The magnetosphere on May 11, 1999, the day the solar wind almost disappeared: I. Current systems

We have examined magnetic field data from Polar MFE to study the magnetospheric current systems on May 11, 1999, when the solar wind density was well below 1 cm(-3) and the IMF was generally weakly northward. We find that the magnetosphere was much more dipolar than usual but the ring current did not disappear. In the inner magnetosphere, the residual field, the observed magnetic field at Polar with the internal field removed, was dominated by the ring current contribution. The magnetic field data agreed well with the Tsyganenko 1996 model with a weak ring current but little magnetopause current. Weak field-aligned currents were observed both in the northern and southern hemisphere on May 11, as expected for northward IMF. The cusp signature of a depressed magnetic field, normally seen by Polar when passing through the high altitude polar cusp, was not seen on May 11, indicating that the energy density of the plasma in the cusp was much smaller than usual, as would be expected if the solar wind source were of lower density.