Description
Energy transfer in magnetised plasmas is fundamentally shaped by the interplay between turbulence and reconnection. Coherent structures are central to intermittent magnetohydrodynamic turbulence, which is observed in situ in the solar wind. We establish a correspondence between the energy transfer rate and the magnetic topology of underlying turbulent fluctuations. Magnetic field topology is classified by the magnetic field gradient tensor, constructed from in situ observations by the four Cluster spacecraft forming a tetrahedron on a scale of ∼40 proton gyro-radii, and energy transfer rates are estimated from third-order structure functions. Energy transfer to decreasing scales, with positive transfer rates significantly larger than the mean, occurs preferentially in hyperbolic magnetic field topology, consistent with reconnection. Energy transfer to increasing scales, with large negative transfer rates, occurs preferentially in magnetic flux ropes. The forward and inverse MHD turbulent cascades are thus carried by current sheet fragmentation in reconnection and flux rope merging, respectively.
