The effects of pressure anisotropy on the tearing mode in a current sheet are studied based on a linear compressible MHD code. It is found that in the presence of a small pressure anisotropy (Pc > Pll)' tearing mode dominates the unstable modes except that the growth rates are enhanced, which is consistent with the recent results; for a large pressure anisotropy the growth rates of unstable modes become even larger, and a mixed mirror-tearing configuration is formed for large k• (wave number along current sheet) implying that mirror instability is responsible for the enhancement of growth rate and modification of the tearing mode. In the presence of a large pressure anisotropy, a new oscillating tearing mode is observed when the wave number k• is small. Furthermore, a large 1/component of the magnetic field is found to be able to cancel the effect of pressure anisotropy, with the resultant mode being the same as that of isotropic tearing instability. 1975, 1978; Drake and Lee, 1977; Killeen and $hestakov, 1978;White, 1983; Lee and It& 1986]. Plasma processes developed in the current sheet may provide an effective mechanism for the conversion of magnetic energy into kinetic energy of particles. The occurrence of current filamentation in the current sheet relaxes the configuration to a lower energy state, which in turn leads to the formation of magnetic islands. This process, known as tearing instability, violates the basic topological restfictions of the infinite conductivity assumption; namely, the process consists of "breaking"and reconnection of the magnetic lines of force. For the tearing mode to occur, a finite resistivity is necessary. In case of collisionless plasma, the role of this resistivity is provided by the inertia of current carriers [$peiser, 1970; Vas•tliunas, 1975; $onnerup, 1979], tearing mode-particle resonance or by microinstabilities driven by the strong currents or gradients in the diffusion region. Reviews of several microinstabilities that are suggested as possible anomalous transport mechanisms in current sheets have been given by $onnerup [1979].For nearly two decades, the tearing instability has been considered as a possible candidate responsible for the reconnection in the magnetotail and magnetopause [Coppi et al., 1966;Galeev and Zelenyi, 1978;Schindler, 1974; •uest and Coroniti, 1981; Lee and Fu, 1985]. As early as 1963, Furth et al. systematically studied the problem of tearing instability analytically by MHD approach. Since then, a substantial amount of research has been done on the linear and nonlinear stages of the instability [ Coppi, 1964; Laval and Pellat, 1967; Dobrowoln•l, 1968; Drake and Lee, 1977; Galeev and gelen•ti, 1975; Terasawa, 1981; Ghen and Palmadesso, 1984; Ambrosiano et al., 1986]. Generally, the tearing instability falls into three regimes which are referred to as collisionless, semicollisionless, and collisional [Drake and Lee, 1977]. In the collisionless regime, anisotropy

Both particle and MHD simulations are performed to study the characteristics of slow shocks in the magnetotail. The particle simulations indicate that switch-off shocks exhibit large amplitude rotational wave trains, while magnetotail slow shocks with an intermediate Mach number MA• < M,: _• 0.98 do not display such rotational wave trains. The MHD simulations show that the spontaneous reconnection process in the near-earth plasma sheet leads to the formation of a pair of slow shocks tailward of the reconnection line (X-line). The properties of slow shocks are found to vary as a function of the distance from X-line due to the formation of plasmoid. Slow shocks in most regions of the magnetotail are found to be non-switch-off shocks with M A• < 0.98. The present results are used to discuss the lack of large amplitude rotational wave trains at slow shocks in the deep magnetotail.

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Interaction between the interplanetary magnetic field and the earth's geomagnetic field at the dayside magnetopause is studied by a global two‐dimensional incompressible MHD simulation. It is found that for a large magnetic Reynolds number (Rm ≥400), the multiple X line reconnection (MXR) becomes the major dayside reconnection process, while for a small magnetic Reynolds number (Rm ≤100), the classical single X line reconnection takes place. Since the magnetic Reynolds number at the dayside magnetopause is probably very high (Rm >10³), it may be suggested that MXR will dominate the dayside reconnection pattern. As a result of the MXR process, magnetic islands are formed on the dayside magnetopause surface. The dependence of the dayside reconnection geometry on the solar wind Alfvén Mach number is also examined.

It is suggested that the existence of a sporadic multiple X line reconnection (MXR) process at the dayside magnetopause could be a source of energy to produce ULF hydromagnetic waves (ƒ ∼ 1–10 mHz) in the dayside magnetopause cusp region. The MXR process is characterized by the repeated formation and convection of magnetic islands and elongated plasma clouds, which compress and distort the adjacent closed geomagnetic field lines. This leads in a natural way to the generation of ULF waves with frequencies in the range 1–10 mHz along cusp geomagnetic field lines. These waves are expected to be appoximately linearly polarized, dominated by variations in the radial magnetic field component near the dayside magnetopause and by variations in the azimuthal magnetic field component near the resonant closed field lines in the magnetosphere. In addition, the field‐aligned currents associated with the MXR process may generate the impulsive magnetic field variations often observed to accompany the continuous variations in the ULF band. Hence the reconnection process may be an additional mechanism, together with the Kelvin‐Helmholtz instability and direct penetration of upstream waves, for the production of hydromagnetic energy in the dayside magnetosphere.