Disentangling space-time variations in Parker Solar Probe solar wind turbulence observations

Abstract:

Single-probe spacecraft observations generally provide temporal information of the solar wind plasma properties as measured in the spacecraft frame. These temporal variations arise from a combination of space and time variations of the fields as the spacecraft moves through the plasma. In order to effectively interpret spacecraft observations, assumptions have to be made to relate these signals to the spatial structure and dynamics of the solar wind. The most common assumption used in the analysis of turbulent signals far from the some is the well known Taylor's Hypothesis, which posits that the temporal variation of spacecraft signals simply arise from the advection of "frozen" structures by the measuring probe. As Parker Solar Probe reaches closer to the Sun, this hypothesis may become less applicable in which case a new methodology is needed to interpret PSP observations beyond TH. In this work we investigate the validity of TH in the first close encounters of Parker Solar Probe, based on a recent model of the space-time correlation of MHD turbulence (Bourouaine & Perez 2019), which has been validated for strong MHD turbulence in high-resolution numerical simulations of Reduced MHD turbulence (Perez & Bourouaine 2020). Our new analysis, which does not rely on TH and only assumes the turbulence to be strongly anisotropic and Alfvénic, establishes that the validity of TH hypothesis only depends on the ratio between the root mean square velocity of the turbulence and the field-perpendicular velocity of the spacecraft in the plasma frame, independent of the Alfvén Mach number. We show that for the first three encounters, TH still holds with reasonable accuracy for some intervals and it completely breaks down for others. We discuss how the new methodology can be applied to the intervals where TH is not applicable.