Excitation of resonant Alfvén waves in the magnetosphere by negative energy surface waves on the magnetopause

Details Excitation of resonant Alfvén waves in the magnetosphere by negative energy surface waves on the magnetopause Excitation of resonant Alfvén waves in the magnetosphere by negative energy surface waves on the magnetopause

Nov 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998jgr...10326573r&link_type=abstract

Journal of Geophysical Research, Volume 103, Issue A11, p. 26573-26584

Physics

14

Magnetospheric Physics: Mhd Waves And Instabilities, Magnetospheric Physics: Magnetopause, Cusp, And Boundary Layers, Magnetospheric Physics: Magnetotail, Magnetospheric Physics: Magnetosphere-Outer

Scientific paper

The instability of the magnetopause caused by the resonant interaction of a negative energy surface wave with Alfvén waves localized in the vicinity of the resonant magnetic surface is considered. The most important property of this instability is that it takes place for flow velocities in the magnetosheath below the Kelvin-Helmholtz (KH) threshold velocity, i.e., when there is no KH instability of the magnetopause. The magnetopause is modeled by an MHD tangential discontinuity with a magnetic-free plasma on the magnetosheath side and cold plasma on the magnetospheric side. It is shown that one of the two surface waves that propagate along the discontinuity when the shear velocity is smaller than the KH threshold velocity is a negative energy wave when the shear velocity is larger than a critical velocity. This negative energy wave propagates tailward in the magnetospheric frame, although it propagates sunward in the magnetosheath frame. When, in addition, a resonant condition is satisfied, the negative energy surface wave resonantly interacts with localized Alfvén waves. This interaction results in growth of both the surface wave and the Alfvén waves. The resonant condition can only be satisfied when the plasma density increases and, consequently, the Alfvén velocity decreases in the direction toward the magnetopause in its vicinity. The increment of the resonant instability is calculated under the assumption that the plasma density changes only in a slab in the vicinity of the magnetopause with the thickness much smaller than the wavelength. The possible observational signatures of the resonant instability are discussed.

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