Ion-Driven Instabilities as Observed by Helios

Instabilities described by linear theory have been studied for several decades as an essential part of the kinetic description of the solar wind. We diagnose unstable behavior of solar wind plasma between 0.3 and 1 au via the Nyquist criterion, applying it to fits of 1.5M proton and alpha particle Velocity Distribution Functions (VDFs) observed by Helios I and II. The variation of the fraction of unstable intervals with radial distance from the Sun is linear, signaling a gradual decline in the activity of unstable modes. When calculated as functions of the solar wind velocity and Coulomb number, we obtain more extreme, exponential trends in the regions where collisions appear to have a notable influence on the VDF. Instability growth rates demonstrate similar behavior, and significantly decrease with Coulomb number. We identify three separate regimes with respect to collisions and their role in regulating instabilities in the solar wind: 1) the collisionless regime, where rapidly growing instabilities dictate the limits of plasma parameters, 2) the transition regime where collisions drive the plasma towards Local Thermodynamic Equilibrium (LTE), significantly decreasing unstable behavior and 3) the collisionally processed regime, where plasma is close to LTE, but occasionally driven towards marginal stability states via local plasma processes. We demonstrate that the observed limits on solar wind plasma parameters are imposed by the interplay between multiple kinds of instabilities and collisions, with the relative importance of these mechanisms varying as the plasma travels further from the Sun.