COMPOSITION OF CORONAL STREAMERS FROM THE SOHO ULTRAVIOLET CORONAGRAPH SPECTROMETER

The Ultraviolet Coronagraph Spectrometer on the SOHO satellite covers the 940-1350 Angstrom range as well as the 470-630 Angstrom range in s econd order. It has detected coronal emission lines of H, N, O, Mg, Al , Si, S, Ar, Ca, Fe, and Ni, particularly in coronal streamers. Resona nce scattering of emission lines from the solar disk dominates the int ensities of a few lines, but electron collisional excitation produces most of the lines observed. Resonance, intercombination and forbidden lines are seen, and their relative line intensities are diagnostics fo r the ionization state and elemental abundances of the coronal gas. Th e elemental composition of the solar corona and solar wind vary, with the abundance of each element related to the ionization potential of i ts neutral atom (First Ionization Potential-FIP). It is often difficul t to obtain absolute abundances, rather than abundances relative to O or Si. In this paper, we study the ionization state of the gas in two coronal streamers, and we determine the absolute abundances of oxygen and other elements in the streamers. The ionization state is close to that of a logT = 62 plasma. The abundances vary among, and even within , streamers. The helium abundance is lower than photospheric, and the FIP effect is present. In the core of a quiescent equatorial streamer, oxygen and other high-FIP elements are depleted by an order of magnit ude compared with photospheric abundances, while they are depleted by only a factor of 3 along the edges of the streamer. The abundances alo ng the edges of the streamer ('legs') resemble elemental abundances me asured in the slow solar wind, supporting the identification of stream ers as the source of that wind component.