Figure captions
Figure 1: Model amplitude reflectivity (vertical axis) vs propagation
magnetic azimuth (horizontal axis). Color marks magnetic dip angle, from
5 deg (blue) to 85 deg (red). Curve for dip angle = 45 deg is dashed.
Model assumes angle-of-incidence is 85 deg, and is averaged over the
band 5 - 20 kHz. (a) Day case. (b) Night case, with labels on curves for
dip angle = 30, 45 deg. The horizontal black line on both panels is
through the minimum of the curve for dip angle = 45. deg in (b). See
Table 1 for parameter values.
Figure 2: Comparison of Atuona detection efficiency against strokes in
two roughly equidistant geographic boxes. (a) Showing the East box, over
northwestern South America. (b) For all strokes in East box, showing the
distribution of fraction of instantaneous path (to Atuona) that is
daylit. This includes all strokes in the East box, not just those
detected by Atuona. Horizontal resolution is 0.01. (c) Detection
efficiency (DE) of Atuona against strokes in East box, as a function of
the instantaneous daylit fraction of the path. (d) Showing the West box.
(e) For all strokes in West box, showing the distribution of fraction of
instantaneous path (to Atuona) that is daylit. (f) Detection efficiency
(DE) of Atuona against strokes in West box, as a function of the
instantaneous daylit fraction of the path. Note the order-of-magnitude
difference in DE scales between (c) East and (f) West boxes.
Figure 3: Geomagnetic setting of each of the ten selected stations. Each
station is shown as a rectangle, either white or black so as to contrast
with the background. The color shading of the background is magnitude of
dip angle, from 0 (black) to 74 deg (red). Only (i) has a different
color scale: 0 (black) to 76 deg (red). The colors are shown within
latitude bands -40 to +40 deg, except -30 to +50 deg for (i) Honolulu.
Also, discrete curves are drawn, where dip angle = -30, -45, +30, and
+45 deg. The white cone at each station’s antipode is excluded from the
analyses, because the Great Circle Paths from strokes within those cones
extend poleward of +/- 55 deg latitude.
Figure 4: Patterns of detection/non-detection for Atuona. (a) Spatial
density of WWLLN-located strokes within the latitude band for which the
Great Circle Path is > 80% sunlit. The maximum for this
density is red, while blue is 1% of the maximum. White areas outside of
the antipodal cone correspond to stroke density less than 1% of the
maximum. The discrete lines are at dip angle = 0, +/- 30, and +/- 45
deg. Neither the stroke density, nor the curves, are shown within the
antipodal cone or outside of the latitude band. (b) Spatial density of
the subset of strokes in whose location Atuona participates, for which
the Great Circle Path is > 80% sunlit. The new maximum is
shown as red, and 1% of this new maximum is shown as blue. (c) Day
model logarithmic reference transmission (see text) versus position, in
four ranges: > -2 (red), from -2 to -3 (yellow), from -3 to
-4 (green), and < -4 (blue). Model is not shown within the
antipodal cone or outside of the latitude band. (d) Similar to (a) but
for Great Circle Paths < 20% sunlit. (e) Similar to (b) but
for Great Circle Paths < 20% sunlit. (f) Similar to (c) but
for night model. (g) Histogram of logarithmic reference transmission.
Black curve: all strokes in latitude band excluding antipodal cone. Red
curve: only those strokes in whose location Atuona participates.
Figure 5: Similar to Figure 4, but for Tahiti station.
Figure 6: Similar to Figure 4, but for Peru station.
Figure 7: Similar to Figure 4, but for Costa Rica station.
Figure 8: Similar to Figure 4, but for Belem station.
Figure 9: Similar to Figure 4, but for Dakar station.
Figure 10: Similar to Figure 4, but for Pune station.
Figure 11: Similar to Figure 4, but for Singapore station.
Figure 12: Similar to Figure 4, but for Honolulu station. Note that
latitude band is -30 to +50 deg, for this station only.
Figure 13: Similar to Figure 4, but for Tel Aviv station.
Figure 14: Distribution of raw amplitude (ADC level) for WWLLN-located
strokes detected by Tel Aviv station. (a) For logarithmic reference
transmission > -2.0. (b) For logarithmic reference
transmission from -2.0 to -2.5.