Abstract
Geomagnetic field data from four observatories and annual field surveys between 2005 and 2015 provide a detailed description of Earth’s magnetic field changes over South Africa, Namibia and Botswana on time scales of less than 1 year. The southern African area is characterized by rapid changes in the secular variation pattern and lies in close proximity to the South Atlantic Anomaly (SAA) where the geomagnetic field intensity is almost 30 % weaker than in other regions at similar latitudes around the globe. Several geomagnetic secular acceleration (SA) pulses (geomagnetic jerks) around 2007, 2010 and 2012 could be identified over the last decade in southern Africa. We present a new regional field model for declination and horizontal and vertical intensity over southern Africa (Southern African REGional (SAREG)) which is based on field survey and observatory data and covering the time interval from 2005 to 2014, i.e. including the period between 2010 and 2013 when no low Earth-orbiting vector field satellite data are available. A comparative evaluation between SAREG and global field models like CHAOS-5, the CHAMP, Orsted and SAC-C model of the Earth's magnetic field and International Geomagnetic Reference Field (IGRF-12) reveals that a simple regional field model based on a relatively dense ground network is able to provide a realistic representation of the geomagnetic field in this area. We particularly note that a global field model like CHAOS-5 does not always indicate similar short-period patterns in the field components as revealed by observatory data, while representing the general secular variation reasonably well during the time interval without near-Earth satellite vector field data. This investigation further shows the inhomogeneous occurrence and distribution of secular variation impulses in the different geomagnetic field components and at different locations in southern African.
Highlights
Geomagnetic secular variation (SV), the change of the magnetic core field, has for a long time been known to proceed in an irregular manner across the globe (Bullard 1948)
Southern Africa lies on the brink of the South Atlantic Anomaly (SAA) where the field intensity is up to 30 % weaker than in comparable latitudes, and the secular variation is strong in this region (e.g. Mandea et al 2007)
We have cross-validated the regional model with the global CHAOS-5 spherical harmonic model, the first model using satellite data and spanning the 2010–2013 gap between the CHAllenging Minisatellite Payload (CHAMP) and Swarm satellite missions, where no near-Earth vector field satellite data are available
Summary
Geomagnetic secular variation (SV), the change of the magnetic core field, has for a long time been known to proceed in an irregular manner across the globe (Bullard 1948). The southern African ground survey data have been used to derive various regional main field and/or secular variations models, using the methods of polynomials for the time intervals 2004–2005 (Kotzé et al 2007) and 2005–2009 (Kotzé 2011; Geese et al 2011), spherical cap harmonic functions for 1975–2000 (Kotzé 2003) and harmonic splines for 1961–2001 (Geese et al 2010) and 2005–2009 (Geese et al 2011). We model main field and secular variation data from the ground station network of the geomagnetic field components declination (D), horizontal intensity (H) and vertical intensity (Z) for the period 2005–2014, using polynomials that can be expressed as a function of latitude and longitude
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