Starting with post World War II studies of fading of radio star sources and continuing with fading of satellite signals of Sputnik, vast quantities of data have built up on the effect of ionospheric irregularities on signals from beyond the F layer. The review attempts to organize the available amplitude and phase scintillation data into equatorial, middle-, and high-latitude morphologies. The effect of magnetic activity, solar sunspot cycle, and time of day is shown for each of these three latitudinal sectors. The effect of the very high levels of solar flux during the past sunspot maximum of 1979-1981 is stressed. During these years unusually high levels of scintillation were noted near the peak of the Appleton equatorial anomaly (∼ ±15° away from the magnetic equator) as well as over polar latitudes. New data on phase fluctuations are summarized for the auroral zone with its sheet-like irregularity structure. One model is now available which will yield amplitude and phase predictions for varying sites and solar conditions. Other models, more limited in their output and use, are also available. The models are outlined with their limitations and data bases noted. New advances in morphology and in understanding the physics of irregularity development in the equatorial and auroral regions have taken place. Questions and unknowns in morphology and in the physics of irregularity development remain. These include the origin of the seeding sources of equatorial irregularities, the physics of development of auroral irregularity patches, and the morphology of F-layer irregularities at middle latitudes.

/pdf/global-morphology-of-ionospheric-scintillations-37hijos7s3.pdf

Global morphology of ionospheric scintillations

An enigma of equatorial research has been the observed seasonal and longitudinal occurrence patterns of equatorial scintillations (and range-type spread F). We resolve this problem by showing that the seasonal maxima in scintillation activity coincide with the times of year when the solar terminator is most nearly aligned with the geomagnetic flux tubes. That is, occurrence of plasma density irregularities responsible for scintillations is most likely when the integrated E-region Pedersen conductivity is changing most rapidly. Hence the hitherto puzzling seasonal pattern of scintillation activity, at a given longitude, becomes a simple deterministic function of the magnetic declination and geographic latitude of the magnetic dip equator. This demonstrated relationship is consistent with equatorial irregularity generation by the collisional Rayleigh-Taylor instability and irregularity growth enhancement by the current convective and (wind-driven) gradient drift instabilities. Some discrepancies in this relationship, however, have been found in scintillation data obtained at lower radio frequencies (below, say, 300 MHz) that suggest the presence of other irregularity-influencing processes. The role of field-aligned currents, associated with the longitudinal gradient in integrated E-region Pedersen conductivity produced at the solar terminator, in equatorial irregularity generation via the current convective instability has not been discussed previously.

Control of the seasonal and longitudinal occurrence of equatorial scintillations by the longitudinal gradient in integrated E region Pedersen conductivity

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

[1] The global distribution of the occurrence rate for density irregularities at 600 km topside ionosphere between ±35° geographic latitudes has been studied with the ROCSAT data during moderate to high solar activity years of 1999 to 2004. The result indicates that the global occurrence distribution of the intermediate-scale (0.1 to 50 km) density irregularities can be grouped into two different populations, one in the equatorial region and the other in the middle-to-subauroral latitude region. The global seasonal/longitudinal (s/l) distribution of equatorial irregularities in the current report reproduces the result of McClure et al. (1998) obtained with the AE-E observations of the mesoscale (50 to 1000 km) plasma bubble structures during high solar activity years of 1978 to 1980, two solar cycles ago. This implies that the density irregularities of different scales from multistage cascading process of the large-scale (>1000 km) gravitational Rayleigh-Taylor instability have manifested in same global s/l distribution pattern. Furthermore, global variation in seeding mechanism and growth condition of the instability process that results in major features in global irregularity pattern seems to persist for past 25 years. In addition, the current result further indicates that an upper latitudinal limit of the equatorial irregularity distribution is located at about ±30°. A different kind of midlatitude irregularity distribution starts to fill in from this dip latitude. In other words, the equatorial density irregularity inside a depleted flux tube can only rise, on statistical average, to an apex height of ∼2000 km. Different magnetic and solar variability effects as well as the local time dependence are noted for the occurrences of density irregularities in the equatorial region versus that at midlatitudes. The occurrence frequency of equatorial density irregularities increases with solar flux intensity; whereas the midlatitude density irregularity is more likely to occur during low solar activity period. The equatorial density irregularities are more likely to occur during periods of low magnetic activity than during magnetic disturbed times. On the other hand, the occurrence of midlatitude density irregularities indicates little dependence on geomagnetic activity. The local time distribution of equatorial irregularity peaks before midnight while the midlatitude irregularity indicates a plateau of high occurrence rate after midnight. Such opposite characteristics in the occurrence pattern between these two spatially separated distributions suggest that different instability mechanisms are operated in two different latitude regions for the occurrence of intermediate-scale density irregularities.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JA011330

Distribution characteristics of topside ionospheric density irregularities: Equatorial versus midlatitude regions

Equatorial scintillations—a review

With the advent of satellite communications systems at frequencies varying from \sim140 MHz to 1600 MHz as well as navigation and ranging systems in the 1200-1600 MHz portions of the spectrum, the effect of equatorial irregularities on fading signals has become of importance. Recent observations of the signal statistics of scintillations at frequencies ranging from 136 MHz to 6 GHz reveals a power law fall off of irregularity sizes. Power spectra are now available for a variety of conditions and for frequencies from VHF to microwaves. During periods of intense equatorial activity, at frequencies to 360 MHz, Rayleigh scattering is frequently experienced. The latitudinal extent of the scintillation irregularity region has been established with a half occurrence width during years of moderate solar flux of plus and minus 12\deg . A correlation of in-situ measurements of irregularities from satellites had revealed the great variations in longitudinal patterns during any season. It has also allowed the \Delta N obtained from in-situ measurements to be utilized to predict scintillation excursions. New facets of scintillation activity in the equatorial region recently reported include weak daytime scintillation and the patchy nature of irregularities (small irregularities embedded in large structures) particularly noted during periods of low solar flux. Future studies will assist in the delineation of the extent of the equatorial region at frequencies from 1.5 to 6 GHz, and in the UHF range. From the viewpoint of the communicator the morphology of scintillations at microwaves has still not been reported from any long term program of measurements.

J. Aarons

Papers

Equatorial scintillations: A review

Equatorial scintillations: A review