Modeling of Coronal EUV Loops Observed with TRACE : I. Hydrostatic SteadyState Solutions with Nonuniform Heating 

Markus J Aschwanden Submitted: 20000818 13:05
Recent observations of coronal loops in EUV wavelengths with the {sl Transition Region and Coronal Explorer (TRACE)} and the {sl Extremeultraviolet Imaging Telescope (EIT)} on the {sl Solar and Heliospheric Observatory (SoHO)} demonstrated three new results that cannot be explained with most of the existing loop models: (1) EUV loops are nearisothermal along their coronal segments, (2) they show an overpressure or overdensity compared with the requirements of steadystate loops with uniform heating, and (3) the brightest EUV loops exhibit extended scale heights up to four times the hydrostatic scale height. These observations cannot be reconciled with the classical RTV (Rosner, Tucker, & Vaiana) model, they do not support models with uniform heating, and partially even violate the requirements of hydrostatic equilibrium.  In this study we conduct numeric calculations of steadystate solutions of the hydrodyn hydrodynamic equations of mass conservation, momentum balance, and energy balance. We calculate some 500 solutions that cover a large parameter space of loop lengths (L approx 4300 Mm), of nonuniform heating functions (with heating scale heights in the range of {lambda}_H approx 1300 Mm), as well as the limit of uniform heating ({lambda}_H gg L). The parameter space can be subdivided into 3 regimes, which contain (1) solutions for stable loops, (2) solutions for unstable loops, and (3) no solutions. Short heating scale heights ({lambda}_{H, Mm} lapprox sqrt{L_{Mm}}) lead to unstable loops. Fitting the hydrostatic solutions to 41 EUV loops observed with {sl TRACE} we find that 60% are dynamically unstable, 30% are near a steadystate equilibrium, and 10% are cooling off. Those loops near steadystate are all found to be heated near the footpoints, with a heating scale height of {lambda}_H=12 pm 5 Mm, covering a fraction {lambda}_H/L=0.2pm0.1 of the loop length. None of the observed loops is consistent with a uniform heating function in steadystate. Because the observed heating scale heights are all found less than a factor of 2 within the instability limit, most of the coronal heating episodes seem to operate with relatively short heating scale heights, producing mostly dynamical dynamically unstable loops and transient brightenings.
Authors: Markus J. Aschwanden, Carolus J. Schrijver, and David Alexander
Projects:

Publication Status: ApJ, subm. (2000 Aug 7)
Last Modified: 20000818 13:05


