Magnetic dynamos in the lab

www.physicstoday.org All astrophysical plasmas are, as far as we know, magnetized and turbulent. They range in size, density, and temperature from relatively small, dense stellar plasmas to enormous, diffuse plasmas in clusters of galaxies. The magnetic fields and the turbulence play important roles in issues as diverse as cosmological structures formation and the origin of cosmic rays. Closer to home, Earth also has a magnetic field, as do most but not all of the other planets. The geomagnetic field makes our planet more hospitable by shielding us from the Sun’s charged-particle wind. We have only a partial understanding of how the turbulent flow of liquid iron in Earth’s outer core generates the geomagnetic field. (Despite being hotter, the inner iron core is solid because it’s under greater pressure.) That level of understanding does not yet make predictive forecasting possible, which is mildly alarming because the geomagnetic field has fallen monotonically throughout recorded history—by approximately 10% since 1838, when Carl Friedrich Gauss published his pioneering global models. We may be headed for a magnetic reversal. The geomagnetic field has reversed many hundreds of times over geological history; the last one happened 780 000 years ago. We can’t be sure about changes in solar radiation during a reversal, when the field is bound to be weak. The origins and dynamics of the dynamo-generated magnetic fields of Earth, the Sun, the gas-giant planets, and nearly every massive astrophysical object are almost certainly controlled by complex turbulent flows of plasmas or conducting liquids. The quest to understand the dynamo process comprises theoretical, computational, and experimental undertakings. Here are some of the outstanding questions: ‣ Why do some planets and stars have strong surface magnetic fields and others do not, and what sets those strengths? ‣ How do stars and galaxies develop large-scale magnetic fields? ‣ What is the role of rotation in the dynamo process? ‣ What determines time-varying behaviors such as reversals of Earth’s field and oscillations of the Sun’s field? ‣ What are the roles of dynamos and magnetic fields in protoplanetary, protostellar, and accretion disks? The issues span many disciplines. Geophysics, astrophysics, plasma physics, and planetary science all claim the dynamo problem as their own. Why, beyond scientific curiosity, do we want a predictive dynamo science? Because we worry about solar storms and the Sun’s role in global climate change. And on a longer time scale, there’s the declining geomagnetic field. From the perspective of two experimentalists, this article attempts to describe the current experimental, theoretical, and computational state of the field. Our view is that true understanding pivots on having a theory that yields predictions testable in the laboratory and extrapolatable to geophysical and astrophysical settings.