Structure and development of the spotless flare on March 16, 1981

The morphological peculiarities of the 1N (N09W22) two-ribbon spotless flare on March 16, 1981, as well as its connection with a magnetic field, have been considered. In contrast to major flares of the active region, this spotless flare is characterized by a large-scale development process, a large distance from the magnetic neutral line, and the absence of the spread of the ribbons. The development of the flare had four periods. At the beginning of each period, a sharp increase in the brightness of the flare was observed along with a simultaneous decrease in the area of the flare ribbons. The areas of the ribbons varied synchronously during all of the periods. However, the situation changed abruptly near the maximum: the area of one of the ribbons increased, whereas the area of the other ribbon decreased. In our opinion, this behavior is a manifestation of real physical processes in the flare source and was a precursor of the beginning of the flare decay. The magnetic field and its topology, as well as the cellular structure of the chromosphere, were primarily responsible for the evolution of the flare. Almost all of the mottles and bright parts of the flare were localized in the immediate vicinity of magnetic hills with field intensities from 80 to 250 G. The main structural elements of the flare have been identified. A phenomenon called the tunnel effect has been revealed: the flare progresses inside a tunnel formed by the system of dark arch structures (filaments). The results indicate that spotless flares apparently constitute a specific class of flare phenomena and the study of them is of great interest for understanding of the origin of solar flares.