How the Oblique Drift Instability Alters Solar Wind Heating and Constrains the Distribution of Solar Wind Observations
Ion-driven plasma instability thresholds, derived from linear theory, constrain the distribution of solar observations in parameter space, defining boundaries of stable plasma parameters. Excursions beyond these thresholds result in the emission of energy, transferred from particles to coherent electromagnetic waves, acting to adjust the system toward a more stable configuration. In this work, we use linear Vlasov--Maxwell theory to define parametric limits for a low-$β$ plasma that contains a drifting proton beam or helium ($α$-particle) population. A sufficiently fast and dense drifting population triggers an Oblique Drift Instability (ODI). This instability decreases the velocity drift between the thermal core proton and secondary populations and prevents the ratio of core thermal to magnetic pressure $β_c$ from decreasing below a minimum value by increasing the temperatures - i.e. heating - of both the core and drifting populations. Our theoretical results are of interest for Parker Solar Probe observations, as they provide an additional mechanism for perpendicular heating of ions active in the sub-\Alfvenic solar wind. The ODI may explain the discrepancy between long-standing expectations of measurements of very low-$β$ plasmas with very large ion temperature anisotropies in the near-Sun environment and in situ observations, where $β$ is consistently measured above a few percent and the secondary ion populations drift faster than the bulk of proton population by no more than approximately the local Alfven speed.