The Balmer 9 and Balmer 11 lines of He ii in the Sun

We identify the Balmer 9 and 11 lines of He ii at 959 Å and 942 Å in solar spectra. These lines are produced mainly by recombination following photoionization of He ii by coronal XUV radiation. From analysis of the line intensities, we confirm the theoretical model of Avrett et al. (1976), who found that an appreciable amount of He⁺⁺ is present at temperatures of 1–2 × 10⁴ K and that the anomalously strong He ii 304 line is produced primarily by collisional excitation. We also confirm the suggestion of Kohl (1977) that the photoionization-recombination process is more important in active regions than in the quiet Sun, and we find that the 304 line is produced largely by recombination in solar flares.     

Solar rotation as measured in EUV chromospheric and coronal lines

Active regions were followed across the disk on OSO 4 spectroheliograms in the Lyman continuum (LC) and in Mg x 625. These observations indicate differential rotation with latitude, but not with height in the atmosphere. The measured equatorial sidereal rotation velocity is 14.7° ±0.2° per day in both chromospheric LC and coronal Mg x, where the quoted error is the standard deviation of a least-squares fit to the data.     

Observational studies of the solar intensity profile in the far infrared and millimeter regions

Observations of the intensity distribution near the solar limb at 2.43 and 22.5 , show the absence of limb brightening to within 1 or 2 arc sec of the limb. Observations at 1.2 mm indicate limb brightening at this wavelength. These results are compared with the Utrecht Reference Photosphere and with existing data on the solar flux in the millimeter range, and suggest that the temperature minimum is broad and extends above ₅₀₀₀ = 2 × 10^–3. A sharp rise of temperature is required above ₅₀₀₀ = 10^–5.     

The extreme ultraviolet emissions of solar flares: A comparison between OSO-6 spectroheliograph observations and SFDs

The time structure and intensity of OSO-6 observations of EUV bursts were studied in relation to the corresponding 10–1030 Å enhancements deduced from SFD data. Impulsive EUV emissions from lines normally emitted from either the chromosphere or from the chromosphere-corona transition region rise simultaneously with the 10–1030 Å flash, to within the time resolution of the OSO-6 observations. Mg × 625 Å also showed concurrent impulsive emissions and a close intensity relation to the 10–1030 Å enhancement. The observational results are consistent with the hypothesis that most of the EUV radiation is being produced thermally in a region of chromospheric density, which is being heated by collisional losses of nonthermal electrons.     

Extreme ultraviolet observations of active regions in the chromosphere and the corona

New observations of solar active regions have been obtained by the Harvard College Observatory EUV spectroheliometer aboard the OSO-IV spacecraft. From the observations we have determined the enhancement in active regions of the emission from ions formed at various temperatures in the chromosphere and corona. The results are in accord with a simple model of active regions, for which the active region pressure is about 5 times the quiet sun pressure; the temperature gradient in the transition zone is about 5 times the quiet sun value; and the coronal temperature above active regions is slightly increased.     

Solar flares in the extreme ultraviolet

Solar-flare observations in the extreme ultraviolet (300–1350 Å) are reported. Some 269 flares observed by the Harvard College Observatory (HCO) experiment on OSO 4 and 211 flares observed by the HCO experiment on OSO 6 have been analyzed. The flares were observed in spectral lines and continua emitted by many ionic species over a temperature range from 10⁴ to 3.5 × 10⁶ K. The EUV data have been correlated with X-ray, H, and radio observations, and a significant number of EUV bursts not associated with reported H, X-ray, or radio bursts have been iden tified and investigated. The results indicate that these latter EUV events are less energetic by about a factor of 2 than EUV bursts associated with — F subflares.     

A geochemical and isotopic assessment of hydraulic connectivity of a stacked aquifer system in the Lisbon Valley,Utah (USA), and critical evaluation of environmental tracers

Hydrogeology Journal - This study investigates hydraulic connectivity of a stacked aquifer system in the Lisbon Valley of southeastern Utah (USA), within the Paradox Basin, where numerous faults...     

The Harvard-Smithsonian reference atmosphere

We present a model of the solar atmosphere in the optical depth range from ₅₀₀₀ = 10^–8 to 25. It combines an improved model of the photosphere that incorporates recent EUV observations with a new model of the quiet lower chromosphere. The latter is based on OSO 4 observations of the Lyman continuum, on infrared observations, and on eclipse electron densities.Our model differs from the Bilderberg Continuum Atmosphere (BCA) in the low chromosphere (₅₀₀₀ < 10^–4), where deviations from local thermodynamic equilibrium in hydrogen and carbon have been taken into account. It also differs in the transition region between the chromosphere and the photosphere (10^–4 < ₅₀₀₀ < 10^–2), where the temperature is lower than in the BCA, and in the convective region (₅₀₀₀ 2), where the temperature is higher than in the BCA.     

Coronal electron density maps for 7 March, 1970, derived from Mgx λ625 spectroheliograms

We have analyzed daily Mgx 625 spectroheliograms acquired by the Harvard College Observatory experiment on OSO-6 for a 28-day period centered on 7 March, 1970, the date of a well-observed total solar eclipse. These data are used to construct maps of the variation across the solar disk of the electron density at the base of the corona. The correspondence of high and low density regions with regions of enhanced and reduced emission in white light and Mgx pictures made during or near the time of the eclipse are described.