Triggered Ion-acoustic Waves in the Solar Wind

For more than 12 hr beginning on 2021 January 18, continuous narrowband electrostatic emissions were observed on the 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⁻¹, 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.