Uncertainties of Solar Wind Speed Determination Using Voyager Energetic Particle Anisotropy Measurements at 85 AU

Details Uncertainties of Solar Wind Speed Determination Using Voyager Energetic Particle Anisotropy Measurements at 85 AU Uncertainties of Solar Wind Speed Determination Using Voyager Energetic Particle Anisotropy Measurements at 85 AU

Dec 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmsh41b..06z&link_type=abstract

American Geophysical Union, Fall Meeting 2004, abstract #SH41B-06

Physics

2104 Cosmic Rays, 2114 Energetic Particles, Heliospheric (7514), 2124 Heliopause And Solar Wind Termination, 2134 Interplanetary Magnetic Fields, 2164 Solar Wind Plasma

Scientific paper

Recent measurements from the LECP experiments on Voyager 1 show that low-energy particles have a very anisotropic pitch-angle distribution in the event of late 2002. In fact, the anisotropy is often so large that it almost appears as a beam of particles streaming along magnetic field lines. Given the large field-aligned pitch angle anisotropy, the Compton-Getting effect needs to be considered very carefully because an uncertainty of magnetic field direction can seriously affect the analysis. In this paper, we will present a calculation of Compton-Getting anisotropy in the presence of field-aligned beams using Lorentz transform of energetic particles with a power law energy spectrum. Our calculation shows that the anisotropy measurements from Voyager 1 can qualitatively fit either a model having a moderate solar wind speed with a Parker spiral magnetic field or a model with an approximately zero solar wind speed in a non-Parker field. The solar wind speed determination crucially depends on three assumptions (1) magnetic field direction, (2) cross-field diffusion anisotropy and (3) background from response to isotropic cosmic rays in the low-energy channels of the instrument. We will discuss how large uncertainties these assumptions may bring to the determination of solar wind speed.

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