THE SUN, OUR DAILY REMINDER OF THE BEAUTY and vigor in which nature manifests itself. For ages the Sun was praised unwaveringly, but advances in science during the last century have allowed us to contemplate its origins and the mechanisms which kept the Sun shining through billions of years. However, even today there are several mechanisms which remain a mystery.
The corona is the outer atmosphere of the Sun which is only visible to the naked eye during a solar eclipse. It forms a glowing halo around the sun which extends millions of kilometers into space. In 1869 observations were made that identified a coronal spectral absorption line at 530.3 nanometers – green1. It took 70 years until Walter Grotrian and Bengt Edlén determined this spectral line belongs to a highly charged atom of iron (Fe13+). In order for 13 electrons to be stripped from an atom of iron the surrounding temperatures must exceed 2 million degrees Celsius. But the temperature of the Sun’s surface does not exceed 6000°C, so how can the outer atmosphere be over 300 times hotter? This question has remained unanswered through today, but now, 70 years after the work of Grotrian and Edlén, we might have the solution.
A critical step towards our current understanding is a NASA satellite launched in 2010 called the Solar Dynamics Observatory (SDO) which aims to study the Sun’s influence on the Earth. In May of 2011 it recorded data which showed how matter and energy can be transferred from the surface of the Sun into the corona. The data was analyzed by a group from Oslo led by Astrophysicist Sven Wedemeyer-Böhm. Their work is published on page 505 in the 486th volume of Nature.
Wedemeyer-Böhm’s group proposes that massive solar tornadoes act as energy channels for heating of the corona2. These enormous magnetically driven tornadoes are roughly the size of Canada and at least 10000 of them are present on the surface of a “quiet-looking” Sun. Their temperatures have been estimated around 1 million degrees Celsius – similar to the observed corona temperature. The Sun’s various atmospheric layers were imaged by measuring spectral lines for different elements present in the solar atmosphere. After stacking these images, Wedemeyer-Böhm’s group noticed the same swirl-like signature in all the images. They concluded the presence of a large vortex structure which extends far beyond the surface of the sun.
Vortex-like flows of gas on the surface of the Sun have been identified in previous studies3 and arise from thermal convection in the layer below1, but now Wedemeyer-Böhm’s group has identified a mechanism which connects these vortex flows to the outer atmosphere – the corona. Magnetic field lines which follow a vortex-like structure underneath the surface extend into the atmosphere of the Sun. These magnetic field lines do not emerge perpendicular to the solar surface and thus cause gas traveling with the field lines to be accelerated along a spiral trajectory into the corona. Wedemeyer-Böhm’s group supported their hypothesis with magneto-fluid-dynamic (MHD) simulations; they calculated peak magnetic field strengths of about 0.1 Tesla – 3000 times stronger than Earth’s magnetic field. The simulation indicated that the accelerated gas has sufficient energy to provide a mechanism for coronal heating. However, it remains unclear how the energy transport mechanisms work in areas on the Sun with extremely strong magnetic fields such as sunspots and solar flares1. Nonetheless, their work has contributed greatly to our current understanding and paves the way for future pursuit of related investigations.
For further information and a movie of the numerical simulations visit the Article and its Supplementary Information at www.nature.com/nature/journal/v486/n7404/full/nature11202.html
Vedran Jelic is a recent graduate from the University of Alberta with a BSc in Engineering Physics. He will be starting his Master’s program in Physics at the University of Alberta with Dr. Frank Hegmann in September. His research focuses on studying nanoscale properties of materials, but though he has a passion for all physics from particle to astronomical.
 Bradshaw, S. J. (2012). “Solar physics: Swirls in the corona.” Nature 486(7404): 476-477.
 Wedemeyer-Bohm, S., E. Scullion, et al. (2012). “Magnetic tornadoes as energy channels into the solar corona.” Nature 486(7404): 505-508.
 Brandt, P. N., G. B. Scharmert, et al. (1988). “Vortex flow in the solar photosphere.” Nature 335(6187): 238-240.