Journal
PHYSICAL REVIEW LETTERS
Volume 120, Issue 20, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevLett.120.205102
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Funding
- NASA [NNX14AR78G, NNX16AG81G]
- NASA [NNX16AG81G, NNX14AR78G, 904247, 674107] Funding Source: Federal RePORTER
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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 gamma slower than instrumental and ionkinetic-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.
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