Coronal Ionization by Two-Step Collision Processes.

We suggest that ionization by electron impact for some coronal ions may take place via the double process of collisional excitation to metastable levels, thence collisional ionization from these excited levels rather than by direct ionization from the ground state. The relative efficiency of this double-collision process depends primarily on the mean decay lifetimes for the excited levels. A specific computation based on estimated upper limits to the mean lifetimes of excited levels in the Fe x-Fe xv sequence shows a flatter spectrum of ion densities and a somewhat lower ionization temperature than is obtained from direct collisional ionization from the ground state, assuming that the abundance of Fe x equals that of Fe xiv. The two-step collision mechanism permits the possibility of a density dependence in the ionization equilibrium.