Observatorio Del Teide Research Articles

On the structure of polar faculae on the Sun

Faculae on the polar caps of the Sun, in short polar faculae (PFe), are investigated. They take part in the magnetic solar cycle. Here, we study the fine structures of PFe, their magnetic fields and their dynamics on short time scales. The observations stem from several periods in 2001 and 2002. They consist of spectropolarimetric data (Stokes I and V) taken in the Fe  6301.5 and 6302.5 A and Fe  6149.3 A lines with the Gregory-Coude Telescope (GCT) and the Vacuum Tower Telescope (VTT) at the Observatorio del Teide on Tenerife. At the VTT, the Gottingen two-dimensional Fabry-Perot spectrometer was used. It allows image reconstruction with speckle methods resulting in spatial resolution of approximately 0. �� 25 for broadband images and 0. �� 5 for magnetograms. The application of singular value decomposition yielded a polarimetric detection limit of |V |≈ 2 × 10 −3 Ic .W e fi nd that PFe, of size of 1 �� or larger, possess substantial fine structure of both brightness and magnetic fields. The brightness and the location of polar facular points change noticeably within 50 s. The facular points have strong, kilo-Gauss magnetic fields, they are unipolar with the same polarity as the global, poloidal magnetic field. The ambient areas, however, exhibit weak flux features of both polarities, as in the quiet Sun near disk center. Strong upflows of 1 km s −1 are detected in the intensity profiles of PFe. The zero-crossings of the Stokes V profiles yield an average velocity of the magnetized plasma of vFT ≈− 0. 4k m s −1 (towards the observer).

Simultaneous Visible and Infrared Spectropolarimetry of a Solar Internetwork Region

We present the first simultaneous infrared (IR) and visible spectropolarimetric observations of a solar internetwork region. The Fe I lines at 6301.5, 6302.5, 15648, and 15652 A were observed, with a lag of only 1 minute, using highly sensitive spectropolarimeters operated in two different telescopes (Vacuum Tower Telescope and THEMIS at the Observatorio del Teide). Some 30% of the observed region shows IR and visible Stokes V signals above noise. These polarization signals indicate the presence of kilogauss (kG) magnetic field strengths (traced by the visible lines) coexisting with sub-kG fields (traced by the infrared lines). In addition, one-quarter of the pixels with signal have visible and IR Stokes V profiles with opposite polarity. We estimate the probability density function of finding each longitudinal magnetic field strength in the region. It has a tail of kG field strengths that accounts for most of the (unsigned) magnetic flux of the region.

On the fractal dimension of small-scale magnetic structures in the Sun

We compare, by means of fractal analyses, the shapes of observed small-scale magnetic structures on the Sun with those of magnetic features resulting from numerical simulations of magnetoconvection. The observations were obtained with the “Gottingen” Fabry-Perot spectrometer at the Vacuum Tower Telescope at the Observatorio del Teide on Tenerife. Magnetograms with 0$\farcs$4–0$\farcs$5 spatial resolution were obtained from two-dimensional Stokes V polarimetry in the $\ion{Fe}{i}$ 6302.5 A line and by image reconstruction with speckle methods. The simulations of magnetoconvection was performed with the MURAM code. It solves the time-dependent MHD equations for a compressible, partly ionized plasma including radiative transfer in LTE. To determine the fractal dimensions the perimeter-area relation is used. We discuss the influence of seeing and noise in the fractal dimension D of the observed magnetograms. A dependence of D on the distance from disk center could not be found. The observations give $D=1.21\pm0.05$ for a pixel size corresponding to 0$\farcs$105, while for the numerical simulations $D=1.38\pm0.07$ for a pixel size of 20.83 km. If we use a yardstick adapted to the spatial resolution the observations give the dimension $D=1.41\pm0.05$ in close agreement with the simulations. This agreement is remarkable since the pixel sizes and spatial resolutions of the simulations and of the observations differ by a factor of 15. The finding supports the view of self-similarity of solar magnetic structures over a large range of scales. In addition, it demonstrates the realism of the simulations and suggests that all important physical processes are included. We discuss our results in comparison with other investigations.

Time series of high resolution photospheric spectra in a quiet region of the sun

A 50 min time series of one-dimensional slit-spectrograms, taken in quiet sun at disk center, observed at the German Vacuum Tower Telescope (Observatorio del Teide), was used to study the global and spatial variations of different line parameters. In order to determine the vertical structure of the photosphere two lines with well separated formation heights have been considered. The data have been filtered of p-modes to isolate the pure convective phenomenon. From our studies of global correlation coefficients and coherence and phase shift analyzes between the several line parameters, the following results can be reported. The convective velocity pattern preserves structures larger than 1.0" up to the highest layers of the photosphere (~ 435 km). However, at these layers, in the intensity pattern only structures larger than 2.0" are still connected with those at the continuum level although showing inverted brightness contrast. This confirms an inversion of temperature that we have found at a height of ~140 km. A possible evidence of gravity waves superimposed to the convective motions is derived from the phase shift analysis. We interpret the behavior of the full width at half maximum and the equivalent width as a function of the distance to the granular borders, as a consequence of enhanced turbulence and/or strong velocity gradients in the intergranular lanes.

Quiet-Sun inter-network magnetic fields observed in the infrared

This paper presents the results of an investigation of the quiet Sun's magnetic field based on high-resolution infrared spectropolarimetric observations obtained with the Tenerife Infrared Polarimeter (TIP) at the German VTT of the Observatorio del Teide. We observed two very quiet regions at disc centre. The seeing was exceptionally good during both observing runs, being excellent during one of them. In both cases the network was intentionally avoided to the extent possible, to focus the analysis on the characteristics of the weak polarization signals of the inter-network regions. We find that the Stokes V profile of Fe  15648 A line in almost 50% of the pixels and Stokes Q and/or U in 20% of the pixels have a signal above 10 −3 (in units of continuum intensity Ic), which is significantly above the noise level of 2−3 × 10 −4 . This implies that we detect fluxes as low as 2 × 10 15 Mx/px. We find evidence that we have detected most of the net flux that is in principle detectable at 1 �� resolution with the Zeeman effect. The observed linear polarization resulting from the transverse Zeeman effect indicates that the magnetic fields have a broad range of inclinations, although most of the pixels show polarization signatures which imply an inclination of about 20 ◦ . Nearly 30% of the selected V-profiles have irregular shapes with 3 or more lobes, suggesting mixed polarities with different LOS velocity within the resolution element. The profiles are classified using a single value decomposition approach. The spatial distribution of the magnetic signal shows that profiles of different classes (having different velocities, splitting, asymmetries) are clustered together and form patches, close to the spatial resolution in size. Most of the field is found to be located in intergranular lanes. The statistical properties of the mainly inter-network field sampled by these observations are presented, showing that most of the observed fields are weak with relatively few kG features. The field strength distribution peaks at 350 G and has a FWHM of 300 G. Other parameters, such as profile asymmetries, filling factors and line-of-sight velocities are also determined and discussed.

Models of a mean granular cell

AbstractFrom inversion of a time series of slit spectra, observed in a quiet region of the solar photosphere, averaged models of a granular cell have been obtained showing the stratification of physical quantities versus optical depth and geometrical height. Furthermore a semi‐empiric dynamic model of a mean granular cell has been derived and the results are presented.

Dynamics of the solar granulation

We study the evolution of the granulation dynamics from the observational point of view. Based on series of excellent spectrograms taken at the VTT, Observatorio del Teide (Tenerife), in 1999, we calculated temporal - spatial maps of the Doppler velocity, line width, and intensity in order to track the dynamical behavior of these observables at different positions along the spectrograph slit. The Doppler velocity map reveals a granular dynamical time - the characteristic time associated with the decay of the Doppler velocity - of approximately 2 min, while the line width map does not show any characteristic time scale but rather a strong intermittence. The intensity map reveals the life time of the granulation as it is given in the literature. The granular dynamical time is practically equal to the value determined from spectrograms taken at the solar minimum 1994; so the dynamical time does not show any change over the solar cycle. The stochastic properties of the Doppler velocity and intensity data samples are studied (i) by means of their statistical moments and (ii) theoretically using presupposed model distributions. For the latter we estimated the distributions' parameters by means of the maximum likelihood method. The histograms of the Doppler velocity variations point to an asymmetric model distribution, while the histograms of the intensity variations infer a symmetric one. The intensity variations can be described well by a Gaussian probability density function, while the Doppler velocity variations are described by the double exponential (Gumbel) distribution, an asymmetric probability function. A remarkable result of the statistical analysis based on both series of observations in 1994 and 1999 is the unambiguous lack of flows with large velocity amplitudes within the intergranular space.

On the brightness and velocity structure of solar granulation

A 45 min time series of two-dimensional spectra has been obtained with the Vacuum Tower Telescope at the Observatorio del Teide in Izana, Tenerife. Scans over the non-magnetic $\ion{Fe}{i}\,5576$ A line of a quiet granular field at disk center were taken simultaneously with a time series of broad band images. From the spectra intensity and velocity maps have been calculated at different line-depths. From the white light images granular shapes have been computed by means of an automatic image segmentation algorithm. A statistical analysis of the intensity and velocity distribution in the detected granular shapes has been carried out. Intensities and velocities are well correlated at low photospheric levels. In the higher photosphere the intensity pattern dissolves whereas the velocities show almost no variation within the probed height interval. The intensity excess of small granules dissolves at lower heights than that of larger ones. The intensity and velocity distribution within the granules depends on the granular size. In smaller structures the maximum intensities and velocities are located close to the granular barycenters whereas for larger granules the maxima are shifted towards the granular boundaries. The width of the transition zone between granules and intergranular lanes is independent on the granular intensity and is constant at approximately $0\farcs 5$. The time evolution of the granular pattern shows a clear dependence of the lifetime of structures on the spatial wavenumber. The e -folding times of the temporal coherences decrease according to a power law with an exponent of $\beta = 3/2$ which is incompatible with the Kolmogorov energy spectrum of homogeneous and isotropic turbulence and might be taken as a hint against the overall turbulent character of granular motions.

Weak magnetic flux features on the Sun

We observed weak magnetic flux in enhanced Ca K network and in its surroundings at various positions on the solar disc ($\cos\theta=1.0\dots0.1$). The Gregory Coude Telescope at the Observatorio del Teide on Tenerife was used together with an image scanner and a Stokes V polarimeter. ${1\over2}(I_\lambda\pm V_\lambda)$ spectrograms in the 6302 A region were taken. A second order flat fielding and smoothing brought the polarimetric sensitivity to a detection limit of 0.001 $I_\mathrm{c}$ (with $I_\mathrm{c}$ = continuum intensity) for exposure times of 300 ms in pixels corresponding to 0$\farcs$ $5\times$0$\farcs$19 ($\widehat=$ slit width $\times$ pixel size of detector). We present correlations between continuum intensity, line center intensity, and magnetic flux and discuss Stokes V profiles. The V amplitude asymmetry from areas with strong flux decreases towards the limb, a change of sign is not identified. The magnetic fields are ordered around strong flux centers with decreasing mean field strength outwards from the centers. In the range of $1\times 10^{16}$ Mx to $5\times 10^{16}$ Mx (in one pixel of $5\times 10^{14}$ cm 2 ) the probability distribution of the magnetic flux is ~$|\Phi|^{-q}$ with $q\approx3$, different from a Gaussian distribution. From this we derive tentativily a radial dependence from the flux centers of the form $\Phi(r)\sim1/r$. For the probability distributions a significant center-to-limb variation is not detected. Together with the center-to-limb behaviour of the asymmetries, we interpret this tentatively as an isotropic orientation of the weak magnetic flux elements and of the gas flows. A vertical orientation of the magnetic fields is also compatible with the measurements, though.

Post-focus instrumentation for GREGOR

GREGOR is a high-resolution solar telescope with an aperture of 1.5 m. It will be equipped with an Adaptive Optics system and is designed for high-precision measurements of magnetic fields and plasma motions in the solar photosphere and chromosphere with a resolution of 70 km on the Sun. GREGOR will replace the Gregory Coudé Telescope at the Observatorio del Teide on Tenerife. In concert with the other solar telescopes at Teide Observatory it will be useful for studying the dynamics of the solar atmosphere and the underlying physical processes. GREGOR will also serve as a test bed for next generation solar telescopes. We discuss briefly the postfocus instrumentation of GREGOR.

Post-focus instrumentation for GREGOR

GREGOR is a high-resolution solar telescope with an aperture of 1.5 m. It will be equipped with an Adaptive Optics system and is designed for high-precision measurements of magnetic fields and plasma motions in the solar photosphere and chromosphere with a resolution of 70 km on the Sun. GREGOR will replace the Gregory Coudé Telescope at the Observatorio del Teide on Tenerife. In concert with the other solar telescopes at Teide Observatory it will be useful for studying the dynamics of the solar atmosphere and the underlying physical processes. GREGOR will also serve as a test bed for next generation solar telescopes. We discuss briefly the postfocus instrumentation of GREGOR.

Analysis of the solar cycle and core rotation using 15 years of Mark-I observations: 1984-1999

High quality observations of the low-degree acoustic modes (p-modes) exist for almost two complete solar cycles using the solar spectrophotometer Mark-I, located at the Observatorio del Teide (Tenerife, Spain) and operating now as part of the Birmingham Solar Oscillations Network (BiSON). We have performed a Fourier analysis of 30 calibrated time-series of one year duration covering a total period of 15 years between 1984 and 1999. Applying different techniques to the resulting power spectra, we study the signature of the solar activity changes on the low-degree p-modes. We show that the variation of the central frequencies and the total velocity power (TVP) changes. A new method of simultaneous fit is developed and a special effort has been made to study the frequency-dependence of the frequency shift. The results confirm a variation of the central frequencies of acoustic modes of about 450 nHz, peak-to-peak, on average for low degree modes between 2.5 and 3.7 mHz. The TVP is anti-correlated with the common activity indices with a decrease of about 20% between the minimum and the maximum of solar cycle 22. The results are compared with those obtained for intermediate degrees, using the LOWL data. The frequency shift is found to increase with the degree with a weak l-dependence similar to that of the inverse mode mass. This verifies earlier suggestions that near surface effects are predominant.

Speckle spectro-polarimetry of solar magnetic structures

We present speckle observations of small-scale magnetic structures on the Sun. They were obtained with the GottingenFabry-Perot interferometer (FPI) in the Vacuum Tower Telescope at the Observatorio del Teide, Tenerife, from quiet and active regions close to disc center. A Stokes V polarimeter was added to the FPI to measure V profiles in the Fe I 6302.5 Aline. The setup allows image reconstruction with speckle methods. The achieved spatial resolution in the magnetograms is -. We describe the observational technique and the data reduction. The results from small-scale magnetic flux elements in a very quiet region, in an active region with pores and abnormal granulation, and in a sunspot with its surroundings are discussed. In the quiet Sun, granular dynamics dominate the time evolution of the magnetic elements. Flux occurs in both bright intergranular points and in dark intergranular spaces. Likewise, with the present spatial resolution, no preference of magnetic flux in abnormal granulation in an active region can be found. Flux occurs in both bright (abnormal) small-scale granules and in the darker spaces in between them. The small sunspot studied has very little magnetic flux in its ambient quiet regions, especially no strong, conspicuous concentrations of returned flux, i.e. of flux with polarity opposite to that in the sunspot.

A new observational strategy in search for solar g-modes

The aim of the present work is the detection of solar g-modes, by means of a new observational strategy based on the exploitation of their spatial and temporal properties. The basic data, obtained at the Observatorio del Teide in 1993, consists of daily solar velocity measurements taken continuously and sequentially at six different and symmetric positions on the solar disk. By correlating the time series resulting from the reduction process, from different solar disk positions and considering the geometrical properties of different modes (l,m) on the Sun's surface, some of these can be selectively eliminated or enhanced. Moreover, the main spectral features present in the resulting power spectra must have precise phase relations if they correspond to global solar g-modes. The severe constraints established by the above properties have been applied to the best observed series (summer 1993). As a result, a discrete series of peaks have been selected that fulfill all the imposed conditions and which can therefore be interpreted as being of solar origin.

Time Evolution of Horizontal Subsurface Flows in the Sun

A ring-diagram analysis technique has been applied to a region of the Sun of about 15 deg2 area close to the disk center at dates chosen for several solar rotations in 1996: April 5, May 1, May 28, June 27, July 23, August 20, August 21, and September 18. The horizontal velocity flows inferred directly beneath the solar surface have thus been obtained for the same region of the Sun at the given dates. The data have been obtained at the Observatorio del Teide and the Tashkent stations with the Taiwan Oscillation Network (TON), consisting of 1080 × 1080 pixel Ca II K-line intensity images. The stability of the results, the stability of the method, and the possible temporal evolution of the subsurface flows have been studied from the results. A rotation curve for the differential radial solar rotation down to the first 27 Mm of depth at the equator is also suggested and qualitatively compared to results obtained by other authors.

Broad-band $JHK(L')$ photometry of a sample of giants with 0.5 $\mathsf{>}$ [Fe/H] $\mathsf{>}$ -3

We present the results of a three-year campaign of broad-band photometry in the near-infrared J, H, K and L 0 bands for a sample of approximately 250 giant stars carried out at the Observatorio del Teide (Tenerife, Spain). Transformations of the Telescopio Carlos S anchez system into/from several currently used infrared systems are extended to the redward part of the colour axis. The linearity of our photometric system in the range 3m ag (Fe/H) > 3. Data of comparable quality previously published have been added to the sample in order to increase the reliability of the relations to be ob- tained. We also provide mean IR colours for giant stars according to spectral type 1 .

On the Reliability of Ring Diagram Analysis

A comparison between the results of applying a ring diagram analysis to two data sets is described in order to check the quality of the two sets and the reliability of the method. The series used have been obtained with two different helioseismological instruments with the same spatial resolution. A section of 30 × 30 deg2 about the disk center has been tracked during 512 minutes over a series of images taken simultaneously by the Taiwan Oscillation Network instrument station at the Observatorio del Teide and the Michelson Doppler Imager on board the Solar and Heliospheric Observatory. Three-dimensional power spectra have been constructed from both series and fitted to get horizontal velocity flows as a function of frequency. Finally, an inversion process has been applied to obtain the depth dependence of the average velocity vector in a depth range of 0-25 Mm below the solar surface. The results show an acceptable correlation between the velocities obtained from the fitting of the spectra of both data sets in spite of the different kinds of noise affecting the two instruments. However, the correlation drops when comparing the velocities obtained after the inversion process based on these results.

Speckle Masking Imaging of Sunspots and Pores

In recent years, speckle interferometry has been successfully applied to various solar phenomena and provides a powerful tool to study solar small-scale structures. The present investigation lays special emphasis on sunspots and sunspot pores. The observations have been performed with the Vacuum Tower Telescope (VTT) at the Observatorio del Teide (Tenerife) in the years from 1992 to 1994. Time series of high-spatial-resolution observations reveal the highly dynamical evolution of sunspot fine structures such as umbral dots, penumbral grains or the small-scale brightenings in the vicinity of sunspots observed in the wings of strong chromospheric absorption lines (moustache phenomenon). The reconstructed images show small-scale structures close to the telescopic diffraction limit of 0.16″ at 550 nm. Furthermore, the high transmission of a Fabry–Perot interferometer (FPI) as the principal optical element of a two-dimensional spectrometer allows one to reconstruct directly images taken within a passband of 0.014 nm.

Aplication of A New Observational Strategy to the Study of Gravitational Solar Modes

The aim of the present work is the detection of solar g-modes, making use of their spatial and temporal properties, by means of a new observational strategy. The basic data, gathered at the Observatorio del Teide in 1993, consists on daily solar velocity measurements taken continuous and sequentially at six different and symmetric positions on the solar disk. By correlating the time series obtained from different positions, and considering the geometrical properties of different modes (l, m) on the Sun‘s surface, some of them can selectively be eliminated or enhanced. In particular, the main spectral features present in the resulting power spectra must have precise phase relations if they correspond to global solar g-modes.

Comparison of Two Fitting Methods for Ring Diagram Analysis of Very HighlSolar Oscillations

A new method of fitting tridimensional power spectra of solar oscillations is described and compared with a previous one whose use has been more common. The new method fits the parameters of the Lorentzian profiles in a bidimensional k - ω diagram constructed from an azimuthal average of the tridimensional one. The horizontal velocities are then determined keeping these parameters fixed, greatly reducing the computation time. Both methods are compared for two radial orders (n = 3, 4) of a tridimensional power spectrum obtained for a region of about 15° square around solar disk center. The images used in this work correspond to a 3 day set of 1080 × 1080 pixel intensity images obtained at the Observatorio del Teide on 1994 November 8-10 with the Taiwanese Oscillation Network (TON) instrument. The results of the fitted velocities agree within the estimated errors for the two methods. The reduction of the computing time obtained with the new method makes it convenient for the ring diagram analysis.