Jorge Enrique Vernazza

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

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.

The quiet sun in the extreme ultraviolet

Observations of the quiet Sun with the Harvard extreme ultraviolet spectrometer on the Skylab mission are reported for the chromosphere, transition region, and corona. The changing structure of the network is examined over the temperature range from 104 to 1.5 x 106 K, and the distribution of intensities in the cell interiors and the network examined from the standpoint of creating characteristic models. Observations of traces of periodic oscillations at 300 s in the cells for T ⩽ 2 x 104 is reported together with the absence of any periodic contributions at higher temperatures or periodic effects at any height in the network elements. Frequent non-periodic brightenings are observed, however, and their characteristics discussed. Observations of the increased thickness of the transition region in coronal holes, as well as other properties are discussed in limited detail. Observations of the centre-to-limb behaviour of transition region and coronal lines are used to construct coronal models, and the assumptions of spherical symmetry evaluated from the intensity data. The effects of spicules on the limb and disk data are discussed in relation to the observations.

Active region flare rates and 8.6 mm brightness temperatures

The relationship between the flaring rates and 8.6 mm brightness temperatures of active regions has been analyzed. We find that as the 8.6 mm brightness temperature of an active region increases, a larger proportion of the energy released by the region in the form of flares is contained in progressively larger flares. At all temperatures subflares are the most frequent event. At intermediate and high temperatures about 10% of the events are flares of importance 1 or larger with flares of importance 2 or larger contributing an increasing fraction of these events as the brightness temperature increases.