“…During times of enhanced D-region electron density, there is an increase in the Lowest Useable Frequency (LUF) (Milan et al, 1998;Uryadov et al, 2019) such that these lower frequency signals are unusable. In more extreme cases, electron density enhancements may allow new propagation modes, for example, enabling Auroral-E propagation (as opposed to F-region) in the auroral oval during strong events (Blagoveshchensky & Sergeeva, 2020;Milan et al, 1997Milan et al, , 1998Uryadov et al, 2019). Studies report electron density enhancements in the F region (higher foF2) prior to the substorm expansion phase (Blagoveshchensky & Kalishin, 2009) and magnetic storm onset (Danilov, 2013;Uryadov et al, 2019).…”