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Unveiling How the Sun’s Corona is Heated

Coronal loops as imaged by the TRACE satellite show gas at 1 million degrees Celsius (NASA/TRACE).

A team of astronomers involving Armagh Observatory and Planetarium (AOP) and the Dublin Institute for Advanced Studies (DIAS) may have solved a long standing problem in solar astrophysics: why does the Sun’s outer tenuous atmosphere (known as the corona) have a temperature of around a million degrees?

The Sun’s outer corona has a temperature of a million degrees and is separated from its cool surface (the photosphere), at 6000 degrees, by only a few hundred kilometres. A major problem in solar physics is understanding why the corona is so much hotter than the Sun’s surface? The answer to this problem lies in the complex magnetic field which is tightly woven within the atmosphere of Sun. Over the last few decades, numerous observatories in space and on the ground, have provided a range of explanations for the origins of this coronal heating. Two competing physical mechanisms are generally accepted. The first is known as magnetic reconnection, which is an explosive release of the magnetic energy, trapped in magnetic fields in the corona (otherwise known as a solar flare). The second is the energy for heating provided by oscillating magnetic fields in the form of waves (otherwise known as Alfvén waves). Detecting these magnetic field oscillations requires advanced numerical simulations combined with the highest resolution images available to astronomers. Moreover, reducing the enormous amount of observational data requires tremendous computing power. Only now has the wave origins behind the heating of the solar corona being seen clearly.

An international team including researches from five countries, and involving the Dublin Institute for Advanced Studies, as well as Armagh Observatory and Planetarium, report on a new discovery made using the Swedish Solar Telescope on La Palma in the Canary Islands. These observations show for the first time the presence of high frequency waves traveling along thin magnetic flux tubes emerging from the Sun. Not only do the waves serve as a substantial source of energy for the corona but they also drive the supersonic solar wind. The work has just been published in Nature Scientific Reports and is led by Dr. A.K. Srivastava from the Department of Physics, Indian Institute of Technology (BHU), India. Other team members are Professor Gerry Doyle from Armagh, Juie Shetye, a PhD student at Armagh, Dr Eamon Scullion from Northumbria University, Professor B.N. Dwivedi from the Department of Physics, Indian Institute of Technology, Varanasi, Professors K. Murawski and Dariusz Wojcik from UMCS, Lublin, Poland,  Dr Marco Stangalini from INAF,  Rome and Professor Tom Ray from the Dublin Institute for Advanced Studies. To carry out the necessary simulations and enormous data reduction, the team made extensive use of Irish Centre for High End Computing (ICHEC) resources.