Abstract
Abstract. We investigate the seasonal, local solar time, and geomagnetic activity variations of the average Doppler velocity measured by an HF digital ionosonde deployed at Bundoora, Australia (145.1° E, 37.7° S, geographic; 49° S magnetic). The Doppler velocities were heavily averaged to suppress the short-term effects (<3 hours) of atmospheric gravity waves, and thereby obtain the diurnal variations attributed to the tidally-driven ionospheric dynamo and electric fields generated by magnetic disturbances. The observed seasonal variations in Doppler velocity were probably controlled by variations in the lower thermospheric winds and ionospheric conductivity above Bundoora and in the magnetically conjugate location. The diurnal variations of the meridional (field-perpendicular) drifts and their perturbations exhibited a complex structure, and were generally smaller than the variations in the zonal drifts. The latter were basically strongly west-ward during the evening to early morning, and weakly east-ward during the late morning to just past noon. The zonal perturbations were strongly enhanced by increasing geomagnetic activity, and closely resembled the perturbation drifts measured by the incoherent scatter radar (ISR) at Millstone Hill (71.5° W, 42.6° N; 57° N). There was also some resemblance between the diurnal variations in the meridional drifts. Overall, the comparisons suggest that with sufficient averaging, Doppler velocities measured with digital ionosondes at mid-latitudes correspond to true ion motions driven by ionospheric electric fields. This is a useful result because apart from the ISRs located in the American-European sector, there are no ground-based instruments capable of measuring electric fields in the mid-latitude ionosphere.Key words. Ionosphere (electric fields and currents; ionosphere atmosphere interactions; mid-latitude ionosphere)
Highlights
We need to understand the physics of electric fields manifesting in the mid-latitude ionosphere and thermosphere to fully describe the global atmospheric response to magnetic storms generated within the Sun
We need to understand the physics of electric fields manifesting in the mid-latitude ionosphere and thermosphere to Correspondence to: M
It is important to understand mid-latitude electric fields because they influence the height of the F-region ionosphere, which might be a sensitive indicator of global climate change in the troposphere (Roble and Dickinson, 1989; Jarvis et al, 1998)
Summary
The basic physics affecting electric fields observed at midlatitudes are summarised as follows: 1. During geomagnetic quiet conditions, the ring current shields the plasmasphere from the magnetospheric electric fields driving steady-state convection. At all times, there must be short-term variability in the dynamo fields associated with irregular neutral winds Perhaps this variability is driven by atmospheric gravity waves which might carry electric fields with them. The Joule heating dissipated during major geomagnetic storms will modify the global thermospheric wind circulation and launch atmospheric gravity waves When these disturbances propagate to mid-latitudes, they will produce longer lasting (∼hours) disturbance dynamo fields (Scherliess and Fejer, 1998). Our knowledge of the thermospheric tides, winds, and ionospheric conductivity variations on a global scale is still incomplete, and modelling the plasmaspheric electric fields driven by the quiet time dynamo at mid-latitudes remains a challenging problem. Despite limitations in our measurements (as with other techniques), the comparisons are promising
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