Effects of polar cap absorption events on VLF transmissions

Abstract

Following a brief review of Hakura's model of a polar cap absorption event (PCA), VLF instrumentation, and data reduction problems, eleven PCAs are studied for their effects on phase and amplitude of VLF transmissions. The eleven PCAs occurred between 24 March 1966 and 6 June 1967. VLF phase anomalies are always advances (Δφ), the magnitude of which increases with increasing geomagnetic latitude of the path, decreasing signal frequency, and decreasing electrical ground conductivity. Signal strength falls by more than 15 dB/1000 km for paths with very low electrical ground conductivity (Greenland and Antarctic Ice Cap), whereas it may increase by a few dB for polar paths over sea water. It is shown that normalization of (Δφ), with respect to frequency and affected length, leads to two distinct linear relationshipsbetween (Δφ) and (log log F/ F 0, where F is the proton flux as measured by satellites. The line of lower slope applies to paths over a highly conducting boundry (sea water), and the line of higher slope to a poorly conducting boundary (ice). The minimum detectable proton flux is 0·5 protons/cm 2 sec ster. The (Δφ)-(log log F/ F 0) relationship implies a decreased sensitivity of (Δφ) to changes in F for larger values of F, an almost uniform VLF reflection height over the polar cap, little influence on reflection height of the shape of the flux spectrum during peak flux, and an upper limit of approximately 25 μsec for a PCA phase anomaly on NPM (26·1 kHz)-Kiruna. Occasionally, irreversible cycle advances were observed on NPG-STO and WWVL-STO. The former are interpreted in terms of enhanced mode conversion and the latter in terms of the tracking of an interfering VLF signal. An event on 13 December 1966 is believed to have been due to electron precipitation from the outer radiation belt.