Ion Scale Electromagnetic Waves in the Inner Heliosphere

TL;DR

This study analyzes ion-scale electromagnetic waves in the inner heliosphere using Parker Solar Probe data, revealing their prevalence, characteristics, and potential generation mechanisms related to plasma instabilities and temperature anisotropy.

Contribution

It provides the first statistical analysis of ion-scale electromagnetic waves in the inner heliosphere, highlighting their occurrence, properties, and possible in-situ generation mechanisms.

Findings

Ion-scale waves observed in 30-50% of intervals.

Average wave amplitude ~4 nT, duration ~20 seconds.

Waves likely generated by plasma instabilities related to temperature anisotropy.

Abstract

Understanding the physical processes in the solar wind and corona which actively contribute to heating, acceleration, and dissipation is a primary objective of NASA's Parker Solar Probe (PSP) mission. Observations of coherent electromagnetic waves at ion scales suggests that linear cyclotron resonance and non-linear processes are dynamically relevant in the inner heliosphere. A wavelet-based statistical study of coherent waves in the first perihelion encounter of PSP demonstrates the presence of transverse electromagnetic waves at ion resonant scales which are observed in 30-50\% of radial field intervals. Average wave amplitudes of approximately 4 nT are measured, while the mean duration of wave events is of order 20 seconds; however long duration wave events can exist without interruption on hour-long timescales. Though ion scale waves are preferentially observed during intervals with a radial mean magnetic field, we show that measurement constraints, associated with single spacecraft sampling of quasi-parallel waves superposed with anisotropic turbulence, render the measured quasi-parallel ion-wave spectrum unobservable when the mean magnetic field is oblique to the solar wind flow; these results imply that the occurrence of coherent ion-scale waves is not limited to a radial field configuration. The lack of strong radial scaling of characteristic wave amplitudes and duration suggests that the waves are generated {\em{in-situ}} through plasma instabilities. Additionally, observations of proton distribution functions indicate that temperature anisotropy may drive the observed ion-scale waves.