In this paper, we investigate the evolution of the magnetic configuration in a famous active region (AR7321) from October 26 to October 27, just on the eve of the occurrence of a 1N/M1.1 flare. The vector photospheric magnetograms are exploited to recover the comprehensive magnetic connectivity patterns and the vertical current system, which are confirmed to bear close spatial correlation with the configuration of the flare observed in different wavelengths displaying the phenomenon from the chromospheric to the coronal level. With these results, we explore the magnetic configuration for the trigger and free energy release in the flare. Our main conclusions are summarized as follows:
The active region has continuous flux emergence which greatly enhanced nonpotentiality in this region; specifically, a newly emerging bipole on Oct. 27 along the inversion line brings in strong shear. This low-lying, strongly sheared loop (L3) pushes a pre-existent overlying loop (L1) and strong free energy is accumulated along this zone of magnetic complexity.
We employ the filtering technique on the observed raw photospheric magnetograms to obtain the vertical current distribution in this active region. It is found that the major current pairs were associated with the emerging flux. Some of the flare kernels show close spatial relationship to the strong current concentration sites. One loop (L1) can be approximately identified as carrying the field-aligned current with both feet rooted in the vertical current concentration sites; while the low-lying loop L3 straddles almost parallel to the magnetic inversion line and current reversing line. One end (A) of L3 was anchored on the edge of the vertical current maximum, and the other end (B) plunging into the reversing zone of longitudinal field and vertical current.
The magnetic connection patterns, deduced from the photospheric vector field, are identified in the space by the flare configuration in multi-waveband observations, and can thus be supposed as magnetic loops projected on the photosphere. We find clear evidence in the observations that the flare is initiated by the interaction between the two loops L3 and L1, on the top (D) of L3, and further energy release is triggered along the L3 loop, especially at the foot regions of A and B. All three bright loci seen in SXR images from the onset to the impulsive phase of the flare are well coaligned with the concentrations of the free magnetic energy, ie., the excess magnetic energy of the observed field to that of a potential field and the ways to release the free energy in these three regions may differ according to their local physics.
Subsequently, the flaring of another region (C) didn't go along the free energy concentration zone but appears a secondary effect from the initial flaring site (loop L3) through the transfer of energy flux. The above scenario of the flare is supported by the previous studies of the same event.
© European Southern Observatory (ESO) 1998
Online publication: July 7, 1998