Interhemispheric observations of nightside ionospheric electric fields in response to IMF B-z and B-y changes and substorm pseudobreakup

HF radar data during equinoctial, small IMF B-y conditions have enabled the ionospheric convection during the substorm growth phase and substorm pseud obreakup to be studied in both hemispheres. This has revealed both conjugat e and non-conjugate convection behaviour during the substorm growth phase b efore and after the pseudobreakup onset. The nightside convection pattern i s found to respond promptly to the southward turning of the interplanetary magnetic field (IMF) which impacts on the dusk flank of the magnetosphere d ue to an inclined phase front in the IMF in the case study presented. The s ubsequent interhemispheric observations of nightside convection are control led by the IMF B-y polarity. The time scale for the response to changes in the IMF B-y component is found to be a little longer than for B-z, and the full impact of the IMF B-y is not apparent in the nightside convection unti l after substorm pseudobreakup has occurred. The pseudobreakup itself is fo und to result in a transitory suppression in the ionospheric electric field in both hemispheres. This flow suppression is very similar to that observe d in HF radar observations of full substorm onset, with the exception of a lack of subsequent poleward expansion.