  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.
