Interferometric studies of spectral lines in the solar corona

A photographic Fabry-Perot interferometer was used to measure the breadth and wavelength of the Fexiv spectral line at 5303 Å in the solar corona, during the eclipse of 7 March, 1970, in Mexico. The observations were consistent with a possible large-scale vortex structure in a bright streamer, in which cooler gases revolved about a hotter core.Work supported by the United States National Science Foundation and by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant AF-AFOSR-68-1496.     

On some flare-sensitive high photospheric and low chromospheric lines

During a stay at the Kitt Peak National Observatory the writer has tried to find an influence of flare radiation on the high photospheric and low chromospheric lines of the area occupied by the flare. Observations have been made in the H region and in the region of the H and K lines. When flare emission is present in sunspots some of the faint (molecular) lines seem to be weakened. When a flare appears near the solar limb some of the Evershed-type (chromospheric) lines are strongly influenced.Kitt Peak National Observatory Contribution No. 481.Visiting Astronomer to the Solar Division, Kitt Peak National Observatory, operated by the Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

Photographic spectrophotometry of selected bright B and Be Stars

Coudé spectra taken in the red region of 49 bright B and Be stars were examined for the behaviour of Hα and the He I λλ 5876 and 6678 line profiles. 51 per cent of the stars were found to show Hα emission to some degree and 22 per cent showed helium emission. Evidence is presented for the existence of drastic changes in the line profile and the radial velocity in some of the stars on a time scale of fractions of a day. Visiting Astronomer, Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, operated by the Association of Universities for Research in Astronomy, under contract with the National Science Foundation.     

A solar spicule model based upon calcium II K line radiative transfer studies

Monte Carlo radiative transfer techniques are used to develop a height-dependent spicule model based upon a more realistic configuration than has hitherto been considered. The spicule is represented by a uniform cylinder, of finite length, standing vertically upon a plane chromosphere. The observed, limb-darkened, anisotropic chromospheric flux incident upon the cylinder is incorporated into the transfer calculations.The resulting model is characterized by a random, line broadening velocity of 20 km/sec, with electron temperature increasing from 6 × 10³ K at the base to about 1.5 × 10⁴ K at 11500 km above the solar surface. The corresponding values of electron density are 8 × 10¹¹ cm^-3 and 4 × 10¹⁰ cm^-3. Contrast curves of the spicule model against the chromospheric background are computed and indicate that spicules should appear both bright and dark on the disk, depending upon their position with respect to the limb, the spectral frequency of observation and the viewing height.This work is based on a Ph.D. thesis submitted to the Department of Astro-Geophysics, University of Colorado.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Spatial and temporal variations of EUV coronal bright points

This paper reports results of an analysis of Skylab observations of coronal bright points made in EUV spectral lines formed in the chromosphere, chromospheric-coronal transition region and corona. The most important result is that the observed bright points exhibited large variations in EUV emission over time scales as short as 5.5 min, the temporal resolution of the data. In most cases strong enhancements in the coronal line were accompanied by strong enhancements in the chromospheric and transition region lines. The intensity variations appear to take place within substructures of the bright points, which most likely consist of miniature loops evolving on time scales of a few minutes. Coronal cooling times derived from the data are consistent with an intermittent, impulsive coronal heating mechanism for bright points.     

Evidence of Chromospheric Activity on HR 1099

We present an observational evidence of the chromospheric activity on HR1099. The IUE spectra of HR 1099 were used for the ultraviolet photometry and for the intensity variation of chromospheric emission line. Ultraviolet light curves were made from the low and high dispersion IUE spectra and were compared with a optical light curve. We have analyzed the ultraviolet and optical light curves by the Wilson and Devinney computing code including dark spots. The intensity variation of Mg II emission line depends on the orbital phase. The maximum intensity of Mg II emission line occurs at the phase of light minimum where dark spot visibility is maximum due to strong chromospheric activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

The solar flares of August 28 and 30, 1966

We describe observations of the class 3+ flare of August 28, 1966, made at the Mount Wilson Observatory. This great proton flare followed the sequence: (1) Precursor flare; (2) Filament eruption; (3) Beginning in penumbra of large spot; (4) Rapid elongation in two strands; (5) Great spray and surface wave; (6) Rapid separation of two strands to maximum brightness; and (7) Slow spread of brightness and decay.The soft X-ray burst coincides with stages 3–6, decaying through stage 7; the hard (> 80 keV) burst coincides, but decays more rapidly.Considering a demi-cylinder of emitting material, the soft X-rays are explained by a 4-million-degree plasma, or at least a large flux of electrons with that amount of energy. Given this flux, the microwave burst is explained by synchrotron emission with the low frequency cut-off due to coronal absorption.The class-2 flare of August 30, 1966, is also discussed.This research was supported by the Advanced Research Projects Agency under the Office of Naval Research Grant N00014-67-C-0140 and the National Aeronautics and Space Administration under Grant NGR 05 002 034.NASA International University Fellow at the California Institute of Technology, 1967–1968, now at Arcetri Astrophysical Observatory.     

On the color of the 26 February 1981 white light flare

We describe visual observations of a white light flare which displayed a pink color in a part of the flare which covered a sunspot umbra. We then show that visible pink tint, if attributable to strong H emission, requires a minimum equivalent emission line width of approximately 140 A, or three times larger than in any flare previously measured. Such extreme line broadening might be interpreted to result from flare penetration to unusually high chromospheric densities ( 10¹⁴ cm^–3), or from anomalous Stark broadening due to turbulent electric fields in an unstable plasma (Spicer and Davis, 1975) at lower density.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Solar oscillations in strong and weak fraunhofer lines over a quiet region

We have analysed a 35-min-long time sequence of spectra in the Caii H line, Nai D1 and D2 lines, and in a large number of strong and weak Fei lines taken over a quiet region at the center of the solar disk. The time series of these spectra have been observed simultaneously in these lines under high spatial, spectral, and temporal resolution at the Vacuum Tower Telescope (VTT) of the Sacramento Peak Observatory. We have derived the line profiles and their central intensity values at the sites of the chromospheric bright points, which are visible in the H line for easy identification. We have done a power spectrum analysis for all the lines, using their central intensity values to determine the period of oscillations. It is shown that the 3 Fei lines, present 23 Å away from the core of the H line representing the pure photospheric lines, Nai D1 and D2 lines, 6 Fei lines at the wings of H line, and Can H line exhibit 5-min, 4.05-min, 3.96-min, and 3.2-min periodicity in their intensity oscillations, respectively. Since all these lines form at different heights in the solar atmosphere from low photosphere to middle chromosphere and show different periodicities in their intensity oscillations, these studies may give an idea about the spatial and temporal relation between the photospheric and chromospheric intensities. Therefore these studies will help to better understand the physical mechanisms of solar oscillations. It is clearly seen that the period of intensity oscillations decreases outward from the low photosphere to the middle chromosphere. Since we have studied a single feature at a time on the Sun (i.e., bright points seen in the H line) in all these spectral lines simultaneously, this may explain about the footpoints of the bright points, the origin of 3-min oscillations, and the relation to other oscillations pertaining to these locations on the Sun. We have concluded that 80% of the bright points are associated with dark elements in the true continuum, and they may seem to have a relationship with the dark intergranular lanes of the photosphere, after carefully examining the brightness (bright threads) extending from the core to the far wings of the H line at the locations of a large number of bright points, using their time sequence of spectra.NRC Resident Research Associate, on leave from Indian Institute of Astrophysics, Bangalore 560034, India.     

Interpretation of XUV spectroheliograms

Some parameters of chromospheric structure are drawn from recently published XUV spectroheliograms. The HeII emission above the limb arises from the small amount of He⁺ still existing at 10⁶°. The larger amounts of He⁺ in the cooler corona at the poles explain the polar cap absorption in 304. The flat distribution of emission in Oiv and Ov, with a sharp spike at the limb, is caused by the rough structure of the chromosphere and the variable excitation in the emitting spicules. The intensity of the Nevii lines shows that the transition zone between chromosphere and corona is very sharp.This research was supported by the National Aeronautics and Space Administration under Grant NASA NGR 05 002 034.     

Oscillations in the lower solar atmosphere at the base of coronal holes

The oscillatory processes in the relatively quiet solar atmosphere, at the base of an extensive coronal hole, have been investigated. The properties of the oscillations in a number of parameters related mainly to the Ca II line intensity have been analyzed in areas belonging to various chromospheric network structures (cells, networks, flocculi, etc.). The goal of this study was to reveal peculiarities of the oscillatory process in the spatial areas located (in projection) at the center of a coronal hole, near its boundary, and at a bright coronal point at various heights of the solar atmosphere (from the photosphere to the middle chromosphere). In most structural elements, the low- and high-frequency components of the spectrum have been found to increase and decrease, respectively, with height. The oscillatory power of the low-frequency oscillations is at a maximum in the areas bordering the bright magnetic network elements. The power of the three-minute, five-minute, and low-frequency oscillations decreases at the centers of the bright chromospheric network. The phase relations point to the propagation of waves mainly at the boundaries of the bright chromospheric network and intermediate (in brightness) network elements. In two of the three investigated regions, the power of the five-minute oscillations (2.4–4.0 mHz) in cells is higher than that of the three-minute ones (5.2–6.8 mHz) at the investigated levels of the quiet solar atmosphere.     

Ca II K emission arches

Arch-like features are often seen in spectrograms of very strong lines near the solar limb when the slit crosses the chromospheric network. We show how earlier kinetic-equilibrium (non-LTE) calculations for Ca ii can be used to predict such features for the K line with two-component atmospheric models.Publication of the Goethe Link Observatory, Indiana University, No. 144Now on leave at the High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado. (NCAR is sponsored by the National Science Foundation.)     

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.     

Measurement of the full Stokes vector of He I 10830 Å

First observations of the full Stokes vector in the upper chromosphere are presented. The He I 10830 Å line, which has been shown to give reliable measurements of the line-of-sight component of the magnetic field vector, has been used for this purpose. It is shown that the difference between the appearance of chromospheric and photospheric magnetic structures observed close to the solar limb is largely due to the difference in height to which they refer and projection effects. The observations do suggest, however, that the magnetic field above sunspot penumbrae is somewhat more vertical in the chromosphere than in the photosphere.The National Optical Astronomy Obervatories are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation     

On the topology of filaments and chromospheric fibrils near sunspots

The similarity between the spiral topology of chromospheric fibrils and filaments observed in H near sunspots and the configuration of an axisymmetric force-free magnetic field is examined. It is suggested that some of the observed features could be interpreted in terms of the configuration of lines of force of an axisymmetric force-free chromospheric magnetic field. Implications of the results of analysis to the possible interpretations of other observed topological features near a sunspot are discussed.Visiting scientist at the High Altitude Observatory.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Sponsored by the Office of Naval Research, Contract No. N 00014-67-C-0290.     

The photospheric network

Spectroheliograms, obtained in certain Fraunhofer lines with the 82-cm solar image at the Kitt Peak National Observatory, show a bright photospheric network having the following properties:

1. It resembles, but does not coincide with, the chromospheric network, the structure of the photospheric network being finer and more delicate than the relatively coarse structure of the chromospheric network.
2. It is exactly cospatial with the network of non-sunspot photospheric magnetic fields.
3. Its visibility in a given photospheric Fraunhofer line is primarily dependent on the states of ionization and excitation from which the line is formed and secondarily dependent on the Zeemansensitivity of the line-being most visible in low-excitation lines of neutral atoms and least visible in high-excitation lines of singly ionized atoms.

We conclude that these magnetic regions of the solar atmosphere are a few hundred degrees hotter than their surroundings, and that they are visible in white light near the limb as photospheric faculae.     

The bright streaks in the Hα disk chromosphere

Evidence is presented demonstrating the existence of a type of chromospheric structure in the form of bright streaks. These are extensions across the solar disk of elongated bright mottles which originate in the central regions of clusters of mottles. They are best observed on good filtergrams at H ± 0.5 Å through comparison with filtergrams at other positions on the line profile. Their length can be as much as 200 sec of arc. The bright streaks appear to be predominantly horizontal loop structures, while the well-known spicules are mainly vertical structures. A bright streak may be well defined or rather diffuse along its length, and many of them are accompanied by darker boundaries or envelopes. It is usual to find a loop of bright streak bridging the central areas of two mottle clusters. It seems that the observed pattern in the space between the chromospheric network at H ± 0.5 Å results partly from the interactions of the bright streaks of different stages of evolution traversing the area in different directions.     

Spatially resolved images and solar irradiance variability

The Sun is the primary source of energy that governs both the terrestrial climate and near-earth space environment. Variations in UV irradiances seen at earth are the sum of global (solar dynamo) to regional (active region, plage, network, bright points and background) solar magnetic activities that can be identified through spatially resolved photospheric, chromospheric and coronal features. In this research, the images of CaII K-line (NSO/Sac Peak) have been analysed to segregate the various chromospheric features. We derived the different indices and estimated their contribution from the time series data to total CaII K emission flux and UV irradiance variability. A part of the important results from this research is discussed in this paper.     

The Ca ii emission lines in quiescent prominences

Observations of the Ca ii H, K, and infrared triplet lines are compared with theoretical predictions from the slab models of Heasley and Milkey (1976). While the theoretical models describe the hydrogen and helium emission spectra of quiescent prominences satisfactorily the predicted Ca ii lines are systematically too bright. The most likely reason for the discrepancy is the inapplicability of the symmetric slab prominence model for lines which become even moderately optically thick in prominences.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Visiting scientist at Kitt Peak National Observatory.