Alternative interpretation of results from Kelvin-Helmholtz vortex identification criteria



Observations of lower density, faster than sheath (LDFTS) plasma at the magnetopause are believed to be specific to rolled-up vortices generated by the Kelvin-Helmholtz instability. Hence, they are used to identify vortices with single-spacecraft measurements. These vortices are expected to occur at the tail-flank magnetopause, beyond the terminator. This fact contrasts with numerous observations of LDFTS plasma far sunward of the terminator. Here we present two alternative explanations for the detection of LDFTS plasma at the dayside magnetopause: (1) the presence of a plasma depletion layer (PDL) readily featuring LDFTS plasma and (2) the plasma velocity pattern of magnetopause surface waves, by which lower/higher-density magnetosheath or PDL plasmas sensed by a wave-observing spacecraft are accelerated/decelerated in magnetosheath flow direction. Even low-latitude boundary layer (LLBL) plasma may be of LDFTS, if the LLBL background flow is antisunward at near-magnetosheath velocities.