1.2 Subproject
34
Sub-Project 1.2
The Morphology of Geomagnetic Field Variations in the Southern African
Region and Links to Global Geomagnetic Field Evolution
Participants
* Coordinator(s)
Institution
Name
Email address
GFZ Potsdam
(GFZ)
Herman Lühr *
Hleuhr@gfz-potsdam.de
Monika Korte
monka@gfz-potsdam.de
Volker Haak
Vhaak@gfz-potsdam.de
Peter R Sutcliffe *
psutcliffe@hmo.ac.za
Pieter B Kotzé
pkotze@hmo.ac.za
Hermanus Magnetic
Observatory
(HMO)
Requested Funding:
Total for the 5-year duration project beginning in 2004: EUR 511 000
Year 1
Year 2
Year 3
Year 4
Year 5
HMO, SA
60 600
31 900
31 900
31 900
31 900
GFZ
75 600
61 800
61 800
61 800
61 800
136 200
93 700
93 700
93 700
93 700
TOTAL
1.2 Subproject
35
Summary
The existence of the geomagnetic field has numerous important
benefits for society; for example, two such benefits are its shielding
against the damaging radiation from space and its implementation in
navigation systems. Presently, the magnetic field of the Earth is
declining at a phenomenal rate. At the current rate of decline, the
dipole field will vanish within about a millennium, which is ten times
faster than the natural decay after a complete switch-off of the
geodynamo. The rapidly decreasing field has evoked the suggestion
that a reversal of the geomagnetic field may have already commenced.
Important indicators about the field variations can be found in the
detailed structure of the magnetic field at the core/mantle boundary.
There have been two prominent patches of reverse magnetic polarity
identified which can account for almost all the present day decrease.
The most intense reverse patch is beneath the southern tip of Africa. A
detailed study of the geomagnetic field in this key region, supported
by the global monitoring of the magnetic field from satellites like
CHAMP, will help to better explain and predict the secular variation
on a larger scale. The southern African subcontinent is therefore an
ideal location for studying the processes responsible for both the
regional and global changes of the geomagnetic field. This will
facilitate a better understanding of field behaviour, the coupling to
other geophysical processes, and the implications for society.
1.2 Subproject
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Scientific Motivation
Rapid Decrease of the Field Intensity
Since the start of systematic magnetic field recordings, some hundred years ago, a continuous
decrease of the Earth’s magnetic dipole moment has been observed. The change in the field
strength is, however, not evenly distributed over the globe. At the Earth’s surface the most
rapid decrease of the main field is observed in the Atlantic region. Southern Africa is in this
regard the continental area where these geomagnetic field changes can be best studied. Since
the establishment of the Hermanus Magnetic Observatory (HMO) in South Africa in 1941, the
total field intensity has decreased by 20%, which is greater than the decrease at any other
magnetic observatory. Presently, the HMO operates three geomagnetic observatories in
southern Africa [see section 7: Current Infrastructure].
Figure 1 shows the changes in field strength as recorded by the three observatories. Although
all the curves show a declining trend, neither the rate of change nor its temporal variation is
identical at the three sites. There are obviously significant gradients in the field decline across
the country, which are smaller in scale than the separation of the recording sites. Part of this
project will thus be to investigate the temporal and spatial structure of the rapid field decrease
below southern Africa with an improved set of observatories.
Total Intensity 1941 - 2002
Mean annual values in nanoteslas
32700
32200
31700
31200
30700
30200
29700
29200
28700
28200
27700
27200
26700
26200
Tsumeb
Hartebeesthoek
Hermanus
25700
1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004
Figure 1: Decrease of the geomagnetic intensity at Hermanus, Hartebeesthoek, and Tsumeb
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Reorientation of the Geomagnetic Field
Besides the field intensity, the orientation of the geomagnetic field in southern Africa is also
changing rapidly [Kotzé, 2003]. In the northwest part of southern Africa the declination of the
magnetic field is propagating eastward (Tsumeb) and in the south-east part westward
(Hermanus and Hartebeesthoek), as shown in Figure 2. This causes a spatial gradient over the
subcontinent which is presently increasing with time. A greater density of observation points
is required in order to resolve the structure of the field orientation and its evolution.
Magnetic Declination 1941 - 2002
60
0
-60
-120
-180
-240
Decl : TSU
Decl : HER / HBK
Mean annual values in min of arc
100
90
Tsumeb. D = -12° + decl
80
Hartebeesthoek. D = -16° + decl
70
Hermanus. D = -22° 48' + decl
60
9.3 '/year
50
11.7 '/year
40
30
20
10
-3.3 '/year
6.3 '/year
0
-10
-20
-30
-40
-50
-2.7 '/year
-60
5.0 '/year
-70
-80
-90
1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 1996 2000 2004
-300
-360
-420
-480
Figure 2: Evolution of the geomagnetic declination at Hermanus, Hartebeesthoek and Tsumeb.
A general picture of the behaviour of the magnetic field components during the past few
decades can be obtained from global magnetic field models. As an example, we have used the
Comprehensive Model (CM3) proposed by Sabaka et al. (2002). Figure 3 shows how the
intensities of the northward (X), eastward (Y) and downward (Z) components have changed
over the past 35 years. Curves have been plotted for each degree of latitude along a profile
from Hermanus (HER) to Tsumeb (TSU). The model predicts a field decrease in all three
components. It is, however, interesting to note that the predicted spatial gradient along the
chosen profile is continuously increasing for both the Y and Z components. The rather
dynamic variation of the magnetic field in this region requires investigation by a denser array
of suitable ground-based observations.
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Figure 3: Prediction of the magnetic field evolution according to the CM3 model (Sabaka et al., 2002) along a profile from
HER (black line) to TSU (orange line) in steps of 1° latitude.
Presently, the geomagnetic field is monitored from space by a fleet of dedicated satellites,
Ørsted, CHAMP and SAC-C. From their measurements global models of the magnetic field
can be derived with an unprecedented accuracy. The secular variation at decadal scale or
longer, however, cannot be well determined due to the fairly short lifetime of these missions.
An important question, therefore, is how well do secular changes derived from satellite
models fit the ground-based measurements in this very dynamic region? If differences are
observed, how can they be explained? Are they due to imperfections in the model? Do
induction anomalies in the ground adversely affect the model? The existence of such an effect
has repeatedly been advocated in the literature (Porstendorfer et al., 1979; Alldredge, 1983),
but so far no convincing observations have been presented. This region with its rapidly
changing internal magnetic field is ideally suited to test the idea that rapid secular variations
can induce currents in highly conductive lateral structures in the lithosphere or upper mantle.
The magnetic signature caused by these confined currents is expected to change at the rate of
the secular variation and not of the field itself. From the results obtained in such an
investigation, it will be possible to decide whether similar studies in other parts of the world
can be expected to be successful.
Improved Modeling of Core Flow Dynamics
The rapid variation of the geomagnetic field is a manifestation of the dynamics of the flow in
the Earth’s outer core. In recent geodynamo models, suggestions have been made about the
kinds of flow required to support sustaining planetary magnetic fields (Gubbins and Roberts,
1987; Glatzmaier and Roberts, 1995). The continuous decrease of the magnetic dipole
moment and the simultaneous increase of multi-pole moments suggest the possibility of an
upcoming reversal of the geomagnetic field (Constable, 1992). It is premature to conclude
that the present-day downward trend of the dipole field will continue until it has disappeared,
particularly as paleomagnetic studies show repeated excursions without a reversal of polarity
1.2 Subproject
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(Nowaczyk and Frederichs, 1999). However, some indications that a reversal may have
commenced, have been presented by Hulot et al. (2002). According to their studies, the
asymmetric state of the geodynamo, which produces large longitudinal differences in field
changes (cf. Fig. 4), is one through which it has to go before the reversal.
Figure 4: Percentage change of the geomagnetic field from 1980 to 2001, as determined by the satellites Magsat and
CHAMP.
Observations of the long-term geomagnetic field changes, called the secular variation, are one
of the few means to estimate the material flows at the core/mantle boundary (Le Huy et al.,
2000). In particular, the radial field component and its temporal variation are of particular
interest for this purpose. Recent studies have identified distinct patches of reversed magnetic
flux at the poles and below Africa which can account for about 90% of the present day field
decrease (Hulot et al., 2002). The most prominent feature in this respect is the growing patch
of reverse magnetic polarity beneath South Africa. To give an indication of the recent
changes, Figure 5 shows the distribution and evolution of the radial magnetic field component
at the core/mantle boundary during the past century. The model used here, Jackson et al.
(2000), shows a region of declining radial field component (blue area) which propagates
north-eastward. At present this patch is just below South Africa. By closely monitoring the
magnetic field changes in the southern part of Africa, we have the opportunity to track this
patch and record its evolution. An objective is to use dedicated measurements for constructing
detailed models of the core flow in this region, which seems to play a key role in the presently
observed magnetic field decline. Only through a better understanding of the dynamic
processes taking place in the Earth’s liquid core, do we stand a chance of improving our
ability to predict changes of the geomagnetic field.
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Figure 5: Secular variation of the radial magnetic field component at the core/mantle boundary for the epochs 1900, 1930
(top) and 1975 and 1990 (bottom) using the model of Jackson et al. (2000). Blue colour marks areas with
diminishing field intensity. Extreme values are ±13 μT/a.
Deepening of the South Atlantic Anomaly
Comparisons of Magsat and CHAMP satellite data indicate that the relative decrease of the
geomagnetic field is particularly strong in the Atlantic and American sector (cf. Fig. 5). This
coincides partly with the South Atlantic Anomaly (SAA), a vaguely-defined, oval-shaped
geographic region centred to the east of Brazil, where the geomagnetic field is significantly
weaker than over the rest of the Earth at equivalent altitudes. It is in this region that the
shielding effect of the magnetic field is severely reduced, thus allowing high energy particles
of the hard radiation belt to penetrate deep into the upper atmosphere to altitudes below
100 km. The hazards of this region appear to lack recognition outside the space science
community, although most spacecraft crossing this region at altitudes below 1000 km have
experienced damage or degradation to some extent [Heitzler et al., 2002].
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Figure 6: Contours of constant magnetic field intensity at 1338 km altitude. Indicated are sites of spacecraft anomalies,
TOPEX (yellow dots) and recent MODIS failure (star) (by courtesy of J.R. Heirtzler, NASA/GSFC).
The continuing, observed weakening of the magnetic field will result in the expansion of the
hazardous SAA region as well as a shift towards southern Africa. The reliable determination
and evaluation of the radiation hazard in this region is of utmost importance to space agencies
and other institutions operating satellites in low-Earth orbit, particularly for the assessment of
radiation risk to humans in space. The planning of satellite missions and the development of
space-borne experiments and infrastructure call for predictions of space environment
conditions about a decade in advance. Estimates of field changes have to be based on the
interpretation of processes taking place in the Earth’s outer core; consequently, results
obtained in the previous section will be required to predict the evolution of the SAA.
Scientific Goals and Societal Benefits
Better Quality Geomagnetic Field Models
One of the immediate practical benefits of this project is that it will enable the HMO to
develop better quality geomagnetic field models for the southern African region (i.e. South
Africa, Namibia, Botswana, Mozambique, and Zimbabwe). This will be of benefit to a
number of non-research organisations and individuals. For example, the Chief Directorate
Surveys and Mapping provides geomagnetic declination information on all maps which they
publish and distribute. The Directorate for Geospatial Information of the South African
National Defence Force and the South African Civil Aviation Authority utilise geomagnetic
declination information in the maintenance of air traffic safety. In addition, the data will be
incorporated into the construction of an International Geomagnetic Reference Field (IGRF)
1.2 Subproject
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model [e.g., Mandea and Macmillan, 2000] generated under the auspices of the International
Association of Geomagnetism and Aeronomy (IAGA). This study is expected to provide
improved projections of the evolution of the magnetic field five years ahead, information
which is incorporated in the IGRF model.
Verification of Satellite Derived Geomagnetic Field Models by Ground Observations
The high quality data from CHAMP have enabled GFZ to generate geomagnetic main field
and secular variation models of much higher resolution than were previously possible. The
extension of the models to shorter wavelength (about 500 km) requires for its verification a
spatial density of ground-based observations which is not supported by the present network of
magnetic observatories. This proposed project will enable a detailed verification of satellite
derived geomagnetic field models from ground observations in this interesting region, besides
the European and north American continents. The continuous measurements will furthermore
serve as a check of the external magnetic field corrections used in the field separation
procedures.
Scientific Capacity Building and Training
The technical implementation of the project and much of the research will be carried out in
South Africa; however, South African technical staff responsible for data processing have
very little experience of satellite data processing. The GFZ, on the other hand, has developed
significant knowledge and expertise in the management and processing of geomagnetic field
data from the CHAMP satellite mission. Inexperienced HMO technical staff will be sent to
GFZ for training and to gain experience in the processing of satellite data. It is also clear
from the scientific motivation that this proposal will lead to numerous research opportunities
and interest by the international research community. Research associated with the project
that is carried out in South Africa will thus be used for the training of students.
Improved understanding and monitoring of the SAA radiation hazard
The deepening SAA could become a major radiation hazard in the future, especially for low
Earth orbiting (LEO) satellites. This project will facilitate contributions to the prediction of
the radiation hazard for satellites in operation and for spacecraft crews.
Implementation and Work Schedule
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Establish an Additional INTERMAGNET Quality Geomagnetic Observatory
Two of the three geomagnetic observing stations which the HMO currently operates in
southern Africa, namely Hermanus (HER) and Hartebeesthoek (HBK), are INTERMAGNET
quality stations. However, the third station, namely Tsumeb (TSU) in northern Namibia, does
not qualify as an INTERMAGNET quality station. The prime reasons for this are the poor
communications link from Tsumeb and the unavailability of dedicated and well trained
technical staff.
When secular variation field surveys are carried out, the data from the nearest observatory are
used to correct for disturbance effects from external sources and to refer the field station data
to a common epoch. It is proposed to establish an additional INTERMAGNET-grade
observatory between Hermanus and Tsumeb for the following reasons:

If it is assumed that an observatory within a distance of 600 km from a secular variation field
station provides an acceptable reference for these corrections [as indicated by the circles drawn
around the observatories in Figure 1], then it is seen that a large area in the Northern Cape and
southern Namibia is not adequately covered. Sufficient coverage could be achieved by
locating an observatory somewhere in the Northern Cape or southern Namibia.

The declination secular variations at the south-eastern and north-western extremes of southern
Africa are currently 10 arc minutes per annum westward and 10 arc minutes per annum
eastward respectively. An observatory located between HER and TSU would help to
accurately monitor the spatial change of secular variation across southern Africa.

A site that appears to be suitable for locating an observatory has been identified in southern
Namibia. It is located at Gamsberg [23o 16’ S, 16o 30’ E], the site of the HESS gamma ray
telescope. It would cover most of the presently uncovered region and is located near the
southern end of the region covered by Tsumeb. It is probably geologically suitable, but may
suffer induction effects from the Damara conductive belt to the north. There are likely to be
excellent infrastructure and technical assistance available.
Increase HMO’s Network of Secular Variation Field Stations
The HMO currently has a network of 10 secular variation field stations distributed over
southern Africa (4 in South Africa, 3 in Namibia, 1 in Botswana, and 2 in Zimbabwe) which
are visited once per year [indicated by the blue dots in Figure 1]. Recent experience has
shown that this number of stations is insufficient to accurately model the secular variation due
to the increasing temporal and spatial gradients. It is proposed that the number of secular
variation field stations be increased to about 40 and that half of these be visited in alternate
years. The present international quality standards [Newitt et al., 1996] as practised during the
southern African repeat station field surveys will also incorporate methods and techniques as
described by Korte and Fredow (2001).
Collaborative Studies of CHAMP Satellite and Ground Based Data
A number of collaborative studies utilizing CHAMP satellite and ground based data are
proposed. These studies will also provide suitable research topics for MSc, PhD, and postdoctoral candidates. Some of the potential research topics are:
1.2 Subproject




44
Geomagnetic field modelling, particularly the comparison of satellite-derived and groundbased tested secular variation models
Studies of the rapid dipole field decrease and rapidly changing geomagnetic declination
Modelling of magnetic fields at the core-mantle interface
Studies of the deepening South Atlantic anomaly
Training of South African HMO Data Processing Staff at GFZ
It is proposed that exchange visits of young staff members be made between GFZ and HMO.
For example, visits by German post-doctoral candidates will assist with the training of
graduate students and visits by South Africans will enable them to gain experience. Technical
staff appointed for the management and processing of data from the magnetometer
experiment on board the SASciSat satellite mission will be sent to GFZ for training.
Estimated Funding Requirements
The project should start as soon as possible, since all the preparations are completed and
because we want to take advantage of an overlap with the CHAMP mission, as far as possible.
The kick-off should be latest at the beginning of the year 2004.
As the duration of the project a five years period is foreseen. Such a time span is regarded as
a minimum for the detection of non-linear magnetic variations coming from the Earth core.
Furthermore, this period fits quite well the active life-time of CHAMP, which is expected to
re-enter by the end of 2007.
It is proposed to establish an additional INTERMAGNET-grade observatory between
Hermanus and Tsumeb at the site of the Gamsberg HESS project due to the excellent
infrastructure and technical assistance available. For this purpose a few non-magnetic
buildings will have to be erected to accommodate the instruments for continuous recording as
well as absolute baseline measurements that have to be performed on a weekly basis.
It is envisaged that the buildings can be constructed within 2 months and once in operation, it
is advisable that the project continues for at least 5 years to obtain good secular variation
results. A technician on site (employed by the HESS project) will be paid on a contract basis
to perform the necessary absolute measurements and do simple maintenance. A technician
from Hermanus will be sent to Gamsberg to do control absolute observations and for major
maintenance tasks.
In addition, it is proposed that the HMO’s present number of secular variation field stations be
increased to about 40 and that half of these be visited in alternate years. The repeat station
services will be adapted to international quality standards such as described by Korte and
Fredow (2001). This will mean extensive travelling by HMO staff members to perform these
field survey measurements, requiring additional funds to cover these expenses. It is advisable
that the project should run for at least 5 years in order to gather sufficient secular variation
measurements.
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Breakdown of funding requirements:
Infrastructure and Instruments (first year)
Building costs at Gamsberg site
Fluxgate vector magnetometer (Model FGE)
Overhauser scalar magnetometer
D/I absolute magnetometer
Data logger (HMO standard)
LEMI fluxgate and logger (field work)
Sub-Total
Estimated Cost
EUR 22 500
EUR 16 300
EUR 12 500
EUR 34 400
EUR 3 800
EUR 10 000
EUR 99 500
Annual Running Costs
Contract with HESS for technician time
Visits to Gamsberg
Extended field survey
Sub-Total
Total
EUR 3 100
EUR 2 500
EUR 2 500
EUR 8 100 p.a.
EUR 107 600
Manpower for research activity
For the scientific evaluation of the measurements and for the magnetic field modelling the
personnel listed below is needed in addition to the available staff members.
At HMO: Evaluation of the repeat station measurements, regional field modelling.
First year
1 MSc student
EUR 5 000
2nd through 5th year
1 MSc student
1 PhD Student
EUR 5 000 p.a.
EUR 17 000 p.a.
At GFZ: Global magnetic field modelling, study of the flow at the core/mantle boundary.
First year
1 PhD student
EUR 20 000
2nd through 5th year
1 PhD student
1 Post Doc
EUR 20 000 p.a.
EUR 40 000 p.a.
Travel costs
There is a close cooperation foreseen between the two institutions. Therefore a trip in each
direction every year is needed. The dwell time in the partner institution will be one to two
weeks. In the first year the travels will be primarily in support of setting up the observatory
and initiating the field survey campaign.
It is foreseen that the scientists employed are coming preferably from South Africa. Each of
them should have at least once during his/her contract period the chance to visit the partner
institution for an exchange of information and a presentation of results. In addition an
exchange of staff members is envisaged for the training of satellite data evaluation.
2 return trips Cape Town – Berlin plus 7 days per diem
EUR 3 600 p.a.
Total cost travel
EUR 18 000
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Collaboration with Other Inkaba Activities
The Earth’s main magnetic field is generated by dynamo action in the outer molten core.
Long-term variations of the geomagnetic field are related to material flows and energy
exchange at the core/mantle boundary. These long-term variations of the geomagnetic field,
particularly the present rapid decrease below South Africa, are expected to be related to a
number of other geophysical processes, such as the existence of a low-velocity seismic
anomaly of global proportion beneath the southern African plate and elevated lower-mantle
temperatures; however, the interconnections between these various phenomena are not
presently understood. In order to gain an understanding of these interconnections there will
need to be collaborative studies utilizing the results arising from this sub-project and:
 SADC Earth and Ocean Monitoring Network
 Epeirogenesis, uplift and erosion
 Mantle and core modelling
 BOSA - Breakup of the South Atlantic from prerift to drift
 Magnetotelluric Survey across the Beatty Anomaly and the southern Cape Conductivity
Belt
Related International Collaborations
The HMO’s collaborations with Africa have traditionally been with the Southern African
Development Community (SADC) countries. The geomagnetic recording station at Tsumeb,
Namibia, is operated in collaboration with the Geological Survey of Namibia and forms part
of the HMO’s observatory network. Observations for input to geomagnetic field models are
made annually at secular variation field stations in Namibia, Botswana, and Zimbabwe with
the assistance of organisations in the respective countries.
The HMO currently participates in a number of formal and informal research collaborations,
which will have a bearing on the work of this sub-project:
 The GeoForschungsZentrum, Potsdam, Germany, on studies of geomagnetic field
variations utilising CHAMP satellite data and the HMO’s ground based data.
 As an INTERMAGNET (INTERnational MAGnetic observatory NETwork) accredited
observatory, the HMO operates instruments, carries out observations, and processes data
in order to produce geomagnetic field data in accordance with INTERMAGNET
standards. The data are distributed to the INTERMAGNET Geomagnetic Information
Nodes (GIN) on a daily basis where data quality is monitored.
 The World Data Centre in Kyoto, Japan, with regard to the computation of the Dst
magnetic storm index.
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Existing Infrastructure
Presently, the HMO operates three geomagnetic observatories in southern Africa. A map showing
the location of these observatories is presented in Figure 7, while Table 1 lists their coordinates.
Figure 7: Map of southern Africa showing the locations of magnetic observatories.
Figure 8: A map showing the various field survey stations (indicated by●) for southern Africa as well as the 3 continuous recording
observatories at Hermanus, Hartebeesthoek and Tsumeb (indicated by ■)
■ Prior to 2000 the Hermanus Magnetic Observatory conducted field surveys at approximately 70 repeat stations located in South Africa, Namibia,
Botswana and Zimbabwe, as shown in Figure 8, at 5 yearly intervals. Since 2000, only 10 stations have been occupied on an annual basis. In order to
expand the current 10 stations to a possible 30 or 40 would therefore require a re-occupation of existing stations used prior to 2000. In some cases,
however, it will be necessary to relocate stations due to urban or industrial expansions, thus observation pillars will have to be re-erected.
1.2 Subproject
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Table 1: Coordinates of geomagnetic observatories operated by the HMO
Observatory
Code
Hermanus
HER
Hartebeesthoek HBK
Tsumeb
TSU
Geographic Coordinates
Geomagnetic Coordinates
Latitude
Longitude
Latitude
Longitude
34.42 S
25.88 S
19.20 S
19.23 E
27.70 E
17.58 E
-42.60
-36.29
-31.07
82.91
95.36
86.86
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