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.