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Ann. Geophys., 24, 2057-2063, 2006
www.ann-geophys.net/24/2057/2006/
© European Geosciences Union 2006
Limits on the core temperature anisotropy of solar wind protons
E. Marsch1, L. Zhao2, and C.-Y. Tu3
1Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany
2Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, Michigan, USA
3Department of Geophysics, Peking University, Beijing, China
Abstract. We analyse the temperature anisotropy of the protons in the solar wind and
thereby concentrate on plasma data obtained in the year 1976 of the Helios 1
and Helios 2 missions. We derive the core proton temperatures  and
 , in the directions perpendicular and parallel to the magnetic
field, as well as the core parallel plasma beta,
 .
The data are separately analysed for two distance ranges,  AU and
 AU, and divided into 24 bins for the plasma beta, in the range from
 to
 , and into 72 bins for the
total temperature anisotropy,
 , which is here
considered in the range from −0.9 to 0.9. The number of spectra in each bin
is determined to obtain distributions. The statistical results are
presented in two-dimensional histograms. For each column we define a critical
upper and lower limit of the anisotropy. The resulting empirical points are
compared with the known theoretical instability thresholds. The protons are
found to come with increasing radial distance closer to the fire-hose instability
threshold.
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