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

[1] Ionospheric scintillations are one of the earliest known effects of space weather. Caused by ionization density irregularities, scintillating signals change phase unexpectedly and vary rapidly in amplitude. GPS signals are vulnerable to ionospheric irregularities and scintillate with amplitude variations exceeding 20 dB. GPS is a weak signal system and scintillations can interrupt or degrade GPS receiver operation. For individual signals, interruption is caused by fading of the in-phase and quadrature signals, making the determination of phase by a tracking loop impossible. Degradation occurs when phase scintillations introduce ranging errors or when loss of tracking and failure to acquire signals increases the dilution of precision. GPS scintillations occur most often near the magnetic equator during solar maximum, but they can occur anywhere on Earth during any phase of the solar cycle. In this article we review the subject of GPS and ionospheric scintillations for scientists interested in space weather and engineers interested in the impact of scintillations on GPS receiver design and use.

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GPS and ionospheric scintillations

A synthetic aperture radar (SAR) can produce high-resolution two-dimensional images of mapped areas. The SAR comprises a pulsed transmitter, an antenna, and a phase-coherent receiver. The SAR is borne by a constant velocity vehicle such as an aircraft or satellite, with the antenna beam axis oriented obliquely to the velocity vector. The image plane is defined by the velocity vector and antenna beam axis. The image orthogonal coordinates are range and cross range (azimuth). The amplitude and phase of the received signals are collected for the duration of an integration time after which the signal is processed. High range resolution is achieved by the use of wide bandwidth transmitted pulses. High azimuth resolution is achieved by focusing, with a signal processing technique, an extremely long antenna that is synthesized from the coherent phase history. The pulse repetition frequency of the SAR is constrained within bounds established by the geometry and signal ambiguity limits. SAR operation requires relative motion between radar and target. Nominal velocity values are assumed for signal processing and measurable deviations are used for error compensation. Residual uncertainties and high-order derivatives of the velocity which are difficult to compensate may cause image smearing, defocusing, and increased image sidelobes. The SAR transforms the ocean surface into numerous small cells, each with dimensions of range and azimuth resolution. An image of a cell can be produced provided the radar cross section of the cell is sufficiently large and the cell phase history is deterministic. Ocean waves evidently move sufficiently uniformly to produce SAR images which correlate well with optical photographs and visual observations. The relationship between SAR images and oceanic physical features is not completely understood, and more analyses and investigations are desired.

Tutorial review of synthetic-aperture radar (SAR) with applications to imaging of the ocean surface

C/NOFS: a Mission to Forecast Scintillations

In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22–23, 1999, with the hourly Dst index being −167 nT at ∼2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of −231 nT at ∼0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS)-based navigation system called Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration for use in the continental United States and their impact on global VHF/UHF communication systems. It is shown that the penetration of transient magnetospheric electric fields equatorward of the shielding region at midlatitudes, which have been well-correlated in the past with rapid changes in the well-known Dst index (or through its recently available high resolution 1-min counterpart the SYM-H index), can cause large increases of total electron content (TEC), TEC fluctuations, and saturated 250-MHz scintillation, and these, in turn, may have significant impacts on WAAS. The local time of Dst changes (and not just Dst magnitude) was found to be very important for WAAS, since the largest effects on TEC are seen near dusk. The prompt penetration of these magnetospheric electric fields all the way to the magnetic equator causes augmentation or inhibition of equatorial spread F. The global ionospheric response to these storms has been obtained from ground-based TEC observations with a GPS network and space-based in situ density and electric field measurements using the Republic of China Satellite-1 (ROCSAT-I) and several Defense Meteorological Satellite Program satellites. These prompt penetration electric fields cause VHF/UHF scintillations and GPS TEC variations at low latitudes in the specific longitude sector for which the early evening period corresponds to the time of rapid Dst variations and maximum Dst phase. The effects of the delayed ionospheric disturbance dynamo and those of decreased magnetospheric convection on postmidnight irregularity generation are shown to be confined to a part of the same longitude range that actively responded to the prompt penetration of electric fields in the early evening sector.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JA001116

Ionospheric effects of major magnetic storms during the International Space Weather Period of September and October 1999: GPS observations, VHF/UHF scintillations, and in situ density structures at middle and equatorial latitudes

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.

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R.S. Allen

Papers

Global morphology of ionospheric scintillations

Observations of scintillations of two satellite beacons near the boundary of the irregularity region