Speedy tsunami seen on Sun's surface

Two satellites have seen "tsunami" spreading on the surface of the Sun after a release of matter into space called a coronal mass ejection (CME).

These tsunami of heightened magnetic field and hot, ionised gas race across the Sun at about 400km per second.

Analysis of the chance sighting, to be published in Solar Physics, allowed the measurement of the magnetic field in "quiet" areas, away from the CME.

Understanding this field may help predict how CMEs will affect the Earth.

And thanks to data from Hinode, one of the two satellites, researchers may have cracked a 70-year-old mystery as to why the Sun's surrounding corona is so much hotter than its surface.

The Japanese satellite Hinode has been studying the Sun since 2006, joined in Earth orbit by the Solar Dynamics Observatory in 2010.

Both satellites look at ultraviolet light from the Sun - colours we cannot see but that give hints as to both the chemical makeup and the extreme physical conditions at and near the Sun's roiling, turbulent surface.

David Long of University College London and colleagues finally spotted what are known as EIT waves after a CME. Like a tsunami emanating from the point of a seismic event, EIT waves are shock waves that carry magnetic fields and hot, ionised "plasma".

"These EIT waves are quite tricky - they're very random and they're relatively rare," Dr Long told BBC News. "We need to be in the right place at the right time; this has been a long time coming."

The SDO satellite was able to capture the ultraviolet light emitted as the wave spread out. From that, the team was able to determine the wave's speed - some 400km per second - and its rough temperature, over a million degrees.

Meanwhile the Hinode satellite returned a high-resolution map of the density of the Sun's surface every 45 seconds.

Using both data sets, the team was able to determine the strength of the magnetic field in the "quiet corona" - a tricky measurement of the Sun in its typical, quiescent state.

"This tells us a lot about the nature of the Sun and what goes on in the atmosphere," Dr Long explained. "These waves are quite important because they're associated with CMEs that fire plasma out into the heliosphere, toward the Earth."

These CMEs can bathe the Earth with fast-moving particles that can disrupt satellite communications or even knock out electrical power here on Earth - but solar scientists struggle to predict their eventual effects.

"Generally we see them when there's a CME coming straight at us - but when it's coming straight at us then it's quite difficult to measure how fast it's coming at us or how strong it is," Dr Long said.

"So by looking at these waves, we should be able to infer how powerful these CMEs are going to be."

More observations of EIT waves will be needed to determine the exact relationship between the waves' and the CMEs' characteristics.

Hot, cold, hot

The Hinode satellite was also crucial for measurements reported at this week's meeting of the Solar Physics Division of the American Astronomical Society.

Michael Hahn and Daniel Wolf Savin of Columbia University in New York, US, used Hinode to peer at similar waves from a "polar coronal hole" - a region where, like the pole of a bar magnet, field lines originate and reach far above the Sun's surface.

They were trying to tackle a puzzle about the temperature of the Sun's surrounding corona.

The temperature at the Sun's core is some 15,000,000C, but its surface is below 6,000C. Yet the corona is known to be at a temperature in excess of 1,000,000C.

How the energy gets into the corona to keep up these temperatures has baffled astronomers for more than half a century.

One idea was that waves of magnetic energy rise from below the Sun's surface, depositing energy into the corona higher up. But what remained unclear was whether the energy was lost on its journey.

Hinode observations of the polar coronal hole have allowed the pair to peek into this interim height and determine how the energy is coupled up from the surface into the corona.

In a preprint on the Arxiv server they show that enough energy is carried by these waves to keep the corona at its searing temperatures.