UVCS Observations of Velocity Shear at Streamer Boundaries in the Corona

Abstract

Measurements of the intensities of the O vi 1032 and 1037 Å spectral lines in the southern solar hemisphere, from 1.5 to 5 R _s ,were made with the SOHO Ultraviolet Coronagraph Spectrometer (UVCS) in May 2000 close to solar maximum. The ratio of the intensity of the two oxygen lines is used as a proxy for solar wind velocity in the inner corona. White light images of the corona taken with the SOHO/LASCO-C2 during the same time period, and used to place the UVCS observations in the context of coronal structures, show streamers extending to high latitudes. The measured radial and azimuthal gradients of the intensity ratio reflect strong velocity shears in the corona with the slowest wind coinciding with the axis of streamers. Comparison of these results with ratios measured with UVCS during solar minimum indicates that the transition from fast to slow wind in the inner corona occurs within 20° of the axis of streamers at both phases of the solar cycle.