Understanding the Latitude Structure of Nitric Oxide in the Mesosphere and Lower Thermosphere

The goal of the proposed work was to understand the latitude structure of nitric oxide in the mesosphere and lower thermosphere. The problem was portrayed by a clear difference between predictions of the nitric oxide distribution from chemical/dynamical models and data from observations made by the Solar Mesosphere Explorer (SMEE) in the early to mid eighties. The data exhibits a flat latitude structure of NO, the models tend to produce at equatorial maximum. The first task was to use the UARS-HALOE data to confirm the SME observations. The purpose of this first phase was to verify the UARS-NO structure is consistent with the SME data. The next task was to determine the cause of the discrepancy between modeled and observed nitric oxide latitude structure. The result from the final phase indicated that the latitude structure in the Photo-Electron (PE) production rate was the most important

F-layer storms and thermospheric composition

This paper discusses the present understanding of ionospheric storms, in particular storm effects in the F-layer and their relationship to changes of thermospheric composition. After summarizing the main effects of storms on the F-layer and the neutral thermosphere, the paper discusses some recent theoretical modelling of how the thermosphere responds to energy inputs at high latitudes, such as occur during storms. The high-latitude inputs set up a thermospheric "storm circulation", with consequent world-wide temperature increases and changes in chemical composition. These composition changes appear quite differently when viewed at fixed heights and at fixed pressure-levels. At fixed heights in the F-layer, there are marked increases of neutral molecular/atomic concentration ratio; but at fixed pressure-levels the increases are confined to high latitudes, with decreases at middle and low latitudes. The results of the modelling cast doubt on the theory that negative F-layer storms at midlatitudes are caused by decreases in the atomic/molecular ratio brought about by dynamical processes originating at high latitudes. The implications for storm theories, and alternative mechanisms for producing negative storm effects, are discussed. It is still considered that the negative storm effects are probably due to photochemical processes. The possibilities include widespread enhancement of the vibrational excitation of molecular nitrogen, induced by soft particle precipitation, or substantial energy inputs at latitudes well equatorward of the auroral ovals

Model predictions of the occurrence of non-Maxwellian plasmas, and analysis of their effects on EISCAT data

The recent identification of non-thermal plasmas using EISCAT data has been made possible by their occurrence during large, short-lived flow bursts. For steady, yet rapid, ion convection the only available signature is the shape of the spectrum, which is unreliable because it is open to distortion by noise and sampling uncertainty and can be mimicked by other phenomena. Nevertheless, spectral shape does give an indication of the presence of non-thermal plasma, and the characteristic shape has been observed for long periods (of the order of an hour or more) in some experiments. To evaluate this type of event properly one needs to compare it to what would be expected theoretically. Predictions have been made using the coupled thermosphere-ionosphere model developed at University College London and the University of Sheffield to show where and when non-Maxwellian plasmas would be expected in the auroral zone. Geometrical and other factors then govern whether these are detectable by radar. The results are applicable to any incoherent scatter radar in this area, but the work presented here concentrates on predictions with regard to experiments on the EISCAT facility

Modelling the response of the thermosphere and ionosphere to geomagnetic storms: Effects of a mid-latitude heat source

The UCL-Sheffield coupled thermosphere/ionosphere model has been used to assess the consequences of a heat source from ring current ion precipitation during a geomagnetic storm. Such a source might in principle cause the thermospheric upwelling needed to produce significant changes in the neutral molecular/atomic ratio, and thus in F2 layer electron density, it having been shown that auroral sources probably cannot produce such changes at mid-latitudes. Some alternative explanations of the F2 layer depletions are considered and largely discounted. The computations show that an O+ source of around 4 mW/m2, as has been detected by satellites, could appreciably change the molecular/atomic ratio, notably in the early morning sector. The response qualitatively agrees with a number of satellite observations of composition and airglow, and with observations of NmF2. If increased in duration and intensity, the source may be able to account for the negative storm phenomena