How does magnetospheric convection relate to the expansion onset of substorms?

Abstract

It is well recognized that magnetospheric and ionospheric convection play a key role in substorm development. Some characteristic implications of the relationship are reviewed and discussed. Southward turning of the IMF or a sudden magnetospheric compression and the associated effects in the magnetotail lead to enhanced earthward plasma flow and to a gradual growth of the ionospheric DP 2 current system. Ionospheric conductivities are enhanced due to increased (mainly Fermi accelerated) electron precipitation. Finally, after an extensive period of convection growth, plasmas in a confined region of the magnetotail become unstable leading to a substorm onset. Occasionally, the entire magnetosphere may experience continuous stable enhanced convection for several hours (up to 10) without clear signatures of magnetospheric substorm-type processes. Impulsive heated plasma beams are observed in the far magnetotail indicating that powerful acceleration processes are in operation. The DP 2 current system in the ionosphere shows a high constantly disturbed level lasting for several hours. The role of ionospheric Hall and Pedersen conductivities is discussed in detail. Three different time constants (ranging from 1 to 1000 min) are identified in the magnetospheric response to convection changes. It is concluded that changes in the tail configuration are needed to start a substorm. Also different types of precipitation mechanisms are active in connection with the various types of magnetospheric response mechanisms. Similarities are found in the wedge-type field-aligned current generation mechanisms during normal substorms and the prolonged stationary magnetospheric convection cases.