The Nature and Velocity of the Sources of Type II Solar Radio Bursts

Abstract

Velocities of the sources of type II bursts are derived from rates of frequency drift using standard density models, both statistically for 21 bursts, and individually for 5 bursts extending over wide frequency ranges. The derived velocities exceed the speed of sound in the magnetic-field.free corona: on the average the velocity decreases with increasing height to a minimum of ,....., 750 kmjs at a little below I Ro' and j,hereafter slowly increases with height. The nature of the type II source is discussed in relation to these velocities, and also in relation to detailed measurements of harmonic ratios and band splitting for the five individual bursts. It is suggested that the type II source is either a strong parallel shock (direction of propagation of shock parallel to magnetic field) or a perpendicular shock. Magnetic field strengths of 2-20 G at 0·5 R 0 above the photosphere, decreasing to 1-10 G at 2 Ro, are derived. Finally, it is shown that theories by which fundamental emission arises in front of the shock, whilst harmonic emission originates in the interior of the shock, are untenable.