Turbulent small-scale kinematic dynamo in the terrestrial magnetosheath

Space plasma turbulence incorporates multi-scale coexisting occurrences of many physical phenomena such as waves, large amplitude field and plasma fluctuations, formation of coherent structures and the large variety of associated energy transfer, transport and conversion processes. For example, magnetic reconnection converts magnetic energy to kinetic and thermal energies and accelerates particles. Contrarily, dynamo action refers to energy conversion processes through which magnetic fields are generated or/and amplified at the expense of kinetic energy. Magnetic reconnection has been extensively studied on the basis of in-situ measurements at large-scale magnetospheric boundaries, in the turbulent magnetosheath and in the solar wind. Dynamo processes have been investigated mainly through numerical studies and in laboratory liquid metal and laser experiments. In-situ observations of dynamo processes require certain physical assumptions to calculate gradients from single-point data in the solar wind. Here we study for the first time the kinematic small-scale dynamo in the turbulent magnetosheath. In the kinematic approach the back reaction of the amplified magnetic field to plasma flows is neglected. Small-scale dynamos can generate or amplify magnetic fields at scales comparable to, or smaller than, the characteristic scales of flow gradients in 3D plasma turbulence. The flow gradients are estimated on the basis of in-situ multi-point MMS measurements. Theoretical predictions and numerical simulation results for the turbulent kinematic dynamo are tested. Specifically, the expected stretching of the magnetic field by velocity gradients, the effect of compressions and the concurrent occurrence of pressure anisotropy instabilities are investigated. The observations show that the magnetosheath data exhibit the expected turbulent dynamo signatures. Since the increase of magnetic field is associated with the loss of kinetic energy, the small-scale dynamo represents an inherent ingredient of plasma turbulence.