3c. Summary of observational results from part 1 of the present study
We presented in JHB1 a method to study the behavior of the inter-station ratio of VLF stroke amplitudes, for strokes that are simultaneously recorded at multiple WWLLN stations. This approach combined numerous recurrent strokes from long-duration lightning clusters to build a time-series of the ratio for a major portion of the UT day. The time variations of the sliding-averaged ratio are dominated by transient excursions coinciding temporally with those periods when the solar terminator is present along one or both of the paths. See, e.g., Figure 2 in JHB1. This strongly motivates a model incorporating significant control by the solar zenith angle.
Our plane-wave model predicts that magnetic-westward propagation has less waveguide transmission than does magnetic-eastward propagation.Crucially, the anisotropy is extremely magnified for a night ionosphere. This anisotropy is modulated by magnetic dip angle: The anisotropy is strongest at low dip angle, and weakest at large dip angle.
To account for solar-zenith-angle control on the waveguide transmission, our model takes a weighted combination of pure-day and pure-night solutions, determined locally for every path element along the Great Circle Path from the lightning to the WWLLN station, and for the exact Universal Time of the stroke .
The model solution based on the plane-wave-reflection theory successfully accounted for the gross features of the solar-terminator transients; see, e.g, Figures 7-10 in JHB1.
Our model predicts, counter-intuitively, that the magnetic-westward attenuation at low magnetic latitude will be much deeper during night than during day conditions. Unfortunately, this suppression of magnetic-westward propagation also largely eliminates the availability of sufficiently numerous recurrent recordings of those signals at our low-latitude stations. Thus the amplitude-ratio method pursued in JHB1 was inherently unable to check on the model’s most intriguing and counter-intuitive prediction.
Thus our method to follow, rather than using lightning detections that exist, is designed to demonstrate the pattern of where and when detections do not exist.