Why is the outer edge of the Sun much hotter than its surface?
Now, a team of scientists from Sheffield and Belfast led by Dr Richard Morton, Northumbria University are close to solve the puzzle.
They have examined the MHD waves using a UK-built solar imager ROSA – Rapid Oscillations of the Solar Atmosphere –,
to observe the chromosphere with a high degree of clarity.
Click on image to enlarge
Magnetic loop structures in the corona of the Sun. The loops highlight the Sun’s magnetic field and are visible
because they support the dense, million degree gas typical of the corona. The image is courtesy of the Science Team for
NASA’s Solar Dynamic Observatory.
They have used cutting-edge solar-imaging technology to observe the Sun’s chromosphere –
a region of the Sun’s atmosphere sandwiched between its surface (photosphere) and outer layer (corona) – to an
unprecedented level of detail.
For years astronomers have looked for the elusive mechanism that causes some stars to have a corona that is
almost 200 times hotter than their photosphere, despite being further away from the heat source at the star’s core.
It is believed that the cause of the increased temperature is due to magnetohydrodynamic (MHD) waves that distribute
the energy generated below the star’s surface to the outer layers of the Sun’s atmosphere
The powerful ROSA tool enabled some of the highest resolution images of the chromosphere to be
obtained, allowing the scientists to study the speed and power of the waves and then estimate the amount of energy
that they transport.
Their calculations confirm that the MHD waves could be responsible for transporting energy from below the
solar surface, out through the chromosphere, into the corona and leading to heating of the outer layers in
excess of a million degrees.
Chromosphere of the solar atmosphere
Bright patches correspond to concentrated magnetic flux. Credit: SOHO
“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," Dr Morton said.
“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.”