Further Insights into the Physics of Ion-Acoustic Waves in the Solar Wind by Comparison of Magnetospheric Multiscale Mission with Parker Solar Probe Measurements Combined with Supporting Theory and Simulation

Parker Solar Probe (PSP) consistently observes large numbers of ion-acoustic waves in the solar wind. These waves have durations on the order of fractions of a second, and their frequency increases and then/or decreases within a single wave packet. Even though the frequency of occurrence of these waves is highly variable within a solar encounter, most of the waves are concentrated in storms of various durations (median ~10 min). Here we present updated findings on the characteristics of the plasma environments where these waves appear in the solar wind. We combine these findings and the phenomenology of these waves with findings from theory and simulation of ion-acoustic waves. This allows us to conjecture about the most probable physical processes taking place inside the wave source regions in the solar wind. In an effort to make more definite claims on these physical processes, we employ observations of these waves from the Magnetospheric Multiscale Mission (MMS) in the near-Earth plasma environment. These multi-point measurements allow us to re-construct the 3D magnetic field and look at triggered particle phase space distributions with accumulation timescales of tens of milliseconds in the source regions of ion-acoustic waves. We examine different ways in which the MMS fields and particle measurements would appear if their spatial and temporal resolutions were similar to those of instruments on-board PSP. We compare the reduced data from MMS to our findings from PSP and supporting theory and simulation, and discuss the implications for the instability mechanisms of ion-acoustic waves in the solar wind.