Empirical Study of Daytime D-Region Attenuation of VLF Waves in the Earth-Ionosphere Waveguide, Using the World-Wide Lightning-Location Network

The sunlit D-region extends lower than the nighttime D-region, and is dominated (except during strong solar X-ray anomalies) by a relatively predictable photo-ionization of the NO molecule by Lyman-α radiation. This sunlit VLF attenuation significantly exceeds the VLF attenuation in the dark D-region.

We describe an approach to fit the daytime attenuation empirically. Our results will be used to test model predictions.The World-Wide Lightning-Location Network (WWLLN) routinely provides estimates of the VLF source energy of lightning strokes anywhere on Earth. These estimates are built by combining WWLLN sensor-station recordings of VLF pulse energy with modeling of the propagation conditions by the Long Wave Propagation Capability (LWPC) code.

In this report, we describe a more primitive use of the sensor-station recordings. We compare the amplitudes of the received VLF pulses with each other, but without any attention to theoretical propagation predictions (e.g. LWPC). Each station has its own stationary (at least over days) system-gain factor, which (without appealing to the LWPC) we do not know. Because we want to study wave attenuation, we may not in the present application use LWPC modeling to "calibrate" the system-gain factors, and must instead think of a way, despite the arbitrary station gains, to estimate from the data the D-region additive attenuation due to daylight. We do this, and as a byproduct also empirically fit the variations of VLF propagation due to geomagnetic inclination and due to wave magnetic propagation azimuth. These results will be compared to model predictions.