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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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