The relation between solar active region location and the lowest frequency of type III emission

We have investigated the relation between the lowest frequencies of type III bursts and the locations of source active regions from which the causal electron beams originate. We chose several active regions that produced coronal mass ejections (CMEs) and flares during their disk passage. The type III bursts considered in this study originated from these eruptions. The CMEs were detected by the Solar and Heliospheric Observatory (SOHO) mission’s Large Angle and Spectrometric coronagraph (LASCO). The type III bursts were observed by the Radio and Plasma Wave Experiment (WAVES) on board Wind. By tracking the type III bursts into the spectral range covered by the Thermal Noise Receiver (TNR) on board Wind, we were able to measure the lowest frequency reached by 104 type III bursts from 17 active regions. In addition, we were able to measure the frequency separation between the type III bursts and the local plasma frequency at the Wind spacecraft as measured by TNR. Because the type III bursts were produced when the source active regions were at various central meridian distances, we could study the lowest frequency at which the type III bursts occurred as a function of the source longitude. We found that the type III emission occurs at frequencies close to the local plasma frequency at the spacecraft when the source active regions are at the disk center or in the western hemisphere. Type III bursts from active regions close to the east limb of the Sun have a higher cutoff frequency. This can be explained by the fact that type III bursts from eastern active regions cannot propagate to the Wind spacecraft because of the intervening denser plasma regions Further investigation is needed to understand the large scatter in the data points, especially those that do not follow the expected tendency: a closer examination would reveal the condition of interplanetary medium and the state of solar wind at the time of these irregular points.