Proton and Alpha Particle Temperature Anisotropies in the Solar Wind with the WIND/SWE Faraday Cup

Details Proton and Alpha Particle Temperature Anisotropies in the Solar Wind with the WIND/SWE Faraday Cup Proton and Alpha Particle Temperature Anisotropies in the Solar Wind with the WIND/SWE Faraday Cup

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sh31a11t&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #SH31A-11

Physics

0654 Plasmas, 2164 Solar Wind Plasma, 7524 Magnetic Fields, 7871 Waves And Instabilities

Scientific paper

Under the assumption that ion distributions in the solar wind take the form of a field aligned, convected bi-Maxwellian, it is possible to measure proton and alpha particle temperature anisotropies. Such anisotropies are seen in solar wind ion spectra collected by the Wind Faraday cup instrument. We have processed ion data for the entire Wind mission to date (1994-2001) using two distinct techniques. Each Faraday cup records the distribution of ions along 18 sunward-facing angles. The first method uses the moments of the proton distribution along these angles and concurrent magnetic field data to determine an effective thermal speed as a function of angle. As we will show, the form of the effective thermal speed yields the temperatures parallel and perpendicular to the magnetic field. The second method employs a non-linear chi-sqares minimization routine which folds a model bi-Maxwellian and the instrument response to determine the best fit to the measured spectrum. This second method is also applied to the alpha particles. We present a quick derivation of the response of a Faraday cup to a bi-Maxwellian distribution, discuss how well this model describes solar wind ions, and compare the results of our two techniques. The parallel and perpendicular thermal speeds as calculated by the two methods show a high level of agreement except in cases of proton double streaming along the direction of the magnetic field line. In this situation, the moment-angle method shows an artificially large value for the thermal speed in the direction parallel to the magnetic field. We will summarize the observed proton and alpha particle temperaures over the course of the mission and discuss variations in anisotropy on a range of timescales.

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