Kelvin-Helmholtz wave at the subsolar magnetopause boundary layer under radial IMF

We present the first observation of the Kelvin-Helmholtz (KH) rolled-up vortex at the dayside magnetopause layers under a radial interplanetary magnetic field (IMF). The study uses measurements of four Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes aligned along the Y_GSE axis about 10 R_E upstream of the Earth and located in different regions of the near-Earth environment. THEMIS C and A serve as monitors of the quiet solar wind and fluctuating magnetosheath conditions, respectively, and THEMIS D and E observe the magnetopause and low-latitude boundary layer (LLBL) crossings. The analysis shows the following: (1) a radial IMF changes to the southward pointing magnetosheath magnetic field; (2) dayside reconnection forms the thin but dense LLBL; (3) a large velocity shear at the LLBL inner edge excites a train of KH waves; and (4) in spite of a short path from the subsolar point (≈5 R_E), one of the KH waves exhibits all features of a fully developed rolled-up vortex.