221 research outputs found
Altitude dependence of nightside Martian suprathermal electron depletions as revealed by MAVEN observations
The MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft is providing new detailed observations of the Martian ionosphere thanks to its unique orbital coverage and instrument suite. During most periapsis passages on the nightside ionosphere suprathermal electron depletions were detected. A simple criterion was implemented to identify the 1742 depletions observed from 16 November 2014 to 28 February 2015. A statistical analysis reveals that the main ion and electron populations within the depletions are surprisingly constant in time and altitude. Absorption by CO2 is the main loss process for suprathermal electrons, and electrons that strongly peaked around 6 eV are resulting from this interaction. The observation of depletions appears however highly dependent on altitude. Depletions are mainly located above strong crustal magnetic sources above 170 km, whereas the depletions observed for the first time below 170 km are globally scattered onto the Martian surface with no particular dependence on crustal fields
Limit cycles in uniform isochronous centers of discontinuous differential systems with four zones
We apply the averaging theory of first order for discontinuous differential systems to study the bifurcation of limit cycles from the periodic orbits of the uniform isochronous center of the differential systems ẋ = -y+x, y = x + xy, and ẋ = -y + xy, y = x + xy, when they are perturbed inside the class of all discontinuous quadratic and cubic polynomials differential systems with four zones separately by the axes of coordinates, respectively. Using averaging theory of first order the maximum number of limit cycles that we can obtain is twice the maximum number of limit cycles obtained in a previous work for discontinuous quadratic differential systems perturbing the same uniform isochronous quadratic center at origin perturbed with two zones separately by a straight line, and 5 more limit cycles than those achieved in a prior result for discontinuous cubic differential systems with the same uniform isochronous cubic center at the origin perturbed with two zones separately by a straight line. Comparing our results with those obtained perturbing the mentioned centers by the continuous quadratic and cubic differential systems we obtain 8 and 9 more limit cycles respectively
An evaluation of some collision models used for Monte Carlo calculations of diatomic rarefied hypersonic flows
Impact of Varying Atmospheric Profiles on Extensive Air Shower Observation: Fluorescence Light Emission and Energy Reconstruction
Several experiments measure the fluorescence light produced by extensive air
showers in the atmosphere. This light is converted into a longitudinal shower
profile from which information on the primary energy and composition is
derived. The fluorescence yield, as the conversion factor between light profile
measured by EAS experiments and physical interpretation of showers, has been
measured in several laboratory experiments. The results, however, differ
considerably. In this article, a model calculation of the fluorescence emission
from relevant band systems of nitrogen in dependence on wavelength and
atmospheric conditions is presented. Different calculations are compared to
each other in combination with varying input parameters. The predictions are
compared with measurements and the altitude-dependence of the fluorescence
yield is discussed in detail.Comment: 21 pages, 5 figures, 4 tables, Astroparticle Physics in pres
Supernova Remnants in the Sedov Expansion Phase: Thermal X-Ray Emission
Improved calculations of X-ray spectra for supernova remnants (SNRs) in the
Sedov-Taylor phase are reported, which for the first time include reliable
atomic data for Fe L-shell lines. This new set of Sedov models also allows for
a partial collisionless heating of electrons at the blast wave and for energy
transfer from ions to electrons through Coulomb collisions. X-ray emission
calculations are based on the updated Hamilton-Sarazin spectral model. The
calculated X-ray spectra are succesfully interpreted in terms of three
distribution functions: the electron temperature and ionization timescale
distributions, and the ionization timescale averaged electron temperature
distribution. The comparison of Sedov models with a frequently used single
nonequilibrium ionization (NEI) timescale model reveals that this simple model
is generally not an appropriate approximation to X-ray spectra of SNRs. We find
instead that plane-parallel shocks provide a useful approximation to X-ray
spectra of SNRs, particularly for young SNRs. Sedov X-ray models described
here, together with simpler plane shock and single ionization timescale models,
have been implemented as standard models in the widely used XSPEC v11 spectral
software package.Comment: 19 pages, 11 figures, submitted to Astrophysical Journa
Cross Sections for Electron Collisions with H2O
Electron collision cross section data for the water molecule are compiled from the literature. Cross sections are collected and reviewed for totalscattering, elastic scattering, momentum transfer, excitations of rotational and vibrational states, electronic excitation, dissociation, ionization,and dissociative attachment. For each of these processes, the recommended values of the cross sections are presented. The literature has beensurveyed up to the end of 2019
Model calculations of electron precipitation induced ionization patches on the nightside of Mars
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94812/1/grl23162.pd
Electron Collisions with CO Molecule: An R-Matrix Study Using a Large Basis Set
Fixed-nuclei R -matrix calculations are performed at the equilibrium geometry of carbon monoxide using the very large cc-pV6Z Gaussian basis set. Results from a close-coupling model involving 27 low-lying target states indicate the presence of three2Σ+ resonances at 10.1 eV (width 0.1 eV), 10.38 eV (0.0005 eV), and 11.15 eV (0.005 eV), a2Δ resonance at 13.3 eV (0.1 eV) and two2Πresonances at 1.9 eV (1.3 eV) and 12.8 eV (0.1 eV). These new results are in very good agreement with many experimental studies but in contrast to a previous calculation using a smaller cc-pVTZ basis set where we found only one2Σ+ resonances at 12.9 eV. This is the first time that any theoretical study has reported these high lying2Σ+ resonances in agreement to experiment and reported detection of a2Δ resonance. Total, elastic and electronic excitation cross sections of CO by electron impact are also presented
Ion beam induced surface pattern formation and stable travelling wave solutions
This article was published in the Journal of Physics: Condensed Matter [© IOP Publishing Ltd.] and the definitive version is available at: http://dx.doi.org/10.1088/0953-8984/25/9/095003The formation of ripple structures on ion bombarded semiconductor surfaces is examined theoretically. Previous models are discussed and a new nonlinear model is formulated, based on the infinitesimal local atomic relocation induced by elastic nuclear collisions in the early stages of collision cascades and an associated density change in the near surface region. Within this framework ripple structures are shown to form without the necessity to invoke surface diffusion or large sputtering as important mechanisms. The model can also be extended to the case where sputtering is important, and it is shown that in this case certain 'magic' angles can occur at which the ripple patterns are most clearly defined. The results are in very good agreement with experimental observations
Correlation-Polarization Effects in Electron/Positron Scattering from Acetylene: A Comparison of Computational Models
Different computational methods are employed to evaluate elastic
(rotationally summed) integral and differential cross sections for low energy
(below about 10 eV) positron scattering off gas-phase CH molecules. The
computations are carried out at the static and static-plus-polarization levels
for describing the interaction forces and the correlation-polarization
contributions are found to be an essential component for the correct
description of low-energy cross section behavior. The local model potentials
derived from density functional theory (DFT) and from the distributed positron
model (DPM) are found to produce very high-quality agreement with existing
measurements. On the other hand, the less satisfactory agreement between the
R-matrix (RM) results and measured data shows the effects of the slow
convergence rate of configuration-interaction (CI) expansion methods with
respect to the size of the CI-expansion. To contrast the positron scattering
findings, results for electron-CH integral and differential cross
sections, calculated with both a DFT model potential and the R-matrix method,
are compared and analysed around the shape resonance energy region and found to
produce better internal agreement
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