We propose to investigate the physical properties of long-living, quasi-stationary coronal structures, known as coronal millimetre wave sources (CMMs).
While solar microwave radiation normally originates within only
2000 - 4000 km from the optical photosphere, by processes such as free-free
bremsstrahlung and gyro-synchrotron radiation, occasionally long lasting
brightenings are observed well beyond the solar limb, at heights ranging
from 5 10
km to more than 2 10
km (
1.1 - 1.3 R
). (e.g.,
Urpo et al. 1986, Kruger et al. 1989).
Typical life times of CMMs range from a few hours to more than one
day. These structures are found to occur in extended active regions and
appear sometimes to be related, both spatially and temporally, with other
phenomena of solar activity, such as long duration soft x-ray emission at
coronal level from quasi-stationary loop systems (LDEs), or with
non-stationary events such as solar flares, occurring some hours before,
and coronal transients.
Presently, the physical parameters of the plasma emitting at millimetric
wavelengths are poorly known. However, from the analysis of the
microwave spectrum inhomogenous source composition could be inferred
(Kruger et al. 1989), probably characterized by the presence of
different components in the
emitting plasma: a low temperature, high density optically thick component
(T 
10
K, n 
10
cm
), and a high
temperature, low density optically thin component (T 
10
K,
n < 10
cm
). The CMMs are thought to be significant
sources also at temperatures < 10
K and therefore a plasma
characterized by a similar temperature
and density distribution should, in principle, also radiate efficiently at
UV wavelengths.
At least those structures located at greater distances from the solar limb should be observed in the lower part of the UVCS FOV. A study of CMMs in the UV spectral region, based on total line intensity and profile analysis, could give complementary information with respect to radio data and help to define their density and temperature distributions unambiguously. This information can be useful in order to relate these phenomena to other coronal processes evolving more rapidly, such as flares, radio bursts and coronal mass ejections (CMEs). Representative values of the physical and geometrical parameters of CMMs, are deduced from a decade of radio observations.
Study of UV Line Emission from Coronal Millimetre Wave Sources (CMMs)
