On behalf of the organizing committee and of the Royal Institute of Technology I wish you welcome to this symposium. I need not stress the importance of the subject which we are going to discuss—as participants in this symposium all of us are aware of it. You also know very well how this field has been opened by Birkeland and Störmer, by Chapman, Cowling and Ferraro, by Hale, Swann, and many others. However, it is not until the last decade that the interest in this subject has become more general. Many different research groups are active but their ideas differ very much, which gives a good reason for meeting and exchanging ideas.