DETERMINATION OF THE FLUX OF MATTER IN A POLAR CORONAL HOLE
VIA COORDINATED OBSERVATIONS BETWEEN SOHO AND EARTH
OBSERVATORIES DURING THE 1998 TOTAL ECLIPSE
S. Patsourakos - J.-C. Vial
Institut d'Astrophysique Spatial, France
Polar coronal holes represent the most convincing site of the origin of the
high-speed solar wind. A precise knowledge of the flux of matter at the
coronal base is extremely important in placing constraints on wind models.
The 1998 total eclipse provides a unique opportunity to put together Earth
observatories and SOHO in order to determine the flux of matter which
requires a knowledge of density and velocity of the coronal plasma.
White-light eclipse-observations still remain an excellent way to determine
the electronic density in the solar corona. Moreover the SOHO's SUMER
spectrometer provides the spectral resolution suited for high resolution
The main characteristics of the SUMER sequence are the following:
(1) 1X300 " slit placed just above the solar limb
(2) spectral band 1025-1045 Å
(3) exposure time 100 sec
(4) line of interest O VI 1031.92 Å.
Our proposed sequence will start and end with the acquisition of two 10
min-reference spectra (covering all the spectral band given above) in order
to derive the wavelength calibration. The sequence's main part will be made
of 100 sec exposures delivering 5 spectral lines OVI 1031.92 Å , Ly 1025.72
Å , OVI 1037.61 Å , CII 1036.33 Å and Fe III 1038.35 Å and it will last in
total for 10 minutes. In this way we will determine the line of sight
velocities for the O VI (2.9 x 10^5 K) line by using the singly ionized
Oxygen lines as well as the Ly-beta line as wavelength tracers. Moreover
the high exposure times used will provide high signal to ratio spectra suited
for further analysis. The contribution of stray-light to the observed signal
up to 1.3 R_sun remains rather at exploitable levels thanks to SUMER's good
scattering characteristics. The combination of the l.o.s. velocities and the
electronic densities will put a lower limit to the flux of matter.