High Minor Ion Outflow Speeds in the Inner Corona and Observed Ion Charge States in Interplanetary Space

Abstract

Flow speeds recently derived from chromospheric/transition region and coronal observations indicate that the solar wind acceleration process takes place at heights in the solar atmosphere much lower than previously imagined. Doppler-dimming observations of the O VI λ1032 and λ1037 spectral lines show that this is not only true for the background electron-proton solar wind but also for minor ions. The limits that these observations place on the flow speed of the O⁺⁵ ions are at least a factor of 3-4 higher than the highest flow speeds expected from earlier minor ion studies. It is likely that other ions reach similar speeds in the inner corona.

The interpretation of in situ charge state observations needs to be reexamined in light of these new velocity measurements. Large flow speeds imply that some species may not be close to equilibrium in the inner corona at any distance. Given a large deviation from equilibrium, the use of charge state ratios to determine equilibrium temperatures may no longer be appropriate.

Using the newest available atomic data together with the most recent observational constraints on electron temperatures, electron densities, and flow speeds, we find models with high minor ion outflow speeds of the order of the O⁺⁵ outflow speed (about 130-230 km s^-1 below 3 R_⊙) that are consistent with charge state observations. We then discuss how minor ion charge state observations can be used in future observational and theoretical studies to place limits on solar wind acceleration and heating mechanisms.