O. Gerlei

A study of surges and flares within an active region

Active region 2684 was observed by the Solar Maximum Mission and ground-based observatories simultaneously for over 12 hours on September 23, 1980. During these observations, recurrent surges were detected above an area with complex parasitic magnetic polarity located at the periphery of the active region. The time evolution of the Hα surges, Civ brightenings and X-ray spikes leads to the conclusion that the energy source is in the corona, from magnetic reconnection. The energy is transported by energetic charged particles along the loops, thereby heating the chromosphere as the particles lose their energy. The divergent motion of the spots corresponding to small dipoles at the base of the surge indicates that there is important magnetic reorganisation. According to the magnetic field-line configuration (large loop or open structures), X-rays can (or cannot) be associated with surges.

Common evolution of adjacent sunspot groups

The evolution of two adjacent bipolar sunspot groups is studied using Debrecen full-disc, white-light photoheliograms and Hα filtergrams as well as Meudon magnetograms. The proper motions of the principal preceding spots of both groups show quite similar patterns; the spots move along almost parallel tracks and change the direction of their motion on the same day at almost the same heliographic longitude. Also, three simultaneous emergences of magnetic flux were observed in both groups. These observations support the idea that these adjacent sunspot groups were magnetically linked below the photosphere. Matching the extrapolated magnetic field lines with the chromospheric fibril structure appears to be different in the two groups since they indicate quite different model solutions for each group, i.e., a near-potential magnetic field configuration in the older group (1) and a twisted force-free field configuration in the younger group (2). The latter configuration could be created by a considerable twist of the main bunch of flux tubes in Group 2, which is reflected in the relative sunspot motions. It is also showed how this twist contributed to the formation of a filament between the two groups.

Sunspot motions and magnetic shears as precusors of flares

Abstract Using full-disc white light photoheliograms, we have studied umbrae motion and variations in sunspot areas in a large activity complex over 4 solar rotations. On the basis of the observational data we illustrate with typical examples to what extent rapid spot motions are associated with flare occurrences.

On the experimental determination of the north direction of a heliogram

A new method to determine the experimental north direction of a heliogram is suggested and a method of reduction for the measurements is given. The accuracy achievable by this method exceeds that generally used.

Effect of electron beams during solar flares

Electron bombardment of the solar atmosphere has two effects: one is to enhance hydrogen recombination emission, the other is to increase the opacity via an increase of H− population. The first effect is the most important in the upper part of the atmosphere and the second in the lower part. We predict that, when enhanced absorption dominates in the part of the atmosphere where radiation originates, there will be a decrease in the white-light emission, leading to a ‘negative flare’, or what we call a ‘Black-Light Flare’. This phenomenon occurs only for a short duration, not more than ∼ 20 s. ‘Black-Light Flares’ have already been observed in the case of flare stars and we suggest here that they could also be present on the Sun, just prior to a White-Light Flare.

On the quasi-homologous limb flares observed on 3 August 1981

Abstract Five limb flares and a large surge, triggered obviously by the second flare, have been investigated on the basis of Hα on- and off-band heliograms and X-ray data obtained during a 6-hour period on 3 August 1981. In addition, in the vicinity of the flare location the sunspots and filaments were followed by observations over several of the former days too. Emphasis was layed on the ascending motion in the course of flare build-up and on the time evolution of the flare energy release using GOES observations in the case of the first and second flares. It was found that their ascending Hα flare kernels, which reached to a height of 20–25×103 km, have been the main center both the source and the starting points of flare loops and perhaps also the main site of the soft X-ray emissions.