5,952 research outputs found
Nonlinear mirror instability
Slow dynamical changes in magnetic-field strength and invariance of the
particles' magnetic moments generate ubiquitous pressure anisotropies in weakly
collisional, magnetized astrophysical plasmas. This renders them unstable to
fast, small-scale mirror and firehose instabilities, which are capable of
exerting feedback on the macroscale dynamics of the system. By way of a new
asymptotic theory of the early nonlinear evolution of the mirror instability in
a plasma subject to slow shearing or compression, we show that the instability
does not saturate quasilinearly at a steady, low-amplitude level. Instead, the
trapping of particles in small-scale mirrors leads to nonlinear secular growth
of magnetic perturbations, . Our theory explains
recent collisionless simulation results, provides a prediction of the mirror
evolution in weakly collisional plasmas and establishes a foundation for a
theory of nonlinear mirror dynamics with trapping, valid up to .Comment: 5 pages, submitte
From Small-Scale Dynamo to Isotropic MHD Turbulence
We consider the problem of incompressible, forced, nonhelical, homogeneous,
isotropic MHD turbulence with no mean magnetic field. This problem is
essentially different from the case with externally imposed uniform mean field.
There is no scale-by-scale equipartition between magnetic and kinetic energies
as would be the case for the Alfven-wave turbulence. The isotropic MHD
turbulence is the end state of the turbulent dynamo which generates folded
fields with small-scale direction reversals. We propose that the statistics
seen in numerical simulations of isotropic MHD turbulence could be explained as
a superposition of these folded fields and Alfven-like waves that propagate
along the folds.Comment: kluwer latex, 7 pages, 7 figures; Proceedings of the International
Workshop "Magnetic Fields and Star Formation: Theory vs. Observations",
Madrid, 21-25 April 2003 -- published version (but the e-print is free of
numerous typos introduced by the publisher
Search of X-ray emission from roAp stars: The case of gamma Equulei
The detection of X-ray emission from Ap stars can be an indicator for the
presence of magnetic activity and dynamo action, provided different origins for
the emission, such as wind shocks and close late-type companions, can be
excluded. Here we report on results for gamma Equu, the only roAp star for
which an X-ray detection is reported in ROSAT catalogs. We use high resolution
imaging in X-rays with Chandra and in the near-infrared with NACO/VLT that
allow us to spatially resolve companions down to ~1" and ~0.06" separations,
respectively. The bulk of the X-ray emission is associated with a companion of
gamma Equu identified in our NACO image. Assuming coevality with the primary
roAp star (~900 Myr), the available photometry for the companion points at a
K-type star with ~0.6 M_sun. Its X-ray properties are in agreement with the
predictions for its age and mass. An excess of photons with respect to the
expected background and contribution from the nearby companion is observed near
the optical position of gamma Equu. We estimate an X-ray luminosity of log L_x
[erg/s] = 26.6 and log(L_x/L_bol) = -7.9 for this emission. A small offset
between the optical and the X-ray image leaves some doubt on its association
with the roAp star. The faint X-ray emission that we tentatively ascribe to the
roAp star is difficult to explain as a solar-like stellar corona due to its
very low L_x/L_bol level and the very long rotation period of gamma Equu. It
could be produced in magnetically confined wind shocks implying a mass loss
rate of ~10^(-14) M_sun/yr or from an additional unknown late-type companion at
separation ~0.4". If confirmed by future deeper X-ray observations this
emission could point at the origin for the presence of radioactive elements on
some roAp stars.Comment: Accepted for publication in Astronomy & Astrophysics (5 pages
The interaction between transpolar arcs and cusp spots
Transpolar arcs and cusp spots are both auroral phenomena which occur when
the interplanetary magnetic field is northward. Transpolar arcs are associated
with magnetic reconnection in the magnetotail, which closes magnetic flux and
results in a "wedge" of closed flux which remains trapped, embedded in the
magnetotail lobe. The cusp spot is an indicator of lobe reconnection at the
high-latitude magnetopause; in its simplest case, lobe reconnection
redistributes open flux without resulting in any net change in the open flux
content of the magnetosphere. We present observations of the two phenomena
interacting--i.e., a transpolar arc intersecting a cusp spot during part of its
lifetime. The significance of this observation is that lobe reconnection can
have the effect of opening closed magnetotail flux. We argue that such events
should not be rare
Diffusion of passive scalar in a finite-scale random flow
We consider a solvable model of the decay of scalar variance in a
single-scale random velocity field. We show that if there is a separation
between the flow scale k_flow^{-1} and the box size k_box^{-1}, the decay rate
lambda ~ (k_box/k_flow)^2 is determined by the turbulent diffusion of the
box-scale mode. Exponential decay at the rate lambda is preceded by a transient
powerlike decay (the total scalar variance ~ t^{-5/2} if the Corrsin invariant
is zero, t^{-3/2} otherwise) that lasts a time t~1/\lambda. Spectra are sharply
peaked at k=k_box. The box-scale peak acts as a slowly decaying source to a
secondary peak at the flow scale. The variance spectrum at scales intermediate
between the two peaks (k_box0). The mixing
of the flow-scale modes by the random flow produces, for the case of large
Peclet number, a k^{-1+delta} spectrum at k>>k_flow, where delta ~ lambda is a
small correction. Our solution thus elucidates the spectral make up of the
``strange mode,'' combining small-scale structure and a decay law set by the
largest scales.Comment: revtex4, 8 pages, 4 figures; final published versio
Model of two-fluid reconnection
A theoretical model of quasi-stationary, two-dimensional magnetic
reconnection is presented in the framework of incompressible two-fluid
magnetohydrodynamics (MHD). The results are compared with recent numerical
simulations and experiment.Comment: 4 pages, 1 figure, accepted to Physical Review Letter
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