Coronal transients are the main evolutionary dynamical structures in the
extended solar corona. Such phenomena are associated with a variation of
the physical parameters (temperature, density and velocity) characterizing
the coronal plasma, occurring over typical time scales ranging from tens of
minutes to hours.
Coronal transients are often related to the expansion of dense plasma
clouds into the interplanetary medium, with outward velocities extending from
less than 100 km s
to more than 1000 km s
, and average values
around 500 km s
: in this case, they are referred as Coronal Mass
Ejections (CMEs). Typical total masses and kinetic energies
of CMEs are 10
- 10
gr and
10
- 10
erg, respectively (Gosling et al. 1976, Hundhausen et
al. 1984, Dryer 1982, Howard et al. 1985).
Presently, knowledge about coronal transients is mainly based on broad--band
white-light observations from satellite instruments (e.g., OSO-7 and Skylab
coronagraphs during solar cycle 20, P 78-1 Solwind, Solar Maximum Mission
C/P during solar cycle 21) in the range 1.6 - 10 solar radii (R
), and
on ground-based instruments in the range 1.2 - 2.2 R
(e.g., HAO
Mauna Loa K coronameter). While such observations allow investigation of the
location of the transient with respect to the solar disk, its morphology
and the temporal variation of the height above solar limb, they
give little information on the physical parameters of the perturbed
coronal region, which are actually poorly understood.
Detailed spectroscopic observations of coronal transients are needed in order to understand the physical properties of the plasma inside the perturbed coronal region, evaluate the influence of the propagation of the transient on the unperturbed coronal plasma (shock formation due to transient propagation, etc.), investigate the association between coronal transients and other forms of solar activity (e.g., prominence eruption and solar flares), and discriminate among the different primary mechanisms proposed to cause coronal transients, with particular reference to the role played by processes involving magnetic field restructuring in the corona, such as magnetic reconnection.
The UVCS primary FOV
(141 arcmin x 42 arcmin) allows coverage of the extended corona from 1.2 to
10 R
:
this range allows the study of coronal transient phenomena at different
heights in the solar atmosphere and/or at different evolutionary stages.
Observations will be performed in both the
HI Ly 
(1130 - 1300 Å) and O VI (930 - 1068 Å) channels, in order
to investigate systematically the dynamical and thermal properties of coronal
mass ejections and, in general, of coronal transients.
>From the analysis of line intensities and profiles, information concerning
plasma temperature and velocity can be obtained. From the line widths
the ``kinetic" temperature of the ions considered can be derived, while
information on the plasma velocity is obtained from the Doppler dimming of the
resonantly scattered line component.
In turn, temperature also affects line intensity, since a change in the thermal
status of the plasma produces a corresponding variation in the
ionization balance.
A detailed study of the CME phenomenon requires the use of a combination of several SOHO instruments (e.g., SUMER, CDS, UVCS, LASCO, EIT) according to the Joint Observing Program designed by the SOHO SWT (coordinator R.A. Harrison).
Coronal Transients in UV Spectral Lines
(1) estimate based on CME angular span
(2) from ground based WL coronagraphs
(3) from space WL coronagraphs