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1.
本文对充满垂直均匀磁场的等温大气内的磁声重力波做了严格的解析分析,并将其通解表述成广义超几何函数的形式。该解可用于对磁大气内振荡现象的进一步数值模拟研究。对解的分析澄清了若干磁声重力波的传播性质。  相似文献   

2.
Intensity variation of cosmic rays near the heliospheric current sheet   总被引：1，自引：0，他引：1  
Cosmic ray intensity variations near the heliospheric current sheet—both above and below it—have been studied during 1964–1976. Superposed epoch analysis of the cosmic ray neutron monitor data with respect to sector boundaries (i.e., heliospheric current sheet crossings) has been performed. In this analysis we have used the data from neutron monitors well distributed in latitude over the Earth's surface. First, this study has been made during the two solar activity minimum periods 1964–1965 and 1975–1976, using the data from Thule (cut-off rigidity 0 GV), Deep River (cut-off rigidity 1.02 GV), Rome (cut-off rigidity 6.32 GV) and Huancayo (cut-off rigidity 13.45 GV) neutron monitors. We have also analyzed the data from Deep River, Rome and Huancayo neutron monitors, for whom we have the data for full period (1964–1976), by dividing the periods according to the changes in solar activity, interplanetary magnetic field polarity and coronal holes. All these studies have shown a negative gradient with respect to heliomagnetic latitude (current sheet). These results have been discussed in the light of theoretical and observational evidences. Suggestions have been given to overcome the discrepancy between the observational and theoretical results. Further, possible explanations for these observational results have been suggested.  相似文献   

3.
The published data on the temperature dependence of the radiative combination of atomic oxygen with nitric oxide at pressures near 1 torr is examined. Arguments are advanced to suggest that radiation near the cut-off wavelength (～ 3875Å) is coming from the unstabilized activated complex, No12. At 4000Å a positive activation energy of 1 kcal mole?1 is deduced. Application of this temperature dependence with the rate coefficient at 5200Å is made to airglow measurements in aurora. The deduced NO concentration is about 109 cm?3, in general agreement with that deduced from the measured NO+/O+2 ratio as well as an auroral model prediction.  相似文献   

4.
The space geodetic technology requires an accurate model of correction of refraction delay by the neutral atmosphere that varies from one observing station to another, and from one azimuth to the next. It is pointed out that under the present condition the astronomical refraction can not yet be directly determined, any correction model because of its high dependence on the assumed atmospheric distribution, is incapable of achieving the required accuracy or of improving the cut-off altitude. In this paper, based on the special properties of the lower latitude meridian circle at Yunnan Observatory and our experience of determining atmospheric refraction therewith, a new method is proposed for improving the accuracy of refraction delay correction. Namely, the measured data of astronomical refraction of an observing station from near zenith to low altitudes in different azimuths are used to evaluate the refractivities and the parameters of the mapping functions, thereby establishing a model of atmospheric refraction delay correction that varies with the observing station and the azimuth. Since it is unnecessary for the new method to adopt any atmospheric distribution model, application of this new method will improve correction accuracy of refraction delay to better than 1mm at zenith and to centimeters at low altitudes, and improve the cut-off altitude to below 5 degrees.  相似文献   

5.
Optical spectral indices of 314 quasars are presented. They are found to cluster around 0=–0.5. These together with 73 optical spectral indices from Richstone and Schmidt form our data set. We have found that the optical spectral indices have a significant correlation with the redshifts while they have no correlation with the optical luminosities. The sharp cut-off near 0 = 1/3 claimed by Richstone and Schmidt is not seen. Instead a cut-off near 0 = 5/2 is indicated. This may be interpreted as an evidence of an optically thick synchrotron source in the Rayleigh-Jeans limit in which the emitted spectrum has an index of 5/2.  相似文献   

6.
Taking pulsars with long pulse nulling as forming a special group, we found that those with pronounced nulling are located on a line near and parallel to the radio cut-off in the 1gP — 1gP diagram. This in turn suggests a reason for the nulling. They are also shown to have smaller than average fields at the light cylinder and to have smaller than average rates of energy dissipation.  相似文献   

7.
Variations of the cosmic ray cut-off rigidities have been observed at mid latitudes during the magnetic storm period 16–18 December 1971. In the present paper the cut-off changes over Europe are determined on an hourly basis from the registrations of 10 European neutron monitor stations. As a first order approximation it is assumed that the observed cut-off variations originate from a spherical current sheet concentric with the Earth and with a current density proportional to the cosine of the geomagnetic latitude. Applying results obtained by Treiman (1953), the radii of the current sphere can then be deduced from the dependences of the relative cut-off rigidity variations on geomagnetic latitude. The sphere is found to be located between 4 and 6 Earth radii during the main phase of the magnetic storm on 17 December 1971. A comparison of these results with in situ measurements carried out in the equatorial plane by Explorer 45 shows good agreement.  相似文献   

8.
It is pointed out that at frequencies near the plasma cut-off frequencies, the corrections to wave refractive indices in a cold plasmaN 0due to the contribution of ions and relativistic effects can be of the same order of magnitude or greater thanN 0. Expressions for wave refractive indicesN taking into account these corrections are derived in a limiting case |N| I. It is shown that the increase in cut-off frequencies due to effects of ions is negligibly small unless the electron plasma frequency is well below the electron gyrofrequency. The decrease of the cut-off frequencies due to relativistic effects is significant ( 1%) only in a rather hot plasma (T e 1 keV), which may be observed in a plasma sheet region of the Earth's magnetosphere and in astrophysical conditions. These effects appear to be particularly important in a strongly anisotropic plasma (the electron perpendicular temperature is noticeably greater than the parallel one).  相似文献   

9.
本文用脉冲星光速圆柱附近磁能的衰减理论研究了毫秒脉冲星的演化。文中从理论上给出了脉冲星到达辐射截止线时的年龄和自转周期,及现在的辐射年龄和从现在辐射年龄到达辐射截止线所需的时间。并用此理论对7颗脉冲双星的演化作了数值计算。  相似文献   

10.
Energetic neutrals in dissociative recombinations near or above the exobase provide an important component of exospheric density and escape fluxes. Plasma thermal velocities provide the main contribution to the velocity spread and an exact integral for the escape flux applicable in marginal cases is found for a simple atmosphere and collisional cut-off. Atomic fragments from recombination of diatomic oxygen and nitrogen ions in the Venus and Mars atmospheres are examined and density integrals derived. The oxygen escape flux on Mars is half that previously estimated and there is very little isotope preference supplementing diffusive separation. However, escape of the heavier 15N isotope is low by a factor two. Reinterpretation of its 75% enrichment as detected by Viking leads to a range 0.4–1.4 mbar for the primeval nitrogen content on Mars.  相似文献   

11.
Gamma-ray emission in pulsar magnetospheres is attributed to synchrotron radiation, which tends to decrease the pitch angle of the particle, being balanced by plasma processes tending to increase the pitch angle. The plasma processes are non-resonant instabilities that drive non-resonant quasilinear diffusion (NQD), thereby pumping energy from waves and the parallel motion of the particle into the perpendicular motion of the particle. It is shown that NQD can maintain the pitch angles for particles near the light-cylinder such that they radiate synchrotron radiation at MeV energies. Compared to conventional emission mechanisms (such as polar cap or outer gap models), the resulting spectrum has a relatively low upper cut-off from about a few to 100 MeV. Possible observational consequences of this mechanism are discussed.  相似文献   

12.
In the present paper we discuss the modifications introduced into the first-order Fermi shock acceleration process due to a finite extent of diffusive regions near the shock or due to boundary conditions leading to an increased particle escape upstream and/or downstream of the shock. In the simple example of the planar shock wave considered we idealize the escape phenomenon by imposing a particle escape boundary at some distance from the shock. The presence of such a boundary (or boundaries) leads to coupled steepening of the accelerated particle spectrum and decreasing of the acceleration time scale. It allows for a semi-quantitative evaluation and, in some specific cases, also for modelling of the observed steep particle spectra as a result of the first-order Fermi shock acceleration. We also note that the particles close to the upper energy cut-off are younger than the estimate based on the respective acceleration time scale. In Appendix A we present a new time-dependent solution for infinite diffusive regions near the shock allowing for different constant diffusion coefficients upstream and downstream of the shock.  相似文献   

13.
《Planetary and Space Science》2007,55(9):1010-1020
In the absence of numerous in situ studies, physical properties of cosmic dust may be derived from observations of their light scattering and thermal properties, through numerical simulations making use of realistic assumptions. Estimations about cometary and interplanetary dust composition, structure, size, as well as about their light scattering and thermal properties, are first summarized. We then present and discuss the numerical simulations we have performed with different types of particles: core-mantle submicron-sized elongated grains (having contributed to the formation of cometary dust), fractal aggregates of such grains (found in cometary comae and in the interplanetary dust cloud), and fractal aggregates of large dust grains (found in cometary dust trails).A very satisfactory fit to the numerous polarimetric observations of comet Hale-Bopp is obtained for a mixture with about 33–60% of organics in mass, with a power law size distribution with an index of (−3) and a radius of 20 μm for the upper cut-off. For the less-constrained polarimetric observations of interplanetary dust near 1 AU, a fit is obtained for a mixture with about 40% of organics in mass, with a similar size distribution and a radius of about 50 μm for the upper cut-off. The ensemble of results obtained for the interplanetary dust strongly suggest that its light scattering and thermal properties stem from the presence of compact and fluffy particles, with compositions ranging from silicates to more absorbing materials, whose contribution decreases with decreasing distance to the Sun.  相似文献   

14.
An analysis of magneto-acoustic-gravity waves in the case of an isothermal atmosphere permeated by a uniform magnetic field is presented. The general solution is expressed in terms of generalized hypergeometric functions. It can be used in numerical simulation of oscillations in a magnetic atmosphere.

It is shown that the elliptically polarized magneto-acoustic-gravity waves consist of a pair of surface waves and a pair of body waves above the cut-off frequency. The body waves along the magnetic field are similar to acoustic waves in an atmosphere and their cut-off frequency is unaffected by magnetic field. The transverse oscillation decreases with height. For the usual boundary condition, the longitudinal oscillation decreases with height; however, in some cases, it may contain terms that increase with height. The solution is singular on a family of ellipses in the frequency - horizontal wave number plane. Near these ellipses, the wave components grow indefinitely.  相似文献   


15.
Most of the unidentified gamma ray sources detected near the Galactic plane by EGRET aboard CGRO are expected to be gamma ray pulsars. We present a study about the detectability and identification of some unidentified EGRET sources with the MAGIC telescope. We list some unidentified gamma ray sources from the third EGRET catalogue to be detected with MAGIC taking into account some important conditions such as the variability parameter of the source, spectral index, inclusion in the GeV catalogue (ApJ 488, 1997, p. 872) and possible associations with known X-ray/radio sources located within the error box of the unidentified gamma ray source. We show the required observation time of these gamma ray pulsar candidates to be detected by MAGIC telescope considering reasonable values of cut-off energy. To be more realistic, we have chosen the zenith angle corresponding to the source culmination in the simulation of the effective area A since the observation time is function of the effective area. In addition to this study, it is very important to consider the extrapolated EGRET flux at MAGIC energies above 30 GeV of the gamma ray pulsar candidates taking the MAGIC sensitivity.  相似文献   

16.
At redshifts z ≳2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Ly α forest. This photoheated gas follows a tight temperature–density relation which introduces a cut-off in the distribution of widths of the Ly α absorption lines ( b -parameters) as a function of column density. We have measured this cut-off in a sample of nine high-resolution, high signal-to-noise ratio quasar spectra and determined the thermal evolution of the intergalactic medium in the redshift range 2.0–4.5. At a redshift z ∼3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of He  ii .  相似文献   

17.
CCD photometry and MERLIN observations of M87 (Virgo A) have been used to study the variations of the radio to optical spectral index along the jet. In general, the radio spectrum is flatter than the optical one and both become steeper with distance from the nucleus. The optical fluxes of the bright knots lie at or below the extrapolation of the radio spectrum. These properties can be explained by power-law spectra at low frequencies that turn down gradually beyond a cut-off frequency. The location of the local cut-off frequency is estimated by fitting the measured fluxes to the theoretical spectrum of a truncated power-law electron energy distribution. The results are interpreted in terms of models of synchrotron emission from shock waves in extragalactic jets.  相似文献   

18.
Observations carried out with the Magneto-Optical Filter at Two Heights (MOTH) experiment show upward-traveling wave packets in magnetic regions with frequencies below the acoustic cut-off. We demonstrate that the frequency dependence of the observed travel times, i.e. the dispersion relation, shows significant differences in magnetic and non-magnetic regions. More importantly, at and above the layer where the Alfvén speed equals the sound speed we do not see the dispersion relation of the slow acoustic mode with a lowered cut-off frequency. Our comparisons with theoretical dispersion relations suggest that this is not the slow acoustic wave type for the upward low-frequency wave. From this we speculate that partial mode conversion from the fast acoustic to the fast magnetic wave might take place.  相似文献   

19.
A simple model of the motion of charged particles in the closed field line magnetic field for L ? 4·5 is used together with Injun 3 measurements of 40 keV precipitated electrons made in the northern hemisphere to estimate theoretically the extent of electron precipitation, the energy input and the 3914 Å airglow in the South Atlantic geomagnetic anomaly. Using average values of the northern hemisphere precipitated electron flux, two regions of significantly enhanced electron precipitation are found in the southern hemisphere. One occurs in the region 10–20°E and 40–50°S, with L ≈ 2, and the second near 30°E and 65°S, with L ≈ 4.5. Approximately 0.04 erg cm?2 sec?1 are deposited by 40 keV electrons for 50 per cent of the time in the first region and half that amount in the second. This increases to ～0·1 and 0·02 erg cm?2 sec?1 respectively for 15 per cent of the time for near sunspot minimum conditions. The results show a gradual increase in precipitation on the western side of the anomaly followed by a rapid increase and sudden cut-off in precipitation within a few degrees west of minimum B. The flux on L = 2 reaches a “spike” in the southern hemisphere ～f35 times greater than the average flux precipitated on L = 2 in the northern hemisphere. This increase in precipitation arises from the loss of “trapped” particles to the atmosphere where the mirror heights are lowest.  相似文献   

20.
Three decades have passed since the supernova SN 1987A was observed in the Large Magellanic Cloud, inside which the product is most likely a neutron star (NS) formed in the core collapse explosion.Although lots of observations with sensitive radio telescopes have taken place, astronomers have not yet detected any evidence for a radio pulsar around the remnant of 1987A. To investigate pulsars inside the SN remnants, we calculate the cut-off oscillation frequency of the plasma around the presumed NS inside SN1987A, as shown to be about 33 GHz at present (2018 CE), which is much higher than the favorite “searching window” (e.g. L-band ∼ 1.4 GHz) of radio pulsar surveys that have been commonly exploited by astronomers. Since radio waves with frequencies lower than the plasma cut-off frequency cannot penetrate the SN remnant media, we suggest that astronomers use higher frequency bands to search for a pulsar in SN 1987A.Furthermore, with the expansion of SN remnant media, we find that the plasma cut-off frequency can decay to the L-band (1.4 GHz) in the future. The strategy of finding a pulsar of SN 1987A is that either the high frequency bands of radio telescopes, or the high energy detections at Gamma-ray and X-ray bands by space satellites are applied.  相似文献   

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