Fine structures of solar type III radio bursts observed in the inner heliosphere by Solar Orbiter

Type III solar radio emissions are intense wave phenomena originating from eruptive events in the solar corona. These emissions are produced by energetic electron beams traveling outward from the Sun along the magnetic Parker spiral. As these beams propagate, they generate intense electrostatic Langmuir waves, which are subsequently converted into freely propagating radio waves. These radio waves, characterized by their distinct dispersed signatures in the time-frequency domain, can be detected throughout the solar system.

However, their propagation is not straightforward. Local density variations in the solar wind plasma cause refraction and, in some cases, reflection of the waves. Such density variations between the source and the observer can obscure low frequency part of the emissions, limiting their propagation to frequencies above the local L-O cutoff frequency. Multi-point observations reveal that the spatial and temporal coincidence of type III emission sources and solar wind density structures modifies the lower-frequency boundary of the detected emissions.

Additionally, some emissions display stripe-like patterns, which are also attributed to local plasma density variations at their source. The fine structures of these low-frequency (<100 kHz) emissions can be resolved with the very high time-frequency resolution of the Radio and Plasma Wave (RPW) instrument onboard the Solar Orbiter. These observations provide valuable insights into the interaction between solar radio emissions and the solar wind plasma environment.