Physical properties of a polar coronal hole from 2 to 5 R sun.

Observations with a white-light coronagraph aboard Skylab are used to determine the boundaries of a coronal hole in the northern polar region and the three-dimensional density structure within the hole between heights of 2 and 5 solar radii. The boundary of the hole is found to be essentially axisymmetric about the polar axis, nearly radial from 3 to 6 solar radii, and located near 25 deg latitude at these heights. The radiances arising from the hole are interpreted as resulting from an axisymmetric density distribution whose logarithmic radial gradient is independent of position within the hole and whose magnitude increases with angular distance away from the hole's axis. The velocity distribution within the hole is obtained from the continuity equation by assuming that the particle flux flowing outward in the hole is similar to that measured for high-speed solar-wind streams at 1 AU, and it is shown that the transition from subsonic to supersonic flow occurs between 2.2 and 3 solar radii.