The Solar Corona above Polar Coronal Holes as Seen by SUMER onSOHO

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Abstract
In order to address two of the principal scientific objectives of the Solar and Heliospheric Observatory (SOHO), studies of the heating mechanisms of the solar corona and the acceleration processes of the solar wind, we deduce electron temperatures, densities, and ion velocities in plumes and interplume regions of polar coronal holes using ultraviolet observations from SUMER (Solar Ultraviolet Measurements of Emitted Radiation) on SOHO. SUMER allows us to study the inner corona up to a distance of about 430,000 km above the limb, or r = 1.6 R☉ (solar radius) from the center of the disk. We find the electron temperatures, Te, to be less than 800,000 K in a plume in the range from r = 1.03 to 1.60 R☉, decreasing with height to ≈ 330,000 K. Near an interplume lane, the electron temperature is also low, but stays between 750,000 and 880,000 K in the same height interval. Doppler widths of O VI lines in plumes are narrower (ΔλD ≈ 150 mÅ, v1/E ≈ 43 km s-1) than in interplume lanes (≈ 190 mÅ, ≈ 55 km s-1). The thermal and turbulent ion speeds, v1/E, of Si7+ reach values up to ≈ 80 km s-1 in the darkest regions outside plumes above the coronal hole. This corresponds to a kinetic ion temperature of 1 × 107 K. A limit of ≈ 18 km s-1 for the bulk speed in plumes below r = 1.2 R☉ is deduced from O VI line shift measurements and consideration of the three-dimensional plume geometry (differential line-of-sight velocities ≤3 km s-1), whereas differential line-of-sight velocities of Mg8+ ions up to 34 km s-1 can be seen in dark regions.