4,782 research outputs found
Magnetic Wreaths and Cycles in Convective Dynamos
Solar-type stars exhibit a rich variety of magnetic activity. Seeking to
explore the convective origins of this activity, we have carried out a series
of global 3D magnetohydrodynamic (MHD) simulations with the anelastic spherical
harmonic (ASH) code. Here we report on the dynamo mechanisms achieved as the
effects of artificial diffusion are systematically decreased. The simulations
are carried out at a nominal rotation rate of three times the solar value
(3), but similar dynamics may also apply to the Sun. Our previous
simulations demonstrated that convective dynamos can build persistent toroidal
flux structures (magnetic wreaths) in the midst of a turbulent convection zone
and that high rotation rates promote the cyclic reversal of these wreaths. Here
we demonstrate that magnetic cycles can also be achieved by reducing the
diffusion, thus increasing the Reynolds and magnetic Reynolds numbers. In these
more turbulent models, diffusive processes no longer play a significant role in
the key dynamical balances that establish and maintain the differential
rotation and magnetic wreaths. Magnetic reversals are attributed to an
imbalance in the poloidal magnetic induction by convective motions that is
stabilized at higher diffusion levels. Additionally, the enhanced levels of
turbulence lead to greater intermittency in the toroidal magnetic wreaths,
promoting the generation of buoyant magnetic loops that rise from the deep
interior to the upper regions of our simulated domain. The implications of such
turbulence-induced magnetic buoyancy for solar and stellar flux emergence are
also discussed.Comment: 21 pages, 16 figures, accepted for publication in Ap
Collisionless energy transfer in kinetic turbulence: field-particle correlations in Fourier space
Turbulence is ubiquitously observed in nearly collisionless heliospheric
plasmas, including the solar wind and corona and the Earth's magnetosphere.
Understanding the collisionless mechanisms responsible for the energy transfer
from the turbulent fluctuations to the particles is a frontier in kinetic
turbulence research. Collisionless energy transfer from the turbulence to the
particles can take place reversibly, resulting in non-thermal energy in the
particle velocity distribution functions (VDFs) before eventual collisional
thermalization is realized. Exploiting the information contained in the
fluctuations in the VDFs is valuable. Here we apply a recently developed method
based on VDFs, the field-particle correlation technique, to a ,
solar-wind-like, low-frequency Alfv\'enic turbulence simulation with well
resolved phase space to identify the field-particle energy transfer in velocity
space. The field-particle correlations reveal that the energy transfer,
mediated by the parallel electric field, results in significant structuring of
the ion and electron VDFs in the direction parallel to the magnetic field.
Fourier modes representing the length scales between the ion and electron
gyroradii show that energy transfer is resonant in nature, localized in
velocity space to the Landau resonances for each Fourier mode. The energy
transfer closely follows the Landau resonant velocities with varying
perpendicular wavenumber and plasma . This resonant signature,
consistent with Landau damping, is observed in all diagnosed Fourier modes that
cover the dissipation range of the simulation.Comment: 31 pages, accepted by JPP, minor improvements compared to v
Turbulence patterns and neutrino flavor transitions in high-resolution supernova models
During the shock-wave propagation in a core-collapse supernova (SN), matter
turbulence may affect neutrino flavor conversion probabilities. Such effects
have been usually studied by adding parametrized small-scale random
fluctuations (with arbitrary amplitude) on top of coarse, spherically symmetric
matter density profiles. Recently, however, two-dimensional (2D) SN models have
reached a space resolution high enough to directly trace anisotropic density
profiles, down to scales smaller than the typical neutrino oscillation length.
In this context, we analyze the statistical properties of a large set of SN
matter density profiles obtained in a high-resolution 2D simulation, focusing
on a post-bounce time (2 s) suited to study shock-wave effects on neutrino
propagation on scales as small as O(100) km and possibly below. We clearly find
the imprint of a broken (Kolmogorov-Kraichnan) power-law structure, as
generically expected in 2D turbulence spectra. We then compute the flavor
evolution of SN neutrinos along representative realizations of the turbulent
matter density profiles, and observe no or modest damping of the neutrino
crossing probabilities on their way through the shock wave. In order to check
the effect of possibly unresolved fluctuations at scales below O(100) km, we
also apply a randomization procedure anchored to the power spectrum calculated
from the simulation, and find consistent results within \pm 1 sigma
fluctuations. These results show the importance of anchoring turbulence effects
on SN neutrinos to realistic, fine-grained SN models.Comment: 19 pages, 8 figures (Major changes in the text, references added,
analysis and figures improved, main results unchanged. To appear in JCAP.
Origin of multi-level switching and telegraphic noise in organic nanocomposite memory devices.
The origin of negative differential resistance (NDR) and its derivative intermediate resistive states (IRSs) of nanocomposite memory systems have not been clearly analyzed for the past decade. To address this issue, we investigate the current fluctuations of organic nanocomposite memory devices with NDR and the IRSs under various temperature conditions. The 1/f noise scaling behaviors at various temperature conditions in the IRSs and telegraphic noise in NDR indicate the localized current pathways in the organic nanocomposite layers for each IRS. The clearly observed telegraphic noise with a long characteristic time in NDR at low temperature indicates that the localized current pathways for the IRSs are attributed to trapping/de-trapping at the deep trap levels in NDR. This study will be useful for the development and tuning of multi-bit storable organic nanocomposite memory device systems
Identifying nonlinear wave interactions in plasmas using two-point measurements: a case study of Short Large Amplitude Magnetic Structures (SLAMS)
A framework is described for estimating Linear growth rates and spectral
energy transfers in turbulent wave-fields using two-point measurements. This
approach, which is based on Volterra series, is applied to dual satellite data
gathered in the vicinity of the Earth's bow shock, where Short Large Amplitude
Magnetic Structures (SLAMS) supposedly play a leading role. The analysis
attests the dynamic evolution of the SLAMS and reveals an energy cascade toward
high-frequency waves.Comment: 26 pages, 13 figure
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