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Astron. Astrophys. 345, 1011-1018 (1999)

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Dynamics of solar active region loops

L.K. Harra-Murnion 1, S.A. Matthews 1, H. Hara 2 and K. Ichimoto 2

1 Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK (lkhm, sam@mssl.ucl.ac.uk)
2 National Astronomical Observatory, Mitaka, Tokyo 181, Japan (hara, ichimoto@solar.mtk.nao.ac.jp)

Received 18 November 1998 / Accepted 1 March 1999


It has been frequently observed that in solar active regions the measured line widths are larger than those based on thermal equilibrium widths. This excess width (characterised as non-thermal velocity, [FORMULA]) has been proposed as a signature of the heating mechanism. The behaviour of the [FORMULA] at coronal temperatures has not produced consistent results with values ranging from 0 to 100 km s-1. We investigate this problem by using joint observations from Norikura Solar Observatory, Japan and the Coronal Diagnostic Spectrometer (CDS) on SOHO. With CDS we determine temperatures, and with Norikura we obtain accurate line widths for Fe XIV ([FORMULA]2 MK). We find that values of [FORMULA] range between 10-20 km s-1. We analyse the dynamical behaviour of different temperature loops and find that for loops with Log [FORMULA] 5.8, the variability [FORMULA] [FORMULA]. This is comparable to the dependence of non-thermal velocity on temperature which we have determined to be [FORMULA] [FORMULA] [FORMULA]. This suggests that the excess line broadening is caused by highly dynamical behaviour in the transition region for active regions. These results have significant implications for potential heating mechanisms and these are discussed.

Key words: Sun: corona – Sun: transition region – Sun: UV radiation – Sun: X-rays, gamma rays

Send offprint requests to: L.K. Harra-Murnion

© European Southern Observatory (ESO) 1999

Online publication: April 28, 1999