A Systematic Survey of H I Lyα and O VI Doublet Profiles Observed in Polar Coronal Holes at Solar Minimum

Abstract

A systematic, comprehensive survey of H I Lyα and O VI doublet profiles in polar coronal holes is performed using spectroscopic observation by the SOHO Ultraviolet Coronagraph Spectrometer (UVCS) at solar minimum in 1996-1997. By decomposing coronal hole profiles into component Gaussians, we determine detailed height distributions of profile parameters for respective components. In particular, we show that for each of the O VI λλ103.2, 103.8 doublet lines, the narrow and broad components have competing intensities within heliocentric height 1.3-2.1 R_solar, and that the considerable profile change occurring in this height range is due to changes not only of the widths of individual components but also of the mutual proportion between the narrow- and the broad-component intensities. We also show that the ratios between the O VI doublet intensities, I_λ103.2/I_λ103.8, for the narrow-narrow and the broad-broad component pairs have contrasting values and exhibit opposing radial dependences; the O VI ratio between the former pair increases with height from 2 at height 1 R_solar to 4 at 1.7 R_solar, while that between the latter decreases with height from 2-3 at <1.8 R_solar to less than 1 at 2.5 R_solar. We discuss implications of our results for the interpretation of the observed broad widths of the O VI doublet.