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1.
Electromagnetic instabilities in high-β plasmas, where β is the ratio of the kinetic plasma energy to the magnetic energy, have a broad range of astrophysical applications. The presence
of temperature anisotropies T
∥
/T
⊥
>1 (where ∥ and ⊥ denote directions relative to the background magnetic field) in solar flares and the solar wind is sustained by the observations
and robust acceleration mechanisms that heat plasma particles in the parallel direction. The surplus of parallel kinetic energy
can excite either the Weibel-like instability (WI) of the ordinary mode perpendicular to the magnetic field or the firehose
instability (FHI) of the circularly polarized waves at parallel propagation. The interplay of these two instabilities is examined.
The growth rates and the thresholds provided by the kinetic Vlasov – Maxwell theory are compared. The WI is the fastest growing
one with a growth rate that is several orders of magnitude larger than that of the FHI. These instabilities are however inhibited
by the ambient magnetic field by introducing a temperature anisotropy threshold. The WI admits a larger anisotropy threshold,
so that, under this threshold, the FHI remains the principal mechanism of relaxation. The criteria provided here by describing
the interplay of the WI and FHI are relevant for the existence of these two instabilities in any space plasma system characterized
by an excess of parallel kinetic energy. 相似文献
2.
K.-I. Nishikawa C. B. Hededal P. E. Hardee G. J. Fishman C. Kouveliotou Y. Mizuno 《Astrophysics and Space Science》2007,307(1-3):319-323
We have applied numerical simulations and modeling to the particle acceleration, magnetic field generation, and emission from
relativistic shocks. We investigate the nonlinear stage of theWeibel instability and compare our simulations with the observed
gamma-ray burst emission. In collisionless shocks, plasma waves and their associated instabilities (e.g., the Weibel, Buneman
and other two-stream instabilities) are responsible for particle (electron, positron, and ion) acceleration and magnetic field
generation. 3-D relativistic electromagnetic particle (REMP) simulations with three different electron-positron jet velocity
distributions and also with an electron-ion plasma have been performed and show shock processes including spatial and temporal
evolution of shocks in unmagnetized ambient plasmas. The growth time and nonlinear saturation levels depend on the initial
jet parallel velocity distributions. Simulations show that the Weibel instability created in the collisionless shocks accelerates
jet and ambient particles both perpendicular and parallel to the jet propagation direction. The nonlinear fluctuation amplitude
of densities, currents, electric, and magnetic fields in the electron-positron shocks are larger for smaller jet Lorentz factor.
This comes from the fact that the growth time of the Weibel instability is proportional to the square of the jet Lorentz factor.
We have performed simulations with broad Lorentz factor distribution of jet electrons and positrons, which is assumed to be
created by photon annihilation. Simulation results with this broad distribution show that the Weibel instability is excited
continuously by the wide-range of jet Lorentz factor from lower to higher values. In all simulations the Weibel instability
is responsible for generating and amplifying magnetic fields perpendicular to the jet propagation direction, and contributes
to the electron’s (positron’s) transverse deflection behind the jet head. This small scale magnetic field structure contributes
to the generation of “jitter” radiation from deflected electrons (positrons), which is different from synchrotron radiation
in uniform magnetic fields. The jitter radiation resulting from small scale magnetic field structures may be important for
understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources
containing relativistic jets and relativistic collisionless shocks. The detailed studies of shock microscopic process evolution
may provide some insights into early and later GRB afterglows. 相似文献
3.
Henrik N. Latter Pierre Lesaffre Steven A. Balbus 《Monthly notices of the Royal Astronomical Society》2009,394(2):715-729
Local simulations of the magnetorotational instability (MRI) in accretion discs can exhibit recurrent coherent structures called channel flows. The formation and destruction of these structures may play a role in the development and saturation of MRI-induced turbulence, and consequently help us understand the time-dependent accretion behaviour of certain astrophysical objects. Previous investigations have revealed that channel solutions are attacked by various parasitic modes, foremost of which is an analogue of the Kelvin–Helmholtz instability. We revisit these instabilities and show how they relate to the classical instabilities of plasma physics, the kink and pinch modes. However, we argue that in most cases channels emerge from developed turbulence and are eventually destroyed by turbulent mixing, not by the parasites. The exceptions are clean isolated channels, which appear in systems near criticality or which emerge from low amplitude initial conditions. These structures can achieve large amplitudes and are only then destroyed, giving rise to eruptive behaviour. 相似文献
4.
Lucas F. Wanex 《Astrophysics and Space Science》2005,298(1-2):337-340
A linear analysis of axial sheared flow in magnetohydrodynamic (MHD) jets with helical magnetic fields is presented. A linearized
set of ideal MHD equations allows the investigation of plasmas with both magnetic shear and flow shear included in the equilibrium
profile. These equations are integrated numerically by following the linear development in time of an initial seed perturbation.
Global instability growth rates are obtained after the numerical solution converges to the fastest growing mode. It is shown
that axial sheared flow reduces the growth of current-driven instabilities in plasma jets with constant magnetic pitch P = rB
z
/B
θ. 相似文献
5.
A summary of total sulfur abundances representative of the Apollo Missions is presented. Lunar crystalline rocks range from
0 to 3100μg S g−1. Lunar soils range from 310 to 1300μg S g−1. Rock mixing models evaluate the distribution of sulfur and define indigenous rock components and extralunar contributions
of sulfur in lunar soils. Extralunar sulfur shows a positive correlation with a CC-1 like meteoritic component and solar wind
derived total carbon content in the Apollo 16 and 17 lunar soils.
Presented at the 25th International Geological Congress, Sydney, Australia, Section 15, Planetology.
Contribution No. 105 from the Center for Meteorite Studies. 相似文献
6.
Nathan C. Hearn Tomasz Plewa R. Paul Drake Carolyn Kuranz 《Astrophysics and Space Science》2007,307(1-3):227-231
We present two- and three-dimensional simulations involving Richtmyer–Meshkov and Rayleigh-Taylor instabilities run with the
adaptive mesh refinement code, flash. Variations in the rate of mixing layer growth due to dimensionality, perturbation modes,
and simulation resolution are explored. These simulations are designed for detailed comparisons with experiments run on the
Omega laser to gain understanding of the mixing processes and to prepare for validation of the Flash code. 相似文献
7.
In core-collapse supernovae, strong blast waves drive interfaces susceptible to Rayleigh–Taylor (RT), Richtmyer–Meshkov (RM),
and Kelvin–Helmholtz (KH) instabilities. In addition, perturbation growth can result from material expansion in large-scale
velocity gradients behind the shock front. Laser-driven experiments are designed to produce a strongly shocked interface whose
evolution is a scaled version of the unstable hydrogen–helium interface in core-collapse supernovae such as SN 1987A. The
ultimate goal of this research is to develop an understanding of the effect of hydrodynamic instabilities and the resulting
transition to turbulence on supernovae observables that remain as yet unexplained.
This paper represents a summary of recent results from a computational study of unstable systems driven by high Mach number
shock and blast waves. For planar multimode systems, compressibility effects preclude the emergence of a regime of self-similar
instability growth independent of the initial conditions (ICs) by allowing for memory of the initial conditions to be retained
in the mix-width at all times. With higher-dimensional blast waves, divergence restores the properties necessary for establishment
of the self-similar state, but achieving it requires very high initial characteristic mode number and high Mach number for
the incident blast wave. Initial conditions predicted by some recent stellar calculations are incompatible with self-similarity. 相似文献
8.
It has been hypothesized that the sustained narrowness observed in the asymptotic cylindrical region of bipolar outflows from
Young Stellar Objects (YSO) indicates that these jets are magnetically collimated. The j
z × B
ϕ force observed in z-pinch plasmas is a possible explanation for these observations. However, z-pinch plasmas are subject
to current driven instabilities (CDI). The interest in using z-pinches for controlled nuclear fusion has lead to an extensive
theory of the stability of magnetically confined plasmas. Analytical, numerical, and experimental evidence from this field
suggest that sheared flow in magnetized plasmas can reduce the growth rates of the sausage and kink instabilities. Here we
propose the hypothesis that sheared helical flow can exert a similar stabilizing influence on CDI in YSO jets. 相似文献
9.
Instabilities in a nonstationary model of self-gravitating disks. I. Bar and ring perturbation modes
The instabilities of bar and ring mode perturbations against the background of a disk oscillating nonlinearly in its own plane
are examined in a disk model which is a nonstationary generalization of the well known Bisnovatyi-Kogan-Zel'dovich model.
Nonstationary analogs corresponding to a dispersion relation are found for these two oscillation modes. The results of the
calculations are presented in the form of critical dependences of the initial virial ratio on the degree of rotation. A comparative
analysis of the growth rates of the gravitational instability for these modes is also carried out. The bar mode instability
occurs if the initial total kinetic energy of the disk is no more than 10.4% of the initial potential energy. The mechanism
is associated with an instability in the radial motions which is aperiodic for small values of the rotation parameter Ω <
0.1, but is otherwise oscillatory. Calculations show that a ring structure can be formed as a result of an instability in
the radial motions if the initial total energy of the model is no more than 5.2% of the initial potential energy, regardless
of the value of Ω.
__________
Translated from Astrofizika, Vol. 51, No. 3, pp. 487–499 (August 2008). 相似文献
10.
Energetic protons haying ring type distributions are shown to generate low-frequency electrostatic waves, propagating nearly transverse to the geomagnetic field lines, in the ring current region by exciting Mode 1 arid Mode 2 nonresonant instabilities and a resonant instability. Mode 1 nonresonant instability has frequencies around ~4 Hz with transverse wavelengths of ~(8–80) km, and it is likely to occur in the region L = (7–8). Mode 2 nonresonant instability can generate frequencies ~(850–1450) Hz with transverse wavelengths ~(2–20) km. The typical frequencies and transverse wavelengths associated with the resonant instability are (950–1250) Hz and (30–65) km. Both the Mode 2 nonresonant instability and the resonant instability can occur in the ring current region with L = (4–6). The low-frequency modes driven by energetic protons could attain maximum saturation electric field amplitude varying from 0.8 mV/m to 70 mV/m. It is suggested that the turbulence produced by the low-frequency modes may cause pitch angle scattering of ring current protons in the region outside the plasmapause resulting in the ring current decay. 相似文献
11.
Yu. A. Fadeyev 《Astronomy Letters》2011,37(1):11-19
The evolution of Population I stars (X = 0.7, Z = 0.02) with initial masses 40M
⊙ ≤ M
ZAMS ≤ 120M
⊙ until core hydrogen exhaustion has been computed. Models of evolutionary sequences have been used as the initial conditions
in solving the equations of radiation hydrodynamics that describe the spherically symmetric motion of a self-gravitating gas.
Stars with initial masses M
ZAMS ≥ 50M
⊙ are shown to become unstable against radial oscillations during the main-sequence evolution. The instability growth rate
and the limit-cycle oscillation amplitude increase as the star evolves and as its initial mass increases. The pulsational
instability is attributable to the iron Z-bump κ mechanism (T ∼ 2 × 105 K). Convection that transfers from 20 to 50% of the total energy flux and, thus, reduces the efficiency of the κ mechanism emerges in the same layers. The periods of the radial oscillations of main-sequence stars lie within the range
from 0.09 to 8 days. The boundaries of the instability region of radial pulsations in the Hertzsprung-Russell diagram have
been determined and observational criteria for revealing pulsating variable main-sequence stars have been proposed. 相似文献
12.
Based on the extended Greenwich – NOAA/USAF catalogue of sunspot groups, it is demonstrated that the parameters describing
the latitudinal width of the sunspot generating zone (SGZ) are closely related to the current level of solar activity, and
the growth of the activity leads to the expansion of the SGZ. The ratio of the sunspot number to the width of the SGZ shows
saturation at a certain level of the sunspot number, and above this level the increase of the activity takes place mostly
due to the expansion of the SGZ. It is shown that the mean latitudes of sunspots can be reconstructed from the amplitudes
of solar activity. Using the obtained relations and the group sunspot numbers by Hoyt and Schatten (Solar Phys.
179, 189, 1998), the latitude distribution of sunspot groups (“the Maunder butterfly diagram”) for the eighteenth and the first half of
the nineteenth centuries is reconstructed and compared with historical sunspot observations. 相似文献
13.
A. S. Baranov 《Astrophysics》2006,49(2):248-254
The electromagnetic instability of an interstellar medium with an arbitrary velocity distribution is examined over the large
scale lengths typical of gas-dust clouds without a significant magnetic field. It is shown that over a moderate time scale
(months and years) these instabilities can develop and that the requirement of stability is satisfied by a narrow class of
distributions that are close to spherical.
__________
Translated from Astrofizika, Vol. 49, No. 2, pp. 289–297 (May 2006). 相似文献
14.
Dynamic interaction between impact melt and fragmented basement at Manicouagan: The suevite connection
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The interface between impact melt rocks and underlying footwall lithologies within the Manicouagan impact structure is defined by a zone of dynamic mixing (<20 m thick). This zone transitions as a continuum from clast‐free to clast‐bearing impact melt rocks, through melt‐bearing breccias to melt‐free breccias. Field observations; microscopy; and major, trace, and rare earth element analysis indicate that the breccias are derived by blending two endmembers during the impact process: impact melt and brecciated footwall. The product is a basal breccia sequence, which locally includes the rock type referred to as suevite. In this occurrence, the suevite is a submelt sheet variety, in contrast to similar lithologies that are developed atop impact melt sheets, or beyond crater rims. Dynamic mixing between impact melt and basal clastic material at Manicouagan is attributed to the initial high‐speed centrifugal outflow of superheated, low viscosity impact melt over underlying fractured and fragmented footwall, and its centripetal return during the earlier stages of the crater modification process. The interaction of two fluids (melt with a mobilized granular medium) possessing contrasting densities, and moving at different velocities, can facilitate shear instabilities and turbulent mixing that may be characteristic of Kelvin–Helmholtz behavior. 相似文献
15.
The evolution of the current sheet in the electric current direction (in the guiding magnetic field direction) is studied
numerically in the 3-D particle-in-cell model with two current sheets and periodic boundary conditions. In the regime with
(where v
D and
are the electric current drift and electron thermal velocities, respectively) the current sheets are unstable owing to the
Buneman and kink instabilities and become strongly fragmented. During their evolution, in addition to an increase of the energy
of the electric field component in the guiding magnetic field direction, the energies of the electric field components in
the perpendicular direction are even more enhanced. In the current sheet the anomalous resistivity (η
anom/η
C∼7×105, where η
C is the classical resistivity) is generated and thus the magnetic field dissipates. Most of the dissipated magnetic energy
is transformed into the electron kinetic energy in the direction of the electric current. The associated electric field accelerates
the electrons from the tail of the distribution function. 相似文献
16.
M. Stupar M. D. Filipović Q. A. Parker G. L. White T. G. Pannuti P. A. Jones 《Astrophysics and Space Science》2007,307(4):423-435
The Parkes–MIT–NRAO (PMN) radio survey has been used to generate a quasi all-sky study of Galactic Supernova Remnants (SNRs)
at a common frequency of 4.85 GHz (λ=6 cm). We present flux densities estimated for the sample of 110 Southern Galactic SNRs (up to δ=−65°) observed with the Parkes 64-m radio telescope and an additional sample of 54 from the Northern PMN (up to δ=+64°) survey undertaken with the Green Bank 43-m (20 SNRs) and 91-m (34 SNRs) radio telescopes. Out of this total sample
of 164 selected SNRs (representing 71% of the currently 231 known SNRs in the Green catalogue) we consider 138 to provide
reliable estimates of flux density and surface brightness distribution. This sub-sample represents those SNRs which fall within
carefully chosen selection criteria which minimises the effects of the known problems in establishing reliable fluxes from
the PMN survey data. Our selection criteria are based on a judicious restriction of source angular size and telescope beam
together with careful evaluation of fluxes on a case by case basis. Direct comparison of our new fluxes with independent literature
values gives excellent overall agreement. This gives confidence in the newly derived PMN fluxes when the selection criteria
are respected. We find a sharp drop off in the flux densities for Galactic SNRs beyond 4 Jy and then a fairly flat distribution
from 5 to 9 Jy, a slight decline and a further flat distribution from 9 to 20 Jy though the numbers of SNR in each Jy bin
are low. We also re-visit the contentious Σ–D (radio surface brightness–SNRs diameter) relation to determine a new power law index for a sub-sample of shell type SNRs
which yields β=−2.2±0.6. This new evaluation of the Σ–D relation, applied to the restricted sample, provides new distance estimates and their Galactic scale height distribution.
We find a peak in the SNR distribution between 7–11 kpc with most restricted to ±100 pc Galactic scale height. 相似文献
17.
The MHD instabilities of a temperature-anisotropic coronal plasma are considered. We show that aperiodic mirror instabilities
of slow MHD waves can develop under solar coronal conditions for weak magnetic fields (B < 1 G) and periodic ion-acoustic instabilities can develop for strong magnetic fields (B > 10 G). We have found the instability growth rates and estimated the temporal and spatial scales of development and decay
of the periodic instability. We show that the instabilities under consideration can play a prominent role in the energy balance
of the corona and may be considered as a large-scale energy source of the wave coronal heating mechanism. 相似文献
18.
Werner M. Neupert 《Solar physics》2011,272(2):319-335
The two-band soft X-ray observations of solar flares made by the Naval Research Laboratory’s (NRL) SOLar RADiation (SOLRAD)
satellites and by the Geostationary Orbiting Environmental Satellites (GOES) operated by the National Oceanic and Atmospheric Administration’s (NOAA) Space Weather Prediction Center have produced
a nearly continuous record of solar flare observations over a period of more than forty years (1969 – 2011). However, early
GOES observations (i.e., GOES-2) and later (GOES-8 and subsequent missions) are not directly comparable due to changes in the conversion of measured
currents to integrated fluxes in the two spectral bands that were adopted: 0.05 – 0.3 (or 0.4) nm, which we refer to as XS
and 0.1 – 0.8 nm (XL). Furthermore, additional flux adjustments, using overlapping data sets, were imposed to provide consistency
of flare-flux levels from mission to mission. This article evaluates the results of these changes and compares experimental
GOES-8/GOES-2 results with changes predicted from modeled flare spectra. The factors by which recent GOES observations can
be matched to GOES-2 are then optimized by adapting a technique first used to extrapolate GOES X-ray fluxes above saturation
using ionospheric VLF radio phase enhancements. A nearly 20% increase in published GOES-8 XL data would be required to match
to GOES-2 XL fluxes, which were based on observed flare spectra. On the other hand, a factor of 1.07 would match GOES-8 and
later flat-spectrum 0.1 – 0.8 nm fluxes to GOES-2 XL if the latter data were converted to a flat-spectrum basis. Finally,
GOES-8 observations are compared to solar soft X-ray estimates made concurrently with other techniques. Published GOES-8 0.1 – 0.8 nm
fluxes are found to be 0.59 of the mean of these other determinations. Rescaling GOES to a realistic flare spectrum and removing
a 30% downward adjustment applied to the GOES-8 measurements during initial data processing would place GOES-8 and later GOES
XL fluxes at 0.94 of this XL mean. GOES-2 on the same scale would lie at about 0.70 of this mean. Significant uncertainties
in the absolute levels of broad band soft X-ray fluxes still remain, however. 相似文献
19.
R.P. Kraft 《Astrophysics and Space Science》1999,265(1-4):153-156
Low metallicity (−3 <=[Fe/H] < = −1) halo field giants exhibit the expected correlation of Na and Mg abundances, based on
the assumption that Na is produced in the same nucleosynthetic sites as are the alpha elements, confirming a result noted
by Sneden (1998). On the other hand, giants in at least some globular clusters (especially M13, but also M15 and NGC 6752)
do not exhibit the Mg vs Na correlation found among halo field giants (Hanson et al., 1998). The very large [Na/Fe]-ratios and widely scattered [Mg/Fe]-ratios found among M13 giants depend, on the average,
on evolutionary state and are probably induced by deep mixing of stellar envelopes through the CNO hydrogen-burning shell.
Why M13 (and M15 and NGC 6752) giants should experience deep mixing whereas field halo giants in the same evolutionary state
mix not at all is an anomaly unexplained by current theories of stellar evolution. By contrast, giants in the outer halo cluster
NGC 7006 show little evidence of deep mixing (Kraft et al., 1998). These differences in the degree of deep mixing among stars in related, but different, stellar populations may be
connected to the so-called `second parameter effect'.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
We consider sterile neutrinos as a component of dark matter in the Milky Way and clusters, and compare their rest mass, decay
rate and the mixing angle. A radiative decaying rate of order Γ∼10−19 s−1 for sterile neutrino rest mass m
s
=18–19 keV can satisfactorily account for the cooling flow problem and heating source in Milky Way center simultaneously.
Also, these ranges of decay rate and rest mass match the prediction of the mixing angle sin 22θ∼10−3 with a low reheating temperature in the inflation model, which enables the sterile-active neutrino oscillation to be visible
in future experiments. However, decaying sterile neutrinos have to be ruled out as a major component of dark matter because
of the high decay rate. 相似文献