Why the outer edge of the Sun hotter than the surface of the Sun?

Sun's surface (Credit: NASA)Researchers have examined the chromosphere of the Sun in a greater detail to reveal the mystery of the hotter outer edge of the sun.

This research has been published online in December issue of the journal Nature Communications.

Chromosphere refers to the lower region of the atmosphere of the Sun or any other star. It is present between the surface of the Sun, i.e. photosphere, and the outer layer of the Sun, i.e. corona. For a long time, astronomers were amazed by this fact that the corona is nearly 200 times hotter than the surface of the star.

It was previously believed that magnetohydrodynamic (MHD) waves distribute energy between the surface and outer layers of the Sun. MHD refers to the movement of the electrically conducting fluids such as plasmas, liquid metals, and salt water or electrolytes.

Scientists have used the most advanced form of solar-imaging technology to study the chromosphere of the Sun and revealed the mechanism behind the hotter outer surface.

In this research, scientists used UK-designed dedicated solar-imaging telescope known as Rapid Oscillations in the Solar Atmosphere, or ROSA, and analyzed the MHD waves to study the chromosphere in high level of detail for the first time.

Through the highest resolution images of the chromosphere, scientists were able to study the speed and the power of the waves enabling them to study the amount of energy transported.  They confirmed that these MHD waves cause the transport of energy from below the solar surface to the corona through the chromosphere resulting in excessively hot outer surface.

Dr Morton, from Solar Physics and Space Plasma Research Centre, School of Mathematics and Statistics, University of Sheffield, said in a statement, “The Sun is our closest star and provides a unique opportunity to study the properties of stars in detail. Stars generate heat through thermonuclear reactions in their core and the temperature decreases towards the star’s surface. However, a significant number of stars have higher temperatures at the outer edges of their atmospheres than they do on their surface.

“Our observations have permitted us to estimate the amount of energy transported by the magnetic waves, and these estimates reveal that the waves’ energy meets the energy requirement for the unexplained temperature increase in the corona.”


Morton, R., Verth, G., Jess, D., Kuridze, D., Ruderman, M., Mathioudakis, M., & Erdélyi, R. (2012). Observations of ubiquitous compressive waves in the Sun’s chromosphere Nature Communications, 3 DOI: 10.1038/ncomms2324