Parallel Electric Fields And Electron Heating Observed In The Young Solar Wind

TL;DR

This study reports the first direct observations of parallel electric fields in the young solar wind, linking them to electron heating and ion acoustic waves, and demonstrating their dominance in solar wind dynamics between 18 and 30 solar radii.

Contribution

It provides direct measurements of parallel electric fields and clarifies their role in electron heating, advancing understanding of solar wind acceleration mechanisms.

Findings

Parallel electric fields are observed during 75% of Parker Solar Probe passes.

Electron heating correlates with the presence of parallel electric fields.

Triggered ion acoustic waves are the dominant wave signal when electric fields are present.

Abstract

The largest electric fields between 18 and 30 solar radii are in narrow band 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 (TIAW), 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 TIAW are further described to conclude that they occur during about 75 percent of the Parker Solar Probe passes through 18 to 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 TIAW is a most significant result because it invalidates other proposed mechanisms that predict heating in this radial range all of the time.