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Probability Density Functions 149 Acknowledgement I am grateful to Dr. K. N. Nagendra for very useful suggestions and comments. References Carroll, T. A., Kopf, M. 2007, A&A, 468, 323 Carroll, T. A., Staude, J. 2003, Astronomische Nachrichten, 324, 392 Carroll, T. A., Staude, J. 2005, Astronomische Nachrichten, 326, 296 de Wijn, A. G., Stenflo, J. O., Solanki, S. K., Tsuneta, S. 2009, Space Sci. Rev., 144, 275 Dolginov, A. Z., Pavlov, G. G. 1972, Soviet Astronomy, 16, 450 Domke, H., Pavlov, G. G. 1979, Ap&SS, 66, 47 Frisch, H., Sampoorna, M., Nagendra, K. N. 2005, A&A, 442, 11 Frisch, H., Sampoorna, M., Nagendra, K. N. 2006a, In: Solar Polarization 4, R. Casini, B. W. Lites (eds.), ASP Conf. Ser., vol. 358, p. 126 Frisch, H., Sampoorna, M., Nagendra, K. N. 2006b, A&A, 453, 1095 Frisch, H., Sampoorna, M., Nagendra, K. N. 2007, Memorie della Societa Astronomica Italiana, 78, 142 Landi degl’Innocenti, E. 1994, In: Solar Surface Magnetism, R. J. Rutten, and C. J. Schrijver (eds.), p. 29 Sampoorna, M., Frisch, H., Nagendra, K. N. 2008a, New Astronomy, 13, 233 Sampoorna, M., Nagendra, K. N., Frisch, H., Stenflo, J. O. 2008b, A&A, 485, 275 Stein, R. F., Nordlund, ˚ A. 2006, ApJ, 642, 1246 Stenflo, J. O. 1987, Solar Phys., 114, 1 Stenflo, J. O., Holzreuter, R. 2002, In: SOLMAG 2002. Proceedings of the Magnetic Coupling of the Solar Atmosphere Euroconference, H. Sawaya-Lacoste (ed.), ESA Special Publication, vol. 505, p. 101 Stenflo, J. O., Holzreuter, R. 2003a, In: Current Theoretical Models and Future High Resolution Solar Observations: Preparing for ATST, A. A. Pevtsov, H. Uitenbroek (eds.), ASP Conf. Ser., vol. 286, p. 169 Stenflo, J. O., Holzreuter, R. 2003b, Astronomische Nachrichten, 324, 397 Vögler, A., Shelyag, S., Schüssler, M., et al. 2005, A&A, 429, 335 Spectropolarimetry with CRISP at the Swedish 1-m Solar Telescope A. Ortiz and L.H.M. Rouppe van der Voort Abstract CRISP (Crisp Imaging Spectro-polarimeter), the new spectropolarimeter at the Swedish 1-m Solar Telescope, opens a new perspective in solar polarimetry. With better spatial resolution (0.13 00 ) than Hinode in the Fe I 6302 ˚ A lines and similar polarimetric sensitivity reached through postprocessing, CRISP complements the SP spectropolarimeter onboard Hinode. We present some of the data that we obtained in our June 2008 campaign and preliminary results from LTE inversions of a pore containing umbral dots. 1 Introduction CRISP (CRisp Imaging Spectro-Polarimeter) is a new imaging spectropolarimeter installed at the Swedish 1-m Solar Telescope (SST, Scharmer et al. 2003) in March 2008. The instrument is based on a dual Fabry-Pérot interferometer system similar to that described by Scharmer (2006). It combines a high spectral resolution, high reflectivity etalon with a low resolution, and low reflectivity etalon. It has been designed as compact as possible, that is, with a minimum of optical surfaces, to avoid straylight as well as possible. For polarimetric studies, nematic liquid crystals are used to modulate the light. These crystals change state in less than 10 ms, which is faster than the CCD readout time. A polarizing beam splitter close to the focal plane splits the beam onto two 1024  1024 synchronized CCDs that measure the two orthogonal polarization states simultaneously. This facilitates a significant reduction of seeing crosstalk in the polarization maps. A third, synchronized, CCD camera records wide-band images through the pre- filter of the Fabry-Pérot system. These images serve as an anchor channel for Multi-Object Multi-Frame Blind Deconvolution (MOMFBD) image restoration (van Noort et al. 2005), which enables near-perfect alignment between the sequen- tially recorded polarization and line position images. For more details on MOMFBD processing of polarization data see van Noort and Rouppe van der Voort (2008). A. Ortiz (  ) and L.H.M. Rouppe van der Voort Institute of Theoretical Astrophysics, University of Oslo, Norway S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3-642-02859-5 11, c  Springer-Verlag Berlin Heidelberg 2010 150 Spectropolarimetry with CRISP at the Swedish 1-m Solar Telescope 151 The etalons can sample spectral lines between 510 and 860 nm. The field of view (FOV) is 70 00  70 00 ; the pixel size is 0.07 00 /pixel. The instrument has been designed to allow diffraction-limited observation at 0.13 00 angular resolution in the Fe I 630nm lines. 2 The June 2008 Campaign Data and Processing The data displayed here were recorded on 12 June 2008 as part of a campaign during June 2008. The target was a pore (AR 10998) located at S09 E24 ( D 0:79). The field of view was 70 00  70 00 . The images recorded correspond to complete Stokes measurements at 15 line positions in steps of 48 m ˚ A, from 336 m ˚ A to +336 m ˚ A, in each of the Fe I lines, 6301.5 and 6302.5 ˚ A. In addition, images were recorded at one continuum wavelength. Each camera operated at 35 Hz frame rate. For each wavelength and Fig. 1 Clockwise:StokesI , Q, V ,andU images taken in the blue wing of Fe I 6302.5 ˚ Aat  D48 m ˚ A, on 12 June 2008 152 A. Ortiz and L.H.M. Rouppe van der Voort LC state, seven images were so recorded per camera. Each sequence for subsequent MOMFBD processing consists of about 870 images per CCD (2,600 in total), recorded during 30 s. The images were divided into overlapping 64  64 pixel sub- fields sampling different isoplanaic patches with overlaps. All images from each subfield were then processed as a single MOMFBD set. They were demodulated with respect to the polarimeter and a detailed telescope polarization model. In addition, the resulting Stokes images were corrected for remaining I to Q, U ,andV crosstalk by subtraction of the Stokes continuum images. Figure 1 shows an example of the resulting Stokes images. The theoretical diffraction limit of the SST is =D D 0:13 00 at 6,303 ˚ A. We measured the real resolution obtained in our June observations by identifying the smallest intensity feature and fitting a Gaussian to it. Figure 2 shows a cut through a bright point with 80 km FWHM for the Gaussian fit. This value is equivalent to 0.11 00 , which is slightly lower than the theoretical resolution 0.13 00 but consistent with it, due to the MOMFBD post-processing performed to the data. We estimated the noise level for the Stokes profiles to be around 2  10 3 for Stokes Q=I c , U=I c and V=I c . 3 Inversions and Results To derive the atmospheric parameters from the observed Stokes images, we use a least-square inversion code, LILIA (Socas-Navarro 2001),basedonLTEatmo- spheres. We assume a one component, laterally homogeneous atmosphere together with stray light contamination. The inversions return nine free parameters as a function of optical depth, including the three components of the magnetic field vector Fig. 2 Cut along brightenings in the stokes I image (thin line)and magnetic field obtained from inversions (thick line). We have fitted a gaussian to the smallest feature we can observe, both in the intensity image and the resulting magnetic field (dotted lines). The fits give us FWHMs of 80 km for I=I c and 227 km for the magnetic field Spectropolarimetry with CRISP at the Swedish 1-m Solar Telescope 153 Fig. 3 Results from the LILIA inversion of a bright point observed in an intergranular lane. Ob- served (solid) and fitted (dashed) I=I c , Q=I c , U=I c ,andV=I c profiles (upper panels), as well as atmospheric parameters (temperature, magnetic field, inclination, and line-of-sight velocity) obtained through the inversion as a function of optical depth (lower panels) (strength, inclination, and azimut), LOS velocity, and temperature among others. We apply the inversion to both the Fe I 6301.5 and 6302.5 ˚ A lines simultaneously. Figure 3 shows an example of the inversion of an individual pixel belonging to a bright point. In this particular case the inversion code yielded a field strength of 1,100 G, inclination of 25 ı , and LOS velocity of 0.6km s 1 , (downflow) at log./ D1:5. Figures 4 and 5 show maps of the obtained magnetic field strength and line-of- sight (LOS) velocity at different heights. Figure 4 shows a micro-pore as well as brightenings produced by emergent magnetic fields. Ribbons (Berger et al. 2004) can be distinguished. Upflows are correlated with the positions of the center of the granules, while downflows are correlated with the intergranular lanes, except in those areas where the magnetic field is emerging, in which velocities are lower due to the supression of convection. Figure 5 presents a pore with several umbral dots and structures within. These brighter umbral structures show lower magnetic field strengths than the darker parts of the umbra as well as higher temperatures. Spectropolarimetry with the NLST K. Sankarasubramanian, S.S. Hasan, and K.E. Rangarajan Abstract India’s National Large Solar Telescope (NLST) will provide opportuni- ties to observe the Sun with high spatial, spectral, and polarimetric resolution. The large aperture also enables high-cadence spectropolarimetry with moderate spatial resolution. A multi-slit spectropolarimeter is planned as one of the back-end instruments for this powerful telescope, primarily to measure vector magnetic fields in both active and quiet regions. An integral-field unit added with the multi-slit spectropolarimeter will enable fast-cadence observation. Here we discuss the scientific requirements for such an instrument, along with advantages and limitations of the concept and preliminary design details. 1 Introduction The National Large Solar Telescope (NLST henceforth) is being planned as a 2 m- class state-of-the-art solar telescope to be installed at a superior site compared to any of the existing solar facilities in India. A state-of-the-art active and adaptive optics system will be incorporated in the telescope design to provide diffraction- limited imaging over the entire wavelength range of interest under favorable seeing conditions. NLST will be one of the best solar observing facilities around the world and be comparable to the next-generation solar facilities elsewhere. It will also provide complementary observations along with current as well as future solar space missions. While space missions can provide uninterrupted coverage of the Sun, the NLST will provide observations with higher spectral, spatial, and polarimetric resolution. At present, the largest solar telescope for solar research in India is the Kodaikanal Tower Tunnel Telescope, which has been in use for the last 35 years. This telescope, along with its high-dispersion spectrograph, is used primarily for spectral K. Sankarasubramanian (  ) Space Astronomy and Instrumentation Division, ISRO Satellite Centre, Bangalore, India S.S. Hasan and K.E. Rangarajan Indian Institute of Astrophysics, Bangalore, India S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3-642-02859-5 12, c  Springer-Verlag Berlin Heidelberg 2010 156 Spectropolarimetry with the NLST 157 and synoptic studies. It has been upgraded with an instrument package for polarimetry at high spectral resolution (Nagaraju et al. 2008). However, this telescope has the disadvantage of large-angle coelostat reflections, which restrict the accuracy of polarization measurements. Also, the site is affected by the regular monsoon seasons and its seeing quality has degraded over the years. There is also a moderate size (50-cm) solar telescope, named the Multi Appli- cation Solar Telescope (MAST), under construction (Venkatakrishnan 2006)atthe Udaipur Solar Observatory, which will be the first new solar facility since several decades in India. However, the smaller aperture size of this telescope, the moderate seeing quality of the site, and the limited wavelength coverage will restrict the usage of this telescope to observations with moderate spatial resolution. In contrast, NLST is planned to study small-scale magneto-hydrodynamical pro- cesses, the dynamical evolution of small-scale magnetic structures, active regions, sunspots, magnetoconvection,the thermodynamics of the chromosphere, and turbu- lent magnetic fields at the highest possible spatial resolution. Table 1 lists the science Table 1 Science requirements and proposed back-end instruments for the NLST Physical process or region to be observed Physical quantity to be measured Telescope or instrument requirement Proposed instrument MHD waves and oscillations Intensity variation of 1% or less; magnetic fields; velocities of 200 ms 1 or less; properties of oscillations High photon flux; spectral resolution of a few m ˚ A; polarization accuracy 0.1% or better; high time cadence High resolution spectrograph; narrow band filters; spectropolarimeter with spatial information; fast cameras Structuring of the solar atmosphere Temperature, velocity, and magnetic field with height Both visible and infrared capabilities Filters and polarimeters for Ca II K, H˛, G-band He I 10830 ˚ A, 1.6 m Active region evolution Velocity and vector magnetic field Fast cadence; FOV 300 00 Spectropolarimeter with spatial information Hanle– Zeeman effect Vector magnetic field Polarization accuracy of 0.01% Spectropolarimeter Photospheric small-scale structures Velocity and vector magnetic field Fast cadence; FOV 30 00 ; polarization accuracy of 0.1%; fast cadence Spectropolarimeter with spatial information Off-limb observations Intensity, velocity, and vector magnetic field Low scattered light; polarization accuracy of 0.01% Spectropolarimeter; high-resolution spectrograph 158 K. Sankarasubramanian et al. Table 2 Specifications for the NLST Physical parameter Value Aperture of primary 2 m Focal length of primary 4 m Optical configuration Three-mirror on-axis Gregorian Field of view (FOV) 300 00 Final focal fatio f-40 Image scale at the science focus 2.6 00 mm 1 Optical quality Better than 0.1 00 over the FOV Wavelength coverage 3,800 ˚ A–2.5 m Polarization accuracy Better than 1 part in 10,000 Scattered light within the telescope Less than 1% Active and adaptive optics Integrated in the telescope Spatial resolution 0.1 00 at 500 nm requirements and the planned back-end instruments that are required to achieve the proposed science objectives. This table is only an indication of the proposed science and is not exhaustive. More details about the NLST and its scientific goals are given in Hasan and NLST Team (2006). A brief technical specifications of the NLST telescope is listed in Table 2. 2 Polarimetry Package Solar spectropolarimetry is the only way to quantitatively study solar magnetic fields. Unfortunately, detecting polarization is a highly difficult task due to its sensi- tive nature to any anisotropic reflection. Any large optical imaging system requires reflective optics. To minimize the effects of reflections on the polarization state of the incident light, the polarization analysis (or the polarimetry) for the NLST will be carried out as early as possible in the optical train. The scientific requirements of the NLST also warrant a very accurate polarization modulation and analyzing unit. These science requirements translate into the following instrument requirements: – Large wavelength coverage, from 3,800 ˚ Ato2.5m. Unfortunately, no current technology is available to have a single polarization modulator covering this ex- tended wavelength range. However, it is feasible with two or three modulators covering a broad range of wavelengths each. – Preferably a fixed package without any moving parts. – High stability over at least a day in order to reduce the need for polarization calibrations. – Polarization accuracy of 10 5 and a precision of a few times 10 4 . – Good optical quality, as it is close to the focus. – A calibration unit located in front of the modulator in the optical path and before any large angle reflections. – Preferably the use of a balanced modulation scheme in order to avoid seeing- induced spurious polarization if off-the-shelf CCD cameras are used. Spectropolarimetry with the NLST 159 – The modulator should have good transmission at all wavelengths from 3,800 ˚ A to 2.5 m as it is located early in the optical path. Two possible options for the polarization packages are being examined for the NLST, keeping in mind that there should be a possibility to do polarimetry over the entire wavelength range of interest (3,800 ˚ A–2.5 m). The first option is a rotating waveplate retarder very similar to the modulator used in SPINOR (Socas-Navarro et al. 2006), but optimized for larger wavelength coverage (using the bi-crystalline or Pancharatnam technique). A bi-crystalline modulator is already in operation at the Dunn Solar Telescope (DST) with limited wavelength coverage (from 500 nm to about 1.6m). The second option is to use liquid crystals. There are design studies for an achromatic liquid crystal modulator, but these are still at an early stage (Gisler et al. 2003). Liquid-crystal variable retarders are an attractive option, but these also will not cover the full wavelength range, and hence at least two or three modulator packages may be required. A detailed comparison between the two options, including their merits and demerits, will be carried out shortly. The most preferred location for the modulator is at the second focus of the on- axis three-mirror Gregorian system. Initial studies indicate that the best position for the analyzer (which is the last optical component in any modulation scheme) is next to the modulator. Then the modulation must be restricted to a single beam scheme due to alignment issues through the whole optical path of the telescope as well as the limitation of adaptive optics systems in handling two beams. Hence, a fast chopping mechanism is necessary if the analyzer is kept at the Gregorian focus. This poses severe requirements on the detector. The CCD must be a fast readout camera or a custom-made camera like the ZIMPOL (Povel et al. 1994)orC 3 PO (Keller 2005). The second best option is to keep the analyzer closer to the detector. This intro- duces cross-talk between the linear polarization (Stokes Q and U ) that varies over the day due to the rotation of the image. This can be overcome by having a rotating analyzer as compensator. Furthermore, using a calibration unit at the Gregorian focus and before the modulator will help to reduce any residual cross-talks in the system. However, this calibration should be robust and highly accurate to achieve the required polarimetric accuracy and precision. A detailed polarization model of the telescope and the polarimetric system will be carried out to bring out the subtle differences between different locations of the modulator and analyzer. In essence, the first option, which is the best one in terms of polarization precision and accuracy, will require a ZIMPOL or C 3 PO-type CCD detector, while the second option, if calibrated to high accuracy, can work even with off-the-shelf CCD detectors in a balanced two-beam modulation scheme. 3 Spectropolarimetry with NLST Table 1 illustrates that all the proposed science goals cannot be met with a single spectropolarimetric instrument due to the large range of field-of-view (FOV) requirements (from 30 00 to 300 00 ), the time cadence (from 1 min to at most sev- 160 K. Sankarasubramanian et al. eral minutes), and the spectral resolution (from 10 to 100m ˚ A). We propose to realize three different instruments (independent or semi-independent) to cover all the science requirements. They are (1) Multi-Slit Imaging Capable (MuSIC) SpectroPolarimeter (hereafter SP) using an integral field unit, (2) Single-slit high spectral resolution SP, and (3) Fabry-Pérot based imaging SP. Given that MAST will realize instruments similar to the second and third type, the priority is given to the first instrument for NLST. 3.1 Multi-slit SP It is obvious that a single-slit SP has the inherent deficiency of low temporal cadence for an intermediate FOV. This is due to the long scanning time required by the moving spectrograph slit. The left column of Fig. 1 illustrates the single-slit spectral imaging concept. The single slit, marked as a dark vertical line in the continuum intensity image (top-left), is scanned across the FOV in order to produce two- dimensional spectropolarimetric mapping. The bottom-left four images are the four Stokes images at the marked slit position. Almost all of the useful solar polarimetric Arcseconds 50 0 5 10 15 20 25 30 10 Arcseconds 15 20 25 30 Arcseconds 50 0 5 10 15 20 25 30 10 Arcseconds 15 20 25 30 0 6300.0 6301.9 6303.7 6305.6 5 10 15 20 25 30 Arcseconds 0 −1.50.0 1.5 5 10 15 20 25 30 Fig. 1 The top two images show red-continuum intensity image of an active region along with vertical markings for a single-slit (top-left) and five slits (top-right). The bottom two figures show the corresponding Stokes spectrum (marked as I , Q, U ,andV in the respective images). Bottom- left is for a single slit and bottom-right for five slits. The single-slit data were obtained using the DLSP (Sankarasubramanian et al. 2004). The multi-slit images are simulated data [...]... de Andalucá, CSIC, Granada, Spain ı ı S.S Hasan and R.J Rutten (eds.), Magnetic Coupling between the Interior and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3- 642 -02859-5 15, c Springer-Verlag Berlin Heidelberg 2010 193 1 94 L.R Bellot Rubio by the umbra In recent years, considerable efforts have been made to understand the energy transport in sunspots Jahn and. .. Steiner et al (2008) obtain spatial and temporal avT erages for the transversal and longitudinal apparent magnetic flux densities, jBapp j L and jBapp j of, respectively, 21.5 G and 5.0 G for run h20 and 10 .4 G and 6.6 G for T L run v10 Thus, the ratio r D hjBapp ji=hjBappji D 4: 3 for h20 and 1:6 for v10 Lites T L et al (2008) obtain from Hinode SP data hjBapp ji D 55 G and hjBapp ji D 11 G resulting in... 1993; Schlichenmaier et al 1998) in which the Evershed flow channels K Ichimoto ( ) Kwasan and Hida Observatories, Kyoto University, Japan S.S Hasan and R.J Rutten (eds.), Magnetic Coupling between the Interior and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3- 642 -02859-5 14, c Springer-Verlag Berlin Heidelberg 2010 186 The Evershed Effect with SOT/Hinode 187 are... diffraction-limited spatial resolution and the contrast in the continuum Recent observational investigations of the dynamics, morphology, and properties of small-scale magnetic field concentrations of the quiet Sun include Berger et al (20 04) ; Langhans et al (20 04) ; Lites and Socas-Navarro (20 04) ; S´ nchez Almeida a et al (20 04) ; Socas-Navarro and Lites (20 04) ; Wiehr et al (20 04) ; Rouppe van der Voort et al... the Hinode space observatory (Kosugi et al 2007) The Japanese Hinode satellite was put in orbit on 22 September 2006 It is not so much the spatial resolution O Steiner ( ) Kiepenheuer-Institut f¨ r Sonnenphysik, Freiburg, Germany u S.S Hasan and R.J Rutten (eds.), Magnetic Coupling between the Interior and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3- 642 -02859-5... multi-slit concept Relative Intensity 1.0 1.0 0.8 0.8 0.6 0.6 0 .4 0 .4 0.2 0.2 0.0 6299 0.0 6300 6301 6302 6303 Wavelength (A) 63 04 6305 –0.2 1081.5 1082.0 1082.5 1083.0 1083.5 10 84. 0 10 84. 5 Wavelength (A) Fig 2 Left: solar intensity from a spectral atlas is plotted as red Three different Gaussian pro˚ files of 2, 4, and 6 A FWHM represent filter passbands Note how the transmission changes at the wings of the... technique between 10 and 100 G This range was narrowed to 4 40 G by Stenflo et al (1998) More precise estimates by Faurobert-Scholl (1993) and Faurobert-Scholl et al (1995) yielded values in the range of 30–60 G in the deep photosphere and 10–20 G in the middle and upper photosphere, where the latter values were increased to 20–30 G by Faurobert et al (2001) The values reported by Stenflo et al and by Faurobert... relations and then uses these to examine how acoustic rays entering regions of strong field split into fast and slow components and the subsequent fates of each Cally (2007) presents the theory in a particularly instructive manner Results from numerical simulations of MAG-wave propagation in three-dimensional space are presented by Cally and Goossens (2008) and Moradi et al (2009) Khomenko and Collados... h20 (heavy) and run v10 (thin) of Steiner et al (2008) v10 and h20 substantially differ in their initial and boundary conditions for the magnetic field Note the different physical meanings of the abscissa and the different units and scales in the ordinates of the two plots Magnetic Coupling in the Quiet Solar Atmosphere 171 a local maximum of the horizontal field component near 500 km height and this is... result Figure 1 shows the horizontal and the vertical magnetic field strengths as a function of height in the atmosphere of the three simulations Left and right boundaries of the left panel correspond to approximately z D 40 0 km and z D 1;000 km of the right panel, respectively Note that the scale of the ordinate is logarithmic and in gauss in the left panel but linear and in mT in the right panel Also . Theoretical Astrophysics, University of Oslo, Norway S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI. Interior and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3- 642 -02859-5 12, c  Springer-Verlag Berlin Heidelberg 2010 156 Spectropolarimetry with the NLST 157 and. Germany S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3- 642 -02859-5 13, c 

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