Narrow-band split frequency decimeter solar burst

A wide-band digital decimetric spectroscope is in the process of development at INPE (Brazil), in conjunction with a 9-m diameter polar mounted antenna. Initially, this spectroscope was operating over a narrow-band 1.6 ± 0.05 MHz in an analogue mode. Here we report a slow drifting 5.0 MHz s^-1, narrow-band 1630-1580 MHz, split frequency solar burst lasting for about 15 s, observed on 15 June, 1991. The separation between the split components is 30 MHz, and the upper split frequency component is more intense than the lower split frequency component. These observed characteristics favour the hypothesis of conversion of plasma waves by combinational scattering on upgoing ion-sound waves in a magnetic loop. Existing strong electron density gradients across the magnetic field in the source region will reduce the free-free absorption of radiation at the fundamental frequency. In order to explain the observed temporal characteristics of the upper and lower split frequency components, the radiation has to propagate 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) 2 × 10⁷ cm; (ii) velocity and length of ion-sound pulse 3.1 × 10⁶ cm s^-1 and 5 × 10⁷ cm, respectively, and (iii) velocity of the exciter of plasma waves (electron or proton beam) 5 × 10⁸ cm s^-1.On leave from the Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia.