Onset of reconnection in the magnetotail

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2744 Magnetotail, 2764 Plasma Sheet, 2788 Storms And Substorms, 7835 Magnetic Reconnection

Scientific paper

The stability of the tearing mode in the magnetotail current sheet with nonzero component normal to the sheet plane remains a subject of persistent debate. It has been shown recently [Sitnov et al., 1998] that the very high stability threshold of the mode, arising from the drift motion of magnetized trapped electrons, may be considerably reduced due to transient electron population and small enough electron-to-ion temperature ratio. This allows the instability gaps inside the WKB region, that is, the wavelength of unstable modes does not need to exceed the inhomogeneity scale along the sheet. This result however has been obtained following the conventional energy principle approach [Lembege and Pellat, 1982] and it did not reveal the specific threshold of the instability. This paper presents the first results of the numerical solution of the tearing eigenvalue problem, which is aimed at obtaining the appropriate stability threshold. The problem is solved using the finite element approach [Sharma, 1983; Brittnacher et al., 1985; Daughton, 1999] combined with the drift-kinetic description of the electron species response [Rutherford and Frieman, 1968; Lembege and Pellat, 1982]. The latter allows to drastically facilitate the problem of integrating along the unperturbed, but still very complicated, particle orbits in the quasineutral sheet. This result confirm the earlier result [Sitnov et al., 1998] that showed instability of the tearing mode in the WKB approximation. This explains in particular the spontaneous reconnection in the tail of Earth's magnetosphere during substorms. Comparison of the theory and simulations with earlier MHD-like analysis [Galeev, 1984] shows that the metastable properties of the quasineutral sheet with respect to tearing perturbations are controlled by the Hall effect and another completely kinetic effect arising from different responses of trapped and transient electron populations.

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