UVCS/SOHO observations of an equatorial streamer above the west limb
of the Sun on October 12, 1996 (Kohl et al. 1997, Solar Phys., 175,
613).  The upper panel shows the streamer in the light of O 5+ ions
(known as O VI) at 103.2 nm.  The lower panel shows the same streamer
in the light from neutral hydrogen atoms (H I) at 121.6 nm (known as
HI Lyman alpha). The images were obtained by interpolating between
observations made at 6 radial positions.  The Y axis gives solar
coordinates in arc min, originating in the center of the disk, where
positive Y axis is the projected heliographic north direction. The X
axis is approximately in the heliographic West direction.  (Note that
the angular diameter of the sun is 32 arc minutes, about 0.5 degree).

Note the striking difference between the two images of the same
streamer: in H I Lyman alpha the maximum of intensity is in the core
of the streamer and the intensity gradually decreases towards the
edges, while in the O VI 103.2 nm line the core is dimmer than the
streamer edges.  This effect was studied by Noci et al. (1997, Adv. in
Space Res., 20, 2219) and Raymond et al. (1997, Solar Phys., 175,
645). They found that it is very likely caused by a lower abundance
of oxygen ions in the core of the streamer, relative to the nearly
photospheric abundance along the edges.  Raymond et al. (1997)
found this relative depletion to be a factor of three in the oxygen
abundance.  The deficit seems to be consistent with a gravitational
settling of O^(5+) ions in the closed magnetic field lines regions
in the center of the streamer.