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  • Solar variability and solar irradiance reconstructions on time scales of

    decades to centuries

    Von der Fakultät für Elektrotechnik, Informationstechnik, Physik

    der Technischen Universität Carolo-Wilhelmina

    zu Braunschweig

    zur Erlangung des Grades einer

    Doktorin der Naturwissenschaften

    (Dr.rer.nat.)

    genehmigte

    Dissertation

    von Laura Antonia Balmaceda

    aus Rosario / Argentinien

  • Bibliografische Information Der Deutschen Bibliothek

    Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.ddb.de abrufbar.

    1. Referentin oder Referent: Dr. Prof. Sami K. Solanki

    2. Referentin oder Referent: Prof. Dr. Karl-Heinz Glassmeier

    eingereicht am: 22 Januar 2007

    mündliche Prüfung (Disputation) am: 11 April 2007

    Copyright c© Copernicus GmbH 2007 ISBN 978-3-936586-68-8

    Copernicus GmbH, Katlenburg-Lindau

    Druck: Schaltungsdienst Lange, Berlin

    Printed in Germany

  • Vorveröffentlichungen der Dissertation

    Teilergebnisse aus dieser Arbeit wurden mit Genehmigung der Fakultät für Physik, vertreten durch den Mentor oder den Betreuer der Arbeit, in folgenden Beiträgen vorab veröf- fentlich:

    Publikationen

    • Balmaceda, L.; Krivova, N.; Solanki, S. K. "Reconstruction of solar irradiance us- ing the Group sunspot number", Adv. Space Res., accepted

    • Krivova, N.; Balmaceda, L.; Solanki, S. K. "Reconstruction of solar total irradiance since 1700 from the surface magnetic flux", A&A, 467, 335-346, 2007

    • Balmaceda, L.; Solanki, S. K.; Krivova, N. "A cross-calibrated sunspot areas time series since 1874", Memorie della Societa Astronomica Italiana, 76, 929, 2005

    Tagungsbeiträge

    • Balmaceda, L.; Solanki, S. K.; Krivova, N. Reconstruction of solar irradiance variations since the Maunder Minimum (Talk). Oberseminar, Technical University Braunschweig, Germany, November 20, 2006.

    • Balmaceda, L.; Solanki, S. K.; Krivova, N. Reconstruction of total solar irradiance variations since the Maunder Minimum (Talk). ISCC-2: Long-term Changes in the Sun and their Effects in the Heliosphere and Planet Earth, Sinaia, Romania, September 13-16, 2006.

    • Balmaceda, L. Solar variability and a possible impact on climate (Talk). 1st Inter- disciplinary Max Planck PhDnet workshop: Global Changes - Environment, Soci- ety & Science, Cologne, Germany, August 17 - 19, 2006.

    • Balmaceda, L. Solar variability and climate changes (Talk). 3rd El Leoncito School of Solar Physics, El Leoncito, San Juan, Argentina, November 28 - December 05, 2005

    • Balmaceda, L.; Solanki, S. K.; Krivova, N. A Homogeneus Sunspot Areas time series since 1874 (Poster). Solar Variability and Earth Climate, Villa Mondragone, Monte Porzio Catone (Rome), Italy, 27 June - 1st July, 2005.

  • Aim at the sun, and you may not reach it; but your arrow will fly far higher than if aimed at an object on a level with yourself.

    Joel Hawes (1789 - 1867)

  • Dedicada a quienes siempre estuvieron a mi lado:

    Oscar, Inés y Virginia

  • Contents

    Summary 11

    1 Introduction 13 1.1 Why study solar variability? . . . . . . . . . . . . . . . . . . . . . . . . 13 1.2 Our magnetic Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.3 Solar irradiance variations . . . . . . . . . . . . . . . . . . . . . . . . . 15

    1.3.1 Total solar irradiance variations . . . . . . . . . . . . . . . . . . 15 1.3.2 Spectral irradiance variations . . . . . . . . . . . . . . . . . . . . 19

    1.4 Sun-climate: is there a link? . . . . . . . . . . . . . . . . . . . . . . . . 20 1.5 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    2 A homogeneus sunspot areas database covering more than 130 years 25 2.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.3 Observational data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.4 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

    2.4.1 On the effect of including offset to calculate cross-calibration fac- tors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

    2.4.2 An alternative method to calculate cross-calibration factors . . . . 30 2.4.3 Error estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

    2.5 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.5.1 Comparison between sunspot areas . . . . . . . . . . . . . . . . 33 2.5.2 Comparison with sunspot number . . . . . . . . . . . . . . . . . 35 2.5.3 Cross-calibrated sunspot area records . . . . . . . . . . . . . . . 37

    2.6 The Photometric Sunspot Index . . . . . . . . . . . . . . . . . . . . . . . 37 2.7 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 39

    3 Reconstruction of solar total and UV irradiance from the sunspot and plage areas 41 3.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.3 Quantifying the active-region contribution to the solar irradiance variations 42

    3.3.1 Sunspot darkening . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.3.2 Facular brightening . . . . . . . . . . . . . . . . . . . . . . . . . 43

    3.4 Reconstruction of irradiance from plage and sunspot areas . . . . . . . . 45 3.5 Comparison with climate records . . . . . . . . . . . . . . . . . . . . . . 47

    9

  • Contents

    3.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.7 Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

    4 Reconstruction of solar total irradiance from the surface magnetic flux 51 4.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.3 Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

    4.3.1 Photospheric magnetic flux . . . . . . . . . . . . . . . . . . . . . 53 4.3.2 Solar irradiance . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.3.3 Parameters and optimization . . . . . . . . . . . . . . . . . . . . 59

    4.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 4.4.1 Standard model . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 4.4.2 A possible shift between the ER and AR evolution . . . . . . . . 65 4.4.3 Constant ER flux . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.4.4 Reconstruction based on the Group sunspot number . . . . . . . . 69

    4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

    5 Outlook: First steps towards a millennial reconstruction of solar irradiance 75 5.1 Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.3 Description of the data . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 5.4 Searching for relations . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

    5.4.1 Total solar irradiance and sunspot number . . . . . . . . . . . . . 78 5.4.2 Total solar irradiance and total magnetic flux . . . . . . . . . . . 80 5.4.3 Total solar irradiance and open magnetic flux . . . . . . . . . . . 81 5.4.4 Open magnetic flux and total magnetic flux . . . . . . . . . . . . 85

    5.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 5.6 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 93

    6 Concluding remarks 95

    Bibliography 97

    Acknowledgements 109

    Lebenslauf 111

    10

  • Summary

    One principal record of solar variability is the time series of "Total Solar Irradiance" which is the total amount of energy per second per unit angle normalized to 1 AU. This quantity is available since November 1978 and the most striking result is that it varies by about 0.1% on time scales of the solar cycle (∼ 11 years). Dark (sunspots) and bright (faculae and the network) features continuously appear and disappear on the solar surface and tra- verse the visible solar disc as the Sun rotates, thus modulating the solar irradiance. Both, sunspot darkening and facular brightening are magnetic phenomena. The observations do not provide any information on longer-term secular trends in the irradiance, however, and models are required to set tighter constraints on this quantity. The aim of this research is to understand the physical mechanisms responsible for solar brightness variations on time scales from the solar cycle to centuries and millennia. The main goal of the thesis is to carry out quantitative modelling of total and spectral irradiance variations based on a model of the surface magnetic flux. In a first step the magnetic flux and irradiance since the Maunder minimum (a time of extended lack of sunspots which coincided with the Lit- tle Ice Age) are reconstructed using a simple but consistent physical model. By using all available data on total and open magnetic flux and total irradiance to constrain the model parameters the increase in total solar irradiance since the Maunder minimum could be estimated to be about 0.1%. Necessary for the study of the possible role of the Sun’s vari- ability on E

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