THEMIS Observations of the Wavy Variations in the Plasma Velocity at the inner edge of the Low-Latitude Boundary Layer

The THEMIS spacecraft provides opportunities to make unique simultaneous observations in the magnetosheath, the magnetospheric boundary layers and the magnetosphere by a string-of-pearls configuration. We have examined how the variations caused by the surface waves on the dayside magnetopause can propagate into the magnetosphere. On July 31, 2007, THEMIS-C and -D observed the wavy variations in the plasma velocity at the inner edge of Low-Latitude Boundary Layer (LLBL) under the northward Interplanetary Magnetic Field (IMF) and quiet solar wind conditions. These wavy velocity variations appeared in the Vx and Vy components. The associated magnetic field had microscopic wavy fluctuations, but no periodic variations were found in simultaneous plasma density and temperature. The hodograms of the velocities perpendicular to the magnetic field line had clear vortex-like structures. From these results, observed plasma velocity variations are Alfvén waves. On the other hand, THEMIS-E also observed the wavy velocity variations at the same region as well as THEMIS-C and -D, but the velocity hodogram did not present the vortex-like structures. Therefore, these wavy velocity variations are local phenomena. THEMIS-B simultaneously observed the diamagnetic-like waves due to the magnetopause undulations in the magnetosheath. This is because the correlation between the magnetic and plasma pressures was clearly out-of-phase. During the interval of these waves, the periodicities of these pressure variations were between 0.5 minute and 1 minute, and consistent with those of the wavy velocity variations. From these results, the fast-mode waves are induced by the magnetopause diamagnetic-like waves, and can propagate into the magnetosphere.