Polar cap study during northward interplanetary magnetic field on 19 January 1998

The interplanetary magnetic field (IMF) was northward for an extended perio d on 19 January 1998. This caused the open polar cap of the ionosphere to b ecome very small and the auroral emission to move poleward. The auroral emi ssion at 630 nm was observed by the meridian scanning photometer located at Eureka near the north magnetic pole. Effects of changes in sign of the daw n-dusk component of the IMF were also observed. A magnetohydrodynamic simul ation model of the magnetosphere and ionosphere was used to study these eve nts. The model was driven using data from the Wind and IMP-8 spacecraft. Th e simulation results show a very small open polar cap indicating that the m agnetosphere is nearly closed. Moreover, in response to the shift from dawn ward to duskward IMF, a narrow strip of closed field breaks off from the da wn boundary and convects across the polar cap and into the dusk boundary. ( C) 2001 American Institute of Physics.