Evidence for nonuniform heating of coronal loops inferred from multithread modeling of TRACE data

The temperature T-e(s) and density structure n(e)(s) of active region loops in EUV observed with TRACE 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, modified by Serio et al. for gravitational stratification (called RTVSp in the following). In a recent Letter, Reale & Peres demonstrated that this method can explain the almost isothermal appearance of TRACE loops (observed by Lent et al.) as derived from the filter-ratio method. From model-fitting of the 171 and 195 Angstrom fluxes of 41 loops, which have loop half-lengths in the range of L = 4-320 Mm, we find that (1) the EUV loops consist of near-isothermal loop threads with substantially smaller temperature gradients than are predicted by the RTVSp model; (2) the loop base pressure, p(0) approximate to 0.3 +/- 0.1 dynes cm(-2), is independent of the loop length L, and It agrees with the RTVSp model for the shortest loops but exceeds the RTVSp 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 approximate to 3 for the largest loops. The data indicate that cool EUV loops in the temperature range of T-e approximate to 0.8-1.6 MK cannot be explained with the static steady state RTVSp 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 +/- 6 Mm. This heating function provides almost uniform heating for small, loops (L less than or similar to 20 Mm), but restricts heating to the foot-points of large loops (L approximate to 50-300 Mm).