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Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 01/05 Podcast
Dynamische Stabilisierung von Hochgeschwindigkeitswolken im galaktischen Halo

Dynamische Stabilisierung von Hochgeschwindigkeitswolken im galaktischen Halo

Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 01/05 vor 23 Jahren

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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.