Physics – Plasma Physics
Scientific paper
Oct 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aps..dppfm1005c&link_type=abstract
American Physical Society, 43rd Annual Meeting of the APS Division of Plasma Physics October 29 - November 2, 2001 Long Beach, C
Physics
Plasma Physics
Scientific paper
Recent solar observations suggest that the solar outer atmosphere is turbulent. In the present talk, I focus on the SOHO/SUMER observations of non-thermal motions --- unresolved plasma motions that are required to explain the line width excess over thermal broadening --- of UV/EUV emitting plasma whose temperature ranges from 10^4 to 10^6 K. It has been found from a thorough investigation of the quiet Sun that the most probable speed of non-thermal motion depends on the formation temperature of the spectral lines: it is smaller than 10 km s-1 in the chromosphere (T<2 × 10^4 K), reaches a peak value of 30 km s-1 in the transition region ( 3 × 10^5 K), and decreases to about 20 km s-1 in the low corona (T>10^6 K). Non-thermal motions are essentially isotropic and remain unresolved at a spatial resolution of 1000 km and at a temporal resolution of a few tens of seconds. Moreover, a careful examination of profiles of the S VI line formed at 2 × 10^5 K has revealed that a fraction of the line-emitting plasma has non-thermal motions of 50 km s-1 or higher values. Non-thermal motions were also measured in each of several spatially resolved loops in an active region. Some loops have the most probable speed of 20 km s-1, others have a much bigger value of 50 km s-1, suggesting that non-thermal motion is an intrinsic property of a loop as temperature is. Non-thermal motions may represent either one of or a combination of three different kinds of physical processes: 1) a superposition of laminar flows along very small magnetic loops, 2) MHD waves, especially Alfvénic waves, and 3) MHD turbulence. Our results seem to be more compatible with the last explanation than the others, and to support the coronal heating by MHD turbulence.
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