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.