Modelling and analysis of Ørsted total field data over Southern Africa

Abstract

Ørsted total intensity data were analysed over the southern African region between 10° South and 45° South in latitude and between 10° East and 45° East in longitude, measured between May 1999 and October 1999. The satellite magnetic field measurements were selected corresponding to magnetic quiet conditions during local day and night times before carrying out an analysis of external fields by subdividing the data into Dst bins 5 nT wide, centred at multiples of 5 nT. The technique of Spherical Cap Harmonic Analysis (SCHA) was then applied to each of these Dst data bins to obtain external field coefficients, after removing the core field by means of the IGRF model. Combining all data sets after removing main ‐ and external fields, enabled the compilation of a total intensity crustal anomaly map over southern Africa at an average satellite altitude of 800 km. The Walvis Ridge and Agulhas anomalies are understood to represent thickened remanently magnetised ocean crust. The Agulhas anomaly however, represents the remnant scar of the process that led to the Gondwana fragmentation.

Spherical Cap Harmonic Analysis Techniques for Mapping High-Latitude Ionospheric Plasma Flow—Application to the Swarm Satellite Mission

“…Satellite data from various missions have been incorporated into single or multi-instrument data sets using techniques not limited to SCHA for magnetic field mapping (e.g. An et al 1998;AMIE;Green et al 2007;Haines 1985b;Kotzé 2002), or for mapping statistical convection patterns based on satellite data collected over a long period of time (e.g. Heppner and Maynard 1987;Rich and Maynard 1989;Hairston and Heelis 1990;Rich and Hairston 1994;Papitashvili et al 1999;Papitashvili and Rich 2002;Weimer 1995).…”

“…Spherical cap harmonic analysis has been used for mapping a wide variety of ionospheric and magnetospheric parameters including secular variation over Canada (Haines 1985c(Haines , 1993, China (Chen et al 2011), Europe (Korte and Haak 2000), Italy (De Santis et al 1997a), the North Atlantic (Pavón-Carrasco et al 2013), South Africa (Nahavo et al 2011) and Spain (Garcia et al 1991;Torta et al 1992); vertical magnetic field anomalies poleward of 40° (Haines 1985b); magnetic field intensity (Kotzé 2002); Birkeland current systems (Green et al 2006); equivalent current systems (Gaya-Pique et al 2008;Haines and Torta 1994); ionospheric total electron content (TEC) (Liu et al 2011(Liu et al , 2014Ghoddousi-Fard et al 2011;Otsuki et al 2011); electric fields (Fiori et al 2010), ionospheric conductance estimates (Green et al 2007); and the critical frequency of the F 2 layer (f o F 2 peak) (De Santis et al 1991Lazo et al 2004). Notably, SCHA algorithms have been incorporated into the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) mapping algorithm (Richmond and Kamide 1988).…”

Hermanus Magnetic Observatory: a historical perspective of geomagnetism in southern Africa

“…The first magnetic survey of the Union of South Africa was carried out by John Carruthers Beattie of the South African College in Cape Town, assisted by John Todd Morrison of the Victoria College in Stellenbosch, between 1898 and 1906 (Beattie and Morrisson, 1903). Measurements of magnetic declination, inclination and horizontal intensity were made using Kew magnetometers and Dover dip circles magnetic declination (Kotzé, 2014). These 336 observations were recently used to derive a spherical cap harmonic field model for southern Africa for 1903.5, valid between 25 and 35 • S and between 17 and 32 • E (Kotzé, 2014).…”

“…Measurements of magnetic declination, inclination and horizontal intensity were made using Kew magnetometers and Dover dip circles magnetic declination (Kotzé, 2014). These 336 observations were recently used to derive a spherical cap harmonic field model for southern Africa for 1903.5, valid between 25 and 35 • S and between 17 and 32 • E (Kotzé, 2014). This model now serves as the first historical regional field model for southern Africa.…”

A proposal for regional modelling at the Earth's surface, R-SCHA2D

“…In fact, the comprehensive model CM4 is doing a good job in that areas between 1960 and 2002.5. It is important to stress that the CM4 is well constrained over Europe but it is reckoned to be less reliable, particularly in the Southern hemisphere or isolated area, like in Antarctica (De Santis et al 2002) or South Africa (Kotzé 2002). Moreover, global models have non-negligible crustal bias that can be corrected for at a regional scale considering jointly aeromagnetic and satellite data over a given region (Thébault et al 2006a).…”