The Compton-Getting effect of energetic particles with an anisotropic pitch-angle distribution:: An application to Voyager 1 results at ∼85 AU

This paper provides a theoretical simulation of anisotropy measurements by the Low-Energy Charged Particle (LECP) experiment on Voyager. The model starts with an anisotropic pitch-angle distribution function in the solar wind plasma reference frame. It includes the effects of both Compton-Getting anisotropy and a perpendicular diffusion anisotropy that possibly exists in the upstream region of the termination shock. The calculation is directly applied to the measurements during the late 2002 particle event seen by Voyager 1. It is shown that the data cannot rule out either the model with zero solar wind speed or the one with a finite speed on a qualitative basis. The determination of solar wind speed using the Compton-Getting effect is complicated by the presence of a large pitch-angle distribution anisotropy and a possible diffusion anisotropy. In most high-energy channels of the LECP instrument, because the pitch-angle distribution anisotropy is so large, a small uncertainty in the magnetic field direction can produce very different solar wind speeds ranging from 0 to > 400 km s(-1). In fact, if the magnetic field is chosen to be in the Parker spiral direction, which is consistent with the magnetometer measurement on Voyager 1, the derived solar wind speed is still close to the supersonic value. Given the uncertainty of the magnetic field direction, only the two lowest energy channels of the LECP instrument can give a definitive result for the solar wind speed. However, these channels contain very high levels of background from their response to isotropic cosmic rays. An uncertainty of just a few percent in the background level can entirely hamper the estimate of solar wind speed.