Plasma Waves near the Electron Cyclotron Frequency in the near-Sun Solar Wind

TL;DR

This study analyzes plasma waves near the electron cyclotron frequency in the near-Sun solar wind, revealing their prevalence, characteristics, and potential role in electron heat flux regulation close to the Sun.

Contribution

It provides the first detailed observations of near-f_ce plasma waves in the near-Sun solar wind, identifying their modes, correlations, and implications for solar wind electron dynamics.

Findings

Waves are concentrated near 0.7 f_ce and f_ce with strong harmonics.

Wave amplitudes and detections increase closer to the Sun.

Waves are correlated with electron properties and magnetic field configurations.

Abstract

Data from the first two orbits of the Sun by Parker Solar Probe reveal that the solar wind sunward of 50 solar radii is replete with plasma waves and instabilities. One of the most prominent plasma wave power enhancements in this region appears near the electron cyclotron frequency (f_ce). Most of this wave power is concentrated in electric field fluctuations near 0.7 f_ce and f_ce, with strong harmonics of both frequencies extending above f_ce. At least two distinct, often concurrent, wave modes are observed, preliminarily identified as electrostatic whistler-mode waves and electron Bernstein waves. Wave intervals range in duration from a few seconds to hours. Both the amplitudes and number of detections of these near-f_ce waves increase significantly with decreasing distance to the Sun, suggesting that they play an important role in the evolution of electron populations in the near-Sun solar wind. Correlations are found between the detection of these waves and properties of solar wind electron populations, including electron core drift, implying that these waves play a role in regulating the heat flux carried by solar wind electrons. Observation of these near-f_ce waves is found to be strongly correlated with near-radial solar wind magnetic field configurations with low levels of magnetic turbulence. A scenario for the growth of these waves is presented which implies that regions of low-turbulence near-radial magnetic field are a prominent feature of solar wind structure near the Sun.