Parker Solar Probe observations of near-$f_{\rm ce}$ harmonics emissions in the near-Sun solar wind and their dependence on the magnetic field direction

TL;DR

This study uses Parker Solar Probe data to analyze near-electron gyrofrequency harmonic emissions in the near-Sun solar wind, revealing their dependence on specific magnetic field directions and fluctuations.

Contribution

It demonstrates the strong correlation between wave emissions and magnetic field orientation, providing new insights into wave generation mechanisms near the Sun.

Findings

Wave emissions occur only within specific magnetic field angle ranges.

Emission amplitudes peak when angles are near the center of these ranges.

Wave intensities correlate with magnetic field angular changes and fluctuations.

Abstract

Wave emissions at frequencies near electron gyrofrequency harmonics are observed at small heliocentric distances below about 40 solar radii and are known to occur in regions with quiescent magnetic fields. We show the close connection of these waves with the large-scale properties of the magnetic field. Near electron gyrofrequency harmonics emissions occur only when the ambient magnetic field points to a narrow range of directions bounded by polar and azimuthal angular ranges in the RTN coordinate system of correspondingly $80^{\circ} \lesssim \theta_B \lesssim 100^{\circ}$ and $10^{\circ} \lesssim \phi_B \lesssim 30^{\circ}$. We show that the amplitudes of wave emissions are highest when both angles are close to the center of their respective angular interval favorable to wave emissions. The intensity of wave emissions correlates with the magnetic field angular changes at both large and small time scales. Wave emissions intervals correlate with intervals of decreases in the amplitudes of broadband magnetic fluctuations at low frequencies of 10Hz-100Hz. We discuss possible generation mechanisms of the waves.