Numerical study on physics on physics of auroral pattern formation and plasma elementary processes

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(1) Our theoretical study on the characteristic pattern of auroras projected on the ionosphere has elucidated fundamental plasma physics for natural phenomena in the magnetosphere/ionosphere system. First, the processes of charge separation in the magnetosphere have been identified as being responsible for the auroral traveling surge, the omega bands, and the ripples. (The causes of these auroral phenomena had long been beyond understanding of theoretical researchers.) Our approach is unique : by using particle simulations we reproduce the temporal/spatial development of auroras on the ionosphere which is in good agreement with the observations. We further apply the simulation-based analysis to the problem of quasi-steady field-aligned currents. Similarly to the case of auroral dynamics, we successfully reproduced the observatioanlly identified pattern of region 1 and region 2 field-aligned current by the two dimensional simulations. This result leads naturally to a new concept that the paired region 1/region2 field-aligned currents can be produced from the plasma distortion in the closed magnetosphere by the solar wind externally applied in the polar cap region.(2) Using the d magnetic field model by Tsyganenko, we numerically obtained the distributions of magnetic drift vectors in the magnetically closed region where the effect of the interplanetary magnetic field is relatively small. From this plot we found that the field-aligned currents in the low-latitude boundary layr (LLBL) is likely to be caused by the pressure gradient of a plasma injected from the magnetosheath. (Paper will be presented at SGEPSS,in March 1995.) We will also argue that some features of the LLBL current system, which have been revealed by recent satellite observations, could not be understood in a model of the viscous interaction between the magnetosphere and the solar wind which had long been believed to be a most likely mechanism for the LLBL current generation.