Physics
Scientific paper
Apr 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011p%26ss...59..402l&link_type=abstract
Planetary and Space Science, Volume 59, Issue 5-6, p. 402-411.
Physics
1
Scientific paper
A hydromagnetic stability problem is solved for the magnetospheric tail inside the solar wind plasma flow. A model magnetotail is used in the form of a plasma cylinder which is inhomogeneous over the radius. For a qualitative analysis of the problem the solution is obtained analytically, in the WKB approximation. The plasma cylinder boundary is assumed to have the form of a tangential discontinuity. A numeric solution was found for a more realistic model, with the boundary in the form of a smooth transition layer. This model cannot simulate such a feature of the actual magnetotail as its being divided into two lobes with opposite magnetic fields. It is capable, however, of simulating the finiteness of the magnetotail cross-section and the inhomogeneous plasma distribution over the radius. It is shown, analytically, that a local instability develops in the boundary when the velocity of the plasma flowing round the magnetosphere exceeds the Alfvén speed at the magnetotail boundary. This conclusion is supported by a numerical solution of the problem for a model with its boundary in the form of a smooth transition layer. The instability increment, however, is much smaller in the latter case. Apart from a local instability of the boundary, unstable global modes are discovered whose amplitude practically does not vary over the magnetotail cross-section. These modes remain unstable for any, however, slow velocities of the plasma flowing round the magnetosphere. When the plasma flow velocity reaches a critical magnitude the global modes of the MHD oscillations become stable. Unstable global modes may be a source of ultra-low-frequency (˜1mHz) oscillations observed in the Earth's nightside magnetosphere.
Leonovich Anatoly S.
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