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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A12, PAGES 29,435–29,450, 2001

Cusp and magnetopause locations in global MHD simulation

M. Palmroth

Finnish Meteorological Institute, Helsinki, Finland


P. Janhunen

Finnish Meteorological Institute, Helsinki, Finland


T. I. Pulkkinen

Finnish Meteorological Institute, Helsinki, Finland


W. K. Peterson

Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, Boulder, Colorado, USA


Abstract

We use the global MHD code Grand Unified Magnetosphere Ionosphere Coupling Simulation (GUMICS-4) to simulate the location and motion of the magnetospheric cusp and the subsolar magnetopause under various interplanetary magnetic field (IMF) directions and solar wind dynamic pressures. We identify the cusp in the simulation from the location of the open/closed field line boundary (OC boundary, equatorward edge of the cusp), determined by direct field line tracing, and several MHD parameters that are expected to react to the entry of the magnetosheath fluid. These parameters include the location of the maximum internal energy density and maximum diamagnetic depression and are collectively called the plasma proxies of the cusp. The simulation results show that with increasingly southward IMF the OC boundary and the plasma proxies move equatorward, as expected. During northward IMF, however, the plasma proxies are located equatorward of the OC boundary and thus on closed field lines. The GUMICS-4 OC boundary location and the statistical plasma cusp location from Polar satellite observations are in good quantitative agreement during southward IMF, but during northward IMF the observed cusp is again several degrees equatorward from the simulated OC boundary. Therefore we conclude that during northward IMF the cusp identification from either the OC boundary or the plasma proxies may become problematic in MHD simulations and discuss the possible physical reasons. The simulation results further indicate that increasing solar wind dynamic pressure shifts the high-altitude cusp slightly equatorward. Furthermore, the increasing pressure also pushes the magnetopause clearly earthward, as expected, while the same field line in the ionosphere shifts only slightly equatorward, implying changes in the field line shape as the pressure changes. In other respects, the simulated magnetopause is in quantitative agreement with the empirical model of Shue et al. [1998].

Received 28 May 2001; accepted 17 August 2001.

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Citation: Palmroth, M., P. Janhunen, T. I. Pulkkinen, and W. K. Peterson (2001), Cusp and magnetopause locations in global MHD simulation, J. Geophys. Res., 106(A12), 29,435–29,450.