The spotless flare of 16 March 1981 – I. Pre-Flare Activations of the Chromospheric Fine Structure

This paper is based on observations in the Hα line with the aim to carry out a detailed study of the spotless flare of importance 1N that was observed at the Baikal Astrophysical Observatory on 16 March 1981. The study focuses on the evolution of the region of interest from the time of its appearance from behind the limb, and on the pre-flare activation of chromospheric features four hours before the flare. The disturbances that preceded the flare spanned an area of about 120 square degrees. The bulk of activations occurred along and near the path of the polarity inversion line (PIL) of the longitudinal component of the magnetic field. The flare was preceded by an eruptive filament, a disturbance of the fine structure of supergranulation cells, and by the formation of dark vortex structures in regions where flare ribbons form; dark mottles in these regions signaled the operation of an oscillatory process with a period of about 3–4 min, and the region where one of the flare ribbons formed showed a `tunnel' of a system of small-scale dark loops. A close association of the chromospheric activations and flare mottles, with the boundaries of the chromospheric and magnetic networks, is established.