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We use magnetic field data from the Cluster mission to estimate the value of the Taylor microscale and the effective magnetic Reynolds number in the interplanetary solar wind. Turbulent cascades can be characterized by the spatial scale at which dissipation begins to impact the local energy transfer, estimated by the Taylor microscale, as well as the separation between the injection and dissipation scales, estimated by the effective magnetic Reynolds number. Estimating the Taylor microscale requires measurements of the autocorrelation function at small separations. The Cluster spacecraft have exceptionally sensitive search coil magnetometers with high time resolution, making them ideal for measuring the Taylor microscale. We obtain a value of 430±20 $430\pm 20$ km; smaller than most previous measurements. We interpret this value as being smaller due to the higher time resolution, enabling the curvature of the autocorrelation function to be measured closer to the origin, giving a more accurate measurement. Combining the Taylor Microscale's computed value with concurrent correlation length measurements, we obtain a value of 150,000±10,000 $150,000\pm 10,000$ for the effective magnetic Reynolds number, which compares well to other observations. The four spacecraft of Cluster also allow directions transverse to the flow to be surveyed. The small separations (7 km) of Clusters 3 and 4 show that the Taylor microscale may vary as a function of direction to the mean magnetic field direction. The observed differences are small, requiring more observations to confirm this anisotropy. Plain Language Summary: The Reynolds number is a key quantity to describe the turbulence level in a fluid flow. In hydrodynamics, the Reynolds number is computed using the molecular viscosity. In the limit of very small values of the Reynolds number Re≪1 $\mathrm{R}\mathrm{e}\ll 1$, the flow is well ordered (or laminar), while in the high limit Re≫1 $\mathrm{R}\mathrm{e}\gg 1$, the flow becomes turbulent. Viscosity cannot be defined in a collisionless medium, so other methods to estimate the Reynolds number must be used. We calculate the Taylor microscale and the correlation length, allowing an estimation of the Reynolds number in the solar wind. Key Points: Merged fluxgate/search coil magnetic field data from Cluster are usedThe merged data are used to obtain the Taylor microscale and the Reynolds numberThe data suggest that electron scale processes may also be important for determining the Taylor microscale [ABSTRACT FROM AUTHOR]

We use density measurements deduced from spacecraft potential to study the power spectral density (PSD) of compressive fluctuations in the solar wind. Typically, plasma measurements do not have a sufficiently high time resolution to resolve density fluctuations down to ion kinetic scales. However, the calibrated spacecraft potential allows for much higher time resolutions to resolve the spectral break between ion inertial and kinetic ranges. We used fast-survey mode data from Magnetospheric MultiScale when the spacecrafts were in the pristine solar wind. The density spectra's morphology differs from the trace magnetic field fluctuations, with a flattening often occurring between inertial and kinetic ranges. We find that the spectral break of the trace magnetic field fluctuations occurs near the expected frequency for cyclotron resonance or magnetic reconnection. Meanwhile, the spectral break at the start of the ion kinetic range for density fluctuations is often at a higher frequency when compared to the trace magnetic field. We discuss possible interpretations for these observations. [ABSTRACT FROM AUTHOR]

Magnetosheath jets are dynamic pressure enhancements observed in the terrestrial magnetosheath. Their generation mechanisms are currently debated but the majority of jets can be linked to foreshock processes. Recent results showed that jets are less numerous when coronal mass ejections (CMEs) cross the magnetosheath and more numerous when stream interaction regions (SIRs) cross it. Here, we show for the first time how the pronounced substructures of CMEs and SIRs are related to jet production. We distinguish between compression and magnetic ejecta (ME) regions for the CME as well as compression region associated with the stream interface and high-speed streams (HSSs) for the SIR. Based on THEMIS and OMNI data covering 2008-2021, we show the 2D probability distribution of jet occurrence using the cone angle and Alfvén Mach number. We compare this distribution with the values within each solar wind (SW) structure. We find that both high cone angles and low Alfvén Mach numbers within CME-MEs are unfavorable for jet production as they may inhibit a well-defined foreshock region. 1D histograms of all parameters show, which SW parameters govern jet occurrence in each SW structure. In terms of the considered parameters the most favorable conditions for jet generation are found for HSSs due to their associated low cone angles, low densities, and low magnetic field strengths., (© 2023. The Authors.)