4a. WWLLN database
The overall epoch for this study is 1 December 2009 through 31 May 2021, Universal Time (UT). Within that overall epoch, numerous WWLLN recording stations began operation, occasionally interrupted operation, and (in a few cases) ceased operation. For much of the overall epoch, at any time WWLLN had > 50 active stations worldwide. At present (2021) the census is > 60 active stations.
Our methodology in the following is to develop statistics on thepatterns of detection and non-detection by selected stations. We use the entire WWLLN network product to define the overall population of WWLLN-located lightning strokes. This population is defined in a separate day file for each UT day. We then focus on ten selected stations located at magnetic low and mid latitudes. We develop statistical maps of the detection/non-detection of the overall WWLLN stroke population, by each of these ten selected stations.
To clarify ”detection”, we mean that a particular stroke is ”detected” by a particular station if that station participates in the solution for that particular stroke’s location . This is a stringent definition of ”detection”, compared to a more permissive definition that the station records a signal (whether or not that signal has the requisite amplitude and signal-to-noise to allow participation in the network solution.) The problem with the permissive definition is that one can never be sure, or even reasonably sure, that a signal is associated with a given stroke, if that signal cannot be used in the position/time solution for that stroke.
Table 2 lists pertinent metadata about the ten selected stations. Three of the stations (Atuona, Tahiti, and Honolulu) are in the Pacific ocean and have dominantly over-seawater paths from abundant lightning in both their Eastern and Western sectors. Another station (Costa Rica) is on a relatively narrow land bridge between major oceans. Two stations (Peru and Dakar) are on the western periphery of lightning-rich continents.
For the statistics on each of the ten selected stations, we define a latitude band within which to include WWLLN strokes. This band is -40 to +40 deg N for nine of the stations, but for Honolulu the band is displaced to -30 to +50 deg N, in order to include the strokes in the northern continental United States. The population of WWLLN strokes within the latitude band is used to detect detection/non-detection by the respective selected station.
The population of WWLLN strokes within the latitude band and available for detection by the respective selected station varies from a maximum of > 2X109 strokes (for both Honolulu and Tel Aviv) down to 3.2X108 strokes (for Belem). This disparity is driven mainly by the difference between the number of operating days for Honolulu (3873) or for Tel Aviv (3987), versus for Belem (566) within the overall epoch.