Since Ferrell (1951) first reported anomalous VHF propagation in equatorial regions, a number of workers have observed the phenomenon during the early afternoon and evening hours, especially the latter. Backscatter experiments in the neighbourhood of the Virgin Islands were carried out by Villard, Stein, and Yeh (1957) employing fixed-frequency radar sounders located at St. Croix and by Dueno (1960) from Mayagiiez. Villard, Stein, and Yeh observed backscatter signals,. at frequencies up to 46� 2 Mc/s, corresponding to slant ranges up to 8000 km between 1500 and 2100 hr local time. These results were attributed to successive reflections from a tilted F layer without intermediate ground reflections (n F propagation). Dueno investigated the long-range trans-equatorial (LRTE) echoes at frequencies up to 49�68 Mc/s over a two-year period. He found a marked seasonal variation in the occurrence, the maxima occurring during the equinoctial periods. Low angle of arrival of the signals was deduced.
Sky maps of red airglow at Townsville show a progressive development of an after mid‐night anomaly from quiescent winter conditions to early summer. The enhancement is most prominent in the northern sky. Comparison is made with similar experiments carried out by other workers at an approximately conjugate station at Maui. The observations are discussed in terms of ionospheric parameters h′ƒ and ƒoF2 indicating the influence of ionospheric tilts and electron density on the results. Electron density appears to have a definite influence.
Early reports of the high‐altitude nuclear explosion over Johnston Island (169.5°W, 16.7°N) on July 9, 1962, show that the [O I] 6300 A airglow emission increased at places remote from the explosion. Odencrantz et al. [1962] observed an increase of the order of 10 rayleighs at China Lake, California (117.6°W, 35.7°N). F. E. Roach (private communication, 1962) has suggested that much higher increases of the order of hundreds of rayleighs might be expected in the magnetic conjugate region to Johnston Island. This prediction met with spectacular confirmation from New Zealand.
The size and brightness of the airglow emitting regions investigated at Townsville (latitude, 19.25°S; longitude, 146.75°E; geomagnetic latitude, 28.4°S) during the period between July and November 1962 undergo systematic diurnal and seasonal changes. Diurnal behavior in the tropical zone is characterized by middle‐of‐the‐night enhancement on some summer nights. Enhanced 6300 A airglow from regions 500 to 1000 km in diameter centered on a single sky location were observed by Barbier et al. [1962] at Maui (latitude, 20.8°N; longitude, 156.5°W; geomagnetic latitude, ∼20°) in the northern summer of 1964. This kind of behavior also occurs at Townsville; it has been established there [Carman and Gibson‐Wilde, 1964] that early summer middle‐of‐the‐night enhancement develops progressively from complete absence in winter.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.