Tower Telescope Research Articles

Two-dimensional spectroscopy of a sunspot

We investigate the properties of the fine structure of a sunspot penumbra based on spectroscopic measurements with high spectral (λ/δλ = 250 000) and high spatial (≈0.5 arcsec) resolution. The magnetically insensitive Fe I 557.6 nm line is used to probe the penumbral atmosphere. The data was taken at the German Vacuum Tower Telescope with the 2D-spectrometer TESOS, taking advantage of the recently installed Kiepenheuer Adaptive Optics System (KAOS). The field of view covers a sunspot located at 23 ◦ off the disk center and its immediate surroundings. The penumbral structure is studied by means of maps computed for the line-of-sight velocity, the line width, the equivalent width and the line depression. Line-of-sight velocities are derived from the Doppler shifts at different bisector levels. From these maps we infer the flow field geometry and study the azimuthal and radial dependences of the line parameters. Our findings can be summarized as follows: (a) the flow pattern has a conspicuous filamentary structure in the deep photospheric layers and is rather diffuse in the high layers. (b) The flow field slightly spreads and fans out with height. (c) The flow geometry confirms the presence of an upflow component in the inner penumbra and a downflow component in the middle and outer penumbra. (d) We find an enhanced brightness of the mid-penumbra (bright ring) in the line wings, but not in the continuum or line core. (e) The azimuthal average of the equivalent width, the line width and the absolute flow velocity increase with radial distance within the penumbra. (f) Small- scale variations of the equivalent width and the line width on the center-side penumbra are co-spatial and correlated with (blue-shifted) fluctuations in the line-of-sight velocity. (g) Inner limb-side penumbral grains are associated with blue-shifts of v ≤− 400 m s −1 , indicating upflows. (h) One umbral dot in our sample is associated with a blue-shift of v = −200 m s −1 .

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.

Imaging spectroscopy of solar pores

Time series of two-dimensional spectra and corresponding broad band images of solar pore regions have been obtained with the Gottingen Fabry-Perot Interferometer at the Vacuum Tower Telescope in Izana, Tenerife. Line bisector shifts have been used for the computation of line-of-sight velocities in the studied regions. Additionally, a local correlation tracking (LCT) algorithm has been applied to obtain horizontal flow velocities from the time series of broad band images. Resulting velocity maps show that within the pore umbrae the flows are almost completely inhibited by the magnetic fields. This also holds for umbral dots and light bridges. On the boundaries of several pore umbrae persistent downflow channels are visible. The structure and temporal evolution of these phenomena have been studied. Finally, the formation of a protopore out of magnetic bright points has been analyzed. The obtained observational results are in good agreement with numerical models and common theoretical scenarios explaining the behaviour and formation of solar pores.

OBSERVATIONS AND SPECTRAL ANALYSES OF SOLAR FLARES

We introduce the two-dimensional spectral observations of solar flares using the Solar Tower Tele-scope of Nanjing University, China. In particular, we introduce three typical events and the methods used to analyze the data. (1) The flare of November 11, 1998, which is a limb flare. We derive the temperature and density within the flaring loop using non-LTE calculations. The results show that the loop top may be hotter and denser than other parts of the loop, which may be a result of magnetic reconnect ion above the loop. (2) The flare of March 10, 2001, which is a white-light flare that shows an emission enhancement at the near infrared continuum. We propose a model of non-thermal electron beam heating plus backwarming to interpret the observations. (3) The flare of September 29, 2002, which shows unusual line asymmetries at one flare kernel. The line asymmetries are caused by an upward moving plasma that is accelerated and heated during the flare development.

Two‐dimensional speckle spectroscopy of Hα features

AbstractIn May 2002, the solar chromosphere was observed with a two‐dimensional spectrometer which is mounted in the German Vacuum Tower Telescope (VTT) at the Observatorio del Teide/Tenerife. The aim of this observation was to investigate the fine structure of the solar chromosphere seen in Hα. We took narrow‐band filtergrams (Δλ ≈ 72 mÅ) by scanning through this line. Broad‐band images taken strictly simultaneously with the narrow‐band filtergrams were restored by speckle methods. The instantaneous optical transfer function from this restoration procedure was used for the reconstruction of the narrow‐band images. Some results of this high spatial resolution observation are presented below.

Speckle spectro‐polarimetry of magnetic structures

AbstractTwo‐dimensional spectrograms were obtained with the Vacuum Tower Telescope, Tenerife, in order to study small‐scale structures and faculae on the Sun. Using the speckle reconstruction method, we obtain high‐resolution images and wavelength scans. Magnetic fields can be studied from Stokes V profiles, and velocity maps are gained by the Doppler shift of the center of gravity of Stokes I. Here some results about small‐scale structures and their magnetic fields are shown.

Evidence for short-period acoustic waves in the solar atmosphere

Short-period acoustic waves are thought to supply the energy for the radiative losses of the non-magnetic chromo- sphere of the Sun and, in general, of late-type stars. Here, we present evidence for the existence of waves in the solar atmosphere with periods in the range of 50 s< P< 100 s. Two-dimensional time sequences with a cadence of 25 s were obtained from quiet Sun disk center in Fei 5434 A. The observations were performed with the Gottingen Fabry-Perot spectrometer in the Vacuum Tower Telescope at the Observatorio del Teide/Tenerife. They are subjected to speckle reconstruction and to a wavelet analysis. The atmospheric ranges forming the velocity signals are narrowed by linear combinations of Doppler maps from wavelengths near line center. The power in the short-period range is concentrated above intergranular spaces. We estimate an acoustic flux into the chromosphere of approximately 310 6 erg cm 2 s 1 , as needed for the chromospheric radiative losses.

2D-spectroscopic observations of $\vec G$-band bright structures in the solar photosphere

We took two-dimensional spectra with the filter spectrometer TESOS at the German Vacuum Tower Telescope, Tenerife, of an absorption line of the CH molecule and a Fe II-line in the G-band at 430.3 nm. We observed a region, close to disk center of the Sun, that showed a lot of structures with enhanced G-band intensity (up to 1.3 times the mean intensity of normal granulation). Our spectroscopic investigation of these structures suggests two classes which differ in their spectroscopic signature: (a) Bright structures caused by significant (up to 40%) weakening of absorption lines of the CH molecule; (b) bright structures only caused by an enhanced continuum intensity. In order to distinguish between those two classes we introduce a Bright Point Index (BPI) defined by the ratio of the normalized line depressions of the Fe II and the CH-line. The bright structures caused by weakening of the CH-lines have high BPI values and are accompanied by downflows. The remaining G-band bright structures have low BPI and are related to granules.

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.

Adaptive optics and multi-conjugate adaptive optics with the VTT

We are currently developing adaptive optics (AO) system with a multi-conjugate extension for the German solar vacuum tower telescope (VTT) at the Teide Observatory on Tenerife.Multi-conjugate adaptive optics (MCAO) is a technique for increasing the field of view by compensating atmospheric turbulence along several, adjacent lines of sight. A conventional AO system compensates only a single line of sight in the direction of the lock point of its wavefront sensor. At larger field angles, the light from the source transverses higher layers of turbulence which are not sampled by the conventional system. Measurements at the VTT indicate that full compensation is typically restricted to a field of about 10 arcsec in diameter at visible wavelengths. An MCAO uses (at least) a second deformable mirror close to the focal plane of the telescope to compensate a larger field. The sun is a privileged target for an MCAO because the wavefront errors at larger field angles are easily measured. We intend to extend our existing AO system with a second deformable mirror and a second wavefront sensor which enables us to extend the compensated field by a factor of three in diameter.We present and discuss our concept.

Mount Wilson Synoptic Magnetic Fields: Improved Instrumentation, Calibration, and Analysis Applied to the 2000 July 14 Flare and to the Evolution of the Dipole Field

This paper describes the current status of the 150 foot solar tower telescope program of synoptic observations with an emphasis on the magnetic field data. A newly installed 24-channel system permits routine intercomparison of magnetic fields measured by the λ676.8 nm line used by the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory and the λ525.0 nm line used by the 150 foot tower. Two important calibration procedures for treatment of saturation and zero-point offset are described. It is demonstrated that solar rotation can be used to extract the east-west component of the slowly evolving, large-scale magnetic field in a stable fashion. This same analysis produces maps of the neutral line configurations that are well correlated with the positions of quiescent prominences. The analysis is applied to the 2000 July 14 flare and shown to demonstrate that the field was sheared due to the westward-moving intrusion of a region just north of the neutral line along which the flaring occurred. A new method for preparing synoptic charts by averaging without differential rotation smearing is presented. These synoptic charts are combined into a new format termed a supersynoptic chart, which makes possible the identification of systematic long-term trends in the magnetic field evolution. Based on these charts, distinct large-scale events of magnetic field bias opposing the old-cycle dipole field are seen. A statistical method using the skewness in the distribution of the polarity bias as a function of longitude is developed. The coincidence between pulses in this skewness and times of rapid change in the Sun's dipole moment is consistent with the idea that a tilt in the orientation of bipolar magnetic regions is responsible for the dipole field reversal. The pulses in skewness are large and limited in number, suggesting the operation of a large-scale instability such as the kink instability.

High-resolution solar spectroscopy with TESOS - Upgrade from a double to a triple system

We present the characteristics and demonstrate the performance of the Triple Etalon SOlar Spectrometer (TESOS) operated at the German Vacuum Tower Telescope (VTT) on Tenerife. The Fabry–Perot interferometer TESOS is ideally suited for precise measurements of photospheric and chromospheric motion. Installed in 1997 and equipped with two etalons, TESOS has recently been completed with a third etalon and upgraded with two high-speed, backside-illuminated CCD cameras. The image scale of 0.089 arc sec pixel−1 is adapted to the resolution of the telescope. The improved system enables frame rates up to 5 frames per second. The spectral resolution of 300 000 allows for spectral diagnostics of weak photospheric lines, including individual CH-lines within the G-band at 430.6 nm.

Spectropolarimetry in a sunspot penumbra

Stokes profiles of sunspot penumbrae show distinct asymmetries, which point to gradients in the velocity field and in the magnetic field. We present spectropolarimetric measurements of the Stokes vector in the neutral iron triplet at 1564.8 nm taken with the Tenerife Infrared Polarimeter (TIP) at the German Vacuum Tower Telescope (VTT) in Tenerife. We report on the peculiarities of the profiles of circularly and linearly polarized light for spots at different heliocentric angles. We elaborate on the spatial dependence of Stokes asymmetries within the penumbra and find for profiles of circularly polarized light: (1) In the center-side penumbra the amplitude difference of Stokes-V exhibits a sign reversal on a radial cut, i.e., in the inner (outer) penumbra the red (blue) lobe is broader and shows a smaller amplitude than the blue (red) lobe. (2) In the outer limb-side penumbra (beyond the magnetic neutral line) the red lobe is broader and of less amplitude than the blue lobe. (3) Along the magnetic neutral line we find abnormal Stokes-V profiles, which consist of more than 2 lobes. This indicates the presence of two polarities. For small heliocentric angles abnormal profiles are also seen beyond the magnetic neutral line in the outer penumbra. (4) Maps of the net circular polarization have the tendency to be antisymmetric with respect to the axis that connects disk center with spot center. This finding is striking, because corresponding maps for Fe I 630.25 are symmetric. For linearly polarized profiles we extract the following features: (5) On the center-side penumbra at a heliocentric angle of 56° a Doppler-shift as high as 5 km s-1 can be directly measured by the splitting of the π-component of the linearly polarized component. (6) In limb-side penumbrae, the profiles of the π-component show the typical asymmetry properties of the Evershed flow as observed in Stokes-I of magnetically insensitive lines. (7) In the outer center- and limb-side penumbrae the center of the π-component is blue-shifted relative to the zero-crossing of the V-profile. Motivated by the moving tube model of Schlichenmaier et al. ([CITE]), we construct simple model atmospheres featuring hot upflows and cool outflows and calculate corresponding synthetic V-profiles. These profiles are compared with our measured ones and with observed V-profiles in Fe I 630.25 from other authors. We find that the synthetic V-profiles can reproduce all essential characteristics of observed V-profiles for both lines.

Penumbral finestructure: need for larger telescopes

We obtained at the same time G-band images at the Dutch Open Telescope (DOT) on La Palma and spectropolarimetric data in the near infrared at the German Vacuum Tower Telescope (VTT) on Tenerife. The spectropolarimetric data show interesting correlations. Bright filaments have a smaller magnetic field strength, and higher Evershed velocities occur in dark structures. This result is in agreement with some previous observations, but also in contradiction to others. However, we suffer from the fact that the resolution limit of the VTT at a wavelength of 1.565 μm corresponds to 400 km. Spatial power spectra derived from the DOT data indicate a typical width of 250 km for penumbral filaments. Obviously a solar telescope with an aperture of at least 1.5 m is needed to obtain sophisticated results for penumbral structures.

Speckle spectro-polarimetry of magnetic structures

High spatial resolution images with spectral and polarimetric information were obtained with the Vacuum Tower Telescope (VTT), Tenerife, in order to study dynamic small scale structures on the Sun. With the Göttingen Fabry-Perotinterferometer (FPI) we observed faculae at different cos θ positions on the Sun. Simultaneously with these narrow-band images, 150 broad-band images were taken from which high resolution images are calculated with the spectral ratio and the speckle masking methods. With the aid of the transfer functions, gained with the speckle recontruction, the narrow-band FPI images can be reconstructed as well. Here our first results are presented.

Penumbral finestructure: need for larger telescopes

We obtained at the same time G-band images at the Dutch Open Telescope (DOT) on La Palma and spectropolarimetric data in the near infrared at the German Vacuum Tower Telescope (VTT) on Tenerife. The spectropolarimetric data show interesting correlations. Bright filaments have a smaller magnetic field strength, and higher Evershed velocities occur in dark structures. This result is in agreement with some previous observations, but also in contradiction to others. However, we suffer from the fact that the resolution limit of the VTT at a wavelength of 1.565 μm corresponds to 400 km. Spatial power spectra derived from the DOT data indicate a typical width of 250 km for penumbral filaments. Obviously a solar telescope with an aperture of at least 1.5 m is needed to obtain sophisticated results for penumbral structures.

A Study of Hydrogen Density in Emerging Flux Loops from a CoordinatedTransition Region and Coronal Explorerand Canary Islands Observation Campaign

During an international ground-based campaign in the Canary Islands coordinated with space instruments (i.e., Transition Region and Coronal Explorer [TRACE]), we observed an active region on 1998 September 10 with high spatial and temporal resolution. New emerging flux in the central part of the active region was observed in magnetograms of the Swedish Vacuum Solar Telescope, La Palma. Emerging loops (arch-filament systems [AFSs]) are well developed in Hα and Ca II according to the observations made at the Vacuum Tower Telescope (VTT) and THEMIS telescope in Tenerife with the Multichannel Subtractive Double Pass (MSDP) spectrographs. The TRACE images obtained at 171 and 195 A show low-emission regions that are easily identified as the individual AFS. They are due to absorption by hydrogen and helium continua in the cool filament plasma. We compare two techniques of measuring the hydrogen density in the cool dense fibrils of AFSs. The first method based on TRACE observations derived the neutral hydrogen column density of the plasma absorbing coronal lines. The second one using Hα line profiles provided by the MSDP spectrographs is based on the cloud model. The results are consistent. We derive also electron density values using Hα lines that are in good agreement with those derived from the 8542 A Ca II line observed with THEMIS (Mein et al.). The three types of observations (TRACE, VTT, THEMIS) are well complementary: absorption of coronal lines giving a good approximation for the maximum value of the neutral hydrogen column density, the Hα line giving a good determination of ne, and the 8542 A Ca II line a good determination of the electronic temperature.

The structure of the photospheric velocity field near Hα filaments

The velocity field of the plasma in the solar photosphere beneath chromospheric Hα filaments is studied. Observations were conducted in 1999–2000 using the magnetograph and tachometer of the tower telescope of the Institute of Terrestrial Magnetism, Ionosphere, and Radio Propagation, recently upgraded to improve both its sensitivity and spatial resolution. The results confirm that, as noted earlier, filaments are frequently found near velocity-inversion lines between regions of upward and downward motion of solar material, and lie predominantly above regions with upward motion of photospheric material. This tendency is characteristic of both the stable filaments of active regions and quiescent filaments far from active regions, though it is more distinct for the former case. The upward motion of photospheric material beneath filaments may play an important role in supporting the filaments against gravity.