01/06/2022 at 10:00 CET
Eduardo Martinez de la Fe
German researchers have replicated for the first time in the laboratory a process that occurs in the solar atmosphere and clarified the mechanism that heats the corona of our star to temperatures much higher than those of its surface.
Researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), a German national laboratory, have laboratory simulated the conditions of the solar atmosphere in a potentially dangerous experiment.
The aim of the experiment was to clear up a mystery: while the core of the Sun reaches 15 million degrees Celsius, its surface is comparatively cold, at only 6,000ºC.
The solar surface even has some even colder areas, with a temperature of 4,000ºC, which are called sunspots.
What is really surprising is that the solar corona, which is the outer part of the Sun’s atmosphere (which we can only see during total eclipses of the star), has a temperature much higher than its surface: almost 2,000,000 degrees.
The protagonists of the new experiment emphasize that one of the great mysteries of solar physics is represented by this difference in temperature.
Magnetic canopy, the key
One clue that can explain it is located in a region of the solar atmosphere just below the corona, where sound waves and certain plasma waves travel at the same speed.
While scientists have long considered these waves to play an essential role in heating the solar corona, there is no consensus on how it occurs.
To delve into this mystery, the German researchers focused on that part of the solar atmosphere that is located immediately below the corona, called magnetic canopy.
The magnetic canopy is a layer of the magnetic field parallel to the solar surface that is found in the lower part of the chromosphere, where the temperature is already around 500,000ºC and where very strong magnetic fields are produced.
In the magnetic canopy, plasma waves, known as Alfvén waves, and magnetic fields, are supposed to heat up the plasma and cause the overheating observed in the solar corona.
In this region of the lower chromosphere, sound and Alfvén waves can easily transform into each other and raise the temperature much higher, explains the director of this research, Frank Stefani, in a statement.
What these researchers have done in their laboratory is to replicate that solar magic point, in which sound and plasma waves are confused, and confirm that a similar process may be taking place in the sun and causing 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 had not been reproduced in a laboratory.
Although this new work provides important data to solve the solar corona heating puzzle, the researchers are planning more detailed numerical analyzes and more experiments to confirm their conclusions.
Also close up
Research on the heating mechanism of the solar corona is also being carried out elsewhere: the Parker Solar Probe and Solar Orbiter space probes are about to gain new insights a short distance from this solar mystery.
Launched in 2018, NASA’s Parker Solar Probe approached 8.5 million kilometers from 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 on the solar surface, of radiation levels in the inner heliosphere, and of the 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 the German scientists.
Mode Conversion and Period Doubling in a Liquid Rubidium Alfvén-Wave Experiment with Coinciding Sound and Alfvén Speeds. F. Stefani et al. Phys. Rev. Lett. 127, 275001, 29 December 2021. DOI: https: //doi.org/10.1103/PhysRevLett.127.275001