdeh.book..249C Altcode: No abstract at ADS Title: Current Sheet Formation and Magnetic Reconnection at 3D Null Points Authors: Pontin, D. I.; Bhattacharjee, A.; Galsgaard, K. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2004ESASP.575..216M Altcode: 2004soho...15..216M No abstract at ADS Title: Fragment Driven Magnetic Reconnection Authors: Galsgaard, K.; Parnell, C. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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>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<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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 1998ASPC..155..371W Altcode: 1998sasp.conf..371W No abstract at ADS Title: Are Prominences Formed by Flux Convergence? Authors: Galsgaard, K.; Longbottom, A. W. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 1995PhDT.......191G Altcode: No abstract at ADS Title: Dynamic behavior and topology of 3D magnetic fields Authors: Galsgaard, K.; Nordlund, Å. Bibcode: 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. Title: Magnetoconvection and magnetoturbulence Authors: Nordlund, Å.; Galsgaard, K.; Stein, R. F. Bibcode: 1994ASIC..433..471N Altcode: No abstract at ADS Title: Numerical Simulations of Magnetic Reconnection in 3-D Authors: Stein, Robert; Galsgaard, Klaus; Nordlund, Aake Bibcode: 1994ASPC...68..210S Altcode: 1994sare.conf..210S No abstract at ADS Title: Deformation of Magnetic Null Points Authors: Galsgaard, K.; Nordlund, A. Bibcode: 1992LNP...399..343G Altcode: 1992esf..coll..343G; 1992IAUCo.133..343G No abstract at ADS Title: Large scale simulations Authors: Nordlund, Ake; Galsgaard, Klaus Bibcode: 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. Title: Vector Potential Magnetic Null Points Authors: Galsgaard, Klaus; Nordlund, Åke Bibcode: 1991LNP...380...89G Altcode: 1991IAUCo.130...89G; 1991sacs.coll...89G No abstract at ADS Title: The Significance of Magnetic Null Points (With 1 Figure) Authors: Galsgaard, K.; Nordlund, Å. Bibcode: 1991mcch.conf..541G Altcode: No abstract at ADS