They are there but they are not seen. Or rather, due to their nature and the multitude of turbulent phenomena that 'populate' the surface of our star, it has always been difficult to confirm their presence. They are the Alfvén waves, a particular type of magnetohydrodynamic wave that plays a crucial role in the transmission of energy in the atmosphere of our star.
Also called torsion magnetic waves, they have their importance in the ability to transport energy over large distances, thanks to their purely magnetic nature. By their characteristic they are produced in the solar atmosphere and can even propagate in interplanetary space and observed close to the Earth.
Despite numerous indirect evidence of the presence of Alfvén waves in the Sun's outermost atmosphere, no direct evidence had been found so far. After more than 50 years now, thanks to a high resolution spectropolarimetric observation of the solar photosphere obtained by the spectropolarimeter of the National Institute of Astrophysics (INAF) IBIS (Interferometric Bidimensional Spectrometer), realized in collaboration with the University of Florence and Rome "Tor Vergata ", it was finally possible to confirm the existence of these particular plasma waves in the solar atmosphere. A discovery that sheds light on the excitation mechanisms linked to the convective motions of the plasma in the solar photosphere. The observations analyzed in the study were obtained when the instrument was installed at the Dunn Solar Telescope in New Mexico, USA. The instrument is now being updated in the INAF laboratories to return to operation in 2022 at the Vacuum Tower Telescope in the Canary Islands, Spain.
The study was led by Marco Stangalini, researcher of the Italian Space Agency (ASI) and associate of INAF and by Prof. Robertus Erdélyi of the University of Sheffield, in collaboration with the University of Rome "Tor Vergata" and with other researchers of national and foreign institutes and universities.
Marco Stangalini, tells us that: “It is the first time that this particular type of waves is directly identified in the lower solar atmosphere, thus making it possible to shed light on the mechanisms of their excitation. What our results show us is that these waves are excited by the convective motions on the surface of the star from where they are able to extract enormous amounts of energy, far greater than those required to heat the outermost layers of our star. The question we now ask ourselves is therefore where does all this energy go ”.
While for the University of Rome "Tor Vergata", Department of Physics, we have
Francesco Berrilli, full professor, who emphasizes: "This result confirms the importance of the study of photospheric dynamics for understanding the complex physical phenomena that occur in the upper layers of the solar atmosphere, often connected to the sources of space weather". “This is evidence that we solar physicists have been looking for for more than 20 years. The next step will consist in using these processes as diagnostics to extract information on the dynamics of our star's atmosphere, "said Dario Del Moro, researcher.
The results of this study, published in the journal "Nature Astronomy", have important implications not only in solar physics - to help experts understand the heating of the corona and the acceleration of the solar wind - but also in high energy astrophysics and in the field of applications such as that of nuclear fusion reactors. Direct evidence of the presence of these particular waves in the solar photosphere is only a first step towards understanding the physical mechanisms associated with them in many different physical and astrophysical contexts.
New steps and further answers are expected from the various infrastructures and missions dedicated to the study of the Sun such as future generation telescopes, such as the 4-meter DKIST telescope (USA) and the next European solar telescope EST, or ESA's Solar Orbiter missions. and NASA's Parker Solar Probe who are providing us with the first great images.
It will be thanks to these formidable instruments and their capacity for simultaneous observation in different spectral regions that we will be able to probe the solar atmosphere at different altitudes and with sensitivity and spatial and temporal resolutions never previously achieved. A dynamic and three-dimensional reconstruction of the strongly magnetized atmosphere of our star will thus be possible, a necessary condition for understanding its behavior in depth.
Paper on Nature Astronomy: https://www.nature.com/articles/s41550-021-01354-8
NASA, Parker Solar Probe: https://www.nasa.gov/content/goddard/parker-solar-probe
Solar Orbiter Missions: https://www.asi.it/en/planets-stars-universe/solar-system-and-beyond/solar-orbiter/