Majority of Solar Wind Intervals Support Ion-Driven Instabilities

K. G. Klein, B. L. Alterman, M. L. Stevens, D. Vech, J. C. Kasper

Research output: Contribution to journalArticlepeer-review

53 Scopus citations


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 He2+ temperature anisotropy with respect to the background magnetic field as well as relative drifts between the proton core, proton beam, and He2+ 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 γ slower than instrumental and ion-kinetic-scale timescales. Unstable spectra are associated with relatively large He2+ drift speeds and/or a departure of the core proton temperature from isotropy; other parametric dependencies of unstable spectra are also identified.

Original languageEnglish (US)
Article number205102
JournalPhysical review letters
Issue number20
StatePublished - May 18 2018

ASJC Scopus subject areas

  • General Physics and Astronomy


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