3b. Disfavoring of nighttime (relative to daytime) magnetic-westward propagation
Note that for abs(dip angle) < 45 deg, the night-profile reflectivity (Figure 1b) for propagation toward magnetic west (270 deg) is less than the day-profile reflectivity (Figure 1a) . This favoring of daytime over nighttime transmission for abs(dip angle) < 45 deg actually applies over a broad azimuth sector centered on magnetic west. Thus, for essentially half of all possible dip angles, and for essentially half of all possible propagation magnetic azimuths, the nighttime reflection is predicted by our model[Jacobson et al. , 2010; Jacobson et al. , 2009]to be disfavored relative to the daytime reflection, and for small dip angle deeply disfavored . This surprising and counter-intuitive feature is not remarked elsewhere in the VLF literature, and thus the burden is on us to provide observational support for this counter-intuitive claim. Intuition would suggest that nighttime propagation should be less lossy than daytime, because the nighttime reference height (85 km in our model) has only 17% as much electron-neutral collision rate as does the daytime reference height (73 km in our model).
The remainder of this article relies on the first part of the study (JHB1) for a detailed development of the model theory. Readers should consult JHB1, which was published as Open Access and hence is without cost to the reader.