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 Tromso). The Tromso 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.

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

Equatorial scintillations—a review

Coordinated in situ measurements of high-resolution (spatial resolution 35 m) irregularity structures in the topside ionosphere using the retarding potential analyzer (RPA) on the AE-E satellite and scintillation measurements using the geostationary satellite Marisat transmissions at 257 MHz and 1.54 GHz from Ascension Island were made in December, 1979 during the recent solar maximum period. The in situ irregularity spectra could be classified into two categories: those having or not having spectral breaks. The first category of spectra is characterized by two spectral indices: one above and the other below a scale-length that lies typically between 500 m and 1 km. The spectral index in the long scale-length end (10 km to ∼1 km) varies between −1 to −1.5, whereas the index in the short scale-length end (∼1 km to 70 m) is between −3 to −3.5. In the second category, the power law spectral index of the irregularities ranges between −2 to −2.5 over the entire scale-length range of 70 m to 10 km. The in situ measurements reveal that in the early evening hours, the E-W gradient scale-lengths can be as small as 35 m, the sampling interval of the RPA instrument. In the pre-midnight and post-midnight hours, the gradient scale length becomes larger, but scale lengths of several hundred meters are frequently encountered. The temporal variation of 1.54-GHz scintillation magnitudes are found to track the E-W bubble structure, scintillations being minimum when the ray traverses the center and large when it crosses the eastern and western walls of the bubble. On the other hand, scintillations at 257 MHz remain saturated during an encounter with a bubble and focussing is often observed, but the autocorrelation interval tracks the bubble structure, being large when the ray path is deep within the bubble where the perturbation is weaker. The power spectra of weak 1.54-GHz scintillations are found to be consistent with the predictions of weak scatter theory based on irregularities having the observed in situ spectra. Significant differences are noted in the spectra of strongly scattered 257-MHz scintillations. In this case the decorrelation bandwidth extends beyond 1 Hz, and the spectral slope of the transitional portion of the spectrum becomes as steep as −6. The moderate to strong GHz scintillations sometimes show a dual slope power spectrum with the lower frequencies exhibiting a slope of −1.5 and the higher frequencies showing a slope of −5.5. The observations of steep structures in irregularities with scale-length of a few hundred meters near midnight, implying relatively slow erosion of sharp gradients are discussed in terms of plasma processes in the topside ionosphere.

High resolution topside in situ data of electron densities and VHF/GHz scintillations in the equatorial region

A high-performance rocket carrying a four-frequency, phase-coherent beacon and full complement of in situ diagnostic instrumentation was launched into active equatorial spread F on July 17, 1979. In this paper we report the results of spectrally analyzing the beacon phase-scintillation and Langmuir probe data. By using simultaneous backscatter data from the Altair radar we were able to establish that the scintillation develops in high-density regions adjacent to the prominent plume structures and associated depletions. In these high-density regions the in situ spectra show a pronounced change in the power law slope near a spatial wavelength of 500 m. Larger scale structures admit a systematically varying power law index that is generally less than 2, in good agreement with a large body of Wideband satellite data and recently analyzed Atmospheric Explorer E data. Smaller-scale structures admit a spectral index much larger than 2. A single, overall power law near k−2 was found only in low-density regions that did not contribute significantly to the scintillation. The results presented here and in a companion paper suggest that refinements in the current theories of equatorial spread F near and above the F region peak are needed.

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Simultaneous Rocket-Borne Beacon and In Situ Measurements of Equatorial Spread F - Intermediate Wavelength Results

[1] GPS L1 C/A signal scintillation data were collected at the equatorial anomaly over a period of three months using five receivers spaced on magnetic east-west and north-south axes to examine the speed, orientation, shape, width, and duration of GPS scintillation fade patterns. The nighttime speeds were primarily eastward in the range of 100–200 m/s with a significant spread to both larger values and negative (westward) values as expected, given the known behavior of ionospheric drifts and GPS signal path movement. The characteristic velocity was found to be small so that the true velocity was equal to the apparent velocity to a very good approximation. The orientation of the scintillation fade patterns was organized by a simple projection model of the magnetic field along the GPS signal path onto the horizontal plane when the signal paths were aligned no closer than 60° from the magnetic field. The shape of the scintillation fade pattern was greatly elongated in the magnetic north-south direction, and no change could be detected over a distance of 1 km. The east-west widths of the scintillation fade patterns were variable, but after normalizing to the elevation angle, accounting for the fade orientation, and eliminating signal paths within 60° of the magnetic field, an organized scale length of about 450 m was determined. The duration of the scintillation fade patterns was examined using the optimal cross-correlation amplitude as a measure of change. For a 5 s duration, 49% of the optimal cross-correlation amplitudes exceed a value of 0.8.

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Size, shape, orientation, speed, and duration of GPS equatorial anomaly scintillations

The time structure of a representative set of weakly and strongly scintillating transionospheric beacon signals is analyzed. Under conditions of weak scatter, the coherence time of the signal intensity is a monotonic function of the Fresnel radius divided by the effective scan velocity. The shape of the function, however, is controlled by the power law index. Data from a Peruvian station show evidence of a slightly steeper spectral distribution than do data from Kwajalein in the Marshall Islands. Under conditions of strong scattering, the intensity coherence time depends only on the perturbation strength. The strong scatter data show remarkably little dispersion when they are plotted against the perturbation strength. The data are all consistent with a phase spectral slope somewhat less than 3, which is independently verified by using phase scintillation data.

C. L. Rino

Papers

The time structure of transionospheric radio wave scintillation