Wave propagation and group velocity

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] The ionosphere's total electron content (TEC) is a parameter widely used in studies of the near-Earth plasma environment. The scientific use of TEC appeared early in the artificial satellite era, and among its many contributions were fundamental insights into how the ionosphere responds to geomagnetic storms. While many excellent reviews of solar-terrestrial disturbances exist in the literature, none have concentrated on the TEC parameter per se. With new TEC data sets increasingly available from the Global Positioning System (GPS), a comprehensive summary of pre-GPS storm studies is needed to set the base for progress in the GPS era. This review summarizes past case studies, describes statistical occurrence pattern, and identifies responsible mechanisms validated via modeling. It presents a new set of results of TEC disturbance patterns during 180 geomagnetic storms to describe seasonal and solar cycle effects. It concludes with a set of open questions that require additional study.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005RG000193

Storms in the ionosphere: Patterns and processes for total electron content

Global propagation of atmospheric gravity waves: A review

T HE ionosphere is an important source of range and range-rate errors for users of the global positioning system (GPS) satellites who require highaccuracy measurements. At times, the range errors of the troposphere and the ionosphere can be comparable, but the variability of the Earth's ionosphere is much larger than that of the troposphere, and it is more difficult to model. The ionospheric range error can vary from only a few meters, to many tens of meters at the zenith, whereas the tropospheric range error at the zenith is generally between two to three meters. Fortunately, the ionosphere is a dispersive medium; that is, the refractive index is a function of the operating frequency, and twofrequency GPS users can take advantage of this property of the ionosphere to measure and correct for the first-order ionospheric range and range-rate effects directly. Unlike the troposphere, the ionosphere can change rapidly in absolute value. Although the range error of the troposphere generally does not change by more than ±10%, even over long periods of time, the ionosphere frequently changes by at least one order of magnitude during the course of each day. The major effects the ionosphere can have on GPS are the following: 1) group delay of the signal modulation, or absolute range error; 2) carrier phase advance, or relative range error; 3) Doppler shift, or range-rate errors; 4) Faraday rotation of linearly polarized signals; 5) refraction or bending of the radio wave; 6) distortion of pulse waveforms; 7) signal amplitude fading or amplitude scintillation; and 8) phase scintillations. In order to understand the reasons for these potential effects on GPS performance, a brief description of the major characteristics of the ionosphere is necessary.

Ionospheric Effects on GPS

In May 1974 a new era in satellite radio beacon studies of the ionosphere opened with the ATS-6 Radio Beacon Experiment. The history of radio beacon studies up to that time is reviewed briefly and the particular features of the ATS-6 beacon are discussed together with the basic theory required to interpret the measurements. The main emphasis is on the ATS-6 beacon experiment but other beacon data are discussed which provide the necessary background. The diurnal and seasonal variations of the total electron content and the plasmaspheric content are presented for the U.S.A. and Europe. In winter the plasmaspheric content over the Western Hemisphere maximizes at night while in Europe and the Pacific it appears to peak near noon. This is thought to be caused by flow of plasma from the local and conjugate ionospheres. Night maxima of total electron content are found showing that they do not arise from depletions of the plasmaspheric content. The plasmaspheric content is highly sensitive to solarterrestrial disturbance, it reaches a minimum on the third day of a storm and may take between 10 and 20 days of partial filling and emptying to recover. Traveling disturbances in U.S.A., Europe, and India show similarities of speeds but not of direction. Beacon observations of micropulsations in total content, tropospheric fluctuations and Fresnel diffraction by intense ionospheric irregularities are discussed together with radio wave scintillations and some applications of beacon radio data to communications and navigation.

Recent progress in satellite radio beacon studies with particular emphasis on the ATS-6 radio beacon experiment

The multifrequency satellite radio beacon enables the measurement of the columnar electron content of the ionosphere and plasmasphere along the ray path and its spatial and temporal structure. Measurements include modulation phase, Faraday rotation, and amplitude. The characteristics of the beacon transmitter and its design are presented together with the design of the Boulder receiver and antennas and the calibration procedures. A shape factor F is defined which depends on the electron density and geomagnetic field distributions. It is found that F varies by about 30% from day to night. It is shown that the ratio of the plasmaspheric content to total content varies from about 0.08 during the day to about 0.35 at night. Other examples which are presented to illustrate the uses of the radio beacon include sunrise effects, solar flare enhancements of total content, and the ionospheric storms of early July 1974.

Some early results from the ATS‐6 Radio Beacon Experiment

Three-dimensional observations of traveling ionospheric disturbances

Nocturnal winter increases in total electron content are observed at Bozeman, Boulder, and Dallas with the ATS 6 radio beacon. Results of this investigation suggest that the increases take place mainly in the ionosphere. Changes in plasmaspheric contents are much smaller. The downward motion of the F2 layer and the east-to-west movement of the nocturnal maximum (NM) suggest the presence of an electric field. A westward electric field will move plasma from higher L shells to lower L shells with smaller volumes and thus increase the plasma pressure, the result being an enhanced flow of plasma from plasmasphere to ionosphere. The total flux required to produce the NM is of the order of 1×1013 m−2 s−1.

K. Davies

Papers

Recent progress in satellite radio beacon studies with particular emphasis on the ATS-6 radio beacon experiment

Some early results from the ATS‐6 Radio Beacon Experiment

Three-dimensional observations of traveling ionospheric disturbances

Nighttime increases in total electron content observed with the ATS 6 radio beacon

Ionospheric disturbances by severe tropospheric weather storms