Whistler Waves and Electron Properties in the Inner Heliosphere: Helios Observations

TL;DR

This study analyzes whistler wave signatures in the inner heliosphere, revealing their increased occurrence with distance and slower solar wind, and links these signatures to electron anisotropy and heat flux variations.

Contribution

It provides new observational insights into the radial and wind-speed dependence of whistler waves and their relation to electron properties in the inner heliosphere.

Findings

Whistler signatures are more common in slow solar wind and increase with distance.

Occurrence of whistlers is lower in fast solar wind and starts beyond 0.6 au.

Electron anisotropy correlates with whistler activity and increases with radial distance.

Abstract

We present the analysis of narrowband whistler wave signatures observed in the inner heliosphere (0.3-1 au). These signatures are bumps in the spectral density in the 10-200 Hz frequency range of the AC magnetic field as measured by the search coil magnetometer on board the Helios1 spacecraft. We show that the majority of whistler signatures are observed in the slow solar wind (<500 km/s) and their occurrence increases with the radial distance (R), from ~3% at 0.3 au to ~10% at 0.9 au. In the fast solar wind (>600 km/s), whistler activity is significantly lower; whistler signatures start to appear for R > 0.6 au and their number increases from ~0.03% at 0.65 au to ~1% at 0.9 au. We have studied the variation of the electron core and halo anisotropy, as well as the electron normalized heat flux as a function of R and of the solar wind speed. We find that, in the slow wind electron core and halo anisotropy is higher than in fast one, and also that these anisotropies increase radially in both types of winds, which is in line with the occurrence of whistler signatures. We hypothesize the existence of a feedback mechanism to explain the observed radial variations of the occurrence of whistlers in relation with the halo anisotropy.