Anisotropic beam model for the spectral observations of radio burst fine structures on 1998 April 15

A fine structure consisting of three almost equidistant frequency bands was observed in the high frequency part of a solar burst on 1998 April 15 by t he spectrometer of Beijing Astronomical Observatory in the range 2.6-3.8 GH z. A model for this event based on beam-anisotropic instability in the sola r corona is presented, Longitudinal plasma waves are excited at cyclotron r esonance and then transformed into radio emission at their second harmonic. The model is in accordance with the observations if we suppose a magnetic field strength in the region of emission generation of about 200 G.