Inertial-Range Energy Transfer Free from Restrictive Assumptions in Turbulent Space Plasmas

In the classical energy cascade scenario, energy is transferred from large to small scales at a constant rate, until it dissipates at the smallest scales. Laws governing the behavior of third-order structure functions in the inertial range, often called third-order laws, are among the few rigorous results about cross-scale energy transfer. Limited by available spacecraft data, energy transfer is most frequently oversimplified under the assumptions of isotropy, incompressibility, homogeneity, etc. This is not altogether convincing especially for the solar wind and magnetosheath turbulence, which is typically anisotropic and inhomogeneous. As the community is presently progressing towards multi-spacecraft constellations, e.g., MMS and HelioSwarm, we revisited several crucial issues pertinent to the inertial-range energy transfer, in particular, the 3D directional dependence (or anisotropy) of energy transfer arising from large-scale magnetic field. To properly account for the full 3D dependence, we applied and further developed direction-averaging and 3D lag-space derivative methods, using both MHD simulations and in situ observations. Both of these methods refine the estimation of energy transfer rate.