Parker Solar Probe evidence for scattering of electrons in the young solar wind by narrowband whistler-mode waves

TL;DR

This study provides evidence that narrowband whistler-mode waves in the young solar wind scatter electrons, reducing heat flux and affecting electron distribution features at distances less than 0.3 AU.

Contribution

It demonstrates the role of narrowband whistler-mode waves in pitch angle scattering of electrons in the young solar wind, a novel insight into solar wind electron dynamics.

Findings

Electron pitch angle broadening correlates with whistler wave intensity.

Heat flux is lower during intervals with strong whistler waves.

Electron distributions show energy-dependent scattering consistent with wave interactions.

Abstract

Observations of plasma waves by the Fields Suite and of electrons by the Solar Wind Electrons Alphas and Protons Investigation (SWEAP) on Parker Solar Probe provide strong evidence for pitch angle scattering of strahl-energy electrons by narrowband whistler-mode waves at radial distances less than ~0.3 AU. We present two example intervals of a few hours that include 8 waveform captures with whistler-mode waves and 26 representative electron distributions that are examined in detail. Two were narrow; 17 were clearly broadened, and 8 were very broad. The two with narrow strahl occurred when there were either no whistlers or very intermittent low amplitude waves. Six of the eight broadest distributions were associated with intense, long duration waves. Approximately half of the observed electron distributions have features consistent with an energy dependent scattering mechanism, as would be expected from interactions with narrowband waves. A comparison of the wave power in the whistler-mode frequency band to pitch angle width and a measure of anisotropy provides additional evidence for the electron scattering by whistler-mode waves. The pitch angle broadening occurs in over an energy range comparable to that obtained for the n=1 (co-streaming) resonance for the observed wave and plasma parameters. The additional observation that the heat flux is lower in the interval with multiple switchbacks may provide clues to the nature of switchbacks. These results provide strong evidence that the heat flux is reduced by narroweband whistler-mode waves scattering of strahl-energy electrons.