1. P Kotzé (1), M Korte (2), M Mandea (2,3)
Modelling of Southern African Secular Variation Observations since 2005
P Kotzé (1), M Korte (2), M Mandea (2,3)
HMO
GFZ German Research Centre for Geosciences
3. Institut de Physique du Globe de Paris

2. COMPASS: Comprehensive Magnetic Processes under the African Southern Subcontinent
Picture supplied by A Geese, GFZ

3. Step 1 Step 2 Observations and Data
Observations at field survey stations
DIFlux , LEMI Fluxgate variometer & Overhauser
D,H,Z
Step 2
Correct for diurnal variations
Use CM4 to correct for external and induced fields
Derive X,Y,Z

4. Modelling of Field Survey Observations
Main Field:
Spherical Cap Harmonic Model
Θ = 20º, k=5: λ = ~ 1646 km
Secular Variation:
Polynomial Model
2nd / 3rd degree

5. Southern African Region
characterised by rapid
variations in X-component SV

6. Rapid variations of
Y-component SV

8. SV Pattern at Hermanus
Plots: First differences of X, Y and D monthly means.
Apply 12 month running means to remove annual variation caused by magnetospheric and ionospheric currents.
What is the importance of jerks?
Jerks occur as a result of jump
in acceleration of fluid motion
at CMB.
2. Electrical conductivity of mantle
limits propagation of jerk.
Occurance of jerks over short periods (SV Impulses) have important consequences for determining electrical conductivity of mantle.

10. Rapid variations in
Declination: :
Eastward change
increased from 5-9 min/y
Westward change
decreased

11. Southern Africa divided into:
Decreasing western region
Increasing eastern region

12. Z-component decreasing
At constant rate between

13. Global IGRF model not suitable
for modelling regional SV,
particularly in Southern Africa
Rapid SV variations necessitate
regional modelling approach

14. Main Field Modelling

15. Conclusions
Prominent secular variation changes from 2005 till 2009 characteristic of geomagnetic jerk occurred in certain areas.
In particular orientation changes as witnessed by changes in declination.
Westward shift in D at HER and HBK slowing down, while Eastward shift at TSU increases.
Secular variation changes over southern Africa could indicate rapid motions at CMB and upper limit on mantle conductivity.