Alexandra Tritschler

The solar chromosphere at high resolution with IBIS. IV. Dual-line evidence of heating in chromospheric network

The structure and energy balance of the solar chromosphere remain poorly known. We used the imaging spectrometer IBIS at the Dunn Solar Telescope to obtain fast-cadence, multi-wavelength profile sampling of Hα and Ca II 854.2 nm over a sizable two-dimensional field of view encompassing quiet-Sun network. We provide a first inventory of how the quiet chromosphere appears in these two lines by comparing basic profile measurements in the form of image displays, temporal-average displays, time slices, and pixel-by-pixel correlations. We find that the two lines can be markedly dissimilar in their rendering of the chromosphere, but that, nevertheless, both show evidence of chromospheric heating, particularly in and around network: Hα in its core width and Ca II 854.2 nm in its brightness. We discuss venues for improved modeling.

THERMAL FINE STRUCTURE AND MAGNETIC FIELDS IN THE SOLAR ATMOSPHERE: SPICULES AND FIBRILS

The relationship between observed structures in the solar atmosphere and the magnetic fields threading them is known only for the solar photosphere, even then imprecisely. We suggest that some of the fine structures in the more tenuous chromosphere and corona—specifically some populations of spicules and fibrils—correspond to warps in two-dimensional sheet-like structures, as an alternative to conventional interpretations in terms of tube-like structures. The sheets are perhaps related to magnetic tangential discontinuities, which Parker has argued arise naturally in low-β conditions. Some consequences of this suggestion, if it can be confirmed, are discussed.

Flow Field Evolution of a Decaying Sunspot

We study the evolution of the flows and horizontal proper motions in and around a decaying follower sunspot based on time sequences of two-dimensional spectroscopic observations in the visible and white-light imaging data obtained over 6 days from 2005 June 7 to 12. During this time period the sunspot decayed gradually to a pore. The spectroscopic observations were obtained with the Fabry-Perot-based Visible-Light Imaging Magnetograph (VIM) in conjunction with the high-order adaptive optics (AO) system operated at the 65 cm vacuum reflector of the Big Bear Solar Observatory (BBSO). We apply local correlation tracking (LCT) to the speckle-reconstructed time sequences of white-light images around 600 nm to infer horizontal proper motions, while the Doppler shifts of the scanned Fe I line at 630.15 nm are used to calculate line-of-sight (LOS) velocities with subarcsecond resolution. We find that the dividing line between radial inward and outward proper motions in the inner and outer penumbra, respectively, survives the decay phase. In particular the moat flow is still detectable after the penumbra disappeared. Based on our observations, three major processes removed flux from the sunspot: (1) fragmentation of the umbra, (2) flux cancelation of moving magnetic features (MMFs; of the same polarity as the sunspot) that encounter the leading opposite polarity network and plages areas, and (3) flux transport by MMFs (of the same polarity as the sunspot) to the surrounding network and plage regions that have the same polarity as the sunspot.

Adaptive Optics at the Big Bear Solar Observatory: Instrument Description and First Observations

In 2004 January, the Big Bear Solar Observatory (BBSO) was equipped with a high-order adaptive optics (AO) system built in collaboration with the National Solar Observatory (NSO) at Sacramento Peak. The hardware is almost identical to the AO system operated at the NSO Dunn Solar Tower (DST), incorporating a 97 actuator deformable mirror, a Shack-Hartmann wave-front sensor with 76 subapertures, and an off-the-shelf digital signal processor system. However, the BBSO optical design is quite different. It had to be adapted to the 65 cm vacuum reflector and the downstream postfocus instrumentation. In this paper, we describe the optical design, demonstrate the AO performance, and use image restoration techniques to illustrate the image quality that can be achieved with the new AO system.

On the fine structure of the quiet solar Ca II K atmosphere

Aims. We investigate the morphological, dynamical, and evolutionary properties of the internetwork and network fine structure of the quiet sun at disk centre. Methods. The analysis is based on a ∼6h time sequence of narrow-band filtergrams centred on the inner-wing Call K 2v reversal at 393.3 nm. To examine the temporal evolution of network and internetwork areas separately we employ a double-Gaussian decomposition of the mean intensity distribution. An autocorrelation analysis is performed to determine the respective characteristic time scales. In order to analyse statistical properties of the fine structure we apply image segmentation techniques. Results. The results for the internetwork are related to predictions derived from numerical simulations of the quiet sun. The average evolutionary time scale of the internetwork in our observations is 52 s. Internetwork grains show a tendency to appear on a mesh-like pattern with a mean cell size of ∼4-5 arcsec. Based on this size and the spatial organisation of the mesh we speculate that this pattern is related to the existence of photospheric downdrafts as predicted by convection simulations. The image segmentation shows that typical sizes of both network and internetwork grains are in the order of 1.6 arcsec.

The Contrast of Magnetic Elements in Synthetic CH- and CN-Band Images of Solar Magnetoconvection

We present a comparative study of the intensity contrast in synthetic CH-band and violet CN-band filtergrams computed from a high-resolution simulation of solar magnetoconvection. The underlying simulation has an average vertical magnetic field of 250 G with kilogauss fields concentrated in its intergranular lanes and is representative of a plage region. To simulate filtergrams typically obtained in CH- and CN-band observations we computed spatially resolved spectra in both bands and integrated these spectra over 1 nm FWHM filter functions centered at 430.5 and 388.3 nm, respectively. We find that the average contrast of magnetic bright points in the simulated filtergrams is lower in the CN band by a factor of 0.96. In comparison, earlier semiempirical modeling and recent observations both estimated that the bright point contrast in the CN band is higher by a factor of 1.4. We argue that the near equality of the bright point contrast in the two bands in the present simulation is a natural consequence of the mechanism that causes magnetic flux elements to be particularly bright in the CN and CH filtergrams, namely, the partial evacuation of these elements and the concomitant weakening of molecular spectral lines in the filter passbands. We find that the rms intensity contrast in the whole field of view of the filtergrams is 20.5% in the G band and 22.0% in the CN band and conclude that this slight difference in contrast is caused by the shorter wavelength of the latter. Both the bright point and rms intensity contrast in the CN band are sensitive to the precise choice of the central wavelength of the filter.

Evidence for a Siphon Flow Ending near the Edge of a Pore

Observations of NOAA AR 9431, taken with the Vacuum Tower at Kitt Peak on 2001 April 18 in the Ca II 854.21 nm line in both circular polarizations, show evidence for a strong supersonic downflow ending near the edge of a magnetic pore. The observed supersonic motion is interpreted as a siphon flow along a magnetic loop connecting a patch of weaker field to the pore of opposite polarity in the same active region. The 854.21 nm line data reveal the upflow at one footpoint of the loop, as well as the acceleration of the flow toward the footpoint at the pore, where the flow reaches line-of-sight velocities of well over 20 km s-1, substantially larger than the critical speed. Numerical radiative transfer modeling of the 854.21 nm line indicates the presence of a strong discontinuity in the flow velocity, which we interpret as evidence for a tube shock in the downwind leg of the siphon.

Fine Structure of the Net Circular Polarization in a Sunspot Penumbra

We present novel evidence for fine structure observed in the net circular polarization (NCP) of a sunspot penumbra based on spectropolarimetric measurements utilizing the Zeeman-sensitive Fe I 630.2 nm line. For the first time we detect filamentary organized fine structure of the NCP on spatial scales that are similar to the inhomogeneities found in the penumbral flow field. We also observe an additional property of the visible NCP, a zero-crossing of the NCP in the outer parts of the center-side penumbra, which has not been recognized before. In order to interpret the observations we solve the radiative transfer equations for polarized light in a model penumbra with embedded magnetic flux tubes. We demonstrate that the observed zero-crossing of the NCP can be explained by an increased magnetic field strength inside magnetic flux tubes in the outer penumbra combined with a decreased magnetic field strength in the background field. Our results strongly support the concept of the uncombed penumbra.

Fabry-Pérot versus slit spectropolarimetry of pores and active network: Analysis of IBIS and Hinode data

We discuss spectropolarimetric measurements of photospheric (Fe I 630.25 nm) and chromospheric (Ca II 854.21 nm) spectral lines in and around small magnetic flux concentrations, including a pore. Our long-term goal is to diagnose properties of the magnetic field near the base of the corona. We compare ground-based two-dimensional spectropolarimetric measurements with (almost) simultaneous space-based slit spectropolarimetry. We address the question of noise and crosstalk in the measurements and attempt to determine the suitability of Ca II measurements with imaging spectropolarimeters for the determination of chromospheric magnetic fields. The ground-based observations were obtained 2008 May 20, with the Interferometric Bidimensional Spectrometer (IBIS) in spectropolarimetric mode operated at the Dunn Solar Telescope at Sunspot, NM. The space observations were obtained with the Spectro-Polarimeter of the Solar Optical Telescope aboard the Japanese Hinode satellite. The agreement between the near-simultaneous co-spatial IBIS and Hinode Stokes-V profiles at 630.25 nm is excellent, with V/I amplitudes compatible to within 1%. The IBIS QU measurements are affected by residual crosstalk from V, arising from calibration inaccuracies, not from any inherent limitation of imaging spectroscopy. We use a Principal Component Analysis to quantify the detected crosstalk. QU profiles with V crosstalk subtracted are in good agreement with the Hinode measurements, but are noisier owing to fewer collected photons. Chromospheric magnetic fields are notoriously difficult to constrain by polarization of Ca II lines alone. However, we demonstrate that high cadence, high angular resolution monochromatic images of fibrils in Ca II and Hα, seen clearly in IBIS observations, can be used to improve the magnetic field constraints, under conditions of high electrical conductivity. Such work is possible only with time series data sets from two-dimensional spectroscopic instruments such as IBIS, under conditions of good seeing.

Progress on the 1.6-meter New Solar Telescope at Big Bear Solar Observatory

The New Solar Telescope (NST) project at Big Bear Solar Observatory (BBSO) now has all major contracts for design and fabrication in place and construction of components is well underway. NST is a collaboration between BBSO, the Korean Astronomical Observatory (KAO) and Institute for Astronomy (IfA) at the University of Hawaii. The project will install a 1.6-meter, off-axis telescope at BBSO, replacing a number of older solar telescopes. The NST will be located in a recently refurbished dome on the BBSO causeway, which projects 300 meters into the Big Bear Lake. Recent site surveys have confirmed that BBSO is one of the premier solar observing sites in the world. NST will be uniquely equipped to take advantage of the long periods of excellent seeing common at the lake site. An up-to-date progress report will be presented including an overview of the project and details on the current state of the design. The report provides a detailed description of the optical design, the thermal control of the new dome, the optical support structure, the telescope control systems, active and adaptive optics systems, and the post-focus instrumentation for high-resolution spectro-polarimetry.