Near‐Relativistic Solar Electrons and Type III Radio Bursts

Recently, it has been found that the inferred injection times of >25 keV electrons are up to 30 minutes later than the start times of the associated type III radio bursts at the Sun. Thus, it has been suggested that the electrons that produce type III bursts do not belong to the same population as those observed above 25 keV. This paper examines the characteristics and circumstances of 79 solar electron beam events measured on the Advanced Composition Explorer (ACE) spacecraft. Particular attention is paid to the very low frequency emissions of the associated radio bursts and the ambient conditions at the arrival times of the electrons at the spacecraft. It is found that the inferred >25 keV electron injection delays are correlated with the times required for the associated radio bursts to drift to the lowest frequencies. This suggests that the electrons responsible for the radio emission, and those observed above 25 keV, are part of a single population and that the electrons both above and below 25 keV are delayed in the interplanetary medium. Further evidence for a single population is the general correspondence between electron and local radio intensities and temporal profiles. It is found that the delays increase with the ambient solar wind density, consistent with the propagation times of the electrons being determined by the characteristics of the interplanetary medium. However, it is known that particle arrival times at 1 AU are a linear function of inverse particle speed. Conventionally, such a relationship is taken to indicate scatter-free propagation when inferred path lengths lie close to 1.2 AU, as they do for the electron events studied here. These conflicting interpretations require further investigation.