4c. Geomagnetic context for the ten selected stations
We now show the geomagnetic context of each of the ten selected stations
and of the propagation paths connecting them to lighting strokes in
their respective latitude band described in Table 2. The geomagnetic
model is the International Reference Geomagnetic Field, or IGRF
[V-MOD , 2010]. Figure 3 contains a separate panel for each of
the ten stations. Color indicates the geomagnetic dip angle’s absolute
value, in deg. Black is dip = 0 deg. For nine of the panels, red is 74
deg, while for one (Figure 3i, Honolulu) the band is offset and the
maximum dip angle is 76 deg. In addition to color-coding, discrete
curves of |dip| = 30, 45 deg are overlaid on the map.
The station is marked with a rectangle symbol, either white or black, to
contrast with its immediate background color. The color shading covers
the latitude band in which strokes are considered for
detection/non-detection by the respective station.
Because of the extremely low transmission of VLF over Antarctic (or, to
a lesser extent, Arctic) ice, we wish to exclude strokes whose Great
Circle paths to the selected station reach further poleward than
geographic latitude +/- 55 deg. This excludes strokes roughly within a
cone centered on the respective station’s antipode, which we blank-out
with white. Thus, for example, in Figure 3(a) the antipode of Atuona is
situated near the southern Red Sea. Each station has its own antipode,
and cone centered there, in which we do not gather statistics regarding
stroke detection by that station.
Recall that the model results (Figure 1) predict a nighttime increase of
attenuation for propagation toward magnetic West, relative to daytime,
for |dip angle| < 45 deg. This nighttime
increase in attenuation toward magnetic West becomes especially severe
for |dip angle| < 30 deg. Figure 3 shows
visually that this band of enhanced nighttime westward attenuation
occupies most of the important lightning prone areas [Christian
et al. , 2003], excepting only the continental United States. That is,
the nighttime disfavoring of magnetic-westward VLF propagation is not
going to be a mere academic curiosity confined to a small region, but
rather applies to most regions of relevance to global VLF lightning
location.