17,028 research outputs found
Mobile Video Object Detection with Temporally-Aware Feature Maps
This paper introduces an online model for object detection in videos designed
to run in real-time on low-powered mobile and embedded devices. Our approach
combines fast single-image object detection with convolutional long short term
memory (LSTM) layers to create an interweaved recurrent-convolutional
architecture. Additionally, we propose an efficient Bottleneck-LSTM layer that
significantly reduces computational cost compared to regular LSTMs. Our network
achieves temporal awareness by using Bottleneck-LSTMs to refine and propagate
feature maps across frames. This approach is substantially faster than existing
detection methods in video, outperforming the fastest single-frame models in
model size and computational cost while attaining accuracy comparable to much
more expensive single-frame models on the Imagenet VID 2015 dataset. Our model
reaches a real-time inference speed of up to 15 FPS on a mobile CPU.Comment: In CVPR 201
Stochastic Acceleration of He and He in Solar Flares by Parallel Propagating Plasma Waves: General Results
We study the acceleration in solar flares of He and He from a thermal
background by parallel propagating plasma waves with a general broken power-law
spectrum that takes into account the turbulence generation processes at large
scales and the thermal damping effects at small scales. The exact dispersion
relation for a cold plasma is used to describe the relevant wave modes. Because
low-energy -particles only interact with small scale waves in the
He-cyclotron branch, where the wave frequencies are below the
-particle gyro-frequency, their pitch angle averaged acceleration time
is at least one order of magnitude longer than that of He ions, which
mostly resonate with relatively higher frequency waves in the proton-cyclotron
(PC) branch. The -particle acceleration rate starts to approach that of
He beyond a few tens of keV nucleon, where -particles can
also interact with long wavelength waves in the PC branch. However, the He
acceleration rate is always smaller than that of He. Consequently, the
acceleration of He is suppressed significantly at low energies, and the
spectrum of the accelerated -particles is always softer than that of
He. The model gives reasonable account of the observed low-energy He
and He fluxes and spectra in the impulsive solar energetic particle events
observed with the {\it Advanced Composition Explorer}. We explore the model
parameter space to show how observations may be used to constrain the model.Comment: 29 pages, 11 Figures, Submitted to Ap
Stochastic Acceleration of 3He and 4He by Parallel Propagating Plasma Waves
Stochastic acceleration of He and He from a thermal background by
parallel propagating turbulent plasma waves with a single power-law spectrum of
the wavenumber is studied. In the model, both ions interact with several
resonant modes. When one of these modes dominates, the acceleration rate is
reduced considerably. At low energies, this happens for He, but not for
He where contributions from the two stronger modes are comparable so that
acceleration of He is very efficient. As a result, the acceleration of
He is suppressed by a barrier below keV nucleon and there
is a prominent quasi-thermal component in the He spectra, while almost all
the injected He ions are accelerated to high energies. This accounts for
the large enrichment of He at high energies observed in impulsive solar
energetic particle events. With reasonable plasma parameters this also provides
a good fit to the spectra of both ions. Beyond MeV nucleon, the
spectrum of He is softer than that of He, which is consistent with the
observed decrease of the He to He ratio with energy. This study also
indicates that the acceleration, Coulomb losses and diffusive escape of the
particles from the acceleration site {\it all} play important roles in shaping
the ion spectra. This can explain the varied spectral shapes observed recently
by the {\it Advanced Composition Explorer}.Comment: 14 pages, 4 figures, Submitted to Apj
Cascade and Damping of Alfv\'{e}n-Cyclotron Fluctuations: Application to Solar Wind Turbulence Spectrum
With the diffusion approximation, we study the cascade and damping of
Alfv\'{e}n-cyclotron fluctuations in solar plasmas numerically. Motivated by
wave-wave couplings and nonlinear effects, we test several forms of the
diffusion tensor. For a general locally anisotropic and inhomogeneous diffusion
tensor in the wave vector space, the turbulence spectrum in the inertial range
can be fitted with power-laws with the power-law index varying with the wave
propagation direction. For several locally isotropic but inhomogeneous
diffusion coefficients, the steady-state turbulence spectra are nearly
isotropic in the absence of damping and can be fitted by a single power-law
function. However, the energy flux is strongly polarized due to the
inhomogeneity that leads to an anisotropic cascade. Including the anisotropic
thermal damping, the turbulence spectrum cuts off at the wave numbers, where
the damping rates become comparable to the cascade rates. The combined
anisotropic effects of cascade and damping make this cutoff wave number
dependent on the wave propagation direction, and the propagation direction
integrated turbulence spectrum resembles a broken power-law, which cuts off at
the maximum of the cutoff wave numbers or the He cyclotron frequency.
Taking into account the Doppler effects, the model can naturally reproduce the
broken power-law wave spectra observed in the solar wind and predicts that a
higher break frequency is aways accompanied with a greater spectral index
change that may be caused by the increase of the Alfv\'{e}n Mach number, the
reciprocal of the plasma beta, and/or the angle between the solar wind velocity
and the mean magnetic field. These predictions can be tested by future
observations
Identification of contrastive and comparable school neighborhoods for childhood obesity and physical activity research
The neighborhood social and physical environments are considered significant factors contributing to children's inactive lifestyles, poor eating habits, and high levels of childhood obesity. Understanding of neighborhood environmental profiles is needed to facilitate community-based research and the development and implementation of community prevention and intervention programs. We sought to identify contrastive and comparable districts for childhood obesity and physical activity research studies. We have applied GIS technology to manipulate multiple data sources to generate objective and quantitative measures of school neighborhood-level characteristics for school-based studies. GIS technology integrated data from multiple sources (land use, traffic, crime, and census tract) and available social and built environment indicators theorized to be associated with childhood obesity and physical activity. We used network analysis and geoprocessing tools within a GIS environment to integrate these data and to generate objective social and physical environment measures for school districts. We applied hierarchical cluster analysis to categorize school district groups according to their neighborhood characteristics. We tested the utility of the area characterizations by using them to select comparable and contrastive schools for two specific studies. RESULTS: We generated school neighborhood-level social and built environment indicators for all 412 Chicago public elementary school districts. The combination of GIS and cluster analysis allowed us to identify eight school neighborhoods that were contrastive and comparable on parameters of interest (land use and safety) for a childhood obesity and physical activity study. CONCLUSION: The combination of GIS and cluster analysis makes it possible to objectively characterize urban neighborhoods and to select comparable and/or contrasting neighborhoods for community-based health studies
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