Parallel Electric Fields and Electron Heating Observed in the Young Solar Wind

Abstract The largest electric fields between 18 and 30 solar radii are in narrowband waves simultaneously observed at a few Hz (somewhat above the local proton gyrofrequency) and a few hundred Hz (far below the lower hybrid frequency), with the higher-frequency wave triggered at specific phases of the lower-frequency wave. This wave pair, called “triggered ion-acoustic waves” (TIAWs), has been shown to both be physical and to occur at times of electron heating. A theory of electron heating and acceleration by the low-frequency wave has been presented. While this theory and the TIAW results strongly suggest the presence of low-frequency electric fields that are parallel to the local magnetic field, such fields have not been directly observed. In this paper, such parallel electric field observations are reported, and TIAWs are further described to conclude that they occur during about 75% of the Parker Solar Probe passes through 18–30 solar radii, and when present, they are the dominant wave signal, lasting for hours. In the presence of these parallel electric fields, electrons are heated while, in their absence, there is no electron heating. That there is no heating between 18 and 30 solar radii in the absence of TIAWs is a most significant result because it invalidates other proposed mechanisms that predict heating in this radial range all of the time.