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.
"Holes" in the solar corona allow a better view into dynamics at the Sun's surface
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.
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