6 Acknowledgements

The author is grateful to the editors for encouraging me to write this review paper and to two anonymous referees for their detailed and constructive comments, which greatly improved the quality of the paper. The discussions with J. Chen, Y. Chen, T.G. Forbes, B.C. Low, S.T. Wu, and J. Zhang are beneficial. PFC has been financially supported by Chinese foundations 2011CB811402 and NSFC (11025314, 10878002, and 10933003) in the past years. SOHO is a project of international cooperation between ESA and NASA. We are also grateful to the BBSO and MLSO teams for making their data available.

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