Interaction of Mercury with the solar wind

We present the structure of the hermean magnetosphere obtained by a global three-dimensional MHD simulation. The magnetic field of Mercury is strong enough to form a permanent magnetosphere under typical solar wind conditions. Mercury does not have a substantial atmosphere or ionosphere which makes the magnetosphere unique in the solar system. We study in detail the hermean magnetosphere for the typical solar wind parameters at perihelion and the nominal Parker spiral interplanetary magnetic field (IMF). Although the magnetosphere of Mercury is qualitatively similar to Earth's magnetosphere, its much smaller size results in many quantitative differences. For example, the magnetic field lines of Mercury are closed only for latitudes less than approximate to 50 degrees, while for Earth the similar latitude will be approximate to 75 degrees. We find that a direct interaction of the solar wind with the surface of the planet and the formation of a "bald" subsolar spot become possible if the solar wind speed is increased by a factor of 2.5 of if density is increased by a factor of 9 as compared with the nominal values. We present the results of our simulation for this case as well, although our conclusion is that direct interaction of Mercury with the solar wind is a rare phenomenon. (C) 2000 Academic Press.