Response of the ionosphere to the passage of acoustic-gravity waves generated by low-altitude nuclear explosions

Abstract

The response of the bottomside ionosphere to the passage of acoustic-gravity waves generated from a large low-altitude explosion (Housatonic) has been observed by means of vertical-incidence ionosondes. Acoustic-gravity wave disturbances propagated essentially horizontally away from the detonation point (G0) with velocities of 610 and 362 m/sec corresponding to the first and second positive peaks of the disturbance. The disturbance was first sensed at each station at times corresponding to 765 m/sec. They were detected up to 4000 km from G0. The geomagnetic field played an important role in the ionosphere response of these gravity waves. For those stations located north of G0 and those south of the geomagnetic equator (group I stations) ionization first slid down the magnetic field lines; whereas, for stations south of G0 but north of the geomagnetic equator (group II stations) ionization was first forced to move up the magnetic field lines. In both cases fluctuations in ionization density at a given height were observed. Considering the height and shape of the ionosphere, the ionization migration caused (1) the ƒ0F2 peak height hm to be lowered as much as 105 km as the disturbance reached its first positive peak overhead and the bottomside ionosphere to be flattened out for group I stations and (2) the opposite behavior for group II stations as the ƒ0F2 disturbance reached its first negative peak. Ionosondes located at distances from 450 to 4000 km from G0 observed fluctuations in electron concentration which at their first peak excursions reached from −20% (for group II stations) to +68% (for group I stations). The observed ƒ0F2 fluctuations appear to fit an approximate theoretical formula especially with regard to their magnetic field direction and amplitude-period dependence. Ionosonde data following the near-surface 58 Mt explosion on October 30, 1961, at Novaya Zemlya also fit this theory.