Possible role of magnetosphere-ionosphere coupling in auroral arc generation

Three models for the magnetosphere-ionosphere coupling feedback instability are considered. The first model is based on demagnetization of hot ions in the plasma sheet. The instability takes place in the global magnetosphere- ionosphere system when magnetospheric electrons drift through a spatial gra dient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetosphe ric electrons move earthward through a radial gradient of hot ions. This le ads to the formation of field-aligned currents. The effect of upward held-a ligned current on particle precipitation and the magnitude of ionospheric c onductivity leads to the instability of this earthward convection and to it s division into convection streams oriented at some angle with respect to t he initial convection direction. The growth rate of the instability is maxi mum for structures with sizes less than the ion Larmor radius in the equato rial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such area, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow area. For such arcs anot her type of instability must be considered. In the other two models the ins tability arises because of the generation of Alfven waves from growing are- like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upwa rd moving Alfven wave. The other model takes into consideration the reflect ion of Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000 km. The growth of structures in both models takes place when th e ionization function associated with upward field-aligned current is shift ed from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different fr om the E X B drift. Although both models may work, the growth rate for the model, based on the modulation of the precipitating accelerated electrons, is significantly larger than that of the model based on the Alfven wave ref lection. This mechanism is suitable for generation of auroral arcs with wid ths of about 1 km and less. The growth rate of the instability can be as la rge as 1 s(-1), and this mechanism enables us to justify the development of auroral arcs only in one ionosphere. It is hardly suitable for excitation of wide and conjugate auroral arcs, but it may be responsible for the forma tion of small-scale structures inside a wide are.