33 research outputs found
The role of the tongue-of-ionization in the formation of the poleward wall of the main trough in the European post-midnight sector
No full textA 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
A study of atmospheric gravity waves and travelling ionospheric disturbances at equatorial latitudes
No full textInternational audienceA global coupled thermosphere-ionosphere-plasmasphere model is used to simulate a family of large-scale imperfectly ducted atmospheric gravity waves (AGWs) and associated travelling ionospheric disturbances (TIDs) originating at conjugate magnetic latitudes in the north and south auroral zones and subsequently propagating meridionally to equatorial latitudes. A 'fast' dominant mode and two slower modes are identified. We find that, at the magnetic equator, all the clearly identified modes of AGW interfere constructively and pass through to the opposite hemisphere with unchanged velocity. At F-region altitudes the 'fast' AGW has the largest amplitude, and when northward propagating and southward propagating modes interfere at the equator, the TID (as parameterised by the fractional change in the electron density at the F2 peak) increases in magnitude at the equator. The amplitude of the TID at the magnetic equator is increased compared to mid-latitudes in both upper and lower F-regions with a larger increase in the upper F-region. The ionospheric disturbance at the equator persists in the upper F-region for about 1 hour and in the lower F-region for 2.5 hours after the AGWs first interfere, and it is suggested that this is due to enhancements of the TID by slower AGW modes arriving later at the magnetic equator. The complex effects of the interplays of the TIDs generated in the equatorial plasmasphere are analysed by examining neutral and ion winds predicted by the model, and are demonstrated to be consequences of the forcing of the plasmasphere along the magnetic field lines by the neutral air pressure wave
A study of atmospheric gravity waves and travelling ionospheric disturbances at equatorial latitudes
No full text
A study of the Joule and Lorentz inputs in the production of atmospheric gravity waves in the upper thermosphere
No full textInternational audienceFirst results of a modelling study of atmospheric gravity waves (AGWs) are presented. A fully-coupled global thermosphere-ionosphere-plasmasphere model is used to examine the relative importance of Lorentz forcing and Joule heating in the generation of AGWs. It is found that Joule heating is the dominant component above 110km. The effects of the direction of the Lorentz forcing component on the subsequent propagation of the AGW are also addressed. It is found that enhancement of zonal E×B forcing results in AGWs at F-region altitudes of similar magnitudes travelling from the region of forcing in both poleward and equatorward directions, whilst enhancement of equatorward meridional E×B forcing results in AGWs travelling both poleward and equatorward, but with the magnitude of the poleward wave severely attenuated compared with the equatorward wav
The dayside high-latitude trough under quiet geomagnetic conditions: Radio tomography and the CTIP model
No full textInternational audienceThe dayside high-latitude trough is a persistent feature of the post-noon wintertime auroral ionosphere. Radio tomography observations have been used to map its location and latitudinal structure under quiet geomagnetic conditions (Kp?2) near winter solstice. The trough is also a clear feature in the ion density distribution of the Coupled Thermosphere-Ionosphere-Plasmasphere model (CTIP) under similar geophysical conditions. Comparisons of the measured and modelled distributions show that the plasma production equatorward of the trough is mainly controlled by solar radiation, but there are also other processes maintaining the equatorward trough-wall that are open to debate. The poleward trough-wall is produced by particle precipitation, but the densities are significantly overestimated by the model. At the trough minimum the observed densities are consistent with low nighttime densities convecting sunward to displace the higher daytime densities, but this is not borne out by the CTIP model. The study shows the potential of combining radio tomography and modelling to interpret the balance of the physical processes responsible for large-scale structuring of the high-latitude ionosphere, and highlights the role of tomographic imaging in validating and developing physical models
A study of the Joule and Lorentz inputs in the production of atmospheric gravity waves in the upper thermosphere
No full text
