258 research outputs found
Generalised-Lorentzian Thermodynamics
We extend the recently developed non-gaussian thermodynamic formalism
\cite{tre98} of a (presumably strongly turbulent) non-Markovian medium to its
most general form that allows for the formulation of a consistent thermodynamic
theory. All thermodynamic functions, including the definition of the
temperature, are shown to be meaningful. The thermodynamic potential from which
all relevant physical information in equilibrium can be extracted, is defined
consistently. The most important findings are the following two: (1) The
temperature is defined exactly in the same way as in classical statistical
mechanics as the derivative of the energy with respect to the entropy at
constant volume. (2) Observables are defined in the same way as in Boltzmannian
statistics as the linear averages of the new equilibrium distribution function.
This lets us conclude that the new state is a real thermodynamic equilibrium in
systems capable of strong turbulence with the new distribution function
replacing the Boltzmann distribution in such systems. We discuss the ideal gas,
find the equation of state, and derive the specific heat and adiabatic exponent
for such a gas. We also derive the new Gibbsian distribution of states. Finally
we discuss the physical reasons for the development of such states and the
observable properties of the new distribution function.Comment: 13 pages, 1 figur
Collisional damping rates for plasma waves
The distinction between the plasma dynamics dominated by collisional
transport versus collective processes has never been rigorously addressed until
recently. A recent paper [Yoon et al., Phys. Rev. E 93, 033203 (2016)]
formulates for the first time, a unified kinetic theory in which collective
processes and collisional dynamics are systematically incorporated from first
principles. One of the outcomes of such a formalism is the rigorous derivation
of collisional damping rates for Langmuir and ion-acoustic waves, which can be
contrasted to the heuristic customary approach. However, the results are given
only in formal mathematical expressions. The present Brief Communication
numerically evaluates the rigorous collisional damping rates by considering the
case of plasma particles with Maxwellian velocity distribution function so as
to assess the consequence of the rigorous formalism in a quantitative manner.
Comparison with the heuristic ("Spitzer") formula shows that the accurate
damping rates are much lower in magnitude than the conventional expression,
which implies that the traditional approach over-estimates the importance of
attenuation of plasma waves by collisional relaxation process. Such a finding
may have a wide applicability ranging from laboratory to space and
astrophysical plasmas.Comment: 5 pages, 2 figures; Published in Physics of Plasmas, volume/Issue
23/6. Publisher: AIP Publishing LLC. Date: Jun 1, 2016. URL:
http://aip.scitation.org/doi/10.1063/1.4953802 Rights managed by AIP
Publishing LL
Cluster Multi-spacecraft Determination of AKR Angular Beaming
Simultaneous observations of AKR emission using the four-spacecraft Cluster
array were used to make the first direct measurements of the angular beaming
patterns of individual bursts. By comparing the spacecraft locations and AKR
burst locations, the angular beaming pattern was found to be narrowly confined
to a plane containing the magnetic field vector at the source and tangent to a
circle of constant latitude. Most rays paths are confined within 15 deg of this
tangent plane, consistent with numerical simulations of AKR k-vector
orientation at maximum growth rate. The emission is also strongly directed
upward in the tangent plane, which we interpret as refraction of the rays as
they leave the auroral cavity. The narrow beaming pattern implies that an
observer located above the polar cap can detect AKR emission only from a small
fraction of the auroral oval at a given location. This has important
consequences for interpreting AKR visibility at a given location. It also helps
re-interpret previously published Cluster VLBI studies of AKR source locations,
which are now seen to be only a subset of all possible source locations. These
observations are inconsistent with either filled or hollow cone beaming models.Comment: 5 pages, 4 figures. Geophys. Res. Letters (accepted
A Note on the Statistical Mechanics of Violent Relaxation of Phase Space Elements of Different Densities
The statistical mechanical investigation of Violent Relaxation of phase space
elements of different densities first derived by Lynden-Bell (1967) is
re-examined. It is found that the mass independence of the equations of motion
of Violent Relaxation calls for a constraint on the volume of the phase space
elements used to formulate the statistical mechanical description of Violent
Relaxation. In agreement with observations of astrophysical objects believed to
have been subject to Violent Relaxation (e.g. clusters of galaxies), the coarse
grained phase space distribution of the final state in the
non-degenerate limit turns into a superposition of Maxwellians of a common
velocity dispersion. Thus, the velocity dispersion problem present in the
investigation of Lynden-Bell (1967) is removed.Comment: AASTeX, 9 pages, uses aas2pp4.sty. Accepted for publication in Ap
Radio relics in cosmological simulations
Radio relics have been discovered in many galaxy clusters. They are believed
to trace shock fronts induced by cluster mergers. Cosmological simulations
allow us to study merger shocks in detail since the intra-cluster medium is
heated by shock dissipation. Using high resolution cosmological simulations,
identifying shock fronts and applying a parametric model for the radio emission
allows us to simulate the formation of radio relics. We analyze a simulated
shock front in detail. We find a rather broad Mach number distribution. The
Mach number affects strongly the number density of relativistic electrons in
the downstream area, hence, the radio luminosity varies significantly across
the shock surface. The abundance of radio relics can be modeled with the help
of the radio power probability distribution which aims at predicting radio
relic number counts. Since the actual electron acceleration efficiency is not
known, predictions for the number counts need to be normalized by the observed
number of radio relics. For the characteristics of upcoming low frequency
surveys we find that about thousand relics are awaiting discovery.Comment: 10 pages, 4 figures, Invited talk at the conference "Diffuse
Relativistic Plasmas", Bangalore, 1-4 March 2011; in press in special issue
of Journal of Astrophysics and Astronom
A river model of space
Within the theory of general relativity gravitational phenomena are usually
attributed to the curvature of four-dimensional spacetime. In this context we
are often confronted with the question of how the concept of ordinary physical
three-dimensional space fits into this picture. In this work we present a
simple and intuitive model of space for both the Schwarzschild spacetime and
the de Sitter spacetime in which physical space is defined as a specified set
of freely moving reference particles. Using a combination of orthonormal basis
fields and the usual formalism in a coordinate basis we calculate the physical
velocity field of these reference particles. Thus we obtain a vivid description
of space in which space behaves like a river flowing radially toward the
singularity in the Schwarzschild spacetime and radially toward infinity in the
de Sitter spacetime. We also consider the effect of the river of space upon
light rays and material particles and show that the river model of space
provides an intuitive explanation for the behavior of light and particles at
and beyond the event horizons associated with these spacetimes.Comment: 22 pages, 5 figure
A Mini-survey of Ultracool Dwarfs at 4.9 GHz
A selection of ultracool dwarfs are known to be radio active, with both
gyrosynchrotron emission and the electron cyclotron maser instability being
given as likely emission mechanisms. To explore whether ultracool dwarfs
previously undetected at 8.5 GHz may be detectable at a lower frequency. We
select a sample of fast rotating ultracool dwarfs with no detectable radio
activity at 8.5 GHz, observing each of them at 4.9 GHz. From the 8 dwarfs in
our sample, we detect emission from 2MASS J07464256+2000321, with a mean flux
level of 286 24 . The light-curve of 2MASS J07464256+2000321, is
dominated towards the end of the observation by a very bright, 100 %
left circularly polarized burst during which the flux reached 2.4 mJy. The
burst was preceded by a raise in the level of activity, with the average flux
being 160 in the first hour of observation rising to
400 in the 40 minutes before the burst. During both periods,
there is significant variability. The detection of 100% circular polarization
in the emission at 4.9 GHz points towards the electron cyclotron maser as the
emission mechanism. However, the observations at 4.9 GHz and 8.5 GHz were not
simultaneous, thus the actual fraction of dwarfs capable of producing radio
emission, as well as the fraction of those that show periodic pulsations is
still unclear, as indeed are the relative roles played by the electron
cyclotron maser instability versus gyrosynchrotron emission, therefore we
cannot assert if the previous non-detection at 8.5 GHz was due to a cut-off in
emission between 4.9 and 8.4 GHz, or due to long term variability
Fast Reconnection in a Two-Stage Process
Magnetic reconnection plays an essential role in the generation and evolution
of astrophysical magnetic fields. The best tested and most robust reconnection
theory is that of Parker and Sweet. According to this theory, the reconnection
rate scales with magnetic diffusivity lambda as lambda^0.5. In the interstellar
medium, the Parker-Sweet reconnection rate is far too slow to be of interest.
Thus, a mechanism for fast reconnection seems to be required. We have studied
the magnetic merging of two oppositely directed flux systems in weakly ionized,
but highly conducting, compressible gas. In such systems, ambipolar diffusion
steepens the magnetic profile, leading to a thin current sheet. If the ion
pressure is small enough, and the recombination of ions is fast enough, the
resulting rate of magnetic merging is fast, and independent of lambda. Slow
recombination or sufficiently large ion pressure leads to slower merging which
scales with lambda as lambda^0.5. We derive a criterion for distinguishing
these two regimes, and discuss applications to the weakly ionized ISM and to
protoplanetary accretion disks.Comment: 21 pages, 13 figures, submitted to Ap
Plasmoid-Induced-Reconnection and Fractal Reconnection
As a key to undertanding the basic mechanism for fast reconnection in solar
flares, plasmoid-induced-reconnection and fractal reconnection are proposed and
examined. We first briefly summarize recent solar observations that give us
hints on the role of plasmoid (flux rope) ejections in flare energy release. We
then discuss the plasmoid-induced-reconnection model, which is an extention of
the classical two-ribbon-flare model which we refer to as the CSHKP model. An
essential ingredient of the new model is the formation and ejection of a
plasmoid which play an essential role in the storage of magnetic energy (by
inhibiting reconnection) and the induction of a strong inflow into reconnection
region. Using a simple analytical model, we show that the plasmoid ejection and
acceleration are closely coupled with the reconnection process, leading to a
nonlinear instability for the whole dynamics that determines the macroscopic
reconnection rate uniquely. Next we show that the current sheet tends to have a
fractal structure via the following process path: tearing, sheet thinning,
Sweet- Parker sheet, secondary tearing, further sheet thinning... These
processes occur repeatedly at smaller scales until a microscopic plasma scale
(either the ion Larmor radius or the ion inertial length) is reached where
anomalous resistivity or collisionless reconnection can occur. The current
sheet eventually has a fractal structure with many plasmoids (magnetic islands)
of different sizes. When these plasmoids are ejected out of the current sheets,
fast reconnection occurs at various different scales in a highly time dependent
manner. Finally, a scenario is presented for fast reconnection in the solar
corona on the basis of above plasmoid-induced-reconnection in a fractal current
sheet.Comment: 9 pages, 11 figures, with using eps.sty; Earth, Planets and Space in
press; ps-file is also available at
http://stesun8.stelab.nagoya-u.ac.jp/~tanuma/study/shibata2001
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