Turbulent dynamo in the terrestrial magnetosheath

Dynamo action refers to energy exchange processes through which magnetic fields are generated or/and amplified at the expense of kinetic energy of the plasma flows in various laboratory, space and astrophysical environments. The generation of the magnetic fields can take place over multiple spatial scales simultaneously. Flows associ- ated with dynamos can generate magnetic fields on scales larger or smaller than the flows themselves. Multi-scale dynamo processes are fundamental for explaining magnetic fields on various scales, from planetary (e.g. geodynamo) to solar/stellar and (extra-)galactic scales. They are also vital for understanding the generation and amplification of magnetic fields by turbulent flows at smaller scales. Although our understanding of dynamos largely stems from theory and simulations, experimental validation of dynamo action remains confined to labora- tory settings. Here we address the question of experimentally verifying the existence of the turbulent dynamo in the Earth’s magnetosheath, a collisionless, high plasma beta environment. The predictions of dynamo theory and simulations, specifically, the expected spatial topology of stretched and folded magnetic fields, the effect of compres- sions, and the occurrence of pressure anisotropy instabilities necessary for the amplification of magnetic fields, are tested. The observations show that the magnetosheath data clearly exhibit the predicted tur- bulent dynamo signatures. Our findings indicate that the terrestrial magnetosheath, with the available high-resolution, multi-point plasma and field measurements, could become the first space-based testbed for validating turbulent dynamo theories and simulations. We expect that direct and more comprehensive statistical observations of the dynamo in the magnetosheath would markedly broaden the scope of both lab- oratory experiments and numerical simulations encompassing spatial scales pertinent to space and astrophysical environment.