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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 24, issue 7
Ann. Geophys., 24, 2057-2063, 2006
https://doi.org/10.5194/angeo-24-2057-2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 24, 2057-2063, 2006
https://doi.org/10.5194/angeo-24-2057-2006
© Author(s) 2006. This work is distributed under
the Creative Commons Attribution 3.0 License.

  09 Aug 2006

09 Aug 2006

Limits on the core temperature anisotropy of solar wind protons

E. Marsch1, L. Zhao2, and C.-Y. Tu3 E. Marsch et al.
  • 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 $T_perp$ and $T_parallel$, in the directions perpendicular and parallel to the magnetic field, as well as the core parallel plasma beta, $beta_parallel$. The data are separately analysed for two distance ranges, $R{leq}0.4$AU and $R{>}0.4$AU, and divided into 24 bins for the plasma beta, in the range from $beta_parallel {=} 0.1$ to $beta_parallel {=}10$, and into 72 bins for the total temperature anisotropy, $A{=}1{-}T_perp/T_parallel$, 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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