Zweijahresbericht 2004/2005 - Bibliothek - GFZ

72
between Earth’s outer core and mantle in order to gain a
better understanding of the geodynamo process. Some
more detail on these results is given in the part of Section
2.3 in this report.
Geomagnetic jerks
Over a very short time span, a number of abrupt changes
in the secular variation have been noted in the series of
magnetic observatories. The cause of these so-called geomagnetic
jerks is not completely known, but they may
reflect the reconfiguration of hydromagnetic motions in
the outer core over small scales and short time-intervals.
These phenomena are difficult to study, because of their
small amplitudes and the overlap of their frequency range
with the effect of solar-dependent external variations.
Moreover, the highly uneven coverage of the globe by
magnetic observatories makes it difficult to study their
geometry and evolution, and whether they are of a global
nature.
One way to overcome the problem of the uneven spatial
distribution of time series is again to turn to satellite data.
A good global coverage is obtained from satellite data in
a short period of time, but satellite data are not very hel-
Fig.9:Secular variation of the radial magnetic field at the core mantle boundary
for epochs 1900, 1930, 1975 and 1990 using the Jackson et al. (2000)
model. The extreme values (red/blue) are +/– 13 µT/yr.
Säkularvariation des radialen Magnetfelds an der Kern-Mantel-Grenze für
die Jahre 1900, 1930, 1975 und 1990 nach dem Modell von Jackson et al.
(2000). Die Extremwerte (rot/blau) sind +/– 13 µT/a.
pful if our interest is focused on a certain location over a
longer period of time. Fig. 8 shows, as an example, all
available CHAMP data over an area of 1° x 1° centered
on the Niemegk Observatory. It is clear that the temporal
resolution, even for a larger area at a fixed position, is not
comparable with what observatories provide as continuous
data. However, this time series can be used for interpolating
the temporal behaviour of the magnetic field. This
kind of plot will be a useful first step in studying secular
variation, and possibly geomagnetic jerks, at a given position
from satellite data.
However, only the joint analysis of observatory and satellite
data can really be useful for the global study of geomagnetic
jerks. To circumvent the spatial and temporal
distribution difficulties, the use of continuous field models
derived from ground-based and satellite data, such as the
Comprehensive Model by Sabaka et al. (2002, 2004), is
one possible solution. Chambodut and Mandea (2005) studied
the temporal and spatial distribution of jerks detected
in these models over the four last decades. The jerks
around 1971, 1980 and 1991 are characterized by a clear
bimodal behaviour of their occurrence date. So far, no
geomagnetic jerk occured during the lifetime of the magnetic
field satellites. A much better description of a jerk
could be provided if it were to occur
during the time of operation of one or preferably
several satellites.
South Atlantic Anomaly
Another interesting feature of the core
field is the so-called South Atlantic
Anomaly. This is a large area of very low
field intensity (less than 20000 nT) over
South America, the southern Atlantic and
southern Africa. Moreover, from MAG-
SAT and CHAMP data we observed that
the field there has been decreasing by
some 8 % during the past 20 years (see
Fig. 7). Recent studies have identified distinct
patches of reversed magnetic flux at
the poles and below Africa which could
be related to the present day field decrease
and might even be a hint that the geodynamo
is heading towards a reversal
(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
recent changes, Fig. 9 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 reversed field direction
(red area) which propagates north-eastward.
At present this patch is just below
South Africa. Moreover, a large longitudinal
difference in field changes is observed,
again with a maximum variation in
Zweijahresbericht 2004/2005 GeoForschungsZentrum Potsdam