The technique of radio tomography has developed during the past fifteen years from a theoretical concept to an established experimental method, used for geophysical investigations ofsolar-terrestrial processes. It also has potential in the mapping and modelling of the ionised atmosphere for application to practical radio systems. The method involves measurement of the electron content of the ionosphere along a large number of intersecting satellite-to-receiver ray paths, with tomographic inversion of the data to give a two-dimensional image of the spatial distribution of plasma density in the region of ray-path intersection. The emphasis in this review is on experimental tomographic observations, which have highlighted the capabilities and potential of the technique. Examples are presented from the equatorial sector where the equatorial anomaly is a significant feature, the mid-latitude sector where radio propagation is often influenced by the presence of the main ionisation trough, and the auroral and polar regions where footprints of solar-terrestrial coupling processes are frequently to be seen.

Radio Tomography: A New Experimental Technique

Data collection for the first ground-based ionospheric tomography campaign in North America was conducted over a 48-hour period in mid-November 1991. The data consist of records of ionospheric total electron content (TEC) from a number of passes of the U. S. Navy Navigation Satellite System spacecraft over a chain of ground-based receiving stations. Data collection and reduction techniques are discussed; these include the determination of absolute TEC from the different phase advances induced by the ionosphere in each component of the dual-frequency spacecraft signal. The use of tomographic methods to reconstruct ionospheric electron densities over a two-dimensional (2-D) region of the Earth's ionosphere at a number of different times is demonstrated. Specifically, two distinct tomographic methods, the algebraic reconstruction technique and a maximum entropy method, are used to mathematically invert the records of TEC. The resulting 2-D contour “maps” of ionospheric electron density are then compared to similar maps produced by the Millstone Hill incoherent backscatter radar facility located at Westford, Massachusetts. Both qualitative and quantitative measures of agreement among the different reconstructions and the radar maps are presented. The behavior of the ionosphere over the course of the experiment is discussed.

Tomographic reconstruction of the ionosphere over North America with comparisons to ground‐based radar

A four-dimensional time-dependent tomographic algorithm, named Multi Instrument Data Analysis System (MIDAS), is used to image the equatorial and low-latitude ionosphere over the central-eastern sides of the Brazilian territory. From differential phase data obtained by a chain of ground-based GPS receiver the total electron content (TEC) is estimated and then, together with a modeled ionosphere from International Reference Ionosphere (IRI) model, the electron density distribution is reconstructed and the parameters of the F2-peak layer are accessed from the images. This paper presents the first study of ionospheric tomography using real dual-frequency data from the Brazilian Network for Continuous GPS Monitoring (RBMC). Ionospheric F2-peak electron density (NmF2) accessed from the images are compared to concurrent measurements from three ionosondes installed across Brazil. One year of data during the solar maximum period from March/2001 to February/2002 is used to analyze the seasonal and hourly variation of the F2-layer peak density. The accuracy with which MIDAS images the electron density during geomagnetic quiet periods is investigated through its correlation and deviation with the ionosonde and IRI model data, respectively. The main aspects of the reconstruction results at the equatorial ionization anomaly (EIA) region over Brazil are highlighted and discussed.

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Tomographic imaging of the equatorial and low-latitude ionosphere over central-eastern Brazil

BO and Bl are parameters used in the International Reference Ionosphere (IRI) model for the calculation of the F region bottom side profiles. For this study, we have used ionograms recorded at Korhogo, an equatorial station. The parameter AH obtained from the magnetic data recorded during the International Equatorial Electrojet Year (IEEY) in West African is used to describe the strength of the equatorial electrojet. This work covers the years 1993 and 1994, two years of low solar activity.

IONOSPHERIC PROFILE PARAMETERS BO AND Bl AND THE EQUATORIAL ELECTROJET

: This report results from a contract tasking Moscow State University as follows: Develop algorithms for the ray nonlinear radiotomography of the ionosphere and radiotomography using middle and high-orbital satellites.

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Development of Algorithms and Programs for the Ray Nonlinear Radiotomography of the Ionosphere and Radiotomography Using Middle and High-Orbital Satellites.

Initial Total Electron Content Results from the Pan American Ionospheric Tomography Campaign.

: The latitudinal extent of the equatorial anomaly has been studied using a theoretical model of the ionosphere which incorporates measured values of vertical E x B drift at the earth's magnetic equator. Realistic values of neutral winds are also included. The equatorial anomaly region, typically between plus and minus 20 degrees magnetic latitude, is that part of the world where the highest values of electron density and Total Electron Content, (TEC), normally occur, and hence is very important to high frequency propagation and to trans-ionospheric propagation effects. During the daytime upward E x B drift at the magnetic equator drives the ionization across field lines to higher latitudes, causing crests in ionization to occur at approximately plus and minus 15 deg dip latitude. The latitude range over which the anomaly makes a significant difference in values of foF2 and TEC is calculated as a percent departure from the case with no equatorial electric field. Results from the model studies with different values of realistic electric fields show that the effects of the anomaly can be highly variable and widespread in latitude and local time.

P. H. Doherty

Papers

Initial Total Electron Content Results from the Pan American Ionospheric Tomography Campaign.

The Latitudinal Extent of the Equatorial Anomaly