Ion-Driven Instabilities in the Inner Heliosphere I: Statistical Trends

Abstract

Instabilities described by linear theory characterize an important form of wave-particle interaction in 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 $\sim 1.5$M proton and $\alpha$ particle Velocity Distribution Functions (VDFs) observed by \emph{Helios I} and \emph{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 find that, for a non-negligible fraction of observations, the proton beam or secondary component might not be detected due to instrument resolution limitations, and demonstrate that the impact of this issue does not affect the main conclusions of this work.

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Cited by in corpus (5)

• Interplay of Turbulence and Proton-Microinstability Growth in Space Plasmas
• Fully Kinetic Simulations of Proton-Beam-Driven Instabilities from Parker Solar Probe Observations
• Ion-Driven Instabilities in the Inner Heliosphere II: Classification and Multi-Dimensional Mapping
• Modeling hot, anisotropic ion beams in the solar wind motivated by the Parker Solar Probe observations near perihelia
• Evolution of an Alfvén Wave-Driven Proton Beam in the Expanding Solar Wind