Non-gaussian probability distribution functions in two dimensional Magnetohydrodynamic turbulence

Intermittency in MHD turbulence has been analyzed using high resolution 2D numerical simulations. We show that the Probability Distribution Functions (PDFs) of the fluctuations of the Elsasser fields, magnetic field and velocity field depend on the scale at hand, that is they are self-affine. The departure of the PDFs from a Gaussian function can be described through the scaling behavior of a single parameter lambda_r^2 obtained by fitting the PDFs with a given curve stemming from the analysis of a multiplicative model by Castaing et al. (1990). The scaling behavior of the parameter lambda_r^2 can be used to extract informations about the intermittency. A comparison of intermittency properties in different MHD turbulent flows is also performed.Comment: 7 pages, with 5 figure

Scaling properties of three-dimensional magnetohydrodynamic turbulence

The scaling properties of three-dimensional magnetohydrodynamic turbulence are obtained from direct numerical simulations of decaying turbulence using $512^3$ modes. The results indicate that the turbulence does not follow the Iroshnikov-Kraichnan phenomenology.In the case of hyperresistivity, the structure functions exhibit a clear scaling range yielding absolute values of the scaling exponents $\zeta_p$. The scaling exponents agree with a modified She-Leveque model $\zeta_p=p/9 + 1 - (1/3)^{p/3}$, corresponding to Kolmogorov scaling but sheet-like geometry of the dissipative structures

Analysis of cancellation in two-dimensional magnetohydrodynamic turbulence

A signed measure analysis of two-dimensional intermittent magnetohydrodynamic turbulence is presented. This kind of analysis is performed to characterize the scaling behavior of the sign-oscillating flow structures, and their geometrical properties. In particular, it is observed that cancellations between positive and negative contributions of the field inside structures, are inhibited for scales smaller than the Taylor microscale, and stop near the dissipative scale. Moreover, from a simple geometrical argument, the relationship between the cancellation exponent and the typical fractal dimension of the structures in the flow is obtained.Comment: 21 pages, 5 figures (3 .jpg not included in the latex file

Statistical anisotropy of magnetohydrodynamic turbulence

Direct numerical simulations of decaying and forced magnetohydrodynamic (MHD) turbulence without and with mean magnetic field are analyzed by higher-order two-point statistics. The turbulence exhibits statistical anisotropy with respect to the direction of the local magnetic field even in the case of global isotropy. A mean magnetic field reduces the parallel-field dynamics while in the perpendicular direction a gradual transition towards two-dimensional MHD turbulence is observed with $k^{-3/2}$ inertial-range scaling of the perpendicular energy spectrum. An intermittency model based on the Log-Poisson approach, $\zeta_p=p/g^2 +1 -(1/g)^{p/g}$, is able to describe the observed structure function scalings.Comment: 4 pages, 3 figures. To appear in Phys.Rev.

Current-sheet formation in incompressible electron magnetohydrodynamics

The nonlinear dynamics of axisymmetric, as well as helical, frozen-in vortex structures is investigated by the Hamiltonian method in the framework of ideal incompressible electron magnetohydrodynamics. For description of current-sheet formation from a smooth initial magnetic field, local and nonlocal nonlinear approximations are introduced and partially analyzed that are generalizations of the previously known exactly solvable local model neglecting electron inertia. Finally, estimations are made that predict finite-time singularity formation for a class of hydrodynamic models intermediate between that local model and the Eulerian hydrodynamics.Comment: REVTEX4, 5 pages, no figures. Introduction rewritten, new material and references adde

Predictors and trajectories of ED visits among patients receiving palliative home care services: Findings from a time series analysis (2013-2017)

Background: Current policies recommend integrating home care and palliative care to enable patients to remain at home and avoid unnecessary hospital admission and emergency department (ED) visits. The Italian health care system had implemented integrated palliative home care (IHPC) services to guarantee a comprehensive, coordinated approach across different actors and to reduce potentially avoidable ED visits. This study aimed to analyze the trajectories of ED visit rates among patients receiving IHPC in the Italian healthcare system, as well as the association between socio-demographic, health supply, and clinical factors. Methods: A pooled, cross-sectional, time series analysis was performed in a large Italian region in the period 2013-2017. Data were taken from two databases of the official Italian National Information System: Home Care Services and ED use. A clinical record is opened at the time a patient is enrolled in IHPC and closed after the last service is provided. Every such clinical record was considered as an IHPC event, and only ED visits that occurred during IHPC events were considered. Results: The 20,611 patients enrolled in IHPC during the study period contributed 23,085 IHPC events; ≥1 ED visit occurred during 6046 of these events. Neoplasms accounted for 89% of IHPC events and for 91% of ED visits. Although there were different variations in ED visit rates during the study period, a slight decline was observed for all diseases, and this decline accelerated over time (b = - 0.18, p = 0.796, 95% confidence interval [CI] = - 1.59;1.22, b-squared = - 1.25, p < 0.001, 95% CI = -1.63;-0.86). There were no significant predictors among the socio-demographic factors (sex, age, presence of a non-family caregiver, cohabitant family members, distance from ED), health supply factors (proponent of IHPC) and clinical factors (prevalent disorder at IHPC entry, clinical symptoms). Conclusion: Our results show that use of ED continues after enrollment in IHPC, but the trend of this use declines over time. As no significant predictive factors were identified, no specific interventions can be recommended on which the avoidable ED visits depend

Evidence for acoustic-like plasmons on epitaxial graphene on Pt(111)

The dispersion and the damping of the sheet plasmon in a graphene monolayer grown on Pt(111) have been studied by using angle-resolved electron energy loss spectroscopy. We found that the dispersion relation of the plasmon mode confined in the graphene sheet is linear, as a consequence of the screening by the metal substrate. Present results demonstrate that the presence of an underlying metal substrate could have striking consequences on the plasmon propagation even in the case of a system which exhibits a weak graphene-substrate interaction. Moreover, we found that Landau damping essentially occurs via interband excitations starting above the Fermi wave vector. On the contrary, intraband transitions do not have a significant influence on the collective mod