02.010 Mystery of sun research

Der Standard
Direct proof: Plasma waves heat up the sun atmosphere, 6.3.2018
derstandard.at Mystery of sun research

Already for a long time researches suspect that Alfvén waves are responsible for the high temperatures in the corona. Now the heating process could be observed for the first time.

An international team of researchers with participation of the Institute for World Space Research (IWF) in Graz came close a mystery of sun research: As the scientists report in "Nature Physics" they succeeded in proving the heating up of the sun atmosphere through plasma waves. According to the Graz researchers it is a breakthrough in the answering of a question, with which science deals for decades.

Although "only" some thousand degrees Celsius are on the sun surface (photosphere), the higher sun atmosphere (corona) reaches temperatures of up to one million degrees Celsius. How the corona plasma is heated up, could until now though not completely be settled.

To further heat up the thinner gas in the corona, not much energy would be necessary fundamentally, but still the mechanism is unclear, which pumps in and distributes the energy in the corona. For a long time researchers suspect that so-called Alfvén waves could be responsible for it. These plasma waves, called after the Swedish physicist and Nobel prize-winner Hannes Alfvén, are formed through vibrations of the magnetic field lines and could transmit the heat on to the electrically loaded particles of the corona according to a common thesis.

Shockwaves on the chromosphere

The Alfvén waves are indeed for more than half a century main focus of laboratory plasma physics and of astrophysics, however their direct proof in the sun atmosphere is difficult, report the researchers around Samuel D. T. Grant of the Astrophysics Research Centre of the British Queen's University Belfast. Grant and colleagues have now presented the first observations of it that Alfvén waves heat up the plasma in the transition layer (chromosphere) between photosphere and corona through the forming of shock fronts.

The proof succeeded through simultaneous, high resolution observations of a sun spot from the year 2014 with the help of the Dunn Solar Telescope (USA) and the Solar Dynamics Observatory of the US space authority Nasa. The international research team could utilize these data. According to the IWF researcher and study co-author Teimuraz Zaqarashvili the observed strong temperature increase during spreading out of the waves was a clear sign for the energy conversion. "The observation of the Alfvén shockwaves is the key for the solution of the mystery, how the corona plasma is heated up", so Zaqarashvili.

The US American Daniel K. Inouye Sun Telescope (DKIST) under construction on the Hawaii volcano Haleakala will, according to the authors, further reveal the fine structure of the shockwaves and make possible an in depth assessment of the role of the Alfvén shocks with the global solar atmospheric heating up. Also through the planned "Solar Orbiter" mission the researches of the IWF hope for further discoveries. In the framework of this Esa mission the sun and the Helios sphere is to be closely examined from a distance of about 45 sun radii. The IWF was entrusted with the antenna calibration, builds the on-board computer for the radio wave instrument and is co-investigator with the magnetometer.

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