3,682 research outputs found
Particle Acceleration in Cosmic Sites - Astrophysics Issues in our Understanding of Cosmic Rays
Laboratory experiments to explore plasma conditions and stimulated particle
acceleration can illuminate aspects of the cosmic particle acceleration
process. Here we discuss the cosmic-ray candidate source object variety, and
what has been learned about their particle-acceleration characteristics. We
identify open issues as discussed among astrophysicists. -- The cosmic ray
differential intensity spectrum is a rather smooth power-law spectrum, with two
kinks at the "knee" (~10^15 eV) and at the "ankle" (~3 10^18 eV). It is unclear
if these kinks are related to boundaries between different dominating sources,
or rather related to characteristics of cosmic-ray propagation. We believe that
Galactic sources dominate up to 10^17 eV or even above, and the extragalactic
origin of cosmic rays at highest energies merges rather smoothly with Galactic
contributions throughout the 10^15--10^18 eV range. Pulsars and supernova
remnants are among the prime candidates for Galactic cosmic-ray production,
while nuclei of active galaxies are considered best candidates to produce
ultrahigh-energy cosmic rays of extragalactic origin. Acceleration processes
are related to shocks from violent ejections of matter from energetic sources
such as supernova explosions or matter accretion onto black holes. Details of
such acceleration are difficult, as relativistic particles modify the structure
of the shock, and simple approximations or perturbation calculations are
unsatisfactory. This is where laboratory plasma experiments are expected to
contribute, to enlighten the non-linear processes which occur under such
conditions.Comment: accepted for publication in EPJD, topical issue on Fundamental
physics and ultra-high laser fields. From review talk at "Extreme Light
Infrastructure" workshop, Sep 2008. Version-2 May 2009: adjust some wordings
and references at EPJD proofs stag
Recommended from our members
Declines in cigarette smoking among US adolescents and young adults: indications of independence from e-cigarette vaping surge
ObjectiveTo compare trends in cigarette smoking and nicotine vaping among US population aged 17-18 years and 18-24 years.MethodsRegression analyses identified trends in ever and current use of cigarettes and e-cigarettes, using three US representative surveys from 1992 to 2022.ResultsFrom 1997 to 2020, cigarette smoking prevalence among those aged 18-24 years decreased from 29.1% (95% CI 27.4% to 30.7%) to 5.4% (95% CI 3.9% to 6.9%). The decline was highly correlated with a decline in past 30-day smoking among those aged 17-18 years (1997: 36.8% (95% CI 35.6% to 37.9%; 2022: 3.0% (95% CI 1.8% to 4.1%). From 2017 to 2019, both ever-vaping and past 30-day nicotine vaping (11.0% to 25.5%) surged among those 17-18 years, however there was no increase among those aged 18-24 years. Regression models demonstrated that the surge in vaping was independent of the decline in cigarette smoking. In the 24 most populous US states, exclusive vaping did increase among those aged 18-24 years, from 1.7% to 4.0% to equivalent to 40% of the decline in cigarette smoking between 2014-15 and 2018-19. Across these US states, the correlation between the changes in vaping and smoking prevalence was low (r=0.11). In the two US states with >US$1/fluid mL tax on e-cigarettes in 2017, cigarette smoking declined faster than the US average.ConclusionsSince 1997, a large decline in cigarette smoking occurred in the US population under age 24 years, that was independent of the 2017-19 adolescent surge in past 30-day e-cigarette vaping. Further research is needed to assess whether the 2014-15 to 2018-19 increase in exclusive vaping in those aged 18-24 years is a cohort effect from earlier dependence on e-cigarette vaping as adolescents
Recommended from our members
Cigarette smoking decline among US young adults from 2000 to 2019, in relation to state-level cigarette price and tobacco control expenditure
ObjectiveTo investigate the association of state-level cigarette price and tobacco control expenditure with the large 2000-2019 decline in cigarette smoking among US 18-24 year-olds.MethodsSmoking behaviour was assessed in the 24 most populous US states using the 1992-2019 Tobacco Use Supplements to the Current Population Survey; association with price and expenditure was tested using adjusted logistic regression. States were ranked by inflation-adjusted average price and tobacco control expenditure and grouped into tertiles. State-specific time trends were estimated, with slope changes in 2001/2002 and 2010/2011.ResultsBetween 2000 and 2010, the odds of smoking among US young adults decreased by a third (adjusted OR, AOR 0.68, 95% CI 0.56 to 0.84). By 2019, these odds were one-quarter of their 2000 level (AOR 0.24, 95% CI 0.19 to 0.31). Among states in the lowest tertile of price/expenditure tobacco control activity, initially higher young adult smoking decreased by 13 percentage points from 2010 to 2018-2019, to a prevalence of 5.6% (95% CI 4.5% to 6.8%), equal to that in the highest tobacco-control tertile of states (6.5%, 95% CI 5.2% to 7.8%). Neither state tobacco control spending (AOR 1.0, 95% CI 0.999 to 1.002) nor cigarette price (AOR 0.96, 95% CI: 0.92 to 1.01) were associated with young adult smoking in statistical models. In 2019, seven states had prevalence over 3 SDs higher than the 24-state mean.ConclusionNational programmes may have filled a gap in state-level interventions, helping drive down the social acceptability of cigarette smoking among young adults across all states. Additional interventions are needed to assist high-prevalence states to further reduce smoking
COSMOCR: A Numerical Code for Cosmic Ray Studies in Computational Cosmology
We present COSMOCR, a numerical code for the investigation of cosmic ray
related studies in computational cosmology. The code follows the diffusive
shock acceleration, the mechanical and radiative energy losses and the spatial
transport of the supra-thermal particles in cosmic environment. Primary cosmic
ray electrons and ions are injected at shocks according to the thermal leakage
prescription. Secondary electrons are continuously injected as a results of p-p
inelastic collisions of primary cosmic ray ions and thermal background nuclei.
The code consists of a conservative, finite volume method with a power-law
sub-grid model in momentum space. Two slightly different schemes are
implemented depending on the stiffness of the cooling terms. Comparisons of
numerical results with analytical solution for a number of tests of direct
interest show remarkable performance of the present code.Comment: 32 pages, 6 figures, Comp. Phys. Comm. in pres
The Global Evolution of Giant Molecular Clouds II: The Role of Accretion
We present virial models for the global evolution of giant molecular clouds.
Focusing on the presence of an accretion flow, and accounting for the amount of
mass, momentum, and energy supplied by accretion and star formation feedback,
we are able to follow the growth, evolution, and dispersal of individual giant
molecular clouds. Our model clouds reproduce the scaling relations observed in
both galactic and extragalactic clouds. We find that accretion and star
formation contribute contribute roughly equal amounts of turbulent kinetic
energy over the lifetime of the cloud. Clouds attain virial equilibrium and
grow in such a way as to maintain roughly constant surface densities, with
typical surface densities of order 50 - 200 Msun pc^-2, in good agreement with
observations of giant molecular clouds in the Milky Way and nearby external
galaxies. We find that as clouds grow, their velocity dispersion and radius
must also increase, implying that the linewidth-size relation constitutes an
age sequence. Lastly, we compare our models to observations of giant molecular
clouds and associated young star clusters in the LMC and find good agreement
between our model clouds and the observed relationship between H ii regions,
young star clusters, and giant molecular clouds.Comment: 23 Pages, 9 Figures. Accepted to Ap
Cerebellar Integrity in the Amyotrophic Lateral Sclerosis - Frontotemporal Dementia Continuum
Amyotrophic lateral sclerosis (ALS) and behavioural variant frontotemporal dementia (bvFTD) are multisystem neurodegenerative disorders that manifest overlapping cognitive, neuropsychiatric and motor features. The cerebellum has long been known to be crucial for intact motor function although emerging evidence over the past decade has attributed cognitive and neuropsychiatric processes to this structure. The current study set out i) to establish the integrity of cerebellar subregions in the amyotrophic lateral sclerosis-behavioural variant frontotemporal dementia spectrum (ALS-bvFTD) and ii) determine whether specific cerebellar atrophy regions are associated with cognitive, neuropsychiatric and motor symptoms in the patients. Seventy-eight patients diagnosed with ALS, ALS-bvFTD, behavioural variant frontotemporal dementia (bvFTD), most without C9ORF72 gene abnormalities, and healthy controls were investigated. Participants underwent cognitive, neuropsychiatric and functional evaluation as well as structural imaging using voxel-based morphometry (VBM) to examine the grey matter subregions of the cerebellar lobules, vermis and crus. VBM analyses revealed: i) significant grey matter atrophy in the cerebellum across the whole ALS-bvFTD continuum; ii) atrophy predominantly of the superior cerebellum and crus in bvFTD patients, atrophy of the inferior cerebellum and vermis in ALS patients, while ALS-bvFTD patients had both patterns of atrophy. Post-hoc covariance analyses revealed that cognitive and neuropsychiatric symptoms were particularly associated with atrophy of the crus and superior lobule, while motor symptoms were more associated with atrophy of the inferior lobules. Taken together, these findings indicate an important role of the cerebellum in the ALS-bvFTD disease spectrum, with all three clinical phenotypes demonstrating specific patterns of subregional atrophy that associated with different symptomology
Quantifying the Impact of Chronic Ischemic Injury on Clinical Outcomes in Acute Stroke With Machine Learning.
Acute stroke is often superimposed on chronic damage from previous cerebrovascular events. This background will inevitably modulate the impact of acute injury on clinical outcomes to an extent that will depend on the precise anatomical pattern of damage. Previous attempts to quantify such modulation have employed only reductive models that ignore anatomical detail. The combination of automated image processing, large-scale data, and machine learning now enables us to quantify the impact of this with high-dimensional multivariate models sensitive to individual variations in the detailed anatomical pattern. We introduce and validate a new automated chronic lesion segmentation routine for use with non-contrast CT brain scans, combining non-parametric outlier-detection score, Zeta, with an unsupervised 3-dimensional maximum-flow, minimum-cut algorithm. The routine was then applied to a dataset of 1,704 stroke patient scans, obtained at their presentation to a hyper-acute stroke unit (St George's Hospital, London, UK), and used to train a support vector machine (SVM) model to predict between low (0-2) and high (3-6) pre-admission and discharge modified Rankin Scale (mRS) scores, quantifying performance by the area under the receiver operating curve (AUROC). In this single center retrospective observational study, our SVM models were able to differentiate between low (0-2) and high (3-6) pre-admission and discharge mRS scores with an AUROC of 0.77 (95% confidence interval of 0.74-0.79), and 0.76 (0.74-0.78), respectively. The chronic lesion segmentation routine achieved a mean (standard deviation) sensitivity, specificity and Dice similarity coefficient of 0.746 (0.069), 0.999 (0.001), and 0.717 (0.091), respectively. We have demonstrated that machine learning models capable of capturing the high-dimensional features of chronic injuries are able to stratify patients-at the time of presentation-by pre-admission and discharge mRS scores. Our fully automated chronic stroke lesion segmentation routine simplifies this process, and utilizes routinely collected CT head scans, thereby facilitating future large-scale studies to develop supportive clinical decision tools
Syntactic comprehension deficits across the FTD-ALS continuum
To establish the frequency, severity, relationship to bulbar symptoms, and neural correlates of syntactic comprehension deficits across the frontotemporal dementia–amyotrophic lateral sclerosis (FTD-ALS) disease spectrum. In total, 85 participants were included in the study; 20 amyotrophic lateral sclerosis (ALS), 15 FTD-ALS, 27 progressive nonfluent aphasia (PNFA), and 23 controls. Syntactic comprehension was evaluated in ALS, FTD-ALS, PNFA, and controls using the Test for Reception of Grammar. Voxel-based morphometry examined neuroanatomical correlates of performance. Syntactic comprehension deficits were detected in 25% of ALS (p = 0.011), 92.9% of FTD-ALS (p < 0.001), and 81.5% of PNFA (p < 0.001) patients. FTD-ALS was disproportionately impaired compared to PNFA. Impaired Test for Reception of Grammar performance was frequent in ALS with early bulbar involvement but did not correlate with bulbar impairment overall. Left peri-insular atrophy correlated with syntactic comprehension deficits. Syntactic comprehension deficits are frequent in FTD-ALS, more severe than in PNFA, and related to left peri-insular atrophy. A significant minority of ALS patients are impaired, but the relationship between bulbar symptoms and syntactic impairment is not understood
Cosmic-ray composition and its relation to shock acceleration by supernova remnants
An overview is given on the present status of the understanding of the origin
of galactic cosmic rays. Recent measurements of charged cosmic rays and photons
are reviewed. Their impact on the contemporary knowledge about the sources and
acceleration mechanisms of cosmic rays and their propagation through the Galaxy
is discussed. Possible reasons for the knee in the energy spectrum and
scenarios for the end of the galactic cosmic-ray component are described.Comment: Invited talk given at the 36th COSPAR Scientific Assembly Beijing,
China, 16 -- 23 July 2006 - submitted to Advances in Space Research -
comments are welcom
From 10 Kelvin to 10 TeraKelvin: Insights on the Interaction Between Cosmic Rays and Gas in Starbursts
Recent work has both illuminated and mystified our attempts to understand
cosmic rays (CRs) in starburst galaxies. I discuss my new research exploring
how CRs interact with the ISM in starbursts. Molecular clouds provide targets
for CR protons to produce pionic gamma rays and ionization, but those same
losses may shield the cloud interiors. In the densest molecular clouds, gamma
rays and Al-26 decay can provide ionization, at rates up to those in Milky Way
molecular clouds. I then consider the free-free absorption of low frequency
radio emission from starbursts, which I argue arises from many small, discrete
H II regions rather than from a "uniform slab" of ionized gas, whereas
synchrotron emission arises outside them. Finally, noting that the hot
superwind gas phase fills most of the volume of starbursts, I suggest that it
has turbulent-driven magnetic fields powered by supernovae, and that this phase
is where most synchrotron emission arises. I show how such a scenario could
explain the far-infrared radio correlation, in context of my previous work. A
big issue is that radio and gamma-ray observations imply CRs also must interact
with dense gas. Understanding how this happens requires a more advanced
understanding of turbulence and CR propagation.Comment: Conference proceedings for "Cosmic-ray induced phenomenology in
star-forming environments: Proceedings of the 2nd Session of the Sant Cugat
Forum of Astrophysics" (April 16-19, 2012). 16 pages, 5 figure
- …