Coordinated observations of solar activity phenomena. I. Multispectral study of an elementary flare.

Details Coordinated observations of solar activity phenomena. I. Multispectral study of an elementary flare. Coordinated observations of solar activity phenomena. I. Multispectral study of an elementary flare.

Jul 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...299..611c&link_type=abstract

Astronomy and Astrophysics, v.299, p.611

Astronomy and Astrophysics

Astrophysics

8

Sun: Activity, Flares, Corona, Chromosphere, Magnetic Fields

Scientific paper

In this paper we present a multispectral analysis of a small flare which may be considered as a sequence of elementary bursts rather than a collective flare phenomenon. A series of 5 short-lived (duration =~1-7s) emission spikes are in fact observed in hard X-ray (HXR). The flare was also observed in soft X-ray (SXR), with 3s temporal resolution, and in the optical range with very high spatial (=~0.5") and temporal (2.7s) resolutions. Sequences of filter images, in Halpha_ +1.5A, He-D_3_ and Na-D_2_ wavelengths were acquired. 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 15s before the flare. This indicates that a density and/or a temperature increase in the flaring loop starts at chromospheric levels, and only after this 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 detectable associated signatures. Apparent contradictions between Halpha_ + 1.5 A and He-D_3_ emissions can be explained assuming atmospheric inhomogeneities, already present within the flaring loop. The flare occurs within an area of positive magnetic polarity, which shows a moderate but constant weighted magnetic shear. No inclusions of the opposite polarity are found in the flaring region, and no changes in the distribution of any of the magnetic field parameters are detected before, 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-associated modifications of the flaring loop atmosphere do not penetrate below the temperature minimum region.

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