The Occurrence Rate of Ion Driven Instabilities in the Solar Wind

Weakly collisional plasmas, including the solar wind, are frequently found in states far from thermodynamic equilibrium and are therefore susceptible to a myriad of instabilities.
Instead of focusing on a single source of free energy or a single kind of unstable mode, we assess the stability of solar wind observations against ion free energy sources using an automated implementation of Nyquist's instability criterion, accounting for the destabilizing effects of proton and alpha temperature anisotropies, temperature disequilibrium, and relative drifts between components. 53.7% of a representative sample of intervals from the Wind spacecraft are unstable, though only 4.5% of the spectra are unstable to long-wavelength instabilities. A majority of the instabilities have maximum growth rates slower than ion-kinetic-scale timescales or the measurement cadence. Instabilities are more likely to arise when a proton beam is resolved, as well as for cases with relatively large alpha drift speeds or a departure of the core proton temperature from isotropy. This automated analysis method promises to offer insight into the role instabilities play in a variety of non-equilibrium plasma systems.