Estimation of Plasma Flow Speeds in the Heliosheath using Voyager-1/LECP

Details Estimation of Plasma Flow Speeds in the Heliosheath using Voyager-1/LECP Estimation of Plasma Flow Speeds in the Heliosheath using Voyager-1/LECP

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufmsh51a1185r&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #SH51A-1185

Physics

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

Scientific paper

We have developed and validated an improved algorithm for extracting plasma flow (ExB) velocities from the scan-plane anisotropies measured by the Low Energy Charged Particle Experiment (LECP) on Voyager-1. We found that the beam-like anisotropies upstream of the termination shock were usually well fit by a Fourier expansion up to and including the second harmonic. Contrary to a common misconception, the second harmonic is not always a "bi-directional" anisotropy. If it is in phase with (rather than out of phase with) the first harmonic, it produces a unidirectional (or beam) anisotropy that is stronger than a simple sinusoidal function. We therefore assume that the ion anisotropy in the plasma frame is described by a second-degree expansion in Legendre polynomials of the pitch cosine with velocity-dependent coefficients. This distribution is then Lorentz-transformed into the spacecraft frame and convolved with the angular response function for the LECP. The resulting equations that relate the count rates in the eight LECP angular sectors can be solved in a least-squares sense for the spherical harmonic coefficients, a procedure that takes into account that one of the eight LECP sectors is blocked. These coefficients are non-linear functions of the plasma flow velocity (V), the velocity (v) of the ions in a given LECP energy channel, and the direction angles of the magnetic field. However, if we assume that the dependence on (V/v) is quadratic, and we make use of the magnetic field directions provided by the VGR-1 magnetometer team, we can then develop an analytic expression for the flow velocity in terms of the LECP sector rates. This is the method utilized by Krimigis et al. (Science, 2005) in their estimates of the heliosheath plasma flow beyond the heliospheric termination shock. We report here on the validation of this technique using computer simulations of the LECP sector response to a gyrotropic ion distribution advected by the plasma flow. For a specified set of spherical harmonic coefficients in the plasma frame (including their velocity dependence), we calculated the seven LECP sector rates. Then, using these as input "data" to our algorithm, we extracted the flow velocity by the procedure described above. We found that the extracted flow velocity agreed with the input velocity to better than 10% for flow velocities of 200 km/s or less. Since the observed downstream anisotropies were weak (well-described by a two-harmonic expansion), and the heliosheath velocities actually extracted were <200 km/s, we consider that our technique produces valid estimates of the plasma flow (ExB) velocity.

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