VOL. 80, NO. 34 JOURNAL OF GEOPHYSICAL RESEARCH DECEMBER 1, 1975 Initial Results From the Search Coil Magnetometer at Siple, Antarctica WILLIAM W. L. TAYLOR AND BODO K. PARADY School of Physics and Astronomy and SpaceScience Center, University of Minnesota Minneapolis, Minnesota 55455 PETER B. LEWIS AND ROGER L. ARNOLDY Physics Department,University of New Hampshire, Durham, New Hampshire 03824 LAURENCE J. CAHILL, JR. School of Physics and A stro. nomyand SpaceScience Center, University of Minnesota Minneapolis, Minnesota 55455 Micropulsations were observed at Siple, Antarctica, throughout 1973. The results of this brief examination of the data generally agree with previous studies. The search coil magnetometer used is most sensitive in the Pc I frequency range,whereits sensitivity approaches the natural background noise. The data are taken on computer-compatible magnetictape, facilitating computer processing. One Pc 1 micropulsation event was examined in detail. Well-known morphologicalcharacteristics of Pc I were found along with the result that duringthe eventa rotation of the principalplaneof polarization into or out of the H, D plane is common. Dispersion analyses of the Pc 1 event allowed a determination of the L value of the magnetospheric propagation path (6.1-6.3) and the equatorial electron density(3.1-6.4 1. INTRODUCTION The research station at Siple, Antarctica (geographic coor- dinates:76øS, 84øW), was designed and built under National Science Foundation sponsorship to provide a mid-latitude (L -- 4) base for conducting active and passive wave-particle in- teraction experiments in the ionosphere and magnetosphere. Similar studies were made earlier at Eights Station in Antarc- tica [Campbell and Silther, 1965; Campbell, 1967a, b]. The Universities of Minnesota and New Hampshire have cooperated in the design, construction, and operation of a three-axis search coil magnetometer system which is now operatingat Siple and recordinggeomagnetic micropulsation data at frequencieslessthan about 10 Hz. Approximately 9 months of data was returned from Siple early in 1974.A preliminary studyof some of the data hasbeen made and will be reported here. We intend this brief report to be an introduction to the Siple searchcoil instrument, to the data processing, and to the micropulsationrecordsthat are available. We hope this report will be usefulto our Siple col- leagues and to other investigators. Section2 describes the ex- periment, its design,capabilities,and performance. The data processing is described in section 3, with an emphasis on com- puter processing. Micropulsation data are discussed in section 4. 2. DESCRIPTION OF THE EXPERIMENT The micropulsation detector consists of three orthogonal permeable core search coils and associated analog and digital electronics. The magnetometer has operatedcontinuously for more than 2 years nearly unattended. The search coils and preamplifiers are mounted in the ice with the Z sensor (up) along the geomagnetic field line, the Y Copyright ¸ 1975 by the American Geophysical Union. sensor (east)horizontal and perpendicular to the Z sensor, and the X sensor (north) along the geomagnetic meridian, perpen- dicular to the Y and Z sensors. The search coils are 500,000 turn coils of number 36 copperwire mountedon 1.8-m-long by 2.5-cm-diameter annealed permalloy cores.Preamplifiers (gain, 103)at the coilsprovide millivolt signallevels for trans- mission to the main electronics in a building 250 rn from the search coils. Automatic periodic calibration is achieved by applyinga triangular current source to calibrationcoils, also wound on the cores. Additional amplification,analog to digital conversion, and digital recording of the data on magnetic tape are done in the main experimentbuilding. After amplificationin the main ex- periment building the frequencyresponse below 2 Hz is vir- tually identicalfor the three axes. The frequency responses for the three axesare shownin Figure 1, whichgives the magnetic field at the sensors corresponding to one digitizationunit. At 1 Hz, one digitization unit corresponds to about 0.4 m3', slightly more than the natural background field of 0.1-1.0 measured, for example,by Lokken et al. [1963] at about 1 Hz. More recent high-sensitivity measurements by Buxton and Fraser-Smith [1974] show that the background field above 1 Hz may frequently be less than 0.1 m3'. The natural background varies with time of day, season, and location; therefore the natural noiselevelmay be aboveor below0.4 at any particular time. The instrumentationused to record the amplified signalsconsists of the following elements: a nine- track digital incremental tape drive; a universal time clock driven by a crystal-controlled oscillator;a recorderinterface; analog to digital (A/D) converters; two filters, detectors, and integrators per, channel, corresponding to Pc 1 (>0.2 Hz)and Pc 2 (0.1-0.2 Hz) frequencies; a system calibrator; and a system control unit which executes the data sampling and recording program. This instrumentation samples and records the amplifiedsignals, the outputs of the Pc detectors, and the 4762