Daytime ionospheric D region sharpness derived from VLF radio atmospherics

[1] We described and applied a technique to measure the local midlatitude daytime ionospheric D region electron density profile sharpness from the Earth‐ionosphere waveguide mode interference pattern in the spectra of radio atmospherics (or sferics for short), which are the high‐power, broadband, very low frequency (VLF, 3–30 kHz) signals launched by lightning discharges. VLF propagation simulations are used to show that the upper VLF frequency spectral minima of sferics on several hundred kilometers long propagation paths depend critically on the effective D region sharpness while depending only weakly on the effective D region height. This enables the straightforward extraction of the sharpness parameter from measured VLF spectra, which generally exhibit well‐defined minima at upper VLF frequencies. By applying this technique, we calculated the profile sharpness during morning, noontime, and afternoon periods in 3 different days using sferics from ∼660–800 km away. The measured sharpness showed a weak dependence on the solar zenith angle, with values between 0.35 and 0.45 km for angles from 20° to 75°. This is different from the previous narrowband measurement since the sharpness derived from narrowband VLF signals highly depends on the solar zenith angle. To better understand this discrepancy, we also used simulations to calculate the equivalent exponential profiles for International Reference Ionosphere (IRI) profiles and the empirical FIRI model profiles. The equivalent exponential profiles can best duplicate the sferic spectral characteristics for IRI and FIRI models. We find that both the magnitudes and solar zenith angle variations of the sharpness for our broadband measurements, previous narrowband measurements, and both models are completely different. This suggests the daytime ionosphere, particularly at larger solar zenith angles, may not be well described by a simple two‐parameter exponential model.