A variable cross-section model of the bow shock of Venus

Magnetohydrodynamics (MHD) parameters like the Alfvenic and the sonic Mach numbers and the direction of the interplanetary magnetic field profoundly affect the interaction of the solar wind with nonmagnetized conducting objects like Venus. The size of the bow shock depends on the two Mach numbers, whereas asymmetries in its shape are governed by the direction of the magnetic field. This paper introduces a new class of bow shock models in which both the shape and the size are controlled by the upstream plasma and field conditions. We use insights from the MHD theory of shocks for point objects and empirical information from actual bow shock crossings to obtain a semiempirical, semitheoretical model of the Cytherean bow shock. The model was developed from a limited data set obtained from the Galileo flyby of Venus but is also in substantial agreement with Pioneer Venus Orbiter observations. It is shown that the dozen bow shock crossing observed by Galileo under steady conditions of solar wind flow and density were caused by changes in the cross section of the bow shock induced by the changing direction of the interplanetary magnetic field.