Ionospheric D Region: VLF‐Measured Electron Densities Compared With Rocket‐Based FIRI‐2018 Model

Abstract

AbstractGround‐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 Ionosphere (IRI‐2016) model.