# HOW HAVE ØRSTED, CHAMP, AND SAC-C IMPROVED OUR KNOWLEDGE OF THE OCEANIC REGIONS?

**M. Purucker ^1^, T. Sabaka ^1^ , N. Olsen ^2^, and S. Maus ^3^**

## ^1^ Raytheon ITSS at Geodynamics Branch

## Goddard Space Flight Center, Greenbelt, MD 20771, USA,

## Email: <purucker@geomag.gsfc.nasa.gov>, sabaka@geomag.gsfc.nasa.gov

## ^2^ Danish Space Research Institute, Copenhagen, Denmark, Email: nio@dsri.dk

## ^3^ GeoForschungsZentrum, Potsdam, Germany, Email: smaus@gfz-potsdam.de

ABSTRACT

This talk presents an overview of the capabilities and accomplishments
of the current generation of magnetic field satellites, especially with
respect to improvements in the oceanic regions. Two types of
improvements are discussed: 1) in our ability to continuously monitor
secular variation in the oceanic regions, and 2) in the characterization
of the anomalous vector field associated with lithospheric
magnetization. Improvements seen in the lithospheric field include both
the ability to resolve subtle features, like the enhanced magnetization
associated with spreading zones, and the ability to improve our
knowledge of isochron location in poorly surveyed parts of the ocean.

INTRODUCTION

In this brief survey, we'd like to provide an overview of the following
topics: 1) secular variation over the North Atlantic Ocean from 1980 to
the present, 2) oceanic vs. continental magnetic fields, and 3)
observations vs. predictions for the North Atlantic and Pacific Oceans.

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SECULAR VARIATION

The earth\'s magnetic field is constantly changing. On the continents we
monitor this change with magnetic observatories. This is currently
almost impossible to do on the oceans (although sea-bottom observatories
may change that) and so satellites are our best source of secular
variation information over the oceans. Prior to Ørsted, CHAMP, and SAC-C
our knowledge of secular variation of the vector magnetic field from
satellites was limited to the Magsat mission, from which it was
difficult to extract secular variation because of its short, 6-month,
lifetime. So we've entered a new age of being able to continuously
monitor global secular variation, our only direct measure of motion in
the fluid outer core, even over the oceans. As an example of our
capability, we show, in Figure 1 (upper left), the accumulated secular
variation in the total field over the western Atlantic Ocean between the
Magsat mission and the early days of Ørsted. Notice that the decrease in
this region amounts to some 5% of the total field, a significant figure.
We can compare that secular variation with that measured from Ørsted
between 2000.0 and 2001.0 and based on a co-estimated main field and
secular variation model, using a color scale that is scaled to the color
scale for the 1980 to 2000 time frame (Figure 1, lower left). We can see
that the area of maximum field decrease is reduced in size. Finally, we
can utilize actual satellite profiles (their locations are shown in red
on the lower left in Figure 1) from the SAC-C satellite to show the
continuation of that secular decrease for a 13 month period beginning in
February of 2001. The observations here have been selected from
magnetically quiet times (the Dst index, a measure of the ring current,
is between 0 and +10 nT) at 22:00 LT and reduced using a main field for
2001.0 Advances in the characterization of secular variation are
continuing, and two of the goals are to characterize short term
phenomena such as oscillations of the core and geomagnetic jerks.

SATELLITE OBSERVATIONS REDUCED TO MODEL

In order to evaluate how Ørsted, CHAMP, and SAC-C have improved our
knowledge of the oceanic lithosphere, we compare what we consider to be
the best model made from earlier data and techniques (the M102389 model
based on Magsat, shown in the left hand column of Table 1) with three
models produced (or published) this year (shown in the right hand
columns of Table 1). We will subsequently refer to these in shorthand as
the observations, in order to contrast them with predictive lithospheric
models based on non-satellite data, to be discussed next. The properties
of these observations are summarized in Table 1.

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PREDICTIVE LITHOSPHERIC MODELS

The Ørsted era has seen the development of predictive models (Table 2)
of induced and remanent magnetization in the oceanic and continental
lithosphere. These models capture the longest wavelength features of the
Cretaceous Quiet Zones, and the continent-ocean magnetization contrast
that overlaps with the core field. Some of these fields are unresolvable
but they have important consequences for interpretation. For example,
Purucker et al. (2002) have recently shown that the large, total-field
anomalies centered over Kentucky, USA, and the south-central USA could
be the manifestations of the magnetic edges of the southern boundaries
of cratonic North America. On other continents, where our knowledge of
the deep lithosphere is not as complete, these models still need
improvement but at the same time offer the promise of a better
understanding of the lithosphere if we can deconvolve the observations.
A similar pattern holds over the oceans, as we will see next.

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We illustrate in Figure 2 how the model is constructed using an example
from the North Atlantic Ocean. We begin in the lower left (Figure 2)
with the induced model, which is based on the volume of the magnetic
crust and its magnetic susceptibility. We show the lithospheric field in
the direction of the main field (total field anomaly). The negative
total field values in the oceanic region flanking Greenland reflect the
contrast in magnetic crustal thickness between Greenland and the
surrounding oceans. Notice the presence of very long wavelengths,
including those not observable because of overlap with the core field.
Those unresolvable wavelengths will be removed in the final step. The
remanent model is shown in the lower right (Figure 2) and is dominated
by the long-wavelength content of the Cretaceous Quiet Zones, and by
new, relatively magnetic crust, in the Davis strait separating Greenland
and Canada. Notice that the field in the northern Davis strait is
opposite to that predicted by the induced model, and hence there will be
some cancellation of fields. The sum of the induced and remanent model
is shown in the upper left (Figure 2). Finally, we restrict the spatial
content of the model to those that can actually be observed by the
satellites, from degrees 15 through 80 or so (Figure 2, upper right).

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OCEANIC VS. CONTINENTAL MAGNETIC FIELDS

The oceanic and continental crust differ in many ways. As a consequence
we might expect there to be a difference in magnetic signature. For
example, they typically differ by a factor of 5 in their thickness.
Because what is sensed at satellite altitude is the product of the
magnetization or susceptibility times the magnetic thickness, a factor
of 5 difference in thickness will translate to a factor of 5 difference
in magnetic field intensity for comparable magnetization or
susceptibilities. Partially counter-balancing the increased thickness of
the continents is the strength of magnetization in the oceans, for
example within the extrusive basaltic layer, especially near the ridge
axis.

In this representation (Figure 3) of the latest observations from Ørsted
and CHAMP, we have masked the oceans in the top figure in order to
highlight the continental magnetizations. Note that the continents
contain the largest (Kursk and Bangui) anomalies. In the lower figure,
we have masked the continents and show the oceanic magnetic fields,
again from the Ørsted and CHAMP observations. Notice that some of the
strongest anomalies are near the continental edges (for example in the
Gulf of Mexico, south of Australia, and southeast of Australia). These
have been interpreted as part of the signature of the continent-ocean
contrast (Gulf of Mexico and south of Australia) or of submerged
micro-continents (Lord Howe Rise). Notice also that much of the Pacific
Ocean is characterized by weak magnetic fields.

The map (not shown) made using the earlier Magsat observations is
similar in both the strength and location of individual continental
anomalies. However, there is a significantly increased magnetic field
over the Pacific, and Indian Ocean regions. We interpret this feature to
result from improvements in modeling the low- and mid-latitude external
fields, and not to any secular change in the lithospheric field over the
oceans.

NORTH ATLANTIC OBSERVATIONS AND PREDICTIONS

The magnetic anomaly field seen by CHAMP, through degree 80, is shown in
Figure 4. We have masked the continents, and show only the oceanic
magnetic fields. Oceanic spreading isochrons are shown with red lines,
and fracture zones with black lines. As we'll see, the North Atlantic
magnetic signatures are dominated by the Cretaceous Quiet Zone and the
continent-ocean contrast.

Shown in Figure 5 are all four sets of satellite observations, with the
oldest, and lowest resolution views of Magsat in the lower right. Ørsted
and CHAMP data go into making the map at the lower left, CHAMP data
alone go into making the map at the upper right, and POGO, Magsat,
Ørsted, and CHAMP go into the map at the upper left. Notice that, with
the exception of the anomaly marked 'A' on the Magsat map, the anomalies
are very similar on all four maps. In the case of 'A', the positive
anomaly is much reduced on the later Ørsted and CHAMP maps, and the
region to the south of 'A' is dominated by a prong of positive magnetic
field in the Ørsted and CHAMP maps, and by a region of negative magnetic
field on the Magsat map.

A comparison of the earlier Magsat observations with the prediction made
from the global induced plus remanent model shows that the predictive
model agrees more closely with the later Ørsted and CHAMP observations
in the vicinity of 'A'. It also allows us a better understanding of the
difference, which is related to a failure to isolate the signature from
the more intensely magnetized mid-ocean ridge in this region. A more
intensely magnetized mid-ocean ridge is predicted from most in-situ
observations but it is a subtle feature and often difficult to observe
in the satellite maps.

While the magnetic signature of the mid-ocean ridge is also observed in
the Ørsted and CHAMP observations shown in Figure 5, it is not as large
as predicted. This is seen in other regions, and in fact some ridges,
like the East Pacific rise, still have no recognizable magnetic
signature from satellite.

EQUATORIAL PACIFIC OBSERVATIONS AND PREDICTIONS

In Figure 6, the predicted field in the equatorial Pacific is shown in
the upper right, along with the separate induced and remanent components
that go into its construction. Notice that we are showing the radial
field now instead of the total field which we showed on the previous
maps of the North Atlantic. This is because the radial field is
especially good at resolving features near the magnetic equator. Notice
especially the predicted boundaries of the Cretaceous Quiet Zone in the
western Pacific. They are shown in Figure 7b (an expanded version of the
upper right image in Figure 6) as anomalies B, C, and D and represent
only the observable wavelength range of the much larger feature seen in
red in the images on the left hand side of Figure 6. It is much more
difficult to observe the radial anomaly field than the total field,
especially at the equator, and Ørsted and CHAMP have made significant
improvements in this respect over Magsat.

![](media/image5.jpeg){width="5.0in" height="4.4in"}In the observations
(not shown), the northernmost 'B' anomaly is resolved in the older
Magsat map, although the more southerly 'C' and 'D' anomalies are not.
In contrast, they are resolved in all three of the current maps.

Looking now at the expected vs observed location of these features, the
edges of the Cretaceous Quiet Zone are systematically east of their
predicted locations on the Ørsted and CHAMP map, with the largest offset
at 'B' and the smallest at 'C'. This pattern is also found on the other
recent maps, and is also true of the 'B' anomaly observed on the Magsat
map. Figures 7a and b show the observed and predicted maps,
respectively, and a comparison of them provides a feel of the offset.
Remember, the black lines on the map are the fracture zones and the red
lines are the seafloor spreading isochrons. In the case of 'B', the
offset amounts to some 700 km in this poorly mapped region of the
Pacific. The next step is to adjust, probably with several iterations,
the locations of the isochrons in the underlying age map and follow the
resulting changes. Additionally it will be important to better
understand the constraints from the sparse marine surveys (NGDC, 1999)
in the vicinity of the Galapagos, Marquesas, and Austral (from north to
south) fracture zones. Finally, we need to explore the long-wavelength
cutoff to ensure that we have not retained any signals of core origin.
This may be especially important in regions where the lithospheric
signal is weak, as in the oceans.

SUMMARY

We draw the following conclusions from an examination of Ørsted, CHAMP
and SAC-C data and supporting models: 1) satellites can now confidently
track secular variation, 2) the magnetic field amplitudes from our best
models still vary significantly, but based on model improvements, we
seem to be converging, 3) the first global models capable of predicting
induced and remanent magnetic fields over the continents and oceans now
exist. 4) the North Atlantic Ocean is well-characterized by the model of
induced and remanent magnetization. Our best observations reduced to a
model now resolve the mid-ocean ridge between 32 and 40 degrees North,
and 5) in the Pacific Ocean there is tentative, tantalizing evidence to
provide the first ever identification of the southeast edge of the
Cretaceous Quiet Zone, and to suggest modifications to that poorly
determined boundary.

ACKNOWLEDGEMENT

This paper is dedicated to the memory of Robert (Bob) Langel and Stephen
Zatman. Bob was the Magsat project scientist and had subsequently worked
closely with our Danish and German colleagues in the planning of Oersted
and CHAMP. Stephen had planned to attend this meeting, and to discuss
his latest insights into torsional oscillations of the fluid core using
observations such as these. He had been a post-doctoral fellow in our
research group and had just begun a profesorial appointment in
geophysics at Washington University when he was killed in an auto crash.
He will be remembered for his outgoing ways and wit.

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