A short wavelength instability in the neutral sheet of the earth's geomagnetic tail |
| |
| Authors: | E C Bowers |
| |
| Institution: | (1) Physics Dept., Imperial College, London, England |
| |
| Abstract: | In an earlier paper, Bowers (1973), ion plasma oscillations were found to be unstable in the steady state developed by Cowley
(1972) for the neutral sheet in the Earth's geomagnetic tail. In this paper a similar stability analysis is carried out but
for a different steady state, suggested by Dungey, with the result that unstable waves with frequencies near the electron
plasma frequency are found. In the Dungey steady state the current necessary for magnetic field reversal is carried by plasma
originating from both the magnetosheath and the lobes of the tail. This modifies the steady state proposed by Alfvén and subsequently
developed by Cowley in which all the current is carried by plasma from the lobes of the tail thereby fixing the cross-tail
potential Φ. With magnetosheath plasma present the value of Φ is no longer fixed solely by parameters in the lobes of the
tail but the cross-tail electric field is still assumed localised in the dusk region of the sheet as in the Cowley model due
to the balance of charge required in the neutral sheet. The value of Φ can be expected to increase as magnetic flux is transported
to the tail which inflates and causes flux annihilation because the magneto-sheath plasma in the neutral sheet has insufficient
pressure to keep the two lobes of the tail apart. The Vlasov-Maxwell set of equations is perturbed and linearised enabling
a critical condition for instability to be found for modes propagating across the tail. Typically, this condition requireseΦ≳KT
m
whereT
m
is the temperature of magnetosheath electrons. The instability occurs in the presence of cold plasma which hasE×B drifted into the neutral sheet from the lobes of the tail. This contrasts with the usual two stream instability which is
stabilised by the cold plasma. Once precipitated the instability may be explosive provided current disruption occurs, for
then a further increase in Φ will result which drives a greater range of wave numbers unstable thereby causing even more turbulence
and an even larger cross-tail electric field. Because of this behaviour the instability may be a trigger for a substorm. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
