Plasmaspheric zonal electric fields and coupling fluxes from the Dunedin VLF Doppler experiment, for 180°E, L ≈ 2.3, at solstice and equinox

Abstract

Group delays and Doppler shifts from ducted whistler-mode signals are measured using the VLF Doppler experiment at Dunedin, New Zealand (45.8°S, 170.5°E). Equatorial zonal electric field and plasmasphere-ionosphere coupling fluxes are determined for L ≈ 2.3 at June solstice and equinox during magnetically quiet periods. The general features of the electric field measured at Dunedin agree with those predicted from ionospheric dynamo theory with a (1,−2) tidal component. Some seasonal variations are observed, with the electric field measured during equinox being smaller and predominantly westward during the night. The electric field at June solstice is also westward during the evening and for part of the night, but turns sharply eastward during the pre-dawn and dawn period at the duct entry site. The June electric field appears to follow a diurnal variation whereas the equinox electric field shows a possible 4-hourly periodic variation. Seasonal variations in the neutral wind pattern, altering the configuration of the ionospheric dynamo field, are the probable cause of the seasonal differences in the electric field. The seasonal variation of the coupling fluxes can be explained by the alteration of the E x B drift pattern, caused by the changes in the electric field.