The aim of this proposal is: i) to determine the coronal abundance of helium and ii) to investigate how the abundances of the main elements change in the outer corona, mainly within streamers above active regions, searching in particular for variations with first ionization potential (FIP).
The coronal helium abundance is uncertain, and the
He abundance measured in situ varies by
factors of 2 to 3. The He II
1085 
line is faint, but it can be used to
determine the abundance with a sufficiently long
integration. Alternatively, one might add up
a large number of exposures taken for other programs
if they include the 1085 line. The example for the
tables is a streamer at 1.5 
with a density
of 
cm
, 
K, and (conservatively) 
cm extent along the line
of sight. Higher temperatures are seen, and they would slightly
reduce the He II, O VI and Ly
intensities and
increase the Mg X and Si XII intensities.
The helium abundance can be determined from
the ratio of the 
1085 intensity to the white
light intensity with only a fairly weak temperature
dependence in the recombination and excitation rates
(roughly 
). An alternative is to measure
the ratio of I(1085) to Ly 
. The temperature can be determined
from the O VI, Mg X, N V and Si XII ratios for a
structure as bright as a streamer, or from an electron
scattering measurement. It is quite possible
that the He abundance in streamers is 2-3 times smaller
than average, because low helium abundances are measured in
the heliosheet. This will be an important comparison with CELIAS.
It may also be possible to detect significant increases in the ratio
of n(He
) to n(He
) during CME events. In situ measurements
of the solar wind show
that this ratio increases by 10
-- 10
during detections of
``non-compressive density enhancement" events.
Following a different approach, the elemental abundances in the corona can be measured using a best fit procedure to evaluate the differential emission measure (DEM) distribution. Line emission at different positions along the solar radius can monitor changes in the abundances when the DEM is evaluated at each height. Measurements should be performed between 1 and 2 solar radii. Several lines can be selected to achieve the purpose; e.g.: N V 1238.8, 1242.8, O VI 1031.9 (1037.6), Ne VI 1006.0, Si XII 499.4, S X 1196.0, Fe XII 1242.0 (1349.4), Al XI 550.0, Mg X 624.9. Nitrogen, Oxygen and Neon are high FIP elements, while Magnesium and Iron are low FIP.
The density and emission measure used for the estimates in the the first table and the photon fluxes in the second column are appropriate to a streamer. The intensities in the first column refer to the quiet corona at 1.5 solar radii.
Abundances of Elements in the Outer Corona
