MHD modeling of coronal loops: the transition region throat

Context. The expansion of coronal loops in the transition region may considerably influence the diagnostics of the plasma emission measure. The cross-sectional area of the loops is expected to depend on the temperature and pressure, and might be sensitive to the heating rate. Aims. The approach here is to study the area response to slow changes in the coronal heating rate, and check the current interpretation in terms of steady heating models. Methods. We study the area response with a time-dependent 2D magnetohydrodynamic (MHD) loop model, including the description of the expanding magnetic field, coronal heating and losses by thermal conduction, and radiation from optically thin plasma. We run a simulation for a loop 50 Mm long and quasi-statically heated to about 4 MK. Results. We find that the area can change substantially with the quasi-steady heating rate, e.g., by ∼40% at 0.5 MK as the loop tem- perature varies between 1 MK and 4 MK, and, therefore, affects the interpretation of the differential emission measure vs. temperature (DEM(T )) curves.