Editorial The science of space weather

This special issue of Annales Geophysicae is devoted to the science of space weather. This subject has become a hot topic in recent years as a result of the growing appreciation that many phenomena in the space environment around the Earth can affect human beings and their technological sys-tems. These phenomena are ultimately driven by activity on the Sun. Thus the science of space weather is deeply concerned with the origins and manifestations of solar ac-tivity and with the propagation of solar-originated effects through the heliosphere to locations where human activities take place. This, of course, is predominantly the Earth and the region of space immediately surrounding it. But our ex-ploration of the solar system means that we must also be aware of space weather effects beyond the Earth- both for the robotic probes currently used for exploration and to be ready for human exploration when that is finally realised. But space weather is not just science. It is concerned with phenomena that can directly affect other human activities, so it is important to understand how the science of space weather can interact with those activities. Many of the peo-ple who run technological systems affected by space weather are interested to know about the science and to explore what it can do to help them. But they want good science, e.g. in-formation on the size and frequency of effects. They will then to use this to see if they can design out space weather effects at reasonable cost. If not, they will then see if there are reasonable precautions that they can take in response to reliable estimates of current or future conditions. The key point is that the practical response to space weather must be reasonable- in particular terms of costs. But to be truly effective that practical response must be un-derpinned by an understanding of the physical mechanisms of space weather phenomena. For example, our current un-derstanding of dayside reconnection is far from perfect but is sufficient to allow us to be confident in using upstream measurements of the solar wind plasma and magnetic field to provide short-term warnings of geomagnetic activity. In prin-ciple, one could base these warnings purely on the statisti