Magnetosheath Struktures

The interaction of the solar wind with (magnetized) obstacles leads to the formation of magnetospheres. Adjacent to the magnetosphere in the sunward direction the magnetosheath develops, a region where the magnetic field strength is enhanced. The main objective of the present project is to study the characteristic features of the magnetosheath and comprises both theoretical modelling (in collaboration with Krasnoyarsk) and data analysis (with New Hampshire). The study of the solar wind flow around a truncated obstacle requires the solution of the set of magnetohydrodynamic equations, which is a set of coupled, non-inear, partial differential equations. This problem can not be solved in general, so that we use the following approach: First, we introduce a double Lagrangian coordinate system, where both plasma stream and magnetic field lines form coordinate lines. Second, the actual region, where the magnetic field is so strong that it must consistently be taken into account, is very thin, so that it can be treated as a thin boundary layer. The different goals of the project are: 1. The terrestrial magnetosheath, where plentiful spacecraft data exist, will be analysed in the context of theoretical modelling 2. The model will be generalized for plasma anisotropy 3. Magnetic clouds will be studied in the frame of the magnetic barrier theory 4. Time-dependent phenomenona will be taken into account 5. The magnetosheath of Venus will be examined, where an old standing discrepancy between data and theoretical modelling should be clarified. We mention that the modelling of physical patterns - the motion of an obstacle through a plasma - is of general plasma physical interest.

The interaction of the solar wind with the Earth`s magnetic field leads to the formation of the so-called magnetosphere, i.e., a cavity filled with hot and dilute plasma, surrounded by the cold and dense solar wind. Since the solar wind is known to be a supersonic flow, a bow shock evolves as the solar wind faces the planetary obstacle Earth. At this discontinuity, the physical parameteres of the solar wind such as density, velocity, temperature, and pressure as well as the magnetic field strength change from upstream to downstream. As a consequence, the decelerated solar wind streams around the magnetosphere, thereby forming the so-called magnetosheath. In the frame of the project the evolution of the physical parameters across the bow shock have been modelled and their characteristic properties in the magnetosheath have been determined and compared with satellite data. A crucial point in this task is the pressure anisotropy, i.e., the pressure parallel with respect to the magnetic field is different to the perpendicular pressure. Theoretical models have been developed for the bow shock and the magnetosheath taking into account the anisotropic nature of the solar wind. It clearly turns out that the performed calculations compare well with satellite observations (e.g., of the American WIND spacecraft). The developed techniques concerning the modelling of the terrestrial magnetosheath have also been applied to planet Venus and to so-called magnetic clouds, which are plasma structures in the solar wind of enhanced magnetic field. These results have been compared to spacecraft data. In the case of Venus it turns out that the inclusion of the magnetic field influence in a consistent way, leads to a much better comparison with data than former models were able to achieve. Subsequent studies deal with time dependent aspects of the magnetosheath, due to time variations in the interplanetary magnetic field and/or pulses of reconnection (transformation of magnetic field energy into plasma energy). We note that in addition to the former defined project aims we broadened our research topic to relevant fields of interest in space physics, e.g., plasma instabilities and acceleration processes in planetary magnetospheres. This work is a useful appendix to the above mentioned work, since it turned out that on the one hand it is of physical importance, and on the other, it can be treated with already developed methods. The studies have been carried out within an international coorporation with Russia, USA, and other countries, were presented at international conferences, and were published in the world-wide most relevant journals in geophysics and plasma physics.