Spherical cap modelling of Ørsted magnetic field vectors over southern Africa

Abstract

Vector magnetic field observations by the Ørsted satellite during geomagnetic quiet conditions around January 1, 2000, have been employed to derive a spherical cap harmonic model (Haines, 1985) over the southern African region between 10• and 45• South in latitude and between 10• and 45• East in longitude. In the process a total of 9283 vector (27949 component) Ørsted observations have been used to derive a spherical cap model with minimum wavelength resolution approximately 1200 km. This model is compared with the global Ørsted magnetic field model (Olsen et al., 2000a) in terms of a fit to observatory and repeat station data over the southern African subcontinent where the Hermanus Magnetic Observatory conducts field surveys at regular 5 year intervals.

The time-varying geomagnetic field of Southern Africa

“…Spherical Cap Harmonic Analysis (SCHA) is a mathematical technique developed specifically to model a potential field and its spatial derivatives, or a general function and its surface derivatives, on a regional scale in order to overcome the non-orthogonality problem in the case of global spheri- cal harmonic models when applied to restricted areas. The SCHA modelling technique has also been used successfully to derive a regional field model over southern Africa, using vector data from the Ørsted satellite mission (Kotzé, 2001). By making every harmonic coefficient a polynomial in time, a SCHA model provides both spatial and temporal continuity, enabling the modelling of secular variation as well.…”

The Hermanus Magnetic Observatory as a space physics facility

“…Figure 2 shows the distribution of the HMO's four magnetic observatories as well as the field stations. Data from two low Earth orbit (LEO) magnetic field satellites, namely, the Danish Ørsted and the German CHAMP satellites, are used to supplement the above mentioned ground data for the derivation and updating of geomagnetic field models [4]. These models are used to provide accurate and up-to-date magnetic field information for navigation purposes as required by the South African Defense Force for safety and security as well as peacekeeping missions in Africa.…”

Polynomial modelling of CHAMP satellite data to investigate rapid secular variation fluctuations over southern Africa during 2003 and 2004

“…Significant current systems exist within and on the boundary of the magnetosphere, resulting in large magnetic fields described by various magnetic activity indices. For this investigation however, we assume that ring current dominates at the mid-latitudes considered and that this can be represented by the D st index (Kotzé, 2001).…”