7,483 research outputs found
Cumulative effect of Weibel-type instabilities in counterstreaming plasmas with non-Maxwellian anisotropies
Counterstreaming plasma structures are widely present in laboratory
experiments and astrophysical systems, and they are investigated either to
prevent unstable modes arising in beam-plasma experiments or to prove the
existence of large scale magnetic fields in astrophysical objects.
Filamentation instability arises in a counterstreaming plasma and is
responsible for the magnetization of the plasma. Filamentationally unstable
mode is described by assuming that each of the counterstreaming plasmas has an
isotropic Lorentzian (kappa) distribution. In this case, the filamentation
instability growth rate can reach a maximum value markedly larger than that for
a a plasma with a Maxwellian distribution function. This behaviour is opposite
to what was observed for the Weibel instability growth rate in a bi-kappa
plasma, which is always smaller than that obtained for a bi-Maxwellian plasma.
The approach is further generalized for a counterstreaming plasma with a
bi-kappa temperature anisotropy. In this case, the filamentation instability
growth rate is enhanced by the Weibel effect when the plasma is hotter in the
streaming direction, and the growth rate becomes even larger. These effects
improve significantly the efficiency of the magnetic field generation, and
provide further support for the potential role of the Weibel-type instabilities
in the fast magnetization scenarios
Demonstrating Universal Scaling in Quench Dynamics of a Yukawa One-Component Plasma
The Yukawa one-component plasma (OCP) is a paradigm model for describing
plasmas that contain one component of interest and one or more other components
that can be treated as a neutralizing, screening background. In appropriately
scaled units, interactions are characterized entirely by a screening parameter,
. As a result, systems of similar show the same dynamics,
regardless of the underlying parameters (e.g., density and temperature). We
demonstrate this behavior using ultracold neutral plasmas (UNP) created by
photoionizing a cold ( mK) gas. The ions in UNP systems are well
described by the Yukawa model, with the electrons providing the screening.
Creation of the plasma through photoionization can be thought of as a rapid
quench from to a final value set by the electron
density and temperature. We demonstrate experimentally that the post-quench
dynamics are universal in over a factor of 30 in density and an order
of magnitude in temperature. Results are compared with molecular dynamics
simulations. We also demonstrate that features of the post-quench kinetic
energy evolution, such as disorder-induced heating and kinetic-energy
oscillations, can be used to determine the plasma density and the electron
temperature.Comment: 10 pages, 12 figures, to be submitted to Physical Review
Heating of the solar wind with electron and proton effects
We examine the effects of including effects of both protons and electrons on the heating of the fast solar wind through two different approaches. In the first approach, we incorporate the electron temperature in an MHD turbulence transport model for the solar wind. In the second approach, we adopt more empirically based methods by analyzing the measured proton and electron temperatures to calculate the heat deposition rates. Overall, we conclude that incorporating separate proton and electron temperatures and heat conduction effects provides an improved and more complete model of the heating of the solar wind
Biodegradable All-Polymer Field-Effect Transistors Printed on Mater-Bi
The growing demand of disposable electronics raises serious concerns for the corresponding increase in the amount of electronic waste, with severe environmental impact. Organic and flexible electronics have been proposed long ago as a more sustainable and energy-efficient technological platform with respect to established ones. Yet, such technology is leading to a drastic increase of plastic waste if common approaches for flexible substrates are followed. In this scenario, biodegradable solutions can significantly limit the environmental impact, actively contributing to eliminate the waste streams (plastic or electronic) associated with disposal of devices. However, achieving suitably scalable processes to pattern mechanically robust organic electronics onto largely available biodegradable substrates is still an open challenge. In this work, all-organic and highly flexible field-effect transistors, inkjet printed onto the biodegradable and compostable commercial substrate Mater-Bi, are demonstrated. Because of the thermal instability of Mater-Bi, no annealing steps are applied, producing devices with limited carrier mobility, yet showing correct n-type behavior and robustness to bending and crumpling. The degradation behavior of the final system shows unaltered biodegradability level according to ISO 14851. These results represent a promising step toward sustainable flexible and large-area electronics, combining energy and materials efficient processes with largely available biodegradable substrates
On the Brightness and Waiting-time Distributions of a Type III Radio Storm observed by STEREO/WAVES
Type III solar radio storms, observed at frequencies below approximately 16
MHz by space borne radio experiments, correspond to the quasi-continuous,
bursty emission of electron beams onto open field lines above active regions.
The mechanisms by which a storm can persist in some cases for more than a solar
rotation whilst exhibiting considerable radio activity are poorly understood.
To address this issue, the statistical properties of a type III storm observed
by the STEREO/WAVES radio experiment are presented, examining both the
brightness distribution and (for the first time) the waiting-time distribution.
Single power law behavior is observed in the number distribution as a function
of brightness; the power law index is approximately 2.1 and is largely
independent of frequency. The waiting-time distribution is found to be
consistent with a piecewise-constant Poisson process. This indicates that
during the storm individual type III bursts occur independently and suggests
that the storm dynamics are consistent with avalanche type behavior in the
underlying active region.Comment: 14 pages, 4 figures, 1 table. Accepted for publication in
Astrophysical Journal Letter
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Magnetoresistance Scaling Reveals Symmetries of the Strongly Correlated Dynamics in BaFe_{2}(As_{1-x}P_{x})_{2}.
The phenomenon of T-linear resistivity commonly observed in a number of strange metals has been widely seen as evidence for the breakdown of the quasiparticle picture of metals. This study shows that a recently discovered H/T scaling relationship in the magnetoresistance of the strange metal BaFe_{2}(As_{1-x}P_{x})_{2} is independent of the relative orientations of current and magnetic field. Rather, its magnitude and form depend only on the orientation of the magnetic field with respect to a single crystallographic axis: the direction perpendicular to the magnetic iron layers. This finding suggests that the magnetotransport scaling does not originate from the conventional averaging or orbital velocity of quasiparticles as they traverse a Fermi surface, but rather from dissipation arising from two-dimensional correlations
Anisotropic Radio-Wave Scattering and the Interpretation of Solar Radio Emission Observations
The observed properties (i.e., source size, source position, time duration, decay time) of solar radio emission produced through plasma processes near the local plasma frequency, and hence the interpretation of solar radio bursts, are strongly influenced by propagation effects in the inhomogeneous turbulent solar corona. In this work, a 3D stochastic description of the propagation process is presented, based on the Fokker-Planck and Langevin equations of radio-wave transport in a medium containing anisotropic electron density fluctuations. Using a numerical treatment based on this model, we investigate the characteristic source sizes and burst decay times for Type III solar radio bursts. Comparison of the simulations with the observations of solar radio bursts shows that predominantly perpendicular density fluctuations in the solar corona are required, with an anisotropy factor ~0.3 for sources observed at around 30 MHz. The simulations also demonstrate that the photons are isotropized near the region of primary emission, but the waves are then focused by large-scale refraction, leading to plasma radio emission directivity that is characterized by a half-width-half-maximum of about 40 degrees near 30 MHz. The results are applicable to various solar radio bursts produced via plasma emission
Anisotropy in the Magnetoresistance Scaling of BaFe(AsP)
Theories of the strange metal, the parent state of many high temperature
superconductors, invariably involve an important role for correlations in the
spin and charge degrees of freedom. The most distinctive signature of this
state in the charge transport sector is a resistance that varies linearly in
temperature, but this phenomenon does not clearly point to one mechanism as
temperature is a scalar quantity that influences every possible mechanism for
momentum relaxation. In a previous work we identified an unusual scaling
relationship between magnetic field and temperature in the in-plane resistivity
of the unconventional superconductor BaFe(AsP), providing
an opportunity to use the vector nature of the magnetic field to acquire
additional clues about the mechanisms responsible for scattering in the strange
metal state. Here we extend this work by investigating other components of the
conductivity tensor under different orientations of the magnetic field. We find
that the scaling phenomenon involves only the out-of-plane component of the
magnetic field and is, strikingly, independent of the direction of the applied
current. This suggests that the origin of the strange magnetotransport is in
the action of the magnetic field on the correlated behavior of spin and charge
degrees of freedom, rather than on the simple cyclotron motion of individual
quasiparticles.Comment: 5 pages, 3 figure
Thermalisation of self-interacting solar flare fast electrons
Most theoretical descriptions of the production of solar flare bremsstrahlung
radiation assume the collision of dilute accelerated particles with a cold,
dense target plasma, neglecting interactions of the fast particles with each
other. This is inadequate for situations where collisions with this background
plasma are not completely dominant, as may be the case in, for example,
low-density coronal sources. We aim to formulate a model of a self-interacting,
entirely fast electron population in the absence of a dense background plasma,
to investigate its implications for observed bremsstrahlung spectra and the
flare energy budget. We derive approximate expressions for the time-dependent
distribution function of the fast electrons using a Fokker-Planck approach. We
use these expressions to generate synthetic bremsstrahlung X-ray spectra as
would be seen from a corresponding coronal source. We find that our model
qualitatively reproduces the observed behaviour of some flares. As the flare
progresses, the model's initial power-law spectrum is joined by a lower energy,
thermal component. The power-law component diminishes, and the growing thermal
component proceeds to dominate the total emission over timescales consistent
with flare observations. The power-law exhibits progressive spectral hardening,
as is seen in some flare coronal sources. We also find that our model requires
a factor of 7 - 10 fewer accelerated electrons than the cold, thick target
model to generate an equivalent hard X-ray flux. This model forms the basis of
a treatment of self-interactions among flare fast electrons, a process which
affords a more efficient means to produce bremsstrahlung photons and so may
reduce the efficiency requirements placed on the particle acceleration
mechanism. It also provides a useful description of the thermalisation of fast
electrons in coronal sources.Comment: 9 pages, 7 figures, accepted for Astronomy & Astrophysics; this
version clarifies arguments around Eqs. (11) and (20
A transonic collisionless model of the solar wind
Because of the semi-collisional nature of the solar wind, the collisionless
or exospheric approach as well as the hydrodynamic one are both inaccurate.
However, the advantage of simplicity makes them useful for enlightening some
basic mechanisms of solar wind acceleration. Previous exospheric models have
been able to reproduce winds that were already nearly supersonic at the
exobase, the altitude above which there are no collisions. In order to allow
transonic solutions, a lower exobase has to be considered, in which case the
protons are experiencing a non-monotonic potential energy profile. This is done
in the present work. In this model, the electron velocity distribution in the
corona is assumed non-thermal. Parametric results are presented and show that
the high acceleration obtained does not depend on the details of the
non-thermal distributions. This acceleration seems, therefore, to be a robust
result produced by the presence of a sufficient number of suprathermal
electrons. A method for improving the exospheric description is also given,
which consists in mapping particle orbits in terms of their invariants of
motion.Comment: 18 pages, 18 figures, accepted for publication in The Astrophysical
Journal (1 May 2004
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