LOWER IONOSPHERIC DISTURBANCES OBSERVED IN LONG-DISTANCE VLF TRANSMISSION AT MIDDLE LATITUDE

An attempt is made to investigate disturbances in the relative phase of long-distance, cesium-controlled, VLF radio wave propagation from NLK (Jim Creek, U.S.A.) to HCM (Nishinomiya, Japan). The propagation path covers a longitudinal range of about 100° and ranges from L~ 1.2 to L ~ 3.2 in invariant latitude. VLF phase disturbances following major geomagnetic storms indicate that the altitudinal and latitudinal extents of ionization enhancement in the D-region depend on the strength of the storm. The after-effect observed in the night-time VLF phase becomes pronounced especially in winter condition of the D-region. There is another type of VLF phase disturbance in the night-time which is not related to storm but associated with the winter anomaly observed in the daytime at middle latitudes (40°-45°N) but below L = 2. This result suggests that the enhanced ionization occurs in the D-region above about 80 km not only in the daytime but also in the night-time for a group of winter anomalous days. It is likely that the main cause of the enhanced ionization is related to the vertical propagation of planetary waves into the mesosphere, resulting in a sudden warming. © 1979.