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Low altitude field-aligned current densities obtained from global\udmagnetospheric simulations are compared with two-dimensional\uddistributions of Birkeland currents at the topside ionosphere\udderived from magnetic field observations by the constellation of\udIridium satellites. We present the analysis of two magnetic cloud\udevents, 17–19 August 2003 and 19–21 March 2001, where the\udinterplanetary magnetic field (IMF) rotates slowly\ud(~10°/h) to avoid time-aliasing in the magnetic\udperturbations used to calculate the Birkeland currents. In the\udAugust 2003 event the IMF rotates from southward to northward while\udmaintaining a negative IMF By during much of the\udinterval. During the March 2001 event the IMF direction varies from\uddawnward to southward to duskward. We find that the distributions of\udthe Birkeland current densities in the simulations agree\udqualitatively with the observations for northward IMF. For southward\udIMF, the dayside Region-1 currents are reproduced in the simulation,\udbut appear on average 5° further poleward of their observed\udlocation, while the nightside Region-1 currents and the Region-2\udcurrents are largely under-represented. Comparison of the observed\udand simulated Birkeland current distributions, which are intimately\udrelated to the plasma drifts at the ionosphere, shows that the\udionospheric convection pattern in the MHD model and its dependence\udon the IMF orientation is essentially correct. The Birkeland total\udcurrents in the simulations are about a factor of 2 larger than\udobserved during southward IMF. For Bz>0 the\uddisparity in the total current is reduced and the simulations for\udpurely northward IMF agree with the observations to within 10%. The\uddisparities in the magnitudes of the Birkeland currents between the\udobservations and the simulation results are a combined effect of the\udsimulation overestimating the ionospheric electric field and of the\udIridium fits underestimating the magnetic perturbations. The\udmarginal difference in the latitude resolution of the ionospheric\udgrids in the simulations and the observations is shown to have only\udsecondary effect on the magnitudes of the Birkeland currents. The\udelectric potentials in the simulation for southward IMF periods are\udtwice as large as those obtained from measurements of the plasma\uddrift velocities by DMSP, implying that the reconnection rates in\udthe simulation are too large

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