On the Solar Origin of Complex Type III–like Radio Bursts Observed at and below 1 MHz

Abstract

By simultaneously analyzing decimetric, metric, and hectometric radio observations, we provide evidence that a class of hectometric type III-like radio events are associated with electrons accelerated during the primary flare energy release process. We do this by demonstrating that there is a good temporal correspondence between the hectometric and decimetric radio emissions that are believed to involve the acceleration of electrons in deep layers of the solar atmosphere at heights estimated to be from 10,000 to 30,000 km (0.014 to 0.043 R☉). This class of hectometric type III-like events may have simple or complex intensity-time profiles. When they have complex profiles of long duration, the decimeter emissions also have complex long-duration profiles. In this latter case they are also often associated with metric type II radio bursts and coronal mass ejections (CMEs). However, we argue on the basis of the observations that the hectometric radio emissions are not necessarily associated with the metric phenomena. Specifically, they are not likely generated by a secondary acceleration process associated with the shock generating a metric type II burst or with a shock driven ahead of the CME despite the fact that the hectometric emission time profiles would (erroneously) be morphologically classified as shock-accelerated (SA) events.