Steady and nonsteady siphon flow in hot coronal loops

Siphon flow in hot coronal loops is examined, in both its steady and dynamic states, in the latter case using a flux-corrected transport simulation. We find that such flows are inhibited by (i) low heating rates, (ii) high pressures (or equivalently densities), (iii) short loop lengths, and (iv) turbulence. In accordance with expectations, we find that small footpoint pressure asymmetries produce steady subsonic flow. However, the standard picture that larger values yield standing shocks is shown to be valid only for sufficiently high heating, long loops, or low pressure (and preferably, low turbulence as well). Values of these parameters more characteristic of active regions produce instead a quasi-periodic "surge" flow when the pressure asymmetry exceeds a critical value at which the temperature gradient at the inflow end reverses sign. These flows are normally subsonic, though examples can he found where the surge is supersonic for a part of each period. The difficulty of driving substantial siphon flows for realistic hot loop models is in accordance with the comparative rarity of observations of these flows. Also, the models indicate that the siphon flow should produce net blueshifts in transition region lines, contrary to observations of net redshifts, which also argues against widespread flows of this type.