An analytical model for Doppler frequency variations of ionospheric HF sounding caused by SSC

Abstract

A theoretical model to interpret the deviations of the Doppler sounding radio frequency during the SSC is proposed. The Doppler response is composed from long‐period deviation with time scale ∼10 min and short‐lived (∼1 min) positive spike. For the interpretation of this spike, the variations of Doppler velocity induced by compressional and Alfvén waves are theoretically estimated on the basis of the thin ionosphere approximation. The analytical model developed goes beyond the older numerical model which has been in use thus far to explain certain facets of the correlation between geomagnetic variations and HF Doppler shifts. This new model provides the possibility to easily estimate the Doppler frequency response to either Alfvén or compresssional modes for extraordinary and ordinary radio waves. A theoretical estimate indicates that, for the same amplitude of the ground magnetic disturbances, a large‐scale compressional mode produces a larger ionospheric response than an Alfvén mode. The plasma vertical displacement caused by the ULF inductive electric field is shown to be the dominating effect. The magnitudes and waveforms of the simultaneously detected ionospheric and magnetic responses on SSC at a low‐latitude site in Japan fit the considered model.