Summary
HF signals that propagate via the ionosphere may exhibit Doppler shifts due to a number of processes that give rise to a time dependent refractive index. This paper examines the role of ULF (1–100mHz) wave energy, incident from the magnetosphere, in causing Doppler shifts in HF signals. We describe a model of ULF wave propagation through the ionosphere that allows the ULF wave mode mix to be specified. It is found that the shear Alfvén mode wave gives the larger Doppler velocities of order 2ms-1 in the F-region, caused mostly by a mechanism that involves the divergence of the electron velocity from the passage of the ULF wave. Pure fast mode perturbations of similar amplitude gives Doppler velocities around ten times smaller. In both cases the role of vertical bulk motion of the electrons is small. An experiment that recorded data from a Doppler sounder and magnetometer is described and the amplitude-ratio and cross-phase of data recorded by the north-south magnetometer sensor and the Doppler sounder are presented. The amplitude-ratio data show a distinctive maximum and the cross-phase a large phase shift around a ULF field line resonance. The paper presents the relevant theory and modelling of this feature and illustrates the dependence on the horizontal spatial structure of the ULF energy.