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The Turbulent Properties of the Sub-Alfv\ enic Solar Wind Measured by the Parker Solar Probe

AuthorZhao, L.; Zank, G.~P.; Telloni, D.; Stevens, M.; Kasper, J.~C.; Bale, S.~D.;
KeywordsParker Data Used; Solar wind; interplanetary turbulence; 1534; 830
AbstractFor the first time, Parker Solar Probe (PSP) observed the sub- Alfv\ enic solar wind where the solar wind bulk speed drops below the local Alfv\ en speed for an extended period of time. Here, we report on the turbulent properties of the sub- Alfv\ enic region. We analyze the turbulence correlation length and the energy transfer and compare the results with nearby super-Alfv\ enic regions. As the Alfv\ en 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 $^\ensuremath\pm$ modes. In the sub-Alfv\ enic 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-Alfv\ enic interval than the nearby downstream super-Alfv\ enic region, which may be due to the lower fluctuation amplitude in the latter super-Alfv\ enic interval. Longer incursions of the PSP in the sub-Alfv\ enic winds in the future will give us better statistics.
Year of Publication2022
Number of PagesL15
Date Publishedapr