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Ann. Geophys., 20, 757-769, 2002
www.ann-geophys.net/20/757/2002/
© European Geosciences Union 2002
On the incidence of Kelvin-Helmholtz instability for mass exchange process at the Earth’s magnetopause
R. Smets1,2, D. Delcourt3, G. Chanteur2, and T. E. Moore1
1NASA/Goddard Space Flight Center, Greenbelt, MD, USA
2CETP/CNRS, Vélizy, France
3CETP/CNRS, Saint Maur des Fossés, France
Correspondence to: R. Smets (rsm@cetp.ipsl.fr)
Abstract. Due to the velocity shear
imposed by the solar wind flowing around the magnetosphere, the magnetopause
flanks are preferred regions for the development of a Kelvin-Helmholtz
instability. Since its efficiency for momentum transfer across the magnetopause
has already been established, we investigate its efficiency for mass transfer.
Using nonresistive magnetohydrodynamic simulations to describe the magnetic
field shape in the instability region, we use test-particle calculations to
analyse particle dynamics. We show that the magnetopause thickness and the
instability wave-length are too large to lead to nonadiabatic motion of thermal
electrons from the magnetosphere. On the other hand, the large mass of H+, He+ and O+ ions leads to such nonadiabatic motion and we thus propose the
Kelvin-Helmholtz instability as a mechanism for either magnetospheric ion
leakage into the magnetosheath or solar wind ion entry in the magnetosphere.
Test-particle calculations are performed in a dimensionless way to discuss the
case of each type of ion. The crossing rate is of the order of 10%. This rate
is anti-correlated with shear velocity and instability wavelength. It increases
with the magnetic shear. The crossing regions at the magnetopause are narrow
and localized in the vicinity of the instability wave front. As a Kelvin-Helmholtz
instability allows for mass transfer through the magnetopause without any
resistivity, we propose it as an alternate process to reconnection for mass
transfer through magnetic boundaries.
Key words. Magnetospheric physics
(magnetopause, cusp and boundary layers; MHD waves and instabilities) – Space
plasma physics (numerical simulation studies)
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