discovered_01_2013

discovered 01.13 FOCUS WWW.Hzdr.DE our experiment, we would like to explore parameter regions that will remain inaccessible to numerical simulations for a long time, true to Richard Feynman’s motto: "What I cannot create I do not understand." A large container is needed, one that we would have rotate about two separate axes. This way, we would set the liquid sodium it contains in motion, allowing it to flow freely - without propellers, apertures, or other built- in components. The flow of the homogenous liquid metal is supposed to set up a magnetic field by itself, similar to the way Earth’s magnetic field is generated. Liquid sodium is the type of material that rather takes some getting used to as it cannot be allowed to come into contact with air or water. However, for purposes of our geophysical and astrophysical experiments, sodium’s high conductivity paired with its low density are a real benefit. In terms of safety, we’re able to draw on our extensive experience with NATAN, our own small sodium facility, and with the large dynamo facility in Riga. It is there that, back in November 1999, the first ever successful hydromagnetic dynamo experiment was conducted - at almost the same time as the dynamo experiment at KIT, the Karlsruhe research center. In 2006, the experiment in French Cadarache followed, to which we also contributed multiple numerical simulations. All of these were important milestones in the experimental investigation of homogeneous dynamos that are responsible for the generation of planetary, stellar, and galactic magnetic fields. In recent years, successful experiments on magnetically induced flow instabilities were added. Such instabilities help explain, among other things, the high speed at which stars and black holes grow. Until now, we were only able to perform these types of experiments using harmless liquid metals that were far worse conductors, however, and now, as part of DRESDYN, we’re planning on continuing these experiments with sodium. Yet the precession dynamo is a whole nother ball game from both a mechanical and safety- related perspective. _PK . Yes, with sodium we’re looking at an entirely different order of magnitude. What’s more, questions of safety with regard to the precession dynamo are infinitely more difficult to solve since current safety methodologies simply aren’t applicable. If anything goes wrong, for instance if a flange should break, we can’t immediately drain the sodium but have to wait until the machine has come to a standstill. Just imagine - eight tons of hot, liquid, flammable sodium inside a container weighing just under 20 tons! This behemoth can be adjusted in its tilt between 90 and 45 degrees in steps of five degrees each and also rotates simultaneously about two axes - all of them are degrees of freedom for reaching the desired flow parameters. We’re looking at a gigantic machine here! What exactly is precession? Why isn’t even a single experiment concerned with it despite the fact that Earth’s precession motion is a hotly debated topic even among climate experts? _FS . This whole topic is rather complex. Changes to Earth’s orbit and angle of tilt of its axis play a crucial role in climate change. But first, let’s have a look at precession, which you can easily study yourself with the help of a small kid’s toy called a spintop: If, during spinning, this spintop is upright, it doesn’t change its own rotational axis and stays in one place despite the rotational motion. If, by contrast, you angle the spintop on a surface when you spin it, the breakdown torque will cause its rotational axis to sway - and that motion is what we call precession. If we now relate this idea to Earth, we find that Earth’s rotational axis is not perpendicular to the level of Earth’s orbit but instead is tilted roughly at a 67-degree angle. Additional forces are exerted on the Earth by the sun and the moon. Which is why, similar to a child’s spintop, it executes a precession motion with a duration of 26,000 years. This is one of the Milankovitch cycles, the others are relating to Earth’s orbit and the tilt of its axis. Both these parameters change over long time scales. It’s a well-established fact that the sequence of ice ages and interglacial periods is triggered to a large extent by such PRECESSION OF EARTH: If you allow a spintop to rotate perpendicular to a surface, the gyroscopic moment will cause the rotating axis to start to sway - this motion is called precession. In the same way, Earth's rotational axis is not perpendicular to Earth's orbit but instead is tilted at a 67 degree angle and both the sun and the moon exert forces on Earth. Which is why it undergoes precession movements similar to a spintop. Diagram: Robert Simmon (NASA GSFC)

discovered_01_2013

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