Ionospheric D region: VLF‐measured Electron Densities compared with Rocket‐Based FIRI‐2018 Model

Ground-based very low frequency (VLF) radio propagation in the Earth-ionosphere waveguide has enabled extensive electron number densities in the D region of the Earth’s ionosphere to be determined, by day typically below heights of 70-80 km and by night in the height range ∼75-90 km. Many rocket-based electron density measurements have also been reported in the literature from ∼60 km upwards using current probes, and radio propagation at a few MHz between the rocket and ground. Recently these rocket measurements have been summarized, and supplemented with D region production-loss modeling, giving rise to a near global model named FIRI-2018 (Faraday-International Reference Ionosphere) which provides electron number densities as functions of height, latitude (<60°), solar zenith angle and F10.7 cm solar flux. These rocket-based electron density values are here compared with corresponding values from VLF measurements, by day at a low-latitude (∼20°) and a high mid-latitude (∼55°), and by night mainly at mid-latitudes. At night the average agreement (over 75-90 km) is remarkably good. By day, at low latitude the agreement is also fairly good (in the common height range ∼60-75 km), with the changes with solar zenith angle being moderately comparable. For daytime high mid-latitudes, the agreement is less satisfactory, particularly at the lowest common altitudes, with the VLF measurements showing the expected effects of cosmic rays much more than the rocket-based values. Overall, we find that the D region description in the FIRI-2018 model is a significant advance on the earlier International Reference