Ionospheric constraints on VHF/UHF communications links during solar maximum and minimum periods

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Abstract
An extensive VHF/UHF scintillation data base covering the frequency range of VHF to a few gigahertz has been utilized to determine the magnitudes of phase and intensity scintillations and their temporal/spatial structures during the sunspot maximum and minimum periods. The equatorial portion of the study has been based on geostationary satellite observations at Huancayo, a station on the magnetic equator, and at Ascension Island, which is an equatorial anomaly station having an extremely disturbed irregularity environment. The high‐latitude part of the study is based on quasistationary satellite measurements at a polar cap location (Thule) and two auroral locations (Goose Bay and Tromsø). The Tromsø observations are augmented with the Defense Nuclear Agency HiLat satellite beacon measurements during the solar minimum period. The data indicate a strong solar cycle control of scintillation activity at all locations, resulting in a drastic reduction of the magnitudes and occurrence of scintillations during the current solar minimum period. This pattern is consistent with both a reduction of F region ionization density and a reduction of irregularity generation in the solar minimum period. At the magnetic equator the magnitude of scintillations at 1.5 GHz seldom exceeds 3 dB with the percentage occurrence > 2 dB varying from 70% during high sunspot conditions to 30% during low sunspot conditions. At the crest of the equatorial anomaly, on the other hand, during the solar maximum in 1979, fades of 20 dB at 1.5 GHz are observed 30% of the time. At a decreased level of solar activity in 1982, a similar occurrence level is obtained at 1.5 GHz for fade levels of only 5 dB. During the solar minimum period, 1.5‐GHz scintillations are virtually absent. Phase scintillation measurements made at Ascension Island indicate that the median value of rms phase deviation is about 5 rad for detrend intervals of 100 s. In the auroral region, during the solar maximum period under magnetically disturbed conditions, the median values of scintillation fades and rms phase deviation (82‐s detrend) at 250 MHz are observed to be 15 dB and 3 rad, respectively. At Thule, located deep within the polar cap, the median values of scintillation fades and rms phase deviation at 250 MHz attain values as large as 20 dB and 4 rad during the sunspot maximum period. Unlike Ascension Island the scintillation activity at high‐latitude stations exhibits a threshold effect and does not decrease until 1983. However, in 1986 with sunspot numbers in the vicinity of 10, fade levels as low as 5 dB at 250 MHz are recorded in the polar cap and auroral stations only 5% of the time. It is noted that at auroral locations the most prominent feature, namely the existence of magnetic L shell‐aligned irregularity sheets, is equally evident at both sunspot maximum and minimum.