This image shows a Coronal Mass Ejection (CME) as observed by two
instruments aboard the SOHO satellite.  The left portion was obtained
with the Extreme Ultraviolet Imaging Telescope (EIT) in the light of
singly ionized helium.  The right portion was obtained by the
Ultraviolet Coronagraph Spectrometer (UVCS) in light scattered by
neutral hydrogen atoms.  The UVCS image was constructed from a time
series of observations as the Coronal Mass Ejection swept across the
UVCS entrance aperture.
 
The ejected material is propelled outwards from the Sun by magnetic
forces at speeds above 100 kilometers per second.  Comparison of
images at different times and measurements of the speed toward the
Earth based on the Doppler shift of the hydrogen spectral line suggest
that the magnetic structure takes the form of a helix, and that it
unwinds as it expands.
 
Coronal gas temperatures are typically 1 to 2 million K, while the
ejected gas temperatures spanned the range 50,000 to 500,000 K.  The
material is bright not because it is denser than they ambient corona,
but because neutral hydrogen and singly ionized helium are far more
abundant at the lower temperatures.
 
A Coronal Mass Ejection may contain a billion tons of tenuous gas.
Those which strike the Earth cause auroral displays at the poles and
sometimes disrupt communications.

(Ciaravella et al. 2000, Astrophys. J., 529, 575)