The Ultraviolet Coronagraph Spectrometer (UVCS) for the Solar
and Heliospheric Observatory (SOHO) mission is designed for
ultraviolet spectroscopy and visible light polarimetry of the
extended solar corona.
The primary scientific objective of the UVCS investigation is to
identify and understand the dominant physical processes controlling
the extended solar corona and the generation of the
solar wind, and to understand the relationship of such processes to solar wind
properties near the Earth.
To progress toward this goal, UVCS is designed to obtain a detailed
description of the physical conditions in the extended corona. Ultraviolet spectroscopic techniques are designed to determine the
random velocity distributions, the densities and the outflow velocities of the
primary particles (i.e., electrons and protons) and of several minor ions.
Chemical abundances as a function of height in the extended corona can be
derived. This
information can be obtained for the large-scale structures such as coronal
holes, helmet streamers and active regions, and for substructures of those
features such as
polar plumes, and the closed magnetic arch structures and
open current sheets of helmet streamers. UVCS can also make detailed
spectroscopic measurements of coronal mass ejections, and the shock
compression and heating of the ambient
coronal plasma through which they propagate.
Heliographic heights from the coronal base to as high as
12 R
can be observed by UVCS. The long duration of SOHO and its
continuous viewing of the Sun allows UVCS to determine the time evolution
of the observed coronal structures.
The primary scientific objectives of the UVCS instrument are:
To investigate mechanisms for accelerating the solar wind,
To identify the dominant acceleration mechanisms, for example, to determine the roles of thermal pressure gradients (Parker type wind), wave-particle interactions, and suprathermal electrons in accelerating the solar wind in different regions,
To test proposed mechanisms for accelerating heavy ions and producing variations in the chemical composition of the solar wind.
To investigate mechanisms for heating the coronal plasma,
To determine if dissipation of energy carried by MHD waves is a dominant source of plasma heating, particularly in magnetically open regions where heating by MHD waves is a strong candidate for coronal heating,
To distinguish mechanisms for heating ions from those heating electrons, particularly in regions where the coronal plasma becomes collisionless,
To determine the radial variation of the heating in a variety of structures in order to empirically constrain heating mechanisms ( e.g. power dissipated as a function of height, characteristic dissipation lengths, dependence of these on physical conditions in, and structure of, a variety of regions with magnetically open and closed configurations),
To locate and characterize coronal sources of the solar wind,
To use tomographic techniques on UVCS spectroscopic limb data to provide global maps of the temperature, density, flow velocity and particle flux (as a function of height) from the low corona, where most of the surface appears to be covered with closed magnetic structures, out to several solar radii above the surface where the magnetically open regions expand to occupy the entire volume and the outflow becomes predominantly radial. These global maps can be used to determine which coronal regions are associated with different types of flows measured far from the Sun by in situ techniques or radio scintillation techniques,
To provide other possible signatures for distinguishing between sources of low and high speed wind: temperatures, densities, and flow velocities of several types of ions and MHD wave amplitudes,
To determine the role of internal structures in coronal holes in generating high speed solar wind. Polar plumes are raylike structures in polar coronal holes which contain a significant fraction of the mass in these regions and hence could be a source of the solar wind outflow from polar regions. The UVCS has the spatial resolution and spectroscopic diagnostic capability for testing this hypothesis.
To investigate coronal phenomena that establish the plasma properties of the solar wind,
To study the physics of the high temperature coronal plasma as it makes a transition from collision-dominated to collisionless conditions. The solar corona is a useful laboratory for studying this fundamental phenomenon. For many processes the corona, which is collision-dominated near the coronal base, becomes collisionless several tenths to several solar radii above the surface,
To acquire critical data on the ionization balance (temperatures, densities,
flow velocities of the electron/proton plasma and heavy ions, and heavy
ion abundances) in the region where the solar wind ionization states
are frozen in, 
.
These data, in combination
with those acquired by in situ instruments on SOHO, will provide
unique, fundamental data for investigating processes controlling the
distribution of heavy ion charge states in the solar wind,
To determine chemical abundances (O, Si, Mg and Fe) and determine their variations in the inner solar wind. Determination of the spatial and temporal variations of the chemical composition at the coronal source of the solar wind is vital to understanding the mechanisms producing the large variations in abundances measured in situ far from the Sun.