Equator-pole differences in the solar chromosphere from Lyman-continuum data

From the analysis of OSO-4 Lyman-continuum spectroheliograms, differences in the brightness and the color temperature between the poles and the equator have been found. These differences are interpreted as resulting from a lower chromospheric density at the poles than at the equator. Two models, one for the poles and one for the equator, giving temperature and density as a function of height, explain the observations. The poles have a lower density and a smaller temperature gradient than the equator does. The differences begin in the middle chromosphere and increase toward the transition zone.