This ominous, dark shape sprawling across the face of the Sun
is a coronal hole --
a low density region extending above
the surface where the solar magnetic field opens freely into
interplanetary space.
Studied extensively
from
space since the 1960s in
ultraviolet and
x-ray light, coronal holes are known to be the source of
the high-speed
solar wind, atoms and electrons
which flow outward along the open
magnetic field lines.
During periods of low activity,
coronal holes typically cover
regions just above the Sun's poles.
But this extensive coronal hole
dominated the Sun's northern hemisphere earlier this week, captured
here in extreme ultraviolet light by cameras onboard
the Solar Dynamics Observatory.
The solar wind streaming from this coronal hole
triggered auroral displays
on planet Earth.
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