An example of the cancellation of magnetic fields during the decay of an active region

A case of cancellation of magnetic fields is observed during the decay of a small active region. Three different sources of information were simultaneously used: high resolution magnetograms, chromospheric Caii filtergrams and transverse velocity fields.A magnetic structure is apparently dragged to the network by the supergranular velocity field while it splits into two. There, they meet another structure with opposite magnetic polarity. After a period of coexistence, the magnetic pairs vanish, leaving no trace of either magnetic or chromospheric structures.Visiting astronomer, Sacramento Peak Observatory, operated by the Association of Universities for Research in Astronomy, Inc. under contract AST-78-17292 with the National Science Foundation.     

Steady flows in active regions observed with the HeI 10830 Å line

We show that the He i 10830 A line gives reliable Doppler shift measurements in the upper chromosphere above active regions. Persistent flow patterns in active regions observed near the solar limb show features previously noted in Dopplergrams using the Civ transition region ultraviolet emission line. Unlike the Civ measurements, however, the He i absorption shows a strong correlation with the line-of-sight velocity images in certain regions of some active regions.Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78-17292 with the United States National Science Foundation.The National Center for Atmospheric Research is operated by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation.Visiting Astronomers, Sacramento Peak Observatory.     

Observations of rotational motion in prominences

The writer reports some recent observations carried out with the McMath Solar Telescope, which can be interpreted as due to rotational motions in prominences. Kitt Peak National Observatory Contribution No. 454. 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.     

The color temperature of a sunspot penumbra

The new Multi-Diode Array and the recently modified Universal Birefringent Filter were used at the Vacuum Tower Telescope at the Sacramento Peak National Observatory to measure the continuum color temperature of a sunspot penumbra between 428.4 and 667.6 nm. The results show that the color temperatures within the penumbral structures closely follow a measure of the wavelength average of the brightness temperature. These observations suggest that, if the dark penumbral filaments overlie a normal quiet photosphere, they are opaque to the radiation from below.Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78-17292 with the National Science Foundation.     

Line strengths for yellow supergiants and cepheids

Line strengths are presented for 15 yellow supergiants and 6 Cepheids. These line strengths are compared with those obtained previously by other investigators. Visiting Astronomer, Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Empirical limb effect curves for the FeI lines λ5250 and λ5576

The center to limb variation of solar spectral line positions, or limb effect, has been measured for the two neutral iron lines at 5250.2 Å and 5576.1 Å. Both limb effect curves show a disk-center reversal of 40–60 ms^–1 and the 5576 data also exhibit a possible second reversal at the limb. The limb effect curves are best described by polynomial expressions rather than by traditional power law expressions.Visiting Student, Kitt Peak National Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Photospheric line asymmetry and granular velocity models

We analyze spectral line profiles obtained from regions of the solar surface exhibiting either an upflow or a downflow on a spatial and temporal scale corresponding to the white-light granulation. The differences between their line bisectors are measured to quantify changes in the asymmetry of the profile resulting from granular motion. The observed bisector differences are compared with differences predicted using conflicting granular models. Models, in which the motion of large, long-lived granules decreases rapidly with increasing height in the photosphere, are compatible with the observed line profile asymmetries.NRC/NAS Resident Research Associate.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.Summer Research Assistant at Sacramento Peak Observatory 1979.     

The measurement of long-period oscillations at Sacramento Peak Observatory and South Pole

A program to measure long-period brightness oscillations at the solar limb has been pursued at Sacramento Peak Observatory for several years. Past improvements in observing technique and data analysis are reviewed. The encouraging results aid in the verification of the reality and the origin of oscillatory signals. However, the main stumbling block to this and other observational programs is the length of observing sequences imposed by the day/night cycle. The South Pole has received considerable attention as a site where extended observations might be possible. Currently, the Sacramento Peak program is developing a South Pole telescope designed for the observing technique and data analysis proven in Sunspot. A review of pertinent South Pole site parameters is given here for other workers who may be considering South Pole observations. Observing sequences longer than 150 hr are possible, though rare. Data sets of this duration are very attractive for solar oscillation studies.Proceedings of the 66th IAU Colloquium: Problems in Solar and Stellar Oscillations, held at the Crimean Astrophysical Observatory, U.S.S.R., 1–5 September, 1981.Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78-17292 with the National Science Foundation.Summer Research Assistant at Sacramento Peak Observatory.     

Magnetic fields in small and young sunspots

Preliminary results of magnetic field measurements in small sunspots from spectrograms obtained with the aid of the McMath Solar Telescope at the Kitt Peak National Observatory are presented. The measured intensities are greater than or equal to about 1200 Gauss. Furthermore, a broadening of the Fei line 6302.508 Å was found in some places of intergranular space. The importance of intergranular space as a possible potential earliest stage of sunspot development is mentioned.Kitt Peak National Observatory, Contribution No. 222.Visiting Astronomer, 1964, Kitt Peak National Observatory.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the Nat. Science Foundation.     

High resolution spectroscopy of the disk chromosphere

We compare temporal power spectra of solar atmospheric oscillations in plage and quiet Sun regions occurring on different parts of a time series of high-quality spectrograms. For periods shorter than 300 s, the oscillation amplitude in the photospheric and low chromospheric parts of the plage is reduced. There is a significant increase in long period power in the chromospheric plage. Our results provide no clear evidence that plages are heated by the dissipation of short-period waves.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST-78-17292 with the National Science Foundation.     

Some comments on the limb shift of solar lines

We searched for a variation with heliographic latitude of the solar limb effect by comparing the relative wavelengths of weak and strong Fraunhofer lines. The blue shifts associated with the limb effect appear 9%±5% larger in the polar radius vector than in an equatorial radius vector at cos = 0.5. This should perhaps be interpreted as an increase with latitude of either solar convection or of convective overshoot. Recent observations of poleward meridional flows of 30m s^–1 should be corrected for this limb effect variation. This correction increases this flow velocity to 70 m s^–1. A search for a similar variation in plages and in network boundaries had negative results, the variation being +1%±5% and -1%±6% respectively.Now at the Multiple Mirror Telescope Observatory, University of Arizona, Tucson, Ariz. 85721, U.S.A. The MMTO is jointly operated by the University of Arizona and the Smithsonian Institution.The Sacramento Peak Observatory is operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.     

Faculae,filigree and calcium bright points

Simultaneous observations of fine structure photospheric features at several wavelengths are described. The observations, which include regions near disk center and at the limb, were obtained using a narrow band calcium filter and selected wavelengths in Mg b₁, the red wing of H and the H continuum using the Universal Birefringent Filter of the Sacramento Peak Observatory.From the disk-center series it is concluded that bright point structures seen in calcium are cospatial with the H filigree and are related in origin.The limb photographs confirm that the calcium network is cospatial with the white light faculae. However, in regions of good seeing it is clear that even at the limb the calcium network patterns consist of individual bright points which are essentially the same as those seen near disk center.Thus it is concluded that the white light faculae, the H filigree and the calcium bright points are different manifestations of the same photospheric features.On leave from Department of Applied Mathematics, University of Sydney, Australia.The Sacramento Peak Observatory is operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.     

A facular model based on the wings of the Ca ii lines

We develop a relatively simple procedure for deriving models of upper photospheric regions based on the damping wings of the Ca ii resonance and infrared triplet lines. The procedure is used to derive a facular model but can also be applied to late-type stars. We compare our model to that of Chapman.Visiting Astronomer at Kitt Peak National Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Staff Member, Laboratory Astrophysics Division, National Bureau of Standards.     

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.     

Absolute spectroscopic determination of solar rotation

We have made absolute measurements at Kitt Peak of the dependence of solar rotation rate upon latitude during the period 17–20 June 1977. We observed the solar potassium line at 7699 Å using an atomic-beam resonance scattering apparatus. The measured sidereal equatorial rotation rate is 13.5±0.5 deg day^–1. Our results are in agreement with those of Howard and Harvey from 1966–68. We intend to use this technique in a long-term program at Oberlin to monitor the equatorial rotation rate.Visiting Astronomer, Kitt Peak National Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

An improved measurement of a spectrogram of a ‘gap’

A spectrogram of the 5250 Å region previously obtained during a period of excellent seeing has been remeasured and calibrated by reference to the preliminary KPNO photometric atlas. This analysis, using the logarithm of opacitance instead of an H-D plot, has reduced some of the effects of scattered light and shows greater facular line contrast.Visiting Scientist, Kitt Peak National Observatory Tucson, Ariz. 85726, U.S.A., which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Heights of formation of non-magnetic solar lines suitable for velocity studies

Heights of formation of lines that do not exhibit Zeeman splitting are calculated using an LTE, partial non-LTE, and full non-LTE approach. Non-magnetic (g=0) lines are valuable for velocity investigations in quiet-Sun magnetic field regions, and a knowledge of their formation heights is useful for obtaining three dimensional velocity profiles in these regions. Presently at Sacramento Peak Observatory. Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Impulsive brightenings/velocity transients in quiescent solar prominences

In some quiescent prominences, areas are found where the H emission profiles are centrally reversed. By combining good spatial, spectral, and temporal resolution, the detailed behavior of these reversal regions has been investigated. Many of the regions show a growth and subsequent decay in the affected area, peak intensity, line width, and depth of the central reversal. Lifetimes of the time-varying reversal features range from 10 to more than 60 min, and they are found near the edges of the prominence fine structure. These events are similar to the impulsive events that the authors discussed in an earlier paper, and may share a common cause. The detailed behavior of the H line profiles is consistent with these reversal features being true self-reversal of the line, indicating unusually high column masses in these areas. Some models of condensation of coronal material to the prominence state predict temporary regions of high density, perhaps high enough to produce the observed reversal. This implies that reversal features are the result of on-going condensation of coronal material into already formed prominences, a result which impacts models of prominence formation and stability.Visiting Astronomer, National Solar Observatory (Sacramento Peak) of National Optical Astronomy Observatories, operated by Association of Universities for Research in Astronomy, Inc. under contract with the National Science Foundation.     

Removing instrumental polarization from infrared solar polarimetric observations

Full Stokes polarimetry is obtained using the National Solar Observatory Vacuum Tower Telescope at Sacramento Peak while observing the magnetically sensitive infrared Fei line at wavelength of 1.56. A technique is described which makes use of the high magnetic resolution in this spectral range to remove instrumental polarization from observed StokesQ, U, andV line profiles.Supported under a USAF/AFOSR research initiative.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.     

Observations of coronal oscillations above an active region

Periodic Doppler width fluctuations have been observed in Fe xiv spectra above an active region. The oscillations have a period of 6.1 ± 0.6 min and a peak-to-peak amplitude of 0.07 ± 0.006 Å. The amplitude of the oscillation increases with height above the limb, and is enhanced at specific heights where we marginally detect line center intensity oscillations. The intensity fluctuations have a period of 6.1 min, an amplitude of 2.0 ± 1.4%, and are 180 ° out of phase with the width oscillations. A comparison region in the quiet corona showed no evidence of oscillatory phenomena.Visiting Student, Sacramento Peak Observatory.NAS/NRC Resident Research Associate.Operated by the Association of Universities for Research in Astronomy Inc., under contract with the National Science Foundation.