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

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

Preliminary results are presented from a campaign of coordinated measurements between the Chatanika radar and the TRIAD satellite to investigate production mechanisms responsible for localized high-latitude scintillations. The radar measured the latitudinal variations of plasma density and electric field while the satellite measured the field-aligned current distribution with latitude. This information was used to calculate the linear growth rate of the current convective instability. We find that field-aligned ionization enhancements, which are a common feature of the auroral F-region, are unstable, with instability growth times of ≤ 6 minutes. The observed plasma configuration, which was already gradient drift unstable on its poleward edge, was further destabilized by a field-aligned current of ∼ 0.8 µA/m². The presence of electron density irregularities produced in the unstable region was verified by observing VHF scintillations on the TRIAD telemetry signal.

Chatanika/Triad observations of unstable ionization enhancements in the auroral F‐region

The linear stability and nonlinear evolution of plasma enhancements in arbitrary ambient electric fields in the diffuse auroral F region ionosphere have been studied using analytical and numerical simulation techniques. Our results indicate that equatorward convecting plasma slabs initially limited in latitudinal extent are primarily destabilized on their poleward sides by a combination of the effects of convection and field-aligned currents. Furthermore, we find that the plasma enhancements break up into primary striationlike structures (elongated in the north-south direction for equatorward convection) which can form and cascade from large (≃100 km) to smaller (≃3 km) scale sizes on the order of an hour. The primary and associated smaller-scale structures can be oriented in either the north-south or east-west (L shell alignment) direction depending on the ambient electric field magnitude and direction. For wave numbers (kx, ky) in Fourier space corresponding to the east-west and north-south directions, respectively, the one-dimensional spatial power spectra of the irregularities in the east-west direction P(kx) ∝ kx−nx with nx ≃ 2-2.5 for 2π/kx between 100 km and 3 km while in the north-south direction, P(ky) ∝ ky−ny with ny ≃ 2 for 2πsol;ky between 256 km and 3 km.

Nonlinear evolution of plasma enhancements in the auroral ionosphere 1. Long wavelength irregularities

. Maps of GPS phase scintillation at high latitudes have been constructed after the first two years of operation of the Canadian High Arctic Ionospheric Network (CHAIN) during the 2008–2009 solar minimum. CHAIN consists of ten dual-frequency receivers, configured to measure amplitude and phase scintillation from L1 GPS signals and ionospheric total electron content (TEC) from L1 and L2 GPS signals. Those ionospheric data have been mapped as a function of magnetic local time and geomagnetic latitude assuming ionospheric pierce points (IPPs) at 350 km. The mean TEC depletions are identified with the statistical high-latitude and mid-latitude troughs. Phase scintillation occurs predominantly in the nightside auroral oval and the ionospheric footprint of the cusp. The strongest phase scintillation is associated with auroral arc brightening and substorms or with perturbed cusp ionosphere. Auroral phase scintillation tends to be intermittent, localized and of short duration, while the dayside scintillation observed for individual satellites can stay continuously above a given threshold for several minutes and such scintillation patches persist over a large area of the cusp/cleft region sampled by different satellites for several hours. The seasonal variation of the phase scintillation occurrence also differs between the nightside auroral oval and the cusp. The auroral phase scintillation shows an expected semiannual oscillation with equinoctial maxima known to be associated with aurorae, while the cusp scintillation is dominated by an annual cycle maximizing in autumn-winter. These differences point to different irregularity production mechanisms: energetic electron precipitation into dynamic auroral arcs versus cusp ionospheric convection dynamics. Observations suggest anisotropy of scintillation-causing irregularities with stronger L-shell alignment of irregularities in the cusp while a significant component of field-aligned irregularities is found in the nightside auroral oval. Scintillation-causing irregularities can coexist with small-scale field-aligned irregularities resulting in HF radar backscatter. The statistical cusp and auroral oval are characterized by the occurrence of HF radar ionospheric backscatter and mean ground magnetic perturbations due to ionospheric currents.

https://angeo.copernicus.org/articles/29/377/2011/angeo-29-377-2011.pdf

C.L. Rino

Papers

The structure of localized nighttime auroral zone scintillation enhancements