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 Letter, we use linear Vlasov-Maxwell theory to define parametric limits for a low-$\beta$ plasma that contains a drifting proton beam or helium ($\alpha$-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 ${\beta }_c$ 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-Alfvénic solar wind. The ODI may explain the discrepancy between long-standing expectations of measurements of very low-$\beta$ plasmas with very large ion temperature anisotropies in the near-Sun environment and in situ observations, where $\beta$ is consistently measured above a few percentages and the secondary ion populations drift faster than the bulk of the proton population by no more than approximately the local Alfvén speed.

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• Alfvén waves
• Kinetic theory
• Plasma instabilities
• Solar wind