4b. Qualitative demonstration of geomagnetic and zenith-angle control over detection
Before embarking on a systematic quantitative analysis, we show a qualitatively clear example of the control over detection exerted by solar zenith angle and by geomagnetic parameters. Figure 2 shows the case of Atuona station, near the mid-Pacific. This example conveniently illustrates the situation for low dip angle and nearly-zonal magnetic propagation azimuth everywhere along the paths eastward and westward to regions of abundant lightning. In Figure 2, the station is a black rectangle symbol. In Figure 2(a), to the East of Atuona is shown a red rectangular box in northwestern South America. We select all WWLLN strokes within that box. For each stroke within that box, we calculate the solar zenith angle (at D-layer height) for all points along the path to Atuona from the stroke, and characterize each stroke by the proportion of the path that is in daylight. Figure 2(b) shows the daylit-fraction distribution of all strokes in the red box of Figure 2(a). The distribution is flat except for roughly equal peaks both at pure-dark (daylit fraction = 0) and at pure-daylit (daylit fraction = 1). We now ask, what is the Atuona detection efficiency (DE) for these strokes, versus the daylit fraction parameter? This is shown in Figure 2(c). The DE peaks toward maximum daylight, and is suppressed (by an order of magnitude) for daylit fraction < 0.6.
Now let us define a ”control” case, which is shown in the right column of Figure 2. Figure 2(d) shows in red a ”West box” over the Australasia sector. It is slightly further from Atuona than the East Box, but is roughly comparable in dip angle along the paths to Atuona. Figure 2(e) shows the daylit-fraction distribution for all strokes in the West box. Figure 2(f) shows the DE for Atuona detection of those strokes, as a function of daylit fraction. Now the DE for eastward propagation is relatively indifferent to daylit fraction, and the median DE for the west box is two orders-of-magnitude higher than the median DE for the East box (Figure 2c), and one order-of-magnitude greater than the maximum DE for the East box (Figure 2c). We note that this is a case where the paths to Atuona from either the West or the East box are everywhere quasi-zonal in magnetic azimuth, and are everywhere at very small dip angle (-20 deg to +20 deg). We chose this because of its convenience for a qualitative exercise like Figure 2.
This example qualitatively demonstrates, within the context of low dip angle, (a) the dramatic difference between propagation at eastward magnetic azimuth versus westward magnetic azimuth, and (b) the extreme favoring of daylit propagation over night propagation for westward magnetic azimuth. This latter feature has not previously been remarked in the VLF literature.