HR: 0830h
AN: SH11C-1116    [PDF]
TI: Estimation of the Solar Wind Velocity at 85 AU from Voyager-1/LECP Energetic Ion Angular Distributions
AU: * Roelof, E C
EM: edmond.roelof@jhuapl.edu
AF: Johns Hopkins U./Applied Physics Lab., 11100 Johns Hopkins Rd., Laurel, MD 20723 United States
AU: Decker, R B
EM: robert.decker@jhuapl.edu
AF: Johns Hopkins U./Applied Physics Lab., 11100 Johns Hopkins Rd., Laurel, MD 20723 United States
AU: Krimigis, S M
EM: tom.krimigis@jhuapl.edu
AF: Johns Hopkins U./Applied Physics Lab., 11100 Johns Hopkins Rd., Laurel, MD 20723 United States
AB: Voyager-1 (V1) encountered a remarkable region of the heliosphere at 85 AU during the last half of 2002 (Krimigis, {\it 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 $\sim$ exp($\alpha$$\mu$) in pitch-cosine ($\mu$) that is convected with the solar wind. We transform it (non-linearly) into the spacecraft frame with a Galilean velocity transformation. We measure a power-law index for the energy spectrum k=1.5. We allow for weak coupling (backscatter) between hemispheres by setting the anisotropy parameter $\alpha$=$\alpha$$_{+}$ for the forward and $\alpha$=$\alpha$$_{-}$ for the backward hemisphere. The time-averaged LECP distributions consistently peak in spin sector 7 (the Sun splits sectors 1 and 8, the latter being blocked by a shield), so we assign the mean direction of the magnetic field to its center and normalize the intensities there. The normalized intensities in the remaining 6 sectors for each LECP channel are then fitted by a least-squares minimization that varies the remaining parameters ($\alpha$$_{+}$, $\alpha$$_{-}$, 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 $\sim$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 $\sim$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.
DE: 2114 Energetic particles, heliospheric (7514)
DE: 2124 Heliopause and solar wind termination
SC: SPA - Solar and Heliospheric Physics [SH]
MN: 2003 Fall Meeting