Abstract. Troughs in the latitudinal distribution of electron density are a well-known feature of the ionosphere from subauroral to polar latitudes. The location and depth of the trough minimum, the width of the feature, and the horizontal gradients in electron density associated with the trough walls are all quantities of interest or concern to practical applications of radio systems involving the ionosphere. In practice, the precise characteristics of trough-like structures have been difficult to monitor using ground-based methods. Ionospheric tomography represents a new development that is maturing into a technique ideally suited to the study of electron density troughs. Results are presented from a variety of observations made during tomographic campaigns in northern Europe. A long-term investigation has been made of the main trough from a network of stations in the United Kingdom. The position of the trough minimum and the wall gradients have been studied on a diurnal basis using tomographic images reconstructed from measurements for a succession of passes of Navy Navigation Satellite System satellites. With stations deployed for more than 6 months, the average behavior has also been studied. Examples are shown of extreme behavior of the trough under very disturbed geomagnetic conditions, during which tomography continues to yield images while the limitations of ionosondes are exposed. Studies of narrow troughs with very steep gradients seen at auroral latitudes have been used to investigate some of the successes and limitations of the tomographic method. Measurements made in the polar cap show the depleted densities of the polar hole in the center of the dawn convection cell and illustrate the power of the tomographic method at high latitudes. Finally, the dayside trough at the high-latitude boundary between corotating and counterstreaming flux tubes in the afternoon sector has been revealed in a tomographic image extending over some 30 ø latitude, made using a chain of six stations in Scandinavia.
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.
The reconstruction of the vertical electrondensity pro®le is a fundamental problem in ionospheric tomography. Lack of near-horizontal ray paths limits the information available on the vertical pro®le, so that the resultant image of electron density is biased in a horizontal sense. The vertical pro®le is of great importance as it aects the authenticity of the entire tomographic image. A new method is described whereby the vertical pro®le is selected using relative total-electroncontent measurements. The new reconstruction process has been developed from modelling studies. A range of background ionospheres, representing many possible peak heights, scale heights and electron densities are formed from a Chapman pro®le on the bottomside with a range of topside pro®les. The iterative reconstruction process is performed on all of these background ionospheres and a numerical selection criterion employed to select the ®nal image. The resulting tomographic images show excellent agreement in electron density when compared with independent veri®cation provided by the EISCAT radar.
Abstract. Radio tomography is a technique for generating images of the spatial structure of ionospheric electron density over a wide area. This paper assesses the potential use of radio tomography in HF oblique propagation and ray tracing applications. Synthetic ionograms produced by ray tracing through tomographic images and ionospheric models have been compared with experimental oblique ionograms from six paths lying close to the image plane in the United Kingdom. In particular, study has been made of the effects of various types of input information used to constrain the vertical electron density structure in the tomographic reconstructions. It was found that use of a fine height resolution (5 km) and incorporation of information from one vertical ionosonde in the reconstruction process makes significant improvements to the overall reliability of the tomographic image. As expected, E layer propagation is better defined using a climatological model than by tomography. However, in comparison with three ionospheric models, use of tomographic images can significantly reduce the RMS error in the determination of the F 2 layer maximum usable frequency. IntroductionWide area imaging of ionospheric electron density is important for both geophysics research and radio system operation. There are a number of complementary techniques, which can be used to achieve this. While scans by incoherent scatter radars can provide images of the ionosphere, widespread use of the technique is limited by cost. Networks of ionosond½s can also be used to generate information about the ionosphere over wide areas, and they are relatively inexpensive. However, ionosond½s are unable to image above the height of peak electron density and are often subject to D region absorption, blankcting sporadic E layers, and scattering of the returned signals. Interpolation between the discrete measurements is also subject to considerable error. Radio tomography is an approach that provides high-resolution images of ionospheric electron concentration, both above and below the F region peak over an Tomographic images are produced by determining the total electron content (TEC) (integrated electron density) along satellite-ground paths by measuring the phase difference between two phase-locked radio transmissions from a satellite in polar orbit. A chain of tomography receivers situated approximately along the meridional orbital plane is used to measure the TEC on a large number of intersecting paths from which a two-dimensional image of electron density may be reconstructed. Reference to recent work on radio tomographic imaging is given by Kersley et al. [1997] and Pryse et al. [1998], and a wider review is given by Leitinger [1999].Ionospheric tomography is a "limited angle" technique since few rays cross the ionosphere at shallow angles of incidence. In consequence, certain a priori conditions must be used to constrain the vertical profile in the reconstruction process. Furthermore, use of additional input information from ionosondes has been shown to ...
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