COORDINATED OBSERVATIONS OF SOLAR-ACTIVITY PHENOMENA .1. MULTISPECTRAL STUDY OF AN ELEMENTARY FLARE

In this paper we present a multispectral analysis of a small flare whi ch may be considered as a sequence of elementary bursts rather than a collective flare phenomenon. A series of 5 short-lived (duration appro ximate to 1-7 s) emission spikes are in fact observed in hard X-ray (S XR). The flare was also observed in soft X-ray (SXR), with 3 s tempora l resolution, and in the optical range with very high spatial (approxi mate to 0.5'') and temporal (2.7 s) resolutions. Sequences of filter i mages, in H-alpha +1.5 Angstrom, He-D-3 and Na-D-2 wavelengths were ac quired. The flare impulsive phase, indicated by the occurrence of the HXR spikes, is preceded by an enhancement in the chromospheric and in the coronal SXR emissions starting, respectively, 40 and 15 s before t he flare. This indicates that a density and/or a temperature increase in the flaring loop starts at chromospheric levels, and only after thi s phase are the 5 separate electron beams sequentially accelerated and stopped in an atmosphere that is already modified. Four of the 5 HXR spikes are temporally associated with chromospheric emission features over small areas of 3'' size, while the fifth spike does not have dete ctable associated signatures. Apparent contradictions between H-alpha + 1.5 Angstrom and He-D-3 emissions can be explained assuming atmosphe ric inhomogeneities, already present within the flaring loop. The flar e occurs within an area of positive magnetic polarity, which shows a m oderate but constant weighted magnetic shear. No inclusions of the opp osite polarity are found in the flaring region, and no changes in the distribution of any of the magnetic field parameters are detected befo re, during and after the flare occurrence. No detectable modifications are measured in the continuum and in all the wavelength points of the Na-D-2 line profile apart from the line core; this means that flare-a ssociated modifications of the flaring loop atmosphere do not penetrat e below the temperature minimum region.