Abstract
Abstract. HF radar data during equinoctial, small IMF By conditions have enabled the ionospheric convection during the substorm growth phase and substorm pseudobreakup to be studied in both hemispheres. This has revealed both conjugate and non-conjugate convection behaviour during the substorm growth phase before and after the pseudobreakup onset. The nightside convection pattern is found to respond promptly to the southward turning of the interplanetary magnetic field (IMF) which impacts on the dusk flank of the magnetosphere due to an inclined phase front in the IMF in the case study presented. The subsequent interhemispheric observations of nightside convection are controlled by the IMF By polarity. The time scale for the response to changes in the IMF By component is found to be a little longer than for Bz, and the full impact of the IMF By is not apparent in the nightside convection until after substorm pseudobreakup has occurred. The pseudobreakup itself is found to result in a transitory suppression in the ionospheric electric field in both hemispheres. This flow suppression is very similar to that observed in HF radar observations of full substorm onset, with the exception of a lack of subsequent poleward expansion.Key words: Ionosphere (auroral ionosphere) - Magnetospheric physics (magnetosphere-ionosphere interactions; storms and substorms)
Highlights
The extensive ®elds-of-view of high frequency (HF) ionospheric radars make them excellent instruments for the investigation of the spatial and temporal develop-The response of magnetospheric and ionospheric convection to changes in the upstream solar wind and IMF has generally been considered to start at the dayside, and subsequently expand towards the nightside (e.g. Etemadi et al, 1988; Todd et al, 1988)
Khan and Cowley (1999) presented a statistical study of ionospheric electric ®elds measured by the EISCAT radar, in which the ionospheric response expands from the dayside to the nightside over a time scale of 10 min
The response has been considered to be a steady evolution, starting near magnetic local noon, and propagating away from this location (Etemadi et al, 1988; Todd et al, 1988; Khan and Cowley, 1999). All three of these studies were consistent with a minimum delay between the upstream conditions and the magnetospheric response occurring close to 1400 magnetic local time (MLT), the shift from local noon being ascribed to the spiral structure of the IMF
Summary
The extensive ®elds-of-view of high frequency (HF) ionospheric radars make them excellent instruments for the investigation of the spatial and temporal develop-. Khan and Cowley (1999) presented a statistical study of ionospheric electric ®elds measured by the EISCAT radar, in which the ionospheric response expands from the dayside to the nightside over a time scale of 10 min. We study the ionospheric electric ®eld response to substorm onset directly with HF radars, examining a pseudobreakup response in both the Northern and Southern Hemisphere for the ®rst time. Such an investigation of the high-latitude convection pattern, observed simultaneously in both hemispheres, can provide the opportunity to examine the time dependent nature of the coupling between the solar wind, magnetosphere and ionosphere during the substorm cycle. A detailed investigation of theow patterns observed in both hemispheres, reveals non-conjugacies which may be attributed to IMF By, and allows an investigation of the time scale on which the magnetosphere responds to such changes
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