An analysis of 1983 observations of facular contrast with an extreme limb photometer

Analysis of facular contrast I/I _qs from Extreme Limb Photometer (ELP) data of the summer of 1983 yield a mean contrast of 0.91 ± 0.19% and 1.57 ± 0.16% for apertures 1 and 2 located at = 0.198 and at = 0.111, respectively. The ratio of the mean contrast in the outer aperture (closer to the limb) to that of the inner one is 1.71 ± 0.40, indicating an increase in the mean facular contrast toward the limb. This result is in agreement with observations made in 1975, 1979, and 1982. The errors are dominated by the random presence of solar active regions. The combined results from all seasons follow an approximately ^–1 curve. Facular excess solar oblateness signals for 1983 are 33.8 ± 6.6 arc ms and 16.5 ± 2.1 arc ms for ELP apertures 1 and 2, respectively, in reasonable agreement with the 1983 excess solar oblateness results of Dicke, Kuhn, and Libbrecht (1985).     

An analysis of full-disk observations of facular contrast in the blue and red

Full-disk images from the Cartesian Full-Disk Telescope no. 2 (CFDT2) were used to study the center-to-limb (CLV) variation of facular contrast in two colors. The CFDT2 images, which have 2.5 arc sec pixels, were obtained during the summer months of 1993, 1994 and 1995. In order to minimize the bias in finding faint facular features in continuum images, we have used coaligned images obtained in the Ca K-line to identify faculae. Faculae were sorted into 20 annular bins of equal width. To reduce the effects of seeing, faculae were not identified closer to the limb than =0.2. The facular pixel contrasts were fitted to various trial functions. The contrast in the blue filter (470.6 nm) rose from 0.122% at disk center to 12.2% at =0.2. The contrast in the red filter (672.3 nm) rose from 0.13% at disk center to 8.16% at =0.2. We have also analyzed the facular contrasts multiplied by their -value to obtain an estimate of facular flux tube contrasts. These flux tube contrasts increased roughly linearly from =0.95 to 0.25. The blue flux tube contrast reached a maximum of 2.48% near =0.25. The red flux tube contrast reached a maximum of 1.59% at =0.2. These contrast values are not corrected for the filling factor. The blue curve leveled off slightly betwen =0.25 and 0.2 while the red curve showed no deviation from its linear trend. These results may provide some support for the hot wall model of facular flux tubes.     

First observations of the granulation at 1.65μ, center to limb variation of the contrast

Brightness fluctuations at 1.65 have been recorded by means of a 64-element array. Infrared photographs clearly show sunspots and granulation with a resolution better than 1. Quantitative analysis of the digitized data is used to compute autocorrelation and power spectrum. Half-width of autocorrelation (405 km) indicates a resolution comparable with the best observations in the visible range. Photographs and quantitative analysis show the existence of a strong contrast variation from the center to the limb. Seeing and instrumental effects are discussed. A model M.T.F. is utilized to compute a foreshortening correction. No attempt is made to get the actual absolute rms value. Nevertheless we find a definite variation of the observed rms which goes from 1.48%±±0.15, at the center, to 1.05%±0.15 at = 0.7 (after foreshortening correction).     

Morphological properties and origin of the photospheric facular granules

From time series of high resolution photographs, morphological properties of the photospheric facular granules were derived. The facular granules are cells of the common granular pattern, brighter than the normal granules when seen between cos = 0.6 and the limb. Their apparent diameter, which decreases towards the limb, is smaller than that of the normal granules: 0.65 and 1.25 respectively at cos = 0.55; their lifetime is 25 min but their bright stage lifetime is only 15 min; they are visible closer to the limb than the normal granules: 1.2 compared to 2–5; the brightening of the facular granules occurs at a faster rate than their fading. From the great similitude of both morphological properties and temperature models of facular and normal granules, it appears possible that the photospheric facular granules are convective cells modified by the presence of a magnetic field of some hundreds Gauss.     

Precise wideband photometry of photospheric faculae with an emphasis on the disk center

The center-to-limb variation of the excess intensity in faculae was obtained for 266 active regions with an accuracy of 10^–3. For this observation full-disk images were obtained with a rotating one-dimensional diode array whose rotation axis was set at the disk center, at the wavelength of 5450 Å with a bandpass of 400 Å. From the center-to-limb variation of excess intensity of active regions the excess effective temperature was found to be 6.4 K on the average where the mean longitudinal magnetic field is 65 G as measured by 5233 Å line. In other words the ratio of the excess radiative flux to the total flux was 0.44% on the average for the present measurements of low spatial resolution of 20.The average excess intensity for 60 active regions near the disk center was found to be 4 × 10^–4 of the quiet Sun intensity. This very low excess brightness averaged over the whole active region, in contrast to the reported high excess brightness of facular points (diameter 0.2) of 0.4, leads to a hypothesis that the background in between facular points in the active region is darker than the true quiet photosphere by 1%. It is further surmised that the inferred darkness of intra facular points is due to partial compensation for excess total irradiance of facular points. This interpretation is also consistent with previous observations of the contrast of facular points near the limb.     

Continuum photometry of solar white-light faculae

We have determined absolute continuum intensities and brightness temperatures of individual facular grains at a spatial resolution limited by the =50 cm aperture of the SVST on La Palma. A facular region at 57° was observed simultaneously in three narrow continuum windows at 450.5, 658.7, and 863.5 nm. We corrected for image degradation by the Earth's atmosphere using the speckle masking method. The brightness temperatures do not exactly follow the Planck law. The differences of T _blue–T _red=220 K and T _ir–T _red=–42 K reflect the wavelength dependence of the continuum formation depth. The (red) temperatures of 250 facular grains show excesses between 250 and 450 K above their undisturbed neighborhood. The wavelength dependence of the relative intensity ratios C= [I ^fac/I ^phot] show a large scatter around mean values of C _blue/C _red=1.075 and C _ir/C _red=0.98. We determined the center-to-limb variation of the 863.5 nm continuum contrast for 0.17>cos>0.39 by measuring 270 grains in reconstructed facular images. The upper envelope of the data points increases linearly to 1.5 at cos=0.17. Application of the mean color dependence yields green contrasts up to C ₅₅₀=1.7, which is far higher than previously observed values. The behaviour for cos>0.17 is estimated from (unreconstructed) frame-selected best images taken over a time interval of 7 hours. Six distinct facular regions clearly discernible during the whole time interval indicate a slight contrast decrease towards the extreme limb. The observed quantities are useful for an adjustment of model calculations and for a discrimination of competing models.     

Pulsar radio emission. I. Oblique rotator

The problem of pulsar radio emission for the case of a coaxial rotator was investigated in our preceding paper [G. S. Sahakian, Astrofizika,38, 143 (1995)]. In this paper it is solved for the realistic case in which the star's magnetic axis does not coincide with its rotational axis (an inclined rotator). It is shown that above the star's magnetic cap a special region, called a magnetic funnel, is formed in which vigorous processes of particle multiplication occur. The height of this region is h 8·10^60.2 ₃₀ ^1/3 R ₆ ^1.3 cm and its radius r(r/c)^0.5 depends little on the inclination angle a ( is the angular rotation rate, is the magnetic moment, R is the star's radius, and r is distance from the center of the star). It is shown that the pulsar radio emission is produced in the magnetic funnel. Here, in the course of active radiative processes, two main particle fluxes with a high ultrarelativistic energy are formed: an upward electron flux and a positron flux falling onto the star's magnetic cap. These main fluxes are accompanied by individual narrow strips of positron and electron fluxes with a relatively low energy, which are fairly powerful, coherent radio sources. Such secondary fluxes are formed immediately after the annihilation of photons of curvature radiation emitted by particles of the main fluxes. The pulsar's radio luminosity is estimated to be L7.4·10^233.52 ₃₀ ^8/3 (a), where (a) is a known function (1 for a<50°). Equating the theoretical and observed radio luminosities L and L₀, we obtain the formula ₃₀P^1.32R ₆ ^0.4 (2.1·10^–27L₀/)^3/8 for the magnetic moment of the pulsar's neutron star, where P is the pulsar's period. The magnetic moments of slow pulsars calculated from this formula turn out to be considerably larger than those of fast pulsars. This means that the masses of slow pulsars are larger, on the average, than those of fast pulsars. The magnetic funnel operates with interruptions, periodically undergoing a discharge, so that the production of pulsar radio emission operates with interruptions. The durations of the production of radio emission and of the interruptions between those processes are on the order of h/c2.7·10^–40.2 ₃₀ ^1/3 sec, i.e., pulsar radio emission has a microstructure. Consequently, a study of the microstructure of the profiles of observed radio pulses enables one to obtain additional information about the magnetic moments of the neutron stars.Translated from Astrofizika, Vol. 39, No. 2, pp. 313–335, April–June, 1996.     

Charge-coupled device imaging spectroscopy of Mars. 1. Instrumentation and data reduction/analysis procedures

This paper describes the collection, reduction, and analysis of 0.4–1.0 m Mars imaging spectroscopy data obtained during the 1988 and 1990 oppositions from Mauna Kea Observatory and provides a general outline for the acquisition and analysis of similar imaging spectroscopy data sets. The U.H. 2.24-m Wide Field Grism CCD Spectrograph was used to collect 13 three-dimensional image cubes covering 90% of the planet south of 50°N in the 0.4–0.8 m region (/=245 at 0.6 m) and covering 55% of the planet south of 50°N in the 0.5–1.0 m region (/=293 at 0.75 m). Spectra extracted from these image cubes reveal the detailed character of the martian near-UV to visible spectrum. Images at red wavelengths reveal the classical albedo markings at 100–500 km spatial resolution while images at blue wavelengths show little surface feature contrast and are dominated by condensate clouds/hazes and polar ice. Many of the data acquisition, reduction, and analysis steps discussed here are new or unique to imaging spectroscopy data sets. These techniques exploit the information contained within the spatial domain of data such as these, thus allowing more traditional point-spectral analysis techniques to be expanded into an imaging format.     

High resolution photographs of the sun near 200 nm

A balloon-borne telescope has been flown two times at altitudes higher than 30 km. We obtained high resolution pictures of the Sun at 200 nm, 210 nm, 310 nm, and 460 nm. The spatial resolution for high contrast objects was 0.5 arc sec.At 460 and 310 nm, granulation and faculae are visible. At 200 nm we find only bright grains of a mean area of 4 arc sec² and a mean contrast of 47%. Though these grains are visible to the center of the disk, we identify them as facular grains. A statistical study of these grains permits me to derive a facular grain model with a temperature excess of 180 K with respect to the surrounding photosphere starting at the altitude of 180 km. This model successfully predicts the grain observed contrast at the different wavelengths and the intensity excess.     

Observational studies of the solar intensity profile in the far infrared and millimeter regions

Observations of the intensity distribution near the solar limb at 2.43 and 22.5 , show the absence of limb brightening to within 1 or 2 arc sec of the limb. Observations at 1.2 mm indicate limb brightening at this wavelength. These results are compared with the Utrecht Reference Photosphere and with existing data on the solar flux in the millimeter range, and suggest that the temperature minimum is broad and extends above ₅₀₀₀ = 2 × 10^–3. A sharp rise of temperature is required above ₅₀₀₀ = 10^–5.     

The ESA project for a Grazing Incidence Solar Telescope (GRIST)

The Grazing Incidence Solar Telescope (GRIST) as it is being studied by the European Space Agency (ESA) is described: A soft X-ray and extreme-ultraviolet (XUV) facility for solar observations with 1 arc sec spatial resolution in a wavelength range extending from 9 to beyond 100 nm. The telescope, a 35° sector of a Wolter, type-II, configuration, will have a focal length of 4 m and a collection area of 280 cm².It is planned that GRIST be flown on joint Spacelab flights with the Solar Optical Telescope (SOT), a NASA facility for the wavelength range, 110 nm to 1 m, which affords a 0.1 arc sec spatial resolution. This would render possible synoptic observations of the solar atmosphere with the highest ever achieved spatial resolution at wavelengths between 9 nm and 1 m.Possible focal-plane instruments and the most important research goals are briefly described.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.On behalf of the ESA Consultants for the Phase-A Study of GRIST: A. H. Gabriel, U. Grossmann-Doerth, M.C.E.H., M. Malinovsky-Arduini, G. Tondello, and H. F. van Beek.     

Airborne total eclipse observation of the extreme solar limb at 400 μm

The total solar eclipse of February 26, 1979 was monitored at far infrared wavelengths from the NASA Lear Jet Observatory flying at 12.9 km in the eclipse shadow. The resultant eclipse curve for radiation within a bandwidth of 20 cm^–1 centered upon 25 cm^–1 (400 m) was measured and analysed at an equivalent angular resolution of 1 arc sec over a 100 arc sec region adjacent to the limb to provide information on the intensity distribution of continuum radiation close to this limb. The curve has been compared to predictions derived from models of the solar atmosphere for the specific geometry of this eclipse, and is shown to match most closely that derived from a uniform distribution of radiation across the disk. This is in distinct contrast to the result obtained in the only other comparable experiment, carried out over Africa in 1973 from a supersonic Concorde aircraft, in which an intense but narrow spike of far infrared radiation at the extreme solar limb was inferred from the data. The absence also in the present observations of the significant limb brightening predicted by the HSRA model (in which homogeneity within the source region is assumed) is in substantial agreement with lower resolution results from mountain altitudes. This result is interpreted as further evidence for the presence in the Sun's lower chromosphere of significant inhomogeneity with a scale size of at least 1000 km at this depth.     

Infrared emission of dust grains in the clusters of galaxies

The observations of the reddening of the distant galaxies and the weak diffuse radiation in the clusters of galaxies can be interpreted as a consequence of the presence of dust grains in the intergalactic medium. When allowance is made for the destruction of the grains in collision with particles of the hot gas, its lifetime is about 10⁷–10⁸ yr at a gas concentrationn _g 10^–3 cm^–3. The detection of the infrared (IR) emission from the galaxy clusters might be the test for the proof of the presence of dust grains in the intergalactic medium. In this paper the estimates of the expected intensities and fluxes of IR emission for the spectral region 50–300 are presented for two galaxy clusters in Coma and Perseus. The parameters of the hot gas spatial distribution are chosen from X-ray observations. Having assumed that intergalactic dust can be ejected only from the galaxies, we used such a model for intergalactic dust grains which explains very well the interstellar dust effects. It is shown that the dust temperature, which is determined from the general energetic balance of the dust grains, can achieve some scores of degrees of Kelvin. Two models of the dust spatial distribution are considered. It is found that the maximum of IR flux for the Coma cluster lies near =100 and the same for the Perseus cluster near 50–70. The total fluxes of IR emission from these clusters are about 10⁵–10⁶ Jy and can be detected by modern observational methods.     

Morphological and evolutional features of Ellerman bombs

Morphological and evolutional features of Ellerman bombs were studied with H filtergrams of two active regions very close to the solar limb. We quantitatively determined the elongated or spike-like shape of the bomb. The mean apparent length of 174 bombs is 1.1 arc sec, while 80% of 204 bombs have a diameter of less than 0.6 arc sec. The mean lifetime of 77 bombs is about 12 min at H - 1.2 Å. The first maximum brightness of a typical bomb is attained, on average in about 2 min. Bombs grow longer in the first brightening phase and their mean upward velocity explains the blue asymmetry of H emission profiles of moustaches.     

Infrared continuum observations of five-minute oscillations

Infrared continuum observations of the Sun at wavelengths between 10 and 30 show a nonisothermal response of the upper photosphere to compression waves associated with the five-minute oscillations. Observations were made with four broad-band filters with effective transmission wavelengths between 10 and 26 and with a 10 aperture. Further observations at submillimeter wavelengths with a 2 aperture did not resolve oscillatory fluctuations of five-minute period.Comparisons with velocity field data of Howard (1976) suggest that the relaxation time of the photosphere exceeds (300/2) seconds at the height of formation of the 26 continuum (_5000Å 10^-2). The photosphere reponds to 3 mHz oscillatory motion with considerably less compression than expected for simple acoustic modes in an adiabatically responsive atmosphere, confirming the evanescent character of the five-minute oscillations.     

Extinction and scattering by several types of silicate sphere of radius 0.05–1.0μm,for the wavelength range 0.21–50μm

The exact calculation of scattering and absorption by various sub-micron sized silicate spheres is presented here, using accurately determined optical constants in the wavelength range from 50 m to 0.21 m. The extinction features near 10 m and 20 m for various samples are discussed. It is found that the ratio of peak extinction at 20 m to that at 10 m is constant for small particles up tor=0.4 m, but is less for particles of radius 1 m. The ratio of maximum extinction in the ultraviolet to that at 10 m decreases with increasing particle size.     

A photometric study of faculae and sunspots between 1.2 and 1.6 μm

We investigate further the interpretation of dark magnetic faculae observed in previous imaging of the solar photosphere at 1.63 m. We show that their contrast at 1.63 m increases with magnetic flux beyond a threshold value of 2 × 10¹⁸ Mx and blends smoothly with the contrast vs flux relation measured at this wavelength for larger structures of sunspot size. Not all facular structures that are bright in Ca K are dark at 1.63 m, apparently because their magnetic flux is not large enough. After correction for blurring, the contrast of the dark faculae observed near the disc center at 1.63 m is approximately 4%. But our observations at 1.23 m, which probe slightly higher photospheric levels, do not show these dark faculae. These results indicate that magnetic flux tubes of diameter as small as 500 km significantly inhibit convective heat flow to the photosphere, much as do sunspot flux tubes of much larger diameter. They also suggest that, in even smaller flux tubes, the inhibition becomes rapidly less significant. Finally, we show that the sunspot-size dependence of umbral infrared contrast versus wavelength that we observe can probably be explained in terms of instrumental blurring. Observations with lower scattered light will be required to determine whether a real decrease of contrast with diameter also plays a role.     

Observations of resonance polarization in Cai λ4227

The resonance polarization in Ca i 4226.73 has been measured close to the solar limb for three values of = cos. The polarization has maxima in the wings at about –500 and + 400 mÅ from the line center, the plane of polarization (electric vector) being parallel to the solar limb. The blue wing is more polarized than the red wing, contrary to earlier results of Brückner (1963). Local minima in the polarization curves occur at the position of blends. The polarization in the wings increases rapidly with decreasing . At = 0.15 the maximum polarization in the blue wing is about 2.4%.The observations are in good qualitative agreement with the recent theory of Dumont et al. (1973), but the observed polarization degrees are significantly smaller than the computed values. This may be explained by the inadequate formula for van der Waals broadening used in the theoretical calculations. Much of the discrepancies between observed and computed values disappear if the recent value of Lites (1974) for the collisional damping constant is used. Further improvements in theory and observation are discussed.     

Spatial offsets between lines and continuum in limb faculae

High-spatial-resolution spectra of limb faculae show spatial displacements of line cores relative to the adjacent continuum at cos<0.35, confirming results from earlier photographic analyses. The observed displacements are geometric measures which probe the upper atmospheric layers of fluxtubes forming facular grains. We selected spectra with highest contrast and smallest width of the facular continuum streaks, in order to avoid clustered structures. The spatial displacements of Stokes-Q and -V maxima were also measured: we find Q-signals spatially located near the continuum, as is expected from their origin in line wings; V-signals – which should give evidence for the existence of horizontal fields – are not found, except for one case of a 'hidden' pore.     

Tides and nutation of the earth: I. Models of an earth with an inelastic mantle and homogeneous,inviscid, liquid core

The use of very-long-baseline radio interferometers during the past 10–15 years has increased the accuracy of amplitude measurements of the Earths forced nutation by more than two orders of magnitude (from 3–5 arc ms to 20 arc s). At the same time, cryogenic gravimeters (which depend for their action on the repulsion of two superconductive rings in a gravitational field) have made it possible to also improve the accuracy of measurements of tidal variations in the gravitational force by two orders of magnitude. This opens up new avenues for the investigation of the mechanical properties of the Earths interior at ultralow frequencies. A brief review is given here of the basic results of interpreting astrometric data and measurements of tidal variations in the gravitational force. Problems suggested by the new observational techniques are formulated.Translated from Astronomicheskii Vestnik, Vol. 38, No. 6, 2004, pp. 542–558.Original Russian Text Copyright © 2004 by Molodensky.