VLF electrical properties of the ice sheet measured at Byrd Station

McMurdo - South Pole

AUGUST 5, 1972

Duggal, S. P., and M. A. Pomerantz. In press. Anisotropies in relativistic cosmic rays from the invisible disk of the sun. Journal of Geophysical Research. Pomerantz, M. A., and S. P. Duggal. 1972. Cosmic ray intensity variations in Antarctica. Antarctic Journal of the U.S., VI1(5): 162-163. Pomerantz, M. A., and S. P. Duggal. 1973. Record-breaking cosmic ray storm stemming from solar activity in August 1972. Nature, 241: 331-332. Pomerantz, M. A., and S. P. Duggal. In press. Remarkable cosmic ray storm and associated relativistic solar particle events of August 1972. Journal of Geophysical Research.

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Figure 3. Plot of the 6-minute average percentage increase from minimum intensity at South Pole and McMurdo Stations during the recovery phase of Forbush decrease 2. The data were normalized by dividing the South Pole values by 1.65. Tracking of the two profiles proves that no burst of relativistic solar cosmic rays accompanied the pulse of low energy particles detected during this period by instruments placed aboard spacecraft. FD-2. Analysis of the high time resolution data disclosed that a burst of about 68 percent was registered in a 10-minute interval at the South Pole. GLE-2 on August 7, 1972, was highly anisotropic and generally appeared to be 'typical," in marked contrast with its highly unusual predecessor (GLE-1) that was characterized by an abnormally steep spectrum, and consequently by a velatively short atmospheric absorption mean free path. A distinct north-south asymmetry occurred during the on-set phase of FD-1 and throughout FD-2, with minimum intensity in the Antarctic. The maximum modulation of the galactic flux during FD-2 occurred somewhere near 60 0 west of the sun-earth line. The pulse-like sharp recovery between 0300 to 0500 Universal Time on August 5 was coincident with a sudden rise in the considerably lower-energy solar particles recorded by detectors aboard spacecraft. However, as illustrated in fig. 3, comparison of data recorded at McMurdo and South Pole Stations proves that the cosmic ray spectrum at this time did not differ from that which prevailed during the cosmic ray storm immediately before and after this period (Pomerantz and Duggal, in press), leading to the undisputable conclusion that no detectable flux of particles with relativistic energies simultaneously was reaching the earth. This work was supported by the National Science Foundation.

References

Duggal, S. P., and M. A. Pomerantz. 1973. Relativistic cosmic rays from the sun's invisible disk. Proceedings of the 13th International Conference on Cosmic Rays. September-October 1973

VLF electrical properties of the ice sheet measured at Byrd Station J . C. ROGERS

Geophysical Institute University of Alaska I, C. PEDEN

Department of Electrical Engineering University of Washington This article summarizes results of the first in situ measurements of the dielectric and loss properties (complex permittivity parameters) of deep antarctic ice, as functions of vertical depth. In 1968, the U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, successfully completed the first drill hole to the bottom of the ice sheet at Byrd Station (Gow ci al.. 1968). The results, summarized here, are of interest to glaciologists as well as to polar scientists studying radio propagation over the ice cap in the VLF range, the polar ionosphere, and the operation Of vi.F antennas in the antarctic environment. During the 1969-1970 austral summer, an electrically short dipole probe with encapsulated instrumentation was used in the 2 164 ' -meter-deep drill hole, to obtain measured probe admittances at 5 discrete VLF frequencies (1.25, 2.5, 5, 10, and 20 kilohertz). Direct measurements were made down to the 1500-meter level, and the dielectric properties later were extrapolated to the bottom of the ice sheet. The latter procedure was necessitated by closure of the lower 30 percent of the drill hole before the experiment could be performed (Peden and Rogers, 197 1). Permittivity properties of the fluid in the drill hole also were measured as functions of depth, as was the hole diameter (Rogers and Peden, 1972). Using these data, the input admittance of the fluid-sheathed probe can be related unambiguously to the relative complex permittivity of the ice external to the sheath (Rogers and Peden, 1973). A simple 3-element equivalent circuit 241

whose frequency-independent capacitor elements experimentally were determined provides a means to reduce the data to the desired form. This work was done at the University of 'Washington, subsequent to the in situ measurement phase, using a laboratory model. Fig. 1 displays the results at three frequencies; the real and imaginary parts of the complex relative permittivity 1 * = ' - j 1 " are plotted as functions of vertical depth. In a lossless material, 1 ' would be referred to as the dielectric constant; 1 " accounts for the electrical losses of the ice. Theoretical values have been calculated for pure ice based on a Debye relaxation mechanism and the temperature profile of the drill hole measured by Gow et al. These values are plotted in fig. 1 as dashed lines. The close relationship between ice temperature and its complex permittivity is apparent from examination of the measured results. It has been determined that other physical variables such as crystal structure and ash content do not influence the measured

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results enough for detection by means of the dipole probe technique (Rogers, 1972). An extrapolation procedure was worked out for the complex permittivities below 1500 meters. Fig. 2 illustrates the results at the three VLF frequencies under consideration, and compares them with the appropriate values calculated for pure polycrystalline ice. It is interesting to compare this study's results with those obtained by other workers using different measurement techniques. Effective bulk average values of the complex permittivity were obtained from surface measurements of the amplitude and phase of the VLF nearfields surrounding the 34-kilometer dipole antenna that ANTARCTIC JOURNAL

formerly was in operation near Byrd Station (Peden, 1972; Peden et al., 1972). Crary and Crombie reported on a related parameter at 16 kilohertz, namely the deduced conductivity of the ice sheet based on the attenuation of vl.F signals traveling over very long paths that included Byrd Station and Rugby, England (1972). Interpolating the permittivities obtained from the in situ measurement to obtain a 16 kilohertz value of ", it is found that the ice in the upper 1500 meters is not as lossy as would be expected from the results of Peden et al., while the deeper ice is lossier. The' results of Crary and Crombie compare most favorably with the extrapolated 2 150-meter results of the drill hole measurement. Clearly the ice sheet can not accurately he represented in the VLF range as a uniform, isotropic, lossy dielectric. Nevertheless, useful approximate values of . * can be obtained from surface measurements in situations where drill holes are not available. The data presented here represent the results of the first in situ measurements of an important electrical parameter of the ice sheet, as functions of depth and of VLF frequency. They are the most accurate presently available for the ice mass covering the interior of Antarctica. The authors gratefully acknowledge the support of the Office of Polar Programs, National Science Foundation, and the assistance of members of the Cold Regions Research and Engineering Laboratory team at Byrd Station, as well as the skilled support of the U.S. Navy. The work was carried out under National Science Foundation grant GV-29356.

References

Crary, J . H., and D. D. Crombie. 1972. Antarctic ice cap attenuation rates of VLF signals determined from short and long great circle paths. Radio Science, VlI(2): 233-238. Gow, A. J . , H. T. Ueda, and D. E. Garfield. 1968. Antarctic ice sheet: preliminary results of first core hole to bedrock. Science, 161: 1011-1013. Peden., I. C. 1972. VLF dielectric and loss properties of the ice sheet at Byrd Station. Antarctic Journal of the U.S., V1I(5): 164-165. Peden, I. C.. and J. C. Rogers. 1971. An experiment for determining the VLF permittivity of deep antarctic ice. IEEE Transactions on Geoscience Electronics, GE-90). Peden, I. C., G. E. Webber, and A. S. Chandler. 1972. Complex permittivity of the antarctic ice sheet in the VLF band. Radio Science, VI1(6): 645-650. Rogers, J . C. 1972. A measurement technique for determining the VLF permittivity of deep antarctic ice using a dipole probe. Ph.D. Dissertation, Xerox University Microfilms, Ann Arbor, Michigan. No. 2087-B. Rogers, J . C.. and I. C. Peden. 1972. Effect of Byrd drill hole diameter variations on in si/u electrical measurement of the ice sheet. Antarctic Journal of the U.S., VII( 5): 165-166. September-October 1973

Rogers, J . C., and I. C. Peden. In press The electrically short sheathed dipole: experimental relationship between its measured admittance and permittivity of the external medium. IF FE Transactiouf on Antenna and Propagation.

Observed ionospheric scintillations ARNOLD J . TUCKER

Applied Research Laboratories University of Texas, Austin The doppler shift of C-W signals, transmitted from geodetic and navigational satellites, was recorded at the Geodetic Satellite Observatory, McMurdo Station. These recorded data are on paper tape and were sent to the U.S., for analysis. Initial analysis of these data consists of comparing each set of experimental data to a set of theoretical data for each satellite pass. The residuals resulting from differences between the experimental and the theoretical data are minimized by a least squares solution. The station position and station frequency are the parameters of the least squares solution. The root mean squares of each set of residuals is computed and is referred to as the filter noise. This filter noise is a measure of the noisiness of the experimental data. A determination of the nominal value for the filter noise can be based on the characteristics of the equipment. Since the performance of the equipment is relatively constant, any variations in these results can be associated with the propagation conditions along the signal path. Using large values of filter noise as a criterion for possible scintillations along the propagation path, specific sets of residual data were selected for more thorough analysis. Plots of the residual data were obtained for each of these sets of data. The segments of each pass that denote noise data were marked and the times of occurrence of the scintillations were determined. The region of the ionosphere where these scintillations occur was assumed to be at an altitude of 300 kilometers. Using the orbital parameters of the satellite and the location of the ground station, the penetration of the propagation path between the satellite and ground station (with the ionosphere at 300 kilometers) was computed for each segment of these passes where there were scintillations. Noted was the spatial location and the time of occurrence of these scintillations. Similar data are being recorded at South Pole Station, at Casey Station, and at Palmer Station. These data will be processed and analyzed, using the methods described above, and combined with the results ftom McMurdo Station to show the spatial and time dependence of the observed scintillations.

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