The goal is to measure the temperature of the upper corona as a function
of height above the solar limb, especially in open--field (coronal hole)
regions. Important questions regarding the role of coronal holes in the
acceleration of the solar wind demand better information on the variation
with height of the coronal temperature. Thermal models of the solar wind
acceleration require high temperatures at the base of the corona (
K). On the contrary, some recent models of the wind
acceleration propose direct transfer of momentum from Alfvén waves to
the medium, without dissipation; that is without requiring high
temperature. The temperature could reach a maximum value (
K) at
a certain height above the solar limb and then fall progressively at
greater heights. Thus the determination of the temperature gradient in the
1 R
-- 3 R
range of heliocentric distance becomes a
critical measurement to obtain constraints on the models of the solar wind
acceleration and contributes directly to one of the main objectives of the
SOHO mission: the understanding of the heating mechanisms of the corona and
the acceleration of the solar wind.
The temperature gradient can be determined by measuring the Doppler widths
of several spectral lines at various positions
above the limb. The combined observation of the resonantly scattered
Ly--
line and of lines emitted by ions
of different chemical elements (N V, O VI, Mg X, Si XII, Fe XII) can help
to correct the line--widths for the mass--motion component (waves,
outflows). Moreover, this combined observation can provide the kinetic
temperatures of electrons, protons and different ionic species, in order to
obtain a more complete knowledge of the thermal properties of the coronal
plasma outflowing from open field regions. In addition, it is useful to
sample various points along the entrance slit of the spectrometer, in order
to investigate possible differences in the thermal properties of
substructures (i.e., plumes) inside the same coronal hole. The wide
latitudinal extension of coronal holes makes this feasible.
Coronal holes can exist at the poles of the Sun, particularly around the
solar minimum when SOHO will be launched. Holes will also exist sometimes
at lower latitudes. However, such low latitude holes will generally have a
much smaller extent in the line of sight. In view of their low emissivity,
their brightness will normally be contaminated by non--hole regions in the
line--of--sight in front of, and behind, the hole. For reliable
observations it is advisable to concentrate on the polar holes, which will not
suffer from this line--of--sight effect, particularly for heliocentric
distances below 3 R
, as recently shown by Spadaro and Ventura
(1992). The morphology and evolution of the selected coronal hole can be
studied on the basis of the synoptic observations of the UVCS and the
images recorded by LASCO.
A detailed Joint Observing Program to measure the temperature gradient in coronal holes which make use of a combination of several SOHO instruments has been designed by the SOHO Science Working Team (SWT) (coordinator A.H. Gabriel).
The Temperature Gradient in Coronal Holes
