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  1. ABSTRACT Long-term observations have shown that black hole X-ray binaries exhibit strong, aperiodic variability on time-scales of a few milliseconds to seconds. The observed light curves display various characteristic features like a lognormal distribution of flux and a linear rms–flux relation, which indicate that the underlying variability process is stochastic in nature. It is also thought to be intrinsic to accretion. This variability has been modelled as inward propagating fluctuations of mass accretion rate, although the physical process driving the fluctuations remains puzzling. In this work, we analyse five exceptionally long-duration general relativistic magnetohydrodynamic (GRMHD) simulations of optically thin, geometrically thick, black hole accretion flows to look for hints of propagating fluctuations in the simulation data. We find that the accretion profiles from these simulations do show evidence for inward propagating fluctuations below the viscous frequency by featuring strong radial coherence and positive time lags when comparing smaller to larger radii, although these time lags are generally shorter than the viscous time-scale and are frequency-independent. Our simulations also support the notion that the fluctuations in $\dot{M}$ build up in a multiplicative manner, as the simulations exhibit linear rms–mass flux relations, as well as lognormal distributions of their mass fluxes. Whenmore »combining the mass fluxes from the simulations with an assumed emissivity profile, we additionally find broad agreement with observed power spectra and time lags, including a recovery of the frequency dependency of the time lags.« less
  2. Abstract The Environmental Effects Assessment Panel of the Montreal Protocol under the United Nations Environment Programme evaluates effects on the environment and human health that arise from changes in the stratospheric ozone layer and concomitant variations in ultraviolet (UV) radiation at the Earth’s surface. The current update is based on scientific advances that have accumulated since our last assessment (Photochem and Photobiol Sci 20(1):1–67, 2021). We also discuss how climate change affects stratospheric ozone depletion and ultraviolet radiation, and how stratospheric ozone depletion affects climate change. The resulting interlinking effects of stratospheric ozone depletion, UV radiation, and climate change are assessed in terms of air quality, carbon sinks, ecosystems, human health, and natural and synthetic materials. We further highlight potential impacts on the biosphere from extreme climate events that are occurring with increasing frequency as a consequence of climate change. These and other interactive effects are examined with respect to the benefits that the Montreal Protocol and its Amendments are providing to life on Earth by controlling the production of various substances that contribute to both stratospheric ozone depletion and climate change.
    Free, publicly-accessible full text available March 1, 2023
  3. ABSTRACT We present 3D general relativistic magnetohydrodynamic simulations of zero angular momentum accretion around a rapidly rotating black hole, modified by the presence of initially uniform magnetic fields. We consider several angles between the magnetic field direction and the black hole spin. In the resulting flows, the mid-plane dynamics are governed by magnetic reconnection-driven turbulence in a magnetically arrested (or a nearly arrested) state. Electromagnetic jets with outflow efficiencies ∼10–200 per cent occupy the polar regions, reaching several hundred gravitational radii before they dissipate due to the kink instability. The jet directions fluctuate in time and can be tilted by as much as ∼30○ with respect to black hole spin, but this tilt does not depend strongly on the tilt of the initial magnetic field. A jet forms even when there is no initial net vertical magnetic flux since turbulent, horizon-scale fluctuations can generate a net vertical field locally. Peak jet power is obtained for an initial magnetic field tilted by 40○–80○ with respect to the black hole spin because this maximizes the amount of magnetic flux that can reach the black hole. These simulations may be a reasonable model for low luminosity black hole accretion flows such as Sgr A* ormore »M87.« less
  4. Recent studies reveal that at large friction Reynolds number delta^+ the outer, inertially-dominated region of the turbulent boundary layer is composed of large scale zones of uniform momentum segregated by narrow fissures of concentrated vorticity. Experiments show that, when scaled by the boundary layer thickness, the fissure thickness is O(1/sqrt{delta^+}), while the dimensional jump in streamwise velocity across each fissure scales in proportion to the friction velocity u_tau. A simple model that exploits these essential elements of the turbulent boundary layer structure at large delta^+ is developed. First, a master wall-normal profile of streamwise velocity is constructed by placing a discrete number of fissures across the boundary layer. The number of fissures and their wall-normal locations follow scalings informed by analysis of the mean momentum equation. The fissures are then randomly displaced in the wall-normal direction, exchanging momentum as they move, to create an instantaneous velocity profile. This process is repeated to generate ensembles of streamwise velocity profiles from which statistical moments are computed. The modelled statistical moments are shown to agree remarkably well with those acquired from direct numerical simulations of turbulent channel flow at large delta^+. In particular, the model robustly reproduces the empirically observed sub-Gaussian behaviour formore »the skewness and kurtosis profiles over a large range of input parameters.« less
  5. Growing complexity and magnitude of the challenges facing humanity require new ways of understanding and operationalizing solutions for more healthy, sustainable, secure, and joyful living. Developed almost contemporaneously but separately, the National Academy of Engineering's 14 Grand Challenges (GCs) and United Nation’s 17 Sustainable Development Goals (GCs) describe and call for solutions to these challenges. During the 2017 meetings for the UNESCO Kick-off for Engineering Report II in Beijing, the Global Grand Challenges Summit in Washington, DC, and the World Engineering Education Forum (WEEF) in Malaysia, we expanded our work to include international perspectives on ways that the GCs and SDGs could be more strongly connected. Within this context we ask, "How can educators integrate best practices to nurture and support development of globally competent students who will reach the goals as the Engineers of 2020?" and "How can connectivity and alignment of curricula to the GCs and SDGs foster students’ development?" Conclusions from the UNESCO’s meeting were that educators and stakeholders still have much to do with respect to sharing the 17 SDGs with engineering audiences around the world. This conclusion was reiterated at WEEF when an informal poll among participants from around the world revealed that knowledge ofmore »both the GCs and the SDGs was not as wide-spread as we had initially assumed. There were several engineering educators who were learning about both of these constructs for the very first time. This led to concerns posed by students participating in the Malaysia conference as part of the Student Platform for Engineering Education Development (World SPEED). The student teams from India, Colombia, Brazil, and Korea acknowledged potential disadvantages associated with learning in the environments created by educators unequipped with knowledge of topics covered by the GCs, and the SDGs. The students were further concerned that their faculty and mentors would not be able to create educational environments that allow for development of intentional learning and conscientious projects associated the GCs and SDGs. The report here will discuss ways that the GCs and SDGs are driving international conversations about engineering curricula, diversity and inclusion, and partnerships for the goals.« less
  6. Abstract This assessment by the Environmental Effects Assessment Panel (EEAP) of the United Nations Environment Programme (UNEP) provides the latest scientific update since our most recent comprehensive assessment (Photochemical and Photobiological Sciences, 2019, 18, 595–828). The interactive effects between the stratospheric ozone layer, solar ultraviolet (UV) radiation, and climate change are presented within the framework of the Montreal Protocol and the United Nations Sustainable Development Goals. We address how these global environmental changes affect the atmosphere and air quality; human health; terrestrial and aquatic ecosystems; biogeochemical cycles; and materials used in outdoor construction, solar energy technologies, and fabrics. In many cases, there is a growing influence from changes in seasonality and extreme events due to climate change. Additionally, we assess the transmission and environmental effects of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is responsible for the COVID-19 pandemic, in the context of linkages with solar UV radiation and the Montreal Protocol.