Dynamische Stabilisierung von Hochgeschwindigkeitswolken im galaktischen Halo
HI observations of high-velocity clouds (HVCs) indicate that they are interacting with their ambient medium. In this thesis, the question on the dynamical and thermal stabilization of a cold dense neutral cloud in a hot, thin, and magnetized ambient halo plasma is addressed. The results of two-dimensional plasma-neutral gas simulations showing the dynamical evolution of neutral gas clouds in a plasma flow are presented. The simulations show the formation of a comet-like head-tail structure combined with a magnetic barrier which exerts a stabilizing pressure on the cloud and hinders hot plasma from diffusing into the cloud. Furthermore, the magnetic barrier significantly reduces the heat conduction between the plasma and the neutral gas. It is demonstrated that a sufficiently large (but small compared to typical halo B-field strengths) component of the halo B-field perpendicular to the cloud's motion effectively stabilizes HVCs against disruption by Kelvin-Helmholtz- or Rayleigh-Taylor-instabilities and against evaporation by heat transfer for periods comparable with typical observed lifetimes of HVCs. The question on the stability of the clouds is addressed in terms of the stripping of cloud material, the temperature profile of the cloud, the dependence of the cloud's stability on the infall angle of the cloud, two-component clouds, and the cloud's morphology. The results are compared with observations of a compact HVC. In addition, the observed strong H/alpha fluxes from cloud edges can be explained by applying the critical velocity effect to the magnetic barrier. In a second part, the thesis addresses the phenomenon of cometary ion-tail disconnection events (DE) in terms of the theory of magnetic reconnection and by means of two-dimensional plasma-neutral gas simulations. The simulations show that either dayside magnetic diffusion and reconnection (when crossing the heliospheric current sheet) and nightside reconnection (without a switch in the magnetic polarity of the solar wind) lead to the disconnection of the plasma tail on the time scale of days, thus reproducing typical observed disconnection events.
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