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.