SOLAR ACTIVE LONGITUDES AND THEIR ROTATION LIYUN ZHANG Department of Physics University of Oulu Finland Academic dissertation to be presented, with the permission of the Faculty of Science of the University of Oulu, for public discussion in the Auditorium L6, Linnanmaa, on 30th November, 2012, at 12 o’clock noon. REPORT SERIES IN PHYSICAL SCIENCES Report No. 78 OULU 2012 ● UNIVERSITY OF OULU Supervisors Prof. Kalevi Mursula University of Oulu, Finland Prof. Ilya Usoskin Sodankylä Geophysical Observatory, University of Oulu, Finland Opponent Dr. Rainer Arlt Leibniz-Institut für Astrophysik Potsdam, Germany Reviewers Prof. Roman Brajša Hvar Observatory, Faculty of Geodesy, University of Zagreb, Croatia Assoc. Prof. Dmitri Ivanovitch Ponyavin St. Petersburg State University, St. Petergoff, Russia Custos Prof. Kalevi Mursula University of Oulu, Finland ISBN 978-952-62-0025-5 ISBN 978-952-62-0026-2 (PDF) ISSN 1239-4327 Oulu University Press Oulu 2012 Zhang, Liyun: Solar active longitudes and their rotation Department of Physical Sciences, University of Oulu, Finland. Report No. 78 (2012) Abstract In this thesis solar active longitudes of X-ray flares and sunspots are studied. The fact that solar activity does not occur uniformly at all heliographic longitudes was noticed by Carrington as early as in 1843. The longitude ranges where solar activity occurs preferentially are called active longitudes. Active longitudes have been found in various manifestations of solar activity, such as sunspots, flares, radio emission bursts, surface and heliospheric magnetic fields, and coronal emissions. However, the active longitudes found when using different rigidly rotating reference frames differ significantly from each other. One reason is that the whole Sun does not rotate rigidly but differentially at different layers and different latitudes. The other reason is that the rotation of the Sun also varies with time. Earlier studies used a dynamic rotation frame for the differential rotation of the Sun and found two persistent active longitudes of sunspots in 1878-1996. However, the migra- tion of active longitudes with respect to the Carrington rotation was treated there rather coarsely. We improved the accuracy of migration to less than one hour. Accordingly, not only the rotation parameters for each class of solar flares and sunspots are found to agree well with each other, but also the non-axisymmetry of flares and sunspots is systematically increased. We also studied the long-term variation of solar surface rotation. Using the improved analysis, the spatial distribution of sunspots in 1876-2008 is analyzed. The statistical evidence for different rotation in the northern and southern hemispheres is greatly im- proved by the revised treatment. Moreover, we have given consistent evidence for the periodicity of about one century in the north-south difference. Keywords: Solar activity, sunspots, X-ray flares, solar rotation, active longitudes. This thesis is dedicated to my biological parents and my adoptive parents for all their love and support! They have been and will always be great sources of encouragement and inspiration throughout my life! 3 Acknowledgements This work has been carried out in the Department of Physics at the University of Oulu, Finland. First, I would like to express my deepest gratitude to Prof. Ilya Usoskin. He very enthusiastically answered my questions at our first meet during the Beijing COSPAR 2006. He has been supporting me with my study and work ever since. He also guided me to write my first scientific paper in 2006. He encouraged me to come to Oulu to get valuable work experience under the guidance of Prof. Kalevi Mursula and himself. In the beginning of my first visit to Oulu, Ilya offered me plenty of help and guidance for managing the new daily life, including how to wait for a bus and how to use the knife and fork at lunch, too much to mention. Next, I wish to express my greatest gratitude to my principal supervisor, Prof. Kalevi Mursula who always has open mind and new ideas. He has always been giving me plenty of support and encouragement to deal with the problems I have had during doing the thesis. Accordingly, the scientific significance of the thesis has been greatly improved. I also sincerely appreciate his fatherly advices on my life. I would like to express my deepest gratitude to my Beijing chief Prof. Huaning Wang, a great scientist in solar physics and solar activity prediction. He has been guiding me through my science career since the very beginning and giving me continuous support. He always reminds me to find out what is the physics behind the phenomenon that I see. The other members of my Beijing group are also deeply acknowledged, especially Prof. Zhanle Du, Xin Huang, Jian Gao, Yanmei Cui and Rong Li, who greatly share my study, work and life. I am very grateful for the funding from the Department of Physics of the Uni- versity of Oulu, the Academy of Finland, the European Commission's Seventh Framework Programme for eHeroes project, and the Key Laboratory of Solar Ac- tivity, National Astronomical Observatories, Chinese Academy of Sciences. I fully acknowledge the pre-examiners of my thesis Prof. Roman Brajˇsaand Assoc. Prof. Dimitri Ponyavin for their valuable comments and advice. I would 5 also like to express my sincere appreciation to Dr. Rainer Arlt who agreed to spare his time to be the opponent. I wish to express my warmest gratitude to all the other members in the Space Physics group for their friendship and help, Reijo Rasinkangas, Leena Kalliopuska, Olesya Yakovchouk, Alessandra Pacini, three Virtanen brothers, Timo Asikainen, Petri Kalliom¨aki,Tomi Karppinen, Lauri Holappa, Ville Maliniemi, Anita Aikio, Ritva Kuula, Tuomo Nygr´en,Kari Kalla, Timo Pitk¨anen,Lei Cai, Perttu Kantola and Raisa Leussu. Members of the Geophysics group are also acknowledged for their warm companionship during the coffee break. In the end I would like to thank my dear sister and brother for their constant support and love during my study and my life. I definitely must thank my beloved husband Ari Ronkainen for everything. He has been strongly supporting me for my work and for my hobbies. I would also like to thank my husband's parents and relatives for their love and support. Oulu, November 30, 2012 Liyun Zhang Original publications This thesis consists of an introduction and four original papers. I L. Zhang, K. Mursula, I. G. Usoskin, and H. N. Wang, Global analysis of active longitudes of solar X-ray flares, J. Atmos. Sol. Terr. Phys., 73, 258- 263, 2011. II L. Zhang, K. Mursula, I. G. Usoskin, and H. N. Wang, Global analysis of active longitudes of sunspots, Astron. Astrophys., 529, A23, 2011. III L. Zhang, K. Mursula, I. Usoskin, H. Wang, and Z. Du, Long-term variation of solar surface differential rotation, Bulletin of Astronomical Society of India, ASI Conference Series, 2, 175, 2011. IV L. Zhang, K. Mursula, and I. Usoskin, Consistent long-term variation in the hemispheric asymmetry of solar rotation, Astron. Astrophys., submitted. In the text, original papers are referred to using roman numerals I-IV. Contents Abstract . 1 Acknowledgements . 5 Original publications . 7 1. Introduction . 11 2. Solar activity . 15 2.1. Sunspots . 15 2.1.1. Wolf sunspot numbers . 17 2.1.2. Group sunspot numbers . 19 2.1.3. Sunspot areas and positions . 20 2.1.4. Cyclicities in solar activity . 21 2.1.5. Grand minima and maxima . 25 2.2. Solar flares . 28 2.2.1. Classification of solar flares . 29 2.2.2. Phases of a typical flare . 31 2.2.3. Radio emissions during solar flares . 34 2.2.4. Flare models . 36 2.3. Coronal mass ejections . 39 2.4. Solar energetic particles acceleration . 41 3. Solar rotation . 43 3.1. Heliographic coordinates . 43 3.2. Surface rotation . 43 3.3. Internal rotation . 45 3.4. Torsional oscillation . 48 3.5. Meridional circulation . 51 4. Active latitudes . 55 5. Active longitudes . 58 5.1. History of active longitude research . 58 5.2. Flip-flop phenomenon . 69 5.3. Dynamo models . 70 5.4. North-South asymmetry . 74 5.4.1. N-S asymmetry in solar activity . 75 5.4.2. N-S asymmetry in solar rotation . 77 6. Summary . 82 1. Introduction As is well known, the Sun is the source of energy that supports the existence of life on the Earth through radiation, which was originally recognized as visible sunlight, while solar electromagnetic radiation covers infrared, visible, and ultraviolet light. Most of the time the Sun appears as a quiet star to the naked-eye, but it is far more active when observed through telescopes, especially through the modern space-borne instruments with high temporal and spatial resolution. Even the quiet Sun appears very active in the observations of the newest space solar telescope - Solar Dynamics Observatory (SDO). Small scale tornadoes or cyclones are found to occur all over the quiet Sun with rotating magnetic fields [Zhang and Liu, 2011] (see the left panel of Fig. 1.1). Later phases of cyclones are observed to be associated with microflares. A huge tornado can be observed before a coronal mass ejection (CME) (see the right panel of Fig. 1.1). There are gradually changing activities and bursting activities on the Sun. The former refer to activities which vary gradually in size and energy and last a rel- atively long time. These include sunspots, plages, faculae, quiet filaments and prominences, coronal holes and streamers, solar wind, and the general radiation. The latter refer to explosive events with sudden and huge energy release. These events include, e.g., flares, CMEs, and explosive filaments and prominences. Solar activity affects the near-Earth space in many different ways. Variation in solar conditions at short time scales, e.g., from a few seconds to days is involved with space weather, while the long-term conditions, from several months to one solar cycle and longer, relate to space climate.