HR: 09:15h
AN: SH41B-06    [Abstracts]
TI: Uncertainties of Solar Wind Speed Determination Using Voyager Energetic Particle Anisotropy Measurements at 85 AU
AU: * Zhang, M
EM: mzhang@fit.edu
AF: Department of Physics and Space Science, Florida Institute of Technology, 150 W. University Blvd., Melbourne, FL 32901 United States
AB: 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.
DE: 2104 Cosmic rays
DE: 2114 Energetic particles, heliospheric (7514)
DE: 2124 Heliopause and solar wind termination
DE: 2134 Interplanetary magnetic fields
DE: 2164 Solar wind plasma
SC: SPA-Solar and Heliospheric Physics [SH]
MN: 2004 AGU Fall Meeting