Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsh21a0733k&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SH21A-0733
Physics
2159 Plasma Waves And Turbulence, 2164 Solar Wind Plasma, 7871 Waves And Instabilities
Scientific paper
We report the results of a statistical study of proton temperature anisotropies in the solar wind using plasma and magnetic field data from the Solar Wind Experiment (SWE) on the Wind satellite. The Faraday Cup plasma instruments on the Wind satellite measure the three-dimensional distribution of the solar wind once every 92 seconds, and have made over 2.2 million measurements to date. Under the assumption that protons in the solar wind have a parallel, T∥ , and perpendicular, Tperpendicular to , temperature relative to the ambient magnetic field, we fit the measurements in each spectrum with a two-temperature convected Maxwellian distribution function. This distribution describes the solar wind well, with an average χ2 per degree of freedom (dof) of 1.05 and with 99% of the spectra yielding χ2/dof < 4. The average errors in our determinations of Tperpendicular to and T∥ are 5% and 8% respectively. We will present our findings on the overall range and limits of the observed temperature anisotropies. We find that 77% of the time T∥ > Tperpendicular to and we show for the first time with long term solar wind measurements that the maximum value of 1-Tperpendicular to /T_∥ is a decreasing function of the parallel plasma beta, β ∥ , for β ∥ > 1, as predicted by an analysis of the firehose instability (Gary, et al., JGR, 103, 14,567, 1998). We compare the limits predicted by the non-resonant and Vlasov resonant firehose instabilities and discuss which fits the observations better. In addition, we find that 19% of the time Tperpendicular to > T∥ . We show for the first time that over years of observation the maximum value of Tperpendicular to /T_∥ -1 is also a decreasing function of β ∥ and compare the observed limits with those predicted by linear plasma theory (Gary, et al., JGR, 102, 27,159, 1997) through the mirror and cyclotron instabilities.
Gary S.
Kasper Justin Christophe
Lazarus Andrew J.
Szabo Andras
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