Ogunmodimu, Olugbenga and Honary, Farideh (2016) Auroral radio absorption : modelling and prediction. PhD thesis, Lancaster University.
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Abstract
Energetic particle precipitation with (>keV) energetic electrons from closed field lines and (>MeV) protons from the solar wind are responsible for enhanced high frequency radiowave absorption in the high latitude ionosphere. Measuring the propagation of radio waves through the ionosphere has been utilised for investigating particle precipitation into the upper atmosphere. Although various methods and models previously proposed for auroral and polar cap absorptions have significantly contributed to our knowledge, there are progressive efforts to improve these models as a result of improved understanding of already made assumptions, development of more efficient equipment and availability of real-time data. This study seeks to contribute to this field. The method utilised combines data from ground-based imaging riometer during solar cycle 23 (1996-2009) and solar wind parameters measured at the L1 point between the Earth and Sun to provide empirical relationship as the basis for a statistical model of auroral absorption. The variation of cosmic noise absorption (CNA) has been modelled using the Akasofu epsilon parameter which characterises the energy transfer between the solar wind and the magnetosphere. The result shows that the absorption model based on the epsilon parameter is reliable for periods with low to moderate solar activity but breaks down during periods of high solar activity such as solar flare and interplanetary coronal mass ejections (ICMEs). Hence, separate models for ICME and flare induced absorptions are produced. The modelled results have been compared with IRIS data. On ICME absorption, key observation shows that absorption associated with ICMEs exhibits different character depending on ICME start times. ICMEs were categorised into day time events (solar zenith of riometer station _ _ 80_) and as night time events (_ _ 100_). Differing absorption signatures were observed for day and night ICME events. This work ranked various solar wind parameters to obtain the best coupling parameter for day and night time absorption. For example, day time ICME model is based on solar wind dynamic pressure and V Bz, while night time ICME model is based on Bz and nV 3. In the case of modelling of solar flare induced absorption, the magnitude and duration of absorption is seen to be dependent on different classes of solar flares. Properties of solar flare such as the rise time, the maximum intensity and decay time were used as the building block of the flare model. Comparing ICME induced absorption with absorption induced by solar flares, it was observed that ICME induced absorption is seen to have longer duration (_ of hours) and stronger magnitude than those associated with solar flares (_ of minutes).