Turbulence in Magnetic Reconnection Jets from Injection to Sub-Ion Scales

We investigate turbulence in magnetic reconnection jets in the Earth's magnetotail using data from the Magnetospheric Multiscale spacecraft. We show that signatures of a limited inertial range are observed in many reconnection jets. The observed turbulence develops on the timescale of a few ion gyroperiods, resulting in intermittent multifractal energy cascade from the characteristic scale of the jet down to the ion scales. We show that at sub-ion scales, the fluctuations are close to monofractal and predominantly kinetic Alfvén waves. The observed energy transfer rate across the inertial range is ∼10^{8}  J kg^{-1} s^{-1}, which is the largest reported for space plasmas so far.

Local dimensionality and inverse persistence analysis of atmospheric turbulence in the stable boundary layer

The dynamics across different scales in the stable atmospheric boundary layer has been investigated by means of two metrics, based on instantaneous fractal dimensions and grounded in dynamical systems theory. The wind velocity fluctuations obtained from data collected during the Cooperative Atmosphere-Surface Exchange Study-1999 experiment were analyzed to provide a local (in terms of scales) and an instantaneous (in terms of time) description of the fractal properties and predictability of the system. By analyzing the phase-space projections of the continuous turbulent, intermittent, and radiative regimes, a progressive transformation, characterized by the emergence of multiple low-dimensional clusters embedded in a high-dimensional shell and a two-lobe mirror symmetrical structure of the inverse persistence, have been found. The phase space becomes increasingly complex and anisotropic as the turbulent fluctuations become uncorrelated. The phase space is characterized by a three-dimensional structure for the continuous turbulent samples in a range of scales compatible with the inertial subrange, where the phase-space-filling turbulent fluctuations dominate the dynamics, and is low dimensional in the other regimes. Moreover, lower-dimensional structures present a stronger persistence than the higher-dimensional structures. Eventually, all samples recover a three-dimensional structure and higher persistence level at large scales, far from the inertial subrange. The two metrics obtained in the analysis can be considered as proxies for the decorrelation time and the local anisotropy in the turbulent flow.

Transition to turbulence in a five-mode Galerkin truncation of two-dimensional magnetohydrodynamics

The chaotic dynamics of a low-order Galerkin truncation of the two-dimensional magnetohydrodynamic system, which reproduces the dynamics of fluctuations described by nearly incompressible magnetohydrodynamic in the plane perpendicular to a background magnetic field, is investigated by increasing the external forcing terms. Although this is the case closest to two-dimensional hydrodynamics, which shares some aspects with the classical Feigenbaum scenario of transition to chaos, the presence of magnetic fluctuations yields a very complex interesting route to chaos, characterized by the splitting into multiharmonic structures of the field amplitudes, and a mixing of phase-locking and free phase precession acting intermittently. When the background magnetic field lies in the plane, the system supports the presence of Alfvén waves thus lowering the nonlinear interactions. Interestingly enough, the dynamics critically depends on the angle between the direction of the magnetic field and the reference system of the wave vectors. Above a certain critical angle, independently from the external forcing, a breakdown of the phase locking appears, accompanied with a suppression of the chaotic dynamics, replaced by a simple periodic motion.

In Situ Observation of Hall Magnetohydrodynamic Cascade in Space Plasma

We present estimates of the turbulent energy-cascade rate derived from a Hall-magnetohydrodynamic (MHD) third-order law. We compute the contribution from the Hall term and the MHD term to the energy flux. Magnetospheric Multiscale (MMS) data accumulated in the magnetosheath and the solar wind are compared with previously established simulation results. Consistent with the simulations, we find that at large (MHD) scales, the MMS observations exhibit a clear inertial range dominated by the MHD flux. In the subion range, the cascade continues at a diminished level via the Hall term, and the change becomes more pronounced as the plasma beta increases. Additionally, the MHD contribution to interscale energy transfer remains important at smaller scales than previously thought. Possible reasons are offered for this unanticipated result.

Turbulence-Driven Ion Beams in the Magnetospheric Kelvin-Helmholtz Instability

The description of the local turbulent energy transfer and the high-resolution ion distributions measured by the Magnetospheric Multiscale mission together provide a formidable tool to explore the cross-scale connection between the fluid-scale energy cascade and plasma processes at subion scales. When the small-scale energy transfer is dominated by Alfvénic, correlated velocity, and magnetic field fluctuations, beams of accelerated particles are more likely observed. Here, for the first time, we report observations suggesting the nonlinear wave-particle interaction as one possible mechanism for the energy dissipation in space plasmas.

Coherent Structures and Spectral Energy Transfer in Turbulent Plasma: A Space-Filter Approach

Plasma turbulence at scales of the order of the ion inertial length is mediated by several mechanisms, including linear wave damping, magnetic reconnection, the formation and dissipation of thin current sheets, and stochastic heating. It is now understood that the presence of localized coherent structures enhances the dissipation channels and the kinetic features of the plasma. However, no formal way of quantifying the relationship between scale-to-scale energy transfer and the presence of spatial structures has been presented so far. In the Letter we quantify such a relationship analyzing the results of a two-dimensional high-resolution Hall magnetohydrodynamic simulation. In particular, we employ the technique of space filtering to derive a spectral energy flux term which defines, in any point of the computational domain, the signed flux of spectral energy across a given wave number. The characterization of coherent structures is performed by means of a traditional two-dimensional wavelet transformation. By studying the correlation between the spectral energy flux and the wavelet amplitude, we demonstrate the strong relationship between scale-to-scale transfer and coherent structures. Furthermore, by conditioning one quantity with respect to the other, we are able for the first time to quantify the inhomogeneity of the turbulence cascade induced by topological structures in the magnetic field. Taking into account the low space-filling factor of coherent structures (i.e., they cover a small portion of space), it emerges that 80% of the spectral energy transfer (both in the direct and inverse cascade directions) is localized in about 50% of space, and 50% of the energy transfer is localized in only 25% of space.

A preliminary study on topical cetirizine in the therapeutic management of androgenetic alopecia

BACKGROUND

Androgenetic alopecia (AGA) is a common form of scalp hair loss that affects up to 50% of males between 18 and 40 years old. Several molecules are commonly used for the treatment of AGA, acting on different steps of its pathogenesis (Minoxidil, Finasteride, Serenoa repens) and show some side effects. In literature, on the basis of hypertrichosis observed in patients treated with analogues of prostaglandin PGF2a, it was supposed that prostaglandins would have an important role in the hair growth: PGE and PGF2a play a positive role, while PGD2 a negative one.

OBJECTIVE

We carried out a pilot study to evaluate the efficacy of topical cetirizine versus placebo in patients with AGA.

PATIENTS AND METHODS

A sample of 85 patients was recruited, of which 67 were used to assess the effectiveness of the treatment with topical cetirizine, while 18 were control patients.

RESULTS

We found that the main effect of cetirizine was an increase in total hair density, terminal hair density and diameter variation from T0 to T1, while the vellus hair density shows an evident decrease. The use of a molecule as cetirizine, with no notable side effects, makes possible a good compliance by patients.

CONCLUSION

Our results have shown that topical cetirizine 1% is responsible for a significant improvement of the initial framework of AGA.

Fast algorithm for a three-dimensional synthetic model of intermittent turbulence

Synthetic turbulence models are useful tools that provide realistic representations of turbulence, necessary to test theoretical results, to serve as background fields in some numerical simulations, and to test analysis tools. Models of one-dimensional (1D) and 3D synthetic turbulence previously developed still required large computational resources. A "wavelet-based" model of synthetic turbulence, able to produce a field with tunable spectral law, intermittency, and anisotropy, is presented here. The rapid algorithm introduced, based on the classic p-model of intermittent turbulence, allows us to reach a broad spectral range using a modest computational effort. The model has been tested against the standard diagnostics for intermittent turbulence, i.e., the spectral analysis, the scale-dependent statistics of the field increments, and the multifractal analysis, all showing an excellent response.

Experimental analysis of intermittency in electrohydrodynamic instability

The properties of turbulent electroconvective fluctuations generated in a nematic liquid crystal under the action of an external oscillating electric field are investigated. In particular, the spectral properties and the scaling behaviour of probability density functions (PDFs) of light intensity fluctuations are considered at different voltages. At intermediate voltage, in the weak turbulent regime, PDFs are Gaussian at large scales and show increasingly enhanced wings at smaller scales, recalling the typical signature of intermittency in isotropic fluid flows. When the voltage is increased, dynamical scattering regimes appear, characterized by increasing complexity. In order to get a quantitative estimate of intermittency, PDFs are modeled through the Castaing distribution, and structure functions are estimated in the framework of Extended Self-Similarity. Results support the generation of small-scale fluctuations through a fragmentation process of large-scale structures. The persistent anisotropic properties of the fluctuations are highlighted by the results.

Collisional thermalization of hydrogen and helium in solar-wind plasma

In situ observations of the solar wind frequently show the temperature of α particles (fully ionized helium) Tα to significantly differ from that of protons (ionized hydrogen) Tp. Many heating processes in the plasma act preferentially on α particles, even as collisions among ions act to gradually establish thermal equilibrium. Measurements from the Wind spacecraft's Faraday cups reveal that, at r=1.0  AU from the Sun, the observed values of the α-proton temperature ratio, θαp≡Tα/Tp, has a complex, bimodal distribution. This study applied a simple model for the radial evolution of θαp to these data to compute expected values of θαp at r=0.1  AU. These inferred θαp values have no trace of the bimodality seen in the θαp values measured at r=1.0  AU but are instead consistent with the actions of the known mechanisms for α-particle preferential heating. This result underscores the importance of collisional processes in the dynamics of the solar wind and suggests that similar mechanisms may lead to preferential α-particle heating in both slow and fast wind.

Cancellation properties in Hall magnetohydrodynamics with a strong guide magnetic field

We present a signed measure analysis of compressible Hall magnetohydrodynamic turbulence with an external guide field. Signed measure analysis allows us to characterize the scaling behavior of the sign-oscillating flow structures and their geometrical properties (fractal dimensions of structures). A reduced numerical model, valid when a strong guide magnetic field is present, is used here. In order to discuss the effect of the Hall term, different values for the ion skin depth are considered in the simulations. Results show that as the Hall term is increased, the fractal dimension of the current and vorticity sheets decreases. This observation, together with previous analysis of the same fields, provides a comprehensive description of the effect of the Hall force on the formation of structures. Two main processes are identified, namely, the widening and unraveling of the sheets.

Phase-synchronization, energy cascade, and intermittency in solar-wind turbulence

The energy cascade in solar wind magnetic turbulence is investigated using MESSENGER data in the inner heliosphere. The decomposition of magnetic field time series in intrinsic functions, each characterized by a typical time scale, reveals phase reorganization. This allows for the identification of structures of all sizes generated by the nonlinear turbulent cascade, covering both the inertial and the dispersive ranges of the turbulent magnetic power spectrum. We find that the correlation (or anticorrelation) of phases occurs between pairs of neighboring time scales, whenever localized peaks of magnetic energy are present at both scales, consistent with the local character of the energy transfer process.

Spatio-temporal dynamics, patterns formation and turbulence in complex fluids due to electrohydrodynamics instabilities

The complex spatio-temporal dynamics generated by electrohydrodynamics instabilities in a nematic liquid crystal under the action of a driving oscillating electric field is investigated. Quasi-stationary convective structures which are visible at large scales are broken into chaotic patterns at higher driving voltages, thus generating small-scale structures. Scaling analysis reveals that these small-scale structures self-organize in a network of subleading structures which are reminescent of convective rolls. This network persists well inside the chaotic regimes, disappearing only at very high voltages, where stochastic dynamical scattering mode takes place.

Anisotropy of spatiotemporal decorrelation in electrohydrodynamic turbulence

Nonlinear straining and random sweeping spatiotemporal decorrelation properties, originally introduced as the main processes for turbulent fluctuations decorrelation in usual fluid flows, have been observed experimentally in anisotropic electroconvective turbulence generated in a nematic liquid crystal under the action of an external oscillating electric field. A transition between both processes occurs when the instability is driven toward states of increasing complexity, thus showing that decorrelation mechanisms in turbulent media are more universal than naively expected. A model for both decorrelation mechanisms is introduced, its comparison with experimental results providing an estimate of the characteristic sweeping velocity.

Scaling laws of turbulence and heating of fast solar wind: the role of density fluctuations

Incompressible and isotropic magnetohydrodynamic turbulence in plasmas can be described by an exact relation for the energy flux through the scales. This Yaglom-like scaling law has been recently observed in the solar wind above the solar poles observed by the Ulysses spacecraft, where the turbulence is in an Alfvénic state. An analogous phenomenological scaling law, suitably modified to take into account compressible fluctuations, is observed more frequently in the same data set. Large-scale density fluctuations, despite their low amplitude, thus play a crucial role in the basic scaling properties of turbulence. The turbulent cascade rate in the compressive case can, moreover, supply the energy dissipation needed to account for the local heating of the nonadiabatic solar wind.

Fractal aggregates evolution of methyl red in liquid crystal

The spontaneous formation of dendritic aggregates is observed in a two-dimensional confined layered system consisting of a film composed of liquid crystal, dye and solvent cast above a polymer substrate. The observed aggregates are promoted by phase separation processes induced by dye diffusion and solvent evaporation. The growth properties of the aggregates are studied through the temporal evolution of their topological properties (surface, perimeter, fractal dimension). The fractal dimension of the completely formed structures, when they are coexistent with different types of structures, is consistent with theoretical and experimental values obtained for Diffusion-Limited Aggregates. Under different experimental conditions (temperature and local dye concentration) the structure forms without interactions with other kinds of structures, and its equilibrium fractal dimension is smaller. The fractal dimension is thus not a universal property of the observed structures, but rather depends on the experimental conditions.

Statistical analysis of random lasing emission properties in nematic liquid crystals

A statistical analysis of random lasing events observed in dye-doped nematic-liquid-crystal films is reported. The occurrence of random laser action in such complex fluids is due to residual resonances in the multiple scattering of spontaneously emitted photons. The Shannon entropy and a local-Poisson test are used here in order to quantitatively characterize the chaotic behavior of laser spikes and gain further understanding of the mechanisms underlying the lasing effect in strongly scattering organized fluids arising by an unexpected interplay of localization and amplification.

Observation of inertial energy cascade in interplanetary space plasma

Direct evidence for the presence of an inertial energy cascade, the most characteristic signature of hydromagnetic turbulence (MHD), is observed in the solar wind by the Ulysses spacecraft. After a brief rederivation of the equivalent of Yaglom's law for MHD turbulence, a linear relation is indeed observed for the scaling of mixed third-order structure functions involving Elsässer variables. This experimental result firmly establishes the turbulent character of low-frequency velocity and magnetic field fluctuations in the solar wind plasma.

Clustering of polarity reversals of the geomagnetic field

Often in nature the temporal distribution of inhomogeneous stochastic point processes can be modeled as a realization of renewal Poisson processes with a variable rate. Here we investigate one of the classical examples, namely, the temporal distribution of polarity reversals of the geomagnetic field. In spite of the commonly used underlying hypothesis, we show that this process strongly departs from a Poisson statistics, the origin of this failure stemming from the presence of temporal clustering. We find that a Lévy statistics is able to reproduce paleomagnetic data, thus suggesting the presence of long-range correlations in the underlying dynamo process.

Proper orthogonal decomposition of solar photospheric motions

The spatiotemporal dynamics of the solar photosphere is studied by performing a proper orthogonal decomposition (POD) of line of sight velocity fields computed from high resolution data coming from the MDI/SOHO instrument. Using this technique, we are able to identify and characterize the different dynamical regimes acting in the system. Low-frequency oscillations, with frequencies in the range 20-130 microHz, dominate the most energetic POD modes (excluding solar rotation), and are characterized by spatial patterns with typical scales of about 3 Mm. Patterns with larger typical scales of approximately 10 Mm, are associated to p-modes oscillations at frequencies of about 3000 microHz.

Intermittency and turbulence in a magnetically confined fusion plasma

We investigate the intermittency of magnetic turbulence as measured in reversed field pinch plasmas. We show that the probability distribution functions of magnetic field differences are not scale invariant; that is, the wings of these functions are more important at the smallest scales, a classical signature of intermittency. We show that scaling laws appear also in a region very close to the external wall of the confinement device, and we present evidences that the observed intermittency increases moving towards the wall.