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First nonlinear force-free field extrapolations of SOLIS/VSM data  

Julia Katharina Thalmann   Submitted: 2008-08-29 09:57

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. 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 minutes and six snapshots during a quiet period. The total magnetic energy in the active region was approximately 3 x 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. 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.

Authors: J. K. Thalmann, T. Wiegelmann, and N.-E. Raouafi
Projects: None

Publication Status: Astronomy and Astrophysics Letters (in press)
Last Modified: 2008-08-29 11:52
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Subject will be restored when possible  

Julia Katharina Thalmann   Submitted: 2008-08-29 09:55

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. 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 minutes and six snapshots during a quiet period. The total magnetic energy in the active region was approximately 3 x 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. 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.

Authors: J. K. Thalmann, T. Wiegelmann, and N.-E. Raouafi
Projects: None

Publication Status: Astronomy and Astrophyscs Letters (in press)
Last Modified: 2008-08-29 09:55
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Subject will be restored when possible  

Julia Katharina Thalmann   Submitted: 2008-03-13 06:04

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. We study the temporal evolution of flaring active region NOAA 10540 and are in particular interested in the free magnetic energy available to power the flares associated with it. We extrapolate 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 are 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 is studied. The coronal magnetic field energy in active region 10540 increases slowly during the three days before a M6.1 flare and drops significantly after it. We estimated the energy which has been 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 a M6.1 flare.

Authors: J. K. Thalmann and T. Wiegelmann
Projects: None

Publication Status: A&A (in press)
Last Modified: 2008-09-23 21:14
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First nonlinear force-free field extrapolations of SOLIS/VSM data
Subject will be restored when possible
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