Imaging of electron density troughs by tomographic techniques

Abstract

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.

Modelling the main ionospheric trough using the Electron Density Assimilative Model (EDAM) with assimilated GPS TEC

“…With the increasing sunlight at dawn, ionization production results in increased densities in the trough and the apparent rapid retreat of the feature to higher latitudes with the trough minimum being typically at 65 • corrected geomagnetic latitude at 08:00 LT in the results of Krankowski et al (2009). A large-scale trough also features in the post-noon high-latitude dayside ionosphere (Kersley et al, 1997). It also comprises a band of lower densities confined in latitude but extended in longitude.…”

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

“…The International Ionospheric Tomography Community (IITC) has brought together several groups with interests in the field to apply the technique to ionospheric imaging, where routine observations by several chains in different longitude sectors are used to provide information on the plasma distribution over a wide geographic region [Kersley et al, 2005]. The present study employs images from tomography chains in Europe [Kersley et al, 1997] and Greenland [Coker et al, 2001] in a study of the large-scale spatial distribution of the ionospheric plasma in the magnetic postnoon sector in winter, under stable conditions of quiet geomagnetic activity (Kp $ 1) and positive B z . To our knowledge this is the first instance in which the distribution of plasma under northward IMF conditions has been imaged over such a large section of the northern polar region.…”

The dayside high-latitude trough under quiet geomagnetic conditions: Radio tomography and the CTIP model

“…Radio tomography (Kunitsyn and Tereshchenko, 2003;Pryse, 2003 and references therein) is a relatively new technique for imaging the spatial distribution of the ionospheric plasma over extended height-versus-latitude planes. It is particularly adept at imaging the plasma distribution in regions of density troughs (Andreeva et al, 1990;Kersley et al, 1997) coverage of the ionosphere at all Universal Times, seasons, geomagnetic and solar conditions. The technique gives wide spatial coverage from a limited number of ground stations and is ideal for monitoring the remote inaccessible regions of the high latitudes.…”