Magnetic Fields and Currents at Mars and Venus.

Abstract:

Venus and Mars have no a global intrinsic magnetic field and solar wind interacts directly with their ionospheres inducing draping magnetospheres. Due to additional currents related to the Hall currents and a different dynamics of different ion species a draping on both planets occurs not symmetrical relative to the direction of the solar wind motional electric field. During solar maximum conditions, the ionosphere of Venus is almost decoupled from solar wind by the magnetic barrier formed by pile-up of the IMF. It is almost unmagnetized except of small-scale magnetic flux ropes. During solar minimum conditions on Venus and most of time on Mars, large-scale magnetic fields are observed deeply in the planetary ionospheres (magnetized ionospheres) raising a question about the origin of these fields. The problem is intimately related to the issue of electric current system and its closure. Mars Express, Venus Express and MAVEN spacecraft have provided us a wealth of in-situ observations of characteristics of induced magnetospheres of Mars and Venus at low altitudes during solar minimum conditions. Although solar wind is terminated at a certain distance from the planets by the magnetic barrier, the thermal ionospheric pressure is not able to balance there the external pressure and the upper ionosphere becomes to be driven into a slow convective motion implying an absence of a static equilibrium in the magnetic barrier. Such a flow carries the magnetic field and the ionosphere becomes magnetized. A static balance is maintained at lower altitudes (≤ 200 km) where the collisional drag force arising from a relative motion between plasma and neutrals balances the external forces. At such altitudes the Hall currents become dominant and produce very asymmetrical distribution of the ionospheric magnetic fields. Structure of the near planet tail also occurs very asymmetrical due to these currents and asymmetrical motion of different ion species. The observations also show that the electric currents which support the magnetic field configuration in the tail are mainly the inertia currents rather than the pressure gradient currents. The 3-D hybrid simulations of the solar wind interaction with Mars and Venus provide us an additional powerful tool for a better insight into the current system of the induced magnetospheres.