The Inability of Steady‐Flow Models to Explain the Extreme‐Ultraviolet Coronal Loops

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Abstract
Recent observations from the Transition Region and Coronal Explorer (TRACE) and the EUV Imaging Telescope (EIT) show that warm (T ≈ 1-1.5 MK) EUV coronal loops in active regions generally have enhanced densities, enhanced pressure scale heights, and flat filter ratio (temperature) profiles in comparison with the predictions of static-equilibrium theory. It has been suggested that mass flows may explain these discrepancies. We investigate this conjecture using one-dimensional hydrodynamic simulations of steady flows in coronal loops. The flows are driven by asymmetric heating that decreases exponentially along the loop from one footpoint to the other. We find that a sufficiently large heating asymmetry can produce density enhancements consistent with a sizable fraction of the observed loops, but that the pressure scale heights are smaller than the corresponding gravitational scale heights, and that the filter ratio profiles are highly structured, in stark contrast to the observations. We conclude that most warm EUV loops cannot be explained by steady flows. It is thus likely that the heating in these loops is time dependent.