The Effect of Solar Wind Expansion and Nonequilibrium Ionization on the Broadening of Coronal Emission Lines

Abstract

When observing spectral lines in the optically thin corona, line-of-sight (LOS) effects can strongly affect the interpretation of the data, especially in regions just above the limb. We present a semiempirical forward model, called the Global Heliospheric Optically thin Spectral Transport Simulation (GHOSTS), to characterize these effects. GHOSTS uses inputs from several other models to compute nonequilibrium ionization states (which include the solar-wind freezing-in effect) for many ions. These are used to generate ensembles of simulated spectral lines that are examined in detail, with emphasis on (1) relationships between quantities derived from observables and the radial variation of the observed quantities, (2) the behavior of thermal and nonthermal components of the line width, and (3) relative contributions of collisionally excited and radiatively scattered photons. We find that rapidly changing temperatures in the low corona can cause ion populations to vary dramatically with height. This can lead to line width measurements that are constant with height (a “plateau” effect) even when the temperature is increasing rapidly, as the plane of sky becomes evacuated and the foreground/background plasma dominates the observation. We find that LOS effects often drive the velocity width to be close to the plane-of-sky value of the wind speed, despite it flowing perpendicularly to the LOS there. The plateau effect can also cause the nonthermal component of the line width to greatly exceed the solar wind velocity at the observation height. Lastly, we study how much of the LOS is significant to the observation, and the importance of including continuum in the solar spectrum when computing the radiatively scattered emission.