A New Four‐Parameter D‐Region Ionospheric Model: Inferences From Lightning‐Emitted VLF Signals

Abstract

AbstractWe present a new four‐parameter model of the D‐region (60–90 km) ionospheric electron density, useful in very low frequency (VLF, 3–30 kHz) remote sensing. VLF waves have a long history of use to indirectly infer D‐region conditions, as they reflect efficiently and thus are sensitive to small changes in the electron density. Most historical efforts use VLF observations along with a forward model of the D‐region and VLF propagation. The ionospheric assumptions in the forward model are altered until the output matches the observation. The most common D‐region model, known as the Wait‐Spies ionosphere, takes the electron density as exponentially increasing with altitude and specifies a height and steepness. This model was designed to capture the VLF propagation variations evident at a single frequency. The real D‐region is likely more complex. The limited number of D‐region rocket passes that have previously been compiled tend to show the existence of a “ledge” somewhere between 70 and 90 km. Broadband VLF signals emitted from lightning allows a more sophisticated parametrization. Using carefully averaged amplitudes and phases of VLF sferics, we formulate a more general four‐parameter D‐region model that includes a ledge discontinuity. Using lightning‐emitted VLF observations along with a theoretical model, we find that this model better describes the ionosphere during the daytime. During the ambient nighttime and during a solar flare the two‐parameter ionosphere may be sufficient, at least for the purposes of calculating broadband VLF propagation, since the ledge either weakens or moves outside the altitude range of VLF sensitivity.