共查询到20条相似文献，搜索用时 843 毫秒
1.
Kyoung W. Min 《Astrophysics and Space Science》1988,150(2):345-355
The recovery phase of the magnetospheric substorm is studied numerically by means of a two-dimensional time-dependent nonlinear resistive MHD code. The initial configuration was chosen from the earlier numerical model in which the magnetospheric substorm was driven by the solar wind plasmas. In order to study the recovery phase, the entering solar wind energy flux was reduced when the magnetospheric substorm was in its expansive phase. The system was found to respond instantly to this change and the result showed many characteristic features related to the recovery phase including the tailward motion of thex-point of the reconnected magnetic field lines and the restoration of a tail-like configuration of the magnetic field. Thex-point moved at almost the same speed of the plasma flow in the upstream region, which was considerably smaller than the speed of the plasma jetting or the speed of the plasmoid. As the recovery phase progressed, the plasma jetting across thex-point was reduced very much in the Earthside region. Although the plasma flow was generally in the Earthward direction in the Earthside region of thex-point, the tailward flow was also found near thex-point. The current density was reduced near thex-point and the neutral sheet was broadened in the recovered region. The plasma sheet also became thick in this region. During the recovery of the substorm, the energy conversion rate, both in the form of plasma acceleration and the Joule heating, was reduced. These results on the recovery phase together with the earlier simulation result on the expansive phase indicate that driven reconnection can be a viable mechanism for the magnetospheric substorm including the recovery phase. 相似文献
2.
Scott Tremaine 《Monthly notices of the Royal Astronomical Society》1999,307(4):877-883
Partially phase-mixed structures in galaxies occupy a complex surface of dimension D in six-dimensional phase space. The appearance of such structures to observers is determined by their projection into a space the dimensionality K of which is determined by the number of observables (e.g. sky position, distance, radial velocity, etc.). We discuss the expected dimensionality of phase-space structures and suggest that the most prominent features in surveys with K D will be stable singularities (catastrophes). The simplest of these are the shells seen in the outer parts of elliptical galaxies. 相似文献
3.
4.
Karen L. Harvey 《Solar physics》1971,16(2):423-430
The explosive phase of a flare can be defined by a simple photometric measurement of H film records of the flare development. Using the quantitative definition, improved correlations are found between the start of the explosive phase and the start of 10.7 cm radio bursts and Sudden Frequency Deviations compared to earlier correlations of the same data using visual estimates of the start of the explosive phase. Explosive development may be confined to only part of a flare. 相似文献
5.
The properties of quintom model are investigated in the isotropic and homogeneous universe as a dynamical system dominated by dark energy including the phantom and quintessence fields. A general discussion about the phase space of spatially non-flat universe is presented. We study the results for the later times without assuming the specific form of the potential. Then, we exhibit an obvious structure for the dynamics of the system. 相似文献
6.
R. H. Dicke 《Solar physics》1988,115(1):171-181
It has previously been shown that the statistics of the phase fluctuation of the sunspot cycle are compatible with the assumption that the solar magnetic field is generated deep in the Sun by a frequency stable oscillator and that the observed substantial phase fluctuation in the sunspot cycle is due to variation in the time required for the magnetic field to move to the solar surface (Dicke, 1978, 1979). It was shown that the observed phase shifts are strongly correlated with the amplitude of the solar cycle. It is shown here that of two empirical models for the transport of magnetic flux to the surface, the best fit to the data is obtained with a model for which the magnetic flux is carried to the surface by convection with the convection velocity proportional to a function of the solar cycle amplitude. The best fit of this model to the data is obtained for a 12-yr transit time. The period obtained for the solar cycle is T = 22.219 ± 0.032 yr. It is shown that the great solar anomaly of 1760–1800 is most likely real and not due to poor data. 相似文献
7.
Dilhan Eryurt-Ezer 《Astrophysics and Space Science》1981,79(2):265-287
The theoretical evolution of a first-generation star of 3M
after the core helium-exhaustion phase has been investigated. The star displays the character of a double shell burning model. Shell hydrogenburning produces energy mostly by the p-p chain reaction. CN-cycle reaction is only operating in the inner edge regions where sufficient amount of carbon is formed by the 3-reactions. Hence, the shell burning time of the star is longer than that of normal stars, thus lengthening the total evolutionary lifetime of the first-generation stars.Prior to carbon-burning phase, the mass of the complete hydrogen-exhausted region is 1.14M
and that of complete helium-exhausted region is 0.83M
. A carbon-oxygen core of about 0.87M
has developed within the star in which the ratio of carbon to oxygen is about 0.85, but decreases down to a value of 0.50 near the boundary of the core. 相似文献
8.
We report here results from a study of X-ray bursts from 3 magnetar candidates (SGR 1806-20, SGR 1900+14 and AXP 1E 2259+586).
We have searched for a pulse phase dependence of the X-ray burst rate from these sources. X-ray light curves were obtained
with the Proportional Counter Array on-board the Rossi X-ray Timing Explorer during the periods of intense burst activity
in these sources. On detailed analysis of the three sources, we found a very significant burst rate for all pulsar phases.
However, some locations appear to produce bursts slightly more often, rendering the non-isotropic distribution. Only in the
case of SGR 1900+14, there is a clear pulse phase dependence of burst rate. 相似文献
9.
A study of the properties of the cosmic radiation of energy - 10 MeV generated by solar flares is reported. Data from four Pioneer spacecraft in interplanetary orbits, and separated by 180° in heliocentric longitude are employed. Attention is restricted to the properties evident at times in excess of 1 day after the occurrence of the parent flare. The anisotropic character of the radiation; the gradients in heliocentric longitude; the decay time constants; and the energy spectra of the radiation are all studied in detail.It is found that the equilibrium anisotropy assumes a direction - 45° E of the satellite-Sun line at very late times. It is suggested that the anisotropy at such times is parallel to E × B. This observation confirms that convection is the determining process in the escape of the solar cosmic rays from the solar system. It indicates that a positive radial gradient of solar cosmic radiation density has builtup at orbit of Earth some 4 days after a flare. This results in an effective convective velocity of approximately 1/2 the solar wind velocity. Direct measurements indicate the presence of strong gradients in heliocentric longitude even at very late times ( 4 days). These gradients are essentially invariant with respect to time, e-folding angles of n - 30° have been observed at - 10 MeV. The presence of these gradients has a major effect on the temporal variation of the cosmic ray flux during the decay phase of the flare effect. Thus, the observed decay time constant is either increased or decreased relative to the convective value depending on the position of the observer relative to the centroid of the cosmic ray population injected by the flare. The effect of the gradient becomes more pronounced at lower energies, and may even exceed the convective removal rate. The observed decay time constant, the characteristics of the anisotropy, and the gradient in longitude are shown to be inter-related as demanded by theory. It is shown that the exponent of the cosmic ray spectrum is dependent on the location of the observer relative to the centroid of the cosmic ray population injected by the parent flare. At a given point in the frame of reference of the cosmic ray population, the spectral exponent is invariant with time.Now at CSIRO, G.P.O. Box 124, Port Melbourne, Victoria 3207, Australia.On leave from Physical Research Laboratory, Ahmedabad, India. 相似文献
10.
Crimean Astrophysical Observatory; Leningrad State University. Translated from Astrofizika, Vol. 28, No. 2, pp. 355–362, March–April, 1988. 相似文献
11.
V. Yu. Terebizh 《Astronomy Letters》2000,26(1):49-60
Phase retrieval of a signal given its intensity is considered as a problem of statistically estimating a set of unknown parameters, the Zernike coefficients. Specifically, the phase problem is presented in the context of classical wave optics in the Fresnel approximation. Investigating the stability in this case suggests first learning if the Zernike coefficients can be restored in principle. If this is indeed the case, it then suggests determining the accuracy of their estimation. The stability of a solution to the phase problem depends, as it does for the other inverse problems, on the spectrum of the Fisher information matrix. An explicit representation of the Fisher matrix is given, and its spectrum is calculated for in-focus and out-of-focus images of a pointlike source. Simulations show that the solutions in the latter case are generally stable, so the coefficients of the Zernike series can be determined with an acceptable accuracy. The principal components, the mutually independent combinations of aberrations that are a generalization of the coefficients of the well-known Karhunen-Loeve decomposition, are calculated. As an example of this approach, the maximum-likelihood method is used to determine the aberrations of the optical system. 相似文献
12.
R. Falciani 《Solar physics》1977,53(1):287-290
13.
Bruce G. Elmegreen 《Astrophysics and Space Science》2003,284(2):819-832
Gas processes affecting star formation are reviewed with an emphasis on gravitational and magnetic instabilities as a source
of turbulence. Gravitational instabilities are pervasive in a multi-phase medium, even for sub-threshold column densities,
suggesting that only an ISM with a pure-warm phase can stop star formation. The instabilities generate turbulence, and this
turbulence influences the structure and timing of star formation through its effect on the gas distribution and density. The
final trigger for star formation is usually direct compression by another star or cluster. The star formation rate is apparently
independent of the detailed mechanisms for star formation, and determined primarily by the total mass of gas in a dense form.
If the density distribution function is a log-normal, as suggested by turbulence simulations, then this dense gas mass can
be calculated and the star formation rate determined from first principles. The results suggest that only 10-4 of the ISM mass actively participates in the star formation process and that this fraction does so because its density is
larger than 105 cm-3, at which point several key processes affecting dynamical equilibrium begin to break down.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
14.
Only during the previous solar cycle have systematic observations begun to be made with the sensitivity and time resolution, and the continuous coverage required to catch the impulsive phase and measure the rapid variations present in many wavelength ranges. Observations in X-rays, gamma-rays, UV, H, and radio wavelengths all reveal rapid variations during the impulsive phase and have contributed to our understanding of the different phenomena involved. Results have been obtained from several spacecraft, from rocket and balloon flights, and from ground-based observations. These are reviewed in the context of a simple single loop flare model with a view to showing what results are consistent with this model and what the major problems are in our understanding of the impulsive phase. New instrumentation planned for observations during the present Cycle 22 will provide a concerted attack on the impulsive phase as part of the Max '91 program. 相似文献
15.
Zdeněk Švestka 《Solar physics》1989,121(1-2):399-417
One has to distinguish between two kinds of the gradual phase of flares: (1) a gradual phase during which no energy is released so that we see only cooling after the impulsive phase (a confined flare), and (2) a gradual phase during which energy release continues (a dynamic flare).The simplest case of (1) is a single-loop flare which might provide an excellent opportunity for the study of cooling processes in coronal loops. But most confined flares are far more complicated: they may consist of sets of unresolved elementary loops, of conglomerates of loops, or they form arcades the components of which may be excited sequentially. Accelerated particles as well as hot and cold plasma can be ejected from the flare site (coronal tongues, flaring arches, sprays, bright and dark surges) and these ejecta may cool more slowly than the source flare itself.However, the most important flares on the Sun are flares of type (2) in which a magnetic field opening is followed by subsequent reconnection of fieldlines that may continue for many hours after the impulsive phase. Therefore, the main attention in this review is paid to the gradual phase of this category of long-decay flares. The following items are discussed in particular: The wide energy range of dynamic flares: from eruptions of quiescent filaments to most powerful cosmic-ray flares. Energy release at the reconnection site and modelling of the reconnection process. The post-flare loops: evidence for reconnection; observations at different wavelengths; energy deposit in the chromosphere, chromospheric ablation, and velocity fields; loops in emission; shrinking loops; magnetic modelling. The gradual phase in X-rays and on radio waves. Post-flare X-ray arches: observations, interpretation, and modelling; relation to metric radio events and mass ejections, multiple-ribbon flares and anomalous events, hybrid events, possible relations between confined and dynamic flares. 相似文献
16.
G.W. Prölss 《Planetary and Space Science》1976,24(6):607-609
A qualitative model of the negative phase of ionospheric storms is presented. Only stations located within an atmospheric disturbance zone of a low ratio will observe a depletion of ionization. The extent of this disturbance zone is determined by geomagnetic coordinates. Thus stations located in the North American and Australian sectors are more liable to observe negative storm effects. On the other hand it is determined by the asymmetric energy injection along the auroral oval. It follows that stations located in the early morning sector during enhanced substorm activity have a greater chance of observing negative storm effects than those situated in the daytime sector. Seasonal and magnetic storm induced changes in the ratio are in phase during summer and out of phase during winter, explaining the seasonal variation of storm effects. 相似文献
17.
Aigen Li Jingyao Hu Lifan Wang Xiaojun Jiang Hongbin Li 《Astrophysics and Space Science》1994,211(2):323-328
From June 19 to August 27, using the 2.16m telescope in the Beijing Astronomical Observatory, we obtained some low resolution spectra of SN1993J. The forbidden emission lines [O I], [Ca II] were very strong and dominated the spectra. However, the H and He lines were very weak. Many fine structures appeared in the H and [O I] lines. We found that the supernova had entered the nebular phase and strong instability and chemical mixing had occurred in the ejecta. 相似文献
18.
《Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science》2001,26(7):495-503
Recent results about the ozone photochemistry in solid matrices (O2, Ar, N2) and water ices are reported. Several areas are reviewed: photochemistry in the visible and UV of ozone monomer, photochemistry of ozone involved in weak complexes, reactivity of atomic oxygen with various partners in nearest neighbour.Although reactions occurring in matrices are not the same as in gas phase, pathways displayed in matrices can bring information for gas phase modelling. 相似文献
19.
B. Kämpfer 《Astrophysics and Space Science》1989,158(1):35-42
The phenomenology of a cosmic-phase transition on the GUT energy scale is considered. Relying on numerical studies of the nucleation theory, we find, within a restricted range of parameters, an approximate power-law expansion accompanied by a large increase of both the scale factor and the comoving entropy. The distinct feature of this particular idea is the fact that inflation and phase conversion of the GUT continuum proceed simultaneously. The generic features of such an idea are explored. 相似文献
20.