Possible role of ion demagnetization in the plasma sheet in auroral arc and substorm generation

Ion demagnetization in the plasma sheet causes the formation of field-align ed current that can trigger a magnetosphere-ionosphere coupling feedback in stability, which may play an important role in substorm and auroral arc gen eration. Since field-aligned currents close ionospheric currents, their mag nitude is controlled by ionospheric conductivity. The cause of instability is the impact of increasing upward field-aligned currents on ionospheric co nductivity, which in turn stimulates an increase in the field-aligned curre nts. When the magnitude of these currents becomes sufficiently large for th e acceleration of precipitating electrons, a feedback mechanism becomes pos sible. Upward field-aligned currents increase the ionospheric conductivity that stimulates an explosion-like increase in field-aligned currents. It is believed that this instability may be related to substorm generation. Dema gnetization of hot ions in the plasma sheet leads to the motion of magnetos pheric electrons through a spatial gradient of ion population. Field-aligne d currents, because of their effect on particle acceleration and the magnit ude of ionospheric conductivity, can also lead to another type of instabili ty associated with the breaking of the earthward convection flow into conve ction streams. The growth rate of this instability is maximum for structure s with sizes less than the ion Larmor radius in the equatorial plane. This may lead to the formation of auroral arcs with widths of the order of 10 km . This instability is able to explain many features of auroral arcs, includ ing their conjugacy in opposite hemispheres. However, it cannot explain ver y narrow (less than 1 km) arcs.