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Evidence for Nonuniform Heating of Coronal Loops Inferred from Multi-Thread Modeling of TRACE Data View all abstracts by submitter

Markus J Aschwanden   Submitted: 2000-05-26 13:04

The temperature Te(s) and density structure n_e(s) of active region loops in EUV observed with {sl TRACE} is modeled with a multi-thread model, synthesized from the summed emission of many loop threads that have a distribution of maximum temperatures and that satisfy the steady-state Rosner-T- Rosner-Tucker-Vaiana (RTV) scaling law, modified by Serio et al. for gravitatio- gravitational stratification (called RTVS_p in the following). In a recent Letter, Reale & Peres demonstrated that this method can explain the almost isothermal appearance of TRACE loops (observed by Lenz et al.) as derived from the filter-ratio method. From model-fitting of the 171 and 195 ang fluxes of 41 loops, which have loop half lengths in the range of L=4-320 Mm, we find: (1) The EUV loops consist of near-isothermal loop threads with substantially smaller temperature gradients than predicted by the RTVS_p model, (2) the loop base pressure, p_0 approx 0.3pm 0.1 dyne cm-2, is independent of the loop length L, it agrees with the RTVS_p model for the shortest loops, but exceeds the RTVS_p model up to a factor of 35 for the largest loops, and (3) the pressure scale height is consistent with hydrostatic equilibrium for the shortest loops, but exceeds the temperature scale height up to a factor of approx 3 for the largest loops. The data indicate that cool EUV loops in the temperature range of Teapprox 0.8-1.6 MK cannot be explained with the static steady-state RTVS_p model in terms of uniform heating, but are fully consistent with Serio's model in the case of nonuniform heating (RTVSph), with heating scale heights in the range of s_H=17 pm 6 Mm. This heating function provides almost uniform heating for small loops (L lapprox 20 Mm), but restricts heating to the footpoints of large loops (Lapprox 50-300 Mm). Chromospheric upflows appear to be the most likely heating source of EUV loops.

Authors: Markus J. Aschwanden, Richard W. Nightingale, and David Alexander
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Publication Status: ApJ 541:1059-1077 (2000 October 1)
Last Modified: 2000-10-06 08:35
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