Harmonic relation of type IIIb-III solar radio bursts in 6.25, 12.5, and 25.0 MHz octaves

This study of type IIIb-III evenmts strongly supports their interpretation as fundamental-harmonic burst pairs. Type IIIb chains and the related type III bursts drift from 12.5 to 6.25 MHz and from 25.0 to 12.5 MHz, respectively, during similar time intervals of 11.1 and 11.0 s, on the average. This harmonic similarity is emphasized by the fact that the drift times of type IIIb chains across the upper octave are significantly less than those of type III bursts in the lower octave, the values being around 6 and 19 s.     

Collisionless deceleration of fast electron streams in the solar coronal plasma

The collisionless deceleration of electron streams responsible for type IIIb bursts has been investigated. For this the difference between the mean velocities of electron streams at plasma levels corresponding to 25 and 12.5 MHz, on one hand, at 12.5 and 6.25 MHz, on the other hand, is estimated. The mean velocity of electron streams between these levels is determined by the time delay in the moments of arrival of radio bursts from these levels. The distance between plasma levels is determined under the assumption that the (statistical) mean velocity of sources of the diffusive type III bursts is constant and equal toc/3 at all considered levels of the solar corona.It is shown that under this assumption the electron streams with the initial velocities of the order of 0.4–0.8c undergo a sufficient deceleration which is characterized by a decrease in their mean velocity by 15–17% between plasma levels at 25 to 6.25 MHz. The stream deceleration becomes more essential with the growth of the initial velocity of the stream. On the other hand, the deceleration disappears when the initial velocity of the stream is of the order of 0.35c. This critical velocityV _s ^* - 0.35c is assumed to define a boundary between two different expansion regimes of fast electrons moving in the solar corona. In the first regime (V _s >V _s ^* ) the induced scattering of plasma waves produces energy losses of the streams. A decrease in the velocities of streams up to the value of the order of 0.35c is due to these losses. In the second regime (V _s -V _s ^* ) a quasilinear expansion of streams is realized. In this case the energy losses of the streams are almost absent.     

Coordinated Observations of the Red Dwarf Flare Star EV Lac in 1993

Results of cooperative observations of the flare star EV Lac in September 1993 are presented. One of the about 30 optical flares detected was powerful enough to permit a quantitative analysis of its intrinsic radiation with the colour-colour technique. Sinusoidal brightness variations due to spottedness of the stellar surface was found to have an amplitude V = 0.^m0.24. Behaviour of the K band stellar brightness during strong and weak U band flares are considered. The upper limits of very fast optical brightness variations were estimated during both a moderate flare and quiet state of the star. No decametric bursts were observed during the campaign that could be certainly attributed to flare activity.