LAUSR

In this study we investigate an event of sharp and transient (≤ 10 min) plasma and magnetic field perturbations observed by Acceleration Reconnection Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) probes in the mid-tail lobe at X ~ –60 RE. This event occurred under northward interplanetary magnetic field (IMF), and these mesoscale perturbations continued for many hours even as all solar wind and IMF parameters remained steady. Main characteristics of these transient perturbations are: (1) An increase in density and plasma pressure; (2) a drop in Bx; (3) an enhancement in the tailward (–Vx) speed; (4) tailward propagation. We conduct a global magnetohyrodamic (MHD) simulation for this event using the observed solar wind/IMF conditions. In the simulation, Kelvin-Helmholtz (K-H) vortices are formed at the near-Earth flank and are convected tailward. The K-H vortices have a two-mode structure with the inner mode extending several RE inside the magnetosphere from the magnetopause. The inner mode vortical flows transport denser mantle plasma from near the magnetopause deeper into the lobe, resulting in localized density and pressure enhancements, while magnetic field changes accordingly with the enhanced flow shear and pressure gradients. As these localized changes pass a fixed location, they create transient perturbations characteristically similar to those observed by ARTEMIS.