829 research outputs found
Extreme Spacecraft Charging in Polar Low Earth Orbit
Spacecraft in low altitude, high inclination (including sun -synchronous) orbits are widely used for remote sensing of the Earth fs land surface and oceans, monitoring weather and climate, communications, scientific studies of the upper atmosphere and ionosphere, and a variety of other scientific, commercial, and military applications. These systems episodically charge to frame potentials in the kilovolt range when exposed to space weather environments characterized by a high flux of energetic (approx.10 fs kilovolt) electrons in regions of low background plasma density. Auroral charging conditions are similar in some ways to the space weather conditions in geostationary orbit responsible for spacecraft charging to kilovolt levels. We first review the physics of space environment interactions with spacecraft materials that control auroral charging rates and the anticipated maximum potentials that should be observed on spacecraft surfaces during disturbed space weather conditions. We then describe how the theoretical values compare to the observational history of extreme charging in auroral environments. Finally, a set of extreme DMSP charging events are described varying in maximum negative frame potential from approx.0.6 kV to approx.2 kV, focusing on the characteristics of the charging events that are of importance both to the space system designer and to spacecraft operators. The goal of the presentation is to bridge the gap between scientific studies of auroral charging and the need for engineering teams to understand how space weather impacts both spacecraft design and operations for vehicles on orbital trajectories that traverse auroral charging environments
Optical signature of sub-gap absorption in the superconducting state of Ba(Fe,Co)2As2
The optical conductivity of Ba(FeCo)As shows a
clear signature of the superconducting gap, but a simple -wave description
fails in accounting for the low frequency response. This task is achieved by
introducing an extra Drude peak in the superconducting state representing
sub-gap absorption, other than thermally broken pairs. This extra peak and the
coexisting -wave response respect the total sum rule indicating a common
origin for the carriers. We discuss the possible origins for this absorption as
(i) quasiparticles due to pair-breaking from interband impurity scattering in a
two band gap symmetry model, which includes (ii) the possible
existence of impurity levels within an isotropic gap model; or (iii) an
indication that one of the bands is highly anisotropic.Comment: 5 pages, 4 figure
Photon emission by an ultra-relativistic particle channeling in a periodically bent crystal
This paper is devoted to a detailed analysis of the new type of the undulator
radiation generated by an ultra-relativistic charged particle channeling along
a crystal plane, which is periodically bent by a transverse acoustic wave, as
well as to the conditions limiting the observation of this phenomenon. This
mechanism makes feasible the generation of electromagnetic radiation, both
spontaneous and stimulated, emitted in a wide range of the photon energies,
from X- up to gamma-rays
ESR study of the single-ion anisotropy in the pyrochlore antiferromagnet Gd2Sn2O7
Single-ion anisotropy is of importance for the magnetic ordering of the
frustrated pyrochlore antiferromagnets Gd2Ti2O7 and Gd2Sn2O7. The anisotropy
parameters for the Gd2Sn2O7 were measured using the electron spin resonance
(ESR) technique. The anisotropy was found to be of the easy plane type, with
the main constant D=140mK. This value is 35% smaller than the value of the
corresponding anisotropy constant in the related compound Gd2Ti2O7.Comment: 8 pages, 3 figure
Phase separation of Bose-Einstein condensates
The zero-temperature system of two dilute overlapping Bose-Einstein
condensates is unstable against long wavelength excitations if the interaction
strength between the distinguishable bosons exceeds the geometric mean of the
like-boson interaction strengths. If the condensates attract each other, the
instability is similar to the instability of the negative scattering length
condensates. If the condensates repel, they separate spatially into condensates
of equal pressure. We estimate the boundary size, surface tension and energy of
the phase separated condensate system and we discuss the implications for
double condensates in atomic traps.Comment: 11 pages, 1 figur
AutoSimulate: (Quickly) Learning Synthetic Data Generation
Simulation is increasingly being used for generating large labelled datasets
in many machine learning problems. Recent methods have focused on adjusting
simulator parameters with the goal of maximising accuracy on a validation task,
usually relying on REINFORCE-like gradient estimators. However these approaches
are very expensive as they treat the entire data generation, model training,
and validation pipeline as a black-box and require multiple costly objective
evaluations at each iteration. We propose an efficient alternative for optimal
synthetic data generation, based on a novel differentiable approximation of the
objective. This allows us to optimize the simulator, which may be
non-differentiable, requiring only one objective evaluation at each iteration
with a little overhead. We demonstrate on a state-of-the-art photorealistic
renderer that the proposed method finds the optimal data distribution faster
(up to ), with significantly reduced training data generation (up to
) and better accuracy () on real-world test datasets than
previous methods.Comment: ECCV 202
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