5. Conclusions
This is the second part of a two-part study of broadband VLF propagation
from lighting strokes to WWLLN stations. The first part of the study
(JHB1) had developed a model for the effects of the ionospheric D-layer
on long-range VLF transmission in the Earth-Ionosphere Waveguide. The
model makes the counter-intuitive prediction that, for dip angles in the
range -30 to +30 deg, propagation toward the west half of magnetic
azimuths will be dramatically worse during conditions of darkness (along
the propagation path) than during conditions of daylight. This feature
had never been remarked before in the literature, although it is in fact
also embedded in the standard LWPC code. We surmise that the reason the
feature had never been remarked is that the LWPC is an end-to-end
treatment that tends to obscure, to the code’s user, the details of
differential transmission at any one point on the propagation path.
Our model had been applied in JHB1 to explaining the inter-station
ratios of signal amplitudes from the same stroke at different stations.
That approach, in common with all virtually all other approaches done by
prior workers, was based on the measurement of VLF amplitudes. However,
we found that the amplitude-based method was inadequate to the test the
model’s counter-intuitive prediction regarding the day/night control of
westerly propagation. That is because the amplitude-based approaches
require detections in order for amplitudes to be determined. ”No
detections, no amplitudes”.
This second part of the study circumvents that problem by adopting an
opposite approach. Rather than use received signal amplitudes as the raw
data, we now use the observed statistical patterns of
detection/non-detection. We compare those patterns to our model’s
predictions of the D-layer contributions to path transmission. By
focusing on the variations between daylit and dark conditions, we also
avoid the confounding effect of ground losses, as the latter are
invariant between daylit and dark conditions.
We highlight the geographical patterns of detection/non-detection from
each of ten selected stations arranged around diverse longitudes. For
each of these stations, we identify strokes whose paths are either
mostly daylit or mostly dark. The patterns of detection/non-detection in
these two special cases are then compared with the predictions of our
transmission model, for either all-lit paths or all-dark paths
respectively. The spatial agreement between observation and model is
good. We then use all the strokes, not just those whose paths are
mostly lit or mostly dark, and calculate the modeled logarithmic
reference transmission along each stroke’s path to the selected station,
taking account of the instantaneous solar zenith angle at each point
along the path. We tally the distribution of logarithmic reference
transmission, both for the parent population of strokes, and for the
subset of strokes that are detected by the selected station. We find
consistently, for all of our ten selected stations, that the detected
subset’s distribution of logarithmic reference transmission is entirely
crowded to the high-transmission end. This suggests that the model’s
predictions of transmission are pertinent.
Finally, and most importantly, our ten case studies robustly demonstrate
that for dip angles in the range -30 to +30 deg, during conditions of
darkness there is dramatically worse transmission from magnetic East to
West then from magnetic West to East, whereas for daylit conditions,
this is much less pronounced. These findings are operationally
significant for long-range lightning detection. For example, WWLLN’s
Pacific stations Atuona, Tahiti, and Honolulu are not able in dark-path
conditions to contribute significantly to locating lightning in South
America, though they are extremely useful over comparable distances with
lightning in Australasia. Similarly, under dark-path conditions, Peru
basically misses the eastern half of its own continent, and Dakar sees
even less of its own continent. For the same reason, during dark-path
conditions, Pune is very good for detecting lightning in Africa but
misses almost all lightning at similar distances in Australasia. These
effects are not subtle, when viewed geographically in terms of areas of
detection and non-detection.