The objective of the UVCS investigation is to answer certain fundamental questions concerning acceleration processes in the corona and their relationship to solar wind properties near the earth. What are the dominant plasma heating and acceleration processes in the solar corona? What is the role of MHD waves? What are the dissipation mechanisms? How are heavy ions accelerated? How is the composition of the solar wind established and what produces the abundance variations seen in the solar wind? Where is the slow speed wind generated? Are coronal holes the only sources of non-transient high speed wind? Closely related to these questions are a variety of other problems. Why do some regions emit low speed wind and others high speed wind? What are the roles of thermal pressure gradients, wave-particle interactions and suprathermal particles in accelerating solar wind from different types of regions? Why are the particle fluxes in high and low speed winds similar? How does the coronal magnetic geometry affect the outflow of the solar wind? What is the role of small-scale structures (polar plumes, spicules) in generating high speed wind? What is the thermal state of gas expelled during coronal mass ejections and what are the plasma conditions in the associated shocks?
To answer such questions, we must determine the physical conditions in coronal regions where the solar wind is accelerated. The number of measured parameters must be sufficient to significantly constrain theoretical solar wind models. Although a substantial amount of data on the electron density structure of the corona already exist, there are only isolated measurements of other critical plasma parameters, except close to the surface (r < 1.3 ).
The UVCS instrument is designed to measure the primary plasma parameters of the solar corona from its base to as far as 12 . It is designed to measure signatures of both thermal and nonthermal processes:
Temperatures: electron temperature (), effective temperatures of protons () and several minor ions (, , , ). The effective temperature is defined as the second moment of the random velocity distribution. The line-of-sight velocity distribution is determined from line profile measurements, and it includes turbulent and wave velocities, as well as thermal motions. The combination of the various effective temperature measurements constrains the velocity amplitude of MHD waves and the population of suprathermal electrons.
Densities: electron and ion densities (, , , , , , ).
Flow velocities: of the electron/proton plasma () and ions (, , ).
UVCS can determine outflow velocities using Doppler shifts and Doppler dimming of spectral lines, and can measure coronal electron temperatures using the electron-scattered Ly- profile or the neutral fraction of hydrogen. Some of the techniques for determining the above parameters from the UVCS measurements are described in the papers listed in Section 11.
The primary scientific objectives of the proposed UVCS instrument are:
To investigate mechanisms for accelerating the solar wind,
.3in 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 distinguish between the effects of waves which damp in the lower corona (acoustic waves, fast mode MHD waves) and waves which survive to heat and accelerate the solar wind beyond the critical point ( e.g. Alfvén waves),
To investigate mechanisms for heating the coronal plasma,
.3in 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 between mechanisms for heating ions from those heating electrons, particularly in regions where the coronal plasma becomes collisionless (r ),
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,
.3in 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. In addition, to use UVCS disk data to provide maps of flow velocities and densities at the coronal base,
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,
.3in 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 (He, N, 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.
The UVCS is a versatile tool for solving these and other significant problems. It can investigate the physics of coronal streamers where transitions between magnetically open and closed structures occur, and where current sheets, which appear to extend far into interplanetary space, originate. It can provide for the first time detailed spectroscopic measurements of coronal mass ejections, the ambient coronal plasma through which they propagate, and shocks which develop in the corona. It can measure the ultraviolet fluxes of several bright stars to directly compare the ultraviolet intensities of the sun and stars with a single, absolutely calibrated instrument.
The UVCS instrument, combined with the other remote sensing instruments and the in situ experiments on SOHO, provides a unique opportunity for addressing these and other important problems.