Substructure of a Kelvin‐Helmholtz Vortex Accompanied by Plasma Transport Under the Northward Interplanetary Magnetic Field

Within a Kelvin-Helmholtz (K-H) vortex at the duskside magnetopause, a substructure characterized by two flux enhancements of cold magnetosheath plasma in a background of hot magnetosphere plasma is observed. The substructure is accompanied by plasma transport across the magnetopause, and the transport region is found to split into two parts: the double peaks in both ion and electron densities, corresponding to the two flux enhancements in spectrograms. There are two decreases in the ion temperature while the electron temperature remains constantly low, suggesting that the substructure is caused by a secondary process within the vortex rather than the secondary crossings of the vortex or the boundary oscillations. A transverse motion of cold plasma is observed in the pitch-angle distribution, demonstrating an example of transverse driving and ongoing plasma transport across the magnetopause. With the convective electric field removed, the perturbed electric field E-1, which is sinusoidal and perpendicular to B, matches the theoretical prediction of the Rayleigh-Taylor (R-T) instability. Such a perturbed electric field is expected to be the electrostatic field that accompanies the R-T instability producing the substructure and drives the plasma transport by E1xB ${\boldsymbol{E}}_{1}\times \boldsymbol{B}$ drift. The observations suggest that this secondary R-T instability within the K-H vortex can drive plasma transport across the magnetopause.