Type III radio bursts in the outer corona

Abstract

Type III solar radio bursts observed from 3.0 to 0.45 MHz with the ATS-II satellite over the period April–October 1967 have been analyzed to derive two alternative models of active region streamers in the outer solar corona. Assuming that the bursts correspond to radiation near the electron plasma frequency, ‘pressure equilibrium’ arguments lead to streamer Model I in which the streamer electron temperature derived from collision damping time falls off much more rapidly than in the ‘average’ corona and the electron density is as much as 25 times the average coronal density at heights of 10 to 50 solar radii (R⊙). In Model II the streamer electron temperature is assumed to equal the average coronal temperature, giving a density enhancement which decreases from a factor of 10 close to the Sun to less than a factor of two at large distances (> 1/4 AU). When the burst frequency drift is interpreted as resulting from the outward motion of a disturbance that stimulates the radio emission, Model I gives a constant velocity of about 0.35c for the exciting disturbance as it moves to large distances, while with Model II, there is a decrease in the velocity to less than 0.2c beyond 10 R⊙.