S xi Emission Lines in Active Region Spectra Obtained with the Solar euv Rocket Telescope and Spectrograph (Serts)

Theoretical electron density sensitive emission line ratios involving a total of eleven 2s ²2p ²–2s2p ³ transitions in Sxi between 187 and 292 Å are presented. A comparison of these with solar active region observations obtained during rocket flights by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals generally good agreement between theory and experiment. However, the 186.87 Å line is masked by fairly strong Fexii emission at the same wavelength, while 239.83 Å is blended with an unknown feature, and 285.58 Å is blended with possibly Niv 285.56 Å. In addition, the 191.23 Å line appears to be more seriously blended with an Fexiii feature than previously believed. The presence of several new Sxi lines is confirmed in the SERTS spectra, at wavelengths of 188.66, 247.14 and 291.59 Å, in excellent agreement with laboratory measurements. In particular, the detection of the 2s ²2p ^{2 3} P ₁ –2s2p ^{3 3} P _0,1 transitions at 242.91 Å is the first time (to our knowledge) that this feature has been identified in the solar spectrum. The potential usefulness of the Sxi line ratios as electron density diagnostics for the solar transition region and corona is briefly discussed.     

Ultra-violet nightglow spectrum from 1900 Å to 3400 Å

Two spectra of the ultra-violet nightglow from 1900 Å to 3400 Å have been recorded by a fast wide-field spectrograph during balloon flights from Aire sur Adour, France on 15 September, 1969 and on 5 October, 1970. These two spectra are composed of theOi line at 2972 Å, of the molecular oxygen Herzberg band systemA ³ _u ⁺ X ³ _g ^– and of a molecular band system that seems to belong to the NO SystemX ² C ² X ⁺, situated at 1991 ű4 Å, 2060 ű4 Å and 2136 ű4 Å. Around 2540 Å, there is absorption by ozone at the altitudes at which the spectra were recorded (35 km and 40 km). We present our calculated value of ozone absorption at 35 km, and the zenith-horizon variation of the nightglow emission.     

Astrophysically useful radiative transition parameters for?the?e1Π–X1Σ+ and 1Σ+–X1Σ+ systems of zirconium oxide

The zirconium oxide (ZrO) is well known for its astrophysical importance. The radiative transition parameters that include Franck-Condon (FC) factor, r-centroid, electronic transition moments, Einstein coefficient, band oscillator strengths, radiative life time and effective vibrational temperature have been estimated for e ¹Π–X ¹Σ⁺ and ¹Σ⁺–X ¹Σ⁺ band systems of ⁹⁰ZrO molecule for the experimentally known vibrational levels using RKR potential energy curves. A reliable numerical integration method has been used to solve the radial Schr?dinger equation for the vibrational wave functions of upper and lower electronic states based on the latest available spectroscopic data and known wavelengths. The estimated radiative transition parameters are tabulated. The effective vibrational temperatures of these band systems of ⁹⁰ZrO molecule are found to be below 4200 K. Hence, the radiative transition parameters help us to ascertain the presence of ⁹⁰ZrO molecule in the interstellar medium, S stars and sunspots.     

The 3410 Å band of the PH molecule in the solar photospheric spectrum

Some weak unidentified solar photospheric lines in the wavelength range: (3400–3465) Å may be due to PH lines of the (0, 0) band of the PH(A ³ _i - X ^{3 -})system. These faint PH molecular lines have resulted an excitation temperature of the order of 4500 K. Using experimental lifetime data for PH in the A ³ _i state, an absorption oscillator strength f ₀₀ = 0.0075 is derived for the 3410 Å band of the PH (A ³ _i - X ^{3 -})system. Accurate line positions, oscillator strength and transition probability for the 4.4 fundamental rotation-vibration band of the PH molecule are obtained. A comparison of positions of some lines of the 4.4 band with those obtained on new tracings of high resolution solar spectra shows many coincidences with weak solar lines.     

Properties of the λ2200 Å interstellar absorber

A strong correlation between the equivalent widths of the 2200 Å absorption band in the interstellar extinction curves and the colour excessesE _B-V of the stars was found. The oscillator strength and the damping constant of the 2200 Å transition were estimated. From these findings some conclusions concerning the identification problem were drawn.     

Possibility of 24MgH+ in the solar atmosphere-high resolution rotation-vibration spectra

High resolution rotation-vibration spectra of v = 1 and 2 sequences of ²⁴MgH⁺ are obtained by a differencing process invoking the known structures of electronic bands observed in the region 214–340 nm. A reversal of R branch (band head formation) is noticed in each of the rotation-vibration bands. The J = 1 J = 0 transition is predicted to be at 376098, 365301, 354286 and 342981 MHz for v = 0,1,2 and 3, respectively, in the ground electronic state ¹⁺. The equilibrium rotational constants B _e, _e and r _e for the X ¹E⁺ state are found 6.4637cm^–1, 0.1899 cm^–1 1.6421 Å, respectively.On leave from Physics Department, Gorakhpur University, Gorakhpur, U.P., India.     

The relative intensities of CI lines in the solar EUV spectrum

The relative intensity of two Ci lines at 1993.6 Å and 1657.4 Å, observed in the limb spectrum of the sun, is a factor 2.6 × 10³ larger than that expected if both lines were optically thin. It is shown that the observed intensity ratio may be explained in terms of the transfer of photons from 1657.4 Å to 1993.6 Å due to a large optical depth in the line at 1657.4 Å. The observed upper limit on the relative intensity of two further lines at 1992.0 Å and 1657.0 Å has been used to show that the line at 1993.6 Å is optically thin. Hence it is shown that (1657.4 Å) = 1300, and (1993.6 Å) = 0.44. These values provide an independent evaluation of optical depths against which chromospheric models may be checked. Assuming a mean temperature of T _e = 8000 °K, and a mean scale height of 350 km, the optical depths lead to a mean hydrogen-particle density of N (H) = 1.4 × 10¹² cm^–3.     

The diatomic molecules BH,BN, and BO in sunspots and the solar abundance of boron

Absorption band spectra of BH and BO have been searched for and not found in spectra of sunspots.Electronic oscillator strengths are available only for the A ¹ -X ^{1 +} system of the BH molecule. The absence of the (0,0) band of BH at 4332 Å reflects a solar abundance of boron logA _B<2.5.The band spectra of BN are several orders of magnitude weaker in sunspots than those of BH and BO.Kitt Peak National Observatory Contribution No. 488.Visiting Astronomer, Solar Division, Kitt Peak National Observatory - Operated by The Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The Violet Opacity in the Red Peculiar Stars – III. Spectral analysis of the SC star GP Ori

Medium-resolution (1.5-2.5 Å) spectrum has been obtained for SC star GP Ori in a wide range of wavelengths from 3730 to 6250 Å. It is characterised mainly by strong atomic lines, as well as moderately strong bands of the CN violet system and CH (0,0) and (0,1) bands at 4315 and 4890 Å. Weak bands of C₂, C₃, and ZrO molecules are observed, however, no evidence is found for the presence of either SiC₂ (Merrill-Sanford bands) or YO. The most prominent atomic lines along with the NaID12 are those of CaII (K,H), CaI at 4227 Å, SrI at 4607 Å, SrII at 4077 and 4215 Å, and BaII at 4554 Å. H seems to be in emission.     

Absorption coefficients for the 727 nm band of methane at 77 K determined by intracavity laser spectroscopy

Methane spectral features in the visible to near-IR region are prominent in the spectra of the outer planets but laboratory data for the appropriate methane conditions are required to interpret the observational data. By use of the intracavity laser spectroscopy technique, a moderately high resolution (500,000) absorption spectrum of the 727 nm band of methane at 77 K is obtained. The methane absorption bands in the visible to near-IR region are very weak, but intracavity laser spectroscopy provides sufficient sensitivity to perform the measurements and to extract quantitative data for methane at low temperatures. Absorption coefficients are determined and are reported as averages at one Å intervals throughout the region 7127–7420 Å. By integrating over the band, an intensity of 753 cm^–1 km^–1 am^–1 is obtained. The results compare well with previous low resolution measurements on methane at room temperature, with gas phase results calculated using the absorption spectrum of liquid methane, and with absorption coefficients derived from methane features observed in the spectra of the outer planets and Titan.     

On the far-ultraviolet radiation of the T Tauri-type stars

It is shown that the far-ultraviolet radiation (shorter than 2000 Å) discovered by ANS observations in the few T Tauri-type stars does not have any relation to the two-photon emission of hydrogen, as suggested by some authors. This is obtained from the observational data of the numerical values of the ratioQ ^*(2q)/Q ^*() for these stars, whereQ ^*(2q) is the complete number of the observed 2q-photons andQ ^*() is the number of observedH-photons. The observational values ofQ ^*(2q)/Q ^*() for four T Tauri-type stars turned out to be in the region of 20–90, while the theoretical value of this relation is 6. Hence, the observed fluxes in the region <2000 Å are 3–15 times larger than the theoretically expected values.The emission discovered in the region <2000 Å is of non-thermal origin, and can be identified with high probability with thetransition radiation. The latter originates as a result of the electromagnetic interaction of so-called fast electrons (E1.5 MeV) with dust particles in the gas-dust clouds surrounding these stars. The theoretical spectral curves of the transition radiation, for a few values of the plasma frequency ₀ for the dust particles, are calculated taking into account also the self-absorption effect of the radiation in the cloud and the absorption in the interstellar medium. Qualitatively, these curves (Figures 2, 3 and 4) are in good accord with the observed spectral distribution curves for the T Tauri-type stars (Figure 1). In particular, in both cases a minimum of radiation flux occurs near to 2200 Å, and a maximum near 1800 Å.The starting point of our analysis has been the concept of the identity of the processes, non-thermal and non-stationary in character, taking place at the time of the flare phenomenon of UV Cet-type stars in one case, and at the generation of continuous emission and the excitation of the emission lines in T Tauri-type stars on the other. In the latter case, the T Tauri-type stars can be regarded aspermanently flaring stars, with a very high frequency of flare events.     

Spectrum of the Extreme Ultraviolet Nightglow as Measured by EURD

After 2 years of operation onboard the Spanish satellite MINISAT, EURDhas achieved an unprecedented success in the observation of the terrestrialnightglow in the EUV, covering a range of 350-1100 Å EURD hasprovided a total of more than 543 hours ofintegration in the long wavelength spectrometer and more than898 hours in the short wavelength one, allowing the achievement of the most detailed atlas of the terrestrial EUV nightglow ever obtained. We present here the spectra obtained, together with the identification of the lines, some of themdetected for the first time in the nightglow. These spectrarepresent an improvement in sensitivity of several orders of magnitude with respect to previous observations. It has beenpossible, for the first time, to identify the complete Lymanseries of atomic hydrogen, resolving up to Lyman-. It hasalso been possible to identify the helium Lyman- line at 537 Å andto detect other lines of the blended Lyman series of helium, at 515 and522 Å. The spectra clearly show the presenceof the OII lines at 617, 644, 673, and 718 Å, previouslyobserved in the dayglow but seen here for the first time in thenightglow. In addition to the recombination continuum of the atomicoxygen at 911 Å, two features of OI have been detected in thenightglow: the 3s ³D_o transition at 989 Å, previouslyobserved by Chakrabarti (1984) and the 2p³4s³S^o transition at 1040 Å, partially overlapped with Lyman , but clearly distinguishable from it. This feature has been seen for the first timein the terrestrial nightglow. The radiative recombination continuum of atomic oxygen at 911 Å, that wasabsent in the observations of the first year of operation of EURD,is now clearly visible. The reasons of the absence of the OIfeature during the first year of operation are still unknown.Anderson et al. (1976) also noticed a strong variation with time of this 911 Å emission.     

Diffuse component of the cosmic far UV radiation and interstellar dust grains

The diffuse far UV radiation ( 1350–1480 Å) observed in the sky region ofl ^II180°, 0°b ^II40° is analyzed in connection with the distributions of stars and dust grains as well as with optical properties of grains. Its intensity (starlight+scattered light) is about 6×10^–7 erg cm^–2 sec^–1 sr^–1 Å^–1 in the direction ofb ^II0° andl ^II180°. The latitude dependence of the intensity is in approximate agreement with the plane parallel slab model of the galaxy with a reasonable set of parameters. The interstellar scattering gives an albedo close to unity and forward phase function of about 0.6, which are not inconsistent with the model of interstellar grains of Wickramasinghe. The upper limit of the extragalactic UV is 2×10^–8 erg cm^–2 sec^–1 sr^–1 Å^–1 in the same region of wave-length.     

Extreme Ultraviolet Emission Lines of ca xv in Solar and Laboratory Spectra

New R-matrix calculations of electron impact excitation rates in Caxv are used to derive theoretical electron density diagnostic emission line intensity ratios involving 2s ²2p ²–2s2p ³ transitions, specifically R ₁=I(208.70 Å)/I(200.98 Å), R ₂=I(181.91 Å)/I(200.98 Å), and R ₃=I(215.38 Å)/I(200.98 Å), for a range of electron temperatures (T _e=10^6.4–10^6.8 K) and densities (N _e=10⁹–10¹³ cm^–3) appropriate to solar coronal plasmas. Electron densities deduced from the observed values of R ₁, R ₂, and R ₃ for several solar flares, measured from spectra obtained with the Naval Research Laboratory's S082A spectrograph on board Skylab, are found to be consistent. In addition, the derived electron densities are in excellent agreement with those determined from line ratios in Caxvi, which is formed at a similar electron temperature to Caxv. These results provide some experimental verification for the accuracy of the line ratio calculations, and hence the atomic data on which they are based. A set of eight theoretical Caxv line ratios involving 2s ²2p ²–2s2p ³ transitions in the wavelength range 140–216 Å are also found to be in good agreement with those measured from spectra of the TEXT tokamak plasma, for which the electron temperature and density have been independently determined. This provides additional support for the accuracy of the theoretical line ratios and atomic data.     

Extreme ultraviolet flashes of solar flares observed via sudden frequency deviations: Experimental results

Properties of solar-flare EUV flashes measured via a type of ionospheric event, called a sudden frequency deviation (SFD), are presented. SFD's are sensitive to bursts of radiation in the 1–1030 Å wavelength range. He ii 303.8 Å, O v 629.7 Å, HL 972.5 Å and C iii 977.0 Å have essentially the same impulsive time dependence as the 1–1030 Å flash responsible for SFD's. Soft X-rays (2–20 Å) and certain EUV lines have a much slower time dependence than the 1–1030 Å flash. Most SFD's have some fine structure, but marked quasi-periodicity in EUV flashes is quite rare. EUV flashes are closely associated with hard X-ray bursts, white-light emission, microwave radio bursts and small bright impulsive kernels in the H flare. The intensity of EUV flashes depends on the central meridian distance of the H flare location; the intensity decreases at the limb. The total energy radiated in the 10–1030 Å flash for the largest events observed is about 10³¹ ergs.     

Evidence for systematic flows in the transition region around prominences

The solar transition region in the neighbourhood of prominences has been studied from observations with the Ultraviolet Spectrometer and Polarimeter of NASA's Solar Maximum Mission satellite. Dopplergrams from observations of the transition-region lines Civ 1548 Å and Siiv 1393 Å, which are formed at about 10⁵ K, give velocity amplitudes typically in the range ± 15 km s^-1. Prominences are found to be located very close to dividing lines between areas of up- and down-draughts in the transition-region. The observed pattern suggests that the 10⁵ K gas flows take place within arcades of magnetic loops, which most likely are part of the supporting magnetic structure for the prominence matter. An additional band of blue-ward Doppler shifts is frequently seen close to quiescent prominences. This may be the source of outward flowing matter along the helmet streamers above filament channels.     

Solar flare X-ray spectra

Results are presented of an investigation of solar flare X-ray spectra in the region 1.70–1.95 Å, obtained aboard the Intercosmos-4 satellite during the maximum of solar activity (October–November, 1970). With the use of 6 high resolution spectra in the region 1.85–1.87 Å the identification of lines due to 18 transitions of 2p 1s type, consisting of the resonance, intercombination and forbidden Fe xxv ion lines and the satellite Fe xxiv lines has been performed. With the use of the recent laboratory data the averaged wavelengths of the lines were obtained confirming the theoretically calculated ones with an accuracy about ± 0.0004 Å. A variable Doppler shift of the Fe xxv resonance lines was observed for the flare of November 16, 1970, which points to hot plasma motions with velocities up to 400 km s^-1.     

Solar X-ray spectra observed from the ‘Intercosmos-4’ satellite and the ‘Vertical-2’ rocket

Results are given of the detailed analysis of fourteen Fe xxv-xxiii lines ( = 1.850–1.870 Å) in the spectra of a solar flare on 16 Nov. 1970. The spectra were obtained with a resolution of about 4 × 10^–4 Å, which revealed lines not previously observed and allowed the measurement of line profiles. The measured values of the wavelengths and emission fluxes are presented and compared with theoretical calculations. The analysis of the contour of the Fe xxv line ( = 1.850 Å) leads to the conclusion that there is unidirectional macroscopic gas motion in the flare region with the velocity (projection on the line of sight) ± 90 km s^–1.Measurements of the 8.42 Å Mg xii and 9.16 Å Mg xi lines in the absence of solar flares indicate prolonged existence of active regions on the solar disk with T _e = 4–6 × 10⁶K and emission measure ME 10⁴⁸ cm^–3. The profile of the Mg xii line indicates a macroscopic ion motion with a velocity up to 100 km s^–1.     

On the excitation of lower levels of singlet helium in quiescent prominences

The paper deals with the excitation of the helium singlet level 2¹ P in the homogeneous and filamentary models of quiescent prominences with following parameters: the optical thickness at the limit of helium Lyman continuum _1c ^M = 0.1–100, T _e = 7000 K, n _e = 5 × 10¹⁰ cm^–3. Assuming a model He atom with seven discrete levels (1¹ S, 2³ S, 2¹ S, 2³ P, 2¹ P, 3³ D, 3¹ D) and the continuum the steady state equations for the levels 2³ S, 2¹ P and the continuum have been solved together with the radiative transfer equations for the line 584 Å and the continuum 504 Å. The variations with depth of the functions n ₂ ³ _S /n ₁ ¹ _S (₁ c), n ₂ ¹ _P /n ₁ ¹ _S (_1c ), and n ⁺ _He n _e /n ₁ ¹ S(_1c ) as well as the intensities of the triplet (D₃, 10830 Å) and singlet (16678, 20581 Å) lines have been calculated. Comparison with observations leads to the following conclusions: (1) The line intensities calculated for filamentary models of prominences agree better with observations than those for homogeneous ones. (2) The helium level 2¹ P is excited by diffuse field 584 Å being formed by recombinations and spontaneous transitions 2¹ P – 1¹ S and escaping from the prominence into the space between the filaments and to the surface. (3) Underpopulation of the singlet level 2¹ P may be explained by combination of weak excitation mechanism (recombinations and formation of the diffuse field 584 Å) and strong deexcitation mechanism (spontaneous transitions into the level 1¹ S).     

The ultraviolet absorption band at 2175 Å: Correlations with other interstellar features

The hump in the ultraviolet part of the interstellar extinction curve is interpreted as a broad diffuse absorption band. Its equivalent width is estimated for 36 stars by means of OAO-2 data. The equivalent widths are correlated with the following parameters: colour excessE(B-V), colour excessE(B – V), depth of the band m _max, equivalent widths of the diffuse bands at 5780 and 6284 Å, and the column density of neutral hydrogenN _HI. The physical parameters half-width and oscillator strength of the band at 2175 Å are estimated.