Emission Line Density Diagnostics

Most emission lines are not sensitive to electron densities common in astrophysical plasmas because the radiative rates connecting the relevant levels are so large that an excited electron promptly transitions to the ground state, emitting photons in the process. However, if the radiative transition rate out of some level in the ion is low, and then electron density is sufficiently high, then collisional excitation or de-excitation may become competitive with radiative transitions.

The following tables list the density-sensitive lines in the LETG and HETG band, along with fits to the emissivivty as a function of the density for various temperatures.