THE ELECTRON DENSITY IN EXPLOSIVE TRANSITION REGION EVENTS OBSERVED BY IRIS

We discuss the intensity ratio of the O IV line at 1401.16 angstrom to the Si IV line at 1402.77 angstrom in Interface Region Imaging Spectrograph (IRIS) spectra. This intensity ratio is important if it can be used to measure high electron densities that cannot be measured using line intensity ratios of two different O IV lines from the multiplet within the IRIS wavelength range. Our discussion is in terms of considerably earlier observations made from the Skylab manned space station and other spectrometers on orbiting spacecraft. The earlier data on the O IV and Si IV ratio and other intersystem line ratios not available to IRIS are complementary to IRIS data. In this paper, we adopt a simple interpretation based on electron density. We adopt a set of assumptions and calculate the electron density as a function of velocity in the Si IV line profiles of two explosive events. At zero velocity the densities are about 2-3 x 10(11) cm(-3), and near 200 km s(-1) outflow speed the densities are about 1012 cm(-3). The densities increase with outflow speed up to about 150 km s(-1) after which they level off. Because of the difference in the temperature of formation of the two lines and other possible effects such as non-ionization equilibrium, these density measurements do not have the precision that would be available if there were some additional lines near the formation temperature of O IV.