Center for Space Research, Massachusetts Institute of Technology, Cambridge

Alpha particle and proton measurements in the solar wind made using the SWE Faraday cup detectors on the WIND spacecraft are reported. Some overall trends observed confirm past observations: the ratios of alpha particle to proton density N_a /N_R , thermal speed W_a /W_R , as well as the differential velocity V_a - V_R (hereafter V_aR ) are generally correlated with bulk solar wind flow speed. The detailed WIND measurements enable us to investigate instances when the alpha-proton differences deviate from these overall general trends. Occasionally, difference velocities as large as 80 km/s were seen, with the ratio of | V_aR | to the Alfvén speed V_A near unity, characteristics more typical of observations at solar distances less than 1 AU. An example is presented where |V_a | - |V_R | reverses sign while | V_aR | stays nearly constant. Comparison of the vector velocities and the magnetic field suggests that the speed reversal is associated with a localized kink in the magnetic field. Finally we show an instance where | V_aR | exceeds the observed wave speed for Alfvénic fluctuations (V_wave=B₀ D V/D B) resulting in alpha particle velocity fluctuations that anti-correlate with the wave. Through this phenomenon has been previously reported in high latitude measurements beyond 1 AU, it is shown here to also occur at 1 AU in the ecliptic.

Geophys. Res. Lett., 23,1183-1186, 1996