Upper Atmosphere PhysicsUniversity of Washington ResearchProgram in Antarctica, 1965-1966

DONALD K. REYNOLDS andH. MYRON SWARM

Department of Electrical EngineeringUniversity of Washington

During 1965-1966, a permanent building wasconstructed at the long-wire site, about 12 milesfrom Byrd Station, and a second dipole antenna, 10miles in length, was laid perpendicularly to the 21-mile antenna. A diagram of the layout of theseantennas is shown in fig. 1. Research was carried

Fig. 1. Diagram of Long-Wire Antennas

out by the University of Washington group on fivedifferent problems, viz.:

VLF ionospheric studies. The D-region, locatedin the height range 50 to 100 kilometers above thesurface of the Earth, is the interface between therelatively un-ionized lower atmosphere and theionosphere. This is a complex and rapidly varyingregion, especially at auroral latitudes, and is char-acterized by very low electron densities, high colli-sion frequencies, and rapid motions. The electrondensity varies rapidly at times of magnetic disturb-ance and may increase enormously to cause the so-called polar-cap absorption events.

Experimental study of the D-region by reflectionof electromagnetic waves has not been successful inthe past owing to limitations in antennas. The long-wire antennas at Byrd Station have removed theseexperimental restrictions. Unlike the extremely nar-

row band vertical antennas used with conventionalVLF stations, the horizontal antennas near ByrdStation have very broad bandwidths and producemaximum radiation in the vertical direction.

A VLF D-region sounding system was installedand put into operation at Byrd Station during the1965-1966 season, and has been in daily operationduring the 1966 winter. The system consists of akeyer unit and transmitter at the long-wire antennasite, and a receiver at the Stanford VLF site. Thekeyer unit generates a sequence of pulses at a succes-sion of frequencies between 3 and 30 kc./s., whichare amplified to peak power levels of about 10kilowatts and applied to the 10-mile antenna. Theenvelopes of the pulses are approximately Gaussian,which reduces to a minimum the spectral width ofthe signal at each operating frequency. Oscilloscopicphotographs of the current wave form fed to theantenna at 5 and 30 kc./s., respectively, are shownin fig. 2. Reflected signals from the ionosphere are

Fig. 2. Current Wave Forms. Left, 5 kc./s.; Right, 30 kc./s.

picked up in a loop antenna at Byrd Station andtransmitted back to the long-wire site via a 450mc./s. telemetry link, where a phase-comparisonsystem is used to measure the propagation delaybetween transmitted and received signals. The rec-ords are currently on film; however, a digital tapesystem is planned for next year.

Results to date from the D-region sounding systemare very encouraging. Ionospheric reflections arebeing received consistently, and a more accuratepicture of the physics of the D-region is emerging.

Continued antenna research. The input impedanceof the 21-mile dipole has been measured periodicallyto determine the effects of temperature and depthof burial under the snow. During January andFebruary 1966, the second dipole antenna, 10 milesin total length, was laid down perpendicularly to the21-mile antenna. Like the longer antenna, the newone is made from the core of RG 17/U cable, con-

September-October, 1966 185

sisting of a polyethylene-covered, solid copper wire.The new antenna increases considerably the capabil-ities of the site, making possible either separate op-eration of the two antennas, or the combinationinto a single antenna system of controllable polariza-tion.

Airborne field-strength measurements. An attemptwas made during February 1966 to measure froman aircraft the radiated fields from the 21-mileantenna. While the aircraft was to have flown to amaximum distance of 500 miles from the center ofthe antenna and measured field strengths at a succes-sion of frequencies in the 1 to 30 kc./s. range, highnoise levels in the airborne receiving equipmentprevented all but local measurements from beingmade. The experiment was not a complete failure,however, as good data were obtained on the currentdistribution along the antenna.

Earth-ionosphere cavity resonances. There hasbeen considerable interest in recent years in theresonances of electromagnetic waves in the Earth-ionosphere cavity. The lowest resonant frequencieshave been calculated to occur at 8.0, 14.1, 20.3, and26.4 cps. In the past, experimental studies of theseresonances have depended entirely on the naturalradiations resulting from lightning activity throughoutthe world. The feasibility of exciting and detectingthese resonances, using man-made signals from the21-mile antenna, is being studied by the Universityof Washington group.

Exospheric electron density program. The objec-tive of this program is to measure the propagationtime of signals transmitted at a series of VLF fre-quencies between Byrd Station and the Seattle,Washington, area via exospheric paths. From thepropagation delay vs. frequency, electron densities inthe exosphere can be calculated. Since the strengthof the signals received in the Northern Hemisphereis expected to be very low, a special binary, codedsequence and correlation-detection scheme is beingemployed. Equipment to key the 100-kilowatt solid-state transmitter at the long-wire site was shippedto Antarctica for the 1965-1966 summer program;however, problems with the transmitter preventedthe keyer from being used, and field modificationswere initiated to adapt the keyer to the lower-powered transmitter used in the D-region sounderprogram.

Receiving equipment will be installed during 1966at an electrically quiet location in the San JuanIslands, about 70 miles north of Seattle, Washington.It is hoped that the field modifications at the long-wire site will be successful and that data can beobtained during 1966.

Ionospheric Forward-Scatter Programin the Antarctic

MARTIN A. POMERANTZBartol Research Foundation

of the Franklin Institute

The three ionospheric forward-scatter circuits:Byrd-Pole, Byrd-McMurdo, and McMurdo-Vostok,operated satisfactorily during the past year. Onlyone polar cap absorption (PCA) event, indicatingthe arrival of solar protons with energies exceeding10 MeV., was observed during the one-year period.The solar particle influx was detected on March 24following the importance 3 solar flare that com-menced at 0233 UT. The records, which displayedunusual characteristics, are being studied in detail.

More than 70 relativistic electron precipitation(REP) events have now been identified in therecords of the Alaskan forward-scatter paths. Theircharacteristics have been established, and generalizedconclusions concerning the spectra of precipitatedelectrons have been deduced. It is noteworthy thatthis phenomenon has never been observed onthe antarctic paths, a fact that is of prime import-ance in confirming our hypothesis concerning themechanism responsible for this new type of radio-wave absorption event. Elaborate calculations now inprogress should make it possible to determine theelectron flux and energy spectrum for any specificevent from the observed radio-wave absorption.

Analysis of recordings of the occurrence of thesporadic E mode of signal propagation has yielded

• — - Frobisher Boy to Great WhalePoleo Byrd t South0900

geomagnetic— Byrd to McMurdo

EKp<IO

II/\. I.%1200 II1..Jii-. I

occurrence-

_\/ \\ \

The Maximum Occurrence of Sporadic E at Two ConjugatePoints Occurs at the Same Local Time if Represented inGeomagnetic Rather Than Geographic Coordinates.

186 ANTARCTIC JOURNAL