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.