Kelvin–Helmholtz Spectral Redistribution by Flow and Density Structure at the Dusk Dayside Magnetopause

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Abstract
We study the development of the Kelvin‐Helmholtz instability (KHI) along the dusk dayside Earth's magnetopause with and without the presence of enhanced plasma density at the shear interface. Using three‐dimensional MAGE global simulations, we establish general KHI growth rate diagrams, which depend sensitively on the transverse profiles of both flow velocity and density. We find multiple KHI onsets promoting various growths along the magnetopause. Their associated KH frequencies and growth rates are found to follow the flow structure and decrease downstream. The magnetopause displacement, caused here by an abrupt southward‐to‐northward IMF turning, shifts KH onsets upstream and promotes density leakage that seeds an additional mixing layer at the interface. The presence of enhanced density modifies KHI frequency response suppressing small‐scale KHI structures while amplifying larger‐scales, as a low‐pass filter. We complement the analysis with dedicated high‐resolution scaled 2‐D Cartesian simulations, Doppler‐corrected for comparison with the global 3‐D results, and find good agreement with the linear KH growth rates. We show that the flow evolution along the magnetopause lowers the KHI frequencies down to Pc5–Pc4 ranges. Density effects further reorganize the frequency range of the KH unstable modes inducing the amplification of Pc6 to low‐Pc5 frequencies, suggesting that KHI can amplify fast magnetosonic waves propagating along standing field lines. The high‐resolution 2‐D simulations suggest that, in the nonlinear stage, the presence of a density pocket can suppress secondary instabilities.
Funding Information
  • International Space Science Institute (25‐635)
  • National Science Foundation (1852977)

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