367 research outputs found
Particle acceleration by circularly and elliptically polarised dispersive Alfven waves in a transversely inhomogeneous plasma in the inertial and kinetic regimes
Dispersive Alfven waves (DAWs) offer, an alternative to magnetic
reconnection, opportunity to accelerate solar flare particles. We study the
effect of DAW polarisation, L-, R-, circular and elliptical, in different
regimes inertial and kinetic on the efficiency of particle acceleration. We use
2.5D PIC simulations to study how particles are accelerated when DAW, triggered
by a solar flare, propagates in transversely inhomogeneous plasma that mimics
solar coronal loop. (i) In inertial regime, fraction of accelerated electrons
(along the magnetic field), in density gradient regions is ~20% by the time
when DAW develops 3 wavelengths and is increasing to ~30% by the time DAW
develops 13 wavelengths. In all considered cases ions are heated in transverse
to the magnetic field direction and fraction of the heated particles is ~35%.
(ii) The case of R-circular, L- and R- elliptical polarisation DAWs, with the
electric field in the non-ignorable transverse direction exceeding several
times that of in the ignorable direction, produce more pronounced parallel
electron beams and transverse ion beams in the ignorable direction. In the
inertial regime such polarisations yield the fraction of accelerated electrons
~20%. In the kinetic regime this increases to ~35%. (iii) The parallel electric
field that is generated in the density inhomogeneity regions is independent of
m_i/m_e and exceeds the Dreicer value by 8 orders of magnitude. (iv) Electron
beam velocity has the phase velocity of the DAW. Thus electron acceleration is
via Landau damping of DAWs. For the Alfven speeds of 0.3c the considered
mechanism can accelerate electrons to energies circa 20 keV. (v) The increase
of mass ratio from m_i/m_e=16 to 73.44 increases the fraction of accelerated
electrons from 20% to 30-35% (depending on DAW polarisation). For the mass
ratio m_i/m_e=1836 the fraction of accelerated electrons would be >35%.Comment: Final accepted version. To appear in Physics of Plasmas, volume 18,
issue 9 (September 2011
Study of the possibility of amplification of a very relativistic synchrotron or gyromagnetic radiation by a noncollective plasma
Amplification of synchrotron radiation by plasm
Weakly Turbulent MHD Waves in Compressible Low-Beta Plasmas
In this Letter, weak turbulence theory is used to investigate interactions
among Alfven waves and fast and slow magnetosonic waves in collisionless
low-beta plasmas. The wave kinetic equations are derived from the equations of
magnetohydrodynamics, and extra terms are then added to model collisionless
damping. These equations are used to provide a quantitative description of a
variety of nonlinear processes, including "parallel" and "perpendicular" energy
cascade, energy transfer between wave types, "phase mixing," and the generation
of back-scattered Alfven waves.Comment: Accepted, Physical Review Letter
Jeans criterion in a turbulent medium
According to the classical Jeans analysis, all the molecular clouds of mass larger than a few 100 M(solar), size larger than about 1pc and kinetic temperature Tk less than 30K are gravitationally unstable. We have shown that in clouds supported by internal supersonic motions, local gravitational instabilities may appear within molecular clouds which are globally stable. The argument is threefold: (1) when the turbulent kinetic energy is included into the internal energy term, the virial equilibrium condition shows that molecular clouds such as those observed, which are gravitationally unstable according to the Jeans criterion, are indeed globally stable if supported by a turbulent velocity field of power spectrum steeper than 3; (2) 2D compressible hydrodynamical simulations show that a supersonic turbulent velocity field generates a turbulent pressure within clouds, the gradients of which stabilize the unstable scales (i.e., the largest scales and the cloud itself) against gravitational collapse; (3) an analysis similar to the Jeans approach but including the turbulent pressure gradient term, gives basically the same results as those given in (1). Clouds of mean density lower than a critical value are found to be stable even though more massive than their Jeans mass. In clouds of mean density larger than that critical value, the gravitational instability appears only over a range of scales smaller than the cloud size, the largest scales being stable. In practice, the observed mean densities are lower than this critical value: the observation of a small number of cores and stars of a few solar masses embedded in clouds of several hundred solar masses can only be understood in terms of small scale density fluctuations of large amplitude generated by the supersonic turbulence which would occasionally overtake the limit of gravitational stability
Are Coronae of Magnetically Active Stars Heated by Flares? III. Analytical Distribution of Superimposed Flares
(abridged) We study the hypothesis that observed X-ray/extreme ultraviolet
emission from coronae of magnetically active stars is entirely (or to a large
part) due to the superposition of flares, using an analytic approach to
determine the amplitude distribution of flares in light curves. The
flare-heating hypothesis is motivated by time series that show continuous
variability suggesting the presence of a large number of superimposed flares
with similar rise and decay time scales. We rigorously relate the amplitude
distribution of stellar flares to the observed histograms of binned counts and
photon waiting times, under the assumption that the flares occur at random and
have similar shapes. Applying these results to EUVE/DS observations of the
flaring star AD Leo, we find that the flare amplitude distribution can be
represented by a truncated power law with a power law index of 2.3 +/- 0.1. Our
analytical results agree with existing Monte Carlo results of Kashyap et al.
(2002) and Guedel et al. (2003). The method is applicable to a wide range of
further stochastically bursting astrophysical sources such as cataclysmic
variables, Gamma Ray Burst substructures, X-ray binaries, and spatially
resolved observations of solar flares.Comment: accepted for publication in Ap
A magnetic thrust action on small bodies orbiting a pulsar
We investigate the electromagnetic interaction of a relativistic stellar wind
with small bodies in orbit around the star. Based on our work on the theory of
Alfv\'en wings to relativistic winds presented in a companion paper, we
estimate the force exerted by the associated current system on orbiting bodies
and evaluate the resulting orbital drift. This Alfv\'enic structure is found to
have no significant influence on planets or smaller bodies orbiting a
millisecond pulsar. %influence on the orbit of bodies around a millisecond
pulsar. On the timescale of millions of years, it can however affect the orbit
of bodies with a diameter of 100 kilometres around standard pulsars with a
period 1 s and a magnetic field T. Kilometer-sized
bodies experience drastic orbital changes on a timescale of years.Comment: accepted for publication in "Astronomy and Astrophysics
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