Observations of Kelvin-Helmholtz Waves in the Earth's Magnetotail Near the Lunar Orbit

Kelvin-Helmholtz waves (KHWs), which have been widely observed at the magnetopause in the region near the Earth, play an essential role in the transport of solar wind plasma and energy into the magnetosphere under dominantly northward interplanetary magnetic field (IMF) conditions. In this study, we present simultaneous observations of KHWs under the northward IMF observed by both the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft in the Earth's magnetotail around the lunar orbit (at X similar to -50R(E), Y similar to 30R(E), dusk side) and the Geotail in the near-Earth space (at X similar to -5R(E), Y similar to -10R(E), dawn side). The KHWs are quantitatively characterized by their dominant period, phase velocity, and wavelength, utilizing wavelet analysis and an approximation of their center-of-mass velocity. Our results suggest that the phase velocity and spatial scale of KHWs may increase as they propagate along the boundary layer toward the tail. Alternatively, the differences between the ARTEMIS and Geotail observations may indicate the possibility of dawn-dusk asymmetry in the excited KHWs in this study. Our results strongly evidence the existence of the development of KHWs in terms of their wave frequency and scale size in the magnetotail and provide insight to the time evolution of KHWs along the magnetopause.