Densities and Velocities in Fast Coronal Mass Ejections: Radiative Pumping of the O vi Doublet

Abstract

In very fast coronal mass ejections (CMEs), it is possible for O VI ions to scatter Ly$β$ photons that originate in the solar chromosphere and for the $λ$1037 transition of O VI to scatter $λ$1032 photons from the solar transition region. This scattering process can be identified by departures of the O VI I1032/I1037 intensity ratio from its collisional value of 2. We report the first detection of this effect in a CME that occurred on 2000 June 28, and we show that this pumping provides a density diagnostic for CMEs faster than 1600 km s-1. At heliocentric distances near 3 Rsolar, this diagnostic is useful for densities in the 5×105-107 cm-3 range.