In Situ Identification of 3D Vortex-Cores using Magnetospheric Multiscale Mission

The Kelvin-Helmholtz (KH) instability can occur in a fluid or a plasma where a velocity shear is generated in a single continuous fluid or plasma, or where a velocity difference is generated across two fluids or plasmas. The KH instability has long been believed to play a key role in plasma transport at the magnetospheric boundaries separating the solar wind and magnetospheric plasmas [Hasegawa et al., 2004; Johnson et al., 2014; A Miura, 1984; A Miura and Pritchett, 1982]. However, the KH instability is the linear instability and recent studies show that subsequent turbulent phenomena generated mainly by long survived free longitudinal vortexes play more important role in plasma transport. Thus, identifying vortices is one of keys to understanding the turbulence and plasma transport in plasma shear layers. Currently, no single precise definition of a vortex is universally accepted, despite the significance of vortices in fluid and plasma dynamics. Several vortex identification methods using Galilean invariance have been proposed by previous researchers [Cai et al., 2018]. By using the four set MMS (Magnetospheric multiscale) satellite velocity data, we Taylor expand the velocity data around the satellites, obtain its first order tensor, and linearly approximate the velocity field. In order to identify 3-D vortex structures, we use the vortex identification criterion that requires the Galilean invariance that is the vortex-core-line inside the MMS tetrahedron. We identified more that 60 turbulent 3D vortexes near magnetopause in dawn side in April, 2018 event (see figure). They have different polarities and 3D structures where longitudinal and transverse vortexes coexists, and, thus, are no longer the KH vortexes.