Search

Results are presented from two multi-instrument case studies showing patches of cold, long-lived plasma in the winter nightside ionosphere during times when the z-component of the Interplanetary Magnetic Field (IMF Bz) was positive. These enhancements were coincident with the antisunward convective plasma drift, flowing from polar to nightside auroral latitudes. In the first case, on 5 December 2005 with IMF By negative, two regions of enhanced electron density were observed extended in MLT in the magnetic midnight sector separated by lower densities near midnight. It is likely that the earlier enhancement originated on the dayside near magnetic noon and was transported to the nightside sector in the convective flow, whilst the later feature originated in the morning magnetic sector. The lower densities separating the two enhancements were a consequence of a pair of lobe cells essentially blocking the direct antisunward cross polar flow from the dayside. A second case study on 4 February 2006 with IMF By positive revealed a single nightside enhancement likely to have originated in the morning magnetic sector. These multi-instrument investigations, incorporating observations by the EISCAT radar facility, the SuperDARN network and radio tomography, reveal that plasma flowing from the dayside can play a significant role in the nightside ionosphere under conditions of IMF Bz positive. The observations are reinforced by simulations of flux-tube transport and plasma decay.

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.

The paper investigates the structure and behaviour of the nighttime ionised (electrified) atmosphere in the polar and auroral regions; the region where the aurora borealis occurs. Of particular interest are plasma structures on horizontal scales of hundreds of kilometres. The observations presented were made by the radiotomography experiment of the University of Wales, Aberystwyth, which has four satellite receiving systems in the high Arctic near the north pole, at Ny Ålesund and Longyearbyen on Svalbard, Bjørnøya (Bear Island) and Tromsø on mainland Norway. Comparisons of tomography images with observations of plasma flow by the international SuperDARN radar suggest that large density plasma produced on the dayside flows across the polar region and into the night sector. The results contribute to the interpretation of physical processes that couple the Earth's environment with space, and are also of interest to users of radio systems where the ionised atmosphere can degrade the propagation of the signals.