IMAGING OF ELECTRON-DENSITY TROUGHS BY TOMOGRAPHIC TECHNIQUES

Troughs in the latitudinal distribution of electron density are a well -known feature of the ionosphere from subauroral to polar latitudes. T he 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 ap plications 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 represe nts 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 norther n Europe. A long-term investigation has been made of the main trough f rom a network of stations in the United Kingdom. The position of the t rough minimum and the wall gradients have been studied on a diurnal ba sis using tomographic images reconstructed from measurements for a suc cession of passes of Navy Navigation Satellite System satellites. With stations deployed for more than 6 months, the average behavior has al so been studied. Examples are shown of extreme behavior of the trough under very disturbed geomagnetic conditions, during which tomography c ontinues to yield images while the limitations of ionosondes are expos ed. Studies of narrow troughs with very steep gradients seen at aurora l latitudes have been used to investigate some of the successes and li mitations of the tomographic method. Measurements made in the polar ca p show the depleted densities of the polar hole in the center of the d awn convection cell and illustrate the power of the tomographic method at high latitudes. Finally, the dayside trough at the high-latitude b oundary between corotating and counterstreaming flux tubes in the afte rnoon sector has been revealed in a tomographic image extending over s ome 30 degrees latitude, made using a chain of six stations in Scandin avia.