title: Geomagnetic diurnal variation studies using global models &amp; observatory data at quiet &amp; moderately disturbed times.
Creators: name: Onovughe, Elvis
Publisher: name: University of Liverpool Repository
Year: 2015

description: In this thesis we investigate and analyse the nature and behaviour of the external field variations of the geomagnetic field for quiet and moderately disturbed days using geomagnetic observatory measurements and field models. We use spherical harmonic modelling, led by available geomagnetic observatory measurements and past models (particularly the Comprehensive Model of Sabaka et al. 2004). As an initial step, we extended the lifespan of the Comprehensive Model (CM4) beyond its 2002.5 lifespan to allow for use of current data. We produced profile plots of the diurnal field and generated global maps of the field and compare these with the CM4 model, to see how well the CM4 model could reasonably predict ground variation of the diurnal field outside its lifespan and for days away from quiet time. The comparison shows that away from quiet time period, the CM4 model is producing more reasonable predictions than expected, despite the lack of active data in the original model dataset. The CM4 model fits the regional type features of the geomagnetic components, but not doing well predicting the short term features during period of rapid variations (seen as ‘wiggles’ in the profile plots) , especially for the X- component. Also, comparing the modelled diurnal maps of the CM4 and observatory data shows that increasing the spherical harmonic degree produces a better match between the CM4 model and the data. Our result reveals that the external field description included in the CM4 model could not sufficiently explain the field variation for days away from quiet time. The CM4 model predicts the Y- and Z- components variations better than the X; this may be due to the fact that the X component is more affected by external field sources. As a result we introduced the use of an additional geomagnetic activity index (the RC index), to enable us to establish the nature of the rapid variation seen in the data for days away from quiet time. We looked for this using eigenanalysis (covariance matrices, eigenvectors and eigenvalues), detrending the data sequences with spline fits, and comparing the observatory data residuals with the RC index values. We also looked at the coherence and correlation between small scale features showing up in the rapid variation, to try to establish the global scale of the variations. We analysed this by simple running average method, correlation and cross-correlation coefficients between the residuals of the observatory data components and the RC index. Our results show that our data for days away from quiet time, particularly the X component, include a strong component in the rapid variations related to large-scale external field variation arising from the magnetospheric ring current. For example we are able to reproduce features in our plots that show a very strong coherence and correlation existing between the X component of our observatory data residuals and the RC index. This is also seen in the same components of the observatories at different locations within the same geographical region, and at some different geographical regions. This allows us to characterise the RC index as being a good representation for rapid variations globally. Also, it makes us optimistic that it may be useful to look at rapid variation observatory results for combined observatory stations as a good technique for remote referencing in aeromagnetic surveying.