Detection of Doppler velocity differences between ions and neutrals in an erupting prominence

We performed a spectroscopic observation of an erupting prominence occurred on the solar limb on 2015 May 8 in He i 7065 Å, O i 7772 Å triplet and Ca ii 8498 Å lines to investigate differences in the Doppler velocity between ions and neutrals in a plasma strongly accelerated by the Lorentz force. We found that the ion–neutral velocity difference between Ca ii and He i reached an order of 15 km s–1. On the other hand, the velocity difference between Ca ii and O i was significantly smaller than that between Ca ii and He i. This result can be interpreted as the formation of O i 7772 Å lines in the erupting prominence is mainly contributed by the recombination from O ii ions through charge transfer with hydrogen atoms, resulting in a behavior close to ions. According to an order estimate of the collisional friction among He i atoms and protons, the observed velocity difference between Ca ii and He i implies the acceleration of the eruption reaches about 150 times the solar gravity. We propose a new method to evaluate the ionization degree of hydrogen from the velocity differences observed in Ca ii, He i, and O i lines.