Interhemispheric contrasts in the ionospheric convection response to changes in the interplanetary magnetic ®eld and substorm activity: a case-study

Interhemispheric contrasts in the ionospheric convection response to variations of the interplanetary magnetic ®eld (IMF) and substorm activity are examined, for an interval observed by the Polar AngloAmerican Conjugate Experiment (PACE) radar system between 1600 and 2100 MLT on 4 March 1992. Representations of the ionospheric convection pattern associated with di€erent orientations and magnitudes of the IMF and nightside driven enhancements of the auroral electrojet are employed to illustrate a possible explanation for the contrast in convection ̄ow response observed in radar data at nominally conjugate points. Ion drift measurements from the Defence Meteorological Satellite Program (DMSP) con®rm these ionospheric convection ̄ows to be representative for the prevailing IMF orientation and magnitude. The location of the ®elds of view of the PACE radars with respect to these patterns suggest that the radar backscatter observed in each hemisphere is critically in ̄uenced by the position of the ionospheric convection reversal boundary (CRB) within the radar ®eld of view and the in ̄uence it has on the generation of the irregularities required as scattering targets by high-frequency coherent radar systems. The position of the CRB in each hemisphere is strongly controlled by the relative magnitudes of the IMF Bz and By components, and hence so is the interhemispheric contrast in the radar observations.