Abstract
Abstract. The main ionospheric trough is a large-scale spatial depletion in the electron density distribution at the interface between the high- and mid-latitude ionosphere. In western Europe it appears in early evening, progresses equatorward during the night, and retreats rapidly poleward at dawn. It exhibits substantial day-to-day variability and under conditions of increased geomagnetic activity it moves progressively to lower latitudes. Steep gradients on the trough-walls on either side of the trough minimum, and their variability, can cause problems for radio applications. Numerous studies have sought to characterize and quantify the trough behaviour. The Electron Density Assimilative Model (EDAM) models the ionosphere on a global scale. It assimilates observations into a background ionosphere, the International Reference Ionosphere 2007 (IRI2007), to provide a full 3-D representation of the ionospheric plasma distribution at specified times and days. This current investigation studied the capability of EDAM to model the ionosphere in the region of the main trough. Total electron content (TEC) measurements from 46 GPS stations in western Europe from September to December 2002 were assimilated into EDAM to provide a model of the ionosphere in the trough region. Vertical electron content profiles through the model revealed the trough and the detail of its structure. Statistical results are presented of the latitude of the trough minimum, TEC at the minimum and of other defined parameters that characterize the trough structure. The results are compared with previous observations made with the Navy Ionospheric Monitoring System (NIMS), and reveal the potential of EDAM to model the large-scale structure of the ionosphere. Keywords. Ionosphere (midlatitude ionosphere; modelling and forecasting) – radio science (ionospheric physics)
Highlights
The main ionospheric trough is a prominent and persistent depletion in the terrestrial ionized atmosphere, residing essentially between the dynamic high-latitude ionosphere on its poleward side and the more benign mid-latitude region on its equatorward side
The black diamonds, corresponding to high Kp, show the trough minimum under disturbed geomagnetic conditions to be near 65◦ N at 16:00 UT and moving progressively equatorward to reach its lowest latitudes of about 52–53◦ N between 02:00 and 05:00 UT
The earlier study used the very-high frequency (VHF) and ultrahigh frequency (UHF) signals from the low Earth orbiting Navy Ionospheric Monitoring System (NIMS) satellites received at four ground stations, with tomographic reconstruction of the measured Total electron content (TEC) providing an image of the ionosphere over a latitude versus height grid during the 20 min duration of each satellite pass
Summary
The main ionospheric trough is a prominent and persistent depletion in the terrestrial ionized atmosphere, residing essentially between the dynamic high-latitude ionosphere on its poleward side and the more benign mid-latitude region on its equatorward side. When observed in Europe, as for example by Krankowski et al (2009), the feature normally first occurs in late afternoon at latitudes poleward of 60◦ latitude It moves progressively equatorward through the night reaching its most equatorward latitude in pre-dawn hours. A large-scale trough features in the post-noon high-latitude dayside ionosphere (Kersley et al, 1997) It comprises a band of lower densities confined in latitude but extended in longitude. This trough has not been studied so widely as the nightside main trough at mid-latitudes, but it has been suggested that it is an extension of the mid-latitude night-time trough back to earlier local times and higher latitudes (Pryse et al, 2005; Moffett and Quegan, 1983). Interest in the trough occurs in applications of ionospheric radio propagation, largely driven by the in-
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