The Earth’s Magnetic Field - Ultra and Extremely Low Frequency Electromagnetic Fields - page 34

Fig. 1.14 Currents in the neutral sheet, field-aligned currents and ionospheric Hall currents that
create eastward (AEJ-E) and westward (AEJ-W) auroral electrojets. A region of current break in
the neutral sheet is also shown. Taken from McPherron et al. ( 1973 )
The above-mentioned reconnection of the magnetic field lines in the neutral sheet
of magnetotail constitutes the dominant bulk of the subject matter of the classic
NENL model (Russell and McPherron 1973 ; Baker et al. 1996 ). In the last decade
considerable attention has been focused on the alternative models for the active
phase of the magnetic substorm. In these models the reconnection of the open
magnetic field flux does not play a vital role. The key to the alternative or CD
(Current Disruption) models is the nonlinear magnetosphere-ionosphere coupling
provided by the magnetic energy which was accumulated in the closed magnetotail
(e.g., see Erickson et al. 2000 ; Lui 2001). In the framework of the CD models the
substorms are basically due to small-scale current disruptions in the near and closed
magnetotail whereas the NENL model assumes that the substorms are caused by
large-scale processes in the middle tail including the reconnection in the open lobes
of the tail.
The concept of the self-organized criticality has been the subject of a great deal
of research during recent years. In these models the substorm is considered as a
stochastic process in the small scale cellular structure with random distribution of
the cell parameters such as the magnetic field. Notice that the observation is actually
indicative of the small-scale structure of the magnetotail field and neutral sheet
current (e.g., Milovanov et al. 1996 ; Uritsky and Pudovkin 1998 and references
herein; Klimas et al. 2000 ).
The numerical simulation of MHD processes in the magnetosphere provides us
with a more comprehensive picture of the substorm development. The interested
reader is referred to the text by Raeder and Maynard ( 2001 ) and Sonnerup et al.
( 2001 ) for a more complete treatise on numerical simulation of magnetospheric
processes.