167 research outputs found
Beam-excited whistler waves at oblique propagation with relation to STEREO radiation belt observations
Isotropic electron beams are considered to explain the excitation of
whistler waves which have been observed by the STEREO satellite in the Earth's radiation belt.
Aside from their large amplitudes (~240 mV/m), another main signature is the strongly
inclined propagation direction relative to the ambient magnetic field. Electron
temperature anisotropy with <I>T</I><sub>e⊥</sub>><I>T</I><sub>e||</sub>, which preferentially generates parallel
propagating whistler waves, can be excluded as a free energy source.
The instability
arises due to the interaction of the Doppler-shifted cyclotron mode ω=−Ω<sub>e</sub>+<I>kV</I><sub>b</sub>cosθ with
the whistler mode in the wave number range
of <I>kc</I>/ω<sub>e</sub>≤1 (θ is the propagation angle
with respect to the background magnetic field direction, ω<sub>e</sub> is the electron plasma frequency
and Ω<sub>e</sub> the electron cyclotron frequency).
Fluid and kinetic dispersion analysis have been used to calculate the growth rate of the
beam-excited whistlers including the most important parameter dependencies.
One is the beam
velocity (<I>V</I><sub>b</sub>) which, for instability, has to be larger than about 2<I>V</I><sub>Ae</sub>, where <I>V</I><sub>Ae</sub> is the
electron Alfvén speed. With increasing <I>V</I><sub>Ae</sub> the propagation angle (θ) of the fastest growing
whistler waves shifts from θ~20° for <I>V</I><sub>b</sub>=2<I>V</I><sub>Ae</sub> to θ~80° for <I>V</I><sub>b</sub>=5<I>V</I><sub>Ae</sub>.
The growth rate is
reduced by finite electron temperatures and disappears if the electron plasma beta (β<sub>e</sub>)
exceeds β<sub>e</sub>~0.2.
In addition, Gendrin modes (<I>kc</I>/ω<sub>e</sub>≈1) are analyzed to determine the conditions
under which stationary nonlinear waves (whistler oscillitons) can exist. The corresponding
spatial wave profiles are calculated using the full nonlinear fluid approach. The results
are compared with the STEREO satellite
observations
Fluid Simulations of Three-Dimensional Reconnection that Capture the Lower-Hybrid Drift Instability
Fluid models that approximate kinetic effects have received attention
recently in the modelling of large scale plasmas such as planetary
magnetospheres. In three-dimensional reconnection, both reconnection itself and
current sheet instabilities need to be represented appropriately. We show that
a heat flux closure based on pressure gradients enables a ten moment fluid
model to capture key properties of the lower-hybrid drift instability (LHDI)
within a reconnection simulation. Characteristics of the instability are
examined with kinetic and fluid continuum models, and its role in the
three-dimensional reconnection simulation is analysed. The saturation level of
the electromagnetic LHDI is higher than expected which leads to strong kinking
of the current sheet. Therefore, the magnitude of the initial perturbation has
significant impact on the resulting turbulence.Comment: 20 pages, 9 figure
Metabolomic profiles are gender, disease and time specific in the interleukin-10 gene-deficient mouse model of inflammatory bowel disease.
Metabolomic profiling can be used to study disease-induced changes in inflammatory bowel diseases (IBD). The aim of this study was to investigate the difference in the metabolomic profile of males and females as they developed IBD. Using the IL-10 gene-deficient mouse model of IBD and wild-type mice, urine at age 4, 6, 8, 12, 16, and 20 weeks was collected and analyzed by nuclear magnetic resonance (NMR) spectroscopy. Multivariate data analysis was employed to assess differences in metabolomic profiles that occurred as a consequence of IBD development and severity (at week 20). These changes were contrasted to those that occurred as a consequence of gender. Our results demonstrate that both IL-10 gene-deficient and wild-type mice exhibit gender-related changes in urinary metabolomic profile over time. Some male-female separating metabolites are common to both IL-10 gene-deficient and control wild-type mice and, therefore, appear to be related predominantly to gender maturation. In addition, we were able to identify gender-separating metabolites that are unique for IL-10 gene-deficient and wild-type mice and, therefore, may be indicative of a gender-specific involvement in the development and severity of the intestinal inflammation. The comparison of the gender-separating metabolomic profile from IL-10 gene-deficient mice and wild-type mice during the development of IBD allowed us to identify changes in profile patterns that appear to be imperative in the development of intestinal inflammation, but yet central to gender-related differences in IBD development. The knowledge of metabolomic profile differences by gender and by disease severity has potential clinical implications in the design of both biomarkers of disease as well as the development of optimal therapies
Particle Energization in an Expanding Magnetized Relativistic Plasma
Using a 2-1/2-dimensional particle-in-cell (PIC) code to simulate the
relativistic expansion of a magnetized collisionless plasma into a vacuum, we
report a new mechanism in which the magnetic energy is efficiently converted
into the directed kinetic energy of a small fraction of surface particles. We
study this mechanism for both electron-positron and electron-ion (mi/me=100, me
is the electron rest mass) plasmas. For the electron-positron case the pairs
can be accelerated to ultra-relativistic energies. For electron-ion plasmas
most of the energy gain goes to the ions.Comment: 7 pages text plus 5 figures, accepted for publication by Physical
Review Letter
A New Type of Plasma Wakefield Accelerator Driven by Magnetowaves
We present a new concept for a plasma wakefield accelerator driven by
magnetowaves (MPWA). This concept was originally proposed as a viable mechanism
for the "cosmic accelerator" that would accelerate cosmic particles to ultra
high energies in the astrophysical setting. Unlike the more familiar Plasma
Wakefield Accelerator (PWFA) and the Laser Wakefield Accelerator (LWFA) where
the drivers, the charged-particle beam and the laser, are independently
existing entities, MPWA invokes the high-frequency and high-speed whistler mode
as the driver, which is a medium wave that cannot exist outside of the plasma.
Aside from the difference in drivers, the underlying mechanism that excites the
plasma wakefield via the ponderomotive potential is common. Our computer
simulations show that under appropriate conditions, the plasma wakefield
maintains very high coherence and can sustain high-gradient acceleration over
many plasma wavelengths. We suggest that in addition to its celestial
application, the MPWA concept can also be of terrestrial utility. A
proof-of-principle experiment on MPWA would benefit both terrestrial and
celestial accelerator concepts.Comment: revtex4, 4 pages, 6 figure
Linear theory of nonlocal transport in a magnetized plasma
A system of nonlocal electron-transport equations for small perturbations in
a magnetized plasma is derived using the systematic closure procedure of V. Yu.
Bychenkov et al., Phys. Rev. Lett. 75, 4405 (1995). Solution to the linearized
kinetic equation with a Landau collision operator is obtained in the diffusive
approximation. The Fourier components of the longitudinal, oblique, and
transversal electron fluxes are found in an explicit form for quasistatic
conditions in terms of the generalized forces: the gradients of density and
temperature, and the electric field. The full set of nonlocal transport
coefficients is given and discussed. Nonlocality of transport enhances electron
fluxes across magnetic field above the values given by strongly collisional
local theory. Dispersion and damping of magnetohydrodynamic waves in weakly
collisional plasmas is discussed. Nonlocal transport theory is applied to the
problem of temperature relaxation across the magnetic field in a laser hot
spot.Comment: 27 pages, 13 figure
Spin wave dynamics and the determination of intrinsic Gilbert damping in locally-excited Permalloy thin films
Time-resolved scanning Kerr effect microscopy has been used to study
magnetization dynamics in Permalloy thin films excited by transient magnetic
pulses generated by a micrometer-scale transmission line structure. The results
are consistent with magnetostatic spin wave theory and are supported by
micromagnetic simulations. Magnetostatic volume and surface spin waves are
measured for the same specimen using different bias field orientations and can
be accurately calculated by k-space integrations over all excited plane wave
components. A single damping constant of Gilbert form is sufficient to describe
both scenarios. The nonuniform pulsed field plays a key role in the spin wave
dynamics, with its Fourier transform serving as a weighting function for the
participating modes. The intrinsic Gilbert damping parameter is most
conveniently measured when the spin waves are effectively stationary.Comment: 5 pages, 4 figures, accepted by Phys. Rev. Let
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