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The remanent magnetisation of meteorite material in the solar system indicates that magnetic fields of several Gauss are present in the protoplanetary disk. It is shown that such relativelystrong magnetic fields can be generated in dusty protoplanetary disks by relative shear motions of the charged dust and the neutral gas components. Self-consistent multi-fluid simulations show that for typical plasma parameters shear flows with collisional momentum transfer between the different components of the dusty plasma result in a very efficient generation of magnetic fields with strengths of about 0.1-1 Gauss on spatial scales of two astronomical units in about one year. Based on the simulations one may predict that future measurements will reveal the strong magnetisation of circumstellar disks around young stellar objects., Comment: to be published in 2003

The process of current filamentation in permanently externally driven, initially globally ideal plasmas is investigated by means of two-dimensional Magnetohydrodynamic (MHD)-simulations. This situation is typical for astrophysical systems like jets, the interstellar and intergalactic medium where the dynamics is dominated by external forces. Two different cases are studied. In one case, the system is ideal permanently and dissipative processes are excluded. In the second case, a system with a current density dependent resistivity is considered. This resistivity is switched on self-consistently in current filaments and allows for local dissipation due to magnetic reconnection. Thus one finds tearing of current filaments and, besides, merging of filaments due to coalescence instabilities. Energy input and dissipation finally balance each other and the system reaches a state of constant magnetic energy in time., Comment: 32 Pages, 13 Figures. accepted, to appear in Physics of Plasmas (049012)

Observations show significant H\alpha-emissions in the Galactic halo near the edges of cold gas clouds of the Magellanic Stream. The source for the ionization of the cold gas is still a widely open question. In our paper we discuss the critical velocity effect as a possible explanation for the observed H\alpha-emission. The critical velocity effect can yield a fast ionization of cold gas if this neutral gas passes through a magnetized plasma under suitable conditions. We show that for parameters that are typical for the Magellanic Stream the critical velocity effect has to be considered as a possible ionization source of high relevance., Comment: 9 pages, 2 figures. accepted, to appear in The Astrophysical Journal