NARROW-BAND SPLIT FREQUENCY DECIMETER SOLAR-BURST

A wide-band digital decimetric spectroscope is in the process of devel opment at INPE (Brazil), in conjunction with a 9-m diameter polar moun ted antenna. Initially, this spectroscope was operating over a narrow- band 1.6 +/- 0.05 MHz in an analogue mode. Here we report a slow drift ing similar to 5.0 MHZ s(-1), narrow-band 16:30-15:80 MHz, split frequ ency solar burst lasting for about 15 s, observed on 15 June, 1991. Th e separation between the split components is similar or equal to 30 MH z, and the upper split frequency component is more intense than the lo wer split frequency component. These observed characteristics favour t he hypothesis of conversion of plasma waves by combinational scatterin g on upgoing ion-sound waves in a magnetic loop. Existing strong elect ron density gradients across the magnetic field in the source region w ill reduce the free-free absorption of radiation at the fundamental fr equency. In order to explain the observed temporal characteristics of the upper and lower split frequency components, the radiation has to p ropagate at some angle to the electron density gradient in the source region Estimated physical parameters of the source and exciters are as follows: (i) maximum source size (height) similar or equal to 2 x 10( 7) cm; (ii) velocity and length of ion-sound pulse similar or equal to 3.1 x 10(6) cm s(-1) and 5 x 10(7) cm, respectively, and (iii) veloci ty of the exciter of plasma waves (electron or proton beam) similar or equal to 5 x 10(8) cm s(-1).