Solar Wind–Magnetosphere Coupling by the Kelvin–Helmholtz Instability at Mercury

Abstract The Kelvin–Helmholtz instability (KHI) has been considered important in the energy transfer and momentum coupling between the solar wind and planetary magnetospheres. To explore this issue, we employ a two-dimensional magnetohydrodynamic simulation to study the nonlinear evolution of the KHI at Mercury’s magnetopause using the parameters derived from a global hybrid simulation of MESSENGER’s first flyby of Mercury. Due to the absence of comprehensive plasma observations of Mercury’s magnetosphere, two scenarios are considered: one with a heavily loaded magnetosphere and the other with a weakly loaded magnetosphere, to demonstrate the development of the KHI under distinct levels of magnetospheric plasma density. Our results indicate that the KHI with a heavily loaded magnetosphere leads to a significantly more turbulent magnetopause and grows into the nonlinear fast-mode plane waves expanding away from the magnetopause. The momentum and energy flux quantified from our simulations reveal that the KHI with a heavily loaded magnetosphere can efficiently transport momentum and energy away from the magnetopause in the presence of the fast-mode plane waves. In the cases with a heavily loaded magnetosphere, observed in the inner magnetosphere, the momentum flux can reach 10 −3 nP a , i.e., about 0.5% of the initial solar-wind dynamic pressure; the energy flux can be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1</mml:mn> <mml:msup> <mml:mn>0</mml:mn> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:mi>erg</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mi>cm</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mi mathvariant="normal">s</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo>,</mml:mo> </mml:math> and the energy density is about 1.5%–3.0% of the initial solar-wind energy. In the cases with a very thin magnetosphere, observed away from the magnetopause, the momentum flux is negligible and the energy flux is smaller without the presence of the fast-mode plane waves in the magnetosphere.