Narrowband oblique whistler-mode waves: Comparing properties observed by Parker Solar Probe at <0.2 AU and STEREO at 1 AU

TL;DR

This study compares narrowband oblique whistler-mode waves observed by Parker Solar Probe inside 0.2 AU with those at 1 AU, revealing their properties, variability, and potential role in regulating electron heat flux in the solar wind.

Contribution

It provides the first detailed comparison of whistler-mode wave properties inside 0.2 AU with those at 1 AU, highlighting differences in intermittency and wave characteristics.

Findings

Waves are more intermittent closer to the Sun.

Whistler waves are associated with beta > 1 and regulate electron heat flux.

Wave angles are often highly oblique, near the resonance cone.

Abstract

AIM: Large amplitude narrowband obliquely propagating whistler-mode waves at frequencies of ~0.2 fce (electron cyclotron frequency) are commonly observed at 1 AU, and are most consistent with the whistler heat flux fan instability. We want to determine whether similar whistler-mode waves occur inside 0.2 AU, and how their properties compare to those at 1 AU. METHODS: We utilize the waveform capture data from the Parker Solar Probe Fields instrument to develop a data base of narrowband whistler waves. The SWEAP instrument, in conjunction with the quasi-thermal noise measurement form Fields, provides the electron heat flux, beta, and other electron parameters. RESULTS: Parker Solar Probe observations inside ~0.3 AU show that the waves are more intermittent than at 1 AU, and are often interspersed with electrostatic whistler/Bernstein waves at higher frequencies. This is likely due to the more variable solar wind observed closer to the Sun. The whistlers usually occur within regions when the magnetic field is more variable and often with small increases in the solar wind speed. The near-sun whistler-mode waves are also narrowband and large amplitude, and associated with beta greater than 1. Wave angles are sometimes highly oblique (near the resonance cone), but angles have been determined for only a small fraction of the events. The association with heat flux and beta is generally consistent with the whistler fan instability although there are intervals where the heat flux is significantly lower than the instability limit. Strong scattering of strahl energy electrons is seen in association with the waves, providing evidence that the waves regulate the electron heat flux..