The Kelvin‐Helmholtz (KH) instability at the Earth's magnetopause is predominantly excited during northward interplanetary magnetic field (IMF). Magnetic reconnection due to KH waves has been suggested as one of the mechanisms to transfer solar wind plasma into the magnetosphere. We investigate KH waves observed at the magnetopause by the Magnetospheric Multiscale (MMS) mission; in particular, we study the trailing edges of KH waves with Alfvénic ion jets. We observe gradual mixing of magnetospheric and magnetosheath ions at the boundary layer. The magnetospheric electrons with energy up to 80 keV are observed on the magnetosheath side of the jets, which indicates that they escape into the magnetosheath through reconnected magnetic field lines. At the same time, the low‐energy (below 100 eV) magnetosheath electrons enter the magnetosphere and are heated in the field‐aligned direction at the high‐density edge of the jets. Our observations provide unambiguous kinetic evidence for ongoing reconnection due to KH waves.
We search the plasma and magnetic field data of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes B and C during 2008 and 2009 for observation evidences of the Kelvin‐Helmholtz instability (KHI). Fourteen KHI events with rolled‐up vortices are identified under the northward interplanetary magnetic field (IMF) at the low‐latitude boundary layer (LLBL). We collect another 42 events reported from the observations of the Geotail, Double Star TC‐1, and Cluster for a statistical study of the KH wave properties. All the 56 rolled‐up KH wave events are quantitatively characterized by the dominant period, phase velocity, and the wavelength. We further explore the relationship between the KH wave period and the solar wind velocity (VSW) and the IMF clock angle. It is found that the KH period tends to be shorter under a higher VSW, and longer with a larger IMF clock angle. The spatial distribution of the KH wavelength shows a longitudinal growth with increasing distance from the subsolar point along the flank magnetopause. The statistical results provide new insights for the development of KH waves and their connection with the interplanetary conditions and deepen our understanding of the KHI at the magnetopause.
[1] We investigate the magnetosphere under radial interplanetary magnetic fields (IMF) by using global magnetohydrodynamic simulations. The magnetosphere-ionosphere system falls into an unexpected state under this specific IMF orientation when the solar wind electric field vanishes. The most important features that characterize this state include (1) magnetic reconnections can still occur, which take place at the equatorward of the cusp in one hemisphere, the tailward of the cusp in the other hemisphere, and also in the plasma sheet; (2) significant north-south asymmetry exists in both magnetosphere and ionosphere; (3) the polar ionosphere mainly presents a weak two-cell convection pattern, with the polar cap potential valued at 30 kV; (4) the whole magnetosphere-ionosphere system stays in a very quiet state, and the AL index does not exceed -70 nT; and (5) the Kelvin-Helmholtz instability can still be excited at both flanks of the magnetosphere. These results imply the controlling role of the IMF direction between the solar wind and magnetosphere interactions and improve our understanding of the solar wind-magnetosphere-ionosphere system.
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