Solar activity and its forecast

An essential problem for solar activity forecasting is understanding the physical basis of solar activity phenomena. During the last decade, interest shifted from a global evaluation of the active centers to a detailed study of their microscopic magnetic components. This allows for the possibility of an identification of the flare sites but we do not understand the physical process of the triggering itself. The X-ray data give us some evidence that during a flare the particles were accelerated in two distinct phases. Obviously other processes of particle acceleration are at work on the sun: transient perturbations of the chromosphere linked to type-III bursts are thought to be connected to an acceleration of subrelativistic electrons which may or may not trigger an optical flare. High time resolution data show that many events are so complex that in the future they will have to be taken into account. Since forecasting centers need a worldwide network, they cooperate through the International Ursigram and World Day Service (IUWDS) : They agree on a permanent program of data exchanges and daily-forecasts and on temporary projects. Several such projects are briefly reported in order to illustrate the variety of the requests that a warning center has to fulfill.