Spatial Scales of the Velocity Shear Layer and Kelvin‐Helmholtz Waves on the Magnetopause: First Statistical Results

Using data taken by the ARTEMIS spacecraft near Moon, we statistically study the properties of Kelvin-Helmholtz (KH) waves and the thicknesses (Delta(Y)) of initial velocity shear layers on the magnetopause at lunar distance. Based on a total of 48 KH wave events and 14 estimates of Delta(Y), the average KH wavelength (lambda(X)) and Delta(Y )are found to be 16.05 R-E and 0.53 R-E at lunar distance, respectively, which are larger than those at near-Earth magnetopause. The lambda(X) to Delta(Y )ratio ranges from 18 to 41 at lunar distance, way beyond the range (6-13) predicted by the linear theory. This result indicates that the KH waves observed at lunar distance are not of the fastest growing mode according to linear theory. This study poses the first statistical study on lambda(X) and Delta(Y),( )providing insights into the generation and development of KH waves at lunar distance. Plain Language Summary Kelvin-Helmholtz (KH) instability typically occurs when a velocity shear exists between the boundary of two layers in a fluid. Its manifestation is KH surface waves or vortices, depending upon the stage in the development of the instability. The solar wind can create a velocity shear along the magnetopause that possibly satisfies the unstable condition for exciting KH waves. Using data observed from spacecraft on the magnetopause near Earth and at lunar distance, we estimate the thickness of the velocity shear that forms on the magnetopause before an instability occurs and the wavelength of KH waves that reach after the instability develops. The estimation shows that the average wavelength for lunar distance is triple larger than that for near-Earth, revealing a significant development in spatial scale of KH waves at lunar distance. The ratio of wavelength to thickness, which can be an indicator of the stage of KH waves, demonstrates that the KH waves at lunar distance are not of the fastest growing mode according to linear theory. A statistical study on the combined wavelength and thickness has never been performed before. The insights we have obtained can intrigue KH researchers for further analyses.