共查询到20条相似文献，搜索用时 31 毫秒
1.
M. Tendler 《Astrophysics and Space Science》1997,256(1-2):205-218
The review addresses transitions into regimes with improved confinement. The E × B paradigm is tested on the example of L–H transitions. It is demonstrated that L–H transitions may emerge either due to an amplification of the diamagnetic drift term caused by enhanced pressure gradient or due to an increase poloidal rotation velocity at the separatrix. In general, it is asserted that the emergence and dynamics of transitions is very sensitive to fine details of prelude plasma profiles. The L–H transition occurs provided the ambipolar electric field changes dramatically within a few centimeters. The dynamics of interest consists of threshold conditions for barrier formation, barrier propagation speed, and profile steepening rates, barrier limits and mechanisms for their relaxation and termination. 相似文献
2.
Speeds of vertical flows in quiescent solar filaments are typically much less than the local Alfvén speed. This is why the flows in filament barbs can be modeled by perturbing a magnetostatic solution describing a balance between the Lorentz force, gravity, and gas pressure in a barb. This approach explains why some of the flows are neither aligned with the magnetic field nor controlled by gravity. Both the observed upflows and the magnetic field dips in barbs are likely to be caused by photospheric magnetic reconnection. 相似文献
3.
4.
The warm current sheet model, and its implications on the temporal behaviour of the geomagnetic tail
J. W. Eastwood 《Planetary and Space Science》1974,22(12):1641-1668
A simple model current sheet is studied numerically. Consistent fields and particle trajectories, and their dependence on electron and proton temperature, convection velocity and normal field, Bz, linking through the current sheet, are presented and discussed. It is shown that the protons, which are the major current carriers, largely retain the decoupling of the motion in the x-y plane from the normal oscillations as in the ‘cold’ current sheet. The positive potential of the current sheet is shown to be sufficient to trap some energetic electrons, the motion of which enables the predominance of energetic electrons towards the dawn side of the tail to be understood. Semi-empirical relationships for the thickness and the potential of the current sheet are obtained.
The consequences of such a current sheet on the behaviour of the geomagnetic tail are investigated. Using Faraday's law and the consistent cross tail electric field it is shown that the effect of a southward turning of the interplanetary field is to lead to a decrease in Bz,an increase of the current sheet conductivity, and a growth of stored field energy, i.e. the current sheet blocks merging. The decrease of the resistance of the current sheet is limited by the finite width of the tail. Finally, it is pointed out that if the conditions which bring about the growth of field energy persist, then the collapse of the field lines characteristic of substorms may occur. 相似文献
5.
By using a one-dimension (one spatial coordinate and three-velocity components), electromagnetic particle simulation code with full ion and electron dynamics, we have studied the acceleration of heavy ions by a nonlinear magnetosonic wave in a multi-ion-species plasma. First, we describe the mechanism of heavy ion acceleration by magnetosonic waves. We then investigate this by particle simulations. The simulation plasma contains four ion species: H, He, O, and Fe. The number density of He is taken to be 10% of that of H, and those of O and Fe are much lower. Simulations confirm that, as in a single-ion-species plasma, some of the hydrogen can be accelerated by the longitudinal electric field formed in the wave. Furthermore, they show that magnetosonic waves can accelerate all the particles of all the heavy species (He, O, and Fe) by a different mechanism, i.e., by the transverse electric field. The maximum speeds of the heavy species are about the same, of the order of the wave propagation speed. These are in good agreement with theoretical prediction. These results indicate that, if high-energy ions are produced in the solar corona through these mechanisms, the elemental compositions of these heavy ions can be similar to that of the background plasma, i.e., the corona. 相似文献
6.
Ionospheric and plasma sheet particle densities, fluxes and bulk velocities along an auroral magnetic field line have been calculated for an ion-exosphere model. It is shown that such a collisionless model accounts for many features observed above the auroral regions. Except for very strong plasma sheet electron precipitation, no large potential difference is needed along the magnetic field lines to account for the usual proton and electron fluxes, their pitch angle distributions, and auroral field aligned currents. 相似文献
7.
The acceleration of relativistic particles is considered during their intersection with hydromagnetic shock fronts in the presence of randomly distributed large-scale magnetic fields. In a series of astronomical objects, the Larmor radius of the relativistic particles exceeds the width of the shock front. In this case there is a change in the adiabatic invariant which results in an increase in the energy of the particle when it crosses the front in any direction. We have proved that the adiabatic part of the energy change will be partially or completely compensated by its reverse change in the weaker regions of the magnetic field. The acceleration mechanism considered is found to be more effective than the Fermi mechanism.If the mean free path of the particles is much less than the distance between the shock fronts, magnetic small-scale fluctuations cause further scattering of the particles. In this case the particles following and crossing the front will return to it. After reversed crossing, a fraction of the particles-defined by the ratio of the front speed to the particle velocity or of the distance between the fronts to the free path — will not return to the front. It is proved that for both large and small free paths the rates at which the particle gains energy are nearly the same. 相似文献
8.
J. L. Caswell Jiyune Yi R. S. Booth D. M. Cragg 《Monthly notices of the Royal Astronomical Society》2000,313(3):599-616
From a search of more than 80 southern class II methanol maser sites, we report measurements of 22 masers at 107.0 GHz and four at 156.6 GHz, mostly new discoveries. Class II sites, recognized by their strong emission at the 6.6-GHz methanol transition, are indirect indicators of new-born massive stars, and several hundred have been documented; only a handful of these had previously been found to exhibit maser emission at the 107.0- or 156.6-GHz transition. The present survey increases the number of known 107.0-GHz masers to 25, providing a sufficiently large sample to assess their general properties. For the stronger ones, our position measurements confirm that, to an accuracy of 5 arcsec, they coincide with the dominant maser emission at 6.6 GHz. Intensity variations exceeding 50 per cent have occurred in some 107.0-GHz maser features that we observed in both 1996 October and 1998 June.
We find that masers are rare at the 156.6-GHz transition. Two new detections increase the total now known to four. Each 156.6-GHz maser is substantially weaker than its corresponding 107.0-GHz maser. Despite the scarcity of masers, our 156.6-GHz spectra at most observed sites show emission, but apparently of a quasi-thermal variety; it is usually accompanied by somewhat weaker thermal emission at 107.0 GHz, and the intensity ratio of the transitions allows us to begin exploration of the physical characteristics of the small molecular clouds (diameter less than 60 mpc) at these sites. The thermal emission thus provides estimates of the environmental conditions that are needed to support strong masing from spots that are apparently embedded within these clouds. 相似文献
We find that masers are rare at the 156.6-GHz transition. Two new detections increase the total now known to four. Each 156.6-GHz maser is substantially weaker than its corresponding 107.0-GHz maser. Despite the scarcity of masers, our 156.6-GHz spectra at most observed sites show emission, but apparently of a quasi-thermal variety; it is usually accompanied by somewhat weaker thermal emission at 107.0 GHz, and the intensity ratio of the transitions allows us to begin exploration of the physical characteristics of the small molecular clouds (diameter less than 60 mpc) at these sites. The thermal emission thus provides estimates of the environmental conditions that are needed to support strong masing from spots that are apparently embedded within these clouds. 相似文献
9.
Complex auroral-absorption events, observed with spaced riometers at high-latitude sites, have been analysed for power spectra and spatial coherence. The coherence distance is typically more than 200 km for the slower fluctuations, but less than 100 km for periodicities less than 9 min. There is evidence that in fact the coherence is controlled by the intensity of the precipitation rather than by the period. The active regions of the magnetosphere that cooperate, i.e. produce coherent particle precipitation—during radio-absorption events have dimensions of the order of one earth radius, the typical size increasing with the level of activity. It is suggested that field aligned currents may affect the scale of coherence. The results of the analysis should be useful in deciding the locations of sites in spaced riometer observations. 相似文献
10.
Harald H. Soleng 《Astrophysics and Space Science》1987,137(2):373-384
Two exact solutions of Einstein's field equations of vacuum are presented and investigated. We will regard the term vacuum fluid as the limiting case of scalar field with an almost constant potential. Considering the four velocity of this fluid we find, that in both solutions there is an anisotropic expansion of the cosmic fluid, but the fluid has vanishing vorticity.We investigate whether shear could prevent the transition into an inflationary era in these models, and the effect of shear on a scalar field is also considered. It is found that shear will speed up the rollover of the scalar field in some Bianchi type-VIII models.Possible initial conditions are discussed in light of the group structures of the models. 相似文献
11.
A. Urban 《Planetary and Space Science》1981,29(12):1353-1365
This paper summarizes ion meaurements in the energy range 0.1–30keV observed during the campaigns “Substorm Phenomena” and “Porcupine”. For a clear survey of the physical processes during extraordinary events, sometimes ion meaurements of higher energies are also taken into account. Generally, the pitch angle distributions were isotropic during all flights except some remarkable events. In general the ion and electron flux intensities correlated, but sometimes revealed a spectral anti-correlation.
Acceleration of the ions by an electrostatic field aligned parallel to the magnetic field could be identified accompanied by intense electron precipitation. On the other hand deceleration of the ions was observed in other field-aligned current sheets which are indicated by the electron and magnetic field measurements. Temporal successive monoenergetic ion variations pointed to energy dispersion and to the location of the source region at 9 RE. Furthermore, ion fluxes higher than those of the electrons were measured at pitch angles parallel to the magnetic field. Each of the examples was observed during different flights. The integral down-going number and energy flux of the ions contributed to the total particle or energy influx between 65% and less than 7% and did not clearly characterize the geophysical launch conditions or auroral activities. 相似文献
12.
In this paper the evolution of a density enhancement under the effect of gravity in a stratified atmosphere is considered
in a 2D simulation. The stratified atmosphere is chosen with a high-density photosphere, transition region and low-density
corona where the enhancement is added in non-equilibrium to the corona. The atmosphere is also threaded with an initially
uniform vertical magnetic field. If sufficiently strong, the magnetic field plays an important role in the evolution of the
density enhancement as it tries to gain equilibrium. It not only enables the density enhancement to maintain its shape as
it falls, but if strong enough results in the density enhancement rebounding a number of times. Therefore both upward and
downward velocities of the enhancement are obtained. In all cases the density enhancement is found to fall with speeds much
less than the free-fall speed and can remain in the corona at least 11 times longer than a free-fall particle. The relevance
of the simulations to the solar atmosphere is then discussed.
Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1005266330720 相似文献
13.
The possibility that the type of magnetohydrodynamic (MHD) discontinuity changes as the plasma flow conditions gradually change is investigated in a general form. The conservation laws in MHD admit such transitions if there exist the so-called transition solutions that simultaneously satisfy two types of discontinuities. These solutions have been sought for. The system of possible transitions between MHD discontinuities obtained on their basis is presented in a clear schematic form. The ultimate general scheme of transitions includes all of the previously described schemes of transitions known to us. The system of discontinuities and transitions between them is studied in a self-consistent solution of the analytical problem of reconnection in a strong magnetic field. 相似文献
14.
V. S. Morozova B. J. Ahmedov A. A. Abdujabbarov A. I. Mamadjanov 《Astrophysics and Space Science》2010,330(2):257-266
Plasma magnetosphere surrounding rotating magnetized neutron star in the braneworld has been studied. For the simplicity of calculations Goldreich-Julian charge density is analyzed for the aligned neutron star with zero inclination between magnetic field and rotation axis. From the system of Maxwell equations in spacetime of slowly rotating star in braneworld, second-order differential equation for electrostatic potential is derived. Analytical solution of this equation indicates the general relativistic modification of an accelerating electric field and charge density along the open field lines by brane tension. The implication of this effect to the magnetospheric energy loss problem is underlined. It was found that for initially zero potential and field on the surface of a neutron star, the amplitude of the plasma mode created by Goldreich-Julian charge density will increase in the presence of the negative brane charge. Finally we derive the equations of motion of test particles in magnetosphere of slowly rotating star in the braneworld. Then we analyze particle motion in the polar cap and show that brane tension can significantly change conditions for particle acceleration in the polar cap region of the neutron star. 相似文献
15.
Because of its proximity to the Sun and its small size, Mercury has not been able to retain its atmosphere and only a thin exosphere surrounds the planet. The exospheric pressure at the planetary surface is approximately 10−10 mbar, set by the Mariner 10 occultation experiment. The existence of gaseous species H, He, and O has been established by Mariner 10. In addition Na, K, and Ca have been observed by ground based instrumentation. Other elements are expected to be found in Mercury's exosphere since the total pressure of the known species is almost two orders of magnitude less than the exospheric pressure.It is intended to measure these exospheric particle densities in situ with an instrument on board of ESA's BepiColombo Mercury Planetary Orbiter (MPO) spacecraft. Since the expected exospheric densities are very small we developed a Monte-Carlo computer model to investigate if such a measurement is feasible along the MPO spacecraft orbit. We model energy and ejection angle distributions of the particles at the surface, with the emission process determining the actual distribution functions. Our model follows the trajectory of each particle by numerical integration until the particle hits Mercury's surface again or escapes from the calculation domain. Using a large set of these trajectories bulk parameters of the exospheric gas are derived, e.g., particle densities for various atomic and molecular species. Our study suggests that a mass spectrometric measurement is feasible and, at least at MPO's periherm, all species that are released from the surface will be observed. 相似文献
16.
J.A. Briggs D.A. Brain M.L. Cartwright J.P. Eastwood J.S. Halekas 《Planetary and Space Science》2011,59(13):1498-1505
Using minimum variance analysis of the circular mapping data from the Mars Global Surveyor (MGS) spacecraft during four selected weeks of observation, we identify 360 magnetic field structures in the Martian topside ionosphere with characteristic signatures of flux ropes. Physical parameters including size, peak field strength, helicity, orientation, and external conditions at the time of each observation are compiled for the events in each population. We observe that Martian flux ropes typically have a peak field amplitude of ∼15 nT and a diameter of ∼80–100 km assuming they are stationary. Flux ropes tend to be aligned approximately parallel to the planetary surface, and perpendicular to the direction from which the solar wind flows. They are more frequently observed during times of low solar wind pressure, but do not show a clear preference for a particular Interplanetary Magnetic Field (IMF) draping direction. Flux rope characteristics of peak field amplitude, diameter, and helicity vary with solar zenith angle. Amplitudes tend to be higher during periods of high solar wind pressure. The events are sorted into three populations based on the location at which they were observed, possibly corresponding to distinct formation mechanisms. Flux ropes observed in eclipse tend to have smaller peak amplitudes and are larger than those observed in sunlight, and are less likely to be oriented parallel to the planetary surface. Proximity to crustal fields does not appear to influence the characteristics of flux ropes observed at the 400 km spacecraft altitude. The frequent observation of flux rope structures near Mars in a variety of locations suggests that the low-altitude plasma environment is quite dynamic, with magnetic shear playing a prominent role in determining magnetic field structure near the planet. 相似文献
17.
It is shown by numerical simulations that enhanced current density can generate double layers, even when the electron drift speed is significantly below the electron thermal speed. The double layer potential is spontaneously produced by the space charge self-consistently developed inside the simulation domain. The particle influxes from the low-potential boundary of our simulation domain are independent of the outfluxes. The potential difference φ0 is shown increase with increasing number density of the injection current. Strong double layers with potential energy eφ0 ? kT0 (the electron thermal energy) are stably formed when the injection electron current much exceeds the thermal current of ambient electrons. The backscattered and mirrored electrons are found to have stabilizing effects on the current-driven double layers. 相似文献
18.
Tyan Yeh 《Solar physics》1977,55(1):241-250
In the coronal-interplanetary space the plasma motion, in a reference frame corotating with the Sun, is aligned with the magnetic field. Just like the solar wind, which is the supersonically expanding flow along open field lines, the flow along closed field lines is mainly driven by the pressure gradient. The flow in the regions of closed field lines is subsonic, being determined by the conditions at the two footpoints of the magnetic flux tube.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
19.
P. R. Wilson 《Solar physics》1977,55(1):35-45
The configuration of a magnetic flux element in a static, compressible, gravitationally stratified plasma is considered. Under isothermal conditions an exact force-free solution is given for a two-dimensional cartesian flux sheath but for an axi-symmetric element, i.e. a flux tube, approximate solutions, applicable only to thin flux tubes, are obtained.When the isothermal condition is relaxed within the flux tube, non-force-free solutions are obtained exhibiting either a temperature excess or deficit on the axis of the flux tube. Using these solutions, the total potential energy of such a tube and its surroundings may be calculated. By comparing this with the total energy of an equivalent system ofn flux tubes the temperature excess or deficit in these tubes corresponding to the adiabatic subdivision of the single tube is determined.It is shown that under non-adiabatic conditions the subdivision process must be endothermic (i.e., external energy is required) if the temperature within the original tube is significantly less than its surroundings but exothermic if the temperatures are comparable. Thus it is conjectured that magnetic structures are less susceptible to subdivision if they are significantly cooler than their surroundings. 相似文献
20.
J. C. Samson 《Planetary and Space Science》1982,30(12):1239-1247
A review of recent experimental results from studies of high latitude Pi 2 pulsations indicates that these pulsations are fundamentally related to the initiation of the auroral breakup and substorm. At high latitudes, the Pi 2's show their peak intensities in the region where the breakup begins and appear to remain in this region after the breakup has spread poleward. In addition, the Pi 2's occur simultaneously with, or before all other ionospheric phenomena associated with the breakup. The field aligned and ionospheric currents associated with the Pi 2 resemble those of a typical substorm, but the ionospheric currents are phase shifted compared to the field aligned current. The periodic oscillations of the Pi 2's are probably caused by a reflection of the initial field aligned current pulse from the auroral ionosphere. This pulse is trapped on dipolar field lines leading to multiple reflections from North and South auroral ionospheres. 相似文献