An interhemispheric study of the ground magnetic and ionospheric electric fields during the substorm growth phase and expansion phase onset

HF radar data during equinoctial, small IMF By conditions have enabled the substorm growth phase to be studied in both hemispheres. This has revealed highly conjugate behavior during the substorm growth phase. The characteristic equatorward progression of the radar backscatter in both hemispheres is well matched by a calculation of the position of the equatorward boundary of the radar backscatter, based on an estimate of the reconnection rate at the magnetopause and the relationship between the cross polar cap potential and the change in polar cap area. Coincident radar backscatter and DMSP precipitation measurements suggest that there is a close relationship between the location of the radar backscatter and structured nightside precipitation. This nightside relationship appears to be analogous to the close relationship between radar backscatter and the targets provided by precipitation at the cusps on the dayside. The high degree of conjugacy during the growth phase mirrors the high degree of conjugacy known to exist between the substorm electrojets. However, in detail, and during the expansion phase, there are significant non-conjugacies displayed in, e.g., the degree of HF absorption in the two hemispheres. This nonconjugacy appears similar to that recently observed in interhemispheric optical data.

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