Author name code: thalmann ADS astronomy entries on 2022-09-14 author:"Thalmann, Julia K." ------------------------------------------------------------------------ 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. Bibcode: 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. Title: Magnetic helicity and energy budget around large confined and eruptive solar flares Authors: Gupta, Manu; Veronig, Astrid; Thalmann, Julia K. Bibcode: 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. Title: Probing the coronal magnetic field with physics informed neural networks Authors: Jarolim, Robert; Podladchikova, Tatiana; Veronig, Astrid; Thalmann, Julia K. Bibcode: 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. Title: The effect of spatial sampling on magnetic field modeling and helicity computation Authors: Thalmann, J. K.; Gupta, M.; Veronig, A. M. Bibcode: 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.
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.
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.
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.
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. 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 Bibcode: 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). 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 Bibcode: 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. Title: Magnetic helicity and energy budget around large confined and eruptive solar flares Authors: Gupta, M.; Thalmann, J. K.; Veronig, A. M. Bibcode: 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.
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).
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, EF/E (energy ratio), the fraction of non-potential (current-carrying) helicity, |HJ|/|HV| (helicity ratio), and the normalized current-carrying helicity, |HJ|/ϕ'2.
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, EF/E and |HJ|/|HV|, and |HJ|/ϕ'2, 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.
Conclusions: If the corona of an AR exhibits characteristic values of ⟨|HJ|/|HV|⟩ > 0.1, ⟨EF/E⟩ > 0.2, and ⟨|HJ|/ϕ'2⟩ > 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 ⟨|HJ|/|HV|⟩ ≲ 0.1, ⟨EF/E⟩ ≲ 0.1, and ⟨|HJ|/ϕ'2⟩ ≲ 0.002. 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. Bibcode: 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.
Aims: In order to understand the initiation and evolution of this particular event, we performed a comprehensive analysis using a combined observational-modeling approach.
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.
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.
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).

Movies are available at https://www.aanda.org 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 Bibcode: 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. 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 Bibcode: 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. Title: Homologous Flaring Activity over a Sunspot Light Bridge in an Emerging Active Region Authors: Louis, Rohan Eugene; Thalmann, Julia K. Bibcode: 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-1, 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. Title: Deducing the reliability of relative helicities from nonlinear force-free coronal models Authors: Thalmann, J. K.; Sun, X.; Moraitis, K.; Gupta, M. Bibcode: 2020A&A...643A.153T Altcode: 2020arXiv200905287T
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, |HJ|/|H𝒱|, a promising candidate to quantity the eruptive potential of solar ARs.
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 |HJ|/|H𝒱|, and to estimate its uncertainty.
Results: In comparison to earlier works, which identified the non-solenoidal contribution to the total magnetic energy, Ediv/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, |Emix|/EJ, s. 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 Ediv/E and |Emix|/EJ, s 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. 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. Bibcode: 2020ApJ...902L..48T Altcode: No abstract at ADS 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. Bibcode: 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}{{J}}| /| {H}{ \mathcal V }| , shows a strong ability to indicate the eruptive potential of a solar AR. However, | {H}{{J}}| /| {H}{ \mathcal V }| 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}{ \mathcal V }/{φ }2, represents no indicator for the eruptivity. 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 Bibcode: 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. Title: Observations of a Footpoint Drift of an Erupting Flux Rope Authors: Zemanová, Alena; Dudík, Jaroslav; Aulanier, Guillaume; Thalmann, Julia K.; Gömöry, Peter Bibcode: 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. Bibcode: 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 & 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 Bibcode: 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.

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., & 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 Bibcode: 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. Bibcode: 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 >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} > 0.5) are predominantly eruptive. Flares originating from underneath the AR dipole field (d_{FC} / d_{PC} < 0.5) tend to be eruptive when they are launched from a compact AR and confined when launched from an extended AR (d_{PC} > 60 Mm). In confined events, the flare-relevant field adjusts its orientation quickly to that of the underlying dipole field with height (δ φ > 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} > 40 Mm) and eruptive flares (h_{crit} < 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 Bibcode: 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 Bibcode: 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}}>0.5) are predominantly eruptive. Flares originating from underneath the AR dipole field (d_{{FC}}/d_{{PC}}<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 Bibcode: 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 Bibcode: 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. Bibcode: 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−1). 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. Bibcode: 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 FC) 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 PC. Flares originating from underneath the AR dipole (d FC/d PC < 0.5) tend to be eruptive if launched from compact ARs (d PC ≤ 60 Mm) and confined if launched from extended ARs. Flares ejected from the periphery of ARs (d FC/d PC > 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 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: 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 Bibcode: 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 × 1023 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 × 1032 erg. Using a magnetic flux estimate of 6 × 1023 Mx for the largest solar AR observed, we find that flares of GOES class ≈X500 could be produced (E bol ≈ 3 × 1033 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. Bibcode: 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. Bibcode: 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-1) than the cooler one (∼130 km s-1). 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. Bibcode: 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. Bibcode: 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” (2015, ApJL, 801, L23) Authors: Thalmann, J. K.; Su, Y.; Temmer, M.; Veronig, A. M. Bibcode: 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 Bibcode: 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. 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 >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 >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 Bibcode: 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. Bibcode: 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 Bibcode: 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” (2012, ApJ, 748, 77) Authors: Sun, Xudong; Hoeksema, J. Todd; Liu, Yang; Wiegelmann, Thomas; Hayashi, Keiji; Chen, Qingrong; Thalmann, Julia Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 2016A&A...588A...6G Altcode: 2016arXiv160202145G
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.
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.
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-1, 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 × 109 cm-3 to 3.16 × 1010 cm-3 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 × 1010 erg s-1 cm-2 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-1 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-1 for the upflowing filament material.
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.

A movie is available at http://www.aanda.org Title: Space Weather and confined CME events Authors: Thalmann, Julia; Temmer, Manuela; Veronig, Astrid; Su, Yang Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 1025 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2013A&A...559A...1S Altcode: 2013arXiv1309.6514S
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.
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.
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-1. 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.
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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 × 1032 erg, about 50% of which is stored below 6 Mm. It decreases by ~0.3 × 1032 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
Results: 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.
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. Bibcode: 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. Bibcode: 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.
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.
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.
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 ∝1025 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. Bibcode: 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. Bibcode: 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 ~1032 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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.
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.
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.
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-1, 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 =Bϕ0/√μ_0ρ represents the Alfvén speed based on the equilibrium poloidal field Bϕ0.
Conclusions: Combination of our measurements with some previous observations of the same kind of oscillations shows good agreement with the proposed interpretation.

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. Bibcode: 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. Bibcode: 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. 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. Bibcode: 2005ESASP.600E.144T Altcode: 2005ESPM...11..144T; 2005dysu.confE.144T No abstract at ADS