4d.1 Approach
We now start a formal comparison of the observed and predicted detection
patterns of each of the ten selected stations. Separately for each
station, we define two cohorts of strokes within the accepted latitude
band. The first cohort contains the strokes whose Great Circle Paths to
the station are > 80% sunlit at the instant of the stroke,
at D-layer height. This first cohort represents mostly-daytime
propagation. The second cohort contains the strokes whose Great Circle
Paths to the station are < 20% sunlit, representing
mostly-nighttime propagation.
In addition to those statistics based on observation, we calculate the
logarithmic reference transmission (Eq. 1 above) for each Great Circle
Path under two artificial conditions: that the entire path be either in
daylight or in darkness. These yield ”day” and ”night” logarithmic
reference transmissions.
Finally, we calculate the instantaneous logarithmic reference
transmission, using the actual instantaneous solar zenith angle at
each point along each path , for all strokes. The distribution of
logarithmic reference transmission shows the strokes that areavailable for the selected station to detect. The
sub-distribution of logarithmic reference transmission only for
the strokes that are detected by the selected station shows the
relationship between detectability (by the selected station) and
logarithmic reference transmission (relative to the selected station).
We would expect that if the model has some correlation to observational
reality, then the strokes detected by the selected station would be
bunched at the high-transmission end of the distribution, and would be
sparse or absent in the low-transmission end of the distribution. On the
other hand, if the model were basically worthless, then there would be
no strong correlation between observed detectability and model-predicted
logarithmic reference transmission.
The reader should keep in mind that stations do not all have the same
effectiveness in detecting lightning [Hutchins , 2014]. We
will call this ”sensitivity”, but this does not mean something so simple
as system gain. Rather, the two most important factors are, first, the
level of background VLF noise affecting the selected station, and,
second, the abundance of nearby lightning [Hutchins , 2014].
The effect of abundant nearby lightning is to reduce the ability of the
station to participate in network detections of distant lightning
strokes. This is because each WWLLN station has a software-adjusted
trigger threshold for capturing a pulse to become a candidate for
participation in a network location of strokes. The threshold is
sluggishly (~2 minutes of inertia) adjusted, so as to
continuously keep the rate of station triggers not greater than 3 per
second. Abundant nearby lightning interacts with this feedback to
increase the trigger level and thus reduce the ability of that station
to trigger on distant lightning.