Transition Zone Lines Research Articles

The intensities and profiles of XUV transition zone lines in a quiet sun region compared to a polar coronal hole

The intensities of XUV transition-zone lines from limb spectra of a quiet-sun region and a polar coronal hole are compared. The spectra were obtained with a slit spectrograph on Skylab and cover a region from -12 sec within the limb to 20 sec above it. The lines selected for comparison are formed at temperatures that range from 36,000 to 220,000 K. Lines of the higher-temperature ions, e.g. O v, are significantly less intense in the coronal hole, and lines of lower-temperature ions show little change. Profiles of selected optically thin transition-zone lines from the quiet-sun and coronal-hole spectra are also shown. The lines are broader than expected in ionization equilibrium, and bulk-motion velocities are deduced from the widths of the lines. There appears to be little, if any, statistically significant difference in the velocities obtained from the quiet-sun region and the coronal hole.

Limb-brightening curves of XUV transition zone lines in the quiet sun and in a polar coronal hole observed from SKYLAB

Solar limb-brightening curves are discussed for XUV spectral lines formed in the upper chromosphere and transition zone of a quiet region and a polar coronal hole. The spectra were recorded with a slit spectrograph on Skylab. The lines considered are emitted from ions formed within the temperature range from 10,000 to 220,000 K. The limb-brightening curves cover a region from -4 sec within the limb to +20 sec above it. The data from 0 sec to +20 sec are compared with predictions based on both homogeneous and inhomogeneous models of the transition zone. The limb-brightening curve of the O I line at 1355.6 A indicates that O I is formed in spicules. The limb brightening of the He II line at 1640.4 A is consistent with a temperature of formation between about 40,000 and 90,000 K for He II.

Ultraviolet Emission Line Profiles of Flares and Active Regions

A preliminary description of ultraviolet spectra of active regions and flares, photographed from Skylab by the Naval Research Laboratory's UV Spectrograph is given. The findings can be summarized as follows: (1) Line profiles of medium ionized lines (transition zone lines) show the most pronounced broadenings and shifts in flares and flare like events. (2) Typical full width at half maximum of these lines correspond to Doppler-velocities of 70 km s-1. (3) Shifts of the same magnitude can be observed. (4) Intersystem lines are not broadened nor shifted. (5) Forbidden coronal lines and intersystem lines become enhanced in the flare spectrum at the moment, when the turbulence seen in the allowed transitions, disappears. (6) A very broad line at 1354.2 Å which appears only in flare spectra, seems to be the forbidden transition 3P1 – 3P0 of Fe XXI.

Flare-Like Ultraviolet Spectra of Active Regions

SummarySome characteristic features of UV spectra which can be seen in flares (Brueckner, this Symposium, p. 135) can also be detected in active regions without the presence of typical other flare phenomena like X-ray enhancement. Another distinct difference between these ‘flare-like’ spectra and flare spectra is the absence of very high temperature ions like Fe XXI in the ‘flare-like’ spectra. They occur in small areas and can be detected as ‘fluctuating Hα bright points’ in broad band Hα. Simultaneously, a strong UV brightening can be seen. As reported by the Skylab crews, these brightenings occur more frequently and quasi-periodic prior to flares. Their spectra show the very broad transition zone lines and often strong line shifts toward the blue or red. Figure 1 shows a selection of typical spectra. One notices a very asymmetric Lα profile in one case. Spectra prior to, during, and after a flare are reproduced in Figure 2. One recognizes that the transition zone instability started intermittently prior to the flare and could be seen during short time intervals long after the flare had ceased.

Observed heights of EUV lines formed in the transition zone and corona

Using high resolution (3–5″) rocket spectroheliograms obtained by NRL, we have analyzed hundreds of small, bright, quiet Sun features to determine the heights of formation of five transition zone lines: He i 584, He ii 304, O v 630, Ne vii 465, and Mg ix 368. The results are in excellent agreement with theoretical model calculations.