Ultraviolet Coronagraph Spectroscopy: A Key Capability for Understanding the Physics of Solar Wind Acceleration

Understanding the physical processes responsible for accelerating the solar wind requires detailed measurements of the collisionless plasma in the extended solar corona. Some key clues about these processes have come from instruments that combine the power of an ultraviolet (UV) spectrometer with an occulted telescope. This combination enables measurements of ion emission lines far from the bright solar disk, where most of the solar wind acceleration occurs. Although the UVCS instrument on SOHO made several key discoveries, many questions remain unanswered because its capabilities were limited. This white paper summarizes these past achievements and also describes what can be accomplished with next-generation instrumentation of this kind. 1. Background and Motivation The hot, ionized outer atmosphere of the Sun is a unique testbed for the study of magnetohydrodynamics (MHD) and plasma physics, with ranges of parameters that are inaccessible on Earth. Although considerable progress has been made during the last few decades, we still do not know the basic physical processes responsible for heating the million-degree solar corona and accelerating the solar wind. Identifying these processes is important not only for understanding the origins and impacts of space weather (e.g., Schwenn 2006; Eastwood 2008), but also for establishing a baseline of knowledge about a well-resolved star that is directly relevant to other astrophysical systems. In order to construct and test theoretical models, a wide range of measurements of relevant plasma parameters must be available. In the low-density, open-field regions that reach into interplanetary space, more parameters need to be measured (in comparison to collision-dominated regions) because the plasma becomes collisionless. In other words, individual particle species—e.g., protons, electrons, helium, and minor ions— can exhibit different properties from one another. Such differences in particle velocity distributions are valuable probes of “microscopic” processes of heating and acceleration. In interplanetary space, such kinetic properties have been measured for decades by in situ particle instruments (e.g., Marsch 1999, 2006; Kasper et al. 2008). However, measurements in the near-Sun regions that are being actively heated and accelerated have been more limited in scope. Remote-sensing measurements of plasma properties in the so-called “extended solar corona” (above about 1.5 R⊙ measured from Sun-center) are difficult to make because the photon emission at large heights is fainter by many orders of magnitude than the emission from the solar disk. Standard telescopes, that do not explicitly block out the solar disk, typically contain enough scattered light from the disk to totally mask the faint off-limb emission. Because the corona is highly ionized, the dominant spectral features are at wavelengths accessible only from space. For these reasons, a series of Ultraviolet Coronagraph Spectrometer (UVCS) instruments have been built and flown on rockets, on a Shuttle-deployed Spartan payload, and as an instrument on the Solar and Heliospheric Observatory (SOHO) spacecraft; see, e.g., Kohl et al. (1978,