SUMER Measurements of Nonthermal Motions: Constraints on Coronal Heating Mechanisms

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Abstract
We have determined nonthermal velocities in the quiet Sun at temperatures between 104 K and 2 × 106 K by measuring the widths of a number of EUV and far-ultraviolet (FUV) lines taken with SUMER on board the SOHO spacecraft. The broadenings owing to the SUMER instrument and the finite opacity in each line have been carefully examined. The nonthermal velocity at temperatures below 2 × 104 K is smaller than 10 km s-1. The velocity increases with temperature, reaches a peak value of 30 km s-1 around 3 × 105 K, and then decreases with the temperature. The coronal nonthermal velocity is about 20 km s-1. There exists a strong correlation between intensity and nonthermal velocity at temperatures 2 × 104-1 × 105 K. The correlation at higher temperatures weakens as temperature increases. Furthermore, there is a spatial correlation between the nonthermal velocities inferred from a set of any two lines with temperatures below 2 × 105 K. Neither significant center-to-limb variation nor meaningful dependence on the integration time was found from the measured nonthermal velocities. We have discovered the existence of high-velocity components in the observed S VI λ933.4 line profiles. The average nonthermal velocity and intensity fraction of this S VI line high-velocity component are found to be 55 km s-1 and 0.25, respectively. Observational characteristics of nonthermal motions carry some problems that should be solved when interpreting observed nonthermal motions in terms of either unresolved loop flows or Alfvén waves. The isotropic and very small scale nature of the observed nonthermal motions appears to be suited to the MHD turbulence interpretation of nonthermal motions. The turbulent heating rates inferred from the measured nonthermal motions can account for the radiative loss throughout the transition region and corona if the nonthermal motions are truly turbulent motions whose mechanical energy is injected at a scale of 1000 km (Kolmogorov-type turbulence) or 15 km (Kraichnan-type turbulence). The existence of high-velocity components at temperatures 6 × 104-2 × 105 K appears as observational evidence supporting nanoflare heating at these temperatures.