NUMERICAL-SIMULATION OF FILLING A MAGNETIC-FLUX TUBE WITH A COLD-PLASMA - ANOMALOUS PLASMA EFFECTS

Large-scale models of plasmaspheric refilling have revealed that durin g the early stage of the refilling counterstreaming ion beams are a co mmon feature. However, the instability of such ion beams and its effec t on refilling remain unexplored. The difficulty with investigating th e effect of ion-beam-driven instability on refilling is that the insta bility and the associated processes are so small-scale that they canno t be resolved in large-scale models. Typically, the instabilities have scale lengths of a few tens of plasma Debye length, which is a few me ters at the most, and the spatial resolution in large-scale models is at least several tens of kilometers. Correspondingly, the temporal sca le of the instability is by several orders of magnitude smaller than t he temporal resolution afforded by the models. In order to learn the b asic effects of ion beam instabilities on refilling, we have performed numerical simulations of the refilling of an artificial magnetic flux tube. The shape and size of the tube are assumed so that the essentia l features of the refilling problem are kept in the simulation and at the same time the small-scale processes driven by the ion beams are su fficiently resolved. We have also studied the effect of commonly found equatorially trapped warm and/or hot plasma on the filling of a flux tube with a cold plasma. When the warm and/or hot plasma consists of a nisotropic ions and isotropic electrons, the potential barrier set up by this plasma has a drastic effect on the flow of the cold ion beams, and hence on the filling. Three types of simulation runs have been pe rformed. In run 1 we have only a cold plasma and we treat ions kinetic ally and electrons are assumed to obey the Boltzmann law. In run 2 bot h electrons and ions in the cold plasma are treated kinetically. Run 3 is similar to run 2, but it includes an equatorially trapped plasma p opulation as mentioned above. A comparison between the results from ru n 1 and run 2 reveals that in the latter type of simulation electron-i on (e-i) and ion-ion (i-i) instabilities occur and significantly modif y the evolution of the plasma density distributions in the flux tube a long with the total plasma content. When the electron dynamics is simp lified by the assumption of the Boltzmann law, both the electron-ion a nd ion-ion instabilities are inhibited. On the other hand, when electr ons are treated kinetically, the e-i instability occurs at an early st age when ion beams are too fast to excite the i-i instability. The for mer instability heats the electrons so that conditions for the latter instability are eventually met. The i-i instability and its nonlinear evolution creates potential structures which significantly modify the filling process. In run 2 filling is enhanced over run 1 due to the tr apping of the plasma in the potential structures. Run 3 with the equat orially trapped plasma consisting of hot anisotropic ions and warm iso tropic electrons shows that the difference in the thermal anisotropy b etween electrons and ions generates electrostatic shocks in the flow o f the initially fast ion beams. The propagating shocks yield extended potential structure in the flux tube. The trapping of plasma in the po tential structure further enhances the filling in run 3 over that seen in run 2.