167 research outputs found
Thermal stability analysis of the fine structure of solar prominences
The linear thermal stability of a 2D periodic structure (alternatively hot and cold) in a uniform magnetic field is analyzed. The energy equation includes wave heating (assumed proportional to density), radiative cooling and both conduction parallel and orthogonal to magnetic lines. The equilibrium is perturbed at constant gas pressure. With parallel conduction only, it is found to be unstable when the length scale 1// is greater than 45 Mn. In that case, orthogonal conduction becomes important and stabilizes the structure when the length scale is smaller than 5 km. On the other hand, when the length scale is greater than 5 km, the thermal equilibrium is unstable, and the corresponding time scale is about 10,000 s: this result may be compared to observations showing that the lifetime of the fine structure of solar prominences is about one hour; consequently, our computations suggest that the size of the unresolved threads could be of the order of 10 km only
Suppression of energetic electron transport in flares by double layers
During flares and coronal mass ejections, energetic electrons from coronal
sources typically have very long lifetimes compared to the transit times across
the systems, suggesting confinement in the source region. Particle-in-cell
simulations are carried out to explore the mechanisms of energetic electron
transport from the corona to the chromosphere and possible confinement. We set
up an initial system of pre-accelerated hot electrons in contact with ambient
cold electrons along the local magnetic field, and let it evolve over time.
Suppression of transport by a nonlinear, highly localized electrostatic
electric field (in the form of a double layer) is observed after a short phase
of free-streaming by hot electrons. The double layer (DL) emerges at the
contact of the two electron populations. It is driven by an ion-electron
streaming instability due to the drift of the back-streaming return current
electrons interacting with the ions. The DL grows over time and supports a
significant drop in temperature and hence reduces heat flux between the two
regions that is sustained for the duration of the simulation. This study shows
transport suppression begins when the energetic electrons start to propagate
away from a coronal acceleration site. It also implies confinement of energetic
electrons with kinetic energies less than the electrostatic energy of the DL
for the DL lifetime, which is much longer than the electron transit time
through the source region
The penetration of plasma clouds across magnetic boundaries : the role of high frequency oscillations
Experiments are reported where a collisionfree plasma cloud penetrates a
magnetic barrier by self-polarization. We here focus on the resulting anomalous
magnetic field diffusion into the plasma cloud, two orders of magnitude faster
than classical, which is one important aspect of the plasma cloud penetration
mechanism. Without such fast magnetic diffusion, clouds with kinetic beta below
unity would not be able to penetrate magnetic barriers at all. Tailor-made
diagnostics has been used for measurements in the parameter range with the
kinetic beta ? 0.5 to 10, and with normalized width w/r(gi) of the order of
unity. Experimental data on hf fluctuations in density and in electric field
has been combined to yield the effective anomalous transverse resistivity
eta(EFF). It is concluded that they are both dominated by highly nonlinear
oscillations in the lower hybrid range, driven by a strong diamagnetic current
loop that is set up in the plasma in the penetration process. The anomalous
magnetic diffusion rate, calculated from the resistivity eta(EFF), is
consistent with single-shot multi-probe array measurements of the diamagnetic
cavity and the associated quasi-dc electric structure. An interpretation of the
instability measurements in terms of the resistive term in the generalized (low
frequency) Ohm's law is given.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Conditions for plasmoid penetration across magnetic barriers
The penetration of plasma clouds, or plasmoids, across abrupt magnetic
barriers (of the scale less than a few ion gyro radii, using the plasmoid
directed velocity) is studied. The insight gained earlier, from experimental
and computer simulation investigations of a case study, is generalised into
other parameter regimes. It is concluded for what parameters a plasmoid should
be expected to penetrate the magnetic barrier through self-polarization,
penetrate through magnetic expulsion, or be rejected from the barrier. The
scaling parameters are n(e), v(0), B(perp), m(i), T(i), and the width w of the
plasmoid. The scaling is based on a model for strongly driven, nonlinear
magnetic field diffusion into a plasma, which is a generalization of the
laboratory findings. The results are applied to experiments earlier reported in
the literature, and also to the proposed application of impulsive penetration
of plasmoids from the solar wind into the Earth's magnetosphere.Comment: 12th International Congress on Plasma Physics, 25-29 October 2004,
Nice (France
Рентгенометрический анализ кинематики L4-L5 И L5-S1 позвоночных сегментов в III стадии дегенеративного процесса
ПОЗВОНОЧНИКА БОЛЕЗНИОСТЕОХОНДРОЗОСТЕОХОНДРИТГРЫЖА МЕЖПОЗВОНОЧНОГО ДИСКАМЕЖПОЗВОНКОВОГО ДИСКА СМЕЩЕНИЕРЕНТГЕНОГРАФИ
Modification of the formation of high-Mach number electrostatic shock-like structures by the ion acoustic instability
The formation of unmagnetized electrostatic shock-like structures with a high Mach number is examined with one- and two-dimensional particle-in-cell (PIC) simulations. The structures are generated through the collision of two identical plasma clouds, which consist of equally hot electrons and ions with a mass ratio of 250. The Mach number of the collision speed with respect to the initial ion acoustic speed of the plasma is set to 4.6. This high Mach number delays the formation of such structures by tens of inverse ion plasma frequencies. A pair of stable shock-like structures is observed after this time in the 1D simulation, which gradually evolves into electrostatic shocks. The ion acoustic instability, which can develop in the 2D simulation but not in the 1D one, competes with the nonlinear process that gives rise to these structures. The oblique ion acoustic waves fragment their electric field. The transition layer, across which the bulk of the ions change their speed, widens and their speed change is reduced. Double layer-shock hybrid structures develop
Nucleation of titanium nanoparticles in an oxygen-starved environment, II: Theory
The nucleation and growth of pure titanium nanoparticles in a low-pressure
sputter plasma has been believed to be essentially impossible. The addition of
impurities, such as oxygen or water, facilitates this and allows the growth of
nanoparticles. However, it seems that this route requires so high oxygen
densities that metallic nanoparticles in the hexagonal aTi-phase cannot be
synthesized. Here we present a model which explains results for the nucleation
and growth of titanium nanoparticles in the absent of reactive impurities. In
these experiments, a high partial pressure of helium gas was added which
increased the cooling rate of the process gas in the region where nucleation
occurred. This is important for two reasons. First, a reduced gas temperature
enhances Ti2 dimer formation mainly because a lower gas temperature gives a
higher gas density, which reduces the dilution of the Ti vapor through
diffusion. The same effect can be achieved by increasing the gas pressure.
Second, a reduced gas temperature has a "more than exponential" effect in
lowering the rate of atom evaporation from the nanoparticles during their
growth from a dimer to size where they are thermodynamically stable, r*. We
show that this early stage evaporation is not possible to model as a
thermodynamical equilibrium. Instead, the single-event nature of the
evaporation process has to be considered. This leads, counter intuitively, to
an evaporation probability from nanoparticles that is exactly zero below a
critical nanoparticle temperature that is size-dependent. Together, the
mechanisms described above explain two experimentally found limits for
nucleation in an oxygen-free environment. First, there is a lower limit to the
pressure for dimer formation. Second, there is an upper limit to the gas
temperature above which evaporation makes the further growth to stable nuclei
impossible.Comment: 32 pages, 7 figure
The triggering of MHD instabilities through photospheric footpoint motions
The results of 3D numerical simulations modelling the twisting of a coronal loop due to photospheric vortex motions are presented. The simulations are carried out using an initial purely axial field and an initial equilibrium configuration with twist, . The non-linear and resistive evolutions of the instability are followed. The magnetic field is twisted by the boundary motions into a loop which initially has boundary layers near the photospheric boundaries as has been suggested by previous work. The boundary motions increase the twist in the loop until it becomes unstable. For both cases the boundary twisting triggers the kink instability. In both cases a helical current structure wraps itself around the kinked central current. This current scales linearly with grid resolution indicating current sheet formation. For the cases studied 35-40% of the free magnetic energy is released. This is sufficient to explain the energy released in a compact loop flare
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