Contribution of chromospheric features to UV irradiance variability from spatially-resolved Ca II K spectroheliograms

We have digitized the Ca ii K spectroheliograms, observed at the National Solar Observatory at Sacramento Peak, for the period 1980 (maximum of solar cycle 21), 1985 (minimum of solar cycle 21), 1987 (beginning of the ascending phase of solar cycle 22), 1988 and 1989 (ascending phase and maximum of solar cycle 22), and 1992 (declining phase of solar cycle 22). A new method for analyzing the K spectroheliograms has been developed and applied to the K images for the time interval of 1992. Using histograms of intensity, we have segregated and measured the cumulative intensity and area of various chromospheric features like the plages, magnetic network and intranetwork elements. Also, the full width at half maximum (FWHM) derived from the histograms has been introduced as a new index for describing the chromospheric activity in the K-line. The full-disk intensity (spatial K index) has been derived from spatially-resolved K images and compared to the spectral K index derived from the line profiles for the full disk. Both the spatial K index and FWHM have been compared to the UV irradiance measured in the Mg ii h and k lines by the NOAA9 satellite and found that they are highly correlated with the Mg ii h and k c/w ratio.NRC Resident Research Associate, on leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

The structure of the temperature minimum region in solar flares and its significance for flare heating mechanisms

We analyze Ca ii K-line profiles of one flare and EUV continuum observations of two other flares in order to infer values for the temperature enhancements (over active region values) produced in the upper photosphere around and above the temperature minimum region. The results, obtained through a partial redistribution calculation of the Ca ii K-line profiles and an LTE approach to the continuum observations, show that the flare temperature minimum is depressed some two scale heights below its preflare level, and that substantial temperature enhancements are produced even at this depth. Estimates for the energy release in these photospheric layers are given, and are found to be comparable with that released in chromospheric H and L emission.We then turn our attention to the investigation of possible heating mechanisms which might be responsible for the observed enhancements. Bombardment by both electrons and protons, and irradiation by soft X-rays, are each considered and found to be largely ineffective, due to the large attenuation of flux by photospheric depths, unless new ideas on the precise nature of these mechanisms are invoked, particularly if the same mechanism is also to explain the observed chromospheric emissions. We therefore conclude that it is most likely that some other mechanism must be advocated in order to explain the observed heating. Possibilities for this are (a) heating by EUV radiation, (b) proton beams with low dispersion energy spectra centered around 10–20 MeV, and (c) localized heating at temperature minimum levels.On leave from: Department of Astronomy, The University, Glasgow G12 8QQ, Scotland, United Kingdom.     

The umbral flash as a magneto-acoustic wave phenomenon

The suggestion that an umbra flash may be caused by a magneto-acoustic wave phenomenon is examined. It is suggested that the flash in Ca ii lines is formed during the compressional stage in a magneto-acoustic wave. The compression which is assumed to be adiabatic will produce a rise in temperature and a corresponding increase in number of Ca ii atoms. The variations in line emission (absorption) coefficient of the Ca ii K-line are calculated on this assumption and are found to be in general agreement with the observed variations. Other observed quantities as proper motion, magnitude of line shift etc., also agree with the wave hypothesis. Further observations which may serve as tests on the wave hypothesis are suggested.     

Characteristics of the Ca ii K-line profiles in the quiet Sun

Feature-to-feature identification is made on simultaneous Ca ii K-line spectrograms (SG) and K_2v spectroheliograms (SHG). The line profiles in plages and in the network boundary nearly always have double-peaked reversal in the core, while those inside the cells present all possibilities: double-peaked, single-peaked on violet side, single-peaked on red side, and unreversed absorption. Statistics of the profiles in the quiet chromosphere show that 50% are K₂₃₂ double-peaked, 20% are K_2v single-peaked, 10% are K_2r single-peaked, and 20% show only incipient reversal or even totally lack any reversal. We call attention to the nontrivial contribution of these absorption profiles which are formed in dark regions shown on SHG's.The physical conditions inferred from different kinds of profiles are briefly discussed.In part to be included in a dissertation to be submitted to the Graduate School, University of Maryland, by S. Y. Liu in partial fulfillment of the requirements for the Ph. D. degree in Astronomy.Visiting Graduate Student, Solar Division, Kitt Peak National Observatory.Visiting Astronomer, Solar Division, Kitt Peak National Observatory.     

Densities and mass motions in transition-zone plasmas in solar flares observed from Skylab

We studied the EUV line spectra of three flare observed with the NRL slit spectrograph on Skylab. The electron densities in the flare transition-zone plasmas are determined from density-sensitive lines of Si iii and O iv. The electron densities in all three flares studied were greatest during the flare maximum with values of the order of 10¹² cm^–3. The density decreases by a factor of 2 to 3 in the decay phase of the flares. The intensities of EUV lines from the flare chromospheric and transition-zone plasmas all are greatly enhanced. In contrast to lines for Oi, Ci, Feii and other chromospheric ions, the lines of Oiv and Nv and other transition-zone lines are not only enhanced but also very much broadened.Fitting of the N v 1242 Å line with a two-gaussian model shows that for two of the flares studied, there is a red-shifted component in addition to an unshifted component. The shifted component in the N v line profiles is interpreted as due to a dynamic and moving plasma with a bulk motion velocity of 12 km s^–1 for one flare and more than 70 km s^–1 for the other. The broadened line profiles indicate that there are large turbulent mass motions with random velocities ranging from 30 to 80 km s^–1.Ball Corporation. Now with NASA/Marshall Space Flight Center.     

A comparison of computed and observed line profiles and flash intensities in the photosphere-chromosphere transition region

This paper mainly deals with the development of a method for the interpretation of eclipse observations in the extreme limb - lower chromosphere transition region. We compute line profiles emitted by a column of 1 cm² cross section along the line of sight and, by integration, profiles and total (flash) intensities from a slice of 1 cm width beyond the Moon's edge, at various heights with respect to the solar limb. The solar models used are mentioned in Section 1; method and approximations are described in Section 2. Calculations have been made for some Fei lines with various equivalent widths (Table I); the results are discussed in Section 3. It appears that the method enables one to distinguish between the various models. Particularly line intensities just inside the limb are very important.We compared our computed results with some observations made by Houtgast (Section 4). The main conclusion is that a BCA-type temperature distribution alone does not agree with observations; we have to assume an excitation temperature of Holweger's type.Investigation performed at the Astronomical Institute, Utrecht; present address: Royal Belgian Observatory, 1180 Brussels.     

Energy spectra of gamma-ray bursts detected by the APEX experiment on phobos mission

Molecular clouds are clumpy on length scales down to the limits of observational resolution. At least some ultracompactHii regions (UCHiiR) may result from the interaction of a young early type star and this type of cloud. The clumps can act as reservoirs of ionized gas distributed within theHii region. These models reproduce the relatively long lifetimes implied by the population statistics of UCHiiR. We present line profile and emission measure plots based on the simplest case where the flow remains supersonic through to a recombination front. The morphology agrees with the shell-like UCHiiR as classified by Churchwell. The predicted line profiles are broad and double peaked with a separation of about 50 km s^–1 for the example given.     

Europium and lanthanum in sunspot and in the undisturbed photosphere

Line profiles of Euii 4129.7 Å and Laii 4086.7 Å were observed photoelectrically and photographically in the photosphere and within sunspots and after correction were compared with calculated profiles. The observed large hyperfine structure broadening and isotope shift of 4129.7 Å confirm the Eu origin of this line and suggest a larger Eu abundance than that recently published by Grevesse and Blanquet (1969). The line profiles within the spot may be well represented by means of the spot model of Hénoux (1969), while older spot models are not able to do it. An abundance difference between spot and photosphere has not been observed.     

The center-to-limb variations of four Ca i lines in the photospheric spectrum at λ6500

We study the center-limb (CL) variation of the average profiles of four Ca i lines near 6500 and compare these observations with synthetic data obtained from several line formation models having different thermal structures, line parameters, LTE and non-LTE conditions, and micro and macroturbulence values, to assess the formation characteristics of our Ca i lines in the solar photosphere.Comparison of numerical results with observations indicates that non-LTE is indispensable to fit the CL variation of the central residual intensity for the line 6493, and anisotropic microturbulence is indispensable to improve the CL behavior of the equivalent widths for all lines. The Ca i line analysis favors a cool photospheric model, but this cannot be disentangled clearly from the effects of non-LTE and small-scale velocity fields on the grounds of the present line formation models.     

Brightness fluctuations in the K-line wings

A power-spectrum and cross-spectrum analysis has been made of measurements of temporal fluctuations of intensity observed in the K-line wing (2.07 Å from line center) and of simultaneous measurements of temporal fluctuations of Doppler displacement of the cores of 3931.122 Fe i and 3933 Ca ii (K₃). The measurements were made in a quiet region near the center of the Sun's disk. We find that the average power spectra of the intensity fluctuations have two significant peaks of about equal strength: one at 0.0033 Hz (300-s period); and one at about 0.001 Hz (1000-s period). The average rms value of these intensity fluctuations is 0.0435±0.0082. Maximum brightness comes before maximum violet displacement of the Fe i line. The mean of the best determined phases is 137° and of all the data 108°. At those places on the Sun where the 300-s oscillations can be identified in the k₃ core, the Doppler displacement of the Fe i line leads that of the K₃ core by a mean phase angle of 27°.     

The Relation of Ca II K FEATURES TO MAGNETIC FIELD

We studied quantitatively the relation between the intensity of Caii K-line bright features and the intensity of the associated magnetic elements using two data sets obtained at the Big Bear Solar Observatory. Both network and intranetwork (IN) structures were considered. Magnetic field changes always affected the K-line emission; for example, the appearance of new bipoles was always followed by enhanced K-line emission. There is an almost linear correlation between the K-line intensity and the magnetic field strength of the stronger network elements (elements with absolute field strength higher than 11–19.5 G). We identified two classes of intranetwork K-line elements: magnetic and non-magnetic ones. The number of the magnetic K-line IN elements above a 1-sigma threshold was only 5%–10% of the number of the non-magnetic ones. The magnetic K-line IN elements were almost 3 to 4 times brighter compared to the non-magnetic elements. On the other hand, the non-magnetic elements were moving with typical velocities of 35–40 km s^–1 while the velocities of the magnetic K-line elements were of the order of 1 km s^–1.     

The solar neutral iron spectrum

Mean profiles of eighteen neutral iron lines of varying strengths were measured at selected positions from the center of the solar disk to the limb. These profiles were obtained by rapid photoelectric scanning of the spectrum with a double-pass spectrometer. The Fe i lines selected are representative of most of the stronger low-lying transitions in the neutral iron atom. In addition to the iron lines, this observational program includes center-to-limb measurements of three Ti ii lines and of the Ca i resonance line 4226.7. The line profiles are presented here in graphical form after correction for instrumental effects and normalization to the local disk center continuum.Both Kitt Peak National Observatory and the National Center for Atmospheric Research are sponsored by the National Science Foundation.     

A three-component model for the formation of the chromospheric Caii K line

A recent two-component model for the formation of the Caii K line in the solar chromosphere put forward by Beebe and Johnson is discussed. Although this model is a great advance on existing one-component models, it is pointed out that observations require a minimum of three components in order to understand the formation of the K₂ peaks.In order to make some progress in the study of multicomponent models an adaptation of the empirical (or analytic) approach is suggested. This relates the line source function directly to observations and places a secondary importance (at this stage) on the synthetic approach to the problem. A model is obtained which is in adequate agreement with observed mean profiles and its features are briefly discussed.     

Absorption effects in sixHii regions

We have measured the profile of the 21-cm line of neutral hydrogen in the direction of sixHii regions (RCW 38, RCW 49, RCW 57 (b), RCW 74, W22, and W37). By comparison with line profiles outside the nebulae we have deduced the absorption profiles corresponding to eachHii region. We compare our results with those of other authors in the 21-cm line, in the line of OH, and in the recombination lines of hydrogen.     

Solar EUV emission line profiles of Si ii and Si iii and their center to limb variations

Spectral line profiles of Si ii and Si iii are presented which were observed both at solar center and near the quiet solar limb with the Naval Research Laboratory EUV spectrograph of ATM/SKYLAB. Absolute intensities and line profiles are derived from the photographic data. A brief discussion is given of their center-to-limb variations and of the optical thickness of the chromosphere in these lines. Nonthermal broadening velocities are found for the optically thin lines from their full width at half maximum intensity (FWHM).Also at High Altitude Observatory for part of this work.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Empirical NLTE analyses of solar spectral lines

The center-to-limb behaviour of the Ba ii 4554 resonance line is analyzed together with data from the extreme limb, flash intensities and profiles of other Ba ii lines. An empirical NLTE method is employed in which the observed profiles are compared with synthesized profiles based on a standard one-dimensional model atmosphere, with the line source function, the barium abundance, the collisional damping and the atmospheric turbulence as free parameters.The line profiles from the extreme limb furnish considerable constraints on the formation of Ba ii 4554. Its wings reverse into emission well inside the solar limb, a phenomenon which cannot be explained by any frequency-independent line source function. Accounting for effects of partially coherent scattering in the line source function is a necessary and adequate step to reproduce the observations both over the disk and near the limb. The form of the empirically derived frequency-dependent line source function is discussed.Results are given for various parameters (gf-values, solar barium abundance and isotope ratios, collisional damping, microturbulence and macroturbulence).The 4554 profile of disk center shows a depression in its blue wing resembling asymetries found in various stellar spectral lines.     

Center limb observations of inhomogeneities in the solar atmosphere

High resolution Ca II K-line spectra obtained at Kitt Peak National Observatory in September 1970 at a dispersion of 20 mm/Å and a spatial resolution approaching 1 arc sec are described.We first investigate the claim made by Zirin, Pasachoff and others that, at high spatial resolution, the K₂ emission occurs either in the red wing or the blue but not both. In fact, it is shown that although some spectra exhibit predominantly singly-peaked emission features, other spectra, at equally high spatial resolution, show mainly doubly-peaked structures. We suggest that both types of spectra may be regarded as normal.Secondly, time series studies reveal at least two cases in which features exhibiting single emission peaks in either the red or the blue wings evolve into doubly-peaked structures in a time scale of 15sec.Finally, two features on the slit are observed to separate in a manner which implies velocities of order 50 km/sec. Other alternative explanations of the phenomena are discussed but are found to be even more implausible than velocities of this magnitude. Nevertheless, we urge the need for independent confirmation of this result.The implications of these observations for inhomogeneous models for the formation of the Caii K-line are discussed.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Spectroscopic observations of zeta Aurigae during the 1971–72 eclipse

The chromospheric phases in the 1971–72 eclipse were observed at wavelength regions containing the K-line of ionized calcium on six nights at ingress and on eight nights at egress at the Okayama Astrophysical Observatory. The linear dispersion of the spectrograms is about 10 Å mm^?1. The spectrograms were measured for line intensities of the K-line and for radial velocities of metallic and hydrogen lines. The results are compared with those obtained in the earlier eclipses. The intensities of the chromospheric K-line during this eclipse were relatively ‘weak’ at both ingress and egress and were similar to those in the 1939–40 eclipse. It seems that the chromospheric activity of ζ Aurigae increased after 1939, reached a maximum at 1950–56, and then decreased again until 1971. The radial velocities of the chromospheric lines of hydrogen atoms, ionized calcium, and some other atoms and ions varied with phase showing almost the same characteristic features as in most earlier eclipses. Intensities and velocities were also measured for satellite lines of the K-line.     

Dynamics of the Orion Nebula

The collision of supersonic gas streams within anHII region is studied. A system of stationary shock waves is set up. Theoretical line profiles through these shocked regions are compared with those obtained in the bright central regions of the Orion Nebula. There is good agreement between the predicted and observed magnitude of line splitting in the forbidden lines of O⁺ and O⁺⁺.     

The small scale velocity field of a quiescent prominence

The small scale velocity field of a large quiescent prominence is studied from simultaneous filtergrams in the red and violet wings of the Ca ii K-line.The average spatial separation of peak velocities is about 3000 km. The duration of individual velocity elements appears to increase with increasing area of the elements. In some positions of the prominence the direction of the velocity persists through the 5 hrs of observations. It is suggested that emitting elements of gas, which flow along the magnetic lines of force, produce peak velocities when they pass through particular locations of the prominence region.There is no clear evidence in the data for oscillations or waves.