Triggered ion acoustic waves in the solar wind

TL;DR

This paper reports the first observation of a new regime of ion-acoustic waves in the solar wind, characterized by repetitive electrostatic emissions correlated with ion beams and electromagnetic fluctuations, observed by Parker Solar Probe.

Contribution

It presents the first detection of a novel regime of ion-acoustic waves in the solar wind, expanding understanding of wave instabilities near the Sun.

Findings

Repetitive electrostatic wave packets observed at ~1.5 Hz.

Correlation between wave emissions, ion beams, and electromagnetic fluctuations.

Identification of a new instability regime of oblique ion-acoustic waves.

Abstract

For more than 12 hours beginning on January 18, 2021, continuous narrowband electrostatic emissions were observed on Parker Solar Probe near 20 solar radii. The observed <1000 Hz frequencies were well below the local ion plasma frequency. Surprisingly, the emissions consisted of electrostatic wave packets with shock-like envelopes, appearing repetitively at a ~1.5 Hz rate. This repetitiveness correlated and was in phase with low frequency electromagnetic fluctuations. The emissions were associated with simultaneously observed ion beams and conditions favorable for ion-acoustic wave excitation, i.e. Te/Ti~5. Based on this information and on their velocity estimates of about 100 km/s, these electrostatic emissions are interpreted as ion-acoustic waves. Their observation demonstrates a new regime of instability and evolution of oblique ion-acoustic waves that have not been reported previously in theory or experiment.