Comparison of Birkeland current observations during two magnetic cloud events with MHD simulations

摘要

Low altitude field-aligned current densities obtained from globalmagnetospheric simulations are compared with two-dimensionaldistributions of Birkeland currents at the topside ionospherederived from magnetic field observations by the constellation ofIridium satellites. We present the analysis of two magnetic cloudevents, 17–19 August 2003 and 19–21 March 2001, where theinterplanetary magnetic field (IMF) rotates slowly(~10°/h) to avoid time-aliasing in the magneticperturbations used to calculate the Birkeland currents. In theAugust 2003 event the IMF rotates from southward to northward whilemaintaining a negative IMF during much of theinterval. During the March 2001 event the IMF direction varies fromdawnward to southward to duskward. We find that the distributions ofthe Birkeland current densities in the simulations agreequalitatively with the observations for northward IMF. For southwardIMF, the dayside Region-1 currents are reproduced in the simulation,but appear on average 5° further poleward of their observedlocation, while the nightside Region-1 currents and the Region-2currents are largely under-represented. Comparison of the observedand simulated Birkeland current distributions, which are intimatelyrelated to the plasma drifts at the ionosphere, shows that theionospheric convection pattern in the MHD model and its dependenceon the IMF orientation is essentially correct. The Birkeland totalcurrents in the simulations are about a factor of 2 larger thanobserved during southward IMF. For >0 thedisparity in the total current is reduced and the simulations forpurely northward IMF agree with the observations to within 10%. Thedisparities in the magnitudes of the Birkeland currents between theobservations and the simulation results are a combined effect of thesimulation overestimating the ionospheric electric field and of theIridium fits underestimating the magnetic perturbations. Themarginal difference in the latitude resolution of the ionosphericgrids in the simulations and the observations is shown to have onlysecondary effect on the magnitudes of the Birkeland currents. Theelectric potentials in the simulation for southward IMF periods aretwice as large as those obtained from measurements of the plasmadrift velocities by DMSP, implying that the reconnection rates inthe simulation are too large.