Brain development continues actively during adolescence. Previous MRI studies have shown complex patterns of apparent loss of grey matter (GM) volume and increases in white matter (WM) volume and ...fractional anisotropy (FA), an index of WM microstructure. In this longitudinal study (mean follow-up=2.5±0.5 years) of 24 adolescents, we used a voxel-based morphometry (VBM)-style analysis with conventional T1-weighted images to test for age-related changes in GM and WM volumes. We also performed tract-based spatial statistics (TBSS) analysis of diffusion tensor imaging (DTI) data to test for age-related WM changes across the whole brain. Probabilistic tractography was used to carry out quantitative comparisons across subjects in measures of WM microstructure in two fiber tracts important for supporting speech and motor functions (arcuate fasciculus AF and corticospinal tract CST). The whole-brain analyses identified age-related increases in WM volume and FA bilaterally in many fiber tracts, including AF and many parts of the CST. FA changes were mainly driven by increases in parallel diffusivity, probably reflecting increases in the diameter of the axons forming the fiber tracts. FA values of both left and right AF (but not of the CST) were significantly higher at the end of the follow-up than at baseline. Over the same period, widespread reductions in the cortical GM volume were found. These findings provide imaging-based anatomical data suggesting that brain maturation in adolescence is associated with structural changes enhancing long-distance connectivities in different WM tracts, specifically in the AF and CST, at the same time that cortical GM exhibits synaptic “pruning”.
River deltas are built by cycles of lobe growth and abrupt channel shifts, or avulsions, that occur within the backwater zone of coastal rivers. Previous numerical models differ on the origin of ...backwater‐scaled avulsion nodes and their consistency with experimental data. To unify previous work, we developed a numerical model of delta growth that includes backwater hydrodynamics, river mouth progradation, relative sea level rise, variable flow regimes, and cycles of lobe growth, abandonment, and reoccupation. For parameter space applicable to lowland deltas, we found that flow variability is the primary mechanism to cause persistent avulsion nodes by focusing aggradation within the backwater zone. Backwater‐scaled avulsion nodes also occur under less likely scenarios of initially uniform bed slopes or during rapid relative sea level rise and marine transgression. Our findings suggest that flow variability is a fundamental control on long‐term delta morphodynamics.
Plain Language Summary
River deltas are important for farming and drinking water, human populations, and diverse wildlife. Rivers on deltas are unstable and abruptly change course every 10–1,000 years. These channel shifts are necessary for sustaining coastal landscapes and also pose significant hazards. Here we present a mathematical model that shows how rivers require occasional floods, similar to what is observed on natural rivers, to give rise to a predictable location where rivers shift their course. Model simulations without floods produce rivers that change course at random locations, unlike natural rivers. Our findings resolve differences in previous studies about the importance of floods and illustrate that occasional floods are necessary for natural delta growth.
Key Points
Rivers on lowland deltas have repeated avulsions at a preferential location at the delta apex that scales with the backwater length
A preferential avulsion node occurs due to flow variability that focuses bed aggradation in the backwater zone
A preferential node under constant discharge simulations in previous work resulted from assumed initial conditions of uniform bed slope
Extensive vertical deformation (>4.5 m) observed at Sierra Negra volcano Galápagos, Ecuador, between 1992 and the 2005 eruption led scientists to hypothesize that repeated faulting events relieved ...magma chamber overpressure and prevented eruption. To better understand the catalyst of the 2005 eruption, thermomechanical models are used to track the stress state and stability of the magma storage system during the 1992–2005 inflation events. Numerical experiments indicate that the host rock surrounding the Sierra Negra reservoir remained in compression with minimal changes in overpressure (~10 MPa) leading up to the 2005 eruption. The lack of tensile failure and minimal overpressure accumulation likely inhibited dike initiation and accommodated the significant inflation without the need for pressure relief through shallow trapdoor faulting events. The models indicate that static stress transfer due to the Mw 5.4 earthquake 3 hr prior to the eruption most likely triggered tensile failure and catalyzed the 2005 eruption.
Plain Language Summary
Tracking the stability of a magma system in the lead up to a volcanic eruption requires investigating both the pressure state of the magma reservoir and stress accumulation in the host rock. New coupled conduit flow‐magma reservoir pressurization models are used to evaluate the evolution of the magma reservoir of Sierra Negra volcano, Galápagos, in the lead up to its 2005 eruption. Stress calculations indicate that the magma reservoir was stable prior to the 2005 eruption and that the eruption was likely triggered by a Mw 5.4 earthquake that occurred 3 hours prior to the event. The new modeling approach has important implications for tracking the stress evolution of magma systems to evaluate future unrest and eruption triggering mechanisms at volcanoes worldwide.
Key Points
New thermomechanical models provide an estimation of magma system stability in the lead up to the 2005 eruption of Sierra Negra
Models suggest that Sierra Negra's magma system was in stable storage prior to eruption with minimal overpressure and no tensile failure
Coulomb static stress calculations indicate that a Mw 5.4 earthquake likely triggered the 2005 eruption
A large source of uncertainty in present understanding of the global carbon cycle is the distribution and dynamics of the soil organic carbon reservoir. Most of the organic carbon in soils is ...degraded to inorganic forms slowly, on timescales from centuries to millennia. Soil minerals are known to play a stabilizing role, but how spatial and temporal variation in soil mineralogy controls the quantity and turnover of long-residence-time organic carbon is not well known. Here we use radiocarbon analyses to explore interactions between soil mineralogy and soil organic carbon along two natural gradients-of soil-age and of climate-in volcanic soil environments. During the first ∼150,000 years of soil development, the volcanic parent material weathered to metastable, non-crystalline minerals. Thereafter, the amount of non-crystalline minerals declined, and more stable crystalline minerals accumulated. Soil organic carbon content followed a similar trend, accumulating to a maximum after 150,000 years, and then decreasing by 50% over the next four million years. A positive relationship between non-crystalline minerals and organic carbon was also observed in soils through the climate gradient, indicating that the accumulation and subsequent loss of organic matter were largely driven by changes in the millennial scale cycling of mineral-stabilized carbon, rather than by changes in the amount of fast-cycling organic matter or in net primary productivity. Soil mineralogy is therefore important in determining the quantity of organic carbon stored in soil, its turnover time, and atmosphere-ecosystem carbon fluxes during long-term soil development; this conclusion should be generalizable at least to other humid environments.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A significant fraction of the energy density of the interstellar medium is in the form of high-energy charged particles (cosmic rays). The origin of these particles remains uncertain. Although it is ...generally accepted that the only sources capable of supplying the energy required to accelerate the bulk of Galactic cosmic rays are supernova explosions, and even though the mechanism of particle acceleration in expanding supernova remnant (SNR) shocks is thought to be well understood theoretically, unequivocal evidence for the production of high-energy particles in supernova shells has proven remarkably hard to find. Here we report on observations of the SNR RX J1713.7 - 3946 (G347.3 - 0.5), which was discovered by ROSAT in the X-ray spectrum and later claimed as a source of high-energy γ-rays of TeV energies (1 TeV = 1012 eV). We present a TeV γ-ray image of the SNR: the spatially resolved remnant has a shell morphology similar to that seen in X-rays, which demonstrates that very-high-energy particles are accelerated there. The energy spectrum indicates efficient acceleration of charged particles to energies beyond 100 TeV, consistent with current ideas of particle acceleration in young SNR shocks.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Earth’s atmosphere is an integral part of the detector in ground-based imaging atmospheric Cherenkov telescope (IACT) experiments and has to be taken into account in the calibration. Atmospheric ...and hardware-related deviations from simulated conditions can result in the mis-reconstruction of primary particle energies and therefore of source spectra. During the eight years of observations with the High Energy Stereoscopic System (H.E.S.S.) in Namibia, the overall yield in Cherenkov photons has varied strongly with time due to gradual hardware aging, together with adjustments of the hardware components, and natural, as well as anthropogenic, variations of the atmospheric transparency. Here we present robust data selection criteria that minimize these effects over the full data set of the H.E.S.S. experiment and introduce the Cherenkov transparency coefficient as a new atmospheric monitoring quantity. The influence of atmospheric transparency, as quantified by this coefficient, on energy reconstruction and spectral parameters is examined and its correlation with the aerosol optical depth (AOD) of independent MISR satellite measurements and local measurements of atmospheric clarity is investigated.
Knowing the site of gamma-ray emission in active galactic nucleus jets will do much for our understanding of the physics of the source. In particular, if the emission region is close to the black ...hole then absorption of gamma-rays with photons from the broad-line region could become significant. Such absorption is predicted to produce two specific spectral breaks in the gamma-ray spectra of Flat Spectrum Radio Quasars (FSRQs). We test this hypothesis using three years of Fermi observations of nine bright FSRQs. A simple power-law fit to the spectrum of each source can be significantly improved by introducing a break, but the break energies are inconsistent with those predicted by the double-absorber model. In some cases the fit can be further improved by a log-parabola. In addition, by dividing the data from each source into two equal epochs we find that the best description of an object's spectrum often varies between a log-parabola and a broken power law.
Aims
Sustained engagement in type 1 diabetes self‐management behaviours is a critical element in achieving improvements in glycated haemoglobin (HbA1c) and minimising risk of complications. ...Evaluations of self‐management programmes, such as Dose Adjustment for Normal Eating (DAFNE), typically find that initial improvements are rarely sustained beyond 12 months. This study identified behaviours involved in sustained type 1 diabetes self‐management, their influences and relationships to each other.
Methods
A mixed‐methods study was conducted following the first two steps of the Behaviour Change Wheel framework. First, an expert stakeholder consultation identified behaviours involved in self‐management of type 1 diabetes. Second, three evidence sources (systematic review, healthcare provider‐generated ‘red flags’ and participant‐generated ‘frequently asked questions’) were analysed to identify and synthesise modifiable barriers and enablers to sustained self‐management. These were characterised according to the Capability‐Opportunity‐Motivation‐Behaviour (COM‐B) model.
Results
150 distinct behaviours were identified and organised into three self‐regulatory behavioural cycles, reflecting different temporal and situational aspects of diabetes self‐management: Routine (e.g. checking blood glucose), Reactive (e.g. treating hypoglycaemia) and Reflective (e.g. reviewing blood glucose data to identify patterns). Thirty‐four barriers and five enablers were identified: 10 relating to Capability, 20 to Opportunity and nine to Motivation.
Conclusions
Multiple behaviours within three self‐management cycles are involved in sustained type 1 diabetes self‐management. There are a wide range of barriers and enablers that should be addressed to support self‐management behaviours and improve clinical outcomes. The present study provides an evidence base for refining and developing type 1 diabetes self‐management programmes.
As soils develop in humid environments, rock-derived elements are gradually lost, and under constant conditions it seems that ecosystems should reach a state of profound and irreversible nutrient ...depletion. We show here that inputs of elements from the atmosphere can sustain the productivity of Hawaiian rainforests on highly weathered soils.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Astronomical adaptive optics (AO) systems are used to increase effective telescope resolution. However, they cannot be used to observe the whole sky since one or more natural guide stars of ...sufficient brightness must be found within the telescope field of view for the AO system to work. Even when laser guide stars are used, natural guide stars are still required to provide a constant position reference. Here, we introduce a technique to overcome this problem by using rotary unmanned aerial vehicles (UAVs) as a platform from which to produce artificial guide stars. We describe the concept that relies on the UAV being able to measure its precise relative position. We investigate the AO performance improvements that can be achieved, which in the cases presented here can improve the Strehl ratio by a factor of at least 2 for a 8 m class telescope. We also discuss improvements to this technique, which is relevant to both astronomical and solar AO systems.
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