Estimation of the Solar Wind Velocity at 85 AU from Voyager-1/LECP Energetic Ion Angular Distributions

Details Estimation of the Solar Wind Velocity at 85 AU from Voyager-1/LECP Energetic Ion Angular Distributions Estimation of the Solar Wind Velocity at 85 AU from Voyager-1/LECP Energetic Ion Angular Distributions

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsh11c1116r&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #SH11C-1116

Physics

2114 Energetic Particles, Heliospheric (7514), 2124 Heliopause And Solar Wind Termination

Scientific paper

Voyager-1 (V1) encountered a remarkable region of the heliosphere at 85 AU during the last half of 2002 (Krimigis, et al., this Conference). Because the plasma instrument on Voyager-1 is inoperative, we extract the solar wind velocity (V) by using its effect upon the intensity anisotropy measured by the Low Energy Charged Particle (LECP) instrument. The V1 LECP measurements indicated significant field-aligned anisotropies strong enough to invalidate the use of the linearized Compton-Getting transformation. Consequently, based on gyrotropic weak-scattering theory we assume an exponential distribution ˜ exp(α μ ) in pitch-cosine (μ ) that is convected with the solar wind. We transform it (non-linearly) into the spacecraft frame with a Galilean velocity transformation. We measure a power-law index for the energy spectrum k=1.5. We allow for weak coupling (backscatter) between hemispheres by setting the anisotropy parameter α =α + for the forward and α =α - for the backward hemisphere. The time-averaged LECP distributions consistently peak in spin sector 7 (the Sun splits sectors 1 and 8, the latter being blocked by a shield), so we assign the mean direction of the magnetic field to its center and normalize the intensities there. The normalized intensities in the remaining 6 sectors for each LECP channel are then fitted by a least-squares minimization that varies the remaining parameters (α +, α -, and V). Thus we extract the solar wind velocity from the LECP angular distributions. The best fits give V=0 over proton energies from 30 keV to 1 MeV, with energy-dependent uncertainties averaging ˜50 km/s. We cannot explain our observations using conventional diffusion-convection (strong-scattering) theory under the assumption that V1 did not leave the normal solar wind and magnetic field. The condition for diffusion-convection equilibrium with no radial streaming in the inertial frame implies a positive radial gradient with a source of particles beyond V1 in order to nullify the solar wind convection. Over the same time period, LECP observes a strong azimuthal anisotropy, which (if the average magnetic field is wound in a Parker sense), corresponds to gradient of increasing intensity as one moves inward along the field. This parallel streaming then implies a source of particles inside the radius of Voyager, but this is inconsistent with the positive radial gradient demanded by the radial transport equation. A quantitative analysis of the latter leads to a mean radial gradient ˜100%/AU with a comparable standard deviation. These are orders of magnitude bigger than gradients usually deduced for the outer heliosphere. We consider it unreasonable that such a configuration could endure there for half a year.

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