Why the Sun’s corona reaches temperatures of quite a few million degrees Celsius is 1 of the fantastic mysteries of photo voltaic physics. A “warm” trail to make clear this impact leads to a region of the photo voltaic ambiance just down below the corona, in which sound waves and particular plasma waves vacation at the similar velocity. In an experiment using the molten alkali metal rubidium and pulsed significant magnetic fields, a team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a German nationwide lab, has designed a laboratory product and for the to start with time experimentally confirmed the theoretically predicted conduct of these plasma waves — so-known as Alfvén waves — as the researchers report in the journal Physical Evaluation Letters.
At fifteen million degrees Celsius, the middle of our Sunshine is unimaginably warm. At its area, it emits its light at a comparatively moderate 6000 degrees Celsius. “It is all the much more astonishing that temperatures of quite a few million degrees suddenly prevail once again in the overlying Sun’s corona,” states Dr. Frank Stefani. His team conducts exploration at the HZDR Institute of Fluid Dynamics on the physics of celestial bodies — including our central star. For Stefani, the phenomenon of corona heating stays 1 of the fantastic mysteries of photo voltaic physics, 1 that retains running by means of his intellect in the form of a incredibly easy issue: “Why is the pot hotter than the stove?”
That magnetic fields participate in a dominant role in heating the Sun’s corona is now commonly recognized in photo voltaic physics. However, it stays controversial no matter if this impact is predominantly because of to a unexpected modify in magnetic discipline constructions in the photo voltaic plasma or to the dampening of distinctive varieties of waves. The new operate of the Dresden team focuses on the so-known as Alfvén waves that come about down below the corona in the warm plasma of the photo voltaic ambiance, which is permeated by magnetic fields. The magnetic fields performing on the ionized particles of the plasma resemble a guitar string, whose playing triggers a wave motion. Just as the pitch of a strummed string improves with its stress, the frequency and propagation velocity of the Alfvén wave improves with the energy of the magnetic discipline.
“Just down below the Sun’s corona lies the so-known as magnetic canopy, a layer in which magnetic fields are aligned mostly parallel to the photo voltaic area. Below, sound and Alfvén waves have roughly the similar velocity and can therefore simply morph into each and every other. We needed to get to precisely this magic point — in which the shock-like transformation of the magnetic energy of the plasma into warmth commences,” states Stefani, outlining his team’s goal.
A dangerous experiment?
Before long just after their prediction in 1942, the Alfvén waves had been detected in to start with liquid-metal experiments and afterwards analyzed in detail in elaborate plasma physics facilities. Only the conditions of the magnetic canopy, viewed as crucial for corona heating, remained inaccessible to experimenters till now. On the 1 hand, in huge plasma experiments the Alfvén velocity is normally a lot greater than the velocity of sound. On the other hand, in all liquid-metal experiments to date, it has been drastically lessen. The rationale for this: the fairly reduced magnetic discipline energy of common superconducting coils with constant discipline of about 20 tesla.
But what about pulsed magnetic fields, these kinds of as all those that can be generated at the HZDR’s Dresden Significant Magnetic Field Laboratory (HLD) with utmost values of virtually a hundred tesla? This corresponds to about two million occasions the energy of the Earth’s magnetic discipline: Would these particularly significant fields enable Alfvén waves to break by means of the sound barrier? By wanting at the properties of liquid metals, it was regarded in progress that the alkali metal rubidium essentially reaches this magic point already at 54 tesla.
But rubidium ignites spontaneously in air and reacts violently with drinking water. The team therefore initially had uncertainties as to no matter if these kinds of a dangerous experiment was recommended at all. The uncertainties were being promptly dispelled, remembers Dr. Thomas Herrmannsdörfer of the HLD: “Our energy provide procedure for running the pulse magnets converts fifty megajoules in a portion of a second — with that, we could theoretically get a professional airliner to consider off in a portion of a second. When I spelled out to my colleagues that a thousandth of this volume of chemical energy of the liquid rubidium does not fret me incredibly a lot, their facial expressions visibly brightened.”
Pulsed by means of the magnetic sound barrier
Nevertheless, it was continue to a rocky highway to the thriving experiment. Because of the pressures of up to fifty occasions the atmospheric air stress generated in the pulsed magnetic discipline, the rubidium melt had to be enclosed in a strong stainless metal container, which an professional chemist, brought out of retirement, was to fill. By injecting alternating latest at the bottom of the container when at the same time exposing it to the magnetic discipline, it was last but not least possible to generate Alfvén waves in the melt, whose upward motion was measured at the expected velocity.
The novelty: when up to the magic discipline energy of 54 tesla all measurements were being dominated by the frequency of the alternating latest sign, precisely at this point a new sign with halved frequency appeared. This unexpected period doubling was in fantastic settlement with the theoretical predictions. The Alfvén waves of Stefani’s team had broken by means of the sound barrier for the to start with time. Despite the fact that not all observed results can nevertheless be spelled out so simply, the operate contributes an crucial detail to fixing the puzzle of the Sun’s corona heating. For the foreseeable future, the researchers are scheduling comprehensive numerical analyses and further experiments.
Research on the heating system of the Sun’s corona is also getting carried out somewhere else: the Parker Solar Probe and Solar Orbiter room probes are about to get new insights at near range.