1,756 research outputs found
Characterization of the nitrogen split interstitial defect in wurtzite aluminum nitride using density functional theory
We carried out Heyd-Scuseria-Ernzerhof hybrid density functional theory plane
wave supercell calculations in wurtzite aluminum nitride in order to
characterize the geometry, formation energies, transition levels and hyperfine
tensors of the nitrogen split interstitial defect. The calculated hyperfine
tensors may provide useful fingerprint of this defect for electron paramagnetic
resonance measurement.Comment: 5 pages, 3 figure
Analytic calculation of radio emission from parametrized extensive air showers:A tool to extract shower parameters
The radio intensity and polarization footprint of a cosmic-ray induced extensive air shower is determined by the time-dependent structure of the current distribution residing in the plasma cloud at the shower front. In turn, the time dependence of the integrated charge-current distribution in the plasma cloud, the longitudinal shower structure, is determined by interesting physics which one would like to extract, such as the location and multiplicity of the primary cosmic-ray collision or the values of electric fields in the atmosphere during thunderstorms. To extract the structure of a shower from its footprint requires solving a complicated inverse problem. For this purposewe have developed a code that semianalytically calculates the radio footprint of an extensive air shower given an arbitrary longitudinal structure. This code can be used in an optimization procedure to extract the optimal longitudinal shower structure given a radio footprint. On the basis of air-shower universality we propose a simple parametrization of the structure of the plasma cloud. This parametrization is based on the results of Monte Carlo shower simulations. Deriving the parametrization also teaches which aspects of the plasma cloud are important for understanding the features seen in the radio-emission footprint. The calculated radio footprints are compared with microscopic CoREAS simulations
A method for high precision reconstruction of air shower Xmax using two-dimensional radio intensity profiles
The mass composition of cosmic rays contains important clues about their
origin. Accurate measurements are needed to resolve long-standing issues such
as the transition from Galactic to extragalactic origin, and the nature of the
cutoff observed at the highest energies. Composition can be studied by
measuring the atmospheric depth of the shower maximum Xmax of air showers
generated by high-energy cosmic rays hitting the Earth's atmosphere. We present
a new method to reconstruct Xmax based on radio measurements. The radio
emission mechanism of air showers is a complex process that creates an
asymmetric intensity pattern on the ground. The shape of this pattern strongly
depends on the longitudinal development of the shower. We reconstruct Xmax by
fitting two-dimensional intensity profiles, simulated with CoREAS, to data from
the LOFAR radio telescope. In the dense LOFAR core, air showers are detected by
hundreds of antennas simultaneously. The simulations fit the data very well,
indicating that the radiation mechanism is now well-understood. The typical
uncertainty on the reconstruction of Xmax for LOFAR showers is 17 g/cm^2.Comment: 12 pages, 10 figures, submitted to Phys. Rev.
Discrete Wirtinger-based inequality and its application
In this paper, we derive a new inequality, which encompasses the discrete Jensen inequality. The new inequality is applied to analyze stability of linear discrete systems with an interval time-varying delay and a less conservative stability condition is obtained. Two numerical examples are given to show the effectiveness of the obtained stability condition
Polarized radio emission from extensive air showers measured with LOFAR
We present LOFAR measurements of radio emission from extensive air showers.
We find that this emission is strongly polarized, with a median degree of
polarization of nearly , and that the angle between the polarization
direction of the electric field and the Lorentz force acting on the particles,
depends on the observer location in the shower plane. This can be understood as
a superposition of the radially polarized charge-excess emission mechanism,
first proposed by Askaryan and the geomagnetic emission mechanism proposed by
Kahn and Lerche. We calculate the relative strengths of both contributions, as
quantified by the charge-excess fraction, for individual air showers. We
find that the measured charge-excess fraction is higher for air showers
arriving from closer to the zenith. Furthermore, the measured charge-excess
fraction also increases with increasing observer distance from the air shower
symmetry axis. The measured values range from for very
inclined air showers at to for almost
vertical showers at . Both dependencies are in qualitative
agreement with theoretical predictions.Comment: 22 pages, 14 figures, accepted for publication in JCA
The radio emission pattern of air showers as measured with LOFAR - a tool for the reconstruction of the energy and the shower maximum
The pattern of the radio emission of air showers is finely sampled with the
Low-Frequency ARray (LOFAR). A set of 382 measured air showers is used to test
a fast, analytic parameterization of the distribution of pulse powers. Using
this parameterization we are able to reconstruct the shower axis and give
estimators for the energy of the air shower as well as the distance to the
shower maximum.Comment: 15 pages, 10 figures, accepted for publication in JCA
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