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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]

In terms of composition and size, Venus is an Earth‐like planet without a global intrinsic magnetic field. The Venusian bow shock and magnetosheath, reaching only 10% of the size of the Earth, are formed from the interaction of the solar wind with the Venusian ionosphere. Energy injection from the solar wind to the Venusian space environment can be studied at the bow shock. In this respect, we hypothesize that the spectral scaling features of magnetic fluctuations near the bow shock represent important information. Nine years (2006–2014) of Venus Express magnetic observations are used to study the spectral features of fluctuations near the bow shock over the frequency range from 0.05 to 0.3 Hz. Our results show that in terms of the spectral scaling features of magnetic fluctuations, the dayside‐nightside shock crossings exhibit a clear asymmetry. Magnetic fluctuations at the dayside shock crossings are associated with spectral indices of ~1, resembling the scaling of 1/f noise. At the nightside shock crossings, the spectral indices are closer to 3/2, resembling the magnetohydrodynamic turbulence scaling. We speculate that the noise at the dayside is the consequence of the newborn strong fluctuations near the dayside bow shock, which can mask or destroy the turbulence convected from the solar wind. The spectral scaling features of magnetic fluctuations at bow shock crossings do not exhibit solar cycle dependence. Although the fluctuations differ downstream of quasi‐parallel or quasi‐perpendicular shocks in the magnetosheath, the shock geometry has no influence on the scaling properties of fluctuations at the dayside‐nightside shocks. Key Points: The distribution of turbulence near the Venusian bow shock has a day‐night side asymmetryThe dayside turbulence is masked or destroyed by the more intensive shock‐induced fluctuationsThe day‐night side asymmetry of the bow shock turbulence is independent on the solar cycle and the type of the bow shock [ABSTRACT FROM AUTHOR]

Abstract: We identify the electron diffusion region (EDR) of a guide field dayside reconnection site encountered by the Magnetospheric Multiscale (MMS) mission and estimate the terms in generalized Ohm's law that controlled energy conversion near the X‐point. MMS crossed the moderate‐shear (∼130°) magnetopause southward of the exact X‐point. MMS likely entered the magnetopause far from the X‐point, outside the EDR, as the size of the reconnection layer was less than but comparable to the magnetosheath proton gyroradius, and also as anisotropic gyrotropic “outflow” crescent electron distributions were observed. MMS then approached the X‐point, where all four spacecraft simultaneously observed signatures of the EDR, for example, an intense out‐of‐plane electron current, moderate electron agyrotropy, intense electron anisotropy, nonideal electric fields, and nonideal energy conversion. We find that the electric field associated with the nonideal energy conversion is (a) well described by the sum of the electron inertial and pressure divergence terms in generalized Ohms law though (b) the pressure divergence term dominates the inertial term by roughly a factor of 5:1, (c) both the gyrotropic and agyrotropic pressure forces contribute to energy conversion at the X‐point, and (d) both out‐of‐the‐reconnection‐plane gradients (∂/∂M) and in‐plane (∂/∂L,N) in the pressure tensor contribute to energy conversion near the X‐point. This indicates that this EDR had some electron‐scale structure in the out‐of‐plane direction during the time when (and at the location where) the reconnection site was observed. [ABSTRACT FROM AUTHOR]

Context. Tangential velocity discontinuity near the boundaries of solar wind magnetic flux tubes results in Kelvin-Helmholtz instability, which might contribute to solar wind turbulence. While the axial magnetic field stabilizes the instability, a small twist in the magnetic field may allow sub-Alfvénic motions to be unstable. Aims. We aim to study the Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind with different configurations of the external magnetic field. Methods. We use magnetohydrodynamic equations in cylindrical geometry and derive the dispersion equations governing the dynamics of twisted magnetic flux tubes moving along its axis in the cases of untwisted and twisted external fields. Then, we solve the dispersion equations analytically and numerically and find thresholds for Kelvin-Helmholtz instability in both cases of the external field. Results. Both analytical and numerical solutions show that the Kelvin-Helmholtz instability is suppressed in the twisted tube by the external axial magnetic field for sub-Alfvénic motions. However, even a small twist in the external magnetic field allows the Kelvin-Helmholtz instability to be developed for any sub-Alfvénic motion. The unstable harmonics correspond to vortices with high azimuthal mode numbers that are carried by the flow. Conclusions. Twisted magnetic flux tubes can be unstable to Kelvin-Helmholtz instability when they move with small speed relative to the main solar wind stream, then the Kelvin-Helmholtz vortices may significantly contribute to the solar wind turbulence. [ABSTRACT FROM AUTHOR]

On the basis of ARTEMIS two-probe mission magnetic reconnection (MR) outflow associated magnetic fluctuations and turbulence are analyzed on 19 February 2011. In the deep-tail, at distances between X = 45 - 51RE, evidence for reconnection associated plasma sheet thinning was found, accompanied by heating of the plasma sheet. Correlated flow and field reversals and the large-scale Hall-effect signatures indicated the presence of the reconnection X-line. Within fast reconnection plasma outflows, magnetic fluctuations exhibit the same spectral scaling features and kinked spectra as magnetic fluctuations in the solar wind or in various parts of geospace. It was shown that the proton scale magnetic fluctuations are constrained by oblique firehose, proton cyclotron and mirror instability thresholds. For parallel plasma β∥ > 1, where the thresholds converge, perpendicular magnetic fluctuations are enhanced. Magnetic compressibility decreases with the distance to the neutral sheet, however, near the instability thresholds it is comparable to the values obtained in the solar wind. [ABSTRACT FROM AUTHOR]

The importance of space weather and its forecasting is growing as interest in studying geoeffective processes in the Sun -- solar wind -- magnetosphere -- ionosphere coupled system is increasing. This paper introduces the proper selection criteria for solar wind magnetic turbulence events during duskward electric field and southward Bz driven geomagnetic storms. Two measures for the strength of solar wind fluctuations were investigated: the standard deviations of magnetic field components and a proxy for the so-called Shebalin anisotropy angles. These measures were compared to the strength of geomagnetic storms obtained from a SYM-H index time series. We found a weak correlation between standard deviation of interplanetary magnetic field GSM component Bz and SYM-H index, and a strong correlation between Shebalin anisotropy angle and the SYM-H index, which can be the result of an increase of probability of magnetic reconnection in fluctuating magnetic fields. [ABSTRACT FROM AUTHOR]

Bursty bulk flow associated magnetic fluctuations exhibit at least three spectral scaling ranges in the Earth's plasma sheet. Two of the three scaling ranges can be associated with multi-scale magnetohydrodynamic turbulence between the spatial scales from ~100 km to several RE (RE is the Earth's radius). These scales include the inertial range and below ~0.5RE a steepened scaling range, theoretically not fully understood yet. It is shown that, in the near-Earth plasma sheet, the inertial range can be robustly identified only if multi-scale quasi stationary (MSQS) data intervals are selected. Multiple bursty flow associated magnetic fluctuations, however, exhibit 1/f type scaling indicating that large-scale fluctuations are controlled by multiple uncorrelated driving sources of the bulk flows (e.g. magnetic reconnection, instabilities). [ABSTRACT FROM AUTHOR]

The observed scale dependence of the probability distributions of the differences of characteristic solar wind variables is analyzed. Intermittency of the turbulent fluctuations at small-scale spatial separations is accompanied by strongly non-Gaussian distributions that turn into a normal distribution for large-scale separation. Conventional theoretical models are subject to insufficient physical justification since nonlocality in turbulence should be based on long-range interactions, provided recently by the bi-kappa distribution in the context of nonextensive thermostatistics. Observed WIND and ACE probability distributions are accurately reproduced for different time lags by the one-parameter bi-kappa functional, a core-halo convolution, where kappa measures the degree of nonlocality or nonextensivity in the system. Gradual decoupling is obtained by enhancing the spatial separation scale corresponding to increasing kappa values, where a Gaussian is approached for infinite kappa. Consequently, long-range interactions introduced on the fundamental level of entropy generalization, are able to provide physically the source of the observed scale dependence of the turbulent fluctuations in the intermittent interplanetary medium. [ABSTRACT FROM AUTHOR]