The role of the tongue‐of‐ionization in the formation of the poleward wall of the main trough in the European post‐midnight sector

Abstract

[1] A series of radio tomography reconstructions from the University of Wales Aberystwyth receiver chains in Scandinavia and the UK, imaging the midnight-dawn sector on 13 December 2001, reveal a persistent large-scale electron density enhancement, which forms the poleward wall of the main ionization trough. Measurements by the European Incoherent Scatter radar (EISCAT) rule out in situ soft-particle precipitation as the main source of the higher densities. SuperDARN plasma drift observations and electric potential patterns place the feature in the dawn cell of the high-latitude convection, leading to the conclusion that the higher density is likely to have originated as photoionization and was convected over the polar cap to the nightside and around toward dawn in a tongue-of-ionization (TOI). Suitable runs of the Coupled ThermosphereIonosphere-Plasmasphere (CTIP) model support this interpretation and also reveal that the formation of the TOI is heavily UT dependent, which would lead to it being most prominent at nighttime in the European sector.Citation: Middleton, H. R., S. E. Pryse, A. G. Wood, and R. Balthazor (2008), The role of the tongue-of-ionization in the formation of the poleward wall of the main trough in the European post-midnight sector,

Modelling the tongue-of-ionisation using CTIP with SuperDARN electric potential input: verification by radiotomography

“…The latitudinal location of the feature also compared well, although the model failed to reproduce the observed gradients on the walls of the enhancement, a likely consequence of the resolution of the model being lower than that of the tomography reconstruction. The model output in the study showed improvement over that obtained by Middleton et al (2008) who used a selected library potential pattern. There were only limited SuperDARN measurements at the time of the B z positive case, and an electric potential pattern from a different time period was used.…”

“…The two case studies with different IMF orientations were chosen to coincide with well-studied radiotomography investigations. The tomography results have already formed the focus of two previous works into the physical processes operating at high latitudes (Middleton et al, , 2008 where the observed features were interpreted by multi-instrument investigations. By choosing such cases, with mature interpretations of the data sets, ambiguity in the comparisons of observations with the model output was less likely.…”

“…During its transit through the polar cap the TOI may be modulated into polar patches and various processes have been proposed for this (for example Anderson et al, 1988;Sojka et al, 1993;Rodger et al, 1994;Milan et al, 2002). A modelling study by Bowline et al (1996) indicated that the TOI is expected to be prominent at Ny-Ålesund in the European sector in the evening under conditions of IMF B y negative, and observations of the remnants of the TOI and polar patches on the nightside in this sector are reported by Middleton et al (2008) and Pryse et al (2006).…”

“…Comparisons of the tomographic observations with measurements by the EISCAT UHF radar at Tromsø on mainland Norway showed that the enhanced densities were cold and at high altitudes, and supported the interpretation of the features being cross-sections through ionisation that had originated at an earlier time and different location, possibly on the dayside, and had been carried by the plasma flow in a tongue-of-ionisation into the region of observations. Further details of the observational results and their interpretation are given by Middleton et al (2008).…”

“…However, there are very few direct comparisons of the ionospheric observations with modelled distributions. Middleton et al (2008) presented multi-instrument observations of a cold high-altitude plasma enhancement on the nightside, interpreted as plasma produced on the dayside and transported across the polar cap in the convective flow into the auroral region. The interpretation was supported by model runs of the CTIP model, although there were disparities in the location and density levels of the observations and model output, with the modelled enhancement being a few degrees latitude equatorward of that observed and of generally slightly higher densities.…”