vestigate the nature of ionospheric irregularities inAntarctica (Bowman 1966b). The angle of elevationof these signals can be determined for EllsworthStation from the interference effects between the di-rect ray and that which travels through the 300-meter thickness of ice and is reflected at the ice-seawater interface. A system of nulls is producedwhich moves to different frequencies for differentoff-vertical angles (Evans, 1961; Bowman, 1966c).Also, many months of antarctic ionograms have beeninspected for clues to possible theories of irregular-ity shapes. An important observation is that al-though the spread-F ionograms often appear as dif-fuse echoes, a structure can be seen through thediffuseness. The structure appears in the form oftraces having the same shape as the main trace, i.e.,near vertical. This and the angle of elevation infor-mation suggest that whether or not a small-scalestructure (i.e., columns of enhanced ionization withdimensions of the order of one kilometer or less)exists, a large-scale structure of the order of tens ofkilometers also exists, giving rise to the observedresolved traces.

Several research problems are currently being in-vestigated. One is an analysis of the topside elec-tron-density morphology during an M-region mag-netic storm. The time history of the electrons willshed light on the phenomenon of F-layer reduction inpeak electron density during ionospheric storms.Another study involves the use of ionogram angleof elevation information to investigate large-scaleirregularities over Ellsworth Station. The time periodis the early evening hours during equinoctial months.These irregularities move towards the Equator withspeeds of several hundreds of kilometers per hour.At the position of maximum disturbance, the elec-tron content can be reduced by a factor as high asten.

ReferencesBowman, G. 1966a. Some geomagnetic and ionospheric ef-

fects in Antarctica prior to storm sudden commence-ments. Wilmington, Mass., Avco Corporation. 28 p.(Antarctic Research and Data Analysis. Scientific report23.)

Bowman, G. 1966b. The nature of ionospheric irregularitiesin Antarctica. Wilmington, Mass., Avco Corporation.(Antarctic Research and Data Analysis. Scientific re-port 24.)

Bowman, G. 1966c. Directional characteristics of ionosondeinterference patterns on the Filchner Ice Shelf. Wilming-ton, Mass., Avco Corporation. (Antarctic Research andData Analysis. Scientific report 25.)

Evans, S. 1961. Polar ionospheric spread echoes and radiofrequency properties of ice shelves. Journal of Geo-physical Research, 66(12): 4137-4141.

Rourke, G. F. 1966. An investigation of summer eveningincreases of F-layer electron density in the region of theAntarctic Peninsula. Wilmington, Mass., Avco Corpora-tion. 43 p. (Antarctic Research and Data Analysis.Scientific report 22.)

Geomagnetism: Plateau Station

JAMES V. HASTINGS and D. J. ELVERSU. S. Coast and Geodetic Survey

Environmental Science Services AdministrationA special magnetograph, differing markedly from

conventional instrumentation, was designed and fab-ricated for use at Plateau Station, at the Fredericks-burg Geomagnetic Center.

The magnetograph consists of a triaxial, satur-able-core, inductor-type (fluxgate) system for meas-uring variations in declination (D), horizontal inten-sity (H), and vertical intensity (Z). Magnetogramsare recorded on two multiple-channel, potentiomet-nc-type recorders. The rapid-run recorder has anoption of eight chart speeds from 0.125 to 16inches per minute and is equipped with proper inputfiltering for recording short-period variations. Thenormal recorder is unfiltered and provides a recordof long-period variations as well as daily variationsand magnetic storm activity. Chart speed of thenormal recorder is 1 inch per hour. Both normaland rapid-run recordings are in real-time.

The sensor mounting is designed to eliminate, bymeans of a level-maintaining suspension, the errorsproduced by differential settling of the instrumentpiers. The sensing unit is housed in a small buildingapproximately 500 feet from the main camp. Thenormal and rapid-run recorders are located in themain camp, allowing the observer to monitor the re-corders and change records daily without leaving themain camp building. The periodic absolute meas-urements of the magnetic elements, for the purposeof maintaining base-line control, are made in asmall building approximately 60 feet from thesensor house. The absolute instruments consist of asuspension-type magnetometer for D, a quartz hori-zontal magnetometer for H, and a proton magnetom-eter for measurements of total intensity (F). Abso-lute Z is computed from H and F.

The installation of housing, piers, and the mag-netograph began in late January 1966 and wascompleted by February 8. A portion of the dataobtained on February 9 and 10 is shown below.

1 -

'HSeptember-October, 1966 189