The phenomenon of scintillation of radio waves propagating through the ionosphere is reviewed in this paper. The emphasis is on propagational aspects, including both theoretical and experimental results. The review opens with a discussion of the motivation for stochastic formulation of the problem. Based on measurements from in-situ, radar, and propagation experiments, ionospheric irregularities ate found to be characterized, in general, by a power-law spectrum. While earlier measurements indicated a spectral index of about 4, there is recent evidence showing that the index may vary with the strength of the irregularity and possibly a two-component spectrum may exist with different spectral indices for large and small structures. Several scintillation theories including the Phase Screen, Rytov, and Parabolic Equation Method (PEM) are discussed next. Statistical parameters of the signal such as the average signal, scintillation index, rms phase fluctuations, correlation functions, power spectra, distributions, etc., are investigated. Effects of multiple scattering are discussed. Experimental results concerning irregularity structures and signal statics are presented. These results are compared with theoretical predictions. The agreements are shown to be satisfactory in a large measure. Next, the temporal behavior of a transionospheric radio signal is studied in terms of a two-frequency mutual coherence function and the temporal moments. Results including numerical simulations are discussed. Finally, some future efforts in ionospheric scintillation studies in the areas of transionospheric communication and space- and geophysics are recommended.

Radio wave scintillations in the ionosphere

A prototype system has been developed to monitor the instantaneous global distribution of ionospheric irregularities, using the worldwide network of Globa Positioning System (GPS) receivers. Case studies in this pape indicate that GPS receiver loss of lock of signal tracking may be associated with strong phase fluctuations. It is shown that a network-based GPS monitoring system will enable us to study the generation and evolution of ionospheric irregularities continuously around the globe under various solar and geophysical conditions, which is particularly suitable for studies of ionospheric storms, and for space weather research and applications.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97GL02273

Monitoring of global ionospheric irregularities using the Worldwide GPS Network

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

The most intense, F region irregularities in the high-latitude ionosphere appear to be produced by convective plasma processes and in particular, by the fluid (gradient drift) interchange instability. Such irregularities are produced by convectively mixing plasma across a mean plasma density gradient with the transport of higher-density plasma into regions of lower-density plasma (and vice versa) leading to the development of an irregularity spectrum that extends in scale from about 10 km down to the ion gyroradius. The mean plasma density gradient that must be present to allow irregularity production by this interchange process appears to be associated with larger-scale (>10 km) plasma structure produced by other means. Because much of the recent progress on this research topic stems from this recognition, a significant portion of this review is dedicated to a description of the characteristics and processes of 10-km plasma structure and their relationships to those of smaller-scale irregularities. From this review, we synthesize a descriptive model of plasma structures in the high-latitude F layer that unifies most of the diverse and independent observations. For the large-scale plasma processes, the model includes (1) the formation of 1000-km-scale “patches” in the polar cap from solar-produced plasma that is transported poleward from lower latitudes; (2) the reconfiguration of patches as they convect into the auroral region and become the latitudinally confined, but longitudinally extended, plasma density enhancements near the equatorward auroral boundary; and (3) the production of localized enhancements and depletions along the poleward auroral boundary by soft-particle precipitation and large but localized electric fields. In the model, the most intense, smaller-scale irregularities are in spatial proximity to these large-scale plasma features, the implication being that the presence of the latter allows formation of the former. The irregularity characteristics are consistent with production by the instability and a morphology controlled by (1) a “slip” velocity (i.e., plasma drift relative to the neutral gas) that is moderately small except in regions of nonuniform plasma convection or under time-varying conditions (e.g., substorms, pulsation events) and (2) a highly conducting auroral E layer that damps irregularity growth and enhances decay. The final irregularity spectrum appears to be produced by (1) global convective processes acting on solar-produced plasma at the largest scales (>50 km), (2) particle precipitation at scales greater than 10 km, (3) perhaps some form of wave activity around 10 km, and (4) the instability at the smaller scales (<10 km).

High-Latitude F-Region Irregularities: A Review and Synthesis

In this paper the weak scatter scintillation theory is reformulated to show explicitly the ramifications of an arbitrarily large ionospheric outer scale. The measured temporal phase spectrum, for example, is effectively truncated at a fixed frequency corresponding to the detrend time or the length of the data interval over which it is measured (whichever is smaller). As a consequence, the rms phase exhibits a complicated dependence on the relative irregularity drift velocity and the propagation geometry. This effect has not been included in previous analyses. By comparison, intensity scintillation data are intrinsically high-pass filtered by the diffraction process. By taking advantage of this fact a simple closed form expression for the S4 intensity scintillation index has been derived. The theory is applied to representative data sets from the Wideband satellite. The interpretation of the ionospheric parameters deduced from the analysis is also discussed.

A power law phase screen model for ionospheric scintillation: 2. Strong scatter

A multifrequency (ten spectral lines between VHF and S band) coherent radio beacon is presently transmitting continuously from a 1000-km, high-inclination orbit for the purpose of characterizing the transionospheric communication channel. Its high phase-reference frequency (2891 MHz) permits direct observation of complex-signal scintillation, and its very stable, sun-synchronous orbit allows repeated pre-midnight observations at low latitudes and near-midnight observations at auroral latitudes. We present here early results of the observations; salient points include the following. First, most of the data are consistent with phase-screen modeling of the production of ionospheric scintillation, including an ƒ−2 frequency dependence for phase variance. Second, propagation theories invoking weak, single scatter seldom are adequate, because even moderate intensity scintillation usually is accompanied by phase fluctuations comparable to or greater than a radian. Third, under conditions producing GHz scintillation (near the geomagnetic equator), lower frequencies show marked diffraction effects, including breakdown of the simple ƒ−2 behavior of phase variance and loss of signal coherenceacross a band as narrow as 11.5 MHz at UHF.

Early results from the DNA Wideband satellite experiment—Complex‐signal scintillation

The most persistent, identifiable feature on intensity and phase scintillation records of coherent beacon signals recorded near Fairbanks, Alaska during several months in 1976 is a narrow region of enhanced scintillation to the south of the station. On high-elevation passes the feature occurs near the magnetic zenith, suggesting that it could be a geometric enhancement produced by field-aligned irregularities. The feature also occurs, however, on medium and low-elevation passes when the minimum off-field angle is several tens of degrees. The observations, therefore, represent evidence for either (1) a hitherto unrecognized irregularity enhancement or layer thickening in the subauroral ionosphere, or (2) longitudinally extended irregularity sheets.

A persistent subauroral scintillation enhancement observed in Alaska

A two-component model for scintillation

: The Wideband Satellite (P76-5) was launched on 22 May 1976 and data have been recorded at three receiving stations since that time. The unique nature of the Wideband signals (coherent transmissions covering VHF to S-band) and the geophysically interesting locations of the receiving stations (covering auroral and equatorial latitudes) have led to a number of new and significant results. These results pertain to both scintillation phenomenology and the physics of striation formulation. This report presents the results of the first year of Wideband operations; work described previously in detailed bimonthly progress reports is only summarized here. Emphasis is placed on aspects not previously described, including: (1) evidence for a geometrical scintillation enhancement due to east-west sheet-like irregularity structures in the auroral zone; (2) spaced-receiver measurements of the diffraction pattern anisotropy and apparent drift motion; (3) total electron content measurements of the paleward boundary of the ionospheric trough; (4) characteristics of equatorial scintillations, particularly the gigahertz scintillation that develop during extremely active periods; and (5) results from the equatorial scintillation campaign conducted in Peru during March 1977. Using the Wideband data base with theoretical work and systems analysis, it should be possible to develop a self- consistent model relating a simple parameterization of the irregularity structure and strength to the observed phase and amplitude scintillations.

https://apps.dtic.mil/sti/pdfs/ADA048801.pdf

E. J. Fremouw

Papers

Early results from the DNA Wideband satellite experiment—Complex‐signal scintillation

A persistent subauroral scintillation enhancement observed in Alaska

A two-component model for scintillation

Wideband Satellite Observations