spacecraft have been analyzed to determine the temperature anisotropy of each ... study, comparing the observed temperature anisotropies to various limits ...
Solar Wind Temperature Anisotropies J. C. Kasper1, A. J. Lazarus1, S. P. Gary2, A. Szabo3 1
MIT/CSR,77 Massachusetts Avenue, Cambridge, USA, 2LANL/NIS-1, M.S. D466, Los Alamos, USA, 3 NASA/GSFC, Code 696, Greenbelt, USA
Abstract. Solar wind proton and alpha spectra from the Faraday Cup portion of the SWE experiment on the Wind spacecraft have been analyzed to determine the temperature anisotropy of each species, under the assumption of convected, bi-Maxwellian distributions. From the start of the mission in late 1994 to date we have collected over 2 million measurements of the anisotropies in the solar wind. This dataset is sufficiently large to conduct a statistical study, comparing the observed temperature anisotropies to various limits imposed by instabilities. Specifically we will discuss the effects of the firehose, mirror, and cyclotron instabilities. With a limit to the proton temperature anisotropy established, we examine several cases where this limit is approached or exceeded and comment on magnetic field activity and alpha parameters during these intervals. In the large plasma beta regime we illustrate evidence of a transition from the cyclotron to the mirror instability as the dominant limit.
INTRODUCTION The first-order departure from a simple Maxwellian velocity distribution function (VDF) of a particle species in the solar wind is due to the existence of a temperature anisotropy. Generally this anisotropy is well described by a convected, field-aligned, biMaxwellian VDF, with two temperatures, T⊥ and T|| perpendicular and parallel to the ambient magnetic field Bo. The solar wind is a collisionless plasma, and one might expect it to satisfy the double adiabatic equations of state, 2 d æ T⊥ ö d æç T|| B ö ç ÷ = 0; ç 2 = 0 dt è B dt è n
(1)
In this case an initially isotropic parcel of solar wind would be expected to develop a temperature anisotropy, R,
R = T⊥ T|| − 1
(2)
which might vary by orders of magnitude. While a radial evolution of the proton anisotropy has been observed [1], generally |R| ≤ unity.
FIGURE 1. Solar wind observations by the Wind spacecraft on April 30, 1997. The alpha particle abundance is very low and may be neglected. Note that the predicted value of T|| is very high and off the scale of the plot for 0500-1500 UT.
Consider the solar wind observations on April 30, 1997 by the Wind spacecraft which are summarized in Figure 1. There was relatively little activity in the first
15 hours of the day; in general the density was increasing while the magnetic field strength decreased. The observed parallel and perpendicular temperatures
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference, edited by M. Velli, R. Bruno, and F. Malara © 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
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a range of values. For R0 mirror [2] and cyclotron [3] instabilities may arise. These growing modes have real frequency ωr with ωr~Ωp for the cyclotron instability and 0
