Abstract
Abstract. 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 gradient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetospheric electrons move earthward through a radial gradient of hot ions. This leads to the formation of field-aligned currents. The effect of upward field-aligned current on particle precipitation and the magnitude of ionospheric conductivity leads to the instability of this earthward convection and to its division into convection streams oriented at some angle with respect to the initial convection direction. The growth rate of the instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such arcs, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow arcs. For such arcs another type of instability must be considered. In the other two models the instability arises because of the generation of Alfven waves from growing arc-like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfven wave. The other model takes into consideration the reflection 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 the ionization function associated with upward field-aligned current is shifted from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different from the E×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 reflection. This mechanism is suitable for generation of auroral arcs with widths of about 1 km and less. The growth rate of the instability can be as large 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 formation of small-scale structures inside a wide arc.Key words: Ionosphere (auroral ionosphere) - Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions)
Highlights
We have examined three models for the magnetosphereionosphere coupling (MIC) feedback instability
Two other models are based on modulation of particle precipitation by ®eld-aligned currents of an Alfven wave generated by an ionospheric inhomogeneity, and on the reection of this wave from the maximum in the Alfven velocity distribution at about 3000 km altitude, respectively
We obtained the dispersion equations for all these three models for the magnetosphere-ionosphere coupling (MIC) feedback instability related to auroral arc generation, which allow us to investigate features of these instabilities and compare their eciencies
Summary
It was shown by many scientists that the magnetospheric convection may be unstable and divided into convection streams (e.g., Sato and Holzer, 1973; Sato, 1978; Leontyev and Lyatsky, 1982; Trakhtengertz and Feldstein, 1984; Lyatsky, 1987; Watanabe and Sato, 1988; Lysak, 1991; Kozlovsky and Lyatsky, 1994, 1999). Sato and Holzer (1973) were the ®rst to propose a selfconsistent model for the magnetospheric convection strati®cation. Sato and Holzer (1973) were the ®rst to propose a selfconsistent model for the magnetospheric convection strati®cation They showed that ®eld-aligned currents of an Alfven wave emitted from an ionospheric inhomogeneity, while reected from the conjugate ionosphere, may lead to an additional increase in ionization inside growing structures and to the development of feedback instability. Lysak, 1991), and a magnetosphere-ionosphere coupling instability including an active role of hot magnetospheric plasma (Lyatsky, 1987; Kozlovsky and Lyatsky, 1994, 1999) All these authors suggested that the convection streams, appearing as a result of this instability, might be responsible for auroral arc generation. Our aim is to present some recent results related to the study of these types of magnetosphere-ionosphere coupling feedback instabilities and discuss their possible application to auroral arc generation
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