Universal Birefringent Filter Research Articles

Evidence for Magnetoconvection in Sunspot Umbral Dots

An analysis of high-resolution Dopplergrams and continuum images of NOAA AR 8350 is presented. The observations were recorded with the universal birefringent filter attached to the Dunn Solar Telescope at the National Solar Observatory, Sunspot, New Mexico. We find upward velocity of the order of 400 m s-1 within umbral dots, surrounded by downward velocity of the order of 300 m s-1. This observation is compatible with the simulations of three-dimensional radiative magnetoconvection with gray radiative transfer in sunspot umbra by Schussler & Vogler, which support the idea that umbral dots appear as a result of magnetoconvection.

Plasma Flows Observed in Magnetic Flux Concentrations and Sunspot Fine Structure Using Adaptive Optics

We present diffraction-limited observations of magnetic flux concentrations and penumbral and umbral fine structure within an active region observed at disk center. We recorded G-band images, magnetograms, Dopplergrams, and narrowband filtergrams, using the Universal Birefringent Filter (UBF) at the Dunn Solar Telescope (DST). The National Solar Observatory (NSO) adaptive optics system at the DST was used to achieve diffraction-limited long-exposure imaging with a high signal-to-noise ratio. The main results can be summarized as follows: Strong and spatially narrow downflows are observed at the edge of magnetic structures, such as small flux concentrations (sometimes also referred to as flux tubes), pores, a light bridge, and the sunspot umbrae. For the particular sunspot observed, we find strong evidence for what appear to be vigorous, small-scale convection patterns in a light bridge. We observe extremely narrow (<02) channels or sheets of downflowing plasma. Flux concentrations as seen in intensity expand from a height close to where the continuum is formed to the height of formation for the G band. These observations indicate that the G band forms in the mid-photosphere. We are able to identify individual penumbral fibrils in our data and find a bright (hot) upflow and a more vertical field structure at the filament footpoint near the umbral boundary. The observations are consistent with a filament geometry in which the field and flow turn to a nearly horizontal, dark structure over a distance of about 02. In the deep photosphere we observe strong upflows of the order of 1 km s-1 in umbral dots. We compare our results with theoretical model predictions.

Universal birefringent filter with a new double passband mode

This article describes the universal birefringent filter (UBF) which will be mounted at Taiwan. The UBF permits observations of solar images, vector magnetic fields and/or line-of-sight velocity fields in any Fraunhofer line in the wavelength region from 4000 A to 7000 A with half width from 0.05 A to 0.14 A. We have tested it at ten Fraunhofer lines under a spectral telescope in Huairou Station of Beijing Astronomical Observatory and obtained the passband profiles. The principles of magnetic field and velocity field measurement with the UBF are also described.

3-Dimensional High-Resolution Solar Spectro-Polarimetry

AbstractIn order to obtain information on the magnetic field on the sun in two spatial dimensions, a spectro-polarimeter of high spatial, spectral, and temporal resolution was built in the German Vacuum Tower Telescope (VTT) at the Observatorio del Teide/Tenerife. The two-dimensional spectrometer in the VTT, using a Universal Birefringent Filter (UBF) and a Fabry-Perot Interferometer (FPI) to obtain narrow-band nltergrams with a spectral resolution of the order of 3.105 (approx. 22 mÅ at CE5; at 6303 Å) and a spatial resolution of 0.2 arcsec/pixel (Bendlin et al. 1992, Bendlin and Volkmer 1993), was extended to work as a spectropolarimeter for measuring Stokes-I and Stokes-V profiles.

A Two-Dimensional Spectrometer for Solar Observation and its recent modifications

AbstractThe instrument presented here is based on the combined use of a Universal Birefringent Filter (UBF) and a Fabry-Perot Interferometer (FPI) to obtain narrow-band filtergrams with a CCD. Scanning through a Fraunhofer line with a sufficient number of wavelength settings is accomplished within a few seconds by tuning only the FPI. The two-dimensional spectrometer in the German Vacuum Tower Telescope (VTT)/Tenerife was used to take observations of high spatial, spectral, and temporal resolution, yielding a wide range of results. Since recently, additional information on the solar magnetic field is obtained with it.

High-resolution spectroscopic imaging of the Sun with a universal birefringent filter and a Fabry-Perot Interferometer

We describe an optical device consisting of a Universal Birefringent Filter and a Fabry-Perot interferometer used in tandem to obtain monochromatic images of the Sun in the visible spectrum (between 4200 and 7000 A). The spectral resolving power is about 300000 and the wavelength stability might reach 0.02 mA at 5000 A (≈1.2 m/s) with an integration time of 1 s. The image is recorded by a CCD camera based on a Thomson THX31159 device with 512×512 square pixels 19 μm in size. A prototype of this instrument was tested at the G. B. Donati Solar Tower of the Arcetri Astrophysical Observatory by obtaining, at fixed time intervals, several series of full disk monochromatic images of the Sun at different wavelengths inside the Fe I 5905.7 A photospheric line. In this paper we describe the instrument and its characteristics and we give an cccount of the preliminary results obtained by measuring the global velocity field on the solar photosphere. These results show that the instrument is well suited for global oscillation measurements.

On the Location of the foot-points of Sub-Arc-Second Magnetic Structures in the Quiet Sun Photosphere

We present the results of our study of a long sequence of high spatial resolution filtergrams obtained with the Universal Birefringent filter (UBF) at the Vacuum Tower Telescope of the National Solar Observatory, Sacramento Peak on July 14, 1987. The UBF has a band pass of 0.25A and was set sequentially in the following wavelengths, and filtergrams were obtained at each of the settings.

Observational maps of the moments of strong line profiles on the solar disk

Using the method of solar bidimensional spectroscopy based on the Universal Birefringent Filter (UBF), we have determined the bidimensional maps of moments of some chromospheric lines. The observational material, referring to a quiet region on the disk center, have been acquired with the UBF of the NSO at Sacramento Peak on Aug. 27,1985. In this note we present the work in progress and the new observational aspects arising from this diagnostic method.

Derivation of the universal wavelength tuning formula for a Lyot birefringent filter

A general method is presented for accurately finding the spectral tuning formula for a Lyot birefringent filter based on a limited number of measured tune solutions, that is, tuning element angles in the filter that provide maximum monochromatic transmission at wavelengths spanning the useful operating range of the filter. The ambiguous determination of retardation from the tuning element angle in a measured tune solution is resolved for each tuning element by extrapolation in the retardation of the next spectrally coarser tuning element assuming that the effective ratio of tuning element thicknesses is slowly varying in wavelength. Applying this to the Sacramento Peak Observatory Universal Birefringent Filter we find: (1) The retardations for adjacent tuning elements made of the same birefringent material are fit by a linear function to within a fraction of the measurement accuracy of the tune solutions over the entire 4000–7000-A spectral range, a precision of ~2 parts in 106. (2) The retardations for a tuning element with quartz compared with a tuning element with calcite birefringent material show excursion from a straight line of ~1 part in 75 and are fit by a quadratic to a fraction of the measurement accuracy of the coarser tuning elements, ~1 part in 5000. (3) The retardation of calcite as a function of inverse wavelength shows excursion from a straight line of ~1 part in 40 and is fit without systematic error by a restricted quintic. Using this parametrization to tune to a selected wavelength gives a tuning element angular positioning accuracy of better than 1° over the entire visible spectrum and an absolute spectral positioning of better than 0.010 A. A spectral stability much better than this is reproduced consistently during a day’s run and from day to day after correction for a uniform temperature variation within the filter determined by monitoring the wavelength of a spectral reference feature, most often the He–Ne laser line.

Bidimensional spectroscopy of the solar chromosphere during the maximum year

Our program was based on simultaneous observations of the same solar region with the Universal Birefringent Filter (UBF) at the Vacuum Tower Telescope (VTT) and with the Big Dome BD Universal Spectrograph (USG). For the UBF images analysis we refer to the 13 June 1980 RG 2502/2511 (N12-E11) observations. Selected pictures present the line profiles for H α, H β , Mg- b 1 and Na- D 2 at some interesting “pixels” of the analyzed AR. We present the detailed photometric morphology of the observed active area and the longitudinal velocity field pattern. A comparison with the corresponding UVSP data is given.

Bidimensional analysis of solar active regions and flares

We describe a method of solar bidimensional spectroscopy exploiting the performances of a Universal Birefringent Filter (UBF) like that of the Sacramento Peak Observatory, which enable an estimate of the profile of some chromospheric lines with moderate spectral resolution (\((\lambda /\Delta \lambda = 2.5 x 10^4 )\)). The numerical inversion technique of Backus and Gilbert has been used to retrieve the estimated line profiles; the capabilities of the proposed method is fully analyzed with some numerical tests and examples. Correction procedures for errors in the positions of the UBF passband, random fluctuations of the exposure times and non-uniform brightness distribution on the filtergrams are also presented. The whole method has been tested on the recovery of quiet atmosphere line profiles and the results derived for the Na D2 line show that the proposed method is completely suitable for many investigations in solar physics.

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 Hα and the Hα continuum using the Universal Birefringent Filter of the Sacramento Peak Observatory.

Observing the sun with a fully tunable Lyot-Ohman filter

We describe the properties of the Zeiss Universal Birefringent Filter and its optical and electronic interface with the Sacramento Peak Observatory'' vacuum telescope. The instrument permits observations of solar intensities, velocities, and/or magnetic fields in rapid succession in any Fraunhofer line in the 410-700-nm wavelength region with high spectral (0.004-0.013 nm) and high spatial (~1/3 sec of arc) resolution.

Properties of the Solar Filigree Structure

A number of observers have noted the presence of bright structures near the cores of the chromospheric rosettes when observed in the far wings of the Hα line (eg Hα ±7/8 Å). Dunn and Zirker observed these bright structures with the highest possible resolution using the Sacramento Peak vacuum solar telescope. They find that these bright regions exhibit a very intricate fine structure which can be followed out much further into the Hα line wing (eg Hα + 2 Å) and even into the continuum. They called this fine structure ‘solar filigree’, the name referring mainly to the collective appearance of the fine structure elements. The elements themselves appear as dot-like structures and frequently also as small wiggly structures called ‘crinkles’. The properties of the filigree structure are summarized as follows: (i)Size: Measured diameter of the crinkles and dots equals 0.25, 0.40 and 0.60″ at Hα + 2 Å, Hα ± 7/8 Å and Hα ±5/8 Å respectively. The telescope resolution equals 0.22″ so that at Hα + 2 Å the structure is extremely small. The drawings in Figure 1 show typical sizes of the crinkles and network patterns in the filigree.(ii)Contrast: Filigree is enhanced in the blue wing of the Hα line. Measured contrast, uncorrected for seeing, equals 5–10%.(iii)Relation to the Granulation: The filigree structures tend to lie between the granules. This is, however, not a strict rule. It seems that in the course of their lifetime the granules move the filigree structures around with velocities of about 1.5 km s-1. Some of the crinkles also seem to wash out temporarily until compressed again by a new granule. The detailed structure of the filigree, therefore, changes significantly over times comparable to the granule lifetime. The overall structure is, however, preserved over much longer periods of time. The granulation pattern when observed in the continuum well outside the Hα line appears very peculiar in that it has substantially decreased in contrast. It appears ‘soft’ similar to granulation washed-out by seeing. This abnormal granulation can be traced over long times (&gt; 30 min) and coincides in location to the filigree location. It is, therefore, definitely real.(iv)Relation to the spicules: The filigree structure falls near the center of the Hα chromospheric rosettes. These rosettes consist of dark elongated mottles which should probably be identified with spicules. There is, therefore, at least a coarse relation between the occurrence of spicules and the filigree. There is no clear evidence that variations in the filigree pattern are related to the generation of spicules. Some spicules seem to originate from the spaces between the crinkles. Too few, however, to conclude a definite relation.(v)Relation to the magnetic field: Beckers studied the filigree with the Universal Birefringent Filter in the magnesium b1 and b2 lines. It is very well visible in the far wing of the lines (eg. b1 ±0.8 Å). When traced into the line core the structures increase somewhat in size, as they do in Hα, and form structures similar to, and perhaps identical with, the so-called photospheric network. In the magnetically sensitive b2 line one sees a one-to-one correspondence between these network structures and the magnetic field so that, at least in the layers seen near the core of the b2 line, there is a one-to-one correspondence between the filigree structures and the enhancements in the magnetic field. Simon and Zirker (Solar Physics, submitted for publication) using a spectrograph also found that the filigree occurs in regions of enhanced magnetic field. However, in contrast to the filter observations, they found the magnetic field regions to be much more diffuse (2–3″) so that there is not a one-to-one spatial correspondence between filigree and magnetic field structure.