Volume 488, Number 3, September IV 2008
|Page(s)||L71 - L74|
|Published online||08 August 2008|
Letter to the Editor
First nonlinear force-free field extrapolations of SOLIS/VSM data
Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany e-mail: [thalmann;wiegelmann]@mps.mpg.de
2 National Solar Observatory, 85719 Tucson, Arizona e-mail: email@example.com
Accepted: 30 July 2008
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 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.
Key words: Sun: magnetic fields / Sun: flares / Sun: corona
© ESO, 2008
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