OBSERVATIONAL TESTS FOR NONEQUILIBRIUM IONIZATION IN THE SOLAR CORONA

Nonequilibrium ionization may be produced by a variety of processes in the solar corona, for example, by mass flows through the large temper ature gradients of the transition region or by impulsive heating and c ooling. Any deviation from equilibrium ionization would have a strong effect on the radiation from the corona and on the interpretation of s olar observations; hence, it is important to determine observational s ignatures of nonequilibrium. The temperature-sensitive line ratios can be used as such signatures. We examine the line ratios: C IV I(1548.2 angstrom)/I(312.4 angstrom), O IV I(789.4 angstrom)/I(554.4 angstrom) , O V I(629.7 angstrom)/I(172.2 angstrom), O VI I(1031.9 angstrom)/I(1 73.0 angstrom) and O VI I(1031.9 angstrom)/I(150.1 angstrom). These li ne ratios are calculated for four coronal loop models that have a stea dy flow and that are known to have significant departures from equilib rium ionization. Our results indicate that, in general, nonequilibrium causes a considerable reduction in the line ratios, more than an orde r of magnitude in the downflowing leg of the loop model with the large st mass flows. We find that the C IV line ratio is the most sensitive to nonequilibrium. We discuss the implications of our results for obse rvations, specifically, the observations expected from the upcoming SO HO mission.