32,931 research outputs found
ROAM: a Radial-basis-function Optimization Approximation Method for diagnosing the three-dimensional coronal magnetic field
The Coronal Multichannel Polarimeter (CoMP) routinely performs coronal
polarimetric measurements using the Fe XIII 10747 and 10798 lines,
which are sensitive to the coronal magnetic field. However, inverting such
polarimetric measurements into magnetic field data is a difficult task because
the corona is optically thin at these wavelengths and the observed signal is
therefore the integrated emission of all the plasma along the line of sight. To
overcome this difficulty, we take on a new approach that combines a
parameterized 3D magnetic field model with forward modeling of the polarization
signal. For that purpose, we develop a new, fast and efficient, optimization
method for model-data fitting: the Radial-basis-functions Optimization
Approximation Method (ROAM). Model-data fitting is achieved by optimizing a
user-specified log-likelihood function that quantifies the differences between
the observed polarization signal and its synthetic/predicted analogue. Speed
and efficiency are obtained by combining sparse evaluation of the magnetic
model with radial-basis-function (RBF) decomposition of the log-likelihood
function. The RBF decomposition provides an analytical expression for the
log-likelihood function that is used to inexpensively estimate the set of
parameter values optimizing it. We test and validate ROAM on a synthetic test
bed of a coronal magnetic flux rope and show that it performs well with a
significantly sparse sample of the parameter space. We conclude that our
optimization method is well-suited for fast and efficient model-data fitting
and can be exploited for converting coronal polarimetric measurements, such as
the ones provided by CoMP, into coronal magnetic field data.Comment: 23 pages, 12 figures, accepted in Frontiers in Astronomy and Space
Science
Viable 3C-SiC-on-Si MOSFET design disrupting current Material Technology Limitations
The cubic polytype (3C-) of Silicon Carbide (SiC) is an emerging semiconductor technology for power devices. The featured isotropic material properties along with the Wide Band Gap (WBG) characteristics make it an excellent choice for power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). Nonetheless, material related limitations originate from the advantageous fact that 3C-SiC can be grown on Silicon (Si) wafers. One of these major limitations is an almost negligible activation of the p-type dopants after ion implantation because the annealing has to take place at relatively low temperatures. In this paper, a novel process flow for a vertical 3C-SiC-on-Si MOSFET is presented to overcome the difficulties that currently exist in obtaining a p-body region through implantation. The proposed design has been accurately simulated with Technology Computer Aided Design (TCAD) process and device software and a comparison is performed with the conventional SiC MOSFET design. The simulated output characteristics demonstrated a reduced on-resistance and at the same time it is shown that the blocking capability can be maintained to the same level. The promising performance of the novel design discussed in this paper is potentially the solution needed and a huge step towards the realisation of 3C-SiC-on-Si MOSFETs with commercially grated characteristics
Data-Optimized Coronal Field Model: I. Proof of Concept
Deriving the strength and direction of the three-dimensional (3D) magnetic
field in the solar atmosphere is fundamental for understanding its dynamics.
Volume information on the magnetic field mostly relies on coupling 3D
reconstruction methods with photospheric and/or chromospheric surface vector
magnetic fields. Infrared coronal polarimetry could provide additional
information to better constrain magnetic field reconstructions. However,
combining such data with reconstruction methods is challenging, e.g., because
of the optical-thinness of the solar corona and the lack and limitations of
stereoscopic polarimetry. To address these issues, we introduce the
Data-Optimized Coronal Field Model (DOCFM) framework, a model-data fitting
approach that combines a parametrized 3D generative model, e.g., a magnetic
field extrapolation or a magnetohydrodynamic model, with forward modeling of
coronal data. We test it with a parametrized flux rope insertion method and
infrared coronal polarimetry where synthetic observations are created from a
known "ground truth" physical state. We show that this framework allows us to
accurately retrieve the ground truth 3D magnetic field of a set of force-free
field solutions from the flux rope insertion method. In observational studies,
the DOCFM will provide a means to force the solutions derived with different
reconstruction methods to satisfy additional, common, coronal constraints. The
DOCFM framework therefore opens new perspectives for the exploitation of
coronal polarimetry in magnetic field reconstructions and for developing new
techniques to more reliably infer the 3D magnetic fields that trigger solar
flares and coronal mass ejections.Comment: 14 pages, 6 figures; Accepted for publication in Ap
Protein-mediated DNA Loop Formation and Breakdown in a Fluctuating Environment
Living cells provide a fluctuating, out-of-equilibrium environment in which
genes must coordinate cellular function. DNA looping, which is a common means
of regulating transcription, is very much a stochastic process; the loops arise
from the thermal motion of the DNA and other fluctuations of the cellular
environment. We present single-molecule measurements of DNA loop formation and
breakdown when an artificial fluctuating force, applied to mimic a fluctuating
cellular environment, is imposed on the DNA. We show that loop formation is
greatly enhanced in the presence of noise of only a fraction of , yet
find that hypothetical regulatory schemes that employ mechanical tension in the
DNA--as a sensitive switch to control transcription--can be surprisingly robust
due to a fortuitous cancellation of noise effects
Transition of amorphous to crystalline oxide film in initial oxide overgrowth on liquid metals
It is important to understand the mechanism of oxidation in the initial stage on the free surface of liquid metals. Mittemeijer and co-workers recently developed a thermodynamic model to study the oxide overgrowth on a solid metal surface. Based on this model, we have developed a thermodynamic model to analyse the thermodynamic stability of oxide overgrowth on liquid metals. The thermodynamic model calculation revealed that the amorphous oxide phase is thermodynamically preferred up to 1.3 and 0.35 nm respectively, in the initial oxide overgrowth on liquid Al and Ga at the corresponding melting point. However, the amorphous phase is thermodynamically unstable in the initial oxide overgrowth on liquid Mg. The thermodynamic stability of amorphous phase in the Al and Ga oxide systems is attributed to lower sums of surface and interfacial energies for amorphous phases, compared to that of the corresponding crystalline phases.Financial support under grant EP/H026177/1 from the EPSRC was used
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