16,470 research outputs found
Prompt energization of relativistic and highly relativistic electrons during a substorm interval: Van Allen Probes observations
Abstract On 17 March 2013, a large magnetic storm significantly depleted the multi-MeV radiation belt. We present multi-instrument observations from the Van Allen Probes spacecraft Radiation Belt Storm Probe A and Radiation Belt Storm Probe B at ~6 Re in the midnight sector magnetosphere and from ground-based ionospheric sensors during a substorm dipolarization followed by rapid reenergization of multi-MeV electrons. A 50% increase in magnetic field magnitude occurred simultaneously with dramatic increases in 100 keV electron fluxes and a 100 times increase in VLF wave intensity. The 100 keV electrons and intense VLF waves provide a seed population and energy source for subsequent radiation belt enhancements. Highly relativistic (\u3e2 MeV) electron fluxes increased immediately at L* ~ 4.5 and 4.5 MeV flux increased \u3e90 times at L* = 4 over 5 h. Although plasmasphere expansion brings the enhanced radiation belt multi-MeV fluxes inside the plasmasphere several hours postsubstorm, we localize their prompt reenergization during the event to regions outside the plasmasphere. Key Points Substorm dynamics are important for highly relativistic electron energization Cold plasma preconditioning is significant for rapid relativistic energization Relativistic / highly relativistic electron energization can occur in \u3c 5 hrs
Seismic modeling using the frozen Gaussian approximation
We adopt the frozen Gaussian approximation (FGA) for modeling seismic waves.
The method belongs to the category of ray-based beam methods. It decomposes
seismic wavefield into a set of Gaussian functions and propagates these
Gaussian functions along appropriate ray paths. As opposed to the classic
Gaussian-beam method, FGA keeps the Gaussians frozen (at a fixed width) during
the propagation process and adjusts their amplitudes to produce an accurate
approximation after summation. We perform the initial decomposition of seismic
data using a fast version of the Fourier-Bros-Iagolnitzer (FBI) transform and
propagate the frozen Gaussian beams numerically using ray tracing. A test using
a smoothed Marmousi model confirms the validity of FGA for accurate modeling of
seismic wavefields.Comment: 5 pages, 8 figure
Aircraft High-Lift Aerodynamic Analysis Using a Surface-Vorticity Solver
This study extends an existing semi-empirical approach to high-lift analysis by examining its effectiveness for use with a three-dimensional aerodynamic analysis method. The aircraft high-lift geometry is modeled in Vehicle Sketch Pad (OpenVSP) using a newly-developed set of techniques for building a three-dimensional model of the high-lift geometry, and for controlling flap deflections using scripted parameter linking. Analysis of the low-speed aerodynamics is performed in FlightStream, a novel surface-vorticity solver that is expected to be substantially more robust and stable compared to pressure-based potential-flow solvers and less sensitive to surface perturbations. The calculated lift curve and drag polar are modified by an empirical lift-effectiveness factor that takes into account the effects of viscosity that are not captured in the potential-flow solution. Analysis results are validated against wind-tunnel data for The Energy-Efficient Transport AR12 low-speed wind-tunnel model, a 12-foot, full-span aircraft configuration with a supercritical wing, full-span slats, and part-span double-slotted flaps
Accelerated cosmological expansion due to a scalar field whose potential has a positive lower bound
In many cases a nonlinear scalar field with potential can lead to
accelerated expansion in cosmological models. This paper contains mathematical
results on this subject for homogeneous spacetimes. It is shown that, under the
assumption that has a strictly positive minimum, Wald's theorem on
spacetimes with positive cosmological constant can be generalized to a wide
class of potentials. In some cases detailed information on late-time
asymptotics is obtained. Results on the behaviour in the past time direction
are also presented.Comment: 16 page
The physical gravitational degrees of freedom
When constructing general relativity (GR), Einstein required 4D general
covariance. In contrast, we derive GR (in the compact, without boundary case)
as a theory of evolving 3-dimensional conformal Riemannian geometries obtained
by imposing two general principles: 1) time is derived from change; 2) motion
and size are relative. We write down an explicit action based on them. We
obtain not only GR in the CMC gauge, in its Hamiltonian 3 + 1 reformulation but
also all the equations used in York's conformal technique for solving the
initial-value problem. This shows that the independent gravitational degrees of
freedom obtained by York do not arise from a gauge fixing but from hitherto
unrecognized fundamental symmetry principles. They can therefore be identified
as the long-sought Hamiltonian physical gravitational degrees of freedom.Comment: Replaced with published version (minor changes and added references
Ionospheric response to the corotating interaction region-driven geomagnetic storm of October 2002
Unlike the geomagnetic storms produced by coronal mass ejections (CMEs), the storms generated by corotating interaction regions (CIRs) are not manifested by dramatic enhancements of the ring current. The CIR-driven storms are however capable of producing other phenomena typical for the magnetic storms such as relativistic particle acceleration, enhanced magnetospheric convection and ionospheric heating. This paper examines ionospheric plasma anomalies produced by a CIR-driven storm in the middle- and high-latitude ionosphere with a specific focus on the polar cap region. The moderate magnetic storm which took place on 14–17 October 2002 has been used as an example of the CIR-driven event. Four-dimensional tomographic reconstructions of the ionospheric plasma density using measurements of the total electron content along ray paths of GPS signals allow us to reveal the large-scale structure of storm-induced ionospheric anomalies. The tomographic reconstructions are compared with the data obtained by digital ionosonde located at Eureka station near the geomagnetic north pole. The morphology and dynamics of the observed ionospheric anomalies is compared qualitatively to the ionospheric anomalies produced by major CME-driven storms. It is demonstrated that the CIR-driven storm of October 2002 was able to produce ionospheric anomalies comparable to those produced by CME-driven storms of much greater Dst magnitude. This study represents an important step in linking the tomographic GPS reconstructions with the data from ground-based network of digital ionosondes
An ecological approach to problems of Dark Energy, Dark Matter, MOND and Neutrinos
Modern astronomical data on galaxy and cosmological scales have revealed
powerfully the existence of certain dark sectors of fundamental physics, i.e.,
existence of particles and fields outside the standard models and inaccessible
by current experiments. Various approaches are taken to modify/extend the
standard models. Generic theories introduce multiple de-coupled fields A, B, C,
each responsible for the effects of DM (cold supersymmetric particles), DE
(Dark Energy) effect, and MG (Modified Gravity) effect respectively. Some
theories use adopt vanilla combinations like AB, BC, or CA, and assume A, B, C
belong to decoupled sectors of physics. MOND-like MG and Cold DM are often
taken as opposite frameworks, e.g. in the debate around the Bullet Cluster.
Here we argue that these ad hoc divisions of sectors miss important clues from
the data. The data actually suggest that the physics of all dark sectors is
likely linked together by a self-interacting oscillating field, which governs a
chameleon-like dark fluid, appearing as DM, DE and MG in different settings. It
is timely to consider an interdisciplinary approach across all semantic
boundaries of dark sectors, treating the dark stress as one identity, hence
accounts for several "coincidences" naturally.Comment: 12p, Proceedings to the 6-th Int. Conf. of Gravitation and Cosmology.
Neutrino section expande
Recommended from our members
Wet solids flow enhancemant
WE used glass beads of different sizes as.a model system to study the flow enhancing properties of Octadecyltrichlorosilane (OTS). 0TS provides Si(CH{sub 2}){sub 17}CH{sub 3} groups that bind with the surface hydrox groups to make it hydrophobic. Experimental data showed, indeed, that surface hydrophobicity promotes the flow of wet granular materials. Mixtures of different percentage of silanized/unsilanized particles were prepared for tensile strength measurements. The tensile strength decreased as more silanized particles were added to the samples. The relationship between dimensionless tensile strength and void fraction followed the correlation found by Pierrat (1994). Contact angles were larger for the silanized particles, as compared with unsilanized ones
Recommended from our members
Wet solids flow enhancement
The objective was to visualize the flow of granular materials in the silo using Nuclear Magnetic Resonance. This was done by introducing traces. Mustard seeds and poppy seeds were used as trace particles. The region sampled was a cylinder 25 mm in diameter and 40 mm in length. Eight slices containing 128 by 128 to 256 by 256 pixels were generated for each image
Post-Prior discrepancies in CDW-EIS calculations for ion impact ionization fully differential cross sections
In this work we present fully differential cross sections (FDCSs)
calculations using post and prior version of CDW--EIS theory for helium single
ionization by 100 MeV C amu and 3.6 MeV amu Au and
Au ions. We performed our calculations for different momentum transfer
and ejected electron energies. The influence of internuclear potential on the
ejected electron spectra is taken into account in all cases. We compare our
calculations with absolute experimental measurements. It is shown that prior
version calculations give better agreement with experiments in almost all
studied cases.Comment: 9 pages, 7 figure
- …