Emission Measure Distribution in Loops Impulsively Heated at the Footpoints

This work is prompted by evidence of sharply peaked emission measure distributions in active stars and by the claims of isothermal loops in solar coronal observations, at variance with the predictions of hydrostatic loop models with constant cross section and uniform heating. We address the problem with loops heated at the footpoints. Since steady heating does not allow static loop model solutions, we explore whether pulse-heated loops can exist and appear as steady loops on a time average. We simulate pulse-heated loops, using the Palermo-Harvard 1-D hydrodynamic code, for different initial conditions corresponding to typical coronal temperatures of stars ranging from intermediate to active [T~(3-10)×106 K]. We find long-lived quasi-steady solutions even for heating concentrated at the footpoints over a spatial region of the order of ~1/5 of the loop half-length and broader. These solutions yield an emission measure distribution with a peak at high temperature, and the cool side of the peak is as steep as ~T5, in contrast to the usual ~T3/2 of hydrostatic models with constant cross section and uniform heating. Such peaks are similar to those found in the emission measure distribution of active stars around 107 K.