共查询到20条相似文献，搜索用时 31 毫秒
1.
Masato Naruse Tetsuya Ito Yutaro Sekimoto Hiroyuki Toba Naohisa Satou Masahiro Sugimoto Wenlei Shan Yoshizou Iizuka Toshiaki Kamba Mamoru Kamikura Yasutaka Serizawa 《Experimental Astronomy》2009,24(1-3):89-107
We have developed a near-field vector beam measurement system covering the range of frequencies from 385 to 500 GHz. The measurement set-up is capable of measurements with dynamic range exceeding 50 dB and amplitude and phase stability respectively of 0.1 dB/h and 1 degree/5 min at room temperature. Beam patterns of the ALMA band 8 corrugated horns and receiver optics block were measured at room temperature and lately compared with physical optics calculations obtained in the far-field. Both co-polar and cross-polar beam patterns of a qualification model of the ALMA band 8 cartridge cooled in a cartridge-test-cryostat have also been measured in the near-field as a detector of a submillimeter vector network analyzer. The measurements presented in this work refer to the lowest, middle and upper frequencies of band 8. The comparisons between software model and experimental measurements at these frequencies show good agreement down to ?30 dB for the main polarization component. The cross-polarization level of the beam propagating through the receiver optics block was also characterized. We found that a cross-polarization level better than ?28 dB can be achieved at all measured frequencies. The measured beam pattern of this receiver corresponds to efficiency of greater than 92% at the sub reflector (diameter of 750 mm) of the ALMA 12 m optics. 相似文献
2.
The effect of realistic ionospheric Hall conductances on axisymmetric toroidal mode hydromagnetic wave resonances is investigated. The toroidal modes couple to evanescent poloidal modes near the ionospheres such that the composite modes resonate at the constant frequencies of the corresponding single-field-shell resonances for zero Hall conductance. A model for these composite modes is developed which has narrow but finite latitudinal resonance widths such as to make the modes valid solutions of the hydromagnetic equations. The modes also suggest that “shell” solutions can realistically describe such properties of real pulsations as frequency, damping, phase variation along the field-line and node-antinode behaviour at the ionospheres. Estimates of ionospheric coupling strength are obtained and compared with magnetospheric coupling strength. It is found that magnetospheric coupling dominates ionospheric coupling for any single non-axisymmetric mode. However, ionospherically coupled axisymmetric modes should be necessary components of the Fourier sum of modes required to model any real pulsation of low to moderate apparent azimuthal wave number.Estimates of the range of magnetospheric coupling strength are obtained for pulsations under a variety of conditions. 相似文献
3.
This paper presents an analytical study of the longitudinal dynamics of a thrusting, lifting, orbital vehicle in a nearly circular orbit. The translational motion is composed of a nonlinear oscillation, or phugoid, and a spiral mode which results in either decay or dilatation of the orbit depending on the perturbed initial conditions. The nonlinear effects on the phugoid period and damping are small in the altitude range considered. Elements of the orbit such as radial distance, velocity, and flight path angle were obtained explicitly as functions of time. The behavior of the variations of these elements is correctly predicted. Explicit expressions for period and damping of the angle-of-attack mode were derived. It is shown that a critical altitude may exist at which the phugoid mode and the angle-of-attack mode have nearly equal periods. Near this resonance altitude linearized solutions are no longer valid and a study of the nonlinear equations shows that there is a strong interactions between the translational and the rotational modes resulting in a switching of the two frequencies of oscillations.This work was supported by NASA Contract No. NASr 54(06). 相似文献
4.
Jayanta Kar 《Astrophysics and Space Science》1987,133(1):29-44
The unrestricted second-order virial tensor formalism has been used to calculate the characteristic frequencies of linear adiabatic oscillations of a composite stellar model having an isothermal core and a polytropic envelope in presence of a weak poloidal magnetic field. The frequencies of the transverse shear mode and the nonradial pulsation mode for both a radiative and a convective envelope (corresponding to polytropic index 3 and 1.5, respectively) alongwith that of the toroidal mode for the radiative envelope get increased in presence of the magnetic field. However, the frequency of the toroidal mode for the convective envelope registers a decrease in presence of the field. The corrections to the various frequencies decrease with increasing values of the parameter characterizing the lowering of the core temperature in presence of the magnetic field. 相似文献
5.
E. K. Majorova 《Astrophysical Bulletin》2011,66(1):90-108
We report the results of computation of the power beam pattern (PB) of the RATAN-600 radio telescope operating in the Southern-sector-with-flat-reflector
(South+flat) mode throughout the entire operating frequency range of the radio telescope with the allowance for diffraction
effects in the space between the secondary, primary, and flat mirrors. We also computed the gain-phase field distributions
at the mirror surfaces when operating in the South+flat mode and in single-sector observations at the close to horizon elevations.
The width of the power beam pattern in the vertical direction is shown to decrease and the maximum of the diagram to shift
towards lower elevations with increasing wavelength. When the radio telescope is operated in the South+flat mode the above
two changes in the power beam pattern begin to show up at shorter wavelengths compared to the case of single-sector operation.
The shift of the power beam pattern maximum is shown to depend on the height at which the antenna is mounted. The power beam
pattern was computed both for the case of a focused antenna and for the case of the off-focus primary emitter. We report the
results of the computation of the PB performed in terms of the Fresnel diffraction and geometrical optics approximations,
and also based on the available experimental data. The diagrams computed using this technique agree well with the measured
power beams. 相似文献
6.
P. P. Dunphy E. L. Chupp M. Popecki D. J. Forrest D. Lopiano T. Shima H. Spinka G. Glass G. Burleson M. Beddo 《Experimental Astronomy》1992,2(4):233-258
We report on the response of a prototype detector to medium energy neutrons. The neutrons were produced by n-p scattering of a neutron beam on a hydrogen target. The measurements provide unique data on the efficiency and response of large NaI scintillators to neutrons in the energy range 36–709 MeV. We apply the results to the high-energy mode of the Gamma-Ray Spectrometer (GRS) on the Solar Maximum Mission satellite by estimating its efficiency for neutron detection. This estimate is compared to earlier Monte Carlo calculations of the GRS efficiency. 相似文献
7.
Electron-acoustic waves are studied with orbital angular momentum (OAM) in an unmagnetized collisionless uniform plasma, whose constituents are the Boltzmann hot electrons, inertial cold electrons and stationary ions. For this purpose, we employ the fluid equations to obtain a paraxial equation in terms of cold electron density perturbations, which admits both the Gaussian and Laguerre–Gaussian (LG) beam solutions. Furthermore, an approximate solution for the electrostatic potential problem is found, which also allows us to express the components of the electric field in terms of LG potential perturbations. Calculating the energy flux of the electron-acoustic waves, an OAM density for these waves is obtained. Numerically, it is found that the parameters, such as, azimuthal angle, radial and angular mode numbers, and the beam waist strongly modify the LG potential profiles associated with electron-acoustic waves. The present results should be helpful to study the trapping and transportation of plasma particles and energy as well as to understand the electron-acoustic mode excitations produced by the Raman backscattering of laser beams in a uniform plasma. 相似文献
8.
T. Takakura 《Solar physics》1979,61(1):161-186
A simulation of normal type III radio bursts has been made in a whole frequency range of about 200 MHz to 30 kHz by the usage of the semi-analytical method as developed in previous papers for the plasma waves excited by a cloud of fast electrons. Three-dimensional plasma waves are computed, though the velocities of fast electrons are assumed to be one-dimensional. Many basic problems about type III radio bursts and associated solar electrons have been solved showing the following striking or unexpected results.Induced scattering of plasma waves, by thermal ions, into the plasma waves with opposite wave vectors is efficient even for a solar electron cloud of rather low number density. Therefore, the second harmonic radio emission as attributed to the coalescence of two plasma waves predominates in a whole range from meter waves to km waves. Fundamental radio emission as ascribed to the scattering of plasma waves by thermal ions is negligibly small almost in the whole range. On the other hand, third harmonic radio emission can be strong enough to be observed in a limited frequency range.If, however, the time integral of electron flux is, for example, 2 × 1013 cm–2 (>5 keV) or more at the height of 4.3 × 1010 cm (
p = 40 MHz) above the photosphere, the fundamental may be comparable with or greater than the second harmonic, but an effective area of cross-section of the electron beam is required to be very small, 1017 cm2 or less, and hence much larger sizes of the observed radio sources must be attributed to the scattering alone of radio waves.The radio flux density expected at the Earth for the second harmonic can increase with decreasing frequencies giving high flux densities at low frequencies as observed, if x-dependence of the cross-sectional area of the electron beam is x
1.5 or less instead of x
2, at least at x 2 × 1012 cm.The second harmonic radio waves are emitted predominantly into forward direction at first, but the direction of emission may reverse a few times in a course of a single burst showing a greater backward emission at the low frequencies.In a standard low frequency model, a total number of solar electrons above 18 keV arriving at the Earth orbit reduces to 12% of the initial value due mainly to the collisional decay of plasma waves before the waves are reabsorbed by the beam electrons arriving later. However, no deceleration of the apparent velocity of exciter appears. A change in the apparent velocity, if any, results from a change in growth rate of the plasma waves instead of the deceleration of individual electrons.Near the Earth, the peak of second harmonic radio flux as emitted from the local plasma appears well after the passage of a whole solar electron cloud through this layer. This is ascribed to the secondary and the third plasma waves as caused in non-resonant regions by the induced scattering of primary plasma waves in a resonant region. 相似文献
9.
The results of nine CCD photometric observing runs on KPD 0629–0016 are presented. During six of the runs measurements were obtained alternately through B and V filters. Four periodicities, for which there is good agreement between the frequencies identified in the B and V data, were extracted: these lie in the range 46–81 min. A fifth lower frequency appears to be definitely present, but is very uncertain due to aliasing. The ratios of the mode amplitudes measured in B and V , and the phase differences between variations in the two colours, are compatible with pulsation theory. 相似文献
10.
Noboru Kaneko Kazuhiko Morita Tetsuya Satoh Kiyotaka Toyama Masaki Nishimura Makoto Maekawa 《Astrophysics and Space Science》2000,274(3):601-641
We reanalyze the propagation of one-dimensional small-amplitude disturbances of given real frequency ω in a radiating and
scattering grey medium using the Eddington approximation, which has been studied previously by us (Kaneko et al., 1976). Numerical results reveals three frequency regimes to be distinguished, and two wave modes always appear in each frequency
regime. The governing equations and analytic solutions are derived for all wave modes using Whitham's method modified into
quadratic form and approximate methods based on radiation thermodynamics. In the high-frequency regime appear the radiation-wave
and adiabatic sound modes, which are damped by opacity and radiative cooling, respectively. Wave patterns in the intermediate-frequency
and low-frequency regimes depend critically on the importance of radiation, for which the criterion is given in terms of the
ratio of total specific heats at constant pressure and constant volume. When the radition overwhelms the matter (radiation-dominated
case), the radiative mode in the intermediate-frequency regime is the constant-volume diffusion mode. When the matter overwhelms
the radiation (matter-dominated case), damped radiation-wave and damped radiation-diffusion modes newly appear between the
radiation-wave and constant-volume diffusion modes. The acoustic mode in the intermediate-frequency regime is the isothermal
sound mode,which is damped by radiative cooling at higher frequencies and by radiation-thermal drag force at lower frequencies.
Two modes appearing in the low-frequency regime are the isentropic radiation-acoustic and constant-pressure diffusion modes.
The absorption coefficient derived for the former is shown to be a radiation-thermodynamic extension of that of Landau and
Lifshitz (1987). The transition frequencies between all adjacent two modes are also derived to discuss the implications of
them.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
The Sardinia Radio Telescope (SRT) is a challeging scientific project managed by the National Institute for Astrophysics (INAF),
it is being developed at 30 km North of the city of Cagliari, Italy. The goal of the SRT project is to build a general purpose,
fully steerable, 64 m diameter radio telescope, capable of operating with high efficiency in the centimeter and millimeter
frequency range (0.3–100 GHz). In portions of this frequency range, especially towards the high end, astronomical observations
can be heavily deteriorated by non-optimal atmospheric conditions, especially by water vapor content. The water molecule permanent
electric dipole in fact, leads to pressure broadened rotational transitions around the 22.23 GHz spectral line. Furthermore,
water vapor’s continuum absorption and emission may influence higher frequency observations too. To a lower degree, cloud
liquid black body radiation can also affect centimeter and millimeter observations. In addition to this, inhomogeneities in
water vapor distributions can cause signal phase errors which introduce a great amount of uncertainty to VLBI mode observations.
The Astronomical Observatory of Cagliari (OA-CA) has obtained historical timeseries of radiosonde profiles conducted at the
airport of Cagliari. Through the radiosonde measurements and an appropriate radiative transfer model, we have performed a
statistical analysis of the SRT site’s atmosphere which accounts for atmospheric opacity at different frequencies, integrated
water vapor (IWV), integrated liquid water (ILW) and cloud cover distributions during the year. This will help to investigate
in which period of the year astronomical observations at different frequencies should be performed preferably. The results
show that, at the SRT site, K-band astronomical observations are possible all year round, the median opacity at 22.23 GHz
is 0.10 Np in the winter (Dec-Jan-Feb) and 0.16 Np in the summer (Jun-Jul-Aug). Integrated water vapor during winter months
ranges, on average, between 7 and 15 mm. Cloud cover is usually not present for more than 36% of the time during the year.
The atmospheric opacity study indicates that observations at higher frequencies (50–100 GHz) may be performed usefully: the
median opacity at 100 GHz is usually below or equal to 0.2 Np in the period that ranges from January to April. 相似文献
12.
W. A. Dziembowski J. Daszy&#;ska-Daszkiewicz A. A. Pamyatnykh 《Monthly notices of the Royal Astronomical Society》2007,374(1):248-255
We use the traditional approximation to describe oscillations with frequencies comparable to the angular rotation rate. Validity of this approximation in application to main-sequence B stars is discussed. Numerical results regarding mode stability and visibility are presented for a model of the Be star HD 163868. For this object, Walker et al. detected a record number of mode frequencies using data from the small space telescope MOST . Our interpretation of these data differs from that of Walker et al. In particular, we interpret peaks in the lowest frequency range as retrograde g modes. We find instability in a large number of modes that remain undetectable because of unfavourable aspect and/or effect of cancellation. There is no clear preference to excitation of prograde modes. 相似文献
13.
S. V. Vorontsov 《Monthly notices of the Royal Astronomical Society》2007,378(4):1499-1506
Accurate measurements of solar p-mode frequencies and frequency splittings at high degree l require an adequate theoretical knowledge of the effects of mode coupling, induced by the variation with latitude of the angular velocity of the solar internal rotation. Earlier results for expansion coefficients of composite solutions (coupling coefficients) are due to Woodard. In this paper, the analysis is extended to allow for the dependence of the differential rotation on depth, and the result is expressed in terms of measurable quantities (the rotational splitting coefficients), which makes it convenient for diagnostic purposes. The analysis is based on the approach of quasi-degenerate perturbation theory, and is extended further to address possible effects of mode coupling in the observational line profiles. It is shown, using approximations applicable at high degree l , that the expected line profiles of composite modes in the observational power spectra are not distorted by mode coupling. 相似文献
14.
The occurrence of oscillatory mode for magnetized fluid in the presence of vertical and horizontal temperature gradients has been studied. In both the cases (highly viscous and low-viscosity fluids) the occurrence of oscillatory solutions to the Bénard problem has been investigated as a function of Prandtl number, magnetic number, resistivity number and the ratio of applied temperature gradients. It is clear that the high Prandtl numbers completely change the character of instability and of the solutions as compared to those appropriate for low Prandtl-number fluids. It has also been shown that small horizontal temperature gradient readily generate oscillatory modes only for real frequencies. The critical Rayleigh number is higher than the critical Rayleigh number of non-magnetized fluid in both the cases-i.e., highly viscous fluid and one of moderate viscosity. 相似文献
15.
The problem of the efficiency of the ion-synchrotron maser proposed by Hoshino and Arons is analyzed in a linear approximation.
A hot, relativistic, electron-positron plasma penetrated by a relativistic ion beam is considered. At the front of the magnetosonic
shock wave an electromagnetic wave is generated, which should be damped on positrons of the plasma. This should, in turn,
result in synchrotron emission from energetic positrons in the high-frequency range, far above the natural frequencies of
the plasma. It is shown that one must allow simultaneously for the conditions of resonance at a high harmonic of the ion-cyclotron
frequency and at the fundamental of the electron-cyclotron frequency. Natural transverse waves are generated in the process,
but within the framework of the linear theory there is no positron acceleration due to the kinetic energy of ions.
Translated from Astrofizika, Vol. 43, No. 3, pp. 389-396, July–September, 2000. 相似文献
16.
Kazunari Iwasaki Toru Tsuribe 《Monthly notices of the Royal Astronomical Society》2008,387(4):1554-1562
A new self-similar solution describing the dynamical condensation of a radiative gas is investigated under a plane-parallel geometry. The dynamical condensation is caused by thermal instability. The solution is applicable to generic flow with a net cooling rate per unit volume and time ∝ρ2 T α , where ρ, T and α are the density, temperature and a free parameter, respectively. Given α, a family of self-similar solutions with one parameter η is found in which the central density and pressure evolve as follows: ρ( x = 0, t ) ∝ ( t c − t )−η/(2−α) and P ( x = 0, t ) ∝ ( t c − t )(1−η)/(1−α) , where t c is the epoch at which the central density becomes infinite. For η∼ 0 the solution describes the isochoric mode, whereas for η∼ 1 the solution describes the isobaric mode. The self-similar solutions exist in the range between the two limits; that is, for 0 < η < 1 . No self-similar solution is found for α > 1 . We compare the obtained self-similar solutions with the results of one-dimensional hydrodynamical simulations. In a converging flow, the results of the numerical simulations agree well with the self-similar solutions in the high-density limit. Our self-similar solutions are applicable to the formation of interstellar clouds (H i clouds and molecular clouds) by thermal instability. 相似文献
17.
Oscillations of magnetic structures in the solar corona have often been interpreted in terms of magnetohydrodynamic waves.
We study the adiabatic magnetoacoustic modes of a prominence plasma slab with a uniform longitudinal magnetic field, surrounded
by a prominence – corona transition region (PCTR) and a coronal medium. Considering linear small-amplitude oscillations, we
deduce the dispersion relation for the magnetoacoustic slow and fast modes by assuming evanescentlike perturbations in the
coronal medium. In the system without PCTR, a classification of the oscillatory modes according to the polarisation of their
eigenfunctions is made to distinguish modes with fastlike or slowlike properties. Internal and external slow modes are governed
by the prominence and coronal properties, respectively, and fast modes are mostly dominated by prominence conditions for the
observed wavelengths. In addition, the inclusion of an isothermal PCTR does not substantially influence the mode frequencies,
but new solutions (PCTR slow modes) are present. 相似文献
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20.
In this paper we reconsider the formulae of tropospheric refraction correction for the Satellite Laser Range technique. From the expansion of the complementary error function, a new continued fraction form of the mapping function at optical frequencies is derived. The correction terms related to the operation frequency of the laser beam are considered in both the zenith delay and the mapping function. The correction for low-elevation satellites is briefly reviewed. The theoretical accuracy of the new mapping function has been analysed via the ray tracing integrals under the standard atmospheric profile. With respect to the radiosonde data, the deviations of the new mapping function are investigated in an elevation range down to near 1°, which is comparable with the results of the Marini–Murray formulae . 相似文献