Solar Wind and Space Weather

Hot coronal plasma cannot be confined by the gravitational field of the Sun and gradually flows out into interplanetary space, forming the solar wind. This chapter provides a brief overview of the characteristics of the solar wind and its manifestations in the heliosphere. There are two types of solar wind: a slow wind with a speed of about 350 km s−1 and a fast wind with speeds of 500–800 km s−1. The fast wind originates in those parts of the corona where magnetic field lines extend from the surface and go into the interplanetary medium (“open field”). The slow wind is formed over areas of closed magnetic fields, field lines of which are fixed at both ends in the photosphere. The interaction of fast and slow solar wind flows leads to compression and deflection of interplanetary magnetic field, which can initiate geomagnetic disturbances on the Earth. However, the strongest geomagnetic storms are caused by the collision of fast CMEs with the Earth’s magnetosphere. This is why forecasting CME arrival is of paramount importance to the entire space weather problem. Some ideas to estimate of the probability of launching the CME in the solar atmosphere are proposed. Several examples of extreme solar storms happened in the twentieth century are described and the possibility of a solar superstorm is discussed.