701 research outputs found
Galactic cosmic rays on extrasolar Earth-like planets: II. Atmospheric implications
(abridged abstract) Theoretical arguments indicate that close-in terrestial
exoplanets may have weak magnetic fields. As described in the companion article
(Paper I), a weak magnetic field results in a high flux of galactic cosmic rays
to the top of the planetary atmosphere. We investigate effects that may result
from a high flux of galactic cosmic rays both throughout the atmosphere and at
the planetary surface. Using an air shower approach, we calculate how the
atmospheric chemistry and temperature change under the influence of galactic
cosmic rays for Earth-like (N_2-O_2 dominated) atmospheres. We evaluate the
production and destruction rate of atmospheric biosignature molecules. We
derive planetary emission and transmission spectra to study the influence of
galactic cosmic rays on biosignature detectability. We then calculate the
resulting surface UV flux, the surface particle flux, and the associated
equivalent biological dose rates. We find that up to 20% of stratospheric ozone
is destroyed by cosmic-ray protons. The reduction of the planetary ozone layer
leads to an increase in the weighted surface UV flux by two orders of magnitude
under stellar UV flare conditions. The resulting biological effective dose rate
is, however, too low to strongly affect surface life. We also examine the
surface particle flux: For a planet with a terrestrial atmosphere, a reduction
of the magnetic shielding efficiency can increase the biological radiation dose
rate by a factor of two. For a planet with a weaker atmosphere (with a surface
pressure of 97.8 hPa), the planetary magnetic field has a much stronger
influence on the biological radiation dose, changing it by up to two orders of
magnitude.Comment: 14 pages, 9 figures, published in A&
Galactic cosmic rays on extrasolar Earth-like planets I. Cosmic ray flux
(abridged abstract) Theoretical arguments indicate that close-in terrestial
exoplanets may have weak magnetic fields, especially in the case of planets
more massive than Earth (super-Earths). Planetary magnetic fields, however,
constitute one of the shielding layers that protect the planet against
cosmic-ray particles. In particular, a weak magnetic field results in a high
flux of Galactic cosmic rays that extends to the top of the planetary
atmosphere. We wish to quantify the flux of Galactic cosmic rays to an
exoplanetary atmosphere as a function of the particle energy and of the
planetary magnetic moment. We numerically analyzed the propagation of Galactic
cosmic-ray particles through planetary magnetospheres. We evaluated the
efficiency of magnetospheric shielding as a function of the particle energy (in
the range 16 MeV E 524 GeV) and as a function of the planetary
magnetic field strength (in the range 0 {M} 10
). Combined with the flux outside the planetary magnetosphere, this
gives the cosmic-ray energy spectrum at the top of the planetary atmosphere as
a function of the planetary magnetic moment. We find that the particle flux to
the planetary atmosphere can be increased by more than three orders of
magnitude in the absence of a protecting magnetic field. For a weakly
magnetized planet (), only particles with energies
below 512 MeV are at least partially shielded. For a planet with a magnetic
moment similar to Earth, this limit increases to 32 GeV, whereas for a strongly
magnetized planet (), partial shielding extends up to 200
GeV. We find that magnetic shielding strongly controls the number of cosmic-ray
particles reaching the planetary atmosphere. The implications of this increased
particle flux are discussed in a companion article.Comment: 10 pages, 9 figures; accepted in A&
Plasmon excitations in graphitic carbon spheres
©1998 The American Physical Society. The electronic version of this article is the complete one and can be found online at: http://link.aps.org/doi/10.1103/PhysRevB.57.15599DOI: 10.1103/PhysRevB.57.15599Electron energy loss spectroscopy in a high-resolution transmission electron microscope has recently been used with success to characterize the electronic properties of closed cage nanometer-size graphitic particles. In the plasmon region, the experimental data reveal interesting size-dependent variations, which are not yet fully understood. The difficulties encountered in the interpretation of the spectra are principally due to the lack of a complete theoretical treatment of the anisotropic dielectric response in nanometer-size particles. In order to obtain a better understanding of the experimental data we propose a model based on nonrelativistic local dielectric response theory for electrons penetrating through a nested concentric-shell fullerene or the so-called ââcarbon onion.ââ The anisotropy of the electronic properties of the sphere is taken into account via the frequency-dependent dielectric tensor of graphite. The model can be applied to simulate electron energy loss spectra as well as line scans through energy filtered images and allows thus a direct comparison to experimental data
Plasmon excitations in carbon onions: Model vs. measurements
©1998 American Institute of PhysicsNon-relativistic local dielectric response theory has proven successful in the interpretation
of Electron Energy Loss data of nanometer-size isotropic particles of different
geometries. In previous work, we have adapted this model to take into account anisotropy as
encountered in the case of carbon onions. We have shown that this anisotropy needs to be
taken into account since important deviations with respect to an isotropic model can be
observed. In this contribution, we report on the first energy filtered images of carbon onions
and compare intensity profiles across the spheres to our calculations
Collective oscillations in a single-wall carbon nanotube excited by fast electrons
©2001 The American Physical Society. The electronic version of this article is the complete one and can be found online at: http://link.aps.org/doi/10.1103/PhysRevB.64.115424DOI: 10.1103/PhysRevB.64.115424Electron energy loss spectroscopy is a well adapted tool for the investigation of the valence excitations of individual nanometer-size particles. The interpretation of the loss spectra of such small particles, however, relies in most cases on a quantitative comparison with simulated excitation probabilities. Here we present a formalism developed for the interpretation of the energy loss data of single-wall carbon nanotubes based on the hydrodynamic theory of plasmon excitations by high-energy electrons. The nanotubes are modeled as a two-dimensional electron gas confined on the circumference of a cylinder. The plasmon excitation probabilities, directly comparable to measurements, are discussed for various parameters
Data scientists
What is a data scientist? How can you become one? How can you form a team of data scientists that fits your organization? In this chapter, we trace the skillset of a successful data scientist and define the necessary competencies. We give a disambiguation to other historically or contemporary definitions of the term, and show how a career as a data scientist might get started. Finally we will answer the above mentioned third question, i.e. how to build analytics teams within a data-driven organization
Evolution of screen use among youth between 2012 and 2020 in Switzerland.
To compare the evolution of screen and Internet use by youths between 2012 and 2020 in Switzerland.
Cross-sectional studies of 10th-graders (age 13-14) were performed in Switzerland in 2012 and 2020, and compared in bivariate and multivariate analyses on sociodemographic, schooling, physical activity, emotional well-being, and screen use variables.
We found that screen use had shifted to smartphones with 71.7 % of youths primarily using this device in 2020 compared to 23.2 % in 2012. In association with this change, young people's screen time had increased dramatically with an odds ratio (OR) of 11.90 for adolescents spending more than 4 h in front of screens in 2020 compared to 2012. No changes were found in the score on the Internet Addiction Test (IAT) to detect problematic screen use and for adolescents' emotional well-being. Furthermore, youths in 2020 engaged in less physical activity lasting 60 min daily, but the frequency of their extracurricular sport participation remained unchanged.
Young people spend more time on screens, especially because of an increase in smartphone use in 2020. However, youths do not seem to show more problematic behaviors regarding screen use, nor has this development affected their emotional well-being. The daily and continuous use of new devices is now an integral part of young people's lives. This process seems to be part of the growth of the digital world. However, Internet and screen addiction scales should be adapted to ensure that adolescents in need of help and counseling are identified
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