Whistler wave occurrence and the interaction with strahl electrons during the first encounter of Parker Solar Probe

TL;DR

This study investigates the occurrence of narrow band whistler waves near the Sun and their interaction with strahl electrons, revealing low occurrence rates, anti-correlations with solar wind speed, and evidence of wave-particle interactions affecting electron pitch-angle distributions.

Contribution

It provides new insights into the properties, occurrence, and interaction mechanisms of whistler waves with strahl electrons during the Parker Solar Probe's first encounter.

Findings

Whistler wave occurrence is very low, less than 1.5%.

Whistler occurrence is anti-correlated with solar wind speed.

Whistler waves cause pitch-angle scattering of strahl electrons.

Abstract

We studied the properties and occurrence of narrow band whistler waves and their interaction with strahl electrons observed between 0.17 and 0.26 au during the first encounter of Parker Solar Probe. We observe that occurrence of whistler waves is low, nearly 1.5% and less than 0.5% in the analyzed peak and average BPF data respectively. Whistlers occur highly intermittently and 80% of the whistlers appear continuously for less than 3 s. Occurrence rate of whistler waves was found to be anti-correlated with the solar wind bulk velocity. The study of the duration of the whistler intervals revealed an anti-correlation between the duration and the solar wind velocity, as well as between the duration and the normalized amplitude of magnetic field variations. The pitch-angle widths (PAWs) of the field-aligned electron population referred to as the strahl are broader by at least 12 degrees during the presence of large amplitude narrow band whistler waves. This observation points towards a EM wave electron interaction, resulting in pitch-angle scattering. PAW of strahl electrons corresponding to the short duration whistlers are higher compared to the long duration whistlers. Parallel cuts through the strahl electron velocity distribution function (VDF) observed during the whistler intervals appear to depart from the Maxwellian shape typically found in the near-Sun strahl VDFs (Bercic et al. 2020). The relative decrease of parallel electron temperature and the increase of PAW for the electrons in strahl energy range suggests that the interaction with whistler waves results in a transfer of electron momentum from the parallel to the perpendicular direction.