Oblique Drift Instability in Low Beta Plasma

Parameters of solar wind velocity distributions are well constrained by thresholds of ion-driven plasma instabilities derived from linear theory. Surpassing these thresholds results in the transfer of energy from particles to coherent electromagnetic waves as the system is altered toward a more stable configuration. We use linear Vlasov-Maxwell theory to describe an Oblique Drift Instability (ODI) that constrains the limits of stable parametric space for a low-beta plasma that contains a drifting proton beam or helium population. This instability decreases the relative drift of secondary populations and prevents beta from decreasing below a minimum value by heating both the core and drifting populations. Our predictions are of interest for Parker Solar Probe (PSP) observations, as they provide an additional mechanism for perpendicular heating of ions active in the vicinity of Alfven surface. The ODI may explain the discrepancy between long-standing expectations of measurements of very low-beta plasmas in the near-Sun environment and in situ observations, where beta is consistently measured above 1%. In parallel, it proposes an interpretation why the drift of the secondary ion populations with respect to the bulk of thermal protons is reduced to no more than approximately the local Alfven speed, as observed in earlier PSP encounters.