1,489 research outputs found
Source of the Bursty Bulk Flow Diffuse Aurora: Electrostatic Cyclotron Harmonic and Whistler Waves in the Coupling of Bursty Bulk Flows to Auroral Precipitation
Electron cyclotron harmonic (ECH) and whistler chorus waves are recognized as the two mechanisms responsible for the resonant waveparticle interactions necessary to precipitate plasma sheet electrons into the ionosphere, producing the diffuse Aurora. Previous work has demonstrated ECH waves dominate electron scattering at L shells >8, while whistler chorus dominates scattering at L shells L 1, consistent with electron betatron acceleration. Here, however, we nd whistler chorus emissions throughout an interval of fast ows where Te,/Te,||< 1. Parallel electron beams account for the enhanced parallel electron temperature and serve as the instability mechanism for the whistler chorus. The parallel electron beams and associated cigarshaped distributions are consistent with Fermi acceleration at dipolarizations in fast ows. We demonstrate that the scattering efciency of the whistler chorus exceeds that of ECH waves, which THEMIS also detects during the fast ows. The obliquity of the whistler waves permits efcient scattering of lowerenergy electrons into the diffuse aurora. We conclude that Fermi acceleration of electrons provides one important freeenergy source for the waveparticle interactions responsible for coupling plasma sheet electrons into the diffuse aurora during substorm conditions
Quenched and Negative Hall Effect in Periodic Media: Application to Antidot Superlattices
We find the counterintuitive result that electrons move in OPPOSITE direction
to the free electron E x B - drift when subject to a two-dimensional periodic
potential. We show that this phenomenon arises from chaotic channeling
trajectories and by a subtle mechanism leads to a NEGATIVE value of the Hall
resistivity for small magnetic fields. The effect is present also in
experimentally recorded Hall curves in antidot arrays on semiconductor
heterojunctions but so far has remained unexplained.Comment: 10 pages, 4 figs on request, RevTeX3.0, Europhysics Letters, in pres
NARC Rayon Replacement Program for the RSRM Nozzle, Phase IV Qualification and Implementation Status
The Space Shuttle NARC Rayon Replacement Program has down-selected Enka rayon as a replacement for the obsolete NARC rayon in the nozzle carbon cloth phenolic (CCP) ablative insulators. Full qualification testing of the Enka rayon-based carbon cloth phenolic is underway, including processing, thmal/structural properties, and hot-fire subscale tests. Required thermal-structural capabilities, together with confidence in erosio/char performance in simulated and subscale hot fire tests such as Wright-Patterson Air Force Base Laser Hardened Materials Evaluation Laboratory testing, NASA-MSFC 24-inch motor tests, NASA-MSFC Solid Fuel Torch - Super Sonic Blast Tube, NASA-MSFC Plasma Torch Test Bed, ATK Thiokol Forty Pound Charge and NASA-MSFC MNASA justified the testing of the new Enka-rayon candidate on full-scale static test motors. The first RSRM full-scale static test motor nozzle, fabricated using the new Enka rayon-based CCP, was successfully demonstrated in June 2004. Two additional static test motors are planned with the new Enka rayon in the next two years along with additional A-basis property characterization. Process variation or "corner-of-the-box" testing together with cured and uncured aging studies are also planned as some of the pre-flight implementation activities with 5-year cured aging studies over-lapping flight hardware fabrication
Hyperfine magnetic field in ferromagnetic graphite
Information on atomic-scale features is required for a better understanding
of the mechanisms leading to magnetism in non-metallic, carbon-based materials.
This work reports a direct evaluation of the hyperfine magnetic field produced
at 13C nuclei in ferromagnetic graphite by nuclear magnetic resonance (NMR).
The experimental investigation was made possible by the results of
first-principles calculations carried out in model systems, including graphene
sheets with atomic vacancies and graphite nanoribbons with edge sites partially
passivated by oxygen. A similar range of maximum hyperfine magnetic field
values (18-21T) was found for all systems, setting the frequency span to be
investigated in the NMR experiments; accordingly, a significant 13C NMR signal
was detected close to this range without any external applied magnetic field in
ferromagnetic graphite
The road travelled : after main‐group elements as transition metals
Since the latter quarter of the twentieth century, main group chemistry has undergone significant advances. Power's timely review in 2010 highlighted the inherent differences between the lighter and heavier main group elements, and that the heavier analogues resemble transition metals as shown by their reactivity towards small molecules. In this concept article, we present an overview of the last 10 years since Power's seminal review, and the progress made for catalytic application. This examines the use of low oxidation state and/or low coordinate group 13 and 14 complexes towards small molecule activation (oxidative addition step in a redox based cycle) and how ligand design plays a crucial role in influencing subsequent reactivity. The challenge in these redox based catalytic cycles still centres on the main group complexes’ ability to undergo reductive elimination, however considerable progress in this field has been reported via reversible oxidative addition reactions. Within the last 5 years the first examples of well‐defined low valent main group catalysts have begun to emerge, representing a bright future ahead for main group chemistry
Enhanced Piezoresponse and Nonlinear Optical Properties of Fluorinated Self-Assembled Peptide Nanotubes
Self-assembled L,L-diphenylalanine (FF) nanostructures offer an attractive platform for photonics and nonlinear optics. The nonlinear optical (NLO) coefficients of FF nanotubes depend on the diameter of the tube [S. Khanra et al. Phys. Chem. Chem. Phys. 19(4), 3084-3093 (2017)]. To further enhance the NLO properties of FF, we search for structural modifications. Here, we report on the synthesis of fluorinated FF dipeptides by replacing one ortho-hydrogen atom in each of the phenyl groups of FF by a fluorine atom. Density-functional theoretical calculations yield insights into minimum energy conformers of fluorinated FF (Fl-FF). Fl-FF self-assembles akin to FF into micron-length tubes. The effects of fluorination are evaluated on the piezoelectric response and nonlinear optical properties. The piezoelectric d15 coefficient of Fl-FF is found to be more than 10 times higher than that of FF nanotubes, and the intensity of second harmonic generation (SHG) polarimetry from individual Fl-FF nanotubes is more than 20 times that of individual FF nanotubes. Furthermore, we obtain SHG images to compare the intensities of FF and Fl-FF tubes. This work demonstrates the potential of fluorine substitution in other self-assembled biomimetic peptides for enhancing nonlinear optical response and piezoelectricity
Plant Genetics and Molecular Biology: An Introduction
The rapidly evolving technologies can serve as a potential growth engine in agriculture as many of these technologies have revolutionized several industries in the recent past. The tremendous advancements in biotechnology methods, cost-effective sequencing technology, refinement of genomic tools, and standardization of modern genomics-assisted breeding methods hold great promise in taking the global agriculture to the next level through development of improved climate-smart seeds. These technologies can dramatically increase our capacity to understand the molecular basis of traits and utilize the available resources for accelerated development of stable high-yielding, nutritious, input-use efficient, and climate-smart crop varieties. This book aimed to document the monumental advances witnessed during the last decade in multiple fields of plant biotechnology such as genetics, structural and functional genomics, trait and gene discovery, transcriptomics, proteomics, metabolomics, epigenomics, nanotechnology, and analytical tools. This book will serve to update the scientific community, academicians, and other stakeholders in global agriculture on the rapid progress in various areas of agricultural biotechnology. This chapter provides a summary of the book, “Plant Genetics and Molecular Biology.
Measurement of polarisation observables in photoproduction off the proton
The reaction is studied in the
photon energy range from threshold. Linearly polarised photon beams from
coherent bremsstrahlung enabled the first measurement of photon beam
asymmetries in this reaction up to MeV. In addition, the
recoil hyperon polarisation was determined through the asymmetry in the weak
decay up to MeV. The data are
compared to partial wave analyses, and the possible impact on the
interpretation of a recently observed cusp-like structure near the
thresholds is discussed.Comment: 6 pages, 5 figures. References [8,9,10,11] which were not on the
original submission are now include
Quasi-free photoproduction of eta-mesons of the neutron
Quasi-free photoproduction of eta-mesons off nucleons bound in the deuteron
has been measured with the CBELSA/TAPS detector for incident photon energies up
to 2.5 GeV at the Bonn ELSA accelerator. The eta-mesons have been detected in
coincidence with recoil protons and recoil neutrons, which allows a detailed
comparison of the quasi-free n(gamma,eta)n and p(gamma,eta)p reactions. The
excitation function for eta-production off the neutron shows a pronounced
bump-like structure at W=1.68 GeV (E_g ~ 1 GeV), which is absent for the
proton.Comment: accepted for publication in Phys. Rev. Let
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