The Spartan ultraviolet coronal spectrometer observed
a helmet streamer in 1993. The measured
HI Ly-
intensities are given in Table 10. The
streamer intensities are about 8 to 10 times that of
the coronal holes observed by the same instrument. Notice
that the streamer intensity is about
50 times higher than that of the coronal hole observed in 1982.
Table 10. Streamer Intensities
* (ph s
cm
sr
)
The OVI intensities provided in Table 10 were
derived from the calculated OVI values for a coronal
hole, which are in Table 3,
and Ly-
intensities and PB values from Spartan.
The prescription was to scale the resonantly scattered
component of OVI in the hole at 1.5 solar radii
by the streamer to hole Ly-
ratio to obtain the
streamer OVI resonant
component at 1.5 solar radii, and scale the
collisional component by the square of Ly-
. Then scale
the resonant component by the PB ratio to obtain
the values for other heights. The collisional
components are scaled by PB squared. The above
approach is based on an assumption that the resonantly
scattered Ly-
and O VI intensities are
proportional to the electron density in regions of
negligible Doppler dimming, and that the
collisional component of O VI is proportional to the square
of the electron density. It is also assumed that
PB is proportional to the electron density. The coronal
hole values at larger heights were not
used in the above because Doppler dimming is expected to
have a large
effect on the resonantly scattered components. (An alternative would have been to scale from the
calculated O VI collisional components at larger heights.)
The above method ignores changes in ionization
balance between the hole and
the streamer and between the base of the streamer and larger heights.
It also ignores changes in the H and O abundances.
Obviously, the above estimates are only a rough approximation.