A possible role of ion demagnetization in substorm generation

A new model for substorm generation based on a stimulated ion diffusion acr oss the magnetic field lines in the magnetotail plasma sheet is studied. Th e essence of the model is as follows: A partial ion demagnetization in the plasma sheer produces outward ion diffusion, which leads to charge separati on and to the formation of field-aligned currents removing these charges. T he magnitude of field-aligned currents is controlled by the,ionospheric con ductivity. The effect of field-aligned currents on the magnitude of the ion ospheric conductivity can lead to the instability, consisting of an increas e in these currents, a sharp increase in the ion diffusion, the dipolarizat ion of the magnetotail magnetic field, and the substorm onset. The model in cludes the following issues: (1) Before the substorm onset the magnetotail plasma sheet approaches the Earth and magnetic field lines are stretched fr om the Earth. (2) Partially demagnetized (because of their large Larmor rad ius) ions in the magnetotail plasma sheet move from the Earth. This produce s a radially directed electric field which restricts the ion escape and pro duces a system of field-aligned currents, with upward current at the bounda ry of ion demagnetization and downward current poleward of this boundary. ( 3) The magnitude of field-aligned currents is controlled by the ionospheric conductivity. For quiet conditions the ionospheric conductivity is small a nd field-aligned currents are weak. An increase in the ion diffusion leads to an increase of field-aligned currents. This stimulates the growth of the ionospheric conductivity, which leads to a new increase in the ion diffusi on and to the substorm onset.