This thesis describes work that has employed both ground- and space-based data to investigate the magnetotail and its response to differing levels of geomagnetic activity. The first study of current sheet motion involved analysing the number of crossings that the Cluster 3 spacecraft made of the cross-tail current sheet, as a measure of current sheet dynamics. Geomagnetic conditions measured during the crossings allowed a comparison of the current sheet dynamics during substorms, magnetic storms and quiet times. The results indicated that more motion of the current sheet is seen during substorms compared to quiet times. In addition, there was evidence that current sheet motion is suppressed when the ring current is enhanced. An analysis of current sheet structure is presented in the next study, which examines the orientation, current density and thickness of the current sheet. The current sheet was found to be more tilted during substorms than quiet times and when magnetic storms were occurring the tilt was reduced. The current density analysis showed larger values during storms compared to quiet and substorm times. The final study investigated how the lobe magnetic field varies according to geomagnetic activity. Results are presented showing evidence of a larger lobe magnetic field during magnetic storms, compared to quiet times and substorms. The thesis work provides a more complete picture of the current sheet and lobes, during different levels of geomagnetic activity, than has been previously shown. There is evidence that substorm occurrence is related to current sheet motion and a highly tilted orientation. The increased lobe field found during magnetic storms may cause a rigidity to the magnetotail that suppresses this motion and its tilting in the YZ plane. The increased lobe field also results in an increased current density in the current sheet during storms.

The problem of generation and maintenance of the solar magnetic field has already been studied in detail, with many studies concentrating on kinematic, convection-driven dynamos. These have, in many cases, been unable to create and sustain a large-scale magnetic field in the parameter regimes required for accurate modelling of the solar interior. This thesis addresses the possibility of an alternative dynamo mechanism, powered by the magnetic buoyancy instability. Such a mechanism is dynamic, rather than kinematic, in nature and as such the electromotive force (emf), which is an averaged product of perturbations, is fully dependent on the form of the field. In this thesis we begin by considering some of the earliest studies of magnetic buoyancy, in particular that of Gilman (1970). The system of perturbation equations from this paper is used to derive an analytical expression for the emf. The large-wavelength limit used in this paper is then examined and the eigenfunctions of a simplified system of perturbation ... '. ~ equations are found to localise in this limit. The emfs resulting from atmospheres unstable to magnetic buoyancy are then numerically calculated using the method of inverse iteration. The symmetries of the emfs and their dependence on the governing parameters of the problem are then studied; finally a sheared component of magnetic field is introduced. This breaks some of the previously existing symmetries of the system and allows us to study the nature of the instability when it must 'untwist' the field in order to propagate.

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).

This thesis is an investigation of the changes in the magnetic field as measured at the surface of the Earth on the time-scale of months to decades. In particular the phenomena of geomagnetic "impulses" or "jerks" are investigated. Vigorous discussion has surrounded these phenomena since they were first suggested to have been of global scale, of short duration and originating within the core (Courtillot et a/, 1978), primarily because of their implications for lower mantle conductivity. A major point of discussion has been whether they were of internal or external origin, and to what extent external fields determine their apparent time-scale. A large quantity of monthly means of the geomagnetic field is analysed here to investigate the contribution from external and induced fields. A model of the disturbance fields on the time-scale of months and years is derived. Using the oa geomagnetic index to represent the temporal dependence, the spatial morphology is found to be primarily dipolar aligned with the Earth's main dipole. This model allows a better representation of the core field to be obtained. Seasonal fluctuations in the field are also quantified. The results are found to be consistent with an insulating mantle down to about 600fcm and a conductivity of about 0.15m-1 to 15m"1 below that. A new method is developed to analyse the time-dependence of the improved representation of the core-field and is applied to a large set of geomagnetic annual means. This method determines the periods oftime for which the field, as measured at different locations, can be represented by a quadratic time-dependence. Such a representation is found to be valid typically for 10 years at a time and valid for 93% of the data. Dates at which the changes from one quadratic time-dependence to another occur are found, to a certain extent, to be globally synchronous. Particular dates when this occurs are found to be 1970, 1978 and 1983, the latter events being similar in character to the 1970 jerk, and are thought to arise from impulses in the third time-derivative of the core field. Spherical harmonic models of the main field with a quadratic time-dependence are then derived for epochs 1965.5,1974.5, 1981.5 and 1986.5 using the technique of stochas-tic inversion. These models are then used to map the changes in secular acceleration associated with the 1970, 1978 and 1983 jerks. The global extent of the 1978 and 1983 jerks have not previously been investigated. The 1983 jerk is found to be much weaker than the others and the 1978 jerk appears anti-correlated with the 1970 jerk. The role of electromagnetic coupling between the core and mantle is considered in the presence of a thin conducting layer at the base of the mantle. Time-dependent torques are computed for the period 1900 to 1980 and found to correlate closely with the torque required to explain the decade fluctuations in the length of day. If electro magnetic coupling is solely responsible for the decade fluctuations then this implies the conductance of the layer must be ~ 7 x 1085. Various other pieces of evidence relating to lower mantle conductivity are also discussed.

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