Abstract
In this thesis, we study an energetic electron atmospheric precipitation event that occurred on 17–19 January 2005, by quantifying the energy distribution, geographic extent and net flux of the precipitating electrons. The electron spectra measured by the IDP instrument onboard the DEMETER satellite provide an indication of the precipitating spectra. These measurements are combined, using subionospheric VLF propagation modelling, with recordings of subionospheric VLF signals on paths that pass beneath the precipitation zone to determine its characteristics. In addition, an overview of the DEMETER satellite, and a study of the effect of a powerful ground-based transmitter on inner belt electrons are also presented.
A detailed examination is made of the measurements taken by the DEMETER electron spectrometer (IDP), with particular focus on the variation in the population of electrons that are sampled by this instrument (e.g. bounce-loss cone, trapped etc.) as a function of the satellite’s geographic location. This provides the first comprehensive description of what this widely-used instrument measures.
As part of the examination of the DEMETER IDP data, we perform a survey of radiation belt drift-loss cone electron enhancements produced by cyclotron resonance with waves from a ground-based transmitter. The transmitter is very clearly identified as the cause of the enhancements through dependence upon transmitter operation, favourable (nighttime) ionospheric conditions above the transmitter, and observation of the enhancement downstream of the transmitter’s location but not significantly upstream of it. The interaction is identified as pitch angle scattering by first-order cyclotron resonance, and the characteristics of the interaction (geomagnetic latitude range, energy range, and scattering efficiency) are then investigated.
DEMETER IDP drift-loss cone measurements of radiation belt electron spectra are used to determine a characteristic spectrum for the 17–19 January 2005 storm. This drift-loss cone energy spectrum is then used as an estimate for the energy distribution of the precipitating (bounce-loss cone) electrons, which cannot be directly measured by DEMETER.
Subionospheric VLF signal amplitude recordings made in Sodankylä, Finland, of transmitters DHO and ICV (in Germany and Italy respectively) are perturbed by precipitation during 17–19 January 2005. Using the subionospheric propagation modelling code LWPC, the subionospheric VLF and energetic precipitation spectra measurements are combined to determine the extent of the precipitation zone in terms of geomagnetic latitude, and the net amount of precipitating flux. Error estimates are given for the extent of the precipitation zone and the amount of precipitating flux. The behaviour of the system in response to variation of the precipitation spectrum is also investigated, using spectra that characterise the periods of the storm when spectrally harder electron fluxes exist. We give explicit expressions for the final estimate of the precipitating electron energy and flux. The ionisation profile which results from this electron precipitation is compared to those in studies of other atmospheric precipitation events, providing an estimate of the likely impact that this event would have on atmospheric ozone levels.