Extreme ultraviolet spectra of solar active regions and their analysis

Extreme ultraviolet spectra of several active regions are presented and analyzed. Spectral intensities of 3 active regions observed with the NRL Skylab XUV spectroheliograph (170–630 Å) are derived. From this data density sensitive line ratios of Mg viii, Si x, S xii, Fe ix, Fe x, Fe xi, Fe xii, Fe xiii, Fe xiv, and Fe xv are examined and typically yield, to within a factor of 2, electron pressures of 1 dyne cm^–2 (n _e T = 6 × 10¹⁵ cm^–3 K). The differential emission measure of the brightest 35 × 35 portion of an active region is obtained between 1.4 × 10⁴ K and 5 × 10⁶ K from HCO OSO-VI XUV (280–1370 Å) spectra published by Dupree et al. (1973). Stigmatic EUV spectra (1170–1710 Å) obtained by the NRL High Resolution Telescope and Spectrograph (HRTS) are also presented. Doppler velocities as a function of position along the slit are derived in an active region plage and sunspot. The velocities are based on an absolute wavelength scale derived from neutral chromospheric lines and are accurate to ±2 km s^–1. Downflows at 10⁵ K are found throughout the plage with typical velocities of 10 km s^–1. In the sunspot, downflows are typically 5 to 20 km s^–1 over the umbra and zero over the penumbra. In addition localized 90 and 150 km s^–1 downflows are found in the umbra in the same 1 × 1 resolution elements which contain the lower velocity downflows. Spectral intensities and velocities in a typical plage 1 resolution element are derived. The velocities are greatest ( 10 km s^–1) at 10⁵ K with lower velocities at higher and lower temperatures. The differential emission measure between 1.3 × 10⁴ K and 2 × 10⁶ K is derived and is found to be comparable to that derived from the OSO-VI data. An electron pressure of 1.4 dynes cm^–2 (n _e T = 1.0 × 10¹⁶ cm^–3 K) is determined from pressure sensitive line ratios of Si iii, O iv, and N iv. From the data presented it is shown that convection plays a major role in determining the structure and dynamics of the active region transition zone and corona.     

Theoretical Neix line ratios compared to solar observations

R-matrix calculations of the 1¹S - 2³S and 1¹S - 2³P electron excitation rates in He - like Cv, Ovii, and Mgxi by Kingston and Tayal are used to interpolate results for Neix. Adoption of these in emission line strength calculations leads to values for the density-sensitiveR ratio very similar to those of Pradhanet al. and Wolfsonet al., although the temperature-sensitiveG ratios are approximately 10 to 20 % lower than those deduced by these authors. However the present theoretical value ofG at the temperature of maximum Neix emission,G(T _m) = 0.82, is in excellent agreement with those observed by the SMM and P78-1 satellites for the 1980, November5 flare (G = 0.83 ± 0.01) and nonflaring active regions (G = 0.80 ± 0.05), respectively.     

Theoretical emission line ratios for O vii in low-density plasmas

New electron excitation rates for O vii calculated by Tayal and Kingston using the R-matrix method are used to determine theoretical emission line strengths. Values of the electron density sensitive ratio R (forbidden line to intercombination line) are found to be very similar to those deduced by other authors. However the temperature sensitive ratios G (intercombination plus forbidden lines to resonance line) are approximately 20% lower than the best previous estimates. The observed value of G for solar active regions (G = 1.0 ± 0.1) predicts an electron temperature in the range 1.1 × 10⁶ K < T _e < 1.8 × 10⁶ K, which overlaps that of maximum O vii emissivity, T _M = 1.8 × 10⁶ K. In addition, the theoretical G versus T _e curve is in excellent agreement with that observed for a Tokamak plasma.     

Coronal survey in X-rays of O vii and Ne ix

We report some results of a rocket experiment flown on 29 April, 1971. A survey of the solar corona was carried out with a pair of collimated Bragg spectrometers to study the resonance, intersystem and forbidden line emission from the helium-like ions O vii (22 Å) and Ne ix (13 Å). In the direction of dispersion the collimator provided a field of view of 1.7. Also, the continuum radiation near 3 Å was monitored by a collimated proportional counter within a view angle of 4.2. The observed X-ray emission came from the general corona, seven plage regions, and one dynamic feature- the late stage of a small flare. From the intensity of the O vii and Ne ix resonance lines the electron temperature and emission measure of the individual emitting regions are derived on the basis of two models, one (a) in which the region is assumed to be isothermal and another (b) in which the emission measure decreases exponentially with increasing temperature. The latter model, which is the most adequate of the two, yields for the electron temperature of the time-varying feature 2–3 × 10⁶ K, for the other active regions 1.5–2.5 × 10⁶ K, and for the general corona 1.3–1.7 × 10⁶ K. The Ne ix emitting regions are about 1.5 times as hot as the O vii regions. The emission measure ranges from 0.4–2.3 × 10⁴⁸ cm^–3 for all active regions and is about 2 × 10⁴⁹ cm^–3 for one hemisphere of the general corona above 10⁶ K. From an analysis of the ratio, R, of the forbidden and intersystem lines of O vii we conclude that none of the regions producing these lines at the time of the rocket flight had electron densities exceeding about 3 × 10⁹ cm^–3. Our data demonstrate a dependence of R upon temperature in agreement with the theory of Blumenthal et al. (1971). The wavelengths for the intersystem, the 1s ²2s ² S ^e–1s2p2s ² P ⁰ satellite, and the forbidden transition show in the case of Ne ix improved agreement with predictions. The observed strength of the satellite lines for both O vii and Ne ix agrees with the predictions of Gabriel's (1972) theory, which attributes their formation to dielectronic recombination.We are saddened to report the death of A. J. Meyerott on 13 November, 1971.     

On the relative residual intensities of the calcium H and K lines

We have observed the solar Caii H and K lines to obtain well-calibrated ratios of their core residual intensities. From three independent calibrations, one using a standard lamp, we conclude that the residual intensity ratio r(K₃)/r(H₃) is 1.048 ± 0.03 in the quiet chromosphere and 1.20 ± 0.03 in a plage region. These ratios correspond closely to those observed in stars with quiet and active chromospheres, respectively. For a chromospheric model suggested by the calcium lines and a four-level Caii ion, we compute H and K line profiles varying the direct collisional coupling and indirect radiative and collisional coupling via the 3 ² D level. We conclude that enhanced chromospheric activity in the sun and late-type stars results more from a steepening of the chromospheric thermal gradient than from a change in density.Kitt Peak National Observatory Contribution No. 530.Of the University of Colorado and the National Bureau of Standards.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The far UV spectrum of the Be star AX Monocerotis

In this paper we study the main features of the far UV spectrum of the binary star AX Mon, observed with the IUE satellite at phase 0.568.Ions indicating a large range of ionization stages, going fromCi,Oi,Ni toSiv,Civ,Nv are present.The spectrum is dominated by shell absorption lines of Feii, Feiii, Siiii,Cii, Alii, Mgii and Niii.Two satellite components are clearly indicated in all these lines except for Niii which presents only one. Their mean velocities are +10±5 km s^–1, –75±10 km s^–1, and –260±15 km s^–1.Red emission wings are observed in the Mgii resonant doublet at 2800 Å, which shows a P Cygni profile. Each of the absorption lines of the Mgii doublet is formed by a narrow component, which is blended with the Mgii interstellar line and a broad component, which shows a complex structure.Broad and asymmetrical profiles are observed for the Siiv,Civ, andNv resonance lines with blue edge velocities about –700±30 km s^–1.     

The winds of WC10 central stars of planetary nebulae

We present the results of an analysis of the winds of two WC10 central stars of planetary nebulae, CPD-56°8032 and He 2-113. These two stars have remarkably similar spectra, although the former exhibits somewhat broader emission line widths. High resolution spectra (up to R=50 000) were obtained in May 1993 for both objects at the 3.9 m AAT, using the UCL Echelle Spectrograph. The fluxes in individual Cii auto-ionising multiplet components, many of which were blended, were derived. Lines originating from auto-ionising resonance states situated in the C²⁺ continuum are very sensitive to the electron temperature, since the population of the these levels is close to LTE. The measured widths and profile shapes of these transitions are presented and are in excellent agreement with those predicted on the basis of their calculated auto-ionising lifetimes. The wind electron temperature is derived for both stars from the ratio of the fluxes in four such transitions (T _e =18 500 K±1 500 K for CPD-56° 8032 andT _e =13 600 K±800 K for He 2-113). Optical depth effects are investigated using normal recombination lines to obtain an independent wind electron temperature estimate in excellent agreement with the dielectronic line analysis.     

EUV observations of the chromospheric network

Extreme ultraviolet observations of a quiet region of the Sun on August 18, 1969, with the Harvard spectroheliometer on OSO 6 indicate that the chromospheric network can be observed in lines of the chromosphere and transition region (T = 8.4 × 10⁵ K) with almost identical structure. At coronal heights, the network changes but some residual structure can still be discerned in Mgx and perhaps Sixii (T = 2.3 × 10⁶ K), although there is little or no evidence remaining in Fexvi (T = = 3.5 × 10⁶ K).     

Evolution du spectre de la Nova Delphini 1967 en 1971 et 1972

Continued spectroscopic observations of Nova Del 67 during 1971 and 1972 show a general decrease of the nebular and coronal emission with respect to the local continuum. The continuous spectrum exhibits a strong Balmer emission (figure 1). Equivalent widths of Hi, Hei, Heii, Oi, [Oi], Oii, [Oii], Oiii, [Oiii], Niii, [Sii], [Aiii], [Neiii], [Nev], [Fevi], [Fevii], [Fex], [Fexi], [Fexiv], [Nixvi], [Nixv] are listed in tables 1 and 2.Figures 5 to 10 show several line profiles, which can be interpreted in terms of a model proposed by Hutchings (1972). Kinematical properties of the nova envelope do not seem to have changed in the 1968–1972 time interval. However, a study of the [Oiii] (4959 Å) line indicates that the physical conditions in the polar blobs in 1972 (T _e=10 660 K,N _e=5,5×10⁵ cm^–3) are different from those prevailing in equatorial rings (T _e=9×100 K,N _e=7×10⁵ cm^–3).

---

---

Le matérial d'observation utilisé pour cette étude a été obtenu à l'aide de téléscopes de 120 cm, 152 cm et 193 cm de l'Observatoire de Haute-Provence (CNRS).     

On the continuum and the absolute magnitude of eta Carinae

The excitation temperature, colour excess and continuum of Car in the visible and near ultraviolet have been determined from the study of the excitation of the ionized iron emission lines. The excitation temperatures of Feii and [Feii] are about 8000K, value which is very much lower than the mean ionization temperature of the elements in Car.E _B-V is about 1^m.1; the absolute visual magnitude is presently –10^m.5±1^m. From the equivalent widths of the emission lines the true continuum between 1.5 and 3.0 ^–1 has been derived. The correspondingB-V is –0^m.14, while the Balmer jump is less than 0^m.5. The continuum appears mainly nebular in origin with a strong contribution of the two-photon emission from the 2s-level of hydrogen in the blue.     

Absolute redshifts in the C iv 1548 Å line in the transition region of the quiet Sun

Observations with the UVSP instrument on the SMM spacecraft were made at the polar limb and disk center for the accurate determination of Doppler shifts of the Civ 1548 Å emission line formed at 10⁵ K in the transition region of the quiet Sun. Individual data points representing 3 arc sec square pixels yield both redshifts and blueshifts, but the mean values from four different days of observations are toward the red. The mean redshifts are in the range 4–8 km s^-1 and are produced by nearly vertically directed flows; the uncertainty associated with the mean values correspond to ±0.5 km s^-1. The redshift increases with brightness of the Civ line.     

Intensity ratios of spectral cores Ca ii K to H across a sunspot

Ratios of the intensity in the core of the Ca ii K line to the intensity of the H line core across a sunspot (SPO 5007) were determined from measurements of spectra made simultaneously with the Echelle spectrograph at the Vacuum Tower Telescope, Sacramento Peak Observatory. The measured values averaged over the neighboring nonspot region, the penumbra and the umbra are found to be 1.13 ± 0.04, 1.19 ± 0.05, and 1.25 ± 0.03, respectively.This work was supported by the U.S.-Republic of Korea Cooperative Science Program (K-24).     

Morphology and spatial distribution of XUV and X-ray emissions in an active region observed from Skylab

We studied the morphology and spatial distribution of loops in an active region, using coordinated observations obtained with both the S082A XUV spectroheliograph and the S056 grazingincidence X-ray telescope on Skylab. The active region loops in the temperature range 5 × 10⁵ –3 × 10⁶ K fall basically into two distinctive groups: the hot loops with temperatures 2–3 × 10⁶ K as observed in coronal lines and X-rays, and the relatively cool loops with temperature 5 × 10⁵ –1 × 10⁶ K as observed in transition-zone lines (Ne vii, Mg ix). The brightest hot coronal loops in the active region are mostly low-lying, compact, closely-packed, and show greater stability than the transition-zone loops, which are fewer in number, large, and slender. The observed aspect ratio of the hot coronal loops is in the range of 0.1 and 0.2, which are almost two orders of magnitude larger than those for the Ne vii loops. Brief discussion of the MHD stability of the loops in terms of the aspect ratio is presented.     

A comparison between Mgii and Caii spectroheliograms

A detailed photometric comparison between a Mgii K filterheliogram and a nearly simultaneous Caii K spectroheliogram is presented. The comparison shows a close correspondence in both location and intensity of the bright features on the Sun with a correlation coefficient of 0.92 ± 0.02 between the Mgii and the Caii intensities in active regions.A small flare is most likely observed in the Mgii heliogram giving a substantial contribution to the recorded intensity.We also estimate theoretically the heights in the solar atmosphere at which the Mgii K and Caii K lines are formed. Taking into account the general shape of the line profiles and the different passbands used in the recordings we arrive at an average height of formation of 1700–1900 km above the photosphere for these particular heliograms.     

The Solar Ultraviolet Spectrum Estimated Using the Mg <Emphasis Type="SmallCaps">ii</Emphasis> Index and Ca <Emphasis Type="SmallCaps">ii</Emphasis> K Disk Activity

As part of a program to estimate the solar spectrum back to the early twentieth century, we have generated fits to UV spectral irradiance measurements from 1 – 410 nm. The longer wavelength spectra (150 – 410 nm) were fit as a function of two solar activity proxies, the Mg ii core-to-wing ratio, or Mg ii index, and the total Ca ii K disk activity derived from ground based observations. Irradiance spectra at shorter wavelengths (1 – 150 nm) where used to generate fits to the Mg ii core-to-wing ratio alone. Two sets of spectra were used in these fitting procedures. The fits at longer wavelengths (150 to 410 nm) were derived from the high-resolution spectra taken by the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) on the Upper Atmospheric Research Satellite (UARS). Spectra measured by the Solar EUV Experiment (SEE) instrument on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite were used for the fits at wavelengths from 1 to 150 nm. To generate fits between solar irradiance and solar proxies, this study uses the above irradiance data, the NOAA composite Mg ii index, and daily Ca ii K disk activity determined from images measured by Big Bear Solar Observatory (BBSO). In addition to the fitting coefficients between irradiance and solar proxies, other results from this study include an estimated relationship between the fraction of the disk with enhanced Ca ii K activity and the Mg ii index, an upper bound of the average solar UV spectral irradiance during periods where the solar disk contains only regions of the quiet Sun, as was believed to be present during the Maunder Minimum, as well as results indicating that slightly more than 60% of the total solar irradiance (TSI) variability occurs between 150 and 400 nm.     

Solar isotopic composition and abundance of europium

High resolution spectra of six photospheric Eu ii lines have been studied using the method of spectrum synthesizing. The isotope ratio is found to be Eu₁₅₃/Eu₁₅₁ = (48 ± 6)/(52 6) and the solar abundance of europium equals log _Eu = 0.7 ± 0.2 in the log _H = 12.00 scale.     

Transitions between the n = 2 and n = 3 levels of Ne VII in the solar spectrum

Theoretical populations of the 2s3l levels of Ne vii are presented for electron temperatures from 2.5 × 10⁵ K to 4 × 10⁶ K and electron densities from 10⁸ cm^–3 to 10¹² cm^–3. These, in conjunction with intensities of previously observed solar Ne vii lines and wavelengths and intensities observed in the laboratory, are used to identify further Ne vii lines in the solar spectrum. The dependence on temperature of intensity ratios such as I(2s2p ¹ P – 2s3d ¹ D)/I(2s2p ³ P – 2s3d ³ D) is demonstrated and the advantages of the small wavelength separation of such lines for solar electron temperature diagnostics are discussed.     

Some Peculiarities of Intensity Oscillations in Ca <Emphasis Type="SmallCaps">ii</Emphasis> Lines under Coronal Holes

Observations of the central intensity of the Ca ii K and 849.8 nm lines are used to derive the ratios of the oscillation power in the frequency ranges of the “five-minute” (W ₅) and “three-minute” (W ₃) oscillations. It is shown that at high significance level ratios, (W ₅/W ₃) >1 at coronal hole bases, and W ₅/W ₃ ≈1 in quiet chromospheric areas far from holes.     

Coronal temperatures and temperature gradients from OSO-7 spectroheliograms

Temperatures and temperature gradients for the outer corona are obtained from brightness gradients of EUV lines that were measured with the spectroheliograph on OSO-7. Brightness gradients show considerable deviations from isothermal model calculations that include collisional excitation and photoexcitation. A negative temperature gradient that gives both positive and negative ion abundance gradients appears to be able to account for the discrepancy. For 284 of Fe xv, perhaps the strongest line from the outer corona, measurements during 1972 appear to be consistent with (i) a temperature near 2.3×10⁶K near the equator at = 1.3±0.1 solar radii from the solar center; (ii) (/T) dT/d values near -0.7 that extend from as low as = 1.2 to about = 1.8. Temperatures from strong lines of Fe xiv and Fe xvi indicate that variations of about ±0.2×10⁶K exist along lines of sight where emission is appreciable. There appears to be some agreement between these results and temperature measurements from ion abundances in the solar wind and Doppler width of 5303.     

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.