Study: solar 'tsunami' gives insight into Sun's magnetic field

Two solar "tsunamis", travelling across the Sun at over a million kilometres per hour, have been used to measure the Sun's magnetic field.

Thought to be caused when the surface of the Sun explodes matter out into space -- events known as coronal mass ejections (CMEs) -- solar tsunamis are waves of ionised gas and magnetic field distortions that travel through the Sun's atmosphere, or corona, sometimes covering the Sun in less than an hour.

By measuring how the tsunami travelled across the Sun, a team from University College London was able to record the strength of the Sun's magnetic field. The research, due to be published in Solar Physics, could help us better understand CMEs, which can disrupt satellites and power grids on Earth.
Although the interior of the Sun is thought to have an extremely powerful magnetic field, the team found that the magnetic field in its atmosphere is about ten times weaker than a fridge magnet.

 Solar tsunamis, also known as "EIT Waves" after the telescope that first detected them in 1997, are notoriously difficult to spot. They propagate radially out from a point on the Sun's surface where a coronal mass ejection has taken place.

"They're only visible when the Sun is quiet," says lead author David Long. "If the Sun is quite active, they're very difficult to spot" as they become hidden by the huge amount of radiation emanating from the Sun.
Even when the Sun is quiet, "you have to look at the right time, in the right place, and have everything going for you," he says.

In 2007 and 2010, Japan's Hinode spacecraft and Nasa's Solar Dynamics Observatory (SDO) did indeed have everything going for them.
Long and his colleagues used the data from the two tsunami events recorded by Hinode and SDO to track the change in velocity and acceleration of the tsunamis.

The way a wave travels can tell you something about the thing it's travelling through. For example, sound travels faster in water than in air, so we can deduce that water is denser than air. For these solar tsunamis, the waves are travelling through the Sun's magnetic field, so if they accelerate or slow down, that tells you something about the density and therefore strength of the magnetic field.
"By looking at how [the waves] deform," says Long. "We can measure the strength of the Sun's magnetic field".
 As the solar tsunamis may be directly caused by CMEs, Long hopes that the research will help our understanding of CMEs, potentially helping us reduce the impact on Earth of explosions of solar matter.