Authors: Mihailo Martinovic (University of Arizona), Kristopher Klein (University of Arizona), Jaye Verniero (NASA GSFC), Leon Ofman (The Catholic University of America), Yogesh (The Catholic University of America), Peter Yoon (University of Maryland), Shabaan M. Shaaban (Qatar University), Benjamin Alterman (NASA GSFC)

Observations show that plasma instability thresholds constrain the observed parameters of solar wind Velocity Distribution Functions (VDFs). These thresholds are derived from linear theory, which defines boundaries of “allowed” plasma parameters (or higher probability states), such as temperature anisotropy, velocity drift of secondary VDF components, or the ratio of thermal to magnetic pressure – plasma beta. Excursions beyond these thresholds result in low probability state that evolves through the emission of energy, transferred from particles to electromagnetic waves, acting to push the system toward a more stable configuration. In this work, we use linear theory to define parametric limits for a low-beta plasma that contains protons and a drifting alpha particle population. A sufficiently fast and dense drifting alpha particle population triggers power emission into the forward-propagating Alfven mode, which in turn alters the proton configuration, preventing the proton beta from decreasing below threshold values. Our theoretical predictions are of interest for Parker Solar Probe observations, showing that long standing expectations of observing very low-beta plasmas with very large temperature anisotropies in the near-Sun environment might be constrained when alpha particles are moving faster than the bulk of proton population by a velocity of the order one Alfven speed.