Comparison of Empirical Models for Polar and Equatorial Coronal Holes

Abstract

We present a self-consistent empirical model for several plasma parameters of a large equatorial coronal hole observed on 1999 November 12 near solar maximum. The model was derived from observations with the Ultraviolet Coronagraph Spectrometer on the Solar and Heliospheric Observatory. In this Letter, we compare the observations of O VI λλ1032, 1037 emission lines with previous observations of a polar coronal hole observed near solar minimum. At the time of the 1999 observations, there was no evidence of large polar coronal holes. The resulting empirical model for the equatorial coronal hole describes the outflow velocities and most probable speeds for O5+, and we compared the derived ion properties with the empirical model for a solar minimum polar coronal hole. The comparison of the empirical models shows that the 1999 equatorial hole has lower O5+ outflow speeds and perpendicular temperatures than its polar counterpart from 1996 to 1997 at heights between 2 and 3 R☉. However, in situ asymptotic speeds of the wind streams coming from the 1996-1997 polar hole and from the 1999 equatorial hole are only ~15% different. Thus, the bulk of the solar wind acceleration must occur above 3 R☉ for the equatorial coronal hole. The equatorial hole also has a higher density than the polar hole at similar heights. It is not yet known whether the higher densities are responsible for the seeming inhibition of the fast ion outflow speeds and extremely large perpendicular temperatures that occur in polar coronal holes at solar minimum. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.