Bringing the sun into the lab
Liquid-metal experiment provides insight into the heating mechanism of
the sun's corona

Date:
January 3, 2022
Source:
Helmholtz-Zentrum Dresden-Rossendorf
Summary:
Why the sun's corona reaches temperatures of several million
degrees Celsius is one of the great mysteries of solar physics. A
'hot' trail to explain this effect leads to a region of the solar
atmosphere just below the corona, where sound waves and certain
plasma waves travel at the same speed. In an experiment using
the molten alkali metal rubidium and pulsed high magnetic fields,
researchers have developed a laboratory model and experimentally
confirmed the theoretically predicted behavior of these plasma
waves.



FULL STORY ==========================================================================
Why the Sun's corona reaches temperatures of several million degrees
Celsius is one of the great mysteries of solar physics. A "hot" trail to explain this effect leads to a region of the solar atmosphere just below
the corona, where sound waves and certain plasma waves travel at the same speed. In an experiment using the molten alkali metal rubidium and pulsed
high magnetic fields, a team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a German national lab, has developed a laboratory model and for
the first time experimentally confirmed the theoretically predicted
behavior of these plasma waves -- so- called Alfve'n waves -- as the researchers report in the journal Physical Review Letters.


==========================================================================
At 15 million degrees Celsius, the center of our Sun is unimaginably
hot. At its surface, it emits its light at a comparatively moderate 6000 degrees Celsius. "It is all the more astonishing that temperatures of
several million degrees suddenly prevail again in the overlying Sun's
corona," says Dr. Frank Stefani. His team conducts research at the
HZDR Institute of Fluid Dynamics on the physics of celestial bodies
-- including our central star. For Stefani, the phenomenon of corona
heating remains one of the great mysteries of solar physics, one that
keeps running through his mind in the form of a very simple question:
"Why is the pot warmer than the stove?" That magnetic fields play a
dominant role in heating the Sun's corona is now widely accepted in
solar physics. However, it remains controversial whether this effect is
mainly due to a sudden change in magnetic field structures in the solar
plasma or to the dampening of different types of waves. The new work of
the Dresden team focuses on the so-called Alfve'n waves that occur below
the corona in the hot plasma of the solar atmosphere, which is permeated
by magnetic fields. The magnetic fields acting 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 increases with its tension,
the frequency and propagation speed of the Alfve'n wave increases with
the strength of the magnetic field.

"Just below the Sun's corona lies the so-called magnetic canopy, a
layer in which magnetic fields are aligned largely parallel to the solar surface. Here, sound and Alfve'n waves have roughly the same speed and can therefore easily morph into each other. We wanted to get to exactly this
magic point -- where the shock-like transformation of the magnetic energy
of the plasma into heat begins," says Stefani, outlining his team's goal.

A dangerous experiment? Soon after their prediction in 1942, the
Alfve'n waves had been detected in first liquid-metal experiments and
later studied in detail in elaborate plasma physics facilities. Only the conditions of the magnetic canopy, considered crucial for corona heating, remained inaccessible to experimenters until now.

On the one hand, in large plasma experiments the Alfve'n speed is
typically much higher than the speed of sound. On the other hand, in all liquid-metal experiments to date, it has been significantly lower. The
reason for this: the relatively low magnetic field strength of common superconducting coils with constant field of about 20 tesla.

But what about pulsed magnetic fields, such as those that can be
generated at the HZDR's Dresden High Magnetic Field Laboratory (HLD)
with maximum values of almost 100 tesla? This corresponds to about two
million times the strength of the Earth's magnetic field: Would these
extremely high fields allow Alfve'n waves to break through the sound
barrier? By looking at the properties of liquid metals, it was known
in advance that the alkali metal rubidium actually reaches this magic
point already at 54 tesla.

But rubidium ignites spontaneously in air and reacts violently with
water. The team therefore initially had doubts as to whether such a
dangerous experiment was advisable at all. The doubts were quickly
dispelled, recalls Dr. Thomas Herrmannsdo"rfer of the HLD: "Our energy
supply system for operating the pulse magnets converts 50 megajoules
in a fraction of a second -- with that, we could theoretically get a
commercial airliner to take off in a fraction of a second.

When I explained to my colleagues that a thousandth of this amount
of chemical energy of the liquid rubidium does not worry me very much,
their facial expressions visibly brightened." Pulsed through the magnetic sound barrier Nevertheless, it was still a rocky road to the successful experiment. Because of the pressures of up to fifty times the atmospheric
air pressure generated in the pulsed magnetic field, the rubidium melt had
to be enclosed in a sturdy stainless steel container, which an experienced chemist, brought out of retirement, was to fill. By injecting alternating current at the bottom of the container while simultaneously exposing
it to the magnetic field, it was finally possible to generate Alfve'n
waves in the melt, whose upward motion was measured at the expected speed.

The novelty: while up to the magic field strength of 54 tesla all
measurements were dominated by the frequency of the alternating current
signal, exactly at this point a new signal with halved frequency
appeared. This sudden period doubling was in perfect agreement with
the theoretical predictions. The Alfve'n waves of Stefani's team had
broken through the sound barrier for the first time. Although not all
observed effects can yet be explained so easily, the work contributes an important detail to solving the puzzle of the Sun's corona heating. For
the future, the researchers are planning detailed numerical analyses
and further experiments.

Research on the heating mechanism of the Sun's corona is also being
carried out elsewhere: the Parker Solar Probe and Solar Orbiter space
probes are about to gain new insights at close range.

========================================================================== Story Source: Materials provided by
Helmholtz-Zentrum_Dresden-Rossendorf. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. F. Stefani, J. Forbriger, Th. Gundrum, T. Herrmannsdo"rfer,
J. Wosnitza.

Mode Conversion and Period Doubling in a Liquid Rubidium
Alfve'n-Wave Experiment with Coinciding Sound and Alfve'n
Speeds. Physical Review Letters, 2021; 127 (27) DOI:
10.1103/PhysRevLett.127.275001 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220103145610.htm
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