The basic physical processes that determine the solar wind will be studied, and global MHD simulation models will be developed.
Present theoretical models are based on fluid equations, where the heat flux is determined by lower moments (i.e. density, flow speed, and temperature). In the collision dominated transition region and inner corona one can use a ``classical'' expression for the heat flux, but protons and other ions become collisionless very close to the Sun and the classical heat flux is invalid .
The so-called gyrotropic equations seem to describe, quite well, both the collision dominated gas close to the Sun and the collisionless ions in the outer corona and in the solar wind. We propose to carry out parameter studies of multi-fluid gyrotropic models of the solar wind. The gyrotropic equations are valid in the limit of a very strong magnetic field, and are suitable for the flow of an ionized gas in a prescribed field. On a longer term the possibility of merging higher-order moment fluid equations with the equations for the magnetic field into an advanced MHD-like description will be studied. This MHD modeling will provide input on the energy balance in the corona-solar wind system and is crucial for the planned studies of the propagation and evolution of CME related MHD shocks and magnetic clouds through the solar wind (see below).
The solar wind outflow also presents a major challenge to numerical modeling since it is a fully three-dimensional, time-dependent physical environment, where regions of supersonic and subsonic speeds coexist in a tenuous, magnetized plasma. Zooming in on details of the wind structure, e.g. at the boundaries of open and closed field line regions or around the heliospheric current sheet, one needs to systematically study plasma dynamics in magnetized, shear flow regions. There has been significant progress in the numerical simulations of (i) stationary, transonic stellar winds ; (ii) MHD bow shocks around idealized obstacles ; (iii) multi-dimensional, magnetized shear layers . To get a global understanding of the physical processes responsible for coronal mass ejections and of the highly dynamic nature of the slow solar wind, a synergy of all these results is required. The flow properties of purely ideal MHD wind solutions will be analyzed in detail, but in a later stage heating and acceleration sources, derived from the above described gyrotropic and higher-order moment studies, will be added to the MHD model. The heliospheric current sheet will be resolved in the simulations using 3D adaptive grid techniques . This will lead to global solar wind models consisting of both stationary wind patterns and dynamic current-vortex sheet regions. These wind models must then demonstrate their stability against large-scale CMEs that involve shock fronts.
These MHD simulations of the ``quiet'' solar wind will be compared with observations of the structure of the heliospheric current sheet, with the main focus on understanding the different spatial scales involved, using ULYSSES and Wind observations. These simulations will also be linked to observations of corotating interaction regions between 1 and 5 AU, focusing on the development of the forward and reverse shocks at their leading and trailing edges. ULYSSES data from the in-ecliptic trajectory period will be extremely valuable in this respect .