Impulsively Accelerated Ions as the Source of Ion Acoustic Waves in Solar Wind

Ion acoustic waves are pervasive at the Earth's bow shock and in regions of active plasmas. Recently, frequency-dispersed ion acoustic-like waves have been observed by Parker Solar Probe in the near-Sun solar wind. These waves are electrostatic, propagate nearly along the magnetic field, and have frequencies on the order of the ion plasma frequency. Frequency-dispersed emissions appear in short (<1 s) bursts and exhibit rising and/or falling tones. This article has a narrow focus, to determine if impulsively accelerated ions are a plausible generation mechanism. We show that velocity dispersion from impulsively accelerated ions can generate a positive slope in the ion distribution that changes in space and time, which can lead to emissions with rising or falling tones given a substantial Doppler shift from the solar wind. The phase velocity is at the velocity of the positive slope, which can differ from the ion acoustic speed, but otherwise these waves are similar to ion acoustic waves. Wave growth is strongest when the positive slope velocity is near the ion acoustic speed. Two mechanisms for impulsive ion acceleration are explored. One mechanism imparts equal energy into the source ions as expected from a parallel potential. The other mechanism imparts equal velocity into the source ions such as that expected from impulsive magnetic reconnection. Both mechanisms result in similar wave characteristics with only subtle differences. Given the persistent appearance of these ion acoustic-like waves, these results suggest that impulsively accelerated ions may be abundant in the near-Sun solar wind.