Solar wind control of the magnetotail

Observations of the geomagnetic tail configuration and its changes in response to the solar wind plasma and the magnetic field are discussed. Particular consideration is given to the role of the interplanetary B(y) component, which is believed to control the location of field lines passing through the tail boundary and thereby the location of local plasma entry. It is shown how the energetically insignificant interplanetary B(z) component controls the degree to which energy is added to the magnetosphere and stored in the tail, and the manner in which internal processes such as tail reconnection release this energy. It is concluded that the magnetotail plays an important role in magnetospheric substorms, although a directly driven component also makes a contribution, particularly prior to intensifications that are associated with tail reconfigurations.