Midlatitude observations of nighttime VLF signal-amplitude anomalies associated with magnetic bays

Abstract

Introduction. The purpose of this letter is to report the observation of VLF propagation anomalies occurring on midlatitude paths during local nighttime. The anomalies appear significant because they are not accompanied by the usual sources of perturbation—solar flares, high-altitude nuclear detonations, or severe magnetic storms. The nighttime occurrence of the anomalies effectively eliminates solar-flare-produced SID's, which are almost exclusively a dayside phenomenon. The changes do, however, resemble those produced during daytime by solar flares [Budden and Ratdiffe, 1937; Bracewell and Straker, 1949; Westfall, 1961; Mitra, 1964]. On rare occasions solar flares produce disturbed conditions on the nightside (see review of February 23, 1956, event by Ortner et al. [I960](, but there are no reports of flare activity at the times under consideration. The character of the anomalies is not unlike that produced by trapped electrons from distant high-altitude nuclear explosions [Zmuda et al., 1964; Sechrist, 1964]. There are, however, no reports of nuclear testing at the times under discussion. The short-term nature of the anomalies is quite different from the long-term delayed changes expected from severe magnetic storms [Bracewell et al., 1951; Burgess, 1964], and no notable storms were reported for several days prior to the anomalies. The signal perturbations to be discussed here more closely resemble perturbations observed in the auroral zone [Bates and Albee, 1965; Reder et al., 1964]. The midlatitude perturbations were accompanied by magnetic bays. Magnetic bays often accompany auroral-zone perturbations, but a direct correlation apparently does not exist in that region.