Evidence for electron excitation of type III radio burst emission

Abstract

Type III radio bursts observed at kilometric wavelengths (≲ 0.35 MHz) by the OGO-5 spacecraft are compared with > 45 keV solar electron events observed near 1 AU by the IMP-5 and Explorer 35 spacecraft for the period March 1968–November 1969. Fifty-six distinct type III bursts extending to ≲ 0.35 MHz (≳ 50 R ⊙ equivalent height above the photosphere) were observed above the threshold of the OGO-5 detector; all but two were associated with solar flares. Twenty-six of the bursts were followed ≲ 40 min later by > 45 keV solar electron events observed at 1 AU. All of these 26 bursts were identified with flares located west of W 09 solar longitude. Of the bursts not associated with electron events only three were identified with flares west of W 09, 18 were located east of W 09 and 7 occurred during times when electron events would be obscured by high background particle fluxes. Thus almost all type III bursts from the western half of the solar disk observed by OGO-5 above a detection flux density threshold of the order of 10−13 Wm−2 Hz−1 at 0.35 MHz are followed by > 45 keV electrons at 1 AU with a maximum flux of ≳ 10 cm−2 s−1 ster−1. If particle propagation effects are taken into account it is possible to account for lack of electron events with the type III bursts from flares east of the central meridian. We conclude that streams of ≈ 10–100 keV electrons are the exciting agent for type III bursts and that these same electrons escape into the interplanetary medium where they are observed at 1 AU. The total number of > 45 keV electrons emitted in association with a strong kilometer wavelength type III burst is estimated to be ⩾ 5 × 1032.