S. E. Pryse

Radio tomography: A new experimental technique

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.

Tomographic reconstruction of ionospheric electron density with European incoherent scatter radar verification

A campaign was conducted in late September 1991 to obtain experimental measurements for use in ionospheric tomography. Four stations receiving signals from the Navy Navigation Satellite System satellites were set up in a meridional chain in Scandinavia covering some 10° of latitude. Measurements of total electron content were made for six satellite passes over a 2-day period coinciding with an extended run of the CP3f latitudinal scan common program of the European incoherent scatter radar. A new reconstruction technique involving the use of two-dimensional basis vectors has been used to convert the total electron content measurements to images of electron density in a height versus latitude plane. Comparisons of the tomographic images and the independent measurements of electron density from the radar show good general agreement. Broadly similar troughs and enhancements are observed by the two techniques, and a latitudinal gradient in the height of the peak density is reproduced in both data sets.

A comparison of reconstruction techniques used in ionospheric tomography

This paper compares the performance of three different reconstruction algorithms used for tomographic imaging of the ionosphere. The test cases chosen make use of experimental electron content observations obtained from a meridional chain of receivers in Scandinavia at times when the European incoherent scatter radar facility provided independent measurements of electron density for verification of the reconstructions. The examples include ionospheres with layers peaking at both anomalously high and very low altitudes, in addition to a smooth horizontally stratified layer. The results expose the limitations of an algorithm based on use of a standard ionospheric model but demonstrate that with appropriate choice of background a priori profiles, reliable images are obtained from both discrete inverse theory and quadratic programing. For these two latter algorithms the resultant reconstructions are essentially independent of the algorithm used. The study highlights the critical role of the range of the background input to the algorithms. A density value at a single grid cell provides sufficient additional a priori information to constrain the imaging of a horizontally stratified ionosphere.

Tomographic imaging of the polar-cap ionosphere over svalbard

Abstract First results are presented from an experiment to image the electron density in the polar-cap ionosphere above Svalbard using tomographic techniques. The results show localised enhanced densities in the cusp and throat region, the elevated densities of a polar patch, relatively low densities in co-rotating plasma flux tubes in darkness at lower latitudes and the depleted densities of the polar hole. The reconstruction of these features, known to occur at polar latitudes, confirms the potential of tomography for imaging the ionosphere at these very high latitudes.

The effects of receiver location in two-station experimental ionospheric tomography

Abstract Experimental results are presented of the imaging of a narrow field-aligned trough in electron density with an associated boundary blob using the techniques of ionospheric tomography, with verification by means of independent measurements from the EISCAT radar. An authentic image is still obtained when observations from only two receiving sites are used if the structure is in the vicinity of the stations. However, the image quality is degraded when the feature, with its strong horizontal gradients, is far from overhead. The results are of importance to the application of ionospheric tomography to the study of spatial structures in electron density over remote and inhospitable regions like the polar caps where the location of ground stations is restricted.

Evidence for the tongue of ionization under northward interplanetary magnetic field conditions

[1] The activities of the International Ionospheric Tomography Community open up new possibilities of simultaneously imaging the large-scale spatial structure of the ionosphere in different longitude sectors. In the study, tomography receiver chains in Scandinavia and Greenland were used to provide a wide view of the plasma density structure in the winter, magnetic postnoon sector under conditions of stable, positive interplanetary magnetic field Bz component. The spatial distributions of the plasma are discussed in light of a high-latitude plasma convection pattern pertinent to the conditions, which is supported by DMSP flow measurements. The observations are consistent with a tongue of dayside photoionization being drawn antisunward by the convection pattern to form an arc of enhanced plasma density around the periphery of the polar cap.

Electron density structures in the polar cap imaged by ionospheric tomography

Abstract Tomographic images are presented of electron density structures obtained from measurements on Svalbard during an experimental campaing in December 1994. Observations near magnetic noon reveal enhanced densities in the cusp region, aligned along the near-vertical geomagnetic field, which may relate to particle precipitation or to the tongue of ionisation extending into the polar cap. Large enhancements in density possibly linked to patches are found in the convective flow of the central polar cap. After magnetic midnight, very low densities are seen in the polar-hole of the dawn cell, where the convecting plasma is in perpetual darkness. Ionisation profiles obtained from the tomographic reconstructions are compared to those of the EISCAT GIVEME model.