Majority of Solar Wind Intervals Support Ion-Driven Instabilities

We perform a statistical assessment of solar wind stability at 1 AU against ion sources of free energy using Nyquist’s instability criterion. In contrast to typically employed threshold models which consider a single free-energy source, this method includes the effects of proton and ${\mathrm{He}}^{2+}$ temperature anisotropy with respect to the background magnetic field as well as relative drifts between the proton core, proton beam, and ${\mathrm{He}}^{2+}$ components on stability. Of 309 randomly selected spectra from the Wind spacecraft, 53.7% are unstable when the ion components are modeled as drifting bi-Maxwellians; only 4.5% of the spectra are unstable to long-wavelength instabilities. A majority of the instabilities occur for spectra where a proton beam is resolved. Nearly all observed instabilities have growth rates $\gamma$ slower than instrumental and ion-kinetic-scale timescales. Unstable spectra are associated with relatively large ${\mathrm{He}}^{2+}$ drift speeds and/or a departure of the core proton temperature from isotropy; other parametric dependencies of unstable spectra are also identified.

Figure 1

(a),(b),(d) The number of unstable modes with ${\gamma }_{\min }>0$ as a function of wave vector $\mathbf{k}{\rho }_p$ for three example spectra. (c) The mean winding number averaged over the 309 observed spectra.

Figure 2

The (${\beta }_{\parallel p},T_{\perp p}/T_{\parallel p}$) distribution of the observed spectra; color indicates the unstable mode density ð, and gray indicates a stable spectrum.

Figure 3

The fraction of observed spectra supporting unstable modes with a growth rate exceeding ${\gamma }_{\min }$. (a) The spectra are divided according to the instability classification presented in the text. (b) The minimum growth rate distribution is rescaled by selected timescales $\tau$.