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31.
32.
We investigate the parameter space of a Parker dynamo with a simple alpha quenching nonlinearity, taking as governing parameters the dynamo number D (D<0) and the ratio of diffusion times in the radial and latitudinal directions in the convective zone. The latter parameter, μ, is connected with the aspect ratio (dimensionless thickness) of the convective zone. We isolate two asymptotic configuration of the dynamo waves excited by the Parker dynamo in the limiting case of strong generation. Apart from the standard case with the solar type dynamo wave travelling from mid-latitudes to the equator, we describe a form of dynamo activity which is basically an anharmonic standing wave. The first situation occurs when μ increases with |D|. With μ fixed and |D| increasing, the second asymptotic configuration occurs. We discuss possibilities of identifying these asymptotic configurations with various types of stellar activity as traced by stellar CaII data. 相似文献
33.
Abstract The radial temperature differences at which the transitions from one wave number to the next occur have been measured with either increasing or decreasing positive radial temperature gradients, at five different rotation rates, with the fluid being always in thermal equilibrium and being in contact with an upper rigid lid. Hysteresis has been observed in all wave number transitions, and also in the transition to upper symmetry. There are, nevertheless, regions in the stability diagram where the wave number is unique. There is an excluded region where the wave number four cannot be obtained through quasi-steady procedure. There is a reversal of the sense of the hysteresis of the transitions. At low ΔT, a wave number transition with increasing radial temperature difference occurs at a higher ΔT, than the same transition with decreasing temperature difference. On the other hand, at large values of ΔT, a wave number transition with increasing radial temperature difference occurs at a lower ΔT, than the same transition with decreasing temperature difference. Wave number transitions with increasing ΔT, occur spontaneously out of amplitude oscillations. Wave number transitions with decreasing ΔT, occur via slow wave splitting in association with phase modulations of the waves. The uniqueness of the wave number in the unique areas of the stability diagram has been confirmed by sudden start experiments. 相似文献
34.
R. Kaiser 《地球物理与天体物理流体动力学》2013,107(1-2):125-135
Abstract Bayly (1993) introduced and investigated the equation (? t + v·▽-η ▽2)S = RS as a scalar analogue of the magnetic induction equation. Here, S(r, t) is a scalar function and the flow field v(r, t) and “stretching” function R(r, t) are given independently. This equation is much easier to handle than the corresponding vector equation and, although not of much relevance to the (vector) kinematic dynamo problem, it helps to study some features of the fast dynamo problem. In this note the scalar equation is considered for linear flow and a harmonic potential as stretching function. The steady equation separates into one-dimensional equations, which can be completely solved and therefore allow one to monitor the behaviour of the spectrum in the limit of vanishing diffusivity. For more general homogeneous flows a scaling argument is given which ensures fast dynamo action for certain powers of the harmonic potential. Our results stress the singular behaviour of eigenfunctions in the limit of vanishing diffusivity and the importance of stagnation points in the flow for fast dynamo action. 相似文献
35.
The standard dynamo models that explain the origin of the large-scale magnetic fields of celestial bodies are related to the view of turbulent or convective flows as a locally statistically homogeneous and isotropic, but not mirror-symmetric, random field. Using an ABC flow, which is a classical example of a flow with deterministic chaos, we ascertain the extent to which the behavior of the magnetic field in such a flow is similar to the behavior of the magnetic field in mirror-asymmetric turbulence. Such a similarity has been found to be achieved if its coefficients A, B, and C are assumed to be random processes. 相似文献
36.
Given the complexity involved in a flux-transport-type dynamo driven by both Babcock – Leighton and tachocline α effects, we present here a step-by-step procedure for building a flux-transport dynamo model calibrated to the Sun as a guide
for anyone who wishes to build this kind of model. We show that a plausible sequence of steps to reach a converged solution
in such a dynamo consists of (i) numerical integration of a classical α – ω dynamo driven by a tachocline α effect, (ii) continued integration with inclusion of meridional circulation to convert the model into a flux-transport dynamo
driven by only a tachocline α effect, (iii) final integration with inclusion of a Babcock – Leighton surface α effect, resulting in a flux-transport dynamo that can be calibrated to obtain a close fit of model output with solar observations. 相似文献
37.
38.
Mausumi Dikpati Peter A. Gilman Giuliana de Toma Siddhartha S. Ghosh 《Solar physics》2007,245(1):1-17
We use the flux-transport dynamo prediction scheme introduced by Dikpati, de Toma, and Gilman (Geophys. Res. Lett.
33, L05102, 2006) to make separate simulations and predictions of sunspot cycle peaks for northern and southern hemispheres. Despite the division
of the data, the skill level achieved is only slightly lower than that achieved for the sum of both hemispheres. The model
shows skill at simulating and predicting the difference in peaks between North and South, provided that difference is more
than a few percent. The simulation and prediction skill is achieved without adjustment to any parameters of the model that
were used when peaks for the sum of North and South sunspot areas was simulated. The results are also very insensitive to
the averaging length applied to the input data, provided the simulations and predictions are for peaks defined by averaging
the observations over at least 13 rotations. However, in its present form, the model is not capable of skillfully simulating
or predicting short-time-scale features of individual solar cycles. 相似文献
39.
D. M. Rust 《Journal of Astrophysics and Astronomy》2000,21(3-4):177-183
Solar filaments are discussed in terms of two contrasting paradigms. The standard paradigm is that filaments are formed by
condensation of coronal plasma into magnetic fields that are twisted or dimpled as a consequence of motions of the fields&#x2019;
sources in the photosphere. According to a new paradigm, filaments form in rising, twisted flux ropes and are a necessary
intermediate stage in the transfer to interplanetary space of dynamo-generated magnetic flux. It is argued that the accumulation
of magnetic helicity in filaments and their coronal surroundings leads to filament eruptions and coronal mass ejections. These
ejections relieve the Sun of the flux generated by the dynamo and make way for the flux of the next cycle. 相似文献
40.
Arnab Rai Choudhuri 《Journal of Astrophysics and Astronomy》2000,21(3-4):373-377
This review provides a historical overview of how research in kinematic solar dynamo modeling evolved during the last few
decades and assesses the present state of research. The early pioneering papers assumed the dynamo to operate in the convection
zone. It was suggested in the 1980s that the dynamo operates in a thin layer at the bottom of the convection zone. Some researchers
in recent years are arguing that the poloidal field is produced near the surface&#x2014;an idea that goes back to Babcock
(1961) and Leighton (1969). 相似文献