The detection of daytime nuclear bursts below 150 km by prompt VLF phase anomalies

The purpose of this paper is to examine theoretically the detectability of daytime nuclear bursts via prompt VLF phase anomalies. Detailed consideration is given to low-fission-yield bursts exploded at altitudes below about 150 km. The distribution of atmospheric ionization due to fission-decay-gamma radiation, which is the most important ionizing mechanism for the situations studied here, is calculated for times up to a minute after the bursts. It is noted that, at the deposition altitudes of interest for daytime VLF detection, the distribution of ionization computed for a time of one second after the burst is typical of that which persists for some tens of seconds. By using these one-second values of ionization and by applying existing techniques, equivalent, nonuniform, sharply bounded, earth-ionosphere cavities are constructed. Currently used VLF propagation theory is then applied to compute the bomb-induced phase anomalies for various burst-transmission path configurations. Detection criteria, based upon phase changes which should be distinguishable from those caused by natural events, are chosen. The corresponding detection ranges (the smallest great-circle distance from the burst to the transmission path) for long daytime VLF transmission paths are computed. Numerical results, which show the detection ranges as functions of the height and magnetic latitude of a nominal 5-kiloton fission-yield device, are presented. The manner in which these ranges scale with fission yield is indicated. The detection ranges are shown to depend very strongly upon the burst altitude, but quite weakly on other parameters. For very low bursts the ranges become extremely small while, for burst heights the order of 100 km or more, they are quite large.