COMPARISON OF PIONEER VENUS AND VENERA BOW SHOCK OBSERVATIONS - EVIDENCE FOR A SOLAR-CYCLE VARIATION

Observations by the Venera 9 and 10 orbiters in 1975‐76 have been used in previous studies to determine the mean location and shape of the Cytherean bow shock. In addition it has also been reported that the shock is found to be more distant from the planet above regions of the ionosheath where draped IMF field lines are oriented perpendicular to the flow as opposed to parallel. An examination of the dependence of shock altitude in the terminator plane on upstream IMF direction using 86 Pioneer Venus orbiter bow shock crossings in 1978‐79 sets an upper limit on this asymmetry of 12% or approximately half that derived earlier from the Venera data. More significantlythe mean distance to the bow shock observed by Pioneer Venus Orbiter is 35% greater than was the case in 1975‐76 near solar minimum. As the growth in effective obstacle radius is an order of magnitude larger than can be accounted for in terms of varying ionopause altitude due to all causesthese results strongly suggest that Venus can absorb significantly more of the incident solar wind plasma during solar minimum when EUV flux is low than during the current epoch in which maximum is approaching. Copyright 1979 by the American Geophysical Union.