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101.
R. T. Schilizzi W. Aldrich B. Anderson A. Bos R. M. Campbell J. Canaris R. Cappallo J. L. Casse A. Cattani J. Goodman H. J. van Langevelde A. Maccafferri R. Millenaar R. G. Noble F. Olnon S. M. Parsley C. Phillips S. V. Pogrebenko D. Smythe A. Szomoru H. Verkouter A. R. Whitney 《Experimental Astronomy》2001,12(1):49-67
102.
The occurrence of permafrost in bedrock in northern Fennoscandia and its dependence on past and presently ongoing climatic variations was investigated with one- (1D) and two-dimensional (2D) numerical models by solving the transient heat conduction equation with latent heat effects included. The study area is characterized by discontinuous permafrost occurrences such as palsa mires and local mountain permafrost. The ground temperature changes during the Holocene were constructed using climatic proxy data. This variation was used as a forcing function at the ground surface in the calculations. Several versions of the present ground temperature were applied, resulting in different subsurface freezing–thawing conditions in the past depending on the assumed porosity and geothermal conditions.Our results suggest that in high altitude areas with a cold climate (present mean annual ground temperature between 0°C and −3°C), there may have been considerable variations in permafrost thickness (ranging from 0 to 150 m), as well as periods of no permafrost at all. The higher is the porosity of bedrock filled with ice, the stronger is the retarding effect of permafrost against climatic variations.Two-dimensional models including topographic effects with altitude-dependent ground temperatures and slope orientation and inclination dependent solar radiation were applied to a case of mountain permafrost in Ylläs, western Finnish Lapland, where bedrock permafrost is known to occur in boreholes to a depth of about 60 m. Modelling suggests complicated changes in permafrost thickness with time as well as contrasting situations on southern and northern slopes of the mountain.Extrapolating the climatic warming of the last 200 years to the end of the next century when the anticipated increase in the annual average air temperature is expected to be about 2 K indicates that the permafrost occurrences in bedrock in northern Fennoscandia would be thawing rapidly in low-porosity formations. However, already a porosity of 5% filled with ice would retard the thawing considerably. 相似文献
103.
W. J. Chaplin G. Houdek Y. Elsworth D. O. Gough G. R. Isaak R. New 《Monthly notices of the Royal Astronomical Society》2005,360(3):859-868
We investigate the frequency dependence of the power spectral density of low-degree solar p modes by comparing measurements with the results of a stochastic-excitation model. In the past it was common practice to use the total power in such investigations. Using the maximum of the power spectral density instead provides a direct comparison with the measured mode heights in the observed power spectrum. This method permits a more careful calibration of the adjustable parameters in the excitation model, a model which we present here, for the first time, in a format that precisely and unambiguously relates the amplitudes of the modes of oscillation to the Reynolds stress in the equilibrium model. We find that errors in the theory of the linear mode damping rates, particularly at low frequency, have a dramatic impact on the predictions of the mode heights in the spectral density, whereas parameter changes in the stochastic excitation model, within a plausible domain of parameter space, have a comparatively small effect. 相似文献
104.
T. T. Chia 《Astrophysics and Space Science》1985,114(1):151-156
In any theory of gravitation that predicts the existence of gravitational waves, it is shown that, in the absence of mass
exchange and mass loss and subjected to a condition, a circular binary system with spherically-symmetrical components cannot
evolve in such a way that the spin angular velocity is always a linear function of the orbital velocity. Probably this relation
between the angular velocities cannot be realized in any time interval. In particular, the system cannot remain in a synchronized
state. Of eight special cases of evolution of this binary system, three cases, where the separation increases, are shown to
be kinematically impossible while a restriction on the variation of a parameter governing the evaluation of the system can
be made in another.
In one of the kinematically allowed special cases the separation increases. 相似文献
105.
Data from the Pulkovo spectrophotometric data base on the absolute quasimonochromatic fluxes from oCet in the 320–1080 nm range are used to determine the physical parameters of this star in different phases of its light curve. The continuum emission layer is found to expand between the phases of the cycle corresponding to the rising and falling branches of the light curve. The average expansion velocity is 32 km/s. By the time the star’s brightness has fallen by roughly three magnitudes, its radius has increased by almost a factor of three. Over this same time the temperature of the layer has fallen from 3000 K to 2200 K. For this expansion velocity, the calculated mass rate loss is ⊙ M /year.__________Translated from Astrofizika, Vol. 48, No. 2, pp. 175–189 (May 2005). 相似文献
106.
Titan has been observed with UVES, the UV-Visual Echelle Spectrograph at the Very Large Telescope, with the aim of characterizing the zonal wind flow. We use a retrieval scheme originally developed for absolute stellar accelerometry [Connes, P., 1985. Astrophys. Space Sci., 110, 211-255] to extract the velocity signal by simultaneously taking into account all the lines present in the spectrum. The method allows to measure the Doppler shift induced at a given point by the zonal wind flow, with high precision. The short-wavelength channel (4200-5200 Å) probes one scale height higher than the long-wavelength one (5200-6200 Å), and we observe statistically significant evidence for stronger winds at higher altitudes. The results show a high dispersion. Globally, we detect prograde zonal winds, with lower limits of 62 and 50 m s−1 at the regions centered at 200 and 170 km altitude, but approximately a quarter of the measurements indicates null or retrograde winds. 相似文献
107.
We present results of investigations into chromospheric velocity oscillations in sunspots, carried out at the Sayan Solar
Observatory. It is shown that the “chevron” structures in the space-time diagrams demonstrate wavetrain properties. Such structures
are indicators of a propagating wave process and they are typical of many sunspots. In the authors’ opinion, three-minute
umbral oscillations are not the source of running penumbral waves (RPW). It is very likely that umbral oscillations and RPW
initially propagate along different magnetic field lines. We explain the decrease in RPW propagation velocity and frequency
in the outer penumbra, as compared with the inner, by the combined action of different frequency modes. To better reveal the
properties of these modes, frequency filtering was used. Our measurements of the RPW (five-minute mode) wavelength and RPW
propagation velocity in different sunspots vary from 12″ to 30″ and from 28 to 60 – 70 km s−1 correspondingly. 相似文献
108.
Report of the International Astronomical Union Division I Working Group on Precession and the Ecliptic 总被引:1,自引:0,他引:1
J. L. Hilton N. Capitaine J. Chapront J. M. Ferrandiz A. Fienga T. Fukushima J. Getino P. Mathews J.-L. Simon M. Soffel J. Vondrak P. Wallace J. Williams 《Celestial Mechanics and Dynamical Astronomy》2006,94(3):351-367
The IAU Working Group on Precession and the Equinox looked at several solutions for replacing the precession part of the IAU
2000A precession–nutation model, which is not consistent with dynamical theory. These comparisons show that the (Capitaine
et al., Astron. Astrophys., 412, 2003a) precession theory, P03, is both consistent with dynamical theory and the solution most compatible with the IAU 2000A
nutation model. Thus, the working group recommends the adoption of the P03 precession theory for use with the IAU 2000A nutation.
The two greatest sources of uncertainty in the precession theory are the rate of change of the Earth’s dynamical flattening,
ΔJ2, and the precession rates (i.e. the constants of integration used in deriving the precession). The combined uncertainties
limit the accuracy in the precession theory to approximately 2 mas cent−2.
Given that there are difficulties with the traditional angles used to parameterize the precession, zA, ζA, and θA, the working group has decided that the choice of parameters should be left to the user. We provide a consistent set of parameters
that may be used with either the traditional rotation matrix, or those rotation matrices described in (Capitaine et al., Astron.
Astrophys., 412, 2003a) and (Fukushima Astron. J., 126, 2003).
We recommend that the ecliptic pole be explicitly defined by the mean orbital angular momentum vector of the Earth–Moon barycenter
in the Barycentric Celestial Reference System (BCRS), and explicitly state that this definition is being used to avoid confusion
with previous definitions of the ecliptic.
Finally, we recommend that the terms precession of the equator and precession of the ecliptic replace the terms lunisolar precession and planetary precession, respectively. 相似文献
109.
We performed polarization observations of giant radio pulses from the millisecond pulsar B1937+21. The observations were carried out in July 2002 with the 64-m Kalyazin radio telescope at a frequency of 600 MHz in two polarization channels with left-and right-hand circular polarizations (RCP and LCP). We used the S2 data acquisition system with a time resolution of 125 ns. The duration of an observing session was 20 min. We detected twelve giant radio pulses with peak flux densities higher than 1000 Jy; five and seven of these pulses appeared in the RCP and LCP channels, respectively. We found no event that exceeded the established detection threshold simultaneously in the two polarization channels. Thus, we may conclude that the detected giant pulses have a high degree of circular polarization, with the frequency of occurrence of RCP and LCP pulses being the same. 相似文献
110.
The Indian team at ISRO has been part of the Whole Earth Telescope (WET) team since 1988 when we first participated in the
campaign on V471 Tau. We have been part of many other observing campaigns over the last decade. This presentation traces the
circumstances leading to our joining the WET team and how useful the coverage from the Indian longitude has been. The results
of several pulsators from the WET runs during which we participated are also described. These include PG1159-035 the prototype
of the GW Vir type of stars, RE J 0751+14 a cataclysmic variable, PG 1336-018 a binary with an sdB pulsator and finally HR
1217 a roAp star. The paper concludes with what the limitations are in our observations and how we can overcome them in the
future. 相似文献