VLF Phase Perturbations Produced by the Variability in Large (V/m) Mesospheric Electric Fields in the 60-70 km Altitude Range

摘要

The large (V/m) mesospheric electric fields have been identified as a possible cause of VLF phase perturbations. These fields affect the fundamental processes that govern the lower D region parameters, primarily the electron temperature and effective collision frequency. The main ionospheric parameter needed to calculate VLF phase perturbations is the low-frequency electron plasma conductivity. This paper discusses the progress made in addressing large (V/m) mesospheric electric fields between 60- and 70-km altitudes since 1990. It focuses on achieving the development of a radio wave technique for sensing large electric fields remotely by using MF radar, and on the fact that the electric field variability leads to the variability of ionospheric conduction contours by a few kilometers in altitude. The statistical analysis of the large mesospheric electric field data acquired in the 60- and 67-km altitude region in Canada and Ukraine suggests that large mesospheric electric fields may occur during about 70% of all the time. However, reasonable assessments of VLF phase perturbations need information on the temporal and especially spatial variability of conduction contours, which remains a major challenge within this problem. First, the technique developed to specify electric fields requires signal-to-noise ratios in excess of a factor of five, which is achieved irregularly with the MF radars used at present. Second, the existing MF radars do not permit the observations of the spatial evolution of these fields at all. The latter problem can be overcome by developing dedicated radar. Meanwhile, colocated VLF phase perturbation measurements and electric field observations by existing MF radars may be combined to produce a pre-intermediate capability. Eventually, a better understanding of the dynamics and mesospheric and ionospheric D-region chemistry, which establish conductivity patterns, will require the combined efforts of the entire scientific community.