Maps of the vertical downward component (sign-changed radial
component) of the core magnetic field (A) and its secular variation (B) as
estimated at the CMB from the GRIMM-3 model up to spherical harmonic degree 13.
Credit: GFZ/Credit: Mandea et al. www.pnas.org/cgi/doi/10.1073/pnas.1207346109
The main field of the Earth’s magnetic field is generated by flows of liquid
iron in the outer core. The Earth’s magnetic field protects us from cosmic
radiation particles. Therefore, understanding the processes in the outer core
is important to understand the terrestrial shield.
Credit: GFZ/Credit: Mandea et al. www.pnas.org/cgi/doi/10.1073/pnas.1207346109
Key to this are measurements of the geomagnetic field
itself. A second, independent access could be represented by the measurement of
minute changes in gravity caused by the fact that the flow in the liquid
Earth’s core is associated with mass displacements. The research group has now
succeeded to provide the first evidence of such a connection of fluctuations in
the Earth’s gravity and magnetic field.
Credit: GFZ/Credit: Mandea et al. www.pnas.org/cgi/doi/10.1073/pnas.1207346109
Evolution of the secular acceleration of the vertical
downward component of the geomagnetic field computed at the Earth’s surface (as
obtained from GRIMM-3 model), which evolves smoothly over the last decade.
This animation shows a patch of acceleration migrating from India around 2003
to the southwest of the Indian Ocean in 2008, while a strong patch of
deceleration grows in the middle Atlantic to reach a maximum in 2006 and then
vanishes rapidly.
Credit: GFZ/Credit: Mandea et al. www.pnas.org/cgi/doi/10.1073/pnas.1207346109
They used magnetic field measurements of the GFZ-satellite
CHAMP and extremely accurate measurements of the Earth’s gravity field derived
from the GRACE mission, which is also under the auspices of the GFZ. “The main
problem was the separation of the individual components of the gravity data
from the total signal,” explains Vincent Lesur from the GFZ German Research
Centre for Geosciences, who is involved in the study.
Computer simulation of the Earth‘s field in a normal period
between reversals. The tubes represent magnetic field lines, blue when the
field points towards the center and yellow when away. The rotation axis of the
Earth is centered and vertical. The dense clusters of lines are within the
Earth’s core
A satellite only measures the total gravity, which consists
of the mass fractions of Earth’s body, water and ice on the ground and in the
air. To determine the mass redistribution by flows in the outer core, the thus
attained share of the total gravity needs to be filtered out. “Similarly, in
order to capture the smaller changes in the outer core, the proportion of the
magnetic crust and the proportion of the ionosphere and magnetosphere need to
be filtered out from the total magnetic field signal measured by the
satellite,” Vincent Lesur explains. The data records of the GFZ-satellite
missions CHAMP and GRACE enabled this for the first time.
Cutaway of the Earth showing the layers to the core
Credit: Wikipedia
During the investigation, the team focused on an area between the Atlantic and the Indian Ocean, as the determined currents flows were the highest here. Extremely fast changes (so-called magnetic jerks) were observed in the year 2007 at the Earth’s surface. These are an indication for sudden changes of liquid flows in the upper outer core and are important for understanding the magneto-hydrodynamics in the Earth’s core. Using the satellite data, a clear signal of gravity data from the Earth’s core could be received for the first time.
Credit: Wikipedia
During the investigation, the team focused on an area between the Atlantic and the Indian Ocean, as the determined currents flows were the highest here. Extremely fast changes (so-called magnetic jerks) were observed in the year 2007 at the Earth’s surface. These are an indication for sudden changes of liquid flows in the upper outer core and are important for understanding the magneto-hydrodynamics in the Earth’s core. Using the satellite data, a clear signal of gravity data from the Earth’s core could be received for the first time.
Fig. S2. Spatial and temporal scales of the physical
processes causing mass variations in the Earth system. The searched-for core
signals, the magnetic jerks, are shown in yellow (adapted from ref. 15).
This results in consequences for the existing conceptual
models. Until now, for example, it was assumed that the differences in the
density of the molten iron in the earth’s core are not large enough to generate
a measurable signal in the earth’s gravitational field. The newly determined
mass flows in the upper outer core allow a new approach to Earth’s core
hydrodynamics.
Original article:
“Recent changes of the Earth’s core derived from satellite
observations of magnetic and gravity fields”, Mioara Mandea, Isabelle Panet, Vincent
Lesur, Olivier de Viron, Michel Diament, and Jean-Louis Le Mouël, PNAS 2012;
doi:10.1073/pnas.1207346109
