209 research outputs found
Study of the possibility of amplification of a very relativistic synchrotron or gyromagnetic radiation by a noncollective plasma
Amplification of synchrotron radiation by plasm
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
Nonstationary driven oscillations of a magnetic cavity
The problem of transition to the steady state of driven oscillations in a magnetic cavity in a cold resistive plasma is addressed. The foot point driving polarized in the inhomogeneous direction is considered, and it is assumed that the cavity length in the direction of the equilibrium magnetic field is much larger than the cavity width in the inhomogeneous direction. The latter assumption enables one to neglect the variation of the magnetic pressure in the inhomogeneous direction, which strongly simplifies the analysis. The explicit solution describing the nonstationary behavior of the magnetic pressure and the velocity is obtained. This solution is used to study the properties of the transition to the steady state of oscillation. The main conclusion is that, in general, there are two different characteristic transitional times. The first time is inversely proportional to the decrement of the global mode. It characterizes the transition to the steady state of the global motion, which is the coherent oscillation of the cavity in the inhomogeneous direction. The second time is the largest of the two times, the first transitional time and the phase-mixing time, which is proportional to the magnetic Reynolds number in 1/3 power. It characterizes the transition to the steady state of the local motion, which is oscillations at the local Alfvén frequencies, and the saturation of the energy damping rate. An example from solar physics shows that, in applications, the second transitional time can be much larger than the first one
Nonlinear Instability of kink oscillations due to shear motions
First results from a high-resolution three-dimensional nonlinear numerical
study of the kink oscillation are presented. We show in detail the development
of a shear instability in an untwisted line-tied magnetic flux tube. The
instability produces significant deformations of the tube boundary. An extended
transition layer may naturally evolve as a result of the shear instability at a
sharp transition between the flux tube and the external medium. We also discuss
the possible effects of the instability on the process of resonant absorption
when an inhomogeneous layer is included in the model. One of the implications
of these results is that the azimuthal component of the magnetic field of a
stable flux tube in the solar corona, needed to prevent the shear instability,
is probably constrained to be in a very specific range
Making the corona and the fast solar wind: a self-consistent simulation for the low-frequency Alfven waves from photosphere to 0.3AU
We show that the coronal heating and the fast solar wind acceleration in the
coronal holes are natural consequence of the footpoint fluctuations of the
magnetic fields at the photosphere, by performing one-dimensional
magnetohydrodynamical simulation with radiative cooling and thermal conduction.
We initially set up a static open flux tube with temperature 10^4K rooted at
the photosphere. We impose transverse photospheric motions corresponding to the
granulations with velocity = 0.7km/s and period between 20 seconds and 30
minutes, which generate outgoing Alfven waves. We self-consistently treat these
waves and the plasma heating. After attenuation in the chromosphere by ~85% of
the initial energy flux, the outgoing Alfven waves enter the corona and
contribute to the heating and acceleration of the plasma mainly by the
nonlinear generation of the compressive waves and shocks. Our result clearly
shows that the initial cool and static atmosphere is naturally heated up to
10^6K and accelerated to 800km/s.Comment: 4 pages, 3 figures, ApJL, 632, L49, corrections of mistypes in
eqs.(3) & (5), Mpeg movie for fig.1 (simulation result) is available at
http://www-tap.scphys.kyoto-u.ac.jp/~stakeru/research/suzuki_200506.mp
An Ab Initio Approach to the Solar Coronal Heating Problem
We present an ab initio approach to the solar coronal heating problem by
modelling a small part of the solar corona in a computational box using a 3D
MHD code including realistic physics. The observed solar granular velocity
pattern and its amplitude and vorticity power spectra, as reproduced by a
weighted Voronoi tessellation method, are used as a boundary condition that
generates a Poynting flux in the presence of a magnetic field. The initial
magnetic field is a potential extrapolation of a SOHO/MDI high resolution
magnetogram, and a standard stratified atmosphere is used as a thermal initial
condition. Except for the chromospheric temperature structure, which is kept
fixed, the initial conditions are quickly forgotten because the included
Spitzer conductivity and radiative cooling function have typical timescales
much shorter than the time span of the simulation. After a short initial start
up period, the magnetic field is able to dissipate 3-4 10^6 ergs cm^{-2} s^{-1}
in a highly intermittent corona, maintaining an average temperature of K, at coronal density values for which emulated images of the Transition
Region And Coronal Explorer(TRACE) 171 and 195 pass bands reproduce observed
photon count rates.Comment: 12 pages, 14 figures. Submitted to Ap
A Contemporary View of Coronal Heating
Determining the heating mechanism (or mechanisms) that causes the outer
atmosphere of the Sun, and many other stars, to reach temperatures orders of
magnitude higher than their surface temperatures has long been a key problem.
For decades the problem has been known as the coronal heating problem, but it
is now clear that `coronal heating' cannot be treated or explained in isolation
and that the heating of the whole solar atmosphere must be studied as a highly
coupled system. The magnetic field of the star is known to play a key role,
but, despite significant advancements in solar telescopes, computing power and
much greater understanding of theoretical mechanisms, the question of which
mechanism or mechanisms are the dominant supplier of energy to the chromosphere
and corona is still open. Following substantial recent progress, we consider
the most likely contenders and discuss the key factors that have made, and
still make, determining the actual (coronal) heating mechanism (or mechanisms)
so difficult
The Thermal Instability of Solar Prominence Threads
The fine structure of solar prominences and filaments appears as thin and
long threads in high-resolution images. In H-alpha observations of filaments,
some threads can be observed for only 5 - 20 minutes before they seem to fade
and eventually disappear, suggesting that these threads may have very short
lifetimes. The presence of an instability might be the cause of this quick
disappearance. Here, we study the thermal instability of prominence threads as
an explanation of their sudden disappearance from H-alpha observations. We
model a prominence thread as a magnetic tube with prominence conditions
embedded in a coronal environment. We assume a variation of the physical
properties in the transverse direction, so that the temperature and density
continuously change from internal to external values in an inhomogeneous
transitional layer representing the particular prominence-corona transition
region (PCTR) of the thread. We use the nonadiabatic and resistive
magnetohydrodynamic equations, which include terms due to thermal conduction
parallel and perpendicular to the magnetic field, radiative losses, heating,
and magnetic diffusion. We combine both analytical and numerical methods to
study linear perturbations from the equilibrium state, focusing on unstable
thermal solutions. We find that thermal modes are unstable in the PCTR for
temperatures higher than 80,000 K, approximately. These modes are related to
temperature disturbances that can lead to changes in the equilibrium due to
rapid plasma heating or cooling. For typical prominence parameters, the
instability time scale is of the order of a few minutes and is independent of
the form of the temperature profile within the PCTR of the thread. This result
indicates that thermal instability may play an important role for the short
lifetimes of threads in the observations.Comment: Accepted for publication in Ap
Impulsive phase flare energy transport by large-scale Alfven waves and the electron acceleration problem
The impulsive phase of a solar flare marks the epoch of rapid conversion of
energy stored in the pre-flare coronal magnetic field. Hard X-ray observations
imply that a substantial fraction of flare energy released during the impulsive
phase is converted to the kinetic energy of mildly relativistic electrons
(10-100 keV). The liberation of the magnetic free energy can occur as the
coronal magnetic field reconfigures and relaxes following reconnection. We
investigate a scenario in which products of the reconfiguration - large-scale
Alfven wave pulses - transport the energy and magnetic-field changes rapidly
through the corona to the lower atmosphere. This offers two possibilities for
electron acceleration. Firstly, in a coronal plasma with beta < m_e/m_p, the
waves propagate as inertial Alfven waves. In the presence of strong spatial
gradients, these generate field-aligned electric fields that can accelerate
electrons to energies on the order of 10 keV and above, including by repeated
interactions between electrons and wavefronts. Secondly, when they reflect and
mode-convert in the chromosphere, a cascade to high wavenumbers may develop.
This will also accelerate electrons by turbulence, in a medium with a locally
high electron number density. This concept, which bridges MHD-based and
particle-based views of a flare, provides an interpretation of the
recently-observed rapid variations of the line-of-sight component of the
photospheric magnetic field across the flare impulsive phase, and offers
solutions to some perplexing flare problems, such as the flare "number problem"
of finding and resupplying sufficient electrons to explain the impulsive-phase
hard X-ray emission.Comment: 31 pages, 6 figure
Luminosity of a quark star undergoing torsional oscillations and the problem of gamma ray bursts
We discuss whether the winding-up of the magnetic field by differential
rotation in a new-born quark star can produce a sufficiently-high, energy,
emission rate of sufficiently long duration to explain long gamma-ray bursts.
In the context of magnetohydrodynamics, we study the torsional oscillations
and energy extraction from a new-born, hot, differentially rotating quark star.
The new-born compact star is a rapid rotator that produces a relativistic,
leptonic wind. The star's torsional oscillation modulates this wind emission
considerably when it is odd and of sufficient amplitude, which is relatively
easy to reach. Odd oscillations may occur just after the formation of a quark
star. Other asymmetries can cause similar effects. The buoyancy of wound-up
magnetic fields is inhibited, or its effects are limited, by a variety of
different mechanisms. Direct electromagnetic emission by the torsional
oscillation in either an outside vacuum or the leptonic wind surrounding the
compact object is found to be insignificant. In contrast, the twist given to
the outer magnetic field by an odd torsional oscillation is generally
sufficient to open the star's magnetosphere. The Poynting emission of the star
in its leptonic environment is then radiated from all of its surface and is
enhanced considerably during these open episodes, tapping at the bulk
rotational energy of the star. This results in intense energy shedding in the
first tens of minutes after the collapse of magnetized quark stars with an
initial poloidal field of order of 10**14 Gauss, sufficient to explain long
gamma-ray bursts.Comment: 16 pages, accepted by Astronomy and Astrophysic
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