I thank the editorial board of the Living Reviews in Solar Physics for the invitation to prepare this review and organizational aid. This work would be impossible without encouraging discussions, as well as direct and indirect help, from my colleagues Gennady Kovaltsov and Sami Solanki, whose invaluable support is acknowledged with my greatest gratitude. I have also greatly benefited from lively and open discussions and debates with Edouard Bard, Jürg Beer, Narendra Bhandari, late Giuliana Cini Castagnoli, Paul Charbonneau, Vincent Courtillot, Cornelis de Jager, Gisella Dreschhoff, Erwin Flückiger, Peter Foukal, Yves Gallet, Agnés Genevey, David Hathaway, Monika Korte, Natalia Krivova, Bernd Kromer, Devendra Lal, Michael Lockwood, Ken McCracken, David Moss, Kalevi Mursula, Raimund Muscheler, Hiroko Miyahara, Heikki Nevanlinna, Valery Ostryakov, Alexander Ruzmaikin, Dieter Schmitt, Manfred Schüssler, Margaret Shea, Don Smart, Ian Snowball, Dmitry Sokoloff, Willie Soon, Carla Taricco, Jóse Vaquero, and many others. I thank Evguenia Usoskina for help with editing this text. I am happy to acknowledge those colleagues who work in this field and make it living and vibrant.
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