Ulysses observations of differential alpha-proton streaming in the solar wind

Data from the solar wind spectrometer on the Ulysses spacecraft are used to study the differential streaming between the alpha particles and protons in the solar wind over the heliographic distance range of 1.3 to 5.4 AU and latitudes from 0 degrees to +/- 80 degrees during the period December 1990 through September 1995. The study is based on 6-hour averages of the parameter V-alpha p = \V-alpha - V-p\ where V-alpha and V-p are the vector velocities of the alpha particles and protons, respectively. it is found that V-alpha p decreases with increasing distance from the Sun and with decreasing solar wind speed. The distance and velocity dependencies can be combined into a single dependence on travel time T from the Sun to the point of observation, with V-alpha p declining, on the average, as T--0.70+/-0.07. After normalization by this travel time factor, there is no residual dependence of V-alpha p on heliographic latitude thus ruling out any rotational effects on either the acceleration or deceleration of the alphas relative to the protons. There is also no significant difference in the normalized values of V-alpha p between quasi-stationary and transient (coronal mass ejection) flows. The ratios V-alpha p/V-wave where V-A is the Alfven speed, and V-alpha p/V-wave, where V-wave is the observed propagation speed of Alfvenic fluctuations, both decline with increasing distance from the Sun, but V-alpha p/V-wave remains in the range of 1.0 to 1.5 out to a travel time of 5 or 10 days. There are weak correlations between the normalized value of V-alpha p and the amplitudes of fluctuations in both the magnitude and the direction of the interplanetary magnetic field. Although V-alpha p anticorrelates strongly with the ratio of the Coulomb collision time to the solar wind expansion time, it is believed that the correlation is not evidence of a cause and effect relation between those two parameters over much of the solar wind regime observed by Ulysses. Where comparisons are possible, the Ulysses data closely agree with extrapolations of the Helios data to greater solar distances.