Triggered Ion-acoustic Waves

Abstract: The Parker Solar Probe is in a solar orbit with a perihelion for orbit 12 at 13.3 solar radii. The electric field experiment on this satellite observes what we call triggered ion-acoustic waves as the most dominant wave mode above a few Hz within the solar radial distance of 15-25 solar radii. In this mode, a few Hz electrostatic wave is typically accompanied by bursts of a few hundred Hz wave whose bursts are phase locked with each low frequency wave period. Plasma density fluctuations with {\Delta}n/n~0.1 accompany these waves and they have no magnetic field component. The wave durations can be hours and their field and density fluctuations are nearly pure sine waves. They are identified as ion-acoustic waves. The low and high frequency waves are measured to have the same phase velocity within experimental uncertainties, which is a requirement associated with their phase locked relationship. From the measured wavelength, the potential associated with the low frequency wave is estimated to be ~10 Volts, which can result in electron heating via the Landau resonance that is in agreement with observations of the core electron temperature increases at times of such waves. Their phase locked relationship and pure frequency are surprising features that characterize a new regime of instability and evolution of ion-acoustic waves that may not have been reported previously. That these waves are an instrumental effect unrelated to natural processes is considered. While this is unlikely, the possibility that these waves are artificial cannot be rule out