A C-level flare observed in an arch filament system: reconnection between pre-existing and emerging field lines?

Abstract

Context. Observations show that solar flares are often caused by the emergence of new magnetic flux from subphotospheric layers and by the interaction of the rising field lines with the ambient magnetic field. In this framework, recent theoretical models of magnetic flux emergence have investigated the effectiveness of magnetic reconnection as a function of the old and new flux system relative orientations. Aims. We aim to compare phenomena that occurred in an active region, before and during a small flare, with the effects of magnetic reconnection between nearly parallel magnetic field lines, foreseen by these models. Methods. We analyzed high resolution photospheric and chromospheric data acquired during a coordinated observational campaign performed with the THEMIS telescope in IPM mode, as well as MDI magnetograms and TRACE 1600 and 171 Å images, to investigate the dynamics and the magnetic configuration of the active region hosting the flare.Results. An emerging arch filament system (AFS) was observed in the area between the two main sunspots: it showed typical upward motion at the arch tops and plasma downward motion at the footpoints. A C-level flare, characterized by a factor of 3 peak enhancement in the GOES X-ray emission with respect to the pre-event background, occurred in this zone, where the configuration of the emerging magnetic field lines showed a small (∼12• ) relative inclination with respect to the old flux system. Conclusions. In an active region (age ≥6 days) a new magnetic flux bundle emerged between the two main polarity spots. It gave rise to the formation of pores in the photosphere and to an AFS in the chromosphere. The interaction between the new and the pre-existing field lines, characterized by a small relative inclination, might have caused a weak reconnection process and given rise to the C-level flare. This result is in broad agreement with numerical simulations predicting very limited reconnection when the two flux systems have an almost parallel orientation.

Quiescent Reconnection Rate Between Emerging Active Regions and Preexisting Field, with Associated Heating: NOAA AR 11112

“…Delay between emergence and reconnection has been reported in previous observations (Longcope, 2005;Zuccarello et al, 2008), and may be a general feature of flux emergence.…”

“…They find evidence of reconnection between 6 and 24 hours after emergence. In yet another study, Zuccarello et al (2008) examined reconnection between an arch filament system that emerged along the PIL in a well established, though not yet diffuse, active region. They determine a time delay of about 10 hours between emergence and the reconnection event, a small C-class flare.…”

Flux emergence within mature solar active regions

“…Later, as the new flux emerges and expands, outflows are found at the other polarity of the older region. As in Zuccarello et al (2008), they find C-class flares.…”

“…This behaviour is not found when the fields are almost parallel. Zuccarello et al (2008) use this result to argue that limited reconnection is responsible for the C-class flare that they observe, as mentioned above. Other theoretical studies that include overlying fields have tended to focus on the anti-parallel case (e.g.…”

“…New magnetic flux can, however, emerge within mature active regions and there have been recent observational studies of this. Zuccarello et al (2008) investigate, via an observational campaign using several different instruments, the emergence of new flux within a mature active region that leads to a flare. The new flux emerges between the two main polarities of the mature region.…”