59 research outputs found
A Method to Compute Three Dimensional Magnetospheric Equilibria with Dipole Tilt and its Application in Estimating Magnetic Flux Tube Volume
In this thesis we describe a new version of a magneto-friction model, which was developed for computing the magnetospheric equilibrium that includes an arbitrary Earth's dipole tilt and interplanetary magnetic field. We also describe the algorithms of this new friction code that trace magnetic field lines, locate the neutral sheet, and identify the magnetopause In addition, we present a generalized theory for calculating magnetic flux tube volume in the magnetotail, in an attempt to generalize the Wolf [2006] empirical formula, and describe a method for estimating flux tube volume from measurements at geosynchronous orbit. This new method has been tested against various equilibrated magnetospheres generated by the new friction code. Although still incomplete, the method exhibits promising features, and is to be completed in the future
Content adaptive sparse illumination for Fourier ptychography
Fourier Ptychography (FP) is a recently proposed technique for large field of
view and high resolution imaging. Specifically, FP captures a set of low
resolution images under angularly varying illuminations and stitches them
together in Fourier domain. One of FP's main disadvantages is its long
capturing process due to the requisite large number of incident illumination
angles. In this letter, utilizing the sparsity of natural images in Fourier
domain, we propose a highly efficient method termed as AFP, which applies
content adaptive sparse illumination for Fourier ptychography by capturing the
most informative parts of the scene's spatial spectrum. We validate the
effectiveness and efficiency of the reported framework with both simulations
and real experiments. Results show that the proposed AFP could shorten the
acquisition time of conventional FP by around 30%-60%
Motion-corrected Fourier ptychography
Fourier ptychography (FP) is a recently proposed computational imaging
technique for high space-bandwidth product imaging. In real setups such as
endoscope and transmission electron microscope, the common sample motion
largely degrades the FP reconstruction and limits its practicability. In this
paper, we propose a novel FP reconstruction method to efficiently correct for
unknown sample motion. Specifically, we adaptively update the sample's Fourier
spectrum from low spatial-frequency regions towards high spatial-frequency
ones, with an additional motion recovery and phase-offset compensation
procedure for each sub-spectrum. Benefiting from the phase retrieval redundancy
theory, the required large overlap between adjacent sub-spectra offers an
accurate guide for successful motion recovery. Experimental results on both
simulated data and real captured data show that the proposed method can correct
for unknown sample motion with its standard deviation being up to 10% of the
field-of-view scale. We have released our source code for non-commercial use,
and it may find wide applications in related FP platforms such as endoscopy and
transmission electron microscopy
Source of the low-altitude hiss in the ionosphere
We analyze the propagation properties of low-altitude hiss emission in the ionosphere observed by DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions). There exist two types of low-altitude hiss: type I emission at high latitude is characterized by vertically downward propagation and broadband spectra, while type II emission at low latitude is featured with equatorward propagation and a narrower frequency band above ∼fcH+. Our ray tracing simulation demonstrates that both types of the low-altitude hiss at different latitude are connected and they originate from plasmaspheric hiss and in part chorus emission. Type I emission represents magnetospheric whistler emission that accesses the ionosphere. Equatorward propagation associated with type II emission is a consequence of wave trapping mechanisms in the ionosphere. Two different wave trapping mechanisms are identified to explain the equatorial propagation of Type II emission; one is associated with the proximity of wave frequency and local proton cyclotron frequency, while the other occurs near the ionospheric density peak
Effects Of Magnetic Drift Shell Splitting On Electron Diffusion In The Radiation Belts
Drift shell splitting in the presence of pitch angle scattering breaks all three adiabatic invariants of radiation belt electron motion and produces new diffusion terms that fully populate the diffusion tensor in the Fokker-Planck equation. The Radbelt Electron Model (REM) solves such a Fokker-Planck equation and is used to investigate the phase space density sources. Our simulation results and theoretical arguments suggest that drift shell splitting changes the phase space location of the source to smaller L shells, which typically reduces outer zone phase space density enhancements, and this reduction has a limit corresponding to two-dimensional local diffusion on a curved surface in the phase space
Large-scale single-photon imaging
Benefiting from its single-photon sensitivity, single-photon avalanche diode
(SPAD) array has been widely applied in various fields such as fluorescence
lifetime imaging and quantum computing. However, large-scale high-fidelity
single-photon imaging remains a big challenge, due to the complex hardware
manufacture craft and heavy noise disturbance of SPAD arrays. In this work, we
introduce deep learning into SPAD, enabling super-resolution single-photon
imaging over an order of magnitude, with significant enhancement of bit depth
and imaging quality. We first studied the complex photon flow model of SPAD
electronics to accurately characterize multiple physical noise sources, and
collected a real SPAD image dataset (64 32 pixels, 90 scenes, 10
different bit depth, 3 different illumination flux, 2790 images in total) to
calibrate noise model parameters. With this real-world physical noise model, we
for the first time synthesized a large-scale realistic single-photon image
dataset (image pairs of 5 different resolutions with maximum megapixels, 17250
scenes, 10 different bit depth, 3 different illumination flux, 2.6 million
images in total) for subsequent network training. To tackle the severe
super-resolution challenge of SPAD inputs with low bit depth, low resolution,
and heavy noise, we further built a deep transformer network with a
content-adaptive self-attention mechanism and gated fusion modules, which can
dig global contextual features to remove multi-source noise and extract
full-frequency details. We applied the technique on a series of experiments
including macroscopic and microscopic imaging, microfluidic inspection, and
Fourier ptychography. The experiments validate the technique's state-of-the-art
super-resolution SPAD imaging performance, with more than 5 dB superiority on
PSNR compared to the existing methods
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