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 * INTRODUCTION

Planetary encounters by numerous spacecraft launched by the USA and USSR have furnished information concerning the solar-wind interaction with the planets Mercury, Venus, Mars, and Jupiter. While direct measurements have indicated a wide range of atmospheric densities and intrinsic magnetic field strengths, the data seem to indicate that the flow pattern around nonmagnetized or weakly-magnetized planets with atmospheres optically thick at ionizing wavelengths (that is, with well-developed ionospheres above the planetary surface) is basically the same as that around a strongly-magnetized planet's magnetosphere, such as the Earth's. The planetary ionosphere apparently presents a hard obstacle to the flow, with bow—shock formation required in the supersonic, super—Alfvénic flow to slow and direct most of the solar—wind plasma around the planetary ionosphere. In this paper, various aspects of the interaction are examined in the context of theoretical models in an attempt to explain observed details of the interaction regions of Venus and Mars.


 * NATURE OF THE OBSTACLE—MECHANICS AND ELECTRODYNAMICS

In order to understand the nature of the obstacle presented to the flow by a planetary ionosphere and to be able to predict the details in the flow field around the planet, two basic boundary conditions must be considered. The first condition specifies the behavior of the magnetic field at the lower boundary of the planetary ionosphere, and the second specifies the penetrating solar-plasma flux into a defined upper boundary (ionopause) of the planetary ionosphere. As will be shown, determination of these conditions allows estimation of the parameters of the flow around the planet and of the dynamic behavior of the planetary ionosphere.

The first condition is that, in the absence of an intrinsic planetary magnetic field, the interplanetary magnetic field must become vanishingly small in the lower atmosphere below the ionosphere. This may be simply understood in terms of magnetic diffusion through the highly-conducting dayside ionosphere. The solar-wind flow around the sides of the planet outside the atmosphere carries interplanetary field lines back at a higher rate than the flow through the ionosphere can carry these same field lines (Dessler, 1968). The result is retardation of flux tubes in the ionosphere, with a given flux tube reaching the bottom of the ionosphere behind the segments of the same flux tube carried by the flow around the ionosphere.

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