Whistler waves in the young solar wind: statistics of amplitude and propagation direction from Parker Solar Probe Encounters 1-11

TL;DR

This study analyzes whistler wave properties in the solar wind using Parker Solar Probe data, revealing how their occurrence and propagation directions vary with distance from the Sun, and linking these patterns to solar wind structures.

Contribution

It introduces a new technique to determine whistler wave propagation directions from spectral data and applies it to a comprehensive set of Parker Solar Probe encounters.

Findings

Whistler wave occurrence peaks between 25-35 RS due to sunward propagating waves.

Anti-sunward whistlers are observed at 30-40 RS.

Sunward and anti-sunward whistlers are equally common at 40-50 RS.

Abstract

In the interplanetary space solar wind plasma, whistler waves are observed in a wide range of heliocentric distance (from ~20 solar radii (RS) to Jupiter's orbit). They are known to interact with solar wind suprathermal electrons (strahl and halo) and to regulate the solar wind heat flux through scattering the strahl electrons. We present the results of applying the technique to determine the whistler wave propagation direction to the spectral data continuously collected by the FIELDS instruments aboard Parker Solar Probe (PSP). The technique was validated based on the results obtained from burst mode magnetic and electric field waveform data collected during Encounter 1. We estimated the effective length of the PSP electric field antennas (EFI) for a variety of solar wind conditions in the whistler wave frequency range and utilized these estimates for determining the whistler wave properties during PSP Encounters 1-11. Our findings show that (1) the enhancement of the whistler wave occurrence rate and wave amplitudes observed between 25-35 RS is predominantly due to the sunward propagating whistler waves population associated with the switchback-related magnetic dips; (2) the anti-sunward or counter-propagating cases are observed at 30-40 RS; (3) between 40-50 RS, sunward and anti-sunward whistlers are observed with comparable occurrence rates; and (4) almost no sunward or counter-propagating whistlers were observed at heliocentric distances above 50 RS.