A theoretical study of the coupling between minor ions and protons in the solar transition region

Abstract

We have used a numerical model to calculate the flow velocities of six minor ion species, using two different background models for the fast solar wind, one with a slow expansion, and one with a very rapidly expanding geometry. By comparing the flow velocity of the minor ions and the protons, we have investigated the degree of coupling between minor ions and the background solar wind in the solar transition region.

Our results show that observations of the Doppler-shift of the ionic emission lines from C(3+), O(3+), O(4+), O(5+) and Ne(7+) reflect the outflow velocity of the background solar wind in the transition region quite well. We also find that the radiation from Mg(8+) may stem from a more extended height region, including regions of very high outflow. The significance of the contribution from regions of high outflow, to the total Doppler-shift of the Mg IX emission line, can be determined through a calculation of the spectral emission profile.

We have also compared the calculated outflow velocities with observations from ten different authors. There are significant discrepancies between calculated outflow velocities, and observations. We find that these discrepancies cannot be resolved by changing the heating of the minor ions without increasing the heating rates to very large values. Our results also indicate that large heating rates lead to a high abundance of low charge states in the solar wind. This is not consistent with observations of charge state densities in-situ. We believe that a background model with an intermediate expansion might provide a better fit to the observations. Finally, we have compared the formation temperatures obtained in the simulations with the ionization equilibrium formation temperatures. We find that the the deviation from ionization equilibrium is significant for all the ions in the rapidly expanding geometry, and for Mg(8+) in the slowly expanding geometry.