Solar prominences in the extreme ultraviolet as observed from the Apollo Telescope Mount

Abstract

Observations of quiescent solar prominences with the Harvard College Observatory spectrometer abroad Skylab show that prominence material is optically thick in the Lyman alpha line and the Lyman continuum. The color temperature of the Lyman continuum has a mean of 6600 K and an upward gradient toward the top of the prominence. The departure coefficient of the ground state of hydrogen is found to be of the order of unity as expected from theory.

The optical depth of the Ciii sheath region is determined directly from the observation of the limb through the prominence and used to infer the mean electron density and the temperature gradient of the sheath. The result implies that the sheath density is about 0.4, and the temperature gradient about 1.4 times the respective value in the Ciii transition zone of the quiet Sun.

The Ciii triplet-singlet ratio for the prominence is found to give a density compatible, within the uncertainty of the atomic parameters, with the density obtained from the optical depth.

The Oviλ1032 line, which is emitted by both the prominence and the surrounding corona, is used to obtain an estimate of the thickness of the outer transition sheath of the prominence.