May 19, 2022


Born to play

Liquid-metal experiment provides insight into the heating mechanism of the sun’s corona — ScienceDaily

Why the Sun’s corona reaches temperatures of a number of million levels Celsius is 1 of the terrific mysteries of photo voltaic physics. A “warm” path to reveal this result sales opportunities to a location of the photo voltaic ambiance just under the corona, where seem waves and specified plasma waves vacation at the similar velocity. In an experiment applying the molten alkali metal rubidium and pulsed higher magnetic fields, a crew from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a German national lab, has produced a laboratory product and for the 1st time experimentally verified the theoretically predicted actions of these plasma waves — so-termed Alfvén waves — as the scientists report in the journal Actual physical Overview Letters.

At 15 million levels Celsius, the middle of our Sunlight is unimaginably warm. At its surface area, it emits its light at a comparatively moderate 6000 levels Celsius. “It is all the much more astonishing that temperatures of a number of million levels abruptly prevail once again in the overlying Sun’s corona,” says Dr. Frank Stefani. His crew conducts research at the HZDR Institute of Fluid Dynamics on the physics of celestial bodies — which includes our central star. For Stefani, the phenomenon of corona heating continues to be 1 of the terrific mysteries of photo voltaic physics, 1 that keeps running through his thoughts in the kind of a extremely uncomplicated concern: “Why is the pot warmer than the stove?”

That magnetic fields participate in a dominant position in heating the Sun’s corona is now widely acknowledged in photo voltaic physics. Nevertheless, it continues to be controversial irrespective of whether this result is predominantly thanks to a unexpected change in magnetic industry structures in the photo voltaic plasma or to the dampening of distinct kinds of waves. The new get the job done of the Dresden crew focuses on the so-termed Alfvén waves that manifest under the corona in the warm plasma of the photo voltaic ambiance, which is permeated by magnetic fields. The magnetic fields acting on the ionized particles of the plasma resemble a guitar string, whose participating in triggers a wave movement. Just as the pitch of a strummed string improves with its rigidity, the frequency and propagation velocity of the Alfvén wave improves with the toughness of the magnetic industry.

“Just under the Sun’s corona lies the so-termed magnetic cover, a layer in which magnetic fields are aligned mainly parallel to the photo voltaic surface area. Here, seem and Alfvén waves have approximately the similar velocity and can consequently quickly morph into every single other. We wanted to get to exactly this magic place — where the shock-like transformation of the magnetic energy of the plasma into heat begins,” says Stefani, outlining his team’s purpose.

A hazardous experiment?

Shortly after their prediction in 1942, the Alfvén waves experienced been detected in 1st liquid-metal experiments and later studied in element in elaborate plasma physics amenities. Only the situations of the magnetic cover, thought of important for corona heating, remained inaccessible to experimenters until eventually now. On the 1 hand, in big plasma experiments the Alfvén velocity is ordinarily significantly bigger than the velocity of seem. On the other hand, in all liquid-metal experiments to day, it has been noticeably reduced. The reason for this: the comparatively low magnetic industry toughness of typical superconducting coils with continuous industry of about twenty tesla.

But what about pulsed magnetic fields, such as people that can be generated at the HZDR’s Dresden Higher Magnetic Industry Laboratory (HLD) with greatest values of virtually 100 tesla? This corresponds to about two million periods the toughness of the Earth’s magnetic industry: Would these really higher fields allow Alfvén waves to split through the seem barrier? By seeking at the qualities of liquid metals, it was recognized in progress that the alkali metal rubidium really reaches this magic place already at 54 tesla.

But rubidium ignites spontaneously in air and reacts violently with h2o. The crew consequently at first experienced uncertainties as to irrespective of whether such a hazardous experiment was recommended at all. The uncertainties have been quickly dispelled, recalls Dr. Thomas Herrmannsdörfer of the HLD: “Our energy provide program for running the pulse magnets converts 50 megajoules in a fraction of a 2nd — with that, we could theoretically get a commercial airliner to take off in a fraction of a 2nd. When I spelled out to my colleagues that a thousandth of this amount of money of chemical energy of the liquid rubidium does not stress me extremely significantly, their facial expressions visibly brightened.”

Pulsed through the magnetic seem barrier

Nevertheless, it was however a rocky street to the profitable experiment. Due to the fact of the pressures of up to fifty periods the atmospheric air stress generated in the pulsed magnetic industry, the rubidium melt experienced to be enclosed in a sturdy stainless metal container, which an professional chemist, brought out of retirement, was to fill. By injecting alternating recent at the bottom of the container even though at the same time exposing it to the magnetic industry, it was eventually probable to make Alfvén waves in the melt, whose upward movement was calculated at the expected velocity.

The novelty: even though up to the magic industry toughness of 54 tesla all measurements have been dominated by the frequency of the alternating recent signal, exactly at this place a new signal with halved frequency appeared. This unexpected time period doubling was in fantastic settlement with the theoretical predictions. The Alfvén waves of Stefani’s crew experienced damaged through the seem barrier for the 1st time. Whilst not all observed consequences can nevertheless be spelled out so quickly, the get the job done contributes an vital element to solving the puzzle of the Sun’s corona heating. For the foreseeable future, the scientists are arranging detailed numerical analyses and additional experiments.

Investigation on the heating system of the Sun’s corona is also becoming carried out in other places: the Parker Photo voltaic Probe and Photo voltaic Orbiter space probes are about to acquire new insights at shut variety.