Abstract
The fine structure in the flow field in the transition region above and surrounding a sunspot is determined fromCIV 1548 Å line profiles, observed with the High Resolution Telescope and Spectrograph (HRTS) during the Spacelab 2 mission. The observed line profiles show one, two, or three distinct velocity components within the resolution element of 1″ × 1″. Supersonic flows occur in small regions where the line profile has two or three components. The line component that shows supersonic speed often is weaker than the subsonic line component, which may explain why some observers have been unable to detect the supersonic flow component. The broadening of individual line components shows non-thermal velocities close to 20 km s−1. This suggests that turbulence is less important than usually considered.
The presence of multiple flows, which also occurs in quiet solar regions, suggests that the transition region above sunspots has a sub-arc-second fine structure, perhaps consisting of thin fibrils. The Evershed flow in the transition region appears to have a correspondingly complex character, possibly consisting of sub- and supersonic siphon flows along the individual fibrils. Time changes in the flow field over 5 min may correspond to characteristic times of individual fine structures. Possible explanations of the net downward directed mass flux are presented.
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Kjeldseth-Moe, O., Brynildsen, N., Brekke, P. et al. Multiple flows and the fine structure of the transition region around sunspots. Sol Phys 145, 257–277 (1993). https://doi.org/10.1007/BF00690655
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DOI: https://doi.org/10.1007/BF00690655