The Turbulent Properties of the Sub-Alfvénic Solar Wind Measured by the Parker Solar Probe

For the first time, Parker Solar Probe (PSP) observed the sub-Alfvénic solar wind where the solar wind bulk speed drops below the local Alfvén speed for an extended period of time. Here, we report on the turbulent properties of the sub-Alfvénic region. We analyze the turbulence correlation length and the energy transfer and compare the results with nearby super-Alfvénic regions. As the Alfvén speed is larger or comparable to the solar wind speed in the intervals studied, we use a modified Taylor's hypothesis to account for wave propagation. We find that the wave propagation speed affects the analysis of the correlation lengths of the forward and backward propagating z± modes. In the sub-Alfvénic region, the correlation length of the z− mode is shorter than that of the outward propagating z+ mode, although the correlation time of the z− mode is about 10 times larger than that of the z+ mode. For the energy transfer, we use both incompressible and compressible formulations to calculate the energy flux based on third-order structure functions. The incompressible energy cascade rates for the forward and backward propagating modes are computed separately using the modified Taylor's hypothesis. The averaged compressible cascade rate is higher in the sub-Alfvénic interval than the nearby downstream super-Alfvénic region, which may be due to the lower fluctuation amplitude in the latter super-Alfvénic interval. Longer incursions of the PSP in the sub-Alfvénic winds in the future will give us better statistics.