E. Mobius et al., Direct evidence of the interstellar gas flow velocity in the pickup ion cut-off as observed with SOHOCELIAS CTOF, GEOPHYS R L, 26(20), 1999, pp. 3181-3184
He+ pickup ions as observed with SOHO CELIAS CTOF have been analyzed for th
e time period DOY 160 - 190, 1996. During this time of the year the Earth i
s on the upwind side of the interstellar gas flow with respect to the sun.
The high-speed cut-off in the frame of the sun is significantly higher v/V-
sw = 2, predicted for pickup ions. The difference increases with lower sola
r wind speeds. This behavior is interpreted as an effect of the local inter
stellar gas flow velocity (inflow at large distances including gravitationa
l acceleration by the sun) on the pickup ion distribution. The neutral velo
city is added to the solar wind velocity in the determination of the pickup
ion cut-off on the upwind side and subtracted on the downwind side of the
gas flow. This new observation will provide a valuable tool to determine th
e interstellar gas flow and will thus complement direct neutral gas measure
ments.