Modelling and Analysis of POGS Data over Southern Africa by Spherical Cap Harmonic Analysis

Abstract

POGS total intensity data were analysed over the Southern African region between 10° South and 45° South in latitude and between 5° East and 40° East in longitude. Satellite data were selected corresponding to magnetically quiet conditions during local night times before carrying out an analysis of external fields by subdividing winter and summer data into Dst bins 5 nT wide, centred at multiples of 5 nT. Spherical Cap Harmonic Analysis (SCHA) was then applied to each of these Dst data bins to obtain external field coefficients corresponding to k = 1. Residual POGS data were downward continued by means of the equivalent layer magnetization model and modelled by SCHA to reveal component data at ground level.

Modelling and analysis of Ørsted total field data over Southern Africa

“…The model just described is not a potential field analysis, but is simply a fit to the residuals, as we did not solve simultaneously for both internal and external fields, with F approximated as a general function of co‐latitude and longitude with no radial dependence.The variation with Dst as obtained in this investigation compares favourably with results obtained by Langel and Estes [1985] using global spherical harmonic analysis for dawn/dusk Magsat data and Kotzé and Barraclough [1997] using winter/summer POGS scalar measurements. It is however difficult to assign error bars to the data in Figure 1.…”

“…Recently, Haines and Newitt [1997] also used SCHA to model the secular variation and main field simultaneously over Canada, using various data sets that include both vector and scalar observations at ground and satellite altitudes. The SCHA modelling technique has also been used successfully to model the external field affecting POGS scalar data [ Kotzé and Barraclough , 1997] in order to identify crustal magnetic anomaly features over southern Africa.…”

“…For each of the day and night Dst data intervals four expansion coefficients were derived, with q 0 0 by far the most significant coefficient, representing the external field at an average satellite altitude of 800 km. Following the approach of [ Langel and Estes , 1985] as well as [ Kotzé and Barraclough , 1997], a regression analysis revealed a clearly linear relationship between the q 0 0 coefficient and Dst for both day and night periods, as displayed in Figure 1: …”

“…This paper discusses the modelling of near‐earth Ørsted total intensity magnetometer data over southern Africa by means of Spherical Cap Harmonic Analysis (SCHA) to identify the external field and the subsequent identification of long‐wavelength crustal anomalies. Previous analysis of Polar Orbiting Geomagnetic Survey (POGS) satellite measurements over the southern African region [ Kotzé and Barraclough , 1997] revealed strong correlations between both continental and oceanic magnetic anomalies and major tectonic features as identified using Magsat data [ Antoine and Moyes , 1992].…”

Spherical cap modelling of Ørsted magnetic field vectors over southern Africa

“…Recently, Haines and Newitt (1997) used SCHA to model the secular variation and main field simultaneously over Canada, using various data sets that include both vector and scalar observations at ground and satellite altitudes. The SCHA modelling technique has also been used successfully to derive a regional field model over southern Africa, using only scalar data from the POGS mission (Kotzé and Barraclough, 1997).…”