Pekka Janhunen

HLLC solver for ideal relativistic MHD

An approximate Riemann solver of Godunov type for ideal relativistic magnetohydrodynamic equations (RMHD) named as HLLC (C denotes contact) is developed. In HLLC the Riemann fan is approximated by two intermediate states, which are separated by the entropy wave. Numerical tests show that HLLC resolves contact discontinuity more accurately than the Harten-Lax-van Leer (HLL) method and an isolated contact discontinuity exactly.

New interpretation of magnetospheric energy circulation

[1]xa0New results from the global MHD simulation GUMICS-4 show that the energy transfer rate from the solar wind into the magnetosphere is a function of also the past values of the IMF. Furthermore, comparison of the energy input rate with the conversion and dissipation rates in the magnetosphere and ionosphere shows that the energy entering the magnetosphere is processed by the system without delay. This and earlier observations lead us to suggest that the role of energy input during the substorm growth phase is to facilitate the configuration change rather than to store energy for later dissipation. While the change of magnetotail stability requires configuration change, the substorm energetics after the onset is quite directly driven by the solar wind energy input.

Hysteresis in solar wind power input to the magnetosphere

[1]xa0We examine the clock angle dependence of the solar wind power input into the magnetosphere using the Grand Unified Magnetosphere Ionosphere Coupling Simulation (GUMICS-4). We compute the power input through the magnetopause systematically in four events where the solar wind parameters are otherwise kept constant, but the interplanetary magnetic field (IMF) clock angle θ is rotated from 0° to 360°. We find that when the clock angle rotates from north to south, the power input is well represented by sin2(θ/2). However, during the return rotation from south to north, the power input in the simulation remains enhanced longer than a sin2(θ/2) dependence would indicate. We suggest that the enhanced energy input during the return rotation may be a consequence of the nature of the reconnection process at the magnetopause.