A study of the plasmaspheric electron content using satellites of the global positioning system

Abstract

The upper atmosphere of the Earth is ionised by electromagnetic radiation and energetic charged particles emanating from the Sun. The ionisation forms the ionosphere of heavy ions (e.g. O) and electrons between about 70 and several hundred kilometres above the Earth's surface. Above this, the protonosphere (H* and electrons) extends to the plasmapause at several Earth radii. The ionosphere and protonosphere together comprise the plasmasphere. The ionisation along a ray-path (defined by the elevation and azimuth angles from a point on the Earth's surface) is measured in terms of the number of free electrons occupying a column of unit area along that ray-path: the total electron content (ThC). At present, the global positioning system of satellites (UPS) potentially offers the most available opportunity for measuring variations in plasmaspheric TEC. An intrinsic problem with the GPS method is that satellites are neither low-orbiting nor geosynchronous, so that space and time variations are not readily separable. Although the plasmasphere extends over a great height range, an infinitely thin slab at the plasmapheric effective height must be assumed in order to convert oblique to zenithal observations. Furthermore, each satellite has a hardware delay, or bias, giving uncertainty in the baseline of the derived electron content. The validity of this conversion process is theprimary consideration of this thesis.

At a latitude of approximately 53°N, Lancashire is well placed as an observing site since UPS satellite orbits are inclined at 55° to the Earth's equatorial plane and, as a result, many UPS tracks pass almost directly overhead, giving true zenithal measurements. This thesis focuses on the question of oblique-to-zenithal correction and related matters. In particular, plasmaspheric effective height (PEH) and satellite bias corrections are determined by measurement of TEC from pairs of satellites, each pair giving simultaneous observations of oblique and zenithal TEC. The bias corrections are applied to the zenithal TEC observations and the continuity of daily TEC plots are then compared with other published bias lists to assess their validity. Additional bias corrections are then determined by extracting theTEC using pairs of satellites with the same elevation at the same time. The Chapman Production Function Model and the Sheffield University Plasmasphere and Ionosphere Model are both used to determine a theoretical value for the PER Further validations are performed by comparing the experimentally determined PEH with those derived from the models. Finally, using the validated PEH and satellite bias corrections, the Lancaster 1993 dataset is converted to a zenithal baseline, and corrected daily TEC plots are produced for days of quiet and disturbed solar activity.

The final results of the project emphasise the need for global standardisation when comparing UPS-derived zenithal TEC maps from geographically remote observing stations. The results indicate (i) that the PEH used in the oblique-to-zenithal thin shell conversion is considerably greater than the commonly adopted value of 350 km. The results are not
conclusive, but it is suggested that a value of approximately 1000 km for PEH is preferred, based on all available evidence. The cause is attributed to the (at times) significant contribution of the ionised hydrogen in the protonosphere. Further results involve bias derivations which indicate (ii) that the bias values vary significantly over a period of several months and that up-to-date biases must be used whenever UPS-derived TEC results are
reduced to zenithal values. Finally, the results suggest that the assumption of spatial isotropy required by the thin shell model is questionable.
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