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\ud 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\ud 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\ud 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.\ud 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\ud to lower mantle conductivity are also discussed
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