The figure below shows images of a coronal mass ejection (CME) taken
with two instruments on the Solar and Heliospheric Observatory (SOHO).
CMEs are large explosive events on the Sun that can release high energy
particles and billions of tons of cooler plasma into interplanetary space.
On the left is a white-light image of the Sun that was taken with the
LASCO/SOHO coronagraph. The white circle represents an outline of the
solar disk. The dark brown area is the part of the solar atmosphere that is
blocked by LASCO's occulting system. On the right side of the figure are
two images of the CME in ultraviolet light. They were reconstructed from
observations with SAO's Ultraviolet Coronagraph Spectrometer (UVCS/SOHO).
The upper UVCS image shows the CME as it appears in light produced by
neutral hydrogen, while the lower image depicts the speeds of neutral
hydrogen atoms moving toward (blue) and away (red) from the observer.
Both of these images use false color, the former to represent the
intensity of the H I Lyman-alpha radiation (ranging from black to orange
to white) and the latter to indicate the direction and speeds of the
hydrogen atoms (dark blue, white and red representing the highest speed
toward the observer, zero speed and the highest speed away from the
observer, respectively). The green line nearest the Sun represents the
location of the base of the ultraviolet images, which is at 2.3 solar
radii from Sun-center.
The upper UVCS image shows the same general morphology as the LASCO
image. The outer portion of this image shows the bright compressed CME
front. The low-intensity cavity (dark area) can be seen just below
that. The bright prominence material is represented in white.
The lower UVCS image shows that the compressed CME front (blue area)
is moving toward the observer with its top part (darker blue area) moving
faster than its lower right side. The trailing prominence material
below the CME front has a more structured pattern of speeds but it is
mainly moving away from the observer.
The UVCS observations provide unique information even though the
reconstructed images are somewhat distorted compared to the LASCO images.
UVCS detected emission from spectral lines of hydrogen, oxygen,
silicon, and carbon in this CME. Line widths and intensities provide
information about proton, electron, and ion temperatures, bulk 3D flows,
elemental abundances, the ionization state of the plasma, and its
helicity (the handedness and geometry of the twisted magnetic field).