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The consequences of a 90° barrier in the scattering of energetic electrons by whistler waves are explored with self-consistent two-dimensional particle-in-cell simulations. In the presence of a 90° scattering barrier, a field-aligned heat flux of energetic electrons will rapidly scatter to form a uniform distribution with pitch angles 0 < θ < 90 ° but with a discontinuous jump at θ = 90 ° to a lower energy distribution of electrons with 90 ° < θ < 180 °. However, simulations reveal that such a distribution contains a large reservoir of free energy that is released to drive large-amplitude, oblique-propagating whistler waves (δ B / B 0 ∼ 0.1). As a result, energetic electrons near a pitch angle 90° experience strong resonance scattering. Nearly half of the energetic electrons in the positive parallel velocity plane cross the 90° barrier and diffuse to negative parallel velocities. Thus, the late-time electron velocity distribution becomes nearly isotropic. This result has implications for understanding the regulation of energetic particle heat flux in space and astrophysical environments, including the solar corona, the solar wind, and the intracluster medium of galaxy clusters. [ABSTRACT FROM AUTHOR]

A set of equations is developed that extends the macroscale magnetic reconnection simulation model kglobal to include particle ions. The extension from earlier versions of kglobal, which included only particle electrons, requires the inclusion of the inertia of particle ions in the fluid momentum equation. The new equations will facilitate the exploration of the simultaneous non-thermal energization of ions and electrons during magnetic reconnection in macroscale systems. Numerical tests of the propagation of Alfvén waves and the growth of firehose modes in a plasma with anisotropic electron and ion pressure are presented to benchmark the new model. [ABSTRACT FROM AUTHOR]

Alzheimer's disease (AD) is a degenerative disorder involving significant memory loss and other cognitive deficits, manifesting as a progression from normal cognitive functioning to mild cognitive impairment to AD. The sooner an accurate diagnosis of probable AD is made, the easier it is to manage symptoms and plan for future therapy. Functional neuroimaging stands to be a useful tool in achieving early diagnosis. Among the many neuroimaging modalities, positron emission tomography (PET) provides direct regional assessment of, among others, brain metabolism, cerebral blood flow, amyloid deposition—all quantities of interest in the characterization of AD. However, there are analytic challenges in identifying early indicators of AD from these high‐dimensional imaging data sets, and it is unclear whether early indicators of AD are more likely to emerge in localized patterns of brain activity or in patterns of correlation between distinct brain regions. Early PET‐based analyses of AD focused on alterations in metabolic activity at the voxel‐level or in anatomically defined regions of interest. Other approaches, including seed‐voxel and multivariate techniques, seek to characterize metabolic connectivity by identifying other regions in the brain with similar patterns of activity across subjects. We briefly review various neuroimaging statistical approaches applied to determine changes in metabolic activity or metabolic connectivity associated with AD. We then present an approach that provides a unified statistical framework for addressing both metabolic activity and connectivity. Specifically, we apply a Bayesian spatial hierarchical framework to longitudinal metabolic PET scans from the Alzheimer's Disease Neuroimaging Initiative. This article is categorized under:Statistical and Graphical Methods of Data Analysis > Analysis of High Dimensional DataStatistical Learning and Exploratory Methods of the Data Sciences > Modeling MethodsStatistical Models > Bayesian Models [ABSTRACT FROM AUTHOR]

To assess the potential survival benefit associated with receipt of definitive treatment (radical prostatectomy or radiation), compared to non-definitive treatment (hormonal therapy or chemotherapy) among men with metastatic prostate cancer. A cohort of men diagnosed with metastatic (T4/M1/N1 or T4/M1) prostate cancer from 1999 to 2013 in the Veterans Health Administration were identified and followed to December 28, 2014. All-cause and prostate cancer-specific mortality were evaluated at 10 years for the T4/M1/N1 cohort and 8 years for the T4/M1/ cohort. The association of definitive treatment (radical prostatectomy or radiation), compared to non-definitive (hormonal therapy or chemotherapy) with both all-cause and prostate cancer-specific mortality was assessed using inverse probability of treatment weighted (IPTW) multivariable survival analyses. The cohort included 2919 with T4/M1/N1 disease and 1479 men with T4/M1 disease. Receipt of definitive treatment was associated with a reduced risk of 10-year all-cause (Hazard Ratio (HR): 0.61; 95% Confidence Interval (CI): 0.57–0.65) and prostate cancer-specific mortality (HR: 0.50; 95% CI: 0.46-0.55) among men diagnosed with T4/M1/N1 met–astatic disease. Definitive treatment was similarly associated with a reduced risk of all-cause (HR: 0.84; 95% CI: 0.77–0.91) and prostate cancer-specific (HR: 0.81; 95% CI: 0.73–0.90) mortality among men diagnosed with T4/M1 only metastatic disease. Definitive treatment may improve survival in men diagnosed with metastatic prostate cancer. [ABSTRACT FROM AUTHOR]

Small-scale mixing drives the diabatic upwelling that closes the abyssal ocean overturning circulation. Indirect microstructure measurements of in situ turbulence suggest that mixing is bottom enhanced over rough topography, implying downwelling in the interior and stronger upwelling in a sloping bottom boundary layer. Tracer release experiments (TREs), in which inert tracers are purposefully released and their dispersion is surveyed over time, have been used to independently infer turbulent diffusivities—but typically provide estimates in excess of microstructure ones. In an attempt to reconcile these differences, Ruan and Ferrari derived exact tracer-weighted buoyancy moment diagnostics, which we here apply to quasi-realistic simulations. A tracer's diapycnal displacement rate is exactly twice the tracer-averaged buoyancy velocity, itself a convolution of an asymmetric upwelling/downwelling dipole. The tracer's diapycnal spreading rate, however, involves both the expected positive contribution from the tracer-averaged in situ diffusion as well as an additional nonlinear diapycnal distortion term, which is caused by correlations between buoyancy and the buoyancy velocity, and can be of either sign. Distortion is generally positive (stretching) due to bottom-enhanced mixing in the stratified interior but negative (contraction) near the bottom. Our simulations suggest that these two effects coincidentally cancel for the Brazil Basin Tracer Release Experiment, resulting in negligible net distortion. By contrast, near-bottom tracers experience leading-order distortion that varies in time. Errors in tracer moments due to realistically sparse sampling are generally small (<20%), especially compared to the O (1) structural errors due to the omission of distortion effects in inverse models. These results suggest that TREs, although indispensable, should not be treated as "unambiguous" constraints on diapycnal mixing. [ABSTRACT FROM AUTHOR]

The abyssal overturning circulation is thought to be primarily driven by small-scale turbulent mixing. Diagnosed water-mass transformations are dominated by rough topography "hotspots," where the bottom enhancement of mixing causes the diffusive buoyancy flux to diverge, driving widespread downwelling in the interior—only to be overwhelmed by an even stronger upwelling in a thin bottom boundary layer (BBL). These water-mass transformations are significantly underestimated by one-dimensional (1D) sloping boundary layer solutions, suggesting the importance of three-dimensional physics. Here, we use a hierarchy of models to generalize this 1D boundary layer approach to three-dimensional eddying flows over realistically rough topography. When applied to the Mid-Atlantic Ridge in the Brazil Basin, the idealized simulation results are roughly consistent with available observations. Integral buoyancy budgets isolate the physical processes that contribute to realistically strong BBL upwelling. The downward diffusion of buoyancy is primarily balanced by upwelling along the sloping canyon sidewalls and the surrounding abyssal hills. These flows are strengthened by the restratifying effects of submesoscale baroclinic eddies and by the blocking of along-ridge thermal wind within the canyon. Major topographic sills block along-thalweg flows from restratifying the canyon trough, resulting in the continual erosion of the trough's stratification. We propose simple modifications to the 1D boundary layer model that approximate each of these three-dimensional effects. These results provide local dynamical insights into mixing-driven abyssal overturning, but a complete theory will also require the nonlocal coupling to the basin-scale circulation. [ABSTRACT FROM AUTHOR]

Much of modern climate science is motivated by the problem of human-caused climate change or its potential solutions, and aims to be “usable” for relevant stakeholders. Sobel (2021) argues in this issue that the expectation for improved climate projections to drive mitigation of greenhouse gas emissions “can now be understood [as] naive about the role of politics, and the power of entrenched interests to inhibit climate action.” While he criticizes this “linear model” of the science–policy interface; he does not elaborate on alternative avenues for scientific advances to spur mitigation. Instead, he encourages physical climate scientists who wish to produce usable results to orient their work towards informing adaptation. He argues that, relative to mitigation science, adaptation science is more likely to be used by stakeholders because the remaining scientific uncertainties are larger and the social barriers to implementation are lower. We join Sobel in calling on physical climate scientists to reflect upon the pathways through which their research improves societal welfare. However, we argue that Sobel’s argument overlooks an important theory of change, namely that mitigation science is politically usable through the non-linear dynamics of social mobilization. Social theories of policy change suggest that organized groups play an outsized role in setting the policy agenda. Grassroots activism on climate, however, has historically been hindered by the abstract nature of climate change, paling in comparison to the lobbying and misinformation campaigns funded by the vested fossil fuel interests. We describe how two recent advances in mitigation science, the Transient Climate Response to cumulative Emissions (TCRE) and Extreme Event Attribution (EEA), have provided social movements with information that allows them to re-frame the climate change problem in a way that attributes blame for the problem, motivates collective action across a diverse coalition of stakeholders, and could plausibly compel policymakers to prioritize the issue in the coming years. Given the utmost importance of mitigation in preventing climate change at the source, we thus advocate for a broader agenda of usable climate research that includes co-production of both mitigation and adaptation science. [ABSTRACT FROM AUTHOR]

Porous materials play a significant role in modern chemistry and materials science; despite recent scientific interest, they have a history dating back to antiquity. Here the authors provide a brief overview of the past that has contributed to their evolution. [ABSTRACT FROM AUTHOR]

Background: Cancer is the second leading cause of death in the United States. Cancer screenings can detect precancerous cells and allow for earlier diagnosis and treatment. Our purpose was to better understand risk factors for cancer screenings and assess the effect of cancer screenings on changes of Cardiovascular health (CVH) measures before and after cancer screenings among patients. Methods: We used The Guideline Advantage (TGA)—American Heart Association ambulatory quality clinical data registry of electronic health record data (n = 362,533 patients) to investigate associations between time-series CVH measures and receipt of breast, cervical, and colon cancer screenings. Long short-term memory (LSTM) neural networks was employed to predict receipt of cancer screenings. We also compared the distributions of CVH factors between patients who received cancer screenings and those who did not. Finally, we examined and quantified changes in CVH measures among the screened and non-screened groups. Results: Model performance was evaluated by the area under the receiver operator curve (AUROC): the average AUROC of 10 curves was 0.63 for breast, 0.70 for cervical, and 0.61 for colon cancer screening. Distribution comparison found that screened patients had a higher prevalence of poor CVH categories. CVH submetrics were improved for patients after cancer screenings. Conclusion: Deep learning algorithm could be used to investigate the associations between time-series CVH measures and cancer screenings in an ambulatory population. Patients with more adverse CVH profiles tend to be screened for cancers, and cancer screening may also prompt favorable changes in CVH. Cancer screenings may increase patient CVH health, thus potentially decreasing burden of disease and costs for the health system (e.g., cardiovascular diseases and cancers). [ABSTRACT FROM AUTHOR]

The emerging view of the abyssal circulation is that it is associated with bottom-enhanced mixing, which results in downwelling in the stratified ocean interior and upwelling in a bottom boundary layer along the insulating and sloping seafloor. In the limit of slowly varying vertical stratification and topography, however, boundary layer theory predicts that these upslope and downslope flows largely compensate, such that net water mass transformations along the slope are vanishingly small. Using a planetary geostrophic circulation model that resolves both the boundary layer dynamics and the large-scale overturning in an idealized basin with bottom-enhanced mixing along a midocean ridge, we show that vertical variations in stratification become sufficiently large at equilibrium to reduce the degree of compensation along the midocean ridge flanks. The resulting large net transformations are similar to estimates for the abyssal ocean and span the vertical extent of the ridge. These results suggest that boundary flows generated by mixing play a crucial role in setting the global ocean stratification and overturning circulation, requiring a revision of abyssal ocean theories. [ABSTRACT FROM AUTHOR]

A new computational model, kglobal, is being developed to explore energetic electron production via magnetic reconnection in macroscale systems. The model is based on the discovery that the production of energetic electrons during reconnection is controlled by Fermi reflection in large-scale magnetic fields and not by parallel electric fields localized in kinetic scale boundary layers. Thus, the model eliminates these boundary layers. However, although the parallel electric fields that develop around the magnetic x-line and associated separatrices are not important in producing energetic electrons, there is a large scale electric field that kickstarts the heating of low-energy electrons and drives the cold-electron return current that accompanies escaping energetic electrons in open systems. This macroscale electric field is produced by magnetic-field-aligned gradients in the electron pressure. We have upgraded kglobal to include this large-scale electric field while maintaining energy conservation. The new model is tested by exploring the dynamics of electron acoustic modes which develop as a consequence of the presence of two electron species: hot kinetic and cold fluid electrons. Remarkably, the damping of electron acoustic modes is accurately captured by kglobal. Additionally, it has been established that kglobal correctly describes the dynamics of the interaction of the parallel electric field with escaping hot electrons through benchmarking simulations with the Particle-In-Cell code p3d. [ABSTRACT FROM AUTHOR]

The role of turbulence in low-β, guide-field reconnection exhausts is explored in 2D reconnection and 2D and 3D Riemann simulations. The structure of the exhaust and associated turbulence is controlled by a pair of rotational discontinuities (RDs) at the exhaust boundary and a pair of slow shocks (SSs) that are generated by counterstreaming ions beams. In 2D, the exhaust develops large-amplitude striations at the ion Larmor radius scale that are produced by electron-beam-driven ion cyclotron waves. The electron beams driving the instability are injected into the exhaust from one of the RDs. However, in 3D Riemann simulations, the additional dimension (in the out-of-plane direction) results in strong Buneman and electron-electron streaming instabilities at the RD which suppress the electron beam formation and therefore the striations in the exhaust. The strength of the streaming instabilities at the RD is controlled by the ratio of the electron thermal speed to Alfvén speed, with the lower thermal speed being more unstable. In the 3D simulations, an ion-ion streaming instability acts to partially thermalize the counterstreaming ion beams at the SSs. This instability is controlled by the ratio of the sound speed to Alfvén speed and is expected to be stable in the low β solar corona. The results suggest that in a guide field reconnection exhaust with 1 ≫ β > m e / m i , the kinetic-scale turbulence that develops will be too weak to play a significant role in energy conversion and particle acceleration. Therefore, the energy conversion will be mostly controlled by laminar physics or multi-x-line reconnection. [ABSTRACT FROM AUTHOR]

Kinetic Riemann simulations have been completed to explore particle heating during guide field reconnection in the low-β environment of the inner heliosphere and the solar corona. The reconnection exhaust is bounded by two rotational discontinuities (RDs), and two slow shocks (SSs) form within the exhaust as in magnetohydrodynamic (MHD) models. At the RDs, ions are accelerated by the magnetic field tension to drive the reconnection outflow as well as flows in the out-of-plane direction. The out-of-plane flows stream toward the midplane and meet to drive the SSs. The SSs differ greatly from those in the MHD model. The turbulence at the shock fronts and both upstream and downstream is weak, and so the shocks are laminar and produce little dissipation. Downstream of the SSs, the counterstreaming ion beams lead to higher density, which leads to a positive potential between the SSs which acts to confine the downstream electrons to maintain charge neutrality. The potential accelerates electrons from upstream of the SSs to the downstream region and traps a small fraction but only modestly increases the downstream electron temperature above the upstream value. In the low-β limit, the released magnetic energy is split between bulk flow and ion heating with little energy going to electrons. That the model produces neither strong electron heating nor an energetic electron component suggests that other mechanisms, such as multiple x-line reconnection, are required to explain energetic electron production in large flares. The model can be tested with the expected data from the Parker Solar Probe. [ABSTRACT FROM AUTHOR]

In their article on "Navigating the Field of Implementation Science Towards Maturity: Challenges and Opportunities," Chambers and Emmons describe the rapid growth of implementation science along with remaining challenges. A significant gap remains in training and capacity building. Formats for capacity building include university degree programs, summer training institutes, workshops, and conferences. In this letter, we describe and amplify on five key areas, including the need to (1) identify advanced competencies, (2) increase the volume and reach of trainings, (3) sustain trainings, (4) build equity focused trainings, and (5) develop global capacity. We hope that the areas we highlight will aid in addressing several key challenges to prioritize in future efforts to build greater capacity in implementation science. [ABSTRACT FROM AUTHOR]

A new computational model suitable for exploring the self-consistent production of energetic electrons during magnetic reconnection in macroscale systems is presented. The equations are based on the recent discovery that parallel electric fields are ineffective drivers of energetic particles during reconnection so that the kinetic scales which control the development of such fields can be ordered out of the equations. The resulting equations consist of a magnetohydrodynamic (MHD) backbone with the energetic component represented by macro-particles described by the guiding center equations. Crucially, the energetic component feeds back on the MHD equations so that the total energy of the MHD fluid and the energetic particles is conserved. The equations correctly describe the firehose instability, whose dynamics plays a key role in throttling reconnection and in controlling the spectra of energetic particles. The results of early tests of the model, including the propagation of Alfvén waves in a system with pressure anisotropy and the growth of firehose modes, establish that the basic algorithm is stable and produces reliable physics results in preparation for further benchmarking with particle-in-cell models of reconnection. [ABSTRACT FROM AUTHOR]

Abstract: We analyze a high‐resolution simulation of magnetopause reconnection observed by the Magnetospheric Multiscale mission and explain the occurrence of strongly localized dissipation with an amplitude more than an order of magnitude larger than expected. Unlike symmetric reconnection, wherein reconnection of the ambient reversed magnetic field drives the dissipation, we find that the annihilation of the self‐generated, out‐of‐plane (Hall) magnetic field plays the dominant role. Electrons flow along the magnetosheath separatrices, converge in the diffusion region, and jet past the X‐point into the magnetosphere. The resulting accumulation of negative charge generates intense parallel electric fields that eject electrons along the magnetospheric separatrices and produce field‐aligned beams. Many of these features match Magnetospheric Multiscale observations. [ABSTRACT FROM AUTHOR]

Abstract: Whistler mode chorus waves are particularly important in outer radiation belt dynamics due to their key role in controlling the acceleration and scattering of electrons over a very wide energy range. The efficiency of wave‐particle resonant interactions is defined by whistler wave properties which have been described by the approximation of plane linear waves propagating through the cold plasma of the inner magnetosphere. However, recent observations of extremely high‐amplitude whistlers suggest the importance of nonlinear wave‐particle interactions for the dynamics of the outer radiation belt. Oblique chorus waves observed in the inner magnetosphere often exhibit drastically nonsinusoidal (with significant power in the higher harmonics) waveforms of the parallel electric field, presumably due to the feedback from hot resonant electrons. We have considered the nature and properties of such nonlinear whistler waves observed by the Van Allen Probes and Time History of Events and Macroscale Interactions define during Substorms in the inner magnetosphere, and we show that the significant enhancement of the wave electrostatic component can result from whistler wave coupling with the beam‐driven electrostatic mode through the resonant interaction with hot electron beams. Being modulated by a whistler wave, the electron beam generates a driven electrostatic mode significantly enhancing the parallel electric field of the initial whistler wave. We confirm this mechanism using a self‐consistent particle‐in‐cell simulation. The nonlinear electrostatic component manifests properties of the beam‐driven electron acoustic mode and can be responsible for effective electron acceleration in the inhomogeneous magnetic field. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we study upwelling pathways and timescales of Circumpolar Deep Water (CDW) in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of CDW upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1° to 0.25° to 0.1°. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading high‐resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops in the 1° model in comparison to 0.9 loops in the 0.1° model. These differences suggest that advective timescales and thus interbasin merging of upwelling CDW may be overestimated by coarse‐resolution models, potentially affecting the skill of centennial scale climate change projections. [ABSTRACT FROM AUTHOR]

The centrosymmetric title complex, [μ-ethane-1,2-diylbis(diphenylphosphane)- κ²P:P′]bis[pentacarbonyltungsten(0)], [W2(C26H24P2)(CO)10], consists of two W(CO)5 moieties bridged by a bis(diphenylphosphanyl)ethane (dppe) ligand. The W0 atom has a slightly distorted octahedral coordination environment consisting of 5 carbonyl ligands and one P atom from the bridging dppe ligand with the nearest W0 atom 5.625 (5) Å away. The complex resides on a center of symmetry. [ABSTRACT FROM AUTHOR]

Background: Biorepository research has substantial societal benefits. This is one of the few studies to focus on male willingness to allow future research use of biospecimens. Methods: This study analyzed the future research consent questions from a prostate cancer biorepository study (N = 1931). The consent form asked two questions regarding use of samples in future studies (1) without and (2) with protected health information (PHI). Yes to both questions of use of samples was categorized as Yes-Always; Yes to without and No to with PHI was categorized as Yes-Conditional; No to without PHI was categorized as Never. We analyzed this outcome to determine significant predictors for consent to Yes-Always vs. Yes-Conditional. Results: 99.33 % consented to future use of samples; 88.19 % consented to future use without PHI, and among those men 10.2 % consented to future use with PHI. Comparing Yes Always and Yes Conditional responses, bivariate analyses showed that race, family history, stage of cancer, and grade of cancer (Gleason), were significant at the a = 0.05 level. Using stepwise multivariable logistic regression, we found that African-American men were significantly more likely to respond Yes Always when compared to White men (p < 0.001). Those with a family history of prostate cancer were significantly more likely to respond Yes Always (p = 0.002). Conclusions: There is general willingness to consent to future use of specimens without PHI among men. [ABSTRACT FROM AUTHOR]

As the solar system moves through the interstellar medium, the solar wind is deflected forming the heliosphere. The standard picture of the heliosphere is a comet-shape like structure with the tail extending for 1000s of astronomical units. This standard picture stems from a view where magnetic forces are negligible and the solar magnetic field is convected passively down the tail. Recently, we showed that the magnetic tension of the solar magnetic field plays a crucial role on organizing the solar wind in the heliosheath into two jet-like structures. The two jets are separated by the interstellar medium that flows between them. The heliosphere then has a "croissant"-like shape where the distance to the heliopause downtail is almost the same as towards the nose. This new view of the heliosphere is in agreement with the energetic neutral atoms maps taken by the Interstellar Boundary Explorer and INCA/CASSINI. We developed as well an analytic model of the heliosheath in the axisymmetric limit that shows how the magnetic tension force is the driver for the north and south jets. We confirmed that the formation of these jets with magnetohydrodynamic (MHD) simulations. The main reason why previous global MHD simulations did not see these jets is due to spurious magnetic dissipation that was present at the heliospheric current sheet. We instead kept the same polarity for the interplanetary (solar) magnetic field in both the northern and southern hemispheres, eliminating spurious magnetic dissipation effects at the heliospheric current sheet. In this paper, we extend these previous results to include additional cases where we used: (a) weaker solar magnetic field; (b) solar magnetic field that reverses polarity at the solar equator in the axisymmetric limit; and (c) slower motion through the interstellar system. We discuss as well future challenges regarding the structure of the heliosphere. [ABSTRACT FROM AUTHOR]

Kinetic simulations of 3D collisionless magnetic reconnection with a guide field show a dramatic enhancement of energetic electron production when compared with 2D systems. In the 2D systems, electrons are trapped in magnetic islands that limit their energy gain, whereas in the 3D systems the filamentation of the current layer leads to a stochastic magnetic field that enables the electrons to access volume-filling acceleration regions. The dominant accelerator of the most energetic electrons is a Fermi-like mechanism associated with reflection of charged particles from contracting field lines. [ABSTRACT FROM AUTHOR]

We compare hybrid (kinetic proton, fluid electron) and particle-in-cell (kinetic proton, kinetic electron) simulations of the solar wind termination shock with parameters similar to those observed by Voyager 2 during its crossing. The steady-state results show excellent agreement between the downstream variations in the density, plasma velocity, and magnetic field. The quasi-perpendicular shock accelerates interstellar pickup ions to a maximum energy limited by the size of the computational domain, with somewhat higher fluxes and maximal energies observed in the particle-in-cell simulation, likely due to differences in the cross-shock electric field arising from electron kinetic-scale effects. The higher fluxes may help address recent discrepancies noted between observations and large-scale hybrid simulations. [ABSTRACT FROM AUTHOR]

The heating of electrons in collisionless magnetic reconnection is explored in particle-in-cell simulations with non-zero guide fields so that electrons remain magnetized. In this regime, electric fields parallel to B accelerate particles directly, while those perpendicular to B do so through gradient- B and curvature drifts. The curvature drift drives parallel heating through Fermi reflection, while the gradient B drift changes the perpendicular energy through betatron acceleration. We present simulations in which we evaluate each of these mechanisms in space and time in order to quantify their role in electron heating. For a case with a small guide field (20% of the magnitude of the reconnecting component), the curvature drift is the dominant source of electron heating. However, for a larger guide field (equal to the magnitude of the reconnecting component) electron acceleration by the curvature drift is comparable to that of the parallel electric field. In both cases, the heating by the gradient B drift is negligible in magnitude. It produces net cooling because the conservation of the magnetic moment and the drop of B during reconnection produce a decrease in the perpendicular electron energy. Heating by the curvature drift dominates in the outflow exhausts where bent field lines expand to relax their tension and is therefore distributed over a large area. In contrast, the parallel electric field is localized near X-lines. This suggests that acceleration by parallel electric fields may play a smaller role in large systems where the X-line occupies a vanishing fraction of the system. The curvature drift and the parallel electric field dominate the dynamics and drive parallel heating. A consequence is that the electron energy spectrum becomes extremely anisotropic at late time, which has important implications for quantifying the limits of electron acceleration due to synchrotron emission. An upper limit on electron energy gain that is substantially higher than earlier estimates is obtained by balancing reconnection drive with radiative loss. [ABSTRACT FROM AUTHOR]

The onset of the acceleration of ions during magnetic reconnection is explored via particle-in-cell simulations in the limit of a strong ambient guide field that self-consistently and simultaneously follow the motions of protons and a particles. Heating parallel to the local magnetic field during reconnection with a guide field is strongly reduced compared with the reconnection of anti-parallel magnetic fields. The dominant heating of thermal ions during guide field reconnection results from pickup behavior of ions during their entry into reconnection exhausts and dominantly produces heating perpendicular rather than parallel to the local magnetic field. Pickup behavior requires that the ion transit time across the exhaust boundary (with a transverse scale of the order of the ion sound Larmor radius) be short compared with the ion cyclotron period. This translates into a threshold in the strength of reconnecting magnetic field that favors the heating of ions with high mass-tocharge. A simulation with a broad initial current layer produces a reconnecting system in which the amplitude of the reconnecting magnetic field just upstream of the dissipation region increases with time as reconnection proceeds. The sharp onset of perpendicular heating when the pickup threshold is crossed is documented. A comparison of the time variation of the parallel and perpendicular ion heating with that predicted based on the strength of the reconnecting field establishes the scaling of ion heating with ambient parameters both below and above the pickup threshold. The relevance to observations of ion heating in the solar corona is discussed. [ABSTRACT FROM AUTHOR]

In observations of flare-heated electrons in the solar corona, a longstanding problem is the unexplained prolonged lifetime of the electrons compared to their transit time across the source. This suggests confinement. Recent particle-in-cell (PIC) simulations, which explored the transport of pre-accelerated hot electrons through ambient cold plasma, showed that the formation of a highly localized electrostatic potential drop, in the form of a double layer (DL), significantly inhibited the transport of hot electrons. The effectiveness of confinement by a DL is linked to the strength of the DL as defined by its potential drop. In this work, we investigate the scaling of the DL strength with the hot electron temperature by PIC simulations and find a linear scaling. We demonstrate that the strength is limited by the formation of parallel shocks. Based on this, we analytically determine the maximum DL strength, and also find a linear scaling with the hot electron temperature. The DL strength obtained from the analytic calculation is comparable to that from the simulations. At the maximum strength, the DL is capable of confining a significant fraction of hot electrons in the source. [ABSTRACT FROM AUTHOR]

The nonlinear development of the strong Buneman instability and the associated fast electron heating in thin current layers with Ωe/ωpe<1 is explored. Phase mixing of the electrons in wave potential troughs and a rapid increase in temperature are observed during the saturation of the instability. We show that the motion of trapped electrons can be described using a Hamiltonian formalism in the adiabatic approximation. The process of separatrix crossing as electrons are trapped and de-trapped is irreversible and guarantees that the resulting electron energy gain is a true heating process. [ABSTRACT FROM AUTHOR]

Recently, 'phase diagrams' of magnetic reconnection were developed to graphically organize the present knowledge of what type, or phase, of reconnection is dominant in systems with given characteristic plasma parameters. Here, a number of considerations that require caution in using the diagrams are pointed out. First, two known properties of reconnection are omitted from the diagrams: the history dependence of reconnection and the absence of reconnection for small Lundquist number. Second, the phase diagrams mask a number of features. For one, the predicted transition to Hall reconnection should be thought of as an upper bound on the Lundquist number, and it may happen for considerably smaller values. Second, reconnection is never 'slow,' it is always 'fast' in the sense that the normalized reconnection rate is always at least 0.01. This has important implications for reconnection onset models. Finally, the definition of the relevant Lundquist number is nuanced and may differ greatly from the value based on characteristic scales. These considerations are important for applications of the phase diagrams. This is demonstrated by example for solar flares, where it is argued that it is unlikely that collisional reconnection can occur in the corona. [ABSTRACT FROM AUTHOR]

The heating and acceleration of ions during magnetic reconnection relevant to coronal heating and flares is explored via particle-in-cell (PIC) simulations and analytic modeling. We show that the dominant heating mechanism of sub-Alvénic ions during reconnection with a guide field, the case of greatest relevance to the corona, results from pickup behavior during the entry into reconnection exhausts, which produces effective thermal speeds of the order of the Alfvén velocity based on the reconnecting magnetic field. There is a mass-to-charge ( M/ Q) threshold for pickup behavior that favors the heating of high- M/ Q ions. Ions below the threshold gain little energy beyond that associated with convective flow. PIC simulations with protons and alphas confirm the pickup threshold. The enhanced heating of high M/ Q ions is consistent with observations of abundance enhancements of such ions in impulsive flares. In contrast to anti-parallel reconnection, the temperature increment during ion pickup is dominantly transverse, rather than parallel, to the local magnetic field. The simulations reveal the dominance of perpendicular heating, which is also consistent with observations. We suggest that the acceleration of ions to energies well above that associated with the Alfvén speed takes place during the interaction with many magnetic islands, which spontaneously develop during 3-D guide-field reconnection. The exploration of particle acceleration in a full 3-D multi-island system remains computationally intractable. Instead we explore ion acceleration in a multi-current layer system with low initial β. Ion energy gain takes place due to Fermi reflection in contracting and merging magnetic islands. Particle acceleration continues until the available magnetic free-energy is significantly depleted so that the pressure of energetic ions approaches that of the reconnecting field. Depending on the strength of the ambient guide field and in spite of the low initial plasma β, the dominance of parallel heating can cause significant regions of the plasma to exceed the marginal firehose condition. [ABSTRACT FROM AUTHOR]

Magnetic islands or flux ropes produced by magnetic reconnection have been observed on the magnetopause, in the magnetotail, and in coronal current sheets. Particle-in-cell simulations of magnetic reconnection with a guide field produce elongated electron current layers that spontaneously produce secondary islands. Here, we explore the seed mechanism that gives birth to these islands. The most commonly suggested theory for island formation is the tearing instability. We demonstrate that in our simulations these structures typically start out, not as magnetic islands, but as electron flow vortices within the electron current sheet. When some of these vortices first form, they do not coincide with closed magnetic field lines, as would be the case if they were islands. Only after they have grown larger than the electron skin depth do they couple to the magnetic field and seed the growth of finite-sized islands. The streaming of electrons along the magnetic separatrix produces the flow shear necessary to drive an electron Kelvin-Helmholtz instability and produce the initial vortices. The conditions under which this instability is the dominant mechanism for seeding magnetic islands are explored. [ABSTRACT FROM AUTHOR]

The structure of shocks that form at the exhaust boundaries during collisionless reconnection of anti-parallel fields is studied using particle-in-cell (PIC) simulations and modeling based on the anisotropic magnetohydrodynamic equations. Large-scale PIC simulations of reconnection and companion Riemann simulations of shock development demonstrate that the pressure anisotropy produced by counterstreaming ions within the exhaust prevents the development of classical Petschek switch-off-slow shocks (SSS). The shock structure that does develop is controlled by the firehose stability parameter &eh;=1-μ0(P|-P⊥)/B2 through its influence on the speed order of the intermediate and slow waves. Here, P| and P⊥ are the pressure parallel and perpendicular to the local magnetic field. The exhaust boundary is made up of a series of two shocks and a rotational wave. The first shock takes &eh; from unity upstream to a plateau of 0.25 downstream. The condition &eh;=0.25 is special because at this value, the speeds of nonlinear slow and intermediate waves are degenerate. The second slow shock leaves &eh;=0.25 unchanged but further reduces the amplitude of the reconnecting magnetic field. Finally, in the core of the exhaust, &eh; drops further and the transition is completed by a rotation of the reconnecting field into the out-of-plane direction. The acceleration of the exhaust takes place across the two slow shocks but not during the final rotation. The result is that the outflow speed falls below that expected from the Walén condition based on the asymptotic magnetic field. A simple analytic expression is given for the critical value of &eh; within the exhaust below which SSSs no longer bound the reconnection outflow. [ABSTRACT FROM AUTHOR]

Simulations of collisionless oblique propagating slow shocks have revealed the existence of a transition associated with a critical temperature anisotropy &eh; = 1 - μ0(P|| - P⊥)/B2 = 0.25 (Y.-H. Liu, J. F. Drake, and M. Swisdak, Phys. Plasmas 18, 062110 (2011)). An explanation for this phenomenon is proposed here based on anisotropic fluid theory, in particular, the anisotropic derivative nonlinear-Schrödinger-Burgers equation, with an intuitive model of the energy closure for the downstream counter-streaming ions. The anisotropy value of 0.25 is significant because it is closely related to the degeneracy point of the slow and intermediate modes and corresponds to the lower bound of the coplanar to non-coplanar transition that occurs inside a compound slow shock (SS)/rotational discontinuity (RD) wave. This work implies that it is a pair of compound SS/RD waves that bound the outflows in magnetic reconnection, instead of a pair of switch-off slow shocks as in Petschek's model. This fact might explain the rareness of in-situ observations of Petschek-reconnection-associated switch-off slow shocks. [ABSTRACT FROM AUTHOR]

The propagation of reconnection signatures and their associated energy are examined using kinetic particle-in-cell simulations and Cluster satellite observations. It is found that the quadrupolar out-of-plane magnetic field near the separatrices is associated with a kinetic Alfvén wave. For magnetotail parameters, the parallel propagation of this wave is super-Alfvénic (V∥ ~ 1500-5500 km/s) and generates substantial Poynting flux (S -~ 10~-10~ W/m2) consistent with Cluster observations of magnetic reconnection. This Poynting flux substantially exceeds that due to frozen-in ion bulk outflows and is sufficient to generate white light aurora in Earth's ionosphere. [ABSTRACT FROM AUTHOR]

During magnetic reconnection, the field lines must break and reconnect to release the energy that drives solar and stellar flares and other explosive events in space and in the laboratory. Exactly how this happens has been unclear, because dissipation is needed to break magnetic field lines and classical collisions are typically weak. Ion-electron drag arising from turbulence, dubbed 'anomalous resistivity', and thermal momentum transport are two mechanisms that have been widely invoked. Measurements of enhanced turbulence near reconnection sites in space and in the laboratory support the anomalous resistivity idea but there has been no demonstration from measurements that this turbulence produces the necessary enhanced drag. Here we report computer simulations that show that neither of the two previously favoured mechanisms controls how magnetic field lines reconnect in the plasmas of greatest interest, those in which the magnetic field dominates the energy budget. Rather, we find that when the current layers that form during magnetic reconnection become too intense, they disintegrate and spread into a complex web of filaments that causes the rate of reconnection to increase abruptly. This filamentary web can be explored in the laboratory or in space with satellites that can measure the resulting electromagnetic turbulence. [ABSTRACT FROM AUTHOR]

A 2-D Riemann problem is designed to study the development and dynamics of the slow shocks that are thought to form at the boundaries of reconnection exhausts. Simulations are carried out for varying ratios of normal magnetic field to the transverse upstream magnetic field (i.e., propagation angle with respect to the upstream magnetic field). When the angle is sufficiently oblique, the simulations reveal a large firehose-sense (P∥>P⊥) temperature anisotropy in the downstream region, accompanied by a transition from a coplanar slow shock to a non-coplanar rotational mode. In the downstream region the firehose stability parameter &eh;=1-μ0(P∥-P⊥)/B2 tends to plateau at 0.25. This balance arises from the competition between counterstreaming ions, which drive &eh; down, and the scattering due to ion inertial scale waves, which are driven unstable by the downstream rotational wave. At very oblique propagating angles, 2-D turbulence also develops in the downstream region. [ABSTRACT FROM AUTHOR]

Mesoscale forecasts for the Los Angeles basin made with the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5) exhibited a moderate to substantial warm temperature bias for extended periods in the summer months. A similar bias also was thought to exist in forecasts made using version 2.2 of the Weather Research and Forecasting Model (WRF v2.2). To address these biases, two sources of anthropogenic moisture were analyzed: commercial irrigation and outdoor domestic water use. These represent substantial amounts of equivalent precipitation that are not accounted for in normal WRF execution. This is especially true for the summer months when little or no precipitation occurs in the area. A method for estimating the temporal and spatial distributions of these two sources was developed and the resulting database was applied to model runs. The addition of these anthropogenic moisture sources is an important source of enthalpy, which results in significant cooling in WRF. However, in the course of the analysis it was determined that the biases in WRF were much smaller than had been thought. Also, despite producing significant cooling, the addition of anthropogenic moisture made only modest improvements in forecast skill, and only for some hours of the day, indicating that more research is necessary on how the physical processes are handled in WRF, and how the anthropogenic moisture is distributed during the forecast period. [ABSTRACT FROM AUTHOR]

This letter describes a statistical model of the dynamics of magnetic islands in very large current layers that develop in space plasma. Two parameters characterize the island distribution: the flux ψ contained in the island and the area A it encloses. The integrodifferential evolution equation for this distribution function is based on rules that govern the small-scale generation of secondary islands, the rates of island growth, and island merging. The numerical solutions of this equation produce island distributions relevant to the magnetosphere and solar corona. The solution of a differential equation for large islands explicitly shows the role merging plays in island growth. [ABSTRACT FROM AUTHOR]

Abstract: Eight polyethers with three to nine stereocentres have been synthesised using chiral base methodology. Examination of the polyethers by CD spectroscopy revealed clear differences between homochiral isomers and isomers with a heterochiral relationship between an ‘inner’ layer of stereocentres and an ‘outer’ layer of stereocentres. A hypothesis to account for these differences has been advanced. [Copyright &y& Elsevier]

Purpose: Prostate cancer (CaP) is disproportionately prevalent among black, compared to white, men. Additionally, men with a family history of CaP have 75% to 80% higher risk of CaP. Therefore we examined racial variation in the association of family history of CaP and self-reported prostate-specific antigen (PSA) testing in the nationally-representative National Health Interview Survey (NHIS). Methods: Data were obtained from the 2005 NHIS, including the Cancer Control Module supplement. We restricted the study sample to men over the age of 40 who reported having “ever heard of a PSA test” (N = 1,744). Men were considered to have a positive family history if either their biological father or at least one biological brother had been diagnosed with CaP. SUDAAN 9.0 was used to perform descriptive and multivariable logistic regression analyses. Results: Men with a family history of CaP were more likely to have a PSA test than those who never had a PSA test (odds ratio [OR] = 1.8; 95% confidence interval [CI]: 1.3–2.5). Among blacks, men with a family history were not significantly more likely to have a PSA test. Conclusions: Despite having the highest risk of cancer, black men with a family history are not screened more than black men without a family history. [Copyright &y& Elsevier]

The results of large-scale, particle-in-cell simulations are presented on the role of Hall electric and magnetic fields on the structure of the electron dissipation region and outflow exhaust during the collisionless magnetic reconnection of antiparallel fields. The simulations reveal that the whistler wave plays the key role in driving the electrons away from the magnetic x-line. Further downstream the electron outflow exhaust consists of a narrow super-Alfvénic jet, which remains collimated far downstream of the x-line, flanked by a pedestal whose width increases monotonically with increasing distance downstream. The open outflow exhaust, which is required for fast reconnection in large systems, is driven by the Hall electric and magnetic fields. Finally, it is the whistler that ultimately facilitates fast reconnection by diverting the electrons flowing toward the current layer into the outflow direction and thereby limiting the length of this layer. The results are contrasted with reconnection in an electron-positron plasma where the Hall fields are absent. The consequence of the expanding outflow exhaust is that, consistent with recent observations, the extended super-Alfvénic electron outflow jet carries a smaller and smaller fraction of the outflowing electrons with increasing distance downstream of the x-line. The results suggest that the structure of the electron current layer and exhaust in simulations might be sensitive to boundary conditions unless the simulation boundary along the outflow direction is sufficiently far from the x-line. [ABSTRACT FROM AUTHOR]

BACKGROUND: The death rate for prostate cancer (PrCA), the most commonly diagnosed cancer in African-American (AA) men, is twice the rate of European-American (EA) men. AA men in South Carolina have the highest age-adjusted death rate in the nation. Studies have shown that treatment offered to AA men with PrCA is systematically different from that offered to EA men. METHODS: Surveys were mailed to 1,866 men in South Carolina with a diagnosis of PrCA. South Carolina men diagnosed with PrCA between 1996 and 2002 were eligible to participate. We performed a descriptive assessment of the factors that influenced PrCA treatment decisions. RESULTS: The treatment choices of AA men were significantly more likely to be influenced by pain and significantly less likely to be influenced by potential for cure compared to EA men. CONCLUSIONS: Providers must be cognizant of the factors that influence treatment, particularly in AA men. Despite the national undertaking to eliminate health disparities, the United States is far from implementing a comprehensive focus on the health of AA men, despite their elevated PrCA morbidity and mortality rates. [ABSTRACT FROM AUTHOR]

It was recently shown that the slow (collisional) Sweet-Parker and the fast (collisionless) Hall magnetic reconnection solutions simultaneously exist for a wide range of resistivities; reconnection is bistable [Cassak, Shay, and Drake, Phys. Rev. Lett., 95, 235002 (2005)]. When the thickness of the dissipation region becomes smaller than a critical value, the Sweet-Parker solution disappears and fast reconnection ensues, potentially explaining how large amounts of magnetic free energy can accrue without significant release before the onset of fast reconnection. Two-fluid numerical simulations extending the previous results for anti-parallel reconnection (where the critical thickness is the ion skin depth) to component reconnection with a large guide field (where the critical thickness is the thermal ion Larmor radius) are presented. Applications to laboratory experiments of magnetic reconnection and the sawtooth crash are discussed. [ABSTRACT FROM AUTHOR]

Although there may be some disagreement as to the precise definition of chaos, it is generally characterized as a nonlinear, deterministic phenomenon. For a class of common chaotic systems, this paper introduces equivalent descriptions in the form of linear, time-invariant (LTI) systems with random inputs. Such models have many advantages as they are much better matched to traditional signal and system theory. The outputs of these LTI systems will be shown to be indistinguishable from the outputs of the corresponding chaotic systems. Specifically, for a given random input the LTI system will generate exactly the same output as the corresponding chaotic map with a certain initial condition. [ABSTRACT FROM AUTHOR]

A long-standing problem in the study of space and astrophysical plasmas is to explain the production of energetic electrons as magnetic fields ‘reconnect’ and release energy. In the Earth's magnetosphere, electron energies reach hundreds of thousands of electron volts (refs 1–3), whereas the typical electron energies associated with large-scale reconnection-driven flows are just a few electron volts. Recent observations further suggest that these energetic particles are produced in the region where the magnetic field reconnects. In solar flares, upwards of 50 per cent of the energy released can appear as energetic electrons. Here we show that electrons gain kinetic energy by reflecting from the ends of the contracting ‘magnetic islands’ that form as reconnection proceeds. The mechanism is analogous to the increase of energy of a ball reflecting between two converging walls—the ball gains energy with each bounce. The repetitive interaction of electrons with many islands allows large numbers to be efficiently accelerated to high energy. The back pressure of the energetic electrons throttles reconnection so that the electron energy gain is a large fraction of the released magnetic energy. The resultant energy spectra of electrons take the form of power laws with spectral indices that match the magnetospheric observations. [ABSTRACT FROM AUTHOR]

This article reviews the current literature on pain in Parkinson's disease to include the anatomy, pathology, and both basic and clinical science research. It explores treatment methods to include medication management as well as deep brain stimulation. [ABSTRACT FROM AUTHOR]

The article explores the historical background of a seminal collection of polyphony held in the Magliabechiano Archive of the Biblioteca Nazionale Centrale in Florence, Italy. The four partbooks contain several Italian, French secular as well as Latin sacred works. It has been assumed that the partbooks were compiled for one of the major families of Florence. Several icons were considered to established the origin of the books.

Magnetic island equilibria resulting from reconnection in magnetohydrodynamic (MHD) simulations are explored in a two-dimensional slab geometry. Magnetic islands are evolved to finite amplitude with a nonzero resistivity. The resistivity and flows are then set to zero and the system is allowed to relax toward equilibrium. A y-type singular current layer in the equilibrium state is identified for all but systems with the smallest values of the tearing mode stability parameter Δ′. It is shown that the length of the equilibrium y line tracks the length of the Sweet–Parker current layer that develops during reconnection. This suggests that the formation of Sweet–Parker current layers during magnetic reconnection in the resistive MHD model is a consequence of the presence of a singularity in post-reconnection state. A threshold in Δ′ for singular behavior is also identified. [ABSTRACT FROM AUTHOR]

The scaling of the reconnection rate is examined in situations in which the equilibrium current supporting a reversed magnetic field has a spatial scale length that is much greater than all nonmagnetohydrodynamic (non-MHD) kinetic scales. In this case, denoted as embedded reconnection, the narrow non-MHD region around the x-line where dissipation is important is embedded inside of a much larger equilibrium current sheet. In this system, the magnetic field just upstream of this non-MHD region, Bd changes significantly during the reconnection process. This wide equilibrium current sheet is contrasted with the very thin equilibrium current sheets of width c/ωpi used in previous simulations to establish the importance of the Hall term in Ohm's law in allowing fast reconnection in large scale collisionless systems. In the present study we lay out a procedure for determining Bd directly from simulation data and use this value to renormalize the reconnection rate using Sweet-Parker-like scaling arguments. Using two-dimensional two-fluid simulations, we find that the time evolution of the reconnection process can he broken into two phases: A developmental phase that is quite long and strongly dependent on system size and presumably the dissipation mechanisms, and a fast asymptotic phase in which the flow velocity into the x-line is on the order of 0.1 of the Alfvén speed based on Bd. The reconnection rate during the asymptotic phase is independent of system size and the majority of island growth and flux reconnection occurs during this phase. The time to reconnect a significant amount of magnetic flux is roughly consistent with solar flare timescales. [ABSTRACT FROM AUTHOR]