Kinetic dissipation and anisotropic heating in a turbulent collisionless plasma

The kinetic evolution of the Orszag-Tang vortex is studied using collisionless hybrid simulations. In the magnetohydrodynamic regime this vortex leads rapidly to broadband turbulence. Significant differences from MHD arise at small scales, where the fluid scale energy dissipates into heat almost exclusively through the magnetic field because the protons are decoupled from the magnetic field. Although cyclotron resonance is absent, the protons heat preferentially in the plane perpendicular to the mean field, as in the corona and solar wind. Effective transport coefficients are calculated.Comment: 4 pages, 4 figures. Submitted to PR

Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence

Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta (ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic shear (strong guide field). Changing the simulation domain size, we find that the ion response varies greatly. For reconnecting regions with scales comparable to the ion Larmor radius, the ions do not respond to the reconnection dynamics leading to ''electron-only'' reconnection with very large quasi-steady reconnection rates. The transition to more traditional ''ion-coupled'' reconnection is gradual as the reconnection domain size increases, with the ions becoming frozen-in in the exhaust when the magnetic island width in the normal direction reaches many ion inertial lengths. During this transition, the quasi-steady reconnection rate decreases until the ions are fully coupled, ultimately reaching an asymptotic value. The scaling of the ion outflow velocity with exhaust width during this electron-only to ion-coupled transition is found to be consistent with a theoretical model of a newly reconnected field line. In order to have a fully frozen-in ion exhaust with ion flows comparable to the reconnection Alfv\'en speed, an exhaust width of at least several ion inertial lengths is needed. In turbulent systems with reconnection occurring between magnetic bubbles associated with fluctuations, using geometric arguments we estimate that fully ion-coupled reconnection requires magnetic bubble length scales of at least several tens of ion inertial lengths

Nonlinear and linear timescales near kinetic scales in solar wind turbulence

The application of linear kinetic treatments to plasma waves, damping, and instability requires favorable inequalities between the associated linear timescales and timescales for nonlinear (e.g., turbulence) evolution. In the solar wind these two types of timescales may be directly compared using standard Kolmogorov-style analysis and observational data. The estimated local (in scale) nonlinear magnetohydrodynamic cascade times, evaluated as relevant kinetic scales are approached, remain slower than the cyclotron period, but comparable to or faster than the typical timescales of instabilities, anisotropic waves, and wave damping. The variation with length scale of the turbulence timescales is supported by observations and simulations. On this basis the use of linear theory—which assumes constant parameters to calculate the associated kinetic rates—may be questioned. It is suggested that the product of proton gyrofrequency and nonlinear time at the ion gyroscales provides a simple measure of turbulence influence on proton kinetic behavior

Conditions for fast magnetic reconnection in astrophysical plasmas

We investigate favourable circumstances for fast magnetic reconnection in astrophysical plasmas based on recent results by Rogers et al. (2001). Given that a critical magnetic field structure with antiparallel field lines exists, our analysis demonstrates that a sufficient condition for fast reconnection is that the ratio of the thermal pressure to the magnetic field pressure $\beta$ should be significantly larger than $2 m_e/m_p$ (twice the ratio of electron mass to proton mass). Using several examples (like the different components of the interstellar medium, the intergalactic medium, active galactic nuclei and jets) we show that in almost all astrophysical plasmas, magnetic reconnection proceeds fast i.e. independent of the resistivity, with a few percent of the Alfv{\'e}n speed. Only for special cases like neutron stars and white dwarfs is $\beta$ smaller than $2 m_e/ m_p$

Reference ranges for left modified myocardial performance index: Systematic review and meta-analysis

INTRODUCTION: The modified myocardial performance index (mod-MPI) is a noninvasive Doppler-derived metric used to evaluate fetal cardiac function. However, the reference ranges for mod-MPI in normal fetuses are not clearly defined, which limits the use of this technology in fetuses with potential cardiac compromise. Thus, we aimed to perform a systematic review and meta-analysis of published mod-MPI reference ranges across gestation. METHODS: The published literature was systematically searched, and all published articles in any language that provided values for the left ventricular mod-MPI obtained in low-risk, singleton fetuses were considered eligible for further review. All retrieved titles and abstracts were independently reviewed by two researchers. Mean and standard deviation by gestational week was extracted or calculated from published data. DerSimonian-Laird random-effects models were used to estimate pooled means and 95% confidence intervals (CIs). RESULTS: The search resulted in 618 unique citations, of which 583 did not meet inclusion criteria, leaving 35 abstracts selected for full-text review. Review of the references of these 35 articles identified another 5 studies of interest. Of the 40 articles reviewed, six met inclusion criteria. There was significant heterogeneity seen in the mod-MPI results reported. Mod-MPI increased as pregnancy progressed in all studies. The pooled mean mod-MPI at 11 weeks\u27 gestation was 0.400 (95% CI 0.374-0.426) and increased to 0.585 (95% CI 0.533-0.637) at 41 weeks\u27 gestation. The increase was linear in 5 of 6 studies, while in 1 study, the mod-MPI was stable until 27 weeks\u27 gestation, and then increased throughout the third trimester. Despite all having trends increasing over pregnancy, there was no study in which all the weekly means fell within the pooled 95% CI. CONCLUSION: While mod-MPI does increase over gestation, the true reference ranges for fetuses remain elusive. Future efforts to further optimize calculation of time intervals possibly via automation are desperately needed to allow for reproducibility of this potentially very useful tool to assess fetal cardiac function

Propagating mode-I fracture in amorphous materials using the continuous random network (CRN) model

We study propagating mode-I fracture in two dimensional amorphous materials using atomistic simulations. We used the continuous random network (CRN) model of an amorphous material, creating samples using a two dimensional analogue of the WWW (Wooten, Winer & Weaire) Monte-Carlo algorithm. For modeling fracture, molecular-dynamics simulations were run on the resulting samples. The results of our simulations reproduce the main experimental features. In addition to achieving a steady-state crack under a constant driving displacement (which had not yet been achieved by other atomistic models for amorphous materials), the runs show micro-branching, which increases with driving, transitioning to macro-branching for the largest drivings. Beside the qualitative visual similarity of the simulated cracks to experiment, the simulation also succeeds in explaining the experimentally observed oscillations of the crack velocity

Finding the needles in the haystack: Generating legal test inputs for object-oriented programs

A test input for an object-oriented program typically consists of asequence of method calls that use the API defined by the programunder test. Generating legal test inputs can be challenging because,for some programs, the set of legal method sequences is much smallerthan the set of all possible sequences; without a formalspecification of legal sequences, an input generator is bound toproduce mostly illegal sequences.We propose a scalable technique that combines dynamic analysis withrandom testing to help an input generator create legal test inputswithout a formal specification, even for programs in whichmost sequences are illegal. The technique uses an example executionof the program to infer a model of legal call sequences, and usesthe model to guide a random input generator towards legal butbehaviorally-diverse sequences.We have implemented our technique for Java, in a tool calledPalulu, and evaluated its effectiveness in creating legal inputsfor real programs. Our experimental results indicate that thetechnique is effective and scalable. Our preliminary evaluationindicates that the technique can quickly generate legal sequencesfor complex inputs: in a case study, Palulu created legal testinputs in seconds for a set of complex classes, for which it took anexpert thirty minutes to generate a single legal input