Abstract Maximum usable frequency (MUF) is an important indicator of the ability of the ionosphere to support over‐the‐horizon propagation of high frequency (HF) radio waves. MUF depends on the location of the transmitter and receiver, and the structure of ionospheric electron density. This study explores short‐ and long‐term variations in the MUF for a cross‐auroral path and a polar cap path using transmitters and receivers in Canada and Greenland. Signals were transmitted at a range of frequencies multiple times per hour, allowing for changes in the MUF to be inferred by analyzing changes in the maximum received frequency (MRF) at the receiver over time. Apart from the expected diurnal variation in the MRF caused by solar photoionization, both paths exhibit reductions in the MRF likely connected to negative storm effects. This is supported by a superposed epoch analysis which demonstrates a reduction in MRF during geomagnetic storms. The polar cap path is shown to experience larger MRF at times when signals interacted with polar cap patches, as indicated by a simple model of patch formation, magnitude azimuthal signal deflections, and Hz Doppler shifts. The polar cap path also exhibits periods of high MRF consistent with reflection off sporadic‐E layers in the polar cap.
Cameron et al. (Fri,) studied this question.