Diagnostic of the Solar Photosphere with High Spatial Resolution Using CH, CN and Continuum Spectral Bands

Diagnostic of the solar photosphere with high spatial resolution using CH, CN and continuum spectral bands Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultaten¨ der Georg-August-Universitat¨ zu Gottingen¨ vorgelegt von Vasily Zakharov aus Moskau / Russland Gottingen¨ 2006 Bibliografische Information Der Deutschen Bibliothek Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet uber¨ http://dnb.ddb.de abrufbar. D7 Referent: Prof. Dr. F. Kneer Korreferent: Prof. Dr. S. K. Solanki Tag der mundlichen¨ Prufung:¨ 24.03.2006 Copyright c Copernicus GmbH 2006 ISBN 3-936586-54-3 Copernicus GmbH, Katlenburg-Lindau Druck: Schaltungsdienst Lange, Berlin Printed in Germany Contents Contents 3 Summary 5 1 Introduction 7 2 High resolution imaging 17 2.1 Phasediversityimagereconstruction . ..... 17 2.2 Observations ................................ 23 2.3 Datareduction................................ 25 2.3.1 Darkframeandflatfield . 25 2.3.2 JPDSreconstruction . 26 2.4 Results.................................... 27 3 High resolution spectroscopy of the solar photosphere 33 3.1 Observations ................................ 33 3.2 Datareductionofthespectra . 36 3.2.1 Darkframe ............................. 37 3.2.2 Flatfield .............................. 37 3.2.3 Dopplershiftandlinewidth . 43 3.2.4 Slitposition............................. 45 3.2.5 Intensity .............................. 46 3.2.6 BrightPointIndex ......................... 48 3.2.7 Linedepression........................... 49 3.3 Spectroscopy of small-scale magnetic features at the disccenter . 49 3.3.1 Mainproperties........................... 49 3.3.2 Individualcases.. .. .. .. .. .. .. .. 54 3.3.3 Intensitycontrast . 60 3.3.4 Brightpointindex ......................... 62 3.3.5 Linedepression........................... 65 3.4 Spectroscopy of small-scale magnetic features near the limb ....... 67 3.4.1 Mainproperties........................... 67 3.4.2 Intensitycontrast . 72 3.4.3 Brightpointindex ......................... 74 3.4.4 Linedepression........................... 77 3.4.5 Strong velocityshiftsseen inupperphotosphere . ..... 78 3 Contents 3.5 Continuumintensity............................. 80 3.6 Spectroscopy of large-scalemagneticphenomena . ....... 80 3.6.1 Pore................................. 80 3.6.2 Sunspot............................... 85 3.6.3 Velocityreference . 85 4 Radiative MHD Simulations 89 4.1 MHDmodel................................. 89 4.2 Radiativetransferandlinesynthesis . ..... 91 4.3 Center-to-limbvariation . 94 4.4 Results ofradiativesimulationsat lowresolution . ......... 97 4.4.1 Limbdarkening........................... 97 4.4.2 CLVofintensityconstrast . 101 4.5 MDIobservations.............................. 103 4.5.1 Radiativesimulations. 106 4.5.2 Results of simulations and comparison with MDI observations . 107 4.6 High resolution structure of the solar surface near the limb ........ 112 4.6.1 Highresolutionobservations . 112 4.6.2 Simulatedfaculae. 114 4.6.3 Originoffaculaeandnarrow,darklanes . 115 Conclusions and outlook 123 A Filter profiles 127 B Contribution functions 131 Bibliography 133 Acknowledgements 143 Lebenslauf 145 4 Summary This thesis describes high-resolution imaging and high-resolution spectroscopic observations in the blue continuum and in molecular bands, their analysis and the interpretation of results. These refer to small-scale photospheric features, e.g. G-band bright points, ribbons, faculae etc. In addition, descriptions are given of the radiative diagnostics in the continuum of three-dimensional MHD simulations that have been carried out in order to study the physical structure of faculae and the center-to-limb variation of their brightness. The projects which have been carried out as part of this thesis and the most important results are: • High-resolution simultaneous observations of the Sun in three spectral bands, the blue continuum (436.4 nm), the G band (430.5 nm) and the violet CN band-head (387.9 nm), have been carried out with the 1-m Swedish Solar Telescope (SST). Near diffraction limited spatial resolution was achieved in the filtergrams by means of the Joint Phase Diverse Speckle and the Multiframe Blind Deconvolution reconstruction techniques. • The contrast of the photospheric bright points was studied in all three observed spectral ranges and compared with the computational predictions by other researches. It was found that on average over observed bright points their contrast in the blue CN band-head images is 1.4 times higher than that in the G-band images. This disagrees with the predictions of 3-D MHD simulations. The reasons for this dis- crepancy are discussed. • High-resolution spectroscopic observations of small-scale magnetic activity mani- festations in the solar photosphere were carried out simultaneously in the blue CN band-head (387.588−388.473 nm) and in a blue spectral band (436.1−436.9 nm) containing absorption lines of CH with the spectrograph installed on the SST. The analysis of the line-core depression of the CN and CH lines and the line-core intensity contrast of the bright points yielded that the bright points appear brighter in the CN than in the CH lines in the same spectral band. • High-resolution narrowband images in the continuum near λ =705.7 nm were obtained with the SST by applying the MFBD reconstruction technique. It is shown that magnetic features observed near the solar limb are accompanied by facular brightenings with a high contrast. The granulation pattern in plages near the limb has a three-dimensional appearance. • Radiative Transfer computations in the continuum at λ =705.7 nm in 3-D MHD models computed by the Max-Planck-Institut für Sonnensystemforschung and Uni- 5 Summary versity of Chicago MHD groups have carried out. The studied models have different amount of average magnetic flux, i.e. 10 G, 200 G and 4000 G as well as MHD simulations of a pore (Cameron et al. 2004). The radiative transfer was computed by means of the STOPRO code. It was shown that concentrations of strong magnetic fields are responsible for the appearance of the 3-D granulation pattern, faculae, narrow dark lanes (at the limbward side of some granules) etc, whose radiative properties are very close to those obtained from the observations. The physical parameters of the atmosphere, which lead to the formation of photospheric phenomena as observed near the limb are discussed. • Radiative Transfer computations in the continuum at λ =676.8 nm and of the full Stokes vector in the Ni I line at λ =676.8 nm in many snapshots of MHD models with different amounts of magnetic flux were carried out with the STOPRO code. The investigation with low spatial resolution of the center-to-limb variation (CLV) of the continuum contrast were analysed. The magnitude of the limb darkening in the simulations was directly compared with the observations by Neckel (1996). The results of Stokes-vector computations of the Ni I line were properly adapted to conditions of the MDI instrument on the SOHO spacecraft and directly compared with the observations obtained with it by Ortiz et al. (2002). 6 1 Introduction The Sun is a special object in stellar astrophysics. It is the most prominent body in our solar system and contains approximately 98% of the total mass in the solar system. The Sun can be observed from Earth with high spatial resolution. Its outer visible layer is called the photosphere, which has a temperature of about 5780 K and emits 99.99 % of the energy generated in the solar interior by nuclear fusion, most of it in thevisiblespectral range. The solar spectrum consists of a continuum with thousands of dark absorption lines superposed. The lines are called Fraunhofer lines, and the solar spectrum is sometimes called the Fraunhofer spectrum. These lines are produced primarily in the photosphere. Spectra of the CH and the CN molecules in the Sun CH is an important molecule in cool stars. Its absorption bands extend from the near- ultraviolet to the mid-infrared with very strong contributions in the visible. The roughly 1 nm wide band near λ =430 nm which contains CH lines is called the G band. At low spectral resolution this band-head resembles a single spectral line to which Fraunhofer (1817) assigned the letter ”G”. Between 429.5 nm<λ<431.5 nm it is dominated by about 746 transitions 90% of them belonging to the A2∆ − X2Π electronic system (Jor- gensen et al. 1996) of the CH molecule. The G band contains also many blends of other species like Fe, Ti, Ni, Cr, Ca, etc. In Fig 1.1 the mean solar spectrum near the G band at the disc center is presented, as observed by a Fourier Transform Spectrometer (FTS, Brault & Neckel 1987) operating at the Mc-Math telescope on Kitt Peak. Another diatomic molecule, cyanogen CN, produces an absorption band in the blue of the solar spectra. The blue CN band-head is located near λ=388 nm and contains absorption lines of the CN molecule owing to transitions of the B2Σ+ − X2Σ+ electronic system. In Fig 1.2 a section of the mean solar spectrum near the blue CN band-head at the disc center taken from the FTS atlas is presented. Additionally, many weak CH lines (log(gf) ≤-5) of the A2∆ − X2Π electronic system and several strong lines of the CH B2Σ− − X2Π system, as well as a number of atomic lines are present in this spectral domain. Magnetic activity of the Sun The magnetic activityof the Sun is responsible for most fascinating structures

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