A study of geomagnetic field time variations over southern Africa using a regional harmonic spline core magnetic field model derived from CHAMP satellite and ground magnetic observations
Date
2019
Authors
Nahayo, Emmanuel
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Abstract
The geomagnetic field spatial variation in Southern Africa is characterised by
a high horizontal gradient, and its study requires high spatial resolution data. Regional
field models are more suitable than global models to study small scale features
of the geomagnetic field variation. They use more dense data, hence a good data
coverage allows a detailed description of the geomagnetic variation in the area of
investigation. In Southern Africa, the ground recording stations are limited (4 magnetic
observatories and 38 geomagnetic repeat stations) and satellite data are needed
for studies where high spatial and temporal resolution data are required. The southern
African region borders the South Atlantic Anomaly (SAA) where the strength
of the magnetic field is approximately 30% weaker compared to other regions of the
same latitudes. In an attempt of supporting geomagnetic field model data users
and investigating the evolution of the South Atlantic Anomaly, a study to develop
and derive a southern African regional field model combining CHAMP satellite data
with ground-based data showed that the regional model over Southern Africa can be
improved by combining both satellite and ground-based data. The internal Earth
processes generating the main field can be studied by modelling the core field using
inversion of geomagnetic field components measured above the surface of the
Earth, and they assist in monitoring the long-term variation of the geomagnetic
field on time scales of 1 year and more, therefore giving valuable information on the
evolution of the SAA in southern Africa.
The study of the geomagnetic field time variation in the southern Africa region
was carried out applying the harmonic splines technique on CHAMP satellite and
ground data that have been recorded between 2001 and 2010. A Southern Africa
Regional Model (SARM) was derived and evaluated using global models, such as
International Geomagnetic Reference Field (IGRF-11) and GFZ Reference Internal Magnetic Model (GRIMM), by calculating the difference between SARM and global
models. The results of this study suggest that the 200 km gridded SARM model
developed from only satellite data is shown to match monthly averaged ground data
to within 1.5%, 1.6% and 0.7% for X, Y and Z, respectively, suggesting the temporal
variations are valid where ground data are not available. In addition, these results
also confirm the earlier findings of the 2007 magnetic jerk and rapid secular variation
fluctuations of 2003 and 2004 on the eastern edge of the SAA which extends 1000
km into South Africa.
CHAMP satellite and relatively densely spaced (300 km) ground-based data measured
over southern African region between 2005 and 2010 are incorporated in a
new regional, harmonic spline based, core field model. Our new SACFM-2 model is
compared to our previously developed SARM model, a regional model based only
on satellite data, and the global CHAOS-6 model developed by Finlay et al. (2016).
The results agree well with the CHAOS-6 model (within 0.4%) in the vertical (Z)
component and total field (F) that are used to investigate the evolution of the South
Atlantic Anomaly in the region. The computed maps of main field of the Z component
and total intensity F show a steady decrease of the field between 2005.5 and
2010.5, reaching an average of 40 nT and 50 nT per year, respectively, in the southwest
of South Africa, indicating evolution of the South Atlantic Anomaly under
Southern Africa and an increase of the geographical area of this feature.
In addition, a southern African lithospheric magnetic model (SALMM) over Southern
Africa was developed based on the Spherical Cap Harmonic Analysis method
using CHAMP satellite measurements at an altitude between 332 km and 365 km,
and between 2007.0 and 2009.0 epochs. This lithospheric magnetic model SALMM
was compared to a regional model developed by Vervelidou et al. (2018) and the
global model MF7. Our lithospheric magnetic model SALMM was interpreted in
terms of regional (100s km) geological features and long-wavelength geological magnetic
anomalies.
Description
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg,
in fulfilment for the degree of Doctor of Philosophy
Johannesburg, October 2019
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Citation
Nahayo, Emmanuel (2019) A study of geomagnetic field time variations over southern Africa using a regional harmonic spline core magnetic field model derived from CHAMP satellite and ground magnetic observations, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/29577>