ducing photographic records of short- and long-pe-riod changes in declination, horizontal intensity,and vertical intensity, are operated at these stations.In addition, absolute instruments, with standardiza-tions traceable to international magnetic standards,are used to maintain base-line control to the highestpossible degree of accuracy.

Secular change patterns in Antarctica portrayedon the latest issues of magnetic charts, as well asisolines of the magnetic elements, have been greatlyimproved in accuracy over similar charts issued afew years ago.

The Byrd and South Pole observatories are among15 widely scattered magnetic observatories oper-ated by C&GS from Antarctica to Point Barrow,Alaska, and from Puerto Rico to Guam.

In combination with the magnetic observatories atByrd and South Pole, C&GS also operates seismo-logical observatories at these stations.

Antarctic Plateau VLF Studies

J. H. CRARYInstitute for Telecommunication Sciences &

AeronomyEnvironmental Science Services Administration

Unusual phase and amplitude changes have beenseen on very-low-frequency (VLF) signals that areapparently the result of abnormal ionization in theD-region created by the effects of electrons precipi -tated from the Van Allen belt and impinging uponthe D-region. These events are believed to occurduring magnetic disturbance, but little is known ofthe details or the conditions under which abnormalD-region ionization is produced. The region of theSouth Atlantic magnetic anomaly should be verysensitive to any such dumping.

The Plateau Station VLF program was plannedbecause of the station's unique position. The greatcircle path from the U. S. Navy VLF transmitterNAA (17.8 kc./s.) at Cutler, Maine, passes veryclose to the center of the South Atlantic magneticanomaly. The path from the U.S. Navy StationNSS (21.4 kc./s.) at Annapolis, Maryland, passesthrough a part of the magnetic anomaly and alsopasses nearly over a VLF recording station at Tucu-man, Argentina. Paths from other VLF transmittingstations pass a considerable distance from the mag-netic anomaly. This fortunate orientation of thepropagation paths of the VLF signals should make

it possible to investigate the effect of the South At-lantic magnetic anomaly on VLF propagation. Inaddition, the proximity of some paths to the receiv-ing station at Tucuman should make it possible toseparate effects occurring in the equatorial and mid-dle-latitude parts of at least some of these pathsfrom those occurring in the anomaly or on the high-latitude portion.

An improved three-channel VLF system for re-cording phase and amplitude on a paper chart wasdesigned and built for Plateau Station. The equip-ment was installed when the station was first set upearly in 1966. The equipment was not operating earlyenough for data to be sent out from the stationthis season, so it is not possible to give any analysisnow. Radio messages indicate that the equipmentis operating satisfactorily. The VLF data gatheredat Plateau Station should be interesting and useful asa tool for the study of electron precipitation phe-nomena and should provide a natural supplement toother VLF data obtained in the Antarctic andnearby regions for high-latitude VLF propagationstudies.

Future plans include the addition of another re-ceiving and recording channel and a digital dataacquisition system, with paper tape output, for theVLF system at Plateau. This equipment is ex-pected to be ready for installation during the 1966-1967 season.

Conjugate Point Investigations

WALLACE H. CAMPBELLInstitute for Telecommunication Sciences &

AeronomyEnvironmental Science Services Administration

Natural field variations in the 0.005 to 5.0 cps.frequency range are recorded at Byrd, Pole, andPlateau Stations. Of particular interest during thepast year was the behavior of a natural field phe-nomenon, called type Pc I geomagnetic micropulsa-tions, at the conjugate locations of Eights Station(discontinued in October 1965) and Quebec, Canada,and Byrd and Great Whale River, Canada. Uniquefield-guided characteristics such as signal polariza-tion, recurrence structure, and spatial distribution areused to verify a hydromagnetic mode of energy pro-pagation. Signal behavior at various solar-terrestrialdisturbance levels is investigated to reveal the under-lying source characteristics. The differing daylightperiods at the two stations of a conjugate pair areused to evaluate the absorption of hydromagneticwaves in the ionosphere.

196 ANTARCTIC JOURNAL