Multi-scale intermittency and energy transfer in the terrestrial foreshock

We investigate the space–time variability of intermittent magnetic turbulence in the terrestrial foreshock using fluxgate magnetometer observations from the Magnetospheric Multiscale (MMS) mission. Intermittency is quantified through sliding-window probability density function analysis and scale-dependent flatness of temporal magnetic field increments, over a broad range (0.2–256 s) of scales. The analysis is complemented by spectral diagnostics of the magnetic time-series. By organizing the analysis in terms of the field-aligned distance from the bow shock and the angle between the interplanetary magnetic field and the shock normal, we resolve systematic differences between quasi-parallel and quasi-perpendicular foreshock regions. The multi-spacecraft character of MMS enables us to directly probe spatial intermittency at the scale of the inter-spacecraft separations (~20 km), and compare spatial and temporal statistics, providing insight into the applicability of the Taylor hypothesis in a highly dynamic foreshock environment. We find that intermittency persists both below and beyond ion temporal scales, with enhanced intermittency in the quasi-parallel foreshock at sub-second scales and a reversal of this trend at larger scales. The latter finding is likely resulted in by intense wave activity. We emphasize that the provisional Plasma Observatory mission would enable our analyses to be extended to a broader range of spatial scales, providing a decisive advance in disentangling spatial and temporal variability and in understanding energy transfer in collisionless space plasmas.

Our study is conducted in the framework of the ESA-supported SWIFT project, which aims to investigate how solar wind dynamics drive turbulence and large-scale current structures within the coupled terrestrial magnetosphere–ionosphere system.