J Smith, J T Steinberg,A J Lazarus
(MIT Center for Space Research,37-680,77 Massachusetts Ave.
Cambridge, MA 02139)
Two periods of alpha particle density enhancement were observed with the SWE instrument on the WIND spacecraft during the Oct. 18-20, 1995, magnetic cloud event. The ratio of alpha particle to proton number density reached 14% in both enhancements. During the first enhancement at the front of magnetic cloud (Oct. 18, 1900 UT to Oct. 19, 0742 UT), we find alpha-proton differential streaming as large as 40 km/s, in contrast to previous work [Borrini et al., 1982] in which local minima in differential flows were observed to accompany enhancement events. The second alpha enhancement, towards the rear of the cloud (Oct. 19, 1912 UT to 2218 UT), is accompanied by little or no differential streaming. Unusually high plasma densities of ~60 cm-3 during the second enhancement and a smaller alpha/proton thermal speed ratio suggest that Coulomb collisions restricted differential flow. We will discuss in further detail the variations of plasma characteristics across the cloud.
Borrini, G., J. T. Gosling, S. J. Bame, W. C. Feldman, Helium Abundance Enhancements in the Solar Wind, J. Geophys. Res., 87, 7370-7378, 1982.
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