Parker Solar Probe evidence for the absence of whistlers close to the Sun to scatter strahl and regulate heat flux

TL;DR

This study uses Parker Solar Probe data to show that narrowband whistler waves are absent close to the Sun, implying alternative mechanisms regulate electron heat flux and distribution in the inner solar environment.

Contribution

It provides the first statistical analysis of whistler wave occurrence from 16 to 130 solar radii, revealing their scarcity inside 28 Rs and their correlation with electron beta and heat flux.

Findings

Whistlers are rarely observed inside ~28 Rs.

Outside 28 Rs, whistlers occur within beta 1-10.

Absence of whistlers near the Sun suggests other plasma processes regulate electron heat flux.

Abstract

Using the Parker Solar Probe FIELDS bandpass filter data and SWEAP electron data from Encounters 1 through 9, we show statistical properties of narrowband whistlers from ~16 Rs to ~130 Rs, and compare wave occurrence to electron properties including beta, temperature anisotropy and heat flux. Whistlers are very rarely observed inside ~28 Rs (~0.13 au). Outside 28 Rs, they occur within a narrow range of parallel electron beta from ~1 to 10, and with a beta-heat flux occurrence consistent with the whistler heat flux fan instability. Because electron distributions inside ~30 Rs display signatures of the ambipolar electric field, the lack of whistlers suggests that the modification of the electron distribution function associated with the ambipolar electric field or changes in other plasma properties must result in lower instability limits for the other modes (including solitary waves, ion acoustic waves) that are observed close to the Sun. The lack of narrowband whistler-mode waves close to the Sun and in regions of either low (<.1) or high (>10) beta is also significant for the understanding and modeling of the evolution of flare-accelerated electrons, and the regulation of heat flux in astrophysical settings including other stellar winds, the interstellar medium, accretion disks, and the intra-galaxy cluster medium