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The
problem of explaining the origin of the Earth's magnetic field
was once ranked by Albert Einstein as among the three most important
unsolved problems in physics. Although today it is widely recognized
that the Earth's magnetic field is generated by a dynamo that
operates in the fluid outer core, the details of how that dynamo
works remain far from understood.
Over
the past 150 years, the main (axial dipole) component of the Earth's
magnetic field has decayed by nearly 10%, a rate ten times faster
than if the dynamo were simply switched off. To that extent, the
dynamo today is operating more as an anti-dynamo, a destroyer of
the dipole part of the field. Intriguingly, this decay rate is characteristic
of magnetic reversals, which paleomagnetic observations have shown
occur on average, though with great variability, about once every
half million years.
Geographically,
the recent dipole decay is largely due to changes in the field beneath
the South Atlantic Ocean. This pattern is connected to the growth
of the South Atlantic Magnetic Anomaly, an area in which the field
at the Earth's surface is now about 35% weaker than would
be expected. This hole in the field has serious implications for
low-Earth-orbit satellite operations since it impacts the radiation
dosage at these altitudes. How much longer will the South Atlantic
Magnetic Anomaly continue to grow? How large will it become? Is
the field reversing? These questions currently cannot be answered
because the mechanism by which the Earth's magnetic field
is generated is only partially understood. But long-term satellite
observations combined with numerical dynamo modeling will advance
our understanding and allow us to model the evolution of this anomaly.
In
addition, satellite observations will enable mapping the three-dimensional
electrical conductivity structure of the Earth, providing important
constraints on the distribution of volatiles within the Earth. Other
important contributions include mapping the magnetization of the
Earth's crust and advancing our understanding of the core's
contribution to the Earth's angular momentum budget.
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