Present rate of uplift in Fennoscandia from GRACE and absolute gravimetry

Fennoscandia is a key region for studying effects of glacial isostatic adjustment. The associated mass variations can be detected by the Gravity Recovery and Climate Experiment (GRACE) satellite mission, which observes the Earth's gravity field since April 2002, as well as by absolute gravimetry field campaigns. Since 2003, annual absolute gravity (AG) measurements have been performed in Fennoscandia by the Institut für Erdmessung (IfE, Institute of Geodesy) of the Leibniz Universität Hannover, Germany, within a multi-national cooperation. This offers a unique opportunity for validation and evaluation of the GRACE results. In this preliminary study, the GRACE results are compared to secular gravity changes based on the surveys from 2004 to 2007 with the FG5-220 gravimeter of the IfE.The results from GRACE monthly solutions provided by different analysis centres show temporal gravity variations in Fennoscandia. The included secular variations are in good agreement with former studies. The uplift centre is located west of the Bothnian Bay, the whole uplift area comprises Northern Europe. Nevertheless, the differences between the GRACE solutions are larger than expected and the different centre-specific processing techniques have a very strong effect on possible interpretations of GRACE results. The comparison of GRACE to the AG measurements reveals that the determined trends fit well with results from GRACE at selected stations, especially for the solution provided by the GFZ. Variations of land hydrology clearly influence results from GRACE and the AG measurements.     

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–Pérot 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.     

Regional gravity field modeling using free-positioned point masses

A two-step free-positioned point mass method is used for regional gravity field modeling together with the remove-compute-restore (RCR) technique. The Quasi-Newton algorithm (L-BFGS-B) is implemented to solve the nonlinear problem with bound constraints in the first step, while in the second step the magnitudes of the point masses are re-adjusted with known positions in the least-squares sense. In order to reach a good representation of the gravity field, a number of parameter sets have to be defined carefully before the computations. The effects of four important parameter sets (depth limits, number of point masses, original/reduced basis functions and optimization directions) are investigated for regional gravity field modeling based on two numerical test cases with synthetic and real data. The results show that the selection of the initial depth and depth limits is of most importance. The number of point masses for obtaining a good fit is affected by the data distribution, while a dependency on the data variability (signal variation) is negligible. Long-wavelength errors in the predicted height anomalies can be reduced significantly by using reduced basis functions, and the radial-direction optimization proves to be stable and reliable for regular and irregular data scenarios. If the parameter sets are defined properly, the solutions are similar to the ones computed by least-squares collocation (LSC), but require fewer unknowns than LSC.     

Synoptic Hα Full-Disk Observations of the Sun from Big Bear Solar Observatory – I. Instrumentation,Image Processing,Data Products,and First Results

The Big Bear Solar Observatory (BBSO) has a long tradition of synoptic full-disk observations. Synoptic observations of contrast enhanced full-disk images in the Caii K-line have been used with great success to reproduce the Hi L irradiance variability observed with the Upper Atmosphere Research Satellite (UARS). Recent improvements in data calibration procedures and image- processing techniques enable us now to provide contrast enhanced H full-disk images with a spatial resolution of approximately 2 and a temporal resolution of up to 3 frames min–1.In this first paper in a series, we describe the instruments, the data calibration procedures, and the image-processing techniques used to obtain our daily H full-disk observations. We also present the final data products such as low- and high-contrast images, and Carrington rotation charts. A time series of an erupting mini- filament further illustrates the quality of our H full-disk observations and motivate one of the future research projects. This lays a solid foundation for our subsequent studies of solar activity and chromospheric fine structures. The high quality and the sunrise- to-sunset operation of the H full-disk observations presented in this paper make them an ideal choice to study statistical properties of mini-filament eruptions, chromospheric differential rotation, and meridional flows within the chromosphere, as well as the evolution of active regions, filaments, flares, and prominences.     

Digitization of sunspot drawings by Spörer made in 1861–1894

Most of our knowledge about the Sun's activity cycle arises from sunspot observations over the last centuries since telescopes have been used for astronomy. The German astronomer Gustav Spörer observed almost daily the Sun from 1861 until the beginning of 1894 and assembled a 33‐year collection of sunspot data covering a total of 445 solar rotation periods. These sunspot drawings were carefully placed on an equidistant grid of heliographic longitude and latitude for each rotation period, which were then copied to copper plates for a lithographic reproduction of the drawings in astronomical journals. In this article, we describe in detail the process of capturing these data as digital images, correcting for various effects of the aging print materials, and preparing the data for contemporary scientific analysis based on advanced image processing techniques. With the processed data we create a butterfly diagram aggregating sunspot areas, and we present methods to measure the size of sunspots (umbra and penumbra) and to determine tilt angles of active regions. A probability density function of the sunspot area is computed, which conforms to contemporary data after rescaling. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Present state and future developments of the European geoid

Regional geoid resp. quasigeoid determinations are nowadays required with an accuracy of ±1 to 10 cm over distances from 100 to some 1000 km in order to meet the demands of geodesy, geophysics, oceanography and engineering. Especially the combination of GPS heighting with classical leveling is one of the primary drivers for precise geoid computations. As a consequence, the IAG International Geoid Commission recognized at its meeting in Milano, 1990, that there is an urgent need for a new European geoid computation. This solution should be significantly improved in spatial resolution and accuracy as compared to presently available models. This led to the decision to form a Subcommission for the Geoid in Europe, and the Institut für Erdmessung (IfE) was asked to serve as a computing center in this project.In the first part of this paper early geoid/quasigeoid computations for the area of Europe as well as more recent results obtained at IfE are summarized. The latter solutions include a gravimetric and an astrogravimetric quasigeoid, which have a spatial resolution of about 20 km and a relative accuracy of some dm. Then the possibilities for an improved European quasigeoid calculation are outlined, considering the availability of new and better global and regional data sets. An overview is given on the procedures currently under study at IfE and on the work performed at IfE since 1990. This work includes the collection and screening of new point gravity and terrain data, some investigations on the use of topographic information available at present, and the calculation of a preliminary quasigeoid solution for central, northern and western Europe including a GPS/leveling control. The paper closes with a survey on future activities at IfE within the European geoid project.     

PEPSI: The high‐resolution échelle spectrograph and polarimeter for the Large Binocular Telescope

PEPSI is the bench‐mounted, two‐arm, fibre‐fed and stabilized Potsdam Echelle Polarimetric and Spectroscopic Instrument for the 2×8.4 m Large Binocular Telescope (LBT). Three spectral resolutions of either 43 000, 120 000 or 270 000 can cover the entire optical/red wavelength range from 383 to 907 nm in three exposures. Two 10.3k×10.3k CCDs with 9‐µm pixels and peak quantum efficiencies of 94–96 % record a total of 92 échelle orders. We introduce a new variant of a wave‐guide image slicer with 3, 5, and 7 slices and peak efficiencies between 92–96 %. A total of six cross dispersers cover the six wavelength settings of the spectrograph, two of them always simultaneously. These are made of a VPH‐grating sandwiched by two prisms. The peak efficiency of the system, including the telescope, is 15 % at 650 nm, and still 11 % and 10 % at 390 nm and 900 nm, respectively. In combination with the 110 m² light‐collecting capability of the LBT, we expect a limiting magnitude of ≈20th mag in V in the low‐resolution mode. The R = 120 000 mode can also be used with two, dual‐beam Stokes IQUV polarimeters. The 270 000‐mode is made possible with the 7‐slice image slicer and a 100‐µm fibre through a projected sky aperture of 0.74″, comparable to the median seeing of the LBT site. The 43 000‐mode with 12‐pixel sampling per resolution element is our bad seeing or faint‐object mode. Any of the three resolution modes can either be used with sky fibers for simultaneous sky exposures or with light from a stabilized Fabry‐Pérot étalon for ultra‐precise radial velocities. CCD‐image processing is performed with the dedicated data‐reduction and analysis package PEPSI‐S4S. Its full error propagation through all image‐processing steps allows an adaptive selection of parameters by using statistical inferences and robust estimators. A solar feed makes use of PEPSI during day time and a 500‐m feed from the 1.8 m VATT can be used when the LBT is busy otherwise. In this paper, we present the basic instrument design, its realization, and its characteristics. Some pre‐commissioning first‐light spectra shall demonstrate the basic functionality. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

New Digital Magnetograph At Big Bear Solar Observatory

A new digital magnetograph system has been installed and tested at Big Bear Solar Observatory. The system uses part of BBSO's existing videomagnetograph (VMG) system: a quarter wave plate, a ferro-electric liquid crystal to switch polarizations, and a 0.25 Å bandpass Zeiss filter tuned at Cai 6103 Å. A new 256×256 pixels, 12-bit Dalsa camera is used as the detector and as the driver to switch the liquid crystal. The data rate of the camera is 90 frames s–1. The camera is interfaced to a Pentium-166 PC with a Tech imaging board for data acquisition and analysis. The computer has 128 MByte of RAM, and up to 700 live images can be stored in memory for quick post-exposure image processing (image selection and alignment). We have significantly improved the sensitivity and spatial resolution over the old BBSO VMG system. In particular: (1) New digital image data are in 12 bits while the video signal is digitized as 8 bits. Polarizations weaker than 1% can not be detected by a single pair subtraction in the video system. The digital system can detect a polarization signal of about 0.3% by a single pair subtraction. (2) Data rate of the digital system is 90 frames s–1, that of the video system is 30 frames s–1. So the time difference between two polarizations is reduced in the new system. Under good seeing conditions, the data rate of 90 frames s–1 ensures that most of the wavefront distortions are frozen and fairly closely the same for the left and right circular polarized image pairs. (3) Magnetograms are constructed after image selection and alignment. We discuss the characteristics of this new system. We present the results of our first tests to reconstruct magnetograms with speckle interferometric techniques. We also present some preliminary results on the comparison of facular/micropore contrasts and magnetic field structure. The experiment with this small detector lays ground for a larger format digital magnetograph system at BBSO, as well as a future Fabry-Pérot system, which will be able to scan across the spectral line.     

High‐order adaptive optical system for Big Bear Solar Observatory

A high‐order Adaptive Optical (AO) system for the 65 cm vacuum telescope of the Big Bear Solar Observatory (BBSO) is presented. The Coudé‐exit of the telescope has been modified to accommodate the AO system and two imaging magnetograph systems for visible‐light and near infrared (NIR) observations. A small elliptical tip/tilt mirror directs the light into an optical laboratory on the observatory's 2^nd floor just below the observing floor. A deformable mirror (DM) with 77 mm diameter is located on an optical table where it serves two wave‐front sensors (WFS), a correlation tracker (CT) and Shack‐Hartman (SH) sensor for the high‐order AO system, and the scientific channels with the imaging magnetographs. The two‐axis tip/tilt platform has a resonance frequency around 3.3 kHz and tilt range of about 2 mrad, which corresponds to about 25″ in the sky. Based on 32 × 32 pixel images, the CT detects image displacements between a reference frame and real‐time frames at a rate of 2 kHz. High‐order wave‐front aberrations are detected in the SH WFS channel from slope measurements derived from 76 sub‐apertures, which are recorded with 1,280 × 1,024 pixel Complex Metal Oxide Semiconductor (CMOS) camera manufactured by Photobit camera. In the 4 × 4 pixel binning mode, the data acquisition rate of the CMOS device is more than 2 kHz. Both visible‐light and NIR imaging magnetographs use Fabry‐Pérot etalons in telecentric configurations for two‐dimensional spectro‐polarimetry. The optical design of the AO system allows using small aperture prefilters, such as interference or Lyot filters, and 70 mm diameter Fabry‐Pérot etalons covering a field‐of‐view (FOV) of about 180″ × 180″.     

Local geoid determination and comparison with GPS results

Ellipsoidal heights have been determined for a test network in Lower Saxony withGPS. TheGPS results, with a relative precision of a few centimeters, have been used to compute quasigeoid heights by substracting leveling heights. This data set is compared to mainly gravimetrically determined quasigeoid heights using least squares collocation techniques. The discrepancies between the two data sets amount to about ±2cm, the maximum interstation distance is about50 km. This agreement shows, thatGPS can be used in combination with gravity information to obtain normal heights withcm-precision.