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On the factors determining the eruptive character of solar flares  

Julia K. Thalmann   Submitted: 2017-12-19 01:24

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 dFC) 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, dPC. Flares originating from underneath the AR dipole (dFC/dPC<0.5) tend to be eruptive if launched from compact ARs (dPC≤60 Mm) and confined if launched from extended ARs. Flares ejected from the periphery of ARs (dFC/dPC>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, hcrit=h(n=1.5), discriminates best between confined (hcrit≳40 Mm) and eruptive flares (hcrit≲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.

Authors: Christian Baumgartner, Julia K. Thalmann, Astrid M. Veronig
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2017-12-20 10:33
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The Confined X-class Flares of Solar Active Region 2192  

Julia K. Thalmann   Submitted: 2015-02-19 01:04

The unusually large NOAA active region 2192, observed in October 2014, 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, also lateral, confinement for a series of large two-ribbon flares originating from the core of the active region. 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 active region 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.

Authors: J. K. Thalmann, Y. Su, M. Temmer, A. M. Veronig
Projects: None

Publication Status: Accepted for publication in The Astrophysical Journal Letters
Last Modified: 2015-02-19 13:05
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Force-free field modeling of twist and braiding-induced magnetic energy in an active-region corona  

Julia K. Thalmann   Submitted: 2013-11-15 04:45

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 corona, has been substantiated by high-resolution observations only recently. 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. 2013 (hereafter CG13). We present nonlinear force-free reconstructions of the associated coronal magnetic field based on vector SDO/HMI magnetograms. We deliver estimates of the free magnetic energy associated to 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 active-region corona being heated by field line braiding. We were able to assess the coronal free energy appropriately by using vector field measurements and attribute the lower energy estimate of CG13 to the underestimated (by a factor of 10) azimuthal field strength. We also quantify the increase of the overall twist of a flare-related flux rope which had been claimed by CG13. From our models we find that the overall twist of the flux rope increased by about half a turn within twelve minutes. Unlike another method, to which we compare our results to, we evaluate the winding of the flux rope's constituent field lines around each other purely based on their modeled coronal 3D field line geometry - to our knowledge for the first time.

Authors: Thalmann, J. K., Tiwari, S. K., Wiegelmann, T.
Projects: Hinode/SOT,SDO-HMI

Publication Status: Accepted for publication in ApJ
Last Modified: 2013-11-17 13:30
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Abstracts by Author
On the factors determining the eruptive character of solar flares
The Confined X-class Flares of Solar Active Region 2192
Force-free field modeling of twist and braiding-induced magnetic energy in an active-region corona

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