Study of the latitudinal dependence of H I Lyman α and O VI emission in the solar corona: Evidence for the superradial geometry of the outflow in the polar coronal holes

Abstract

We study the latitudinal distribution of the H I Lyman α and O VI (103.2 nm and 103.7 nm) line emission during the period of the Whole Sun Month campaign (August 10 to September 8, 1996) when the Sun was close to the minimum of its activity. The H I Lyman α and O VI line intensities appeared to be almost constant with latitude within the polar coronal holes and have abrupt increases toward the streamer region. We found that both north and south polar coronal holes had similar line intensities and line‐of‐sight velocities, as well as kinetic temperatures of H0 and O5+. The dependence of these parameters on latitude and radius is provided. We derived boundaries of the polar coronal holes based on the H I Lyman α and O VI line intensity distributions for several days during the Whole Sun Month campaign. We found that the polar coronal hole boundaries clearly have a superradial geometry with diverging factor ƒmax ranging from 6.0 to 7.5, and they are consistent with boundaries previously derived from the electron density distributions. We also found that, in general, they are not symmetric with respect to the heliographic poles, and their size and geometry change over periods of days. The H I Lyman α, O VI (103.2 nm), and the O VI (103.7 nm) line intensities showed similar boundaries within the uncertainties of our data. We modeled the latitudinal distribution of the H I Lyman α and O VI (103.2 nm and 103.7 nm) line intensities in the south polar coronal hole on August 17, 1996, assuming the coronal plasma outflow along either purely radial or nonradial flux tubes. A comparison of model predictions with the observed distributions shows evidence that the outflow velocity vectors follow nonradial intensity pattern.