Magnetospheric Research ELF/VLF Programme
The position at which the overwintering station was established (at an altitude of 1700 m) had special advantages for the measurement of movements in the magnetosphere and changes in the precipitation of particles from it, as shown by ELF/VLF signals. This formed part of a major international exercise, of interest not only to UK scientists but also to South African, American, French and Argentinean organizations.
The programme had been formulated by Dr K. Bullough of Professor Kaiser’s Department of Space Physics, University of Sheffield, England. Suitable mobile equipment was designed and constructed at Sheffield. It consisted of twin-loop antennae, preamplifier and receiver, tape recorder, Venner Oscillator, rechargeable battery power supplies, time code generator and goniometer.
This not only enabled ELF/VLF signals to be recorded, but provided a direction finding capability showing where the signals come from. By operating simultaneously with other stations, it was possible to fix the point at which the signals penetrated the lower part of the ionosphere.
Any irregularities in the ionosphere diffuse rapidly along the tubes of force and thus generate radio guides linking one hemisphere with the other. Movements in the high atmosphere cause these tubes to move and thus it is possible to watch the circulation in the magnetosphere from the ground using naturally generated signals. The inner part of the magnetosphere co-rotates with the earth. The outer part forms a comet tail extending beyond the moon, which is approximately constant in form relative to the sun. The solar wind blows the outer part of this tail away from the sun setting up a circulation with an inward flow near the centre of the tail. Changes in movement with time or position can cause acceleration of particles and thus precipitation of high energy particles into the ionosphere which are then visible as aurora.
The most interesting and complex movements occur near the boundary between the co-rotating part of the magnetosphere, which has high electron density, causing great delay of the signals, and the relatively rarified plasma of the rail. The mean position of this is strategically placed relative to Halley Bay (British Antarctic Survey), Siple (U.S. base), SANAE (South African base) and the expedition’s overwintering station, Ryvingen.
There is a major international effort to study such changes using an east-west chain along a constant magnetic latitude. It is very desirable that cross bearings from widely separated stations should be supplemented by cross bearings between a close triangle of stations. SANAE, Halley Bay and Ryvingen formed a useful triangle since Ryvingen is about 120 kms from SANAE and both are 6oo kms from Halley Bay.
Observations made at the unmanned station at SEAL, 120 kms from Halley Bay during the relatively short periods in which this station was operating, have shown that interesting differences in behaviour can occur over a range of only 120 kms and that this separation is good for studying movements of ducts. A permanent station of the same type as the mobile station exists at Halley Bay and similar equipment of American design was recently installed at Siple and Palmer (U.S. bases) and SANAE. Thus observations made at Ryvingen can, in principle, be used in conjunction with SANAE as a short base system, with Halley as a medium base system using identical equipment at both ends, and with Palmer and Siple as a long based system. This would complement the planned cooperation between Siple, Palmer, Halley Bay and SANAE. It is therefore to be expected that more than one of the national groups working in this field will be interested in using the data obtained at Ryvingen.
One of the major reasons for the international interest in the study of movements using ELF/VLF signals in the Weddel Sea sector of Antarctica is that conditions are exceptionally favourable for the generation and propagation for such signals. During winter between one and two orders of magnitude wore signals are received than in the northern hemisphere during its local winter, and the great skewness of the invariant magnetic pole relative to the geographical pole in the south causes the circulation pattern to swing over an abnormally wide range of geographical latitude. Thus it is relatively easy to identify, and also to follow the movements of individual ducts for many hours, and also to obtain sufficient observations of each duct to minimise the random errors inherent in any radio direction finding system.
In addition, the existence of much man-made radiation in these bands from the industrial zones of the eastern USA and Canada causes strong amplification of signals moving from north to south, thus enabling ducts to be followed over several thousands of kilometres, instead of a few hundred as is normal in the northern hemisphere.
Bullough K. and Sagredo JL. 1973
VLF goniometer observations at Halley Bay Antarctica — I. The equipment and measurements of signals bearing.
Planet. Space Sci. 21 899—912.
Sagredo JL. and Bullough K. 1973
VLF goniometer observations at Halley Bay, Antarctica II. Magnetic structure deduced from whistler observations.
Planet. Space Sci. 21 913—23.
Kaiser TR. and Orr D. and Smith AJ. 1977
VLF electromagnetic phenomena: Whistlers and micropulsations.
Phil. Trans. R. Society London Bz79 226—38.