Empirical Results on the Magnetic Prandtl Number in the Slow Solar Wind Based on In Situ Measurements

Abstract The magnetic Prandtl number in the slow solar wind is estimated by using magnetic and velocity Reynolds numbers. The Prandtl number quantifies the ratio of kinetic to magnetic diffusion rates in a plasma, and indicates which process dominates the transport dynamics in a fluid. We use a combination of the correlation scale and the Taylor microscale to estimate the Reynolds numbers. We find Reynolds numbers of and . The magnetic Prandtl number is the ratio of kinetic to magnetic Reynolds numbers and is found to be: at 1 au. This novel result suggests the ratio of magnetic/kinetic diffusion is approximately unity in the slow wind, implying that the energy held by magnetic and velocity fields, for similar wavenumber ranges, is approximately equal, consistent with the approximately Alfvénic nature of the interval.