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.