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This paper describes observations of the reliability of the ionospheric HF communication channel within and close to the northern auroral zone. Automatic equipment has been developed to measure reliability and virtual reflection height as a function of frequency in an hourly cycle. Measurements have been made over two propagation paths within Norway. The data show diurnal and seasonal variations as well as dependence of the measured parameters on geomagnetic conditions. The observations have been compared with predictions obtained from the Ionospheric Communication Enhanced Profile Analysis and Circuit Prediction Program (ICEPAC) prediction code. This code includes a model of the high‐latitude and polar ionospheres and their dependence upon geomagnetic disturbances. The results of the comparison show that ICEPAC represents an improvement over previous codes but that transmission losses are not properly included.

This paper reports the results of the first concerted effort to study the effects of solar proton events (SPEs} on ELF propagation using a manmade source (i.e., the Wisconsin Transmitter Facility (WTF)). The major result is the observation of substantially increased effective attenuation rates along high altitude paths during SPEs. The largest values of inferred attenuation rate during SPE disturbed propagation conditions were t'or the WTF/Gulf of Alaska path (2.51 dB/Mm at night, and 2.66 dB/Mm during the day). In fact, during the 23 November 1982 minimum nighttime field strength period of 0630 to 0830 GMT, the effective attenuation rate for this path exceeded 3 dB/Mm. These attenuation rates are substantially higher than those measured during ambient propagation conditions on WTF/eastand-northeast paths (.,-1.0 dB/Mm at night, and ~ 1.25 dB/Mm during the day). The probable reason for the higher effective attenuation rates observed on the WTF/Gult' of Alaska path is that this path is nearly tangent to the exterior boundary of the disturbed polar cap. That geometry causes lateral refraction of the transverse electromagnetic (TEM) mode and thus shadow zones. there were substantial changes in their rates of oc~ the New London Laboratory (NLL) of NUSC for currence. However, there was no polar cap "black-signal demodulation and decoding. The AN/BSR-1 out" of the spherics, and their amplitude reduction receiver is composed of an AN/UYK-20 minicompuappeared to be no greater than the reductions oc-ter, a signal timing and interference unit (STIU), a casionally observed at lower latitudes in experiments rubidium frequency time standard, two magnetic tape recorders, and a preamplifier. The message Copyright 1987 by the American Geophysical Union. output is on a teletype (TTY), which is used to con-Paper number 6S0613. trO1 the receiver. Because the receiver must operate in 0048-6604/87/006s-0613508.00 real time, the reduction procedure, itself, takes 12 111 112 KATAN AND BANNISTER: ELF PROPAGATION DURING SOLAR PROTON EVENTS hours. Through this process, estimates of absolute nister, 1975] signal and effective-noise levels, (The effective-noise spectrum level (in dBA/m-(1 Hz) •/2) is defined as the 20 log H• ,-, K + 20 log E -spectrum level of ELF noise at the signal frequency -10 log [a sin (p/a)] + 20 log [F(qb)/B] (I} divided by the improvement (in snr), using nonlinear processing [Evans and Gr•qths, 1974].) signal-to-where K =--140.7 dB at 76 Hz and E= noise ratios (snr), and relative signal-phase estimates [ho(trEw)l/a(X/•l/2 ]-• is defined as the earth. are obtained. Depending on the snr, these estimates ionosphere waveguide excitation factor. Note that E can be obtained every 224, 448, and 896 s relative to is inversely proportional to the product of the possible message start times, which occur every 896 s fective ionospheric reflecting height, h 0 (in kilome. referenced to 0000 GMT each day. From these data, ters) times (trEw) 1/2. Also, the effective ELF attenuation rates and earth-ß ß O • e • ß © ß ß © ß ß 0 FIEL...

In March and April 1973, the 76-Hz magnetic field strength, produced by the Wisconsin Test Facility elevated and (newly installed) buried north-south antennas, was measured in Wisconsin and Texas. The principal result obtained is that there is no measurable difference in performance between the two antennas. o WEST OF CLAM LAKE MEASUREMENTS 8 EAST OF CLAM LAKE MEASUREMENTS -40

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