Recovery of streamlines in the flank low‐latitude boundary layer

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We present first results of a novel technique for producing a two‐dimensional (2‐D) map of the velocity field from single‐spacecraft observations of the bulk plasma parameters and magnetic field. For flow transverse to a unidirectional magnetic field, the MHD equations of motion can be reduced to a Grad‐Shafranov‐type (GS‐type) equation for the stream function (Sonnerup et al., 2006), provided that the plasma structure is 2‐D and time‐independent when seen in its proper frame. We show how this equation can be used to recover the flow field in regions surrounding a spacecraft path, as was first done in the GS reconstruction of the magnetic field by Sonnerup and Guo (1996). The new method is benchmarked by use of an exact solution of the GS‐type equation and further by use of synthetic data from 2‐D MHD simulations of the Kelvin‐Helmholtz instability (KHI). Streamline maps of reasonable accuracy can be generated even when temporal evolution during the KHI development expected at the flank magnetopause is present. Application of the technique to a Geotail encounter with a train of KH waves in the low‐latitude flank boundary layer indicates that a chain of vortices (each of size ∼3 RE by 1 RE) existed and moved tailward along the magnetopause.