They reproduce in the laboratory what happens in the Sun’s atmosphere – CVBJ
06/01/2022 at 10:00 CET
Eduardo Martinez de la Fé
German researchers have reproduced for the first time in the laboratory a process that occurs in the solar atmosphere and have clarified the mechanism that heats the corona of our star to temperatures much higher than those on its surface.
Researchers at Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a German national laboratory, simulated the conditions of the solar atmosphere in a laboratory in a potentially dangerous experiment.
The aim of the experiment was to unravel a mystery: while the core of the Sun reaches 15 million degrees Celsius, its surface is relatively cold, at only 6,000 ° C.
The solar surface even has even cooler areas, with a temperature of 4000 ° C, which are called sunspots.
What is really surprising is that the solar corona, which is the outer part of the Sun’s atmosphere (which can only be seen during total star eclipses), has a temperature well above its surface: nearly 2,000,000 degrees.
The protagonists of the new experiment point out that one of the great mysteries of solar physics is represented by this temperature difference.
Magnetic awning, the key
One clue that can explain this is in a region of the solar atmosphere just below the corona, where sound waves and some plasma waves travel at the same speed.
While scientists have long considered these waves to play a critical role in heating the solar corona, there is no consensus on how this happens.
To unravel this mystery, German researchers focused on that part of the solar atmosphere immediately below the corona, called magnetic awning.
The magnetic canopy is a layer of the magnetic field parallel to the solar surface which is found in the lower part of the chromosphere, where the temperature is already around 500,000 ° C and where very strong magnetic fields are produced.
In the magnetic canopy, plasma waves, called Alfvén waves, and magnetic fields are believed to heat the plasma and cause the overheating seen in the solar corona.
In this region of the lower chromosphere, sound and Alfven waves can easily transform and raise the temperature much higher, said research director Frank Stefani in a statement.
What these researchers did in their lab was to reproduce solar magic point, in which sound and plasma waves are confused, and confirms that a similar process can take place in the sun and cause the corona to heat up.
To achieve this, Stefani and his team used a dangerous molten rubidium, an alkali metal, and subjected it to high magnetic fields: the laboratory model experimentally confirmed the theoretical behavior of Alfvén waves for the first time.
In the same way that playing a guitar string triggers a wave motion, the frequency and speed of the Alfvén waves increase with the strength of the magnetic field present in the magnetic canopy and heat up the solar corona.
Alfvén waves were first predicted in 1942, after being detected in liquid metal experiments and studied in plasma physics laboratories.
Until now, the conditions in the sun’s magnetic canopy that produce corona heating have not been reproduced in the laboratory.
While this new work provides important data for solving the solar corona heating puzzle, the researchers are planning more detailed numerical analyzes and more experiments to confirm their findings.
Research on the solar corona heating mechanism is also being carried out elsewhere: the Parker Solar Probe and Solar Orbiter space probes are about to gain new knowledge at a short distance from this solar mystery.
Launched in 2018, NASA’s Parker solar probe approached 8.5 million kilometers of the solar surface last November, in order to track how energy and heat move through the solar corona and explore what accelerates the solar wind and solar energetic particles.
Solar Orbiter, launched in 2020, is a scientific solar observation satellite developed by the European Space Agency (ESA) in collaboration with NASA.
Its objective is to make detailed measurements of the magnetic field at the solar surface, radiation levels in the inner heliosphere and solar wind, as well as to make observations of the Sun’s polar regions from high latitude orbits.
Both missions will provide information that will complement the experimental observations of German scientists.
Mode conversion and period doubling in an Alfvven wave experiment of liquid rubidium with coincident sound and Alfven velocities. F. Stefani et al. Phys. Rev. Lett. 127, 275001, December 29, 2021. DOI: https: //doi.org/10.1103/PhysRevLett.127.275001