The structure and development of the spotless flare of 16 March 1981

Morphological peculiarities of 1N (N09W22) two-ribbon spotless flare of March 16, 1981 are considered along with its connection with a magnetic field. As opposed to major flares of the active region, this spotless flare was characterized by large-scale development process, considerable remoteness from the magnetic field neutral line, and by the absence of spread of ribbons. The flare evolution had four periods. At the beginning of each period, a sharp increase in flare brightness along with simultaneous decrease in the area of flare ribbons was observed. During all periods ribbon areas were changing synchronously but once, near the maximum the situation abruptly changed: The area of one of ribbons increased and of the other one decreased. In our opinion, this could be considered as physical processes really taken place in the flare source, and it was the predictor of the beginning of the flare decay. The magnetic field and its topology, as well as cellular structure of the chromosphere determined the flare evolution. Almost all flare nodes and bright parts were localized in the immediate vicinity of magnetic hills with the intensity from 80 to 250 G. Main structural elements of the flare were distinguished. A phenomenon called as tunnel effect was found out. It means that the flare progresses inside the tunnel formed by the system of dark arch structures (filaments). The study findings are evidence in favour of an assumption that spotless flares most likely present a specific class of flare phenomena, and the study of them is of great interest for understanding a flare origin on the Sun.