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

TL;DR

This study models the coronal magnetic field in active regions using nonlinear force-free extrapolations from SOLIS/VSM data, revealing energy dynamics associated with small and large solar flares.

Contribution

It is the first to use SOLIS/VSM vector magnetograph data for nonlinear force-free field extrapolations to analyze active region magnetic energy evolution during flares.

Findings

Small flares release a minor fraction of magnetic energy.

Active regions with larger free magnetic energy produce more intense flares.

Coronal magnetic energy decreases post-flare, approaching potential field configuration.

Abstract

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 $3 \times 10^{25}$ 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.