Velocity-space signatures of energy transfer for ion-acoustic instabilities

Context. Observations by Parker Solar Probe (PSP) of electrostatic waves suggest that electrostatic instabilities, including the ion-ion-acoustic instability (IIAI) frequently observed in the inner heliosphere, play an important role in plasma heating and particle acceleration. Aims. Our aim is to explore the application of single spacecraft diagnostics to the IIAI, in anticipation of its application to two of the current missions operating in the inner heliosphere, PSP and Solar Orbiter. Methods. We applied the field-particle correlation (FPC) technique to fully kinetic simulations of IIAI. We characterized the conversion of energy between the electric field and particle species, allowing the differentiation between oscillatory and secular energy transfer to and from the particles and highlighting the role of resonant energy exchange. We then identified the characteristic IIAI signatures for the proton and electron distributions, and related them to our previous knowledge of IIAI onset and energy exchange mechanisms. Results. Applying the FPC technique to our simulations that were run in a parameter regime compatible with solar wind conditions, we identified IIAI signatures that would enable efficient recognition of IIAI in observations. This task is left for future missions, since the timescale over which IIAI signatures develop is too fast for the sampling rates of current missions.