Emission Measure Distribution in Loops Impulsively Heated at the Footpoints

This work is prompted by the evidence of sharply peaked emission measuredistributions in active stars, and by the claims of isothermal loops in solarcoronal observations, at variance with the predictions of hydrostatic loopmodels with constant cross-section and uniform heating. We address the problemwith loops heated at the foot-points. Since steady heating does not allowstatic loop models solutions, we explore whether pulse-heated loops can existand appear as steady loops, on a time average. We simulate pulse-heated loops,using the Palermo-Harvard 1-D hydrodynamic code, for different initialconditions corresponding to typical coronal temperatures of stars ranging fromintermediate to active ($T \sim 3$--$10 \times 10^6$ K). We find long-livedquasi-steady solutions even for heating concentrated at the foot-points over aspatial region of the order of $\sim 1/5$ of the loop half length andbroader.These solutions yield an emission measure distribution with a peak athigh temperature, and the cool side of the peak is as steep as $\sim T^{5}$, incontrast to the usual $\sim T^{3/2}$ of hydrostatic models with constantcross-section and uniform heating. Such peaks are similar to those found in theemission measure distribution of active stars around $10^7$ K.Comment: 9 pages, 6 figures, accepted for publication in The Astrophysical Journa