Inertial‐Range Energy Transfer Free From Isotropic Assumption in Turbulent Space Plasma

Abstract The idea of an energy cascade in the inertial range is often invoked in turbulent space plasmas to estimate the energy dissipation rate. Laws governing the behavior of third‐order structure functions in the inertial range, so‐called third‐order laws, are among the few rigorous theoretical results quantifying cross‐scale energy transfer. The widely used third‐order‐law derived rate assumes isotropy, which fundamentally conflicts with the anisotropic nature of space plasmas. Elementary questions persist regarding how such anisotropic energy cascades can be quantified using multi‐spacecraft constellations. As the heliospheric community increasingly progresses toward multi‐spacecraft, multi‐scale constellations, such as Plasma Observatory and HelioSwarm, we revisit these crucial issues pertinent to accurately measuring the inertial‐range energy transfer. Here, we make a systematic comparison between two methods: direction‐averaging (DA) and lag polyhedral derivative ensemble (LPDE) to determine the full three‐dimensional (3D) dependence of cross‐scale energy transfer. We find that DA exhibits both polar and azimuthal dependence, but is insensitive to spacecraft configuration. By contrast, LPDE is strongly affected by spacecraft separation and tetrahedral shape, while being comparatively insensitive to the sampling trajectory. Our findings have direct implications for current and future multi‐spacecraft missions. Both DA and LPDE will provide crucial information on the nature of turbulence in space and astrophysics.