On the association of large-scale X-ray brightenings and solar flares

MANDRINI, C.H.; MACHADO, M.E.

Iguazú

Congreso; International Astronomical Union Colloquium 133; 1991

International Astronomical Union

We have studied SMM Hard X-ray Imaging Spectrometer (HXIS) observations of large scale brightenings (LSBs) associated with confined flares that developed in complex active regions. These structures have sizes (> 1010 cm) similar to the so-called ?giant arches?, that were discovered in HXIS images hours after the onset of two-ribbon flares. We have analyzed seven events, four observed after flares that took place in active region (AR) 2372 (NOAA number) during April 7 and 8, 1980 and three from AR 2779 during November 11 and 12, 1980. Both ARs have the same magnetic configuration, two main spots with a reverse polarity region in between; in this type of configuration four topologically distinct sets of field lines are present. The three-dimensional field topology of AR 2372 was derived, using magnetograph data from April 6, in one of our recent works. This computations show the presence of a separator above the intermediate bipolar region and lines of force joining the main leading and trailing spots over it. These lines would correspond to the large loops observed in X-rays. Thus, these large scale features appear as pre-existing magnetic structures, whose brightening can be due to heating by internal energy dissipation or by energy leakage at the separator region. In some of our examples we were able to study simultaneously the temporal evolution of the flare and large loops parameters, we found that the LSBs temperature reaches its maximum during, or slightly after, the impulsive phase in hard X-rays and the emission measure some tens of minutes later; in all cases the LSB maximum temperature is higher than the flare's. We demonstrated that this can be due to the injection of suprathermal particles that are accelerated at the separator region. We have observed other characteristics in these large features, like a temperature stratification in the April LSBs and a growth with time in the November events. Our findings concerning confined flares and LSBs have led us to propose an alternative scenario for the origin of giant arches. In this view giant arches appear as the result of the sequential energization, due to reconnection, of a conglomerate of pre-existing large scale loops.