Physics – Plasma Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsh51d..05m&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SH51D-05
Physics
Plasma Physics
[7827] Space Plasma Physics / Kinetic And Mhd Theory, [7829] Space Plasma Physics / Kinetic Waves And Instabilities, [7836] Space Plasma Physics / Mhd Waves And Instabilities, [7839] Space Plasma Physics / Nonlinear Phenomena
Scientific paper
Various dynamic processes in the expanding solar wind produce a range of non-thermal ion and electron properties, which include different velocities, different temperatures, and non-unity temperature anisotropies for each particle species. If these deviations become too severe, a kinetic microinstability can arise, in which exponentially-growing wave modes scatter the particles. In this way, instabilities help to limit the departures of plasma from thermal equilibrium. The work presented here specifically relates to instabilities driven by ion temperature anisotropy: the cyclotron, mirror, and parallel and oblique firehose instabilities. Previous studies have provided strong evidence that instabilities limit proton temperature anisotropy and that the two-dimensional (i.e., mirror and oblique firehose) instabilities are significantly more active in this regard than the one-dimensional (i.e., cyclotron and parallel firehose) instabilities. However, this work considers instabilities driven by alpha-particle temperature anisotropy, which have received far less attention in the literature. Linear Vlasov analysis was used to derive the expected threshold conditions for these alpha-particle instabilities. When data from the Wind spacecraft's Faraday cups was compared to these threshold conditions, it was found to be consistent with instabilities (particularly the mirror and oblique firehose) constraining alpha-particle temperature anisotropy in the solar wind. While this result may seem counterintuitive because of the low alpha-particle relative abundance, the theoretical analysis indicates that the low collision rates in the solar wind often allow protons and alpha-particles to resonate separately and with different wave modes. These results provide new insight into how instabilities redistribute energy in the plasma and have important implication for the interpretation of the observations of minor-ions in the solar wind and corona.
Gary Peter S.
Kasper Justin Christophe
Maruca Bennett A.
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