Evidence for Nonuniform Heating of Coronal Loops Inferred from Multithread Modeling of TRACE Data

Abstract

The temperature and density structure of active region loops in EUV observed with T RACE T e (s) n e (s) is modeled with a multithread model, synthesized from the summed emission of many loop threads that have a distribution of maximum temperatures and that satisfy the steady state Rosner-Tucker-Vaiana (RTV) scaling law, modiÐed by Serio et al. for gravitational stratiÐcation (called in the following). RTVS p In a recent Letter, Reale & Peres demonstrated that this method can explain the almost isothermal appearance of T RACE loops (observed by Lenz et al.) as derived from the Ðlter-ratio method. From model-Ðtting of the 171 and 195 Ñuxes of 41 loops, which have loop half-lengths in the range of A Ž L \ 4È320 Mm, we Ðnd that (1) the EUV loops consist of near-isothermal loop threads with substantially smaller temperature gradients than are predicted by the model ; (2) the loop base pressure, RTVS p dynes cm~2, is independent of the loop length L , and it agrees with the model for p 0 B 0.3 ^ 0.1 RTVS p the shortest loops but exceeds the model up to a factor of 35 for the largest loops ; and (3) the RTVS p pressure scale height is consistent with hydrostatic equilibrium for the shortest loops but exceeds the temperature scale height up to a factor of B3 for the largest loops. The data indicate that cool EUV loops in the temperature range of MK cannot be explained with the static steady state T e B 0.8È1.6 model in terms of uniform heating but are fully consistent with SerioÏs model in the case of RTVS p nonuniform heating with heating scale heights in the range of Mm. This heating (RTVS ph), s H \ 17 ^ 6 function provides almost uniform heating for small loops Mm), but restricts heating to the foot-(L [ 20 points of large loops (L B 50È300 Mm).