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Author name code: thalmann
ADS astronomy entries on 2022-09-14
author:"Thalmann, Julia K." 

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Title: The Importance of Method Redundancy in Studying Pre-Eruption
    Evolution in Solar Active Regions
Authors: Georgoulis, Manolis K.; Pariat, Etienne; Liu, Yang; Thalmann,
   Julia K.
2022cosp...44.1358G    Altcode:
  In a recent synergistic work stemming from a prior International
  Space Science Institute (ISSI) Working Group, the evolution of
  magnetic helicity in an intensely eruptive solar active region was
  studied using several different helicity calculation methods. This
  was the first time all these methods were tested on real solar data,
  without the possibility of a ground truth. Focusing on the pre-eruption
  evolution prior to an eruptive X-class flare (SOL2006-12-13T02:14X3.4)
  in NOAA active region (AR) 10930, we reveal a more complex picture than
  what any single method might convey. Through imperfect but overall
  converging calculations from different methods, we find artifacts
  that could mislead conclusions. More importantly, we find evidence of
  competing physical tendencies in the active region whose omission could
  lead to counterintuitive, hence misleading, again, conclusions. While
  for the Sun we have the capability to use different data and methods
  for related purposes, this is not the case for other eruptive stars,
  which is a fact calling for robust modeling approaches, relying
  on scarce and indirect observations of stellar magnetic fields and
  CME properties. Confluence of any data available and modeling could
  offer the redundancy needed to critically assess partial findings
  and reconcile them into a physically consistent picture of stellar
  eruptions, quite possibly with qualitative / quantitative similarities
  and differences from the eruptions of our own Sun.

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Title: Magnetic helicity and energy budget around large confined
    and eruptive solar flares
Authors: Gupta, Manu; Veronig, Astrid; Thalmann, Julia K.
2022cosp...44.2424G    Altcode:
  In order to better understand the underlying process and prerequisites
  for solar activity, it is essential to study the time evolution of
  the coronal magnetic field of solar active regions (ARs), which is
  associated to flare activity and leads to large coronal mass ejections
  (CMEs). We investigate the coronal magnetic energy and helicity budgets
  of ten solar ARs around the times of large flares. In particular,
  we are interested in a possible relation of the derived quantities
  to the particular type of flares that the AR produces, i.e., whether
  they are associated with a CME or are confined. Using an optimization
  approach, we employed time series of 3D nonlinear force-free magnetic
  field models for each target AR, covering a time span of several hours
  around the time of occurrence of large solar flares (GOES class M1.0
  and larger). We subsequently computed the 3D magnetic vector potentials
  associated to the model 3D coronal magnetic field using a finite-volume
  method. This allows us to correspondingly compute the coronal magnetic
  energy and helicity budgets (so-called extensive quantities), as well as
  related intensive proxies, such as the relative contribution of free
  magnetic energy (the energy ratio), the fraction of non-potential
  (current-carrying) helicity, and the normalized current-carrying
  helicity. The extensive quantities of flare-productive ARs cover a
  broad range of magnitudes, with no apparent relation to the potential
  of an AR to produce a CME-associated flare. In contrast, we find
  the intensive proxies (the energy ratio, the helicity ratio, and
  the normalized current-carrying helicity) to be distinctly different
  for ARs that produce CME-associated large flares compared to those
  which produce confined flares. Thus, for the majority of ARs in our
  sample, characteristic pre-flare levels of the intensive proxies allow
  statements regarding the likelihood of subsequent CME-productivity.

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Title: Probing the coronal magnetic field with physics informed
    neural networks
Authors: Jarolim, Robert; Podladchikova, Tatiana; Veronig, Astrid;
   Thalmann, Julia K.
2022cosp...44.2463J    Altcode:
  While the photospheric magnetic field of our Sun is routinely
  measured, its extent into the upper solar atmosphere (the corona)
  remains elusive. In this study, we present a novel approach for coronal
  magnetic field extrapolation using physics informed neural networks. The
  neural network is optimized to match observations of the photospheric
  magnetic field vector at the bottom-boundary, while simultaneously
  satisfying the force-free and divergence-free equations in the entire
  simulation volume. We demonstrate that our method can account for
  noisy data and deviates from the physical model where the force-free
  magnetic field assumption cannot be satisfied. We utilize meta-learning
  concepts to simulate the evolution of the active region 11158. Our
  simulation of 5 days of observations at full cadence, requires less
  than 13 hours of total computation time. The derived evolution of the
  free magnetic energy and helicity in the active region, shows that
  our model captures flare signatures, and that the depletion of free
  magnetic energy spatially aligns with the observed EUV emission. Our
  method provides the ability to perform magnetic field extrapolations
  in quasi real-time, which can be used for space weather monitoring,
  studying pre-eruptive structures and as initial condition for MHD
  simulations. The flexibility in terms of data and the possibility of
  extending the underlying physical model, offers great potential for
  the field of magnetic field simulations.

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Title: The effect of spatial sampling on magnetic field modeling
    and helicity computation
Authors: Thalmann, J. K.; Gupta, M.; Veronig, A. M.
2022A&A...662A...3T    Altcode: 2022arXiv220409267T
  Context. Nonlinear force-free (NLFF) modeling is regularly used to
  indirectly infer the 3D geometry of the coronal magnetic field,
  which is not otherwise accessible on a regular basis by means of
  direct measurements. <BR /> Aims: We study the effect of binning in
  time-series NLFF modeling of individual active regions (ARs) in order to
  quantify the effect of a different underlying spatial sampling on the
  quality of modeling as well as on the derived physical parameters. <BR
  /> Methods: We apply an optimization method to sequences of Solar
  Dynamics Observatory (SDO) Helioseismic and Magnetic Imager (HMI) vector
  magnetogram data at three different plate scales for three solar active
  regions to obtain nine NLFF model time series. From the NLFF models,
  we deduce active-region magnetic fluxes, electric currents, magnetic
  energies, and relative helicities, and analyze those with respect
  to the underlying spatial sampling. We calculate various metrics to
  quantify the quality of the derived NLFF models and apply a Helmholtz
  decomposition to characterize solenoidal errors. <BR /> Results: At
  a given spatial sampling, the quality of NLFF modeling is different
  for different ARs, and the quality varies along the individual model
  time series. For a given AR, modeling at a certain spatial sampling is
  not necessarily of superior quality compared to that performed with a
  different plate scale. Generally, the NLFF model quality tends to be
  higher for larger pixel sizes with the solenoidal quality being the
  ultimate cause for systematic variations in model-deduced physical
  quantities. <BR /> Conclusions: Optimization-based modeling using
  SDO/HMI vector data binned to larger pixel sizes yields variations
  in magnetic energy and helicity estimates of ≲30% on overall,
  given that concise checks ensure the physical plausibility and high
  solenoidal quality of the tested model. Spatial-sampling-induced
  differences are relatively small compared to those arising from other
  sources of uncertainty, including the effects of applying different
  data calibration methods, those of using vector data from different
  instruments, or those arising from application of different NLFF
  methods to identical input data.

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Title: The 2019 International Women's Day Event: A Two-step Solar
    Flare with Multiple Eruptive Signatures and Low Earth Impact
Authors: Dumbovic, Mateja; Veronig, Astrid; Podladchikova, Tatiana;
   Thalmann, Julia; Chikunova, Galina; Dissauer, Karin; Magdalenic,
   Jasmina; Temmer, Manuela; Guo, Jingnan; Samara, Evangelia
2021AGUFMSH32A..08D    Altcode:
  We present a detailed analysis of an eruptive event that occurred on
  early 2019 March 8 in active region AR 12734, to which we refer as the
  International Women's day event. The event under study is intriguing in
  several aspects: 1) low-coronal eruptive signatures come in ”pairs” (a
  double-peak flare, two coronal dimmings, and two EUV waves); 2) although
  the event is characterized by a complete chain of eruptive signatures,
  the corresponding coronagraphic signatures are weak; 3) although
  the source region of the eruption is located close to the center of
  the solar disc and the eruption is thus presumably Earth-directed,
  heliospheric signatures are very weak with little Earth-impact. We
  analyze a number of multi-spacecraft and multi-instrument (both
  remote-sensing and in situ) observations, including Soft X-ray,
  (extreme-) ultraviolet (E)UV), radio and white-light emission, as well
  as plasma, magnetic field and particle measurements. We employ 3D NLFF
  modeling to investigate the coronal magnetic field configuration in and
  around the active region, the GCS model to make a 3D reconstruction of
  the CME geometry and the 3D MHD numerical model EUHFORIA to model the
  background state of the heliosphere. Our results indicate two subsequent
  eruptions of two systems of sheared and twisted magnetic fields,
  which merge already in the upper corona and start to evolve further
  out as a single entity. The large-scale magnetic field significantly
  influences both, the early and the interplanetary evolution of the
  structure. During the first eruption the stability of the overlying
  field was disrupted which enabled the second eruption. We find that
  during the propagation in the interplanetary space the large-scale
  magnetic field, i.e. , the location of heliospheric current sheet
  between the AR and the Earth likely influences propagation and the
  evolution of the erupted structure(s).

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Title: Magnetic Helicity Estimations in Models and Observations of
    the Solar Magnetic Field. IV. Application to Solar Observations
Authors: Thalmann, J. K.; Georgoulis, M. K.; Liu, Y.; Pariat, E.;
   Valori, G.; Anfinogentov, S.; Chen, F.; Guo, Y.; Moraitis, K.; Yang,
   S.; Mastrano, Alpha; ISSI Team on Magnetic Helicity
2021ApJ...922...41T    Altcode: 2021arXiv210808525T
  In this ISSI-supported series of studies on magnetic helicity in the
  Sun, we systematically implement different magnetic helicity calculation
  methods on high-quality solar magnetogram observations. We apply
  finite-volume, discrete flux tube (in particular, connectivity-based)
  and flux-integration methods to data from Hinode's Solar Optical
  Telescope. The target is NOAA Active Region 10930 during a 1.5-day
  interval in 2006 December that included a major eruptive flare
  (SOL2006-12-13T02:14X3.4). Finite-volume and connectivity-based methods
  yield instantaneous budgets of the coronal magnetic helicity, while
  the flux-integration methods allow an estimate of the accumulated
  helicity injected through the photosphere. The objectives of our work
  are twofold: a cross-validation of methods, as well as an interpretation
  of the complex events leading to the eruption. To the first objective,
  we find (i) strong agreement among the finite-volume methods, (ii)
  a moderate agreement between the connectivity-based and finite-volume
  methods, (iii) an excellent agreement between the flux-integration
  methods, and (iv) an overall agreement between finite-volume- and
  flux-integration-based estimates regarding the predominant sign and
  magnitude of the helicity. To the second objective, we are confident
  that the photospheric helicity flux significantly contributed to the
  coronal helicity budget and that a right-handed structure erupted from
  a predominantly left-handed corona during the X-class flare. Overall,
  we find that the use of different methods to estimate the (accumulated)
  coronal helicity may be necessary in order to draw a complete picture
  of an active region corona, given the careful handling of identified
  data (preparation) issues, which otherwise would mislead the event
  analysis and interpretation.

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Title: Magnetic helicity and energy budget around large confined
    and eruptive solar flares
Authors: Gupta, M.; Thalmann, J. K.; Veronig, A. M.
2021A&A...653A..69G    Altcode: 2021arXiv210608781G
  Context. In order to better understand the underlying processes
  and prerequisites for solar activity, it is essential to study the
  time evolution of the coronal magnetic field of solar active regions
  (ARs) associated with flare activity. <BR /> Aims: We investigate the
  coronal magnetic energy and helicity budgets of ten solar ARs around
  the times of large flares. In particular, we are interested in a
  possible relation of the derived quantities to the particular type of
  the flares that the AR produces, namely, whether they are associated
  with a CME or whether they are confined (i.e., not accompanied by a
  CME). <BR /> Methods: Using an optimization approach, we employed time
  series of 3D nonlinear force-free magnetic field models of ten ARs,
  covering a time span of several hours around the time of occurrence
  of large solar flares (GOES class M1.0 and larger). We subsequently
  computed the 3D magnetic vector potentials associated to the model 3D
  coronal magnetic field using a finite-volume method. This allows us
  to correspondingly compute the coronal magnetic energy and helicity
  budgets, as well as related (intensive) quantities such as the
  relative contribution of free magnetic energy, E<SUB>F</SUB>/E (energy
  ratio), the fraction of non-potential (current-carrying) helicity,
  |H<SUB>J</SUB>|/|H<SUB>V</SUB>| (helicity ratio), and the normalized
  current-carrying helicity, |H<SUB>J</SUB>|/ϕ'<SUP>2</SUP>. <BR />
  Results: The total energy and helicity budgets of flare-productive
  ARs (extensive parameters) cover a broad range of magnitudes, with
  no obvious relation to the eruptive potential of the individual
  ARs, that is, whether or not a CME is produced in association with
  the flare. The intensive eruptivity proxies, E<SUB>F</SUB>/E and
  |H<SUB>J</SUB>|/|H<SUB>V</SUB>|, and |H<SUB>J</SUB>|/ϕ'<SUP>2</SUP>,
  however, seem to be distinctly different for ARs that produce
  CME-associated large flares compared to those which produce confined
  flares. For the majority of ARs in our sample, we are able to identify
  characteristic pre-flare magnitudes of the intensive quantities
  that are clearly associated with subsequent CME-productivity. <BR />
  Conclusions: If the corona of an AR exhibits characteristic values of
  ⟨|H<SUB>J</SUB>|/|H<SUB>V</SUB>|⟩ &gt; 0.1, ⟨E<SUB>F</SUB>/E⟩
  &gt; 0.2, and ⟨|H<SUB>J</SUB>|/ϕ'<SUP>2</SUP>⟩ &gt; 0.005, then
  the AR is likely to produce large CME-associated flares. Conversely,
  confined large flares tend to originate from ARs that exhibit
  coronal values of ⟨|H<SUB>J</SUB>|/|H<SUB>V</SUB>|⟩ ≲ 0.1,
  ⟨E<SUB>F</SUB>/E⟩ ≲ 0.1, and ⟨|H<SUB>J</SUB>|/ϕ'<SUP>2</SUP>⟩
  ≲ 0.002.

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Title: 2019 International Women's Day event. Two-step solar flare
    with multiple eruptive signatures and low Earth impact
Authors: Dumbović, M.; Veronig, A. M.; Podladchikova, T.; Thalmann,
   J. K.; Chikunova, G.; Dissauer, K.; Magdalenić, J.; Temmer, M.; Guo,
   J.; Samara, E.
2021A&A...652A.159D    Altcode: 2021arXiv210615417D
  Context. We present a detailed analysis of an eruptive event that
  occurred on 2019 March 8 in the active region AR 12734, which we
  refer as the International Women's Day event. The event under study
  is intriguing based on several aspects: (1) low-coronal eruptive
  signatures come in `pairs', namely, there is a double-peaked flare,
  two coronal dimmings, and two extreme ultraviolet (EUV) waves; (2)
  although the event is characterized by a complete chain of eruptive
  signatures, the corresponding coronagraphic signatures are weak;
  and (3) although the source region of the eruption is located close
  to the center of the solar disc and the eruption is thus presumably
  Earth-directed, heliospheric signatures are very weak with very weak
  Earth impact. <BR /> Aims: In order to understand the initiation and
  evolution of this particular event, we performed a comprehensive
  analysis using a combined observational-modeling approach. <BR />
  Methods: We analyzed a number of multi-spacecraft and multi-instrument
  (both remote-sensing and in situ) observations, including soft X-ray,
  EUV, radio and white-light emission, as well as plasma, magnetic field,
  and particle measurements. We employed 3D nonlinear force-free modeling
  to investigate the coronal magnetic field configuration in and around
  the active region, the graduated cylindrical shell model to make a 3D
  reconstruction of the CME geometry, and the 3D magnetohydrodynamical
  numerical model EUropean Heliospheric FORecasting Information Asset
  to model the background state of the heliosphere. <BR /> Results:
  Our results reveal a two-stage C1.3 flare, associated with two
  EUV waves that occur in close succession and two-stage coronal
  dimmings that evolve co-temporally with the flare and type II and
  III radio bursts. Despite its small GOES class, a clear drop in
  magnetic free energy and helicity is observed during the flare. White
  light observations do not unambiguously indicate two separate CMEs,
  but rather a single entity most likely composed of two sheared and
  twisted structures corresponding to the two eruptions observed in the
  low corona. The corresponding interplanetary signatures are that of
  a small flux rope swith indications of strong interactions with the
  ambient plasma, which result in a negligible geomagnetic impact. <BR
  /> Conclusions: Our results indicate two subsequent eruptions of
  two systems of sheared and twisted magnetic fields, which already
  begin to merge in the upper corona and start to evolve further out
  as a single entity. The large-scale magnetic field significantly
  influences both the early and the interplanetary evolution of the
  structure. During the first eruption, the stability of the overlying
  field was disrupted, enabling the second eruption. We find that during
  the propagation in the interplanetary space the large-scale magnetic
  field, that is, the location of heliospheric current sheet between the
  AR and the Earth, is likely to influence propagation, along with the
  evolution of the erupted structure(s). <P />Movies are available at <A
  href="https://www.aanda.org/10.1051/0004-6361/202140752/olm">https://www.aanda.org</A>

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Title: Interpretable Solar Flare Forecasting with Deep Learning
Authors: Jarolim, Robert; Podladchikova, Tatiana; Veronig, Astrid;
   Thalmann, Julia K.; Hofinger, -Markus; Narnhofer, -Dominik; Pock,
   -Thomas; Schopper, Tobias
2021cosp...43E1036J    Altcode:
  Solar flares and coronal mass ejections (CMEs) are the main drivers
  for severe space weather disturbances on Earth and other planets. While
  the geo-effects of CMEs give us a lead time of about 1 to 4 days, the
  effects of flare induced enhanced radiation and flare-accelerated solar
  energetic particles (SEPs) are very immediate, approximately 8 and 20
  minutes, respectively. Thus, predictions of solar flare occurrence at
  least several hours ahead are of high importance for the mitigation
  of severe space weather effects. Observations and simulations of solar
  flares suggest that the structure and evolution of the active region's
  magnetic field is a key component for energetic eruptions. However,
  the main changes are assumed to happen in the coronal fields, whereas
  current measurements are mostly restricted to the photospheric magnetic
  field. We present an automatic flare prediction deep learning algorithm
  based on the HMI photospheric line-of-sight magnetic field and its
  temporal evolution together with the coronal evolution as observed by
  multi-wavelengths EUV filtergrams from the AIA instrument onboard the
  Solar Dynamics Observatory. As input to our deep learning model we use
  the magnetograms and EUV filtergrams with a cadence of 10 minutes over
  a 40 minutes time interval from pre-identified active regions. The
  neural network predicts X, M and C class flares up to 3 hours ahead,
  hereby the network assigns probabilities for the flare occurrence to
  consecutive time frames of 20 minutes. From this setup the network
  learns independently to identify features in the imaging data based
  on the dynamic evolution of the coronal structure and the photospheric
  magnetic field evolution, which may hint at flare occurrence in the near
  future. In order to overcome the "black box problem" of machine-learning
  algorithms, and thus to allow for physical interpretation of the network
  findings, we employ an attention mechanism at multiple resolution
  scales, which enables the network to focus on relevant regions within
  the spatio-temporal domain. This allows us to extract the emphasized
  regions, which reveal the neural network interpretation of the flare
  onset conditions. Our novel approach combines the performance of neural
  network predictions with the benefit of a direct interpretation of
  the relevant physical features.

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Title: Estimating the magnetic flux within an eruptive flux rope
Authors: Temmer, Manuela; Rodriguez, Luciano; Dissauer, Karin; Veronig,
   Astrid; Tschernitz, Johannes; Thalmann, Julia K.; Hinterreiter, Jürgen
2021cosp...43E1741T    Altcode:
  Erupting magnetic flux ropes develop into coronal mass ejections (CMEs)
  as they evolve and finally propagate into interplanetary space. Those
  large scale eruptions are observed to be frequently related to dynamic
  surface phenomena such as coronal waves and dimming regions. The better
  we are able to estimate initial CME parameters such as kinematics,
  geometry, and magnetic properties, the more precisely we can feed
  state-of-the-art CME propagation models and with that improve CME
  forecasting. In that respect, we report on a well-observed flare-CME
  event from 1 October 2011 focusing on the dynamic evolution of the
  CME and its embedded magnetic field. Using combined STEREO and SDO
  observations together with nonlinear force-free (NLFF) modeling we
  derive separately the flare reconnection and dimming flux. We find
  that already before the start of the impulsive flare phase magnetic
  reconnection was ongoing, that added magnetic flux to the flux rope
  before its final eruption. As the dimming evolves over a longer time
  span than the flaring phase, we find that the dimming flux increases by
  more than 25% after the end of the flare. This indicates that magnetic
  flux is still added to the flux rope after eruption and that the derived
  flare reconnection flux is most probably a lower limit for estimating
  the magnetic flux within the flux rope.

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Title: Homologous Flaring Activity over a Sunspot Light Bridge in
    an Emerging Active Region
Authors: Louis, Rohan Eugene; Thalmann, Julia K.
2021ApJ...907L...4L    Altcode: 2020arXiv201207454L
  Sunspot light bridges are known to exhibit a variety of dynamic and
  persistent phenomena such as surges, small-scale jets, etc., in the
  chromosphere and transition region. While it has generally been proposed
  that magnetic reconnection is responsible for this small-scale dynamism,
  persistent flaring activity lasting several hours from the same spatial
  location on a sunspot light bridge has rarely been reported. We combine
  observations from the Atmospheric Imaging Assembly and the Helioseismic
  Magnetic Imager on board the Solar Dynamics Observatory to investigate
  homologous flaring activity over a small sunspot light bridge in
  an emerging flux region. The homologous flares all produced broad,
  collimated jets including a B6.4 class flare. The jets rise at a speed
  of about 200 km s<SUP>-1</SUP>, reach projected heights of about 98
  Mm, and emerge from the same spatial location for nearly 14 hrs, after
  which they cease completely. A nonlinear force-free extrapolation of
  the photospheric magnetic field shows a low-lying flux rope connecting
  the light bridge to a remote opposite-polarity network. The persistent
  flares occur as a result of the rapid horizontal motion of the leading
  sunspot that causes the relatively vertical magnetic fields in the
  adjacent umbra to reconnect with the low-lying flux rope in the light
  bridge. Our results indicate that the flaring ceases once the flux
  rope has lost sufficient twist through repeated reconnections.

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Title: Deducing the reliability of relative helicities from nonlinear
    force-free coronal models
Authors: Thalmann, J. K.; Sun, X.; Moraitis, K.; Gupta, M.
2020A&A...643A.153T    Altcode: 2020arXiv200905287T
  <BR /> Aims: We study the relative helicity of active region (AR) NOAA
  12673 during a ten-hour time interval centered around a preceding X2.2
  flare (SOL2017-09-06T08:57) and also including an eruptive X9.3 flare
  that occurred three hours later (SOL2017-09-06T11:53). In particular,
  we aim for a reliable estimate of the normalized self-helicity of
  the current-carrying magnetic field, the so-called helicity ratio,
  |H<SUB>J</SUB>|/|H<SUB>𝒱</SUB>|, a promising candidate to quantity
  the eruptive potential of solar ARs. <BR /> Methods: Using Solar
  Dynamics Observatory Helioseismic and Magnetic Imager vector magnetic
  field data as an input, we employ nonlinear force-free (NLFF) coronal
  magnetic field models using an optimization approach. The corresponding
  relative helicity, and related quantities, are computed using a
  finite-volume method. From multiple time series of NLFF models based
  on different choices of free model parameters, we are able to assess
  the spread of |H<SUB>J</SUB>|/|H<SUB>𝒱</SUB>|, and to estimate
  its uncertainty. <BR /> Results: In comparison to earlier works,
  which identified the non-solenoidal contribution to the total magnetic
  energy, E<SUB>div</SUB>/E, as selection criterion regarding the required
  solenoidal quality of magnetic field models for subsequent relative
  helicity analysis, we propose to use in addition the non-solenoidal
  contribution to the free magnetic energy, |E<SUB>mix</SUB>|/E<SUB>J,
  s</SUB>. As a recipe for a reliable estimate of the relative magnetic
  helicity (and related quantities), we recommend to employ multiple NLFF
  models based on different combinations of free model parameters, to
  retain only those that exhibit smallest values of both E<SUB>div</SUB>/E
  and |E<SUB>mix</SUB>|/E<SUB>J, s</SUB> at a certain time instant,
  to subsequently compute mean estimates, and to use the spread of the
  individually contributing values as an indication for the uncertainty.

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Title: Erratum: "On the Reliability of Magnetic Energy and Helicity
    Computations Based on Nonlinear Force-free Coronal Magnetic Field
    Models" (2019, ApJL, 880, L6)
Authors: Thalmann, Julia K.; Linan, L.; Pariat, E.; Valori, G.
2020ApJ...902L..48T    Altcode:
  No abstract at ADS

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Title: Magnetic Helicity Budget of Solar Active Regions Prolific of
    Eruptive and Confined Flares
Authors: Thalmann, Julia K.; Moraitis, K.; Linan, L.; Pariat, E.;
   Valori, G.; Dalmasse, K.
2019ApJ...887...64T    Altcode: 2019arXiv191006563T
  We compare the coronal magnetic energy and helicity of two solar active
  regions (ARs), prolific in major eruptive (AR 11158) and confined
  (AR 12192) flaring, and analyze the potential of deduced proxies
  to forecast upcoming flares. Based on nonlinear force-free (NLFF)
  coronal magnetic field models with a high degree of solenoidality,
  and applying three different computational methods to investigate
  the coronal magnetic helicity, we are able to draw conclusions
  with a high level of confidence. Based on real observations of two
  solar ARs we checked trends regarding the potential eruptivity of
  the active-region corona, as suggested earlier in works that were
  based on numerical simulations, or solar observations. Our results
  support that the ratio of current-carrying to total helicity, |
  {H}<SUB>{{J</SUB>}}| /| {H}<SUB>{ \mathcal V </SUB>}| , shows a strong
  ability to indicate the eruptive potential of a solar AR. However, |
  {H}<SUB>{{J</SUB>}}| /| {H}<SUB>{ \mathcal V </SUB>}| does not seem to
  be indicative for the magnitude or type of an upcoming flare (confined
  or eruptive). Interpreted in the context of earlier observational
  studies, our findings furthermore support that the total relative
  helicity normalized to the magnetic flux at the NLFF model’s lower
  boundary, {H}<SUB>{ \mathcal V </SUB>}/{φ }<SUP>2</SUP>, represents
  no indicator for the eruptivity.

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Title: A Hot Cusp-shaped Confined Solar Flare
Authors: Hernandez-Perez, Aaron; Su, Yang; Thalmann, Julia; Veronig,
   Astrid M.; Dickson, Ewan C.; Dissauer, Karin; Joshi, Bhuwan; Chandra,
   Ramesh
2019ApJ...887L..28H    Altcode: 2019arXiv191110859H
  We analyze a confined flare that developed a hot cusp-like structure
  high in the corona (H ∼ 66 Mm). A growing cusp-shaped flare arcade
  is a typical feature in the standard model of eruptive flares, caused
  by magnetic reconnection at progressively larger coronal heights. In
  contrast, we observe a static hot cusp during a confined flare. Despite
  an initial vertical temperature distribution similar to that in eruptive
  flares, we observe a distinctly different evolution during the late
  (decay) phase, in the form of prolonged hot emission. The distinct
  cusp shape, rooted at locations of nonthermal precursor activity, was
  likely caused by a magnetic field arcade that kinked near the top. Our
  observations indicate that the prolonged heating was a result of slow
  local reconnection and an increased thermal pressure near the kinked
  apexes due to continuous plasma upflows.

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Title: Observations of a Footpoint Drift of an Erupting Flux Rope
Authors: Zemanová, Alena; Dudík, Jaroslav; Aulanier, Guillaume;
   Thalmann, Julia K.; Gömöry, Peter
2019ApJ...883...96Z    Altcode: 2019arXiv190802082Z
  We analyze the imaging observations of an M-class eruptive flare of 2015
  November 4. The pre-eruptive Hα filament was modeled by the nonlinear
  force-free field model, which showed that it consisted of two helical
  systems. Tether-cutting reconnection involving these two systems led
  to the formation of a hot sigmoidal loop structure rooted in a small
  hook that formed at the end of the flare ribbon. Subsequently, the hot
  loops started to slip away from the small hook until it disappeared. The
  loops continued slipping and the ribbon elongated itself by several
  tens of arcseconds. A new and larger hook then appeared at the end of
  the elongated ribbon with hot and twisted loops rooted there. After
  the eruption of these hot loops, the ribbon hook expanded and later
  contracted. We interpret these observations in the framework of
  the recent three-dimensional (3D) extensions to the standard solar
  flare model predicting the drift of the flux rope footpoints. The hot
  sigmoidal loop is interpreted as the flux rope, whose footpoints drift
  during the eruption. While the deformation and drift of the new hook can
  be described by the model, the displacement of the flux rope footpoint
  from the filament to that of the erupting flux rope indicate that the
  hook evolution can be more complex than those captured by the model.

---------------------------------------------------------
Title: On the Reliability of Magnetic Energy and Helicity Computations
    Based on Nonlinear Force-free Coronal Magnetic Field Models
Authors: Thalmann, Julia K.; Linan, L.; Pariat, E.; Valori, G.
2019ApJ...880L...6T    Altcode: 2019arXiv190701179T
  We demonstrate the sensitivity of magnetic energy and helicity
  computations regarding the quality of the underlying coronal magnetic
  field model. We apply the method of Wiegelmann &amp; Inhester to a
  series of Solar Dynamics Observatory/Helioseismic and Magnetic Imager
  vector magnetograms, and discuss nonlinear force-free (NLFF) solutions
  based on two different sets of the free model parameters. The two
  time series differ from each other concerning their force-free and
  solenoidal quality. Both force- and divergence-freeness are required
  for a consistent NLFF solution. Full satisfaction of the solenoidal
  property is inherent in the definition of relative magnetic helicity
  in order to ensure gauge independence. We apply two different magnetic
  helicity computation methods to both NLFF time series and find that
  the output is highly dependent on the level to which the NLFF magnetic
  fields satisfy the divergence-free condition, with the computed magnetic
  energy being less sensitive than the relative helicity. Proxies for
  the nonpotentiality and eruptivity derived from both quantities are
  also shown to depend strongly on the solenoidal property of the NLFF
  fields. As a reference for future applications, we provide quantitative
  thresholds for the force- and divergence-freeness, for the assurance
  of reliable computation of magnetic energy and helicity, and of their
  related eruptivity proxies.

---------------------------------------------------------
Title: Invisibility of Solar Active Region Umbra-to-Umbra Coronal
Loops: New Evidence that Magnetoconvection Drives Solar-Stellar
    Coronal Heating
Authors: Moore, Ronald L.; Tiwari, Sanjiv; Thalmann, Julia; Panesar,
   Navdeep; Winebarger, Amy
2019AAS...23410603M    Altcode:
  How magnetic energy is injected and released in the solar corona,
  keeping it heated to several million degrees, remains elusive. The
  corona is shaped by the magnetic field that fills it and the heating
  of the corona generally increases with increasing strength of the
  field. For each of two bipolar solar active regions having one or
  more sunspots in each of the two main opposite-polarity domains of
  magnetic flux, from comparison of a nonlinear force-free model of the
  active region's three-dimensional coronal magnetic field to observed
  extreme-ultraviolet coronal loops, we find that (1) umbra-to-umbra
  loops, despite being rooted in the strongest magnetic flux at both ends,
  are invisible, and (2) the brightest loops have one foot in a sunspot
  umbra or penumbra and the other foot in another sunspot's penumbra or
  in unipolar or mixed-polarity plage. The invisibility of umbra-to-umbra
  loops is new evidence that magnetoconvetion drives solar-stellar coronal
  heating: evidently, the strong umbral field at both ends quenches the
  magnetoconvection and hence the heating. Broadly, our results indicate
  that depending on the field strength in both feet, the photospheric feet
  of a coronal loop on any convective star can either engender or quench
  coronal heating in the body of the loop. <P />This work was supported
  by funding from the Heliophysics Division of NASA's Science Mission
  Directorate, from NASA's Postdoctoral Program, and from the Austrian
  Science Fund. The results have been published in The Astrophysical
  Journal Letters (Tiwari, S. K., Thalmann, J. K., Panesar, N. K., Moore,
  R. L., &amp; Winebarger, A. R. 2017, ApJ Letters, 843:L20).

---------------------------------------------------------
Title: Pre-eruption Processes: Heating, Particle Acceleration, and the
    Formation of a Hot Channel before the 2012 October 20 M9.0 Limb Flare
Authors: Hernandez-Perez, Aaron; Su, Yang; Veronig, Astrid M.;
   Thalmann, Julia; Gömöry, Peter; Joshi, Bhuwan
2019ApJ...874..122H    Altcode: 2019arXiv190208436H
  We report a detailed study of the pre-eruption activities that led to
  the occurrence of an M9.0 flare/CME event on 2012 October 20 in NOAA
  AR 11598. This includes the study of the preceding confined C2.4 flare
  that occurred on the same AR ∼25 minutes earlier. We observed that the
  M9.0 flare occurred as a consequence of two distinct triggering events
  well separated in time. The first triggering episode occurred as early
  as ∼20 minutes before the onset of the M9.0 flare, evidenced by the
  destabilization and rise of a pre-existing filament to a new position of
  equilibrium at a higher coronal altitude during the decay phase of the
  C2.4 flare. This brought the system to a magnetic configuration where
  the establishment of the second triggering event was favorable. The
  second triggering episode occurred ∼17 minutes later, during the
  early phase of the M9.0 flare, evidenced by the further rise of the
  filament and successful ejection. The second trigger is followed by a
  flare precursor phase, characterized by nonthermal emission and the
  sequential formation of a hot channel as shown by the SDO/AIA DEM
  (differential emission measure) maps, the RHESSI X-ray images and
  spectra. These observations are suggestive of magnetic reconnection
  and particle acceleration that can explain the precursor phase and can
  be directly related to the formation of the hot channel. We discuss
  the triggering mechanisms, their implications during the early and
  precursor phases and highlight the importance of early activities and
  preceding small confined flares to understand the initiation of large
  eruptive flares.

---------------------------------------------------------
Title: Which factors of an active region determine whether a strong
    flare will be CME associated or not?
Authors: Baumgartner, Christian; Thalmann, Julia K.; Veronig, Astrid M.
2018csc..confE..10B    Altcode:
  We study how the magnetic field determines whether a strong flare
  launched from an active region (AR) will be eruptive or confined,
  i.e. associated with a coronal mass ejection (CME) or not. To this aim,
  we selected all large flares that were observed by the SDO HMI and
  AIA instruments during the period 2011 to 2015 within 50° from the
  disk center. In total, our data set comprises 44 flares of GOES class
  &gt;M5.0. Out of these, 12 events were confined (7 M and 5 X-flares) and
  32 were eruptive (18 M- and 14 X-flares). We used 3D potential magnetic
  field models to study their location within the host AR (using the flare
  distance from the flux-weighted AR center, d_{FC}) and the strength
  of the overlying coronal field (via decay index n). We also present a
  first systematic study of the orientation of the coronal magnetic field
  changing with height, using the orientation φ of the flare-relevant
  polarity inversion line as a measure. We analyzed all quantities with
  respect to the size of the underlying active-region dipole field,
  defined by the distance between the flux-weighted opposite-polarity
  centers, d_{PC}. We find that flares originating from the periphery
  of an AR dipole field (d_{FC} / d_{PC} &gt; 0.5) are predominantly
  eruptive. Flares originating from underneath the AR dipole field (d_{FC}
  / d_{PC} &lt; 0.5) tend to be eruptive when they are launched from
  a compact AR and confined when launched from an extended AR (d_{PC}
  &gt; 60 Mm). In confined events, the flare-relevant field adjusts its
  orientation quickly to that of the underlying dipole field with height
  (δ φ &gt; 40° between the surface and the apex of the active-region
  dipole field), in contrast to eruptive events where it changes more
  slowly. The critical height for torus instability discriminates best
  between confined (h_{crit} &gt; 40 Mm) and eruptive flares (h_{crit}
  &lt; 40 Mm). It discriminates better than δ φ, implying that the decay
  of the confining field plays a stronger role in the eruptive/confined
  character of a flare than its orientation at different heights.

---------------------------------------------------------
Title: Magnetic reconnection fluxes in solar flares and their
    implications for solar and stellar superflares
Authors: Veronig, Astrid; Tschernitz, Johannes; Thalmann, Julia K.;
   Hinterreiter, Jürgen; Pötzi, Werner
2018cosp...42E3538V    Altcode:
  We study the energy release process of a set of 51 solar flares
  which span almost four orders of magnitude in flare energy, from GOES
  class B3 to X17. 19 events of our sample are eruptive, i.e. have a
  CME associated, and 32 are confined (no CME associated). We use Hα
  filtergrams from Kanzelhöhe Observatory together with SDO HMI and SOHO
  MDI magnetograms to derive magnetic reconnection fluxes and reconnection
  rates. We find that the flare reconnection flux is strongly correlated
  with the peak of the GOES 1-8 Å soft X-ray flux (r=0.9, in log-log
  space), both for confined and eruptive flares. In the largest events,
  up to ≈50% of the total magnetic flux of the host active region
  (AR) is involved in the flare magnetic reconnection. Based on these
  findings, we extrapolate the properties of the largest flares that may
  be launched from our present day's Sun. A complex solar AR that hosts
  a magnetic flux of 2\cdot 10^{23} {Mx}, which is supported by the
  largest active-region magnetic fluxes directly measured, is capable
  of producing an X80 flare (corresponding to a bolometric energy of
  about 7 \cdot 10^{32} ergs). Using a magnetic flux estimate of 6\cdot
  10^{23} {Mx} for the largest solar AR observed, we find that flares
  of GOES class ≈X500 could be produced (E_{bol} ≈ 3 \cdot 10^{33}
  ergs). Our results lie on the lower end of the energies of superflares
  on solar-type stars recently detected in Kepler data. Furthermore, they
  suggest that the present day's Sun is capable of producing flares and
  related space weather events more than an order of magnitude stronger
  than observed in the past.

---------------------------------------------------------
Title: Which factors of an active region determine whether a flare
    will be eruptive or confined?
Authors: Veronig, Astrid; Thalmann, Julia K.; Baumgartner, Christian
2018cosp...42E3539V    Altcode:
  We study how the magnetic field determines whether a strong flare
  launched from an active region (AR) will be eruptive or confined. To
  this aim, we analyzed 44 flares above GOES class M5.0 that occurred
  during 2011-2015. We used 3D potential magnetic field models to study
  their location within the host AR (using the flare distance from the
  flux-weighted AR center, d_{{FC}}) and the strength of the overlying
  coronal field (via decay index n). We also present a first systematic
  study of the orientation of the coronal magnetic field changing
  with height, using the orientation φ of the flare-relevant polarity
  inversion line as a measure. We analyzed all quantities with respect to
  the size of the underlying active-region dipole field, characterized
  by the distance between the flux-weighted opposite-polarity centers,
  d_{{PC}}. We find that flares originating from the periphery
  of an active-region dipole field (d_{{FC}}/d_{{PC}}&gt;0.5) are
  predominantly eruptive. Flares originating from underneath the AR
  dipole field (d_{{FC}}/d_{{PC}}&lt;0.5) tend to be eruptive when they
  are launched from a compact AR (d_{{PC}}≤60 Mm) and confined when
  launched from an extended AR. In confined events the flare-relevant
  field adjusts its orientation quickly to that of the underlying dipole
  field with height (Δφ≳40° between the surface and the apex of
  the active-region dipole field), in contrast to eruptive events where
  it changes more slowly with height. The critical height for torus
  instability, h_{{crit}}=h(n=1.5), discriminates best between confined
  (h_{{crit}}≳40 Mm) and eruptive flares (h_{{crit}}≲40 Mm). It
  discriminates better than Δφ, implying that the decay of the confining
  field plays a stronger role than its orientation at different heights.

---------------------------------------------------------
Title: Characteristics of ribbon evolution and reconnection electric
    fields in Hα two-ribbon flares
Authors: Hinterreiter, Jürgen; Veronig, Astrid; Thalmann, Julia;
   Tschernitz, Johannes; Pötzi, Werner
2018EGUGA..20.9819H    Altcode:
  We perform a statistical study of magnetic reconnection related
  parameters in Hα two-ribbon flares. 50 flare events, including 19
  eruptive flares (i.e. associated to a coronal mass ejection) and 31
  confined flares (i.e. CME-less) are analyzed, which are distributed
  over a wide range of GOES classes (from B3 to X17). The maximum ribbon
  separation, ribbon-separation velocity, mean magnetic-field strength,
  and reconnection electric field (i.e., local reconnection rate) are
  derived from Hα filtergrams obtained at Kanzelhöhe Observatory in
  combination with co-registered SOHO MDI and SDO HMI magnetograms. We
  find that the ribbon separation of eruptive flares correlates with the
  GOES flux and is statistically larger than that of confined flares,
  whereas no dependence was found for the maximum ribbon-separation
  velocity and the GOES flux. The local reconnection rate strongly
  correlates with the GOES flux. In addition, eruptive flares with a
  stronger peak reconnection electric field tend to be accompanied by
  faster CMEs. The estimated reconnection-related proxies for confined
  and eruptive events, however, appear in the form of two distinct
  but largely overlapping populations. This suggests that there is no
  significant difference in the underlying reconnection process.

---------------------------------------------------------
Title: Combining remote-sensing image data with in-situ measurements
    supported by modeling for Earth-affecting CME events
Authors: Temmer, Manuela; Thalmann, Julia; Dissauer, Karin; Veronig,
   Astrid; Tschernitz, Johannes; Hinterreiter, Jürgen; Rodriguez, Luciano
2018EGUGA..20.3999T    Altcode:
  We analyze the well observed flare-CME event from October 1, 2011
  and cover the complete chain of action - from the Sun to Earth. We
  study in detail the solar surface and atmosphere (SDO and ground-based
  instruments) associated to the flare/CME and also track the off-limb CME
  signatures in interplanetary space (STEREO-SoHO). This is complemented
  by surface magnetic field information and 3D coronal magnetic field
  modeling. From in-situ measurements (Wind), we extract the corresponding
  ICME characteristics. Results show that the flare reconnection flux is
  most probably a lower limit for estimating the magnetic flux within the
  flux rope as 1) magnetic reconnection processes were already ongoing
  before the start of the impulsive flare phase and 2) the dimming flux
  increased by more than 25% after the end of the flare, indicating that
  magnetic flux was still added to the flux rope after eruption. When
  comparing this to the in-situ axial magnetic flux of the magnetic cloud,
  we find that it is reduced by at least 75%, referring to substantial
  erosion in interplanetary space. Careful inspection of on-disk features
  associated with CMEs are essential for interpreting such scenarios.

---------------------------------------------------------
Title: Statistical Properties of Ribbon Evolution and Reconnection
    Electric Fields in Eruptive and Confined Flares
Authors: Hinterreiter, J.; Veronig, A. M.; Thalmann, J. K.; Tschernitz,
   J.; Pötzi, W.
2018SoPh..293...38H    Altcode: 2018arXiv180103370H
  A statistical study of the chromospheric ribbon evolution in Hα
  two-ribbon flares was performed. The data set consists of 50 confined
  (62%) and eruptive (38%) flares that occurred from June 2000 to
  June 2015. The flares were selected homogeneously over the Hα and
  Geostationary Operational Environmental Satellite (GOES) classes, with
  an emphasis on including powerful confined flares and weak eruptive
  flares. Hα filtergrams from the Kanzelhöhe Observatory in combination
  with Michelson Doppler Imager (MDI) and Helioseismic and Magnetic
  Imager (HMI) magnetograms were used to derive the ribbon separation,
  the ribbon-separation velocity, the magnetic-field strength, and
  the reconnection electric field. We find that eruptive flares reveal
  statistically larger ribbon separation and higher ribbon-separation
  velocities than confined flares. In addition, the ribbon separation
  of eruptive flares correlates with the GOES SXR flux, whereas no clear
  dependence was found for confined flares. The maximum ribbon-separation
  velocity is not correlated with the GOES flux, but eruptive flares
  reveal on average a higher ribbon-separation velocity (by ≈ 10 km
  s<SUP>−1</SUP>). The local reconnection electric field of confined
  (c c =0.50 ±0.02 ) and eruptive (c c =0.77 ±0.03 ) flares correlates
  with the GOES flux, indicating that more powerful flares involve
  stronger reconnection electric fields. In addition, eruptive flares
  with higher electric-field strengths tend to be accompanied by faster
  coronal mass ejections.

---------------------------------------------------------
Title: On the Factors Determining the Eruptive Character of Solar
    Flares
Authors: Baumgartner, Christian; Thalmann, Julia K.; Veronig, Astrid M.
2018ApJ...853..105B    Altcode: 2017arXiv171205106B
  We investigated how the magnetic field in solar active regions (ARs)
  controls flare activity, i.e., whether a confined or eruptive flare
  occurs. We analyzed 44 flares of GOES class M5.0 and larger that
  occurred during 2011-2015. We used 3D potential magnetic field models to
  study their location (using the flare distance from the flux-weighted
  AR center d <SUB>FC</SUB>) and the strength of the magnetic field in
  the corona above (via decay index n and flux ratio). We also present a
  first systematic study of the orientation of the coronal magnetic field,
  using the orientation φ of the flare-relevant polarity inversion line
  as a measure. We analyzed all quantities with respect to the size of
  the underlying dipole field, characterized by the distance between
  the opposite-polarity centers, d <SUB>PC</SUB>. Flares originating
  from underneath the AR dipole (d <SUB>FC</SUB>/d <SUB>PC</SUB> &lt;
  0.5) tend to be eruptive if launched from compact ARs (d <SUB>PC</SUB>
  ≤ 60 Mm) and confined if launched from extended ARs. Flares ejected
  from the periphery of ARs (d <SUB>FC</SUB>/d <SUB>PC</SUB> &gt; 0.5)
  are predominantly eruptive. In confined events, the flare-relevant field
  adjusts its orientation quickly to that of the underlying dipole with
  height (Δφ ≳ 40° until the apex of the dipole field), in contrast
  to eruptive events where it changes more slowly with height. The
  critical height for torus instability, h <SUB>crit</SUB> = h(n = 1.5),
  discriminates best between confined (h <SUB>crit</SUB> ≳ 40 Mm)
  and eruptive flares (h <SUB>crit</SUB> ≲ 40 Mm). It discriminates
  better than Δφ, implying that the decay of the confining field plays
  a stronger role than its orientation at different heights.

---------------------------------------------------------
Title: Reconnection Fluxes in Eruptive and Confined Flares and
    Implications for Superflares on the Sun
Authors: Tschernitz, Johannes; Veronig, Astrid M.; Thalmann, Julia K.;
   Hinterreiter, Jürgen; Pötzi, Werner
2018ApJ...853...41T    Altcode: 2017arXiv171204701T
  We study the energy release process of a set of 51 flares (32 confined,
  19 eruptive) ranging from GOES class B3 to X17. We use Hα filtergrams
  from Kanzelhöhe Observatory together with Solar Dynamics Observatory
  HMI and Solar and Heliospheric Observatory MDI magnetograms to derive
  magnetic reconnection fluxes and rates. The flare reconnection flux
  is strongly correlated with the peak of the GOES 1-8 Å soft X-ray
  flux (c = 0.92, in log-log space) for both confined and eruptive
  flares. Confined flares of a certain GOES class exhibit smaller ribbon
  areas but larger magnetic flux densities in the flare ribbons (by a
  factor of 2). In the largest events, up to ≈50% of the magnetic flux
  of the active region (AR) causing the flare is involved in the flare
  magnetic reconnection. These findings allow us to extrapolate toward the
  largest solar flares possible. A complex solar AR hosting a magnetic
  flux of 2 × 10<SUP>23</SUP> Mx, which is in line with the largest
  AR fluxes directly measured, is capable of producing an X80 flare,
  which corresponds to a bolometric energy of about 7 × 10<SUP>32</SUP>
  erg. Using a magnetic flux estimate of 6 × 10<SUP>23</SUP> Mx for
  the largest solar AR observed, we find that flares of GOES class
  ≈X500 could be produced (E <SUB>bol</SUB> ≈ 3 × 10<SUP>33</SUP>
  erg). These estimates suggest that the present day’s Sun is capable
  of producing flares and related space weather events that may be more
  than an order of magnitude stronger than have been observed to date.

---------------------------------------------------------
Title: Invisibility of Solar Active Region Umbra-to-Umbra Coronal
Loops: New Evidence that Magnetoconvection Drives Solar-Stellar
    Coronal Heating
Authors: Tiwari, S. K.; Thalmann, J. K.; Panesar, N. K.; Moore, R. L.;
   Winebarger, A. R.
2017AGUFMSH43A2789T    Altcode:
  Coronal heating generally increases with increasing magnetic field
  strength: the EUV/X-ray corona in active regions is 10-100 times more
  luminous and 2-4 times hotter than that in quiet regions and coronal
  holes, which are heated to only about 1.5 MK, and have fields that are
  10-100 times weaker than that in active regions. From a comparison of
  a nonlinear force-free model of the three-dimensional active region
  coronal field to observed extreme-ultraviolet loops, we find that (1)
  umbra-to-umbra coronal loops, despite being rooted in the strongest
  magnetic flux, are invisible, and (2) the brightest loops have one
  foot in an umbra or penumbra and the other foot in another sunspot's
  penumbra or in unipolar or mixed-polarity plage. The invisibility of
  umbra-to-umbra loops is new evidence that magnetoconvection drives
  solar-stellar coronal heating: evidently, the strong umbral field at
  both ends quenches the magnetoconvection and hence the heating. Our
  results from EUV observations and nonlinear force-free modeling of
  coronal magnetic field imply that, for any coronal loop on the Sun or
  on any other convective star, as long as the field can be braided by
  convection in at least one loop foot, the stronger the field in the
  loop, the stronger the coronal heating.

---------------------------------------------------------
Title: Observational and Model Analysis of a Two-ribbon Flare Possibly
    Induced by a Neighboring Blowout Jet
Authors: Joshi, Bhuwan; Thalmann, Julia K.; Mitra, Prabir K.; Chandra,
   Ramesh; Veronig, Astrid M.
2017ApJ...851...29J    Altcode: 2017arXiv171008099J
  In this paper, we present unique observations of a blowout coronal jet
  that possibly triggered a two-ribbon confined C1.2 flare in bipolar
  solar active region NOAA 12615 on 2016 December 5. The jet activity
  initiates at chromospheric/transition region heights with a small
  brightening that eventually increases in volume, with well-developed
  standard morphological jet features, viz., base and spire. The spire
  widens up with a collimated eruption of cool and hot plasma components,
  observed in the 304 and 94 Å channels of AIA, respectively. The speed
  of the plasma ejection, which forms the jet’s spire, was higher
  for the hot component (∼200 km s<SUP>-1</SUP>) than the cooler one
  (∼130 km s<SUP>-1</SUP>). The NLFF model of coronal fields at the
  pre- and post-jet phases successfully reveals openings of previously
  closed magnetic field lines with a rather inclined/low-lying jet
  structure. The peak phase of the jet emission is followed by the
  development of a two-ribbon flare that shows coronal loop emission in
  HXRs up to ∼25 keV energy. The coronal magnetic fields rooted at the
  location of EUV flare ribbons, derived from the NLFF model, demonstrate
  the pre-flare phase to exhibit an “X-type” configuration, while
  the magnetic fields at the post-flare phase are more or less oriented
  parallel. Comparisons of multi-wavelength measurements with the magnetic
  field extrapolations suggest that the jet activity likely triggered
  the two-ribbon flare by perturbing the field in the interior of the
  active region.

---------------------------------------------------------
Title: Generation Mechanisms of Quasi-parallel and Quasi-circular
    Flare Ribbons in a Confined Flare
Authors: Hernandez-Perez, Aaron; Thalmann, Julia K.; Veronig, Astrid
   M.; Su, Yang; Gömöry, Peter; Dickson, Ewan C.
2017ApJ...847..124H    Altcode: 2017arXiv170808612H
  We analyze a confined multiple-ribbon M2.1 flare (SOL2015-01-29T11:42)
  that originated from a fan-spine coronal magnetic field configuration,
  within active region NOAA 12268. The observed ribbons form in
  two steps. First, two primary ribbons form at the main flare site,
  followed by the formation of secondary ribbons at remote locations. We
  observe a number of plasma flows at extreme-ultraviolet temperatures
  during the early phase of the flare (as early as 15 minutes before
  the onset) propagating toward the formation site of the secondary
  ribbons. The secondary ribbon formation is co-temporal with the
  arrival of the pre-flare generated plasma flows. The primary ribbons
  are co-spatial with Ramaty High Energy Spectroscopic Imager (RHESSI)
  hard X-ray sources, whereas no enhanced X-ray emission is detected at
  the secondary ribbon sites. The (E)UV emission, associated with the
  secondary ribbons, peaks ∼1 minute after the last RHESSI hard X-ray
  enhancement. A nonlinear force-free model of the coronal magnetic field
  reveals that the secondary flare ribbons are not directly connected to
  the primary ribbons, but to regions nearby. Detailed analysis suggests
  that the secondary brightenings are produced due to dissipation of
  kinetic energy of the plasma flows (heating due to compression), and
  not due to non-thermal particles accelerated by magnetic reconnection,
  as is the case for the primary ribbons.

---------------------------------------------------------
Title: The Causes of Quasi-homologous CMEs
Authors: Liu, Lijuan; Wang, Yuming; Liu, Rui; Zhou, Zhenjun; Temmer,
   M.; Thalmann, J. K.; Liu, Jiajia; Liu, Kai; Shen, Chenglong; Zhang,
   Quanhao; Veronig, A. M.
2017ApJ...844..141L    Altcode: 2017arXiv170608878L
  In this paper, we identified the magnetic source locations of 142
  quasi-homologous (QH) coronal mass ejections (CMEs), of which 121
  are from solar cycle (SC) 23 and 21 from SC 24. Among those CMEs, 63%
  originated from the same source location as their predecessor (defined
  as S-type), while 37% originated from a different location within the
  same active region as their predecessor (defined as D-type). Their
  distinctly different waiting time distributions, peaking around 7.5 and
  1.5 hr for S- and D-type CMEs, suggest that they might involve different
  physical mechanisms with different characteristic timescales. Through
  detailed analysis based on nonlinear force-free coronal magnetic field
  modeling of two exemplary cases, we propose that the S-type QH CMES
  might involve a recurring energy release process from the same source
  location (by magnetic free energy replenishment), whereas the D-type
  QH CMEs can happen when a flux tube system is disturbed by a nearby CME.

---------------------------------------------------------
Title: Erratum: “The Confined X-class Flares of Solar Active Region
2192” (<A href="http://doi.org/10.1088/2041-8205/801/2/L23">2015,
    ApJL, 801, L23</A>)
Authors: Thalmann, J. K.; Su, Y.; Temmer, M.; Veronig, A. M.
2017ApJ...844L..27T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: On Flare-CME Characteristics from Sun to Earth Combining
    Remote-Sensing Image Data with In Situ Measurements Supported
    by Modeling
Authors: Temmer, Manuela; Thalmann, Julia K.; Dissauer, Karin;
   Veronig, Astrid M.; Tschernitz, Johannes; Hinterreiter, Jürgen;
   Rodriguez, Luciano
2017SoPh..292...93T    Altcode: 2017arXiv170300694T
  We analyze the well-observed flare and coronal mass ejection (CME)
  from 1 October 2011 (SOL2011-10-01T09:18) covering the complete chain of
  effects - from Sun to Earth - to better understand the dynamic evolution
  of the CME and its embedded magnetic field. We study in detail the
  solar surface and atmosphere associated with the flare and CME using the
  Solar Dynamics Observatory (SDO) and ground-based instruments. We also
  track the CME signature off-limb with combined extreme ultraviolet
  (EUV) and white-light data from the Solar Terrestrial Relations
  Observatory (STEREO). By applying the graduated cylindrical shell
  (GCS) reconstruction method and total mass to stereoscopic STEREO-SOHO
  (Solar and Heliospheric Observatory) coronagraph data, we track
  the temporal and spatial evolution of the CME in the interplanetary
  space and derive its geometry and 3D mass. We combine the GCS and
  Lundquist model results to derive the axial flux and helicity of
  the magnetic cloud (MC) from in situ measurements from Wind. This is
  compared to nonlinear force-free (NLFF) model results, as well as to
  the reconnected magnetic flux derived from the flare ribbons (flare
  reconnection flux) and the magnetic flux encompassed by the associated
  dimming (dimming flux). We find that magnetic reconnection processes
  were already ongoing before the start of the impulsive flare phase,
  adding magnetic flux to the flux rope before its final eruption. The
  dimming flux increases by more than 25% after the end of the flare,
  indicating that magnetic flux is still added to the flux rope after
  eruption. Hence, the derived flare reconnection flux is most probably a
  lower limit for estimating the magnetic flux within the flux rope. We
  find that the magnetic helicity and axial magnetic flux are lower in
  the interplanetary space by ∼ 50% and 75%, respectively, possibly
  indicating an erosion process. A CME mass increase of 10% is observed
  over a range of ∼4 -20 R<SUB>⊙</SUB>. The temporal evolution of
  the CME-associated core-dimming regions supports the scenario that
  fast outflows might supply additional mass to the rear part of the CME.

---------------------------------------------------------
Title: New Evidence that Magnetoconvection Drives Solar-Stellar
    Coronal Heating
Authors: Tiwari, Sanjiv K.; Thalmann, Julia K.; Panesar, Navdeep K.;
   Moore, Ronald L.; Winebarger, Amy R.
2017ApJ...843L..20T    Altcode: 2017arXiv170608035T
  How magnetic energy is injected and released in the solar
  corona, keeping it heated to several million degrees, remains
  elusive. Coronal heating generally increases with increasing magnetic
  field strength. From a comparison of a nonlinear force-free model
  of the three-dimensional active region coronal field to observed
  extreme-ultraviolet loops, we find that (1) umbra-to-umbra coronal
  loops, despite being rooted in the strongest magnetic flux, are
  invisible, and (2) the brightest loops have one foot in an umbra or
  penumbra and the other foot in another sunspot’s penumbra or in
  unipolar or mixed-polarity plage. The invisibility of umbra-to-umbra
  loops is new evidence that magnetoconvection drives solar-stellar
  coronal heating: evidently, the strong umbral field at both ends
  quenches the magnetoconvection and hence the heating. Broadly, our
  results indicate that depending on the field strength in both feet,
  the photospheric feet of a coronal loop on any convective star can
  either engender or quench coronal heating in the loop’s body.

---------------------------------------------------------
Title: Magnetic Helicity Estimations in Models and Observations of
    the Solar Magnetic Field. III. Twist Number Method
Authors: Guo, Y.; Pariat, E.; Valori, G.; Anfinogentov, S.; Chen,
   F.; Georgoulis, M. K.; Liu, Y.; Moraitis, K.; Thalmann, J. K.; Yang, S.
2017ApJ...840...40G    Altcode: 2017arXiv170402096G
  We study the writhe, twist, and magnetic helicity of different
  magnetic flux ropes, based on models of the solar coronal magnetic
  field structure. These include an analytical force-free Titov-Démoulin
  equilibrium solution, non-force-free magnetohydrodynamic simulations,
  and nonlinear force-free magnetic field models. The geometrical
  boundary of the magnetic flux rope is determined by the quasi-separatrix
  layer and the bottom surface, and the axis curve of the flux rope is
  determined by its overall orientation. The twist is computed by the
  Berger-Prior formula, which is suitable for arbitrary geometry and
  both force-free and non-force-free models. The magnetic helicity is
  estimated by the twist multiplied by the square of the axial magnetic
  flux. We compare the obtained values with those derived by a finite
  volume helicity estimation method. We find that the magnetic helicity
  obtained with the twist method agrees with the helicity carried by the
  purely current-carrying part of the field within uncertainties for
  most test cases. It is also found that the current-carrying part of
  the model field is relatively significant at the very location of the
  magnetic flux rope. This qualitatively explains the agreement between
  the magnetic helicity computed by the twist method and the helicity
  contributed purely by the current-carrying magnetic field.

---------------------------------------------------------
Title: Magnetic helicity estimations in models and observations of
    the solar magnetic field
Authors: Valori, Gherardo; Pariat, Etienne; Anfinogentov, Sergey;
   Chen, Feng; Georgoulis, Manolis; Guo, Yang; Liu, Yang; Moraitis,
   Kostas; Thalmann, Julia K.; Yang, Shangbin
2017EGUGA..19.3692V    Altcode:
  Magnetic helicity, as one of the few conserved quantities
  in magneto-hydrodynamics, is often invoked as the principle
  driving the generation and structuring of magnetic fields in a
  variety of environments, from dynamo models in stars and planets,
  to post-disruption reconfigurations of tokamak's plasmas. Most
  particularly magnetic helicity has raised the interest of solar
  physicists, since helicity is suspected to represent a key quantity for
  the understanding of solar flares and the generation of coronal mass
  ejections. In recent years, several methods of estimation of magnetic
  helicity have been proposed and already applied to observations and
  numerical simulations. However, no systematic comparison of accuracy,
  mutual consistency, and reliability of such methods has ever been
  performed. We present the results of the first benchmark of several
  finite-volume methods in estimating magnetic helicity in 3D test
  models. In addition to finite volume methods, two additional methods
  are also included that estimate magnetic helicity based either on the
  field line's twist, or on the field's values on one boundary and an
  inferred minimal volume connectivity. The employed model tests range
  from solutions of the force-free equations to 3D magneto-hydrodynamical
  numerical simulations. Almost all methods are found to produce the same
  value of magnetic helicity within few percent in all tests. However,
  methods show differences in the sensitivity to numerical resolution and
  to errors in the solenoidal property of input fields. Our benchmark of
  finite volume methods allows to determine the reliability and precision
  of estimations of magnetic helicity in practical cases. As a next step,
  finite volume methods are used to test estimation methods that are
  based on the flux of helicity through one boundary, in particular
  for applications to observation-based models of coronal magnetic
  fields. The ultimate goal is to assess if and how can helicity be
  meaningfully used as a diagnostic of the evolution of magnetic fields
  in the solar atmosphere.

---------------------------------------------------------
Title: Advances in solar flare science through modeling of the
    magnetic field in the solar atmosphere (Arne Richter Award for
    Outstanding ECSs Lecture)
Authors: Thalmann, Julia K.
2017EGUGA..19.2310T    Altcode:
  Ever since we know of the phenomenon of solar flares and coronal mass
  ejections, we try to unravel the secrets of the underlying physical
  processes. The magnetic field in the Sun's atmosphere is the driver
  of any solar activity. Therefore, the combined study of the surface
  (photosphere) magnetic field and the magnetic field in the atmosphere
  above (the chromosphere and corona) is essential. At present, direct
  measurements of the solar magnetic field are regularly available only
  for the solar surface, so that we have to rely on models to reconstruct
  the magnetic field in the corona. Corresponding model-based research
  on the magnetic field within flaring active regions is inevitable
  for the understanding of the key physical processes of flares and
  possibly associated mass ejections, as well as their time evolution. I
  will focus on recent advances in the understanding of the magnetic
  processes in solar flares based on quasi-static force-free coronal
  magnetic field modeling. In particular, I will discuss aspects
  such as the structure (topology) of the coronal magnetic field, its
  flare-induced reconfiguration, as well as the associated modifications
  to the inherent magnetic energy and helicity. I will also discuss the
  potential and limitations of studies trying to cover the complete chain
  of action, i.e., to relate the (magnetic) properties of solar flares
  to that of the associated disturbances measured in-situ at Earth,
  as induced by flare-associated coronal mass ejections after passage
  of the interplanetary space separating Sun and Earth. Finally, I will
  discuss future prospects regarding model-based research of the coronal
  magnetic field in the course of flares, including possible implications
  for improved future flare forecasting attempts.

---------------------------------------------------------
Title: Magnetic reconnection rates in solar flares and implications
    for "superflares"
Authors: Veronig, Astrid; Tschernitz, Johannes; Hinterreiter, Jürgen;
   Thalmann, Julia
2017EGUGA..19.4751V    Altcode:
  We present a statistical study of magnetic reconnection rates and
  fluxes to study the energy release process in solar flares. Our data
  set covers 50 events, including 19 eruptive flares (i.e. flares
  associated with a coronal mass ejection) and 31 confined flares
  (i.e. not associated with a coronal mass ejection). The events under
  study are distributed over a wide range of GOES classes, from B to
  &gt;X10. Magnetic reconnection rates and fluxes are derived from the
  flare ribbon evolution studied in Halpha filtergrams from Kanzelhöhe
  Observatory and co-registered photospheric line-of-sight magnetic
  field maps from HMI/SDO and MDI/SOHO. We find a distinct correlation
  between the total flare reconnection flux with the GOES peak flux for
  both eruptive and confined flares. In the largest events, the flare
  reconnection fluxes may reach up to &gt;30% of the total active region
  magnetic flux. The implications of the distinct correlations obtained
  are discussed with respect to the recently detected superflares on
  solar-like stars and the largest flares expected on the Sun.

---------------------------------------------------------
Title: Flare-CME characteristics from Sun to Earth combining
    observations and modeling
Authors: Temmer, Manuela; Thalmann, Julia K.; Dissauer, Karin;
   Veronig, Astrid M.; Tschernitz, Johannes; Hinterreiter, Jürgen;
   Rodriguez, Luciano
2017EGUGA..19.1942T    Altcode:
  We analyze the well observed flare-CME event from October 1, 2011
  (SOL2011-10-01T09:18) covering the complete chain of action - from
  Sun to Earth - for a better understanding of the dynamic evolution
  of the CME and its embedded magnetic field. We study in detail the
  solar surface and atmosphere from SDO and ground-based instruments
  associated to the flare-CME and also track the CME signature offlimb
  from combined EUV and white-light data with STEREO. By applying 3D
  reconstruction techniques (GCS, total mass) to stereoscopic STEREO-SoHO
  coronagraph data, we track the temporal and spatial evolution of the
  CME in interplanetary space and derive its geometry and 3D-mass. We
  combine the GCS and Lundquist model results to derive the axial flux
  and helicity of the MC from in situ measurements (Wind). This is
  compared to nonlinear force-free (NLFF) model results as well as to
  the reconnected magnetic flux derived from the flare ribbons (flare
  reconnection flux) and the magnetic flux encompassed by the associated
  dimming (dimming flux). We find that magnetic reconnection processes
  were already ongoing before the start of the impulsive flare phase,
  adding magnetic flux to the flux rope before its final eruption. The
  dimming flux increases by more than 25% after the end of the flare,
  indicating that magnetic flux is still added to the flux rope after
  eruption. Hence, the derived flare reconnection flux is most probably a
  lower limit for estimating the magnetic flux within the flux rope. We
  obtain that the magnetic helicity and axial magnetic flux are reduced
  in interplanetary space by ∼50% and 75%, respectively, possibly
  indicating to an erosion process. A mass increase of 10% for the CME
  is observed over the distance range from about 4-20 Rs. The temporal
  evolution of the CME associated core dimming regions supports the
  scenario that fast outflows might supply additional mass to the rear
  part of the CME.

---------------------------------------------------------
Title: Arcade Implosion Caused by a Filament Eruption in a Flare
Authors: Wang, Juntao; Simões, P. J. A.; Fletcher, L.; Thalmann,
   J. K.; Hudson, H. S.; Hannah, I. G.
2016ApJ...833..221W    Altcode: 2016arXiv161005931W
  Coronal implosions—the convergence motion of plasmas and entrained
  magnetic field in the corona due to a reduction in magnetic
  pressure—can help to locate and track sites of magnetic energy
  release or redistribution during solar flares and eruptions. We report
  here on the analysis of a well-observed implosion in the form of an
  arcade contraction associated with a filament eruption, during the
  C3.5 flare SOL2013-06-19T07:29. A sequence of events including the
  magnetic flux-rope instability and distortion, followed by a filament
  eruption and arcade implosion, lead us to conclude that the implosion
  arises from the transfer of magnetic energy from beneath the arcade
  as part of the global magnetic instability, rather than due to local
  magnetic energy dissipation in the flare. The observed net contraction
  of the imploding loops, which is found also in nonlinear force-free
  field extrapolations, reflects a permanent reduction of magnetic
  energy underneath the arcade. This event shows that, in addition to
  resulting in the expansion or eruption of an overlying field, flux-rope
  instability can also simultaneously implode an unopened field due to
  magnetic energy transfer. It demonstrates the “partial opening of
  the field” scenario, which is one of the ways in 3D to produce a
  magnetic eruption without violating the Aly-Sturrock hypothesis. In
  the framework of this observation, we also propose a unification of
  three main concepts for active region magnetic evolution, namely the
  metastable eruption model, the implosion conjecture, and the standard
  “CSHKP” flare model.

---------------------------------------------------------
Title: Magnetic Helicity Estimations in Models and Observations of
the Solar Magnetic Field. Part I: Finite Volume Methods
Authors: Valori, Gherardo; Pariat, Etienne; Anfinogentov, Sergey;
   Chen, Feng; Georgoulis, Manolis K.; Guo, Yang; Liu, Yang; Moraitis,
   Kostas; Thalmann, Julia K.; Yang, Shangbin
2016SSRv..201..147V    Altcode: 2016SSRv..tmp...68V; 2016arXiv161002193V
  Magnetic helicity is a conserved quantity of ideal magneto-hydrodynamics
  characterized by an inverse turbulent cascade. Accordingly, it
  is often invoked as one of the basic physical quantities driving
  the generation and structuring of magnetic fields in a variety of
  astrophysical and laboratory plasmas. We provide here the first
  systematic comparison of six existing methods for the estimation of
  the helicity of magnetic fields known in a finite volume. All such
  methods are reviewed, benchmarked, and compared with each other,
  and specifically tested for accuracy and sensitivity to errors. To
  that purpose, we consider four groups of numerical tests, ranging
  from solutions of the three-dimensional, force-free equilibrium, to
  magneto-hydrodynamical numerical simulations. Almost all methods are
  found to produce the same value of magnetic helicity within few percent
  in all tests. In the more solar-relevant and realistic of the tests
  employed here, the simulation of an eruptive flux rope, the spread
  in the computed values obtained by all but one method is only 3 %,
  indicating the reliability and mutual consistency of such methods in
  appropriate parameter ranges. However, methods show differences in the
  sensitivity to numerical resolution and to errors in the solenoidal
  property of the input fields. In addition to finite volume methods,
  we also briefly discuss a method that estimates helicity from the
  field lines' twist, and one that exploits the field's value at one
  boundary and a coronal minimal connectivity instead of a pre-defined
  three-dimensional magnetic-field solution.

---------------------------------------------------------
Title: Erratum: “Evolution of Magnetic Field and Energy in A
    Major Eruptive Active Region Based on SDO/HMI Observation” (<A
href="http://dx.doi.org/10.1088/0004-637x/748/2/77 ">2012, ApJ,
    748, 77</A>)
Authors: Sun, Xudong; Hoeksema, J. Todd; Liu, Yang; Wiegelmann,
   Thomas; Hayashi, Keiji; Chen, Qingrong; Thalmann, Julia
2016ApJ...828...65S    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Temporal and Spatial Relationship of Flare Signatures and
    the Force-free Coronal Magnetic Field
Authors: Thalmann, J. K.; Veronig, A.; Su, Y.
2016ApJ...826..143T    Altcode: 2016arXiv160503703T
  We investigate the plasma and magnetic environment of active
  region NOAA 11261 on 2011 August 2 around a GOES M1.4 flare/CME
  (SOL2011-08-02T06:19). We compare coronal emission at the (extreme)
  ultraviolet and X-ray wavelengths, using SDO AIA and RHESSI
  images, in order to identify the relative timing and locations of
  reconnection-related sources. We trace flare ribbon signatures at
  ultraviolet wavelengths in order to pin down the intersection
  of previously reconnected flaring loops in the lower solar
  atmosphere. These locations are used to calculate field lines from
  three-dimensional (3D) nonlinear force-free magnetic field models,
  established on the basis of SDO HMI photospheric vector magnetic
  field maps. Using this procedure, we analyze the quasi-static time
  evolution of the coronal model magnetic field previously involved
  in magnetic reconnection. This allows us, for the first time, to
  estimate the elevation speed of the current sheet’s lower tip during
  an on-disk observed flare as a few kilometers per second. A comparison
  to post-flare loops observed later above the limb in STEREO EUVI images
  supports this velocity estimate. Furthermore, we provide evidence for
  an implosion of parts of the flaring coronal model magnetic field,
  and identify the corresponding coronal sub-volumes associated with
  the loss of magnetic energy. Finally, we spatially relate the build
  up of magnetic energy in the 3D models to highly sheared fields,
  established due to the dynamic relative motions of polarity patches
  within the active region.

---------------------------------------------------------
Title: Suppression of heating of coronal loops rooted in opposite
    polarity sunspot umbrae
Authors: Tiwari, Sanjiv K.; Thalmann, Julia; Moore, Ronald; Panesar,
   Navdeep; Winebarger, Amy
2016shin.confE..61T    Altcode:
  EUV observations of active region (AR) coronae reveal the presence
  of loops at different temperatures. To understand the mechanisms that
  result in hotter or cooler loops, we study a typical bipolar AR, near
  solar disk center, which has moderate overall magnetic twist and at
  least one fully developed sunspot of each polarity. From AIA 193 and
  94 Å images we identify many clearly discernible coronal loops that
  connect plage or a sunspot of one polarity to an opposite-polarity
  plage region. The AIA 94 Å images show dim regions in the umbrae of
  the sunspots. To see which coronal loops are rooted in a dim umbral
  area, we performed a non-linear force-free field (NLFFF) modeling
  using photospheric vector magnetic field measurements obtained with
  the Heliosesmic Magnetic Imager (HMI) onboard SDO. The NLFFF model,
  validated by comparison of calculated model field lines with observed
  loops in AIA 193 and 94 Å, specifies the photospheric roots of the
  model field lines. Some model coronal magnetic field lines arch from
  the dim umbral area of the positive-polarity sunspot to the dim umbral
  area of a negative-polarity sunspot. Because these coronal loops are
  not visible in any of the coronal EUV and X-ray images of the AR, we
  conclude they are the coolest loops in the AR. This result suggests
  that the loops connecting opposite polarity umbrae are the least heated
  because the field in umbrae is so strong that the convective braiding
  of the field is strongly suppressed.

---------------------------------------------------------
Title: Suppression of heating of coronal loops rooted in opposite
    polarity sunspot umbrae
Authors: Tiwari, Sanjiv K.; Thalmann, Julia K.; Moore, Ronald L.;
   Panesar, Navdeep; Winebarger, Amy R.
2016SPD....47.0336T    Altcode:
  EUV observations of active region (AR) coronae reveal the presence
  of loops at different temperatures. To understand the mechanisms that
  result in hotter or cooler loops, we study a typical bipolar AR, near
  solar disk center, which has moderate overall magnetic twist and at
  least one fully developed sunspot of each polarity. From AIA 193 and
  94 A images we identify many clearly discernible coronal loops that
  connect plage or a sunspot of one polarity to an opposite-polarity
  plage region. The AIA 94 A images show dim regions in the umbrae of
  the spots. To see which coronal loops are rooted in a dim umbral area,
  we performed a non-linear force-free field (NLFFF) modeling using
  photospheric vector magnetic field measurements obtained with the
  HMI onboard SDO. After validation of the NLFFF model by comparison of
  calculated model field lines and observed loops in AIA 193 and 94, we
  specify the photospheric roots of the model field lines. The model field
  then shows the coronal magnetic loops that arch from the dim umbral
  areas of the opposite polarity sunspots. Because these coronal loops
  are not visible in any of the coronal EUV and X-ray images of the AR,
  we conclude they are the coolest loops in the AR. This result suggests
  that the loops connecting opposite polarity umbrae are the least
  heated because the field in umbrae is so strong that the convective
  braiding of the field is strongly suppressed.We hypothesize that the
  convective freedom at the feet of a coronal loop, together with the
  strength of the field in the body of the loop, determines the strength
  of the heating. In particular, we expect the hottest coronal loops
  to have one foot in an umbra and the other foot in opposite-polarity
  penumbra or plage (coronal moss), the areas of strong field in which
  convection is not as strongly suppressed as in umbra. Many transient,
  outstandingly bright, loops in the AIA 94 movie of the AR do have this
  expected rooting pattern. We will also present another example of AR
  in which we find a similar rooting pattern of coronal loops.

---------------------------------------------------------
Title: Exceptions to the rule: the X-flares of AR 2192 Lacking
    Coronal Mass Ejections
Authors: Thalmann, J. K.; Su, Y.; Temmer, M.; Veronig, A. M.
2016ASPC..504..203T    Altcode:
  NOAA Active region (AR) 2192, that was present on the Sun in October
  2014, was the largest region which occurred since November 1990
  (see Figure 1). The huge size accompanied by a very high activity
  level, was quite unexpected as it appeared during the unusually weak
  solar cycle 24. Nevertheless, the AR turned out to be one of the most
  prolific flaring ARs of cycle 24. It produced in total 6 X, 29 M, 79
  C flares during its disk passage from October 18-29, 2014 (see Figure
  2). Surprisingly, all flares greater than GOES class M5 and X were
  confined, i.e. had no coronal mass ejections (CME) associated. All
  the flare events had some obvious similarity in morphology, as they
  were located in the core of the AR and revealed only minor separation
  motion away from the neutral line but a large initial separation of
  the conjugate flare ribbons. In the paper by Thalmann et al. (2015)
  we describe the series of flares and give details about the confined
  X1.6 flare event from October 22, 2014 as well as the single eruptive
  M4.0 flare event from October 24, 2014. The study of the X1.6 flare
  revealed a large initial separation of flare ribbons together with
  recurrent flare brightenings, which were related to two episodes of
  enhanced hard X-ray emission as derived from RHESSI observations. This
  suggests that magnetic field structures connected to specific regions
  were repeatedly involved in the process of reconnection and energy
  release. Opposite to the central location of the sequence of confined
  events within the AR, a single eruptive (M4.0) event occurred on
  the outskirt of the AR in the vicinity of open magnetic fields. Our
  investigations revealed a predominantly north-south oriented magnetic
  system of arcade fields overlying the AR that could have preserved
  the magnetic arcade to erupt, and consequently kept the energy release
  trapped in a localized volume of magnetic field high up in the corona
  (as supported by the absence of a lateral motion of the flare ribbons
  and the recurrent brightenings within them). We conclude that the
  background magnetic field configuration is an essential parameter
  for deriving the "eruptiveness" of flare events. Sun et al. (2015)
  supports this conclusion and derived for this AR a quite slow
  decay of the strength of the overlying magnetic field (decay index;
  see Török &amp; Kliem 2005). Interestingly, our magnetic field
  modellings revealed no flux rope inherent to the AR, indicating that
  further investigations are needed. In a recent paper by Veronig $
  Polanec (2015), who investigated in more detail the X-flares using
  also ground-based observations in Hα from Kanzelhöhe Observatory
  (Pötzi et al. 2015), it was shown that such confined events could be
  explained by the emerging-flux model, where newly emerging small flux
  tubes reconnect with pre-existing large coronal loops.

---------------------------------------------------------
Title: Chromospheric evaporation flows and density changes deduced
    from Hinode/EIS during an M1.6 flare
Authors: Gömöry, P.; Veronig, A. M.; Su, Y.; Temmer, M.; Thalmann,
   J. K.
2016A&A...588A...6G    Altcode: 2016arXiv160202145G
  <BR /> Aims: We study the response of the solar atmosphere during a GOES
  M1.6 flare using spectroscopic and imaging observations. In particular,
  we examine the evolution of the mass flows and electron density together
  with the energy input derived from hard X-ray (HXR) in the context of
  chromospheric evaporation. <BR /> Methods: We analyzed high-cadence
  sit-and-stare observations acquired with the Hinode/EIS spectrometer
  in the Fe xiii 202.044 Å (log T = 6.2) and Fe xvi 262.980 Å (log T =
  6.4) spectral lines to derive temporal variations of the line intensity,
  Doppler shifts, and electron density during the flare. We combined these
  data with HXR measurements acquired with RHESSI to derive the energy
  input to the lower atmosphere by flare-accelerated electrons. <BR />
  Results: During the flare impulsive phase, we observe no significant
  flows in the cooler Fe xiii line but strong upflows, up to 80-150 km
  s<SUP>-1</SUP>, in the hotter Fe xvi line. The largest Doppler shifts
  observed in the Fe xvi line were co-temporal with the sharp intensity
  peak. The electron density obtained from a Fe xiii line pair ratio
  exhibited fast increase (within two minutes) from the pre-flare level
  of 5.01 × 10<SUP>9</SUP> cm<SUP>-3</SUP> to 3.16 × 10<SUP>10</SUP>
  cm<SUP>-3</SUP> during the flare peak. The nonthermal energy flux
  density deposited from the coronal acceleration site to the lower
  atmospheric layers during the flare peak was found to be 1.34 ×
  10<SUP>10</SUP> erg s<SUP>-1</SUP> cm<SUP>-2</SUP> for a low-energy
  cut-off that was estimated to be 16 keV. During the decline flare phase,
  we found a secondary intensity and density peak of lower amplitude
  that was preceded by upflows of ~15 km s<SUP>-1</SUP> that were
  detected in both lines. The flare was also accompanied by a filament
  eruption that was partly captured by the EIS observations. We derived
  Doppler velocities of 250-300 km s<SUP>-1</SUP> for the upflowing
  filament material. <BR /> Conclusions: The spectroscopic results
  for the flare peak are consistent with the scenario of explosive
  chromospheric evaporation, although a comparatively low value of the
  nonthermal energy flux density was determined for this phase of the
  flare. This outcome is discussed in the context of recent hydrodynamic
  simulations. It provides observational evidence that the response
  of the atmospheric plasma strongly depends on the properties of the
  electron beams responsible for the heating, in particular the steepness
  of the energy distribution. The secondary peak of line intensity and
  electron density detected during the decline phase is interpreted as a
  signature of flare loops being filled by expanding hot material that
  is due to chromospheric evaporation. <P />A movie is available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201527403/olm">http://www.aanda.org</A>

---------------------------------------------------------
Title: Space Weather and confined CME events
Authors: Thalmann, Julia; Temmer, Manuela; Veronig, Astrid; Su, Yang
2016EGUGA..18.7517T    Altcode:
  The unusually large NOAA active region (AR) 2192, observed in October
  and November 2014, was outstanding in its productivity of major flares
  (GOES class M5 and larger). During the time when the AR faced Earth,
  major Space Weather events would have been expected. However, none of
  the X-flares was associated to a coronal mass ejection. Observational
  evidence for the confinement of the flare are large initial separation
  of the flare ribbons, together with an almost absent growth in ribbon
  separation. The low dynamic of the ribbons also suggests a reconnection
  site high up in the corona. From NLFF modeling we show that the
  arcade overlying the AR had a predominantly north-south oriented
  magnetic system, which served as a strong, also lateral, confinement
  for the flares at the core of the active region. From the magnetic
  field modeling we derived the decay of the constraining background,
  and it was found that the overlying field was only slowly decaying
  with height. We conclude that observational data of the solar surface,
  especially of flare ribbon dynamics as well as magnetic field models
  support Space Weather predictions.

---------------------------------------------------------
Title: The exceptional aspects of the confined X-class flares of
    solar active region 2192
Authors: Thalmann, Julia K.; Su, Yang; Temmer, Manuela; Veronig,
   Astrid M.
2016IAUS..320...60T    Altcode: 2016arXiv160503712T
  During late October 2014, active region NOAA 2192 caused an unusual high
  level of solar activity, within an otherwise weak solar cycle. While
  crossing the solar disk, during a period of 11 days, it was the source
  of 114 flares of GOES class C1.0 and larger, including 29 M- and 6
  X-flares. Surprisingly, none of the major flares (GOES class M5.0
  and larger) was accompanied by a coronal mass ejection, contrary to
  statistical tendencies found in the past. From modeling the coronal
  magnetic field of NOAA 2192 and its surrounding, we suspect that the
  cause of the confined character of the flares is the strong surrounding
  and overlying large-scale magnetic field. Furthermore, we find evidence
  for multiple magnetic reconnection processes within a single flare,
  during which electrons were accelerated to unusual high energies.

---------------------------------------------------------
Title: The Influence of Spatial resolution on Nonlinear Force-free
    Modeling
Authors: DeRosa, M. L.; Wheatland, M. S.; Leka, K. D.; Barnes, G.;
   Amari, T.; Canou, A.; Gilchrist, S. A.; Thalmann, J. K.; Valori,
   G.; Wiegelmann, T.; Schrijver, C. J.; Malanushenko, A.; Sun, X.;
   Régnier, S.
2015ApJ...811..107D    Altcode: 2015arXiv150805455D
  The nonlinear force-free field (NLFFF) model is often used to
  describe the solar coronal magnetic field, however a series of
  earlier studies revealed difficulties in the numerical solution of the
  model in application to photospheric boundary data. We investigate
  the sensitivity of the modeling to the spatial resolution of the
  boundary data, by applying multiple codes that numerically solve the
  NLFFF model to a sequence of vector magnetogram data at different
  resolutions, prepared from a single Hinode/Solar Optical Telescope
  Spectro-Polarimeter scan of NOAA Active Region 10978 on 2007 December
  13. We analyze the resulting energies and relative magnetic helicities,
  employ a Helmholtz decomposition to characterize divergence errors, and
  quantify changes made by the codes to the vector magnetogram boundary
  data in order to be compatible with the force-free model. This study
  shows that NLFFF modeling results depend quantitatively on the spatial
  resolution of the input boundary data, and that using more highly
  resolved boundary data yields more self-consistent results. The
  free energies of the resulting solutions generally trend higher
  with increasing resolution, while relative magnetic helicity values
  vary significantly between resolutions for all methods. All methods
  require changing the horizontal components, and for some methods also
  the vertical components, of the vector magnetogram boundary field in
  excess of nominal uncertainties in the data. The solutions produced
  by the various methods are significantly different at each resolution
  level. We continue to recommend verifying agreement between the modeled
  field lines and corresponding coronal loop images before any NLFFF
  model is used in a scientific setting.

---------------------------------------------------------
Title: Two-fluid 2.5D code for simulations of small scale magnetic
    fields in the lower solar atmosphere
Authors: Piantschitsch, Isabell; Amerstorfer, Ute; Thalmann, Julia
   Katharina; Hanslmeier, Arnold; Lemmerer, Birgit
2015IAUGA..2250036P    Altcode:
  Our aim is to investigate magnetic reconnection as a result of the
  time evolution of magnetic flux tubes in the solar chromosphere. A
  new numerical two-fluid code was developed, which will perform a
  2.5D simulation of the dynamics from the upper convection zone up
  to the transition region. The code is based on the Total Variation
  Diminishing Lax-Friedrichs method and includes the effects of
  ion-neutral collisions, ionisation/recombination, thermal/resistive
  diffusivity as well as collisional/resistive heating. What is innovative
  about our newly developed code is the inclusion of a two-fluid model
  in combination with the use of analytically constructed vertically
  open magnetic flux tubes, which are used as initial conditions for
  our simulation. First magnetohydrodynamic (MHD) tests have already
  shown good agreement with known results of numerical MHD test problems
  like e.g. the Orszag-Tang vortex test, the Current Sheet test or the
  Spherical Blast Wave test. Furthermore, the single-fluid approach will
  also be applied to the initial conditions, in order to compare the
  different rates of magnetic reconnection in both codes, the two-fluid
  code and the single-fluid one.

---------------------------------------------------------
Title: The exceptional aspects of the confined X-Flares of Solar
    Active Region 2192
Authors: Thalmann, Julia K.; Su, Yang; Temmer, Manuela; Veronig, Astrid
2015IAUGA..2215645T    Altcode:
  Active region NOAA 2192 showed an outstanding productivity
  of major (GOES class M5 and larger) two-ribbon flares lacking
  eruptive events. None of the X-flares was associated to a coronal
  mass ejection. The major confined flares on 2014 October 22 and 24
  originated from the active-region core and were prohibited to develop
  an associated mass ejection due to the confinement of the overlying
  strong magnetic field. In contrast, the single eruptive M-flare on
  October 24 originated from the outer parts of the active region, in the
  neighborhood of open large-scale fields, which allowed for the observed
  mass ejection. Analysis of the spacial and temporal characteristics
  of the major confined flares revealed exceptional aspects, including a
  large initial separation of the confined flares' ribbons and an almost
  absent growth in ribbon separation, suggesting a reconnection site
  high up in the corona. Furthermore, detailed analysis of a confined
  X-flare on October 22 provides evidence that magnetic field structures
  were repeatedly involved in magnetic reconnection, that a large number
  of electrons was accelerated to non-thermal energies but that only a
  small fraction out of these accelerated electrons was accelerated to
  high energies. We conclude the latter due to the unusual steepness
  of the associated power law spectrum. Finally, we demonstrate that
  a considerable portion of the magnetic energy released during the
  X-flare was consumed by the non-thermal flare energy.

---------------------------------------------------------
Title: Evidence of suppressed heating of coronal loops rooted in
    opposite polarity sunspot umbrae
Authors: Tiwari, Sanjiv K.; Thalmann, Julia K.; Winebarger, Amy R.;
   Panesar, Navdeep K.; Moore, Ronald
2015TESS....120404T    Altcode:
  Observations of active region (AR) coronae in different EUV wavelengths
  reveal the presence of various loops at different temperatures. To
  understand the mechanisms that result in hotter or cooler loops, we
  study a typical bipolar AR, near solar disk center, which has moderate
  overall magnetic twist and at least one fully developed sunspot of
  each polarity. From AIA 193 and 94 A images we identify many clearly
  discernible coronal loops that connect opposite-polarity plage or
  a sunspot to a opposite-polarity plage region. The AIA 94 A images
  show dim regions in the umbrae of the spots. To see which coronal
  loops are rooted in a dim umbral area, we performed a non-linear
  force-free field (NLFFF) modeling using photospheric vector magnetic
  field measurements obtained with the Heliosesmic Magnetic Imager (HMI)
  onboard SDO. After validation of the NLFFF model by comparison of
  calculated model field lines and observed loops in AIA 193 and 94 A,
  we specify the photospheric roots of the model field lines. The model
  field then shows the coronal magnetic loops that arch from the dim
  umbral area of the positive-polarity sunspot to the dim umbral area of a
  negative-polarity sunspot. Because these coronal loops are not visible
  in any of the coronal EUV and X-ray images of the AR, we conclude they
  are the coolest loops in the AR. This result suggests that the loops
  connecting opposite polarity umbrae are the least heated because the
  field in umbrae is so strong that the convective braiding of the field
  is strongly suppressed.From this result, we further hypothesize that
  the convective freedom at the feet of a coronal loop, together with the
  strength of the field in the body of the loop, determines the strength
  of the heating. In particular, we expect the hottest coronal loops
  to have one foot in an umbra and the other foot in opposite-polarity
  penumbra or plage (coronal moss), the areas of strong field in which
  convection is not as strongly suppressed as in umbrae. Many transient,
  outstandingly bright, loops in the AIA 94 A movie of the AR do have
  this expected rooting pattern.

---------------------------------------------------------
Title: The Confined X-class Flares of Solar Active Region 2192
Authors: Thalmann, J. K.; Su, Y.; Temmer, M.; Veronig, A. M.
2015ApJ...801L..23T    Altcode: 2015arXiv150205157T
  The unusually large active region (AR) NOAA 2192, observed in 2014
  October, was outstanding in its productivity of major two-ribbon flares
  without coronal mass ejections. On a large scale, a predominantly
  north-south oriented magnetic system of arcade fields served as a strong
  top and lateral confinement for a series of large two-ribbon flares
  originating from the core of the AR. The large initial separation of
  the flare ribbons, together with an almost absent growth in ribbon
  separation, suggests a confined reconnection site high up in the
  corona. Based on a detailed analysis of the confined X1.6 flare on
  October 22, we show how exceptional the flaring of this AR was. We
  provide evidence for repeated energy release, indicating that the
  same magnetic field structures were repeatedly involved in magnetic
  reconnection. We find that a large number of electrons was accelerated
  to non-thermal energies, revealing a steep power-law spectrum, but
  that only a small fraction was accelerated to high energies. The total
  non-thermal energy in electrons derived (on the order of 10<SUP>25</SUP>
  J) is considerably higher than that in eruptive flares of class X1,
  and corresponds to about 10% of the excess magnetic energy present in
  the active-region corona.

---------------------------------------------------------
Title: The magnetic field in the solar atmosphere
Authors: Wiegelmann, Thomas; Thalmann, Julia K.; Solanki, Sami K.
2014A&ARv..22...78W    Altcode: 2014arXiv1410.4214W
  This publication provides an overview of magnetic fields in the solar
  atmosphere with the focus lying on the corona. The solar magnetic field
  couples the solar interior with the visible surface of the Sun and with
  its atmosphere. It is also responsible for all solar activity in its
  numerous manifestations. Thus, dynamic phenomena such as coronal mass
  ejections and flares are magnetically driven. In addition, the field
  also plays a crucial role in heating the solar chromosphere and corona
  as well as in accelerating the solar wind. Our main emphasis is the
  magnetic field in the upper solar atmosphere so that photospheric and
  chromospheric magnetic structures are mainly discussed where relevant
  for higher solar layers. Also, the discussion of the solar atmosphere
  and activity is limited to those topics of direct relevance to the
  magnetic field. After giving a brief overview about the solar magnetic
  field in general and its global structure, we discuss in more detail
  the magnetic field in active regions, the quiet Sun and coronal holes.

---------------------------------------------------------
Title: Force-free Field Modeling of Twist and Braiding-induced
    Magnetic Energy in an Active-region Corona
Authors: Thalmann, J. K.; Tiwari, S. K.; Wiegelmann, T.
2014ApJ...780..102T    Altcode: 2013arXiv1311.3413T
  The theoretical concept that braided magnetic field lines in the solar
  corona may dissipate a sufficient amount of energy to account for the
  brightening observed in the active-region (AR) corona has only recently
  been substantiated by high-resolution observations. From the analysis
  of coronal images obtained with the High Resolution Coronal Imager,
  first observational evidence of the braiding of magnetic field lines
  was reported by Cirtain et al. (hereafter CG13). We present nonlinear
  force-free reconstructions of the associated coronal magnetic field
  based on Solar Dynamics Observatory/Helioseismic and Magnetic Imager
  vector magnetograms. We deliver estimates of the free magnetic energy
  associated with a braided coronal structure. Our model results suggest
  (~100 times) more free energy at the braiding site than analytically
  estimated by CG13, strengthening the possibility of the AR corona
  being heated by field line braiding. We were able to appropriately
  assess the coronal free energy by using vector field measurements and
  we attribute the lower energy estimate of CG13 to the underestimated
  (by a factor of 10) azimuthal field strength. We also quantify the
  increase in the overall twist of a flare-related flux rope that was
  noted by CG13. From our models we find that the overall twist of the
  flux rope increased by about half a turn within 12 minutes. Unlike
  another method to which we compare our results, we evaluate the
  winding of the flux rope's constituent field lines around each other
  purely based on their modeled coronal three-dimensional field line
  geometry. To our knowledge, this is done for the first time here.

---------------------------------------------------------
Title: Two-Fluid 2.5D MHD-Code for Simulations in the Solar Atmosphere
Authors: Piantschitsch, I.; Amerstorfer, U.; Thalmann, J.; Utz, D.;
   Hanslmeier, A.; Bárta, M.; Thonhofer, S.; Lemmerer, B.
2014CEAB...38...59P    Altcode:
  We investigate magnetic reconnection due to the evolution of magnetic
  flux tubes in the solar chromosphere. We developed a new numerical
  two-fluid magnetohydrodynamic (MHD) code which will perform a 2.5D
  simulation of the dynamics from the upper convection zone up to the
  transition region. Our code is based on the Total Variation Diminishing
  Lax-Friedrichs scheme and makes use of an alternating-direction implicit
  method, in order to accommodate the two spatial dimensions. Since we
  apply a two-fluid model for our simulations, the effects of ion-neutral
  collisions, ionization/recombination, thermal/resistive diffusivity
  and collisional/resistive heating are included in the code. As initial
  conditions for the code we use analytically constructed vertically open
  magnetic flux tubes within a realistic stratified atmosphere. Initial
  MHD tests have already shown good agreement with known results of
  numerical MHD test problems like e.g. the Orszag-Tang vortex test.

---------------------------------------------------------
Title: Twisting solar coronal jet launched at the boundary of an
    active region
Authors: Schmieder, B.; Guo, Y.; Moreno-Insertis, F.; Aulanier, G.;
   Yelles Chaouche, L.; Nishizuka, N.; Harra, L. K.; Thalmann, J. K.;
   Vargas Dominguez, S.; Liu, Y.
2013A&A...559A...1S    Altcode: 2013arXiv1309.6514S
  <BR /> Aims: A broad jet was observed in a weak magnetic field area
  at the edge of active region NOAA 11106 that also produced other
  nearby recurring and narrow jets. The peculiar shape and magnetic
  environment of the broad jet raised the question of whether it was
  created by the same physical processes of previously studied jets
  with reconnection occurring high in the corona. <BR /> Methods:
  We carried out a multi-wavelength analysis using the EUV images
  from the Atmospheric Imaging Assembly (AIA) and magnetic fields
  from the Helioseismic and Magnetic Imager (HMI) both on-board the
  Solar Dynamics Observatory, which we coupled to a high-resolution,
  nonlinear force-free field extrapolation. Local correlation tracking
  was used to identify the photospheric motions that triggered the jet,
  and time-slices were extracted along and across the jet to unveil its
  complex nature. A topological analysis of the extrapolated field was
  performed and was related to the observed features. <BR /> Results:
  The jet consisted of many different threads that expanded in around 10
  minutes to about 100 Mm in length, with the bright features in later
  threads moving faster than in the early ones, reaching a maximum speed
  of about 200 km s<SUP>-1</SUP>. Time-slice analysis revealed a striped
  pattern of dark and bright strands propagating along the jet, along with
  apparent damped oscillations across the jet. This is suggestive of a
  (un)twisting motion in the jet, possibly an Alfvén wave. Bald patches
  in field lines, low-altitude flux ropes, diverging flow patterns, and a
  null point were identified at the basis of the jet. <BR /> Conclusions:
  Unlike classical λ or Eiffel-tower-shaped jets that appear to be caused
  by reconnection in current sheets containing null points, reconnection
  in regions containing bald patches seems to be crucial in triggering
  the present jet. There is no observational evidence that the flux
  ropes detected in the topological analysis were actually being ejected
  themselves, as occurs in the violent phase of blowout jets; instead,
  the jet itself may have gained the twist of the flux rope(s) through
  reconnection. This event may represent a class of jets different from
  the classical quiescent or blowout jets, but to reach that conclusion,
  more observational and theoretical work is necessary.

---------------------------------------------------------
Title: Comparison of Force-free Coronal Magnetic Field Modeling
    Using Vector Fields from Hinode and Solar Dynamics Observatory
Authors: Thalmann, J. K.; Tiwari, S. K.; Wiegelmann, T.
2013ApJ...769...59T    Altcode: 2013arXiv1304.3619T
  Photospheric magnetic vector maps from two different instruments
  are used to model the nonlinear force-free coronal magnetic field
  above an active region. We use vector maps inferred from polarization
  measurements of the Solar Dynamics Observatory/Helioseismic and Magnetic
  Imager (HMI) and the Solar Optical Telescope's Spectropolarimeter (SP)
  on board Hinode. Besides basing our model calculations on HMI data,
  we use both SP data of original resolution and scaled down to the
  resolution of HMI. This allows us to compare the model results based
  on data from different instruments and to investigate how a binning
  of high-resolution data affects the model outcome. The resulting
  three-dimensional magnetic fields are compared in terms of magnetic
  energy content and magnetic topology. We find stronger magnetic fields
  in the SP data, translating into a higher total magnetic energy
  of the SP models. The net Lorentz forces of the HMI and SP lower
  boundaries verify their force-free compatibility. We find substantial
  differences in the absolute estimates of the magnetic field energy but
  similar relative estimates, e.g., the fraction of excess energy and
  of the flux shared by distinct areas. The location and extension of
  neighboring connectivity domains differ and the SP model fields tend
  to be higher and more vertical. Hence, conclusions about the magnetic
  connectivity based on force-free field models are to be drawn with
  caution. We find that the deviations of the model solution when based
  on the lower-resolution SP data are small compared to the differences
  of the solutions based on data from different instruments.

---------------------------------------------------------
Title: Force-free coronal magnetic field modeling using vector fields
    from Hinode and SDO
Authors: Thalmann, Julia K.; Tiwari, Sanjiv K.; Wiegelmann, Thomas
2013EGUGA..15.1368T    Altcode:
  Given the lack of routine direct measurements of the magnetic
  field in the solar corona, force-free reconstruction methods are
  a promising tool for the diagnostics of the magnetic structure
  there. Routine photospheric magnetic field measurements which monitor
  the temporal evolution of an active region and contain information on
  the non-potentiality of the field above are used as an input. Based on
  the assumption that magnetic forces dominate the solar atmosphere, these
  models allow estimates of the total and free magnetic energy content and
  the structure of the magnetic field above active regions. The outcome
  of force-free field modeling strongly depends on the vector magnetic
  field data used as boundary condition. We compare the model results
  based on simultaneously observed vector maps from the Helioseismic and
  Magnetic Imager (HMI) on board Solar Dynamics Observatory and from the
  Solar Optical Telescope Spectropolarimeter (SP) on board Hinode. We
  find substantial differences in the absolute estimates of the magnetic
  field energy but very similar relative estimates, e.g., the fraction
  of energy to be set free during an eruption or the fraction of flux
  linking distinct areas within an active region. Our study reveals that
  only relative estimates of coronal physical quantities from force-free
  models might be save and conclusions about the magnetic field topology
  might be drawn with caution.

---------------------------------------------------------
Title: On the Comparison of Nonlinear Force-free Models Based on
    Vector-magnetograms from Different Instruments
Authors: Thalmann, J. K.; Wiegelmann, T.; Tiwari, S. K.; Sun, X.
2012AGUFMSH41C2120T    Altcode:
  We investigate the three-dimensional structure of the magnetic field in
  the outer solar atmosphere with the help of photospheric magnetic vector
  maps based on measurements from the Helioseismic and Magnetic Imager
  (HMI) on board the Solar Dynamics Observatory and of the Solar Optical
  Telescope Spectral-polarimeter (SP) on board the Hinode spacecraft. HMI
  and SP magnetic vector maps of NOAA AR 11382 on 21-22 December 2011
  are used as lower boundary condition for nonlinear force-free field
  reconstructions. We compare the resulting three-dimensional coronal
  magnetic field models in terms of the energy content, the magnetic
  pressure, the vertical distribution of the magnetic field and
  associated electric current density, as well as the magnetic field
  line configuration and compare the latter to the loops visible in
  coronal images from the SDO Atmospheric Imaging Assembly.

---------------------------------------------------------
Title: How Should One Optimize Nonlinear Force-Free Coronal Magnetic
    Field Extrapolations from SDO/HMI Vector Magnetograms?
Authors: Wiegelmann, T.; Thalmann, J. K.; Inhester, B.; Tadesse, T.;
   Sun, X.; Hoeksema, J. T.
2012SoPh..281...37W    Altcode: 2012SoPh..tmp...67W; 2012arXiv1202.3601W
  The Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
  Observatory (SDO) provides photospheric vector magnetograms with
  a high spatial and temporal resolution. Our intention is to model
  the coronal magnetic field above active regions with the help of
  a nonlinear force-free extrapolation code. Our code is based on an
  optimization principle and has been tested extensively with semianalytic
  and numeric equilibria and applied to vector magnetograms from Hinode
  and ground-based observations. Recently we implemented a new version
  which takes into account measurement errors in photospheric vector
  magnetograms. Photospheric field measurements are often affected by
  measurement errors and finite nonmagnetic forces inconsistent for use
  as a boundary for a force-free field in the corona. To deal with these
  uncertainties, we developed two improvements: i) preprocessing of the
  surface measurements to make them compatible with a force-free field,
  and ii) new code which keeps a balance between the force-free constraint
  and deviation from the photospheric field measurements. Both methods
  contain free parameters, which must be optimized for use with data from
  SDO/HMI. In this work we describe the corresponding analysis method
  and evaluate the force-free equilibria by how well force-freeness and
  solenoidal conditions are fulfilled, by the angle between magnetic
  field and electric current, and by comparing projections of magnetic
  field lines with coronal images from the Atmospheric Imaging Assembly
  (SDO/AIA). We also compute the available free magnetic energy and
  discuss the potential influence of control parameters.

---------------------------------------------------------
Title: Nonlinear Force-free Field Modeling of a Solar Active Region
    Using SDO/HMI and SOLIS/VSM Data
Authors: Thalmann, J. K.; Pietarila, A.; Sun, X.; Wiegelmann, T.
2012AJ....144...33T    Altcode: 2012arXiv1206.1141T
  We use SDO/HMI and SOLIS/VSM photospheric magnetic field measurements
  to model the force-free coronal field above a solar active region,
  assuming magnetic forces dominate. We take measurement uncertainties
  caused by, e.g., noise and the particular inversion technique, into
  account. After searching for the optimum modeling parameters for the
  particular data sets, we compare the resulting nonlinear force-free
  model fields. We show the degree of agreement of the coronal field
  reconstructions from the different data sources by comparing the
  relative free energy content, the vertical distribution of the magnetic
  pressure, and the vertically integrated current density. Though the
  longitudinal and transverse magnetic flux measured by the VSM and
  HMI is clearly different, we find considerable similarities in the
  modeled fields. This indicates the robustness of the algorithm we use
  to calculate the nonlinear force-free fields against differences and
  deficiencies of the photospheric vector maps used as an input. We also
  depict how much the absolute values of the total force-free, virial,
  and the free magnetic energy differ and how the orientation of the
  longitudinal and transverse components of the HMI- and VSM-based model
  volumes compare to each other.

---------------------------------------------------------
Title: Evolution of Magnetic Field and Energy in a Major Eruptive
    Active Region Based on SDO/HMI Observation
Authors: Sun, Xudong; Hoeksema, J. Todd; Liu, Yang; Wiegelmann,
   Thomas; Hayashi, Keiji; Chen, Qingrong; Thalmann, Julia
2012ApJ...748...77S    Altcode: 2012arXiv1201.3404S
  We report the evolution of the magnetic field and its energy in NOAA
  active region 11158 over five days based on a vector magnetogram series
  from the Helioseismic and Magnetic Imager (HMI) on board the Solar
  Dynamic Observatory (SDO). Fast flux emergence and strong shearing
  motion led to a quadrupolar sunspot complex that produced several
  major eruptions, including the first X-class flare of Solar Cycle
  24. Extrapolated nonlinear force-free coronal fields show substantial
  electric current and free energy increase during early flux emergence
  near a low-lying sigmoidal filament with a sheared kilogauss field
  in the filament channel. The computed magnetic free energy reaches a
  maximum of ~2.6 × 10<SUP>32</SUP> erg, about 50% of which is stored
  below 6 Mm. It decreases by ~0.3 × 10<SUP>32</SUP> erg within 1 hr
  of the X-class flare, which is likely an underestimation of the actual
  energy loss. During the flare, the photospheric field changed rapidly:
  the horizontal field was enhanced by 28% in the core region, becoming
  more inclined and more parallel to the polarity inversion line. Such
  change is consistent with the conjectured coronal field "implosion" and
  is supported by the coronal loop retraction observed by the Atmospheric
  Imaging Assembly (AIA). The extrapolated field becomes more "compact"
  after the flare, with shorter loops in the core region, probably because
  of reconnection. The coronal field becomes slightly more sheared in the
  lowest layer, relaxes faster with height, and is overall less energetic.

---------------------------------------------------------
Title: Nonlinear Force-Free Extrapolation of Vector Magnetograms
    into the Corona
Authors: Thalmann, J. K.; Wiegelmann, T.; Sun, X.; Hoeksema, J. T.;
   Liu, Y.; Tadesse, T.
2011AGUFMSH33C..05T    Altcode:
  To investigate the structure and evolution of the coronal magnetic
  field, we extrapolate measurements of the photospheric magnetic
  field vector into the corona based on the force-free assumption. A
  complication of this approach is that the measured photospheric
  magnetic field is not force-free and that one has to apply a
  preprocessing routine in order to achieve boundary conditions suitable
  for the force-free modelling. Furthermore the nonlinear force-free
  extrapolation code takes errors in the photospheric field data into
  account which occur due to noise, incomplete inversions or ambiguity
  removing techniques. Within this work we compare extrapolations from
  SDO/HMI and SOLIS vector magnetograms and explain how to find optimum
  parameters for handling the data of a particular instrument. The
  resulting coronal magnetic field lines are quantitatively compared
  with coronal EUV-images from SDO/AIA.

---------------------------------------------------------
Title: Evolution of Magnetic Field and Energy in A Major Eruptive
    Active Region Based on SDO/HMI Observation
Authors: Sun, Xudong; Hoeksema, Todd; Liu, Yang; Wiegelmann, Thomas;
   Hayashi, Keiji; Chen, Qingrong; Thalmann, Julia
2011sdmi.confE..63S    Altcode:
  We report the evolution of magnetic field and its energy in NOAA
  AR 11158 based on a vector magnetogram series from the Helioseismic
  and Magnetic Imager (HMI). Fast flux emergence and strong shearing
  motion created a quadrupolar sunspot complex that produced several
  major eruptions, including the first X-class flare of solar cycle
  24. Extrapolated non-linear force-free coronal field shows substantial
  electric current and free energy increase during early flux emergence
  along a newly-formed, low-lying filament with a typical 1000 G field
  strength and 0.45 Mm^(-1) alpha-parameter at its center. The computed
  magnetic free energy reaches a maximum of 2.62E32 erg, about 50%
  stored below 6 Mm. This free energy decreases by 0.33E32 erg within
  1 hour of the X-class flare, which is likely an underestimation of
  the actual energy loss. During the flare, photospheric field changed
  rapidly: the horizontal field was enhanced by 28% in the AR core
  region. Such change is consistent with the conjectured coronal field
  "implosion", and is in line with both the reconnection signatures
  and the coronal loop retraction observed by the Atmospheric Image
  Assembly (AIA). Extrapolation indicates that the coronal field relaxes
  more rapidly with height after the flare and becomes overall less
  energetic. These preliminary results demonstrate the capability to
  quantitatively study the AR field topology and energetics using SDO
  data- although difficulties still abound.

---------------------------------------------------------
Title: Estimating the Relative Helicity of Coronal Magnetic Fields
Authors: Thalmann, J. K.; Inhester, B.; Wiegelmann, T.
2011SoPh..272..243T    Altcode:
  To quantify changes of the solar coronal field connectivity during
  eruptive events, one can use magnetic helicity, which is a measure of
  the shear or twist of a current-carrying (non-potential) field. To
  find a physically meaningful quantity, a relative measure, giving
  the helicity of a current-carrying field with respect to a reference
  (potential) field, is often evaluated. This requires a knowledge of the
  three-dimensional vector potential. We present a method to calculate
  the vector potential for a solenoidal magnetic field as the sum of a
  Laplacian part and a current-carrying part. The only requirements are
  the divergence freeness of the Laplacian and current-carrying magnetic
  field and the sameness of their normal field component on the bounding
  surface of the considered volume.

---------------------------------------------------------
Title: Monitoring free magnetic energy in erupting active regions
Authors: Wiegelmann, Thomas; Thalmann, Julia; Jing, Ju; Wang, Haimin
2010cosp...38.2960W    Altcode: 2010cosp.meet.2960W
  In solar eruptions, like flares and coronal mass ejections, free
  magnetic energy stored in the solar corona is converted into kinetic
  energy. Unfortunately the coronal magnetic field cannot be measured
  directly. We can, however, reconstruct the coronal magnetic field
  from measurements of the photospheric magnetic field vector under
  the reasonable assumption of a force-free coronal plasma. With
  a procedure dubbed preprocessing we derive force-free consistent
  boundary conditions, which are extrapolated into the solar corona
  with a nonlinear force-free extrapolation code. The resulting 3D
  coronal magnetic field allows us to derive the magnetic topology and
  to computed the magnetic energy as well as an upper limited of the
  free energy available for driving eruptive phenomena. We apply our
  code to measurements from several ground based vector magnetographs,
  e.g. the Solar Flare Telescope, SOLIS and the Big Bear Solar
  Observatory. Within our studies we find a clear relationship between
  the stored magnetic energy and the strength of eruptions. In most cases
  not the entire free energy is converted to kinetic energy, but only a
  fraction. Consequently, the post-flare magnetic field configuration
  is usually not entirely current free, but significantly closer to a
  potential field as before the flare.

---------------------------------------------------------
Title: Evolution of coronal magnetic fields
Authors: Thalmann, Julia Katharina
2010PhDT.......222T    Altcode:
  No abstract at ADS

---------------------------------------------------------
Title: Nonlinear Force-Free Magnetic Field Modeling of AR 10953:
    A Critical Assessment
Authors: De Rosa, Marc L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
   Lites, B. W.; Aschwanden, M. J.; Amari, T.; Canou, A.; McTiernan,
   J. M.; Régnier, S.; Thalmann, J. K.; Valori, G.; Wheatland, M. S.;
   Wiegelmann, T.; Cheung, M. C. M.; Conlon, P. A.; Fuhrmann, M.;
   Inhester, B.; Tadesse, T.
2009SPD....40.3102D    Altcode:
  Nonlinear force-free field (NLFFF) modeling seeks to provide accurate
  representations of the structure of the magnetic field above solar
  active regions, from which estimates of physical quantities of interest
  (e.g., free energy and helicity) can be made. However, the suite of
  NLFFF algorithms have failed to arrive at consistent solutions when
  applied to (thus far, two) cases using the highest-available-resolution
  vector magnetogram data from Hinode/SOT-SP (in the region of the
  modeling area of interest) and line-of-sight magnetograms from
  SOHO/MDI (where vector data were not available). One issue is that
  NLFFF models require consistent, force-free vector magnetic boundary
  data, and vector magnetogram data sampling the photosphere do not
  satisfy this requirement. Consequently, several problems have arisen
  that are believed to affect such modeling efforts. We use AR 10953
  to illustrate these problems, namely: (1) some of the far-reaching,
  current-carrying connections are exterior to the observational field
  of view, (2) the solution algorithms do not (yet) incorporate the
  measurement uncertainties in the vector magnetogram data, and/or (3)
  a better way is needed to account for the Lorentz forces within the
  layer between the photosphere and coronal base. In light of these
  issues, we conclude that it remains difficult to derive useful and
  significant estimates of physical quantities from NLFFF models.

---------------------------------------------------------
Title: A Critical Assessment of Nonlinear Force-Free Field Modeling
    of the Solar Corona for Active Region 10953
Authors: De Rosa, Marc L.; Schrijver, Carolus J.; Barnes, Graham;
   Leka, K. D.; Lites, Bruce W.; Aschwanden, Markus J.; Amari, Tahar;
   Canou, Aurélien; McTiernan, James M.; Régnier, Stéphane; Thalmann,
   Julia K.; Valori, Gherardo; Wheatland, Michael S.; Wiegelmann, Thomas;
   Cheung, Mark C. M.; Conlon, Paul A.; Fuhrmann, Marcel; Inhester,
   Bernd; Tadesse, Tilaye
2009ApJ...696.1780D    Altcode: 2009arXiv0902.1007D
  Nonlinear force-free field (NLFFF) models are thought to be viable
  tools for investigating the structure, dynamics, and evolution of
  the coronae of solar active regions. In a series of NLFFF modeling
  studies, we have found that NLFFF models are successful in application
  to analytic test cases, and relatively successful when applied
  to numerically constructed Sun-like test cases, but they are less
  successful in application to real solar data. Different NLFFF models
  have been found to have markedly different field line configurations
  and to provide widely varying estimates of the magnetic free energy in
  the coronal volume, when applied to solar data. NLFFF models require
  consistent, force-free vector magnetic boundary data. However,
  vector magnetogram observations sampling the photosphere, which is
  dynamic and contains significant Lorentz and buoyancy forces, do not
  satisfy this requirement, thus creating several major problems for
  force-free coronal modeling efforts. In this paper, we discuss NLFFF
  modeling of NOAA Active Region 10953 using Hinode/SOT-SP, Hinode/XRT,
  STEREO/SECCHI-EUVI, and SOHO/MDI observations, and in the process
  illustrate three such issues we judge to be critical to the success of
  NLFFF modeling: (1) vector magnetic field data covering larger areas
  are needed so that more electric currents associated with the full
  active regions of interest are measured, (2) the modeling algorithms
  need a way to accommodate the various uncertainties in the boundary
  data, and (3) a more realistic physical model is needed to approximate
  the photosphere-to-corona interface in order to better transform the
  forced photospheric magnetograms into adequate approximations of nearly
  force-free fields at the base of the corona. We make recommendations
  for future modeling efforts to overcome these as yet unsolved problems.

---------------------------------------------------------
Title: Magnetic Field Extrapolation of Flaring Active Regions
Authors: Thalmann, J. K.; Wiegelmann, T.
2009CEAB...33..131T    Altcode:
  The solar corona is structured by magnetic fields. As direct
  measurements of the coronal magnetic field are not routinely available,
  it is extrapolated from photospheric vector magnetograms. When magnetic
  flux emerges from below the solar surface and expands into the corona,
  the coronal magnetic field is destabilized, leading to explosive
  phenomena like flares or coronal mass ejections. Our aim is to get
  insights in the coronal magnetic field structure in active regions and
  to study its temporal evolution. We are in particular interested to
  investigate the magnetic configuration of active regions in the course
  of flares. Therefore, we study the temporal evolution of the flaring
  active regions NOAA 10540 and NOAA 10960 as observed in January 2004 and
  June 2007, respectively. We are in particular interested in the free
  magnetic energy available to power the flares associated with it. To
  investigate AR 10540 we used photospheric vector magnetograms measured
  with the Solar Flare Telescope VectorMagnetograph and for AR 10960 we
  used data provided by the Synoptic Optical Long-term Investigations of
  the Sun VectorSpectroMagnetograph. We extrapolated these measurements
  into the corona with the help of a nonlinear force-free field model
  based on a well-tested multigrid-like optimization code with which
  we were able to estimate the energy content of the 3D coronal fields,
  as well as an upper limit for its free magnetic energy.

---------------------------------------------------------
Title: Nonlinear Force-Free Magnetic Field Modeling of the Solar
Corona: A Critical Assessment
Authors: De Rosa, M. L.; Schrijver, C. J.; Barnes, G.; Leka, K. D.;
   Lites, B. W.; Aschwanden, M. J.; McTiernan, J. M.; Régnier, S.;
   Thalmann, J.; Valori, G.; Wheatland, M. S.; Wiegelmann, T.; Cheung,
   M.; Conlon, P. A.; Fuhrmann, M.; Inhester, B.; Tadesse, T.
2008AGUFMSH41A1604D    Altcode:
  Nonlinear force-free field (NLFFF) modeling promises to provide accurate
  representations of the structure of the magnetic field above solar
  active regions, from which estimates of physical quantities of interest
  (e.g., free energy and helicity) can be made. However, the suite of
  NLFFF algorithms have so far failed to arrive at consistent solutions
  when applied to cases using the highest-available-resolution vector
  magnetogram data from Hinode/SOT-SP (in the region of the modeling
  area of interest) and line-of-sight magnetograms from SOHO/MDI (where
  vector data were not been available). It is our view that the lack of
  robust results indicates an endemic problem with the NLFFF modeling
  process, and that this process will likely continue to fail until (1)
  more of the far-reaching, current-carrying connections are within the
  observational field of view, (2) the solution algorithms incorporate
  the measurement uncertainties in the vector magnetogram data, and/or
  (3) a better way is found to account for the Lorentz forces within
  the layer between the photosphere and coronal base. In light of these
  issues, we conclude that it remains difficult to derive useful and
  significant estimates of physical quantities from NLFFF models.

---------------------------------------------------------
Title: Evolution of two Flaring Active Regions With CME Association
Authors: Thalmann, J. K.; Wiegelmann, T.
2008AGUFMSH23B1642T    Altcode:
  We study the coronal magnetic field structure of two active regions, one
  during solar activity minimum (June 2007) and another one during a more
  active time (January 2004). The temporal evolution was explored with the
  help of nonlinear force-free coronal magnetic field extrapolations of
  SOLIS/VSM and NAOJ/SFT photospheric vector magnetograms. We study the
  active region NOAA 10960 observed on 2007 June 7 with three SOLIS/VSM
  snapshots taken during a small C1.0 flare of time cadence 10 minutes
  and six snapshots during a quiet period. The total magnetic energy in
  the active region was approximately 3 × 1025 J. Before the flare the
  free magnetic energy was about 5~% of the potential field energy. A part
  of this excess energy was released during the flare, producing almost
  a potential configuration at the beginning of the quiet period. The
  return to an almost potential structure can be assigned to a CME as
  recorded by the SoHO/LASCO instrument on 2007 June 07 around 10 minutes
  after the flare peaked, so that whatever magnetic helicity was bodily
  removed from the structure. This was compared with active region 10540
  observed on 2004 January 18 -- 21, which was analyzed with the help
  of vector magnetograph data from the Solar Flare Telescope in Japan
  of time cadence of about 1 day. The free energy was Efree≈ 66~%
  of the total energy which was sufficiently high to power a M6.1 flare
  on January 20, which was associated with a CME 20 minutes later. The
  activity of AR 10540 was significantly higher than for AR 10960,
  as was the total magnetic energy. Furthermore, we found the common
  feature that magnetic energy accumulates before the flare/CME and a
  significant part of the excess energy is released during the eruption.

---------------------------------------------------------
Title: First nonlinear force-free field extrapolations of SOLIS/VSM
    data
Authors: Thalmann, J. K.; Wiegelmann, T.; Raouafi, N. -E.
2008A&A...488L..71T    Altcode: 2008arXiv0809.1428T
  Aims: We study the coronal magnetic field structure inside active
  regions and its temporal evolution. We attempt to compare the magnetic
  configuration of an active region in a very quiet period with that
  for the same region during a flare. <BR />Methods: Probably for
  the first time, we use vector magnetograph data from the Synoptic
  Optical Long-term Investigations of the Sun survey (SOLIS) to model
  the coronal magnetic field as a sequence of nonlinear force-free
  equilibria. We study the active region NOAA 10960 observed on 2007
  June 7 with three snapshots taken during a small C1.0 flare of time
  cadence 10 min and six snapshots during a quiet period. <BR />Results:
  The total magnetic energy in the active region was approximately 3 ×
  10<SUP>25</SUP> J. Before the flare the free magnetic energy was about
  5% of the potential field energy. A part of this excess energy was
  released during the flare, producing almost a potential configuration
  at the beginning of the quiet period. <BR />Conclusions: During the
  investigated period, the coronal magnetic energy was only a few percent
  higher than that of the potential field and consequently only a small
  C1.0 flare occurred. This was compared with an earlier investigated
  active region 10540, where the free magnetic energy was about 60% higher
  than that of the potential field producing two M-class flares. However,
  the free magnetic energy accumulates before and is released during
  the flare which appears to be the case for both large and small flares.

---------------------------------------------------------
Title: Preprocessing of Hinode/SOT Vector Magnetograms for Nonlinear
    Force-Free Coronal Magnetic Field Modeling
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
   M. L.; Metcalf, T. R.
2008ASPC..397..198W    Altcode: 2008arXiv0801.2884W
  The solar magnetic field is key to understanding the physical processes
  in the solar atmosphere. Nonlinear force-free codes have been shown
  to be useful in extrapolating the coronal field from underlying vector
  boundary data (for an overview see Schrijver et al. (2006)). However,
  we can only measure the magnetic field vector routinely with high
  accuracy in the photosphere with, e.g., Hinode/SOT, and unfortunately
  these data do not fulfill the force-free consistency condition as
  defined by Aly (1989). We must therefore apply some transformations
  to these data before nonlinear force-free extrapolation codes can be
  legitimately applied. To this end, we have developed a minimization
  procedure that uses the measured photospheric field vectors as input
  to approximate a more chromospheric like field (The method was dubbed
  preprocessing. See Wiegelmann et al. (2006) for details). The procedure
  includes force-free consistency integrals and spatial smoothing. The
  method has been intensively tested with model active regions (see
  Metcalf et al. 2008) and been applied to several ground based vector
  magnetogram data before. Here we apply the preprocessing program to
  photospheric magnetic field measurements with the Hinode/SOT instrument.

---------------------------------------------------------
Title: Evolution of the flaring active region NOAA 10540 as a sequence
    of nonlinear force-free field extrapolations
Authors: Thalmann, J. K.; Wiegelmann, T.
2008A&A...484..495T    Altcode:
  Context: The solar corona is structured by magnetic fields. As direct
  measurements of the coronal magnetic field are not routinely available,
  it is extrapolated from photospheric vector magnetograms. When
  magnetic flux emerges from below the solar surface and expands into
  the corona, the coronal magnetic field is destabilized, leading to
  explosive phenomena like flares or coronal mass ejections. <BR />Aims:
  We study the temporal evolution of the flaring active region NOAA
  10540 and are in particular interested in the free magnetic energy
  available to power the flares associated with it. <BR />Methods: We
  extrapolated photospheric vector magnetograms measured with the Solar
  Flare Telescope, located in Tokyo, into the corona with the help of a
  nonlinear force-free field model. This coronal magnetic field model is
  based on a well-tested multigrid-like optimization code with which we
  were able to estimate the energy content of the 3D coronal field, as
  well as an upper limit for its free magnetic energy. Furthermore, the
  evolution of the energy density with height and time was studied. <BR
  />Results: The coronal magnetic field energy in active region 10540
  increases slowly during the three days before an M6.1 flare and drops
  significantly after it. We estimated the energy that was set free
  during this event as ∝10<SUP>25</SUP> J. A sequence of nonlinear
  force-free extrapolations of the coronal magnetic field shows a build
  up of magnetic energy before a flare and release of energy during the
  flare. The drop in magnetic energy of the active region is sufficient
  to power an M6.1 flare.

---------------------------------------------------------
Title: Non-Linear Force-Free Field Modeling of a Solar Active Region
    Around the Time of a Major Flare and Coronal Mass Ejection
Authors: De Rosa, M. L.; Schrijver, C. J.; Metcalf, T. R.; Barnes,
   G.; Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann,
   T.; Wheatland, M.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
   M.; Kusano, K.; Régnier, S.; Thalmann, J.
2008AGUSMSP31A..06D    Altcode:
  Solar flares and coronal mass ejections are associated with rapid
  changes in coronal magnetic field connectivity and are powered by
  the partial dissipation of electrical currents that run through
  the solar corona. A critical unanswered question is whether the
  currents involved are induced by the advection along the photosphere
  of pre-existing atmospheric magnetic flux, or whether these currents
  are associated with newly emergent flux. We address this problem by
  applying nonlinear force-free field (NLFFF) modeling to the highest
  resolution and quality vector-magnetographic data observed by the
  recently launched Hinode satellite on NOAA Active Region 10930 around
  the time of a powerful X3.4 flare in December 2006. We compute 14
  NLFFF models using 4 different codes having a variety of boundary
  conditions. We find that the model fields differ markedly in geometry,
  energy content, and force-freeness. We do find agreement of the best-fit
  model field with the observed coronal configuration, and argue (1)
  that strong electrical currents emerge together with magnetic flux
  preceding the flare, (2) that these currents are carried in an ensemble
  of thin strands, (3) that the global pattern of these currents and
  of field lines are compatible with a large-scale twisted flux rope
  topology, and (4) that the ~1032~erg change in energy associated with
  the coronal electrical currents suffices to power the flare and its
  associated coronal mass ejection. We discuss the relative merits of
  these models in a general critique of our present abilities to model
  the coronal magnetic field based on surface vector field measurements.

---------------------------------------------------------
Title: Nonlinear Force-free Field Modeling of a Solar Active Region
    around the Time of a Major Flare and Coronal Mass Ejection
Authors: Schrijver, C. J.; DeRosa, M. L.; Metcalf, T.; Barnes, G.;
   Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann, T.;
   Wheatland, M. S.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann,
   M.; Kusano, K.; Régnier, S.; Thalmann, J. K.
2008ApJ...675.1637S    Altcode: 2007arXiv0712.0023S
  Solar flares and coronal mass ejections are associated with rapid
  changes in field connectivity and are powered by the partial dissipation
  of electrical currents in the solar atmosphere. A critical unanswered
  question is whether the currents involved are induced by the motion of
  preexisting atmospheric magnetic flux subject to surface plasma flows or
  whether these currents are associated with the emergence of flux from
  within the solar convective zone. We address this problem by applying
  state-of-the-art nonlinear force-free field (NLFFF) modeling to the
  highest resolution and quality vector-magnetographic data observed
  by the recently launched Hinode satellite on NOAA AR 10930 around
  the time of a powerful X3.4 flare. We compute 14 NLFFF models with
  four different codes and a variety of boundary conditions. We find
  that the model fields differ markedly in geometry, energy content,
  and force-freeness. We discuss the relative merits of these models in
  a general critique of present abilities to model the coronal magnetic
  field based on surface vector field measurements. For our application
  in particular, we find a fair agreement of the best-fit model field
  with the observed coronal configuration, and argue (1) that strong
  electrical currents emerge together with magnetic flux preceding the
  flare, (2) that these currents are carried in an ensemble of thin
  strands, (3) that the global pattern of these currents and of field
  lines are compatible with a large-scale twisted flux rope topology,
  and (4) that the ~10<SUP>32</SUP> erg change in energy associated with
  the coronal electrical currents suffices to power the flare and its
  associated coronal mass ejection.

---------------------------------------------------------
Title: Can We Improve the Preprocessing of Photospheric Vector
    Magnetograms by the Inclusion of Chromospheric Observations?
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
   M. L.; Metcalf, T. R.
2008SoPh..247..249W    Altcode: 2008arXiv0801.2707W; 2008SoPh..tmp...27W
  The solar magnetic field is key to understanding the physical processes
  in the solar atmosphere. Nonlinear force-free codes have been shown to
  be useful in extrapolating the coronal field upward from underlying
  vector boundary data. However, we can only measure the magnetic
  field vector routinely with high accuracy in the photosphere, and
  unfortunately these data do not fulfill the force-free condition. We
  must therefore apply some transformations to these data before nonlinear
  force-free extrapolation codes can be self-consistently applied. To
  this end, we have developed a minimization procedure that yields a more
  chromosphere-like field, using the measured photospheric field vectors
  as input. The procedure includes force-free consistency integrals,
  spatial smoothing, and - newly included in the version presented here
  - an improved match to the field direction as inferred from fibrils
  as can be observed in, for example, chromospheric Hα images. We test
  the procedure using a model active-region field that included buoyancy
  forces at the photospheric level. The proposed preprocessing method
  allows us to approximate the chromospheric vector field to within a few
  degrees and the free energy in the coronal field to within one percent.

---------------------------------------------------------
Title: Nonlinear force-free field models
Authors: Wiegelmann, Thomas; Thalmann, Julia; Inhester, Bernd
2008cosp...37.3462W    Altcode: 2008cosp.meet.3462W
  The photospheric magnetic field vector is routinely measured with high
  accuracy from ground based and space born instruments. We use these
  measurements to prescribe suitable boundary conditions for modelling
  the coronal magnetic field. Because of the low-beta plasma the magnetic
  field is in lowest order assumed to be force-free in the corona and
  upper chromosphere, but not in the high-beta photosphere. We developed
  a program package which contains a preprocessing program and a nonlinear
  force-free coronal magnetic extrapolation code. Both programs are based
  on optimization principles. The preprocessing routine uses the measured
  photospheric vector magnetogram as input and approximates the magnetic
  field vector in the force-free upper chromosphere. These data are used
  as boundary condition for a nonlinear force-free extrapolation of the
  coronal magnetic field. We applied our method to study the temporal
  evolution of a flaring active region as a sequence of nonlinear
  force-free equilibria. We found that magnetic energy was build up
  before the occurance of a flare and released after it. Furthermore,
  the 3D-magnetic field model allows us to trace the temporal evolution
  of the energy flows in the flaring region.

---------------------------------------------------------
Title: Nonlinear Force-Free Field Extrapolation of NOAA AR 0696
Authors: Thalmann, J. K.; Wiegelmann, T.
2007AGUFMSH13A1095T    Altcode:
  We investigate the 3D coronal magnetic field structure of NOAA AR 0696
  in the period of November 09-11, 2004, before and after an X2.5 flare
  (occurring around 02:13 UT on November 10, 2004). The coronal magnetic
  field dominates the structure of the solar corona and consequently plays
  a key role for the understanding of the initiation of flares. The most
  accurate presently available method to derive the coronal magnetic
  field is currently the nonlinear force-free field extrapolation
  from measurements of the photospheric magnetic field vector. These
  vector-magnetograms were processed from stokes I, Q, U, and V
  measurements of the Big Bear Solar Observatory and extrapolated into
  the corona with the nonlinear force-free optimization code developed by
  Wiegelmann (2004). We analyze the corresponding time series of coronal
  equilibria regarding topology changes of the 3D coronal magnetic field
  during the flare. Furthermore, quantities such as the temporal evolution
  of the magnetic energy and helicity are computed.

---------------------------------------------------------
Title: Can we Improve the Preprocessing of Photospheric
    Vectormagnetograms by the Inclusion of Chromospheric Observations?
Authors: Wiegelmann, T.; Thalmann, J. K.; Schrijver, C. J.; De Rosa,
   M. L.; Metcalf, T. R.
2007AGUFMSH51C..02W    Altcode:
  The solar magnetic field is key to understanding the physical
  processes in the solar atmosphere. Unfortunately, we can measure
  the magnetic field vector routinely with high accuracy only in the
  photosphere with, e.g., Hinode/SOT and in future with SDO/HMI. These
  measurements are extrapolated into the corona under the assumption
  that the field is force-free. That condition is not fulfilled in the
  photosphere, but is in the chromosphere and corona. In order to make
  the observed boundary data consistent with the force-free assumption,
  we therefore have to apply some transformations before nonlinear
  force-free extrapolation codes can be legitimately applied. We develop
  a minimization procedure that uses the measured photospheric field
  vectors as input to approximate a more chromospheric-like field. The
  procedure includes force-free consistency integrals, spatial smoothing,
  and - newly included in the version presented here - an improved match
  to the field direction as inferred from fibrils as can be observed in,
  e.g., chromospheric H-alpha images. We test the procedure using a model
  active-region field that included buoyancy forces at the photospheric
  level. We apply the combined preprocessing and nonlinear force-free
  extrapolation method to compute the coronal magnetic field in an active
  region measured with the Hinode/SOT instrument.

---------------------------------------------------------
Title: Large amplitude oscillatory motion along a solar filament
Authors: Vršnak, B.; Veronig, A. M.; Thalmann, J. K.; Žic, T.
2007A&A...471..295V    Altcode: 2007arXiv0707.1752V
  Context: Large amplitude oscillations of solar filaments is a phenomenon
  that has been known for more than half a century. Recently, a new mode
  of oscillations, characterized by periodical plasma motions along
  the filament axis, was discovered. <BR />Aims: We analyze such an
  event, recorded on 23 January 2002 in Big Bear Solar Observatory Hα
  filtergrams, to infer the triggering mechanism and the nature of the
  restoring force. <BR />Methods: Motion along the filament axis of a
  distinct buldge-like feature was traced, to quantify the kinematics of
  the oscillatory motion. The data were fitted by a damped sine function
  to estimate the basic parameters of the oscillations. To identify the
  triggering mechanism, morphological changes in the vicinity of the
  filament were analyzed. <BR />Results: The observed oscillations of the
  plasma along the filament were characterized by an initial displacement
  of 24 Mm, an initial velocity amplitude of 51 km s<SUP>-1</SUP>,
  a period of 50 min, and a damping time of 115 min. We interpret
  the trigger in terms of poloidal magnetic flux injection by magnetic
  reconnection at one of the filament legs. The restoring force is caused
  by the magnetic pressure gradient along the filament axis. The period of
  oscillations, derived from the linearized equation of motion (harmonic
  oscillator) can be expressed as P=π√{2}L/v_Aϕ≈4.4L/v_Aϕ, where
  v_Aϕ =B<SUB>ϕ0</SUB>/√μ_0ρ represents the Alfvén speed based
  on the equilibrium poloidal field B<SUB>ϕ0</SUB>. <BR />Conclusions:
  Combination of our measurements with some previous observations of
  the same kind of oscillations shows good agreement with the proposed
  interpretation. <P />Movie to Fig. 1 is only available in electronic
  form at http://www.aanda.org

---------------------------------------------------------
Title: Analysis of the Flare Wave Associated with the 3B/X3.8 Flare
    of January 17, 2005
Authors: Thalmann, J. K.; Veronig, A. M.; Temmer, M.; Vršnak, B.;
   Hanslmeier, A.
2007CEAB...31..187T    Altcode:
  The flare wave associated with the 3B/X3.8 flare and coronal mass
  ejection (CME) of January 17, 2005 are studied using imaging data
  in the Hα and EUV spectral channels. Due to the high-cadence Hα
  observations from Kanzelhöhe Solar Observatory (KSO), a distinct
  Moreton wave can be identified in ∼40 Hα frames over a period
  of ∼7 minutes. The associated coronal EIT wave is identifiable in
  only one EUV frame and appears close to the simultaneously observed
  Moreton wave front, indicating that they are closely associated
  phenomena. Beside the morphology of the wave across the solar disc
  (covering an angular extend of ∼130°), the evolution in different
  directions is studied to analyse the influence of a coronal hole (CH)
  on the wave propagation. The Moreton wave shows a decelerating character
  which can be interpreted in terms of a freely propagating fast-mode MHD
  shock. The parts of the wave front moving towards the CH show a lower
  initial and mean speed, and a greater amount of deceleration than the
  segments moving into the undisturbed direction. This is interpreted
  as the tendency of high Alfvén velocity regions to influence the
  propagation of wave packets.

---------------------------------------------------------
Title: Interaction of a Moreton/EIT Wave and a Coronal Hole
Authors: Veronig, Astrid M.; Temmer, Manuela; Vršnak, Bojan; Thalmann,
   Julia K.
2006ApJ...647.1466V    Altcode: 2006astro.ph..4613V
  We report high-cadence Hα observations of a distinct Moreton wave
  observed at Kanzelhöhe Solar Observatory associated with the 3B/X3.8
  flare and coronal mass ejection (CME) event of 2005 January 17. The
  Moreton wave can be identified in about 40 Hα frames over a period of
  7 minutes. The EIT wave is observed in only one frame, but the derived
  propagation distance is close to that of the simultaneously measured
  Moreton wave fronts, indicating that they are closely associated
  phenomena. The large angular extent of the Moreton wave allows us to
  study the wave kinematics in different propagation directions with
  respect to the location of a polar coronal hole (CH). In particular, we
  find that the wave segment whose propagation direction is perpendicular
  to the CH boundary (“frontal encounter”) is stopped by the CH, which
  is in accordance with observations reported from EIT waves. However,
  we also find that at a tongue-shaped edge of the coronal hole, where
  the front orientation is perpendicular to the CH boundary (the wave
  “slides along” the boundary), the wave signatures can be found up
  to 100 Mm inside the CH. These findings are briefly discussed in the
  frame of recent modeling results.

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Title: Wave Phenomena Associated with the X3.8 Flare/cme of
    17-JAN-2005
Authors: Temmer, M.; Veronig, A.; Vršnak, B.; Thalmann, J.;
   Hanslmeier, A.
2005ESASP.600E.144T    Altcode: 2005ESPM...11..144T; 2005dysu.confE.144T
  No abstract at ADS