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971.
Hidden Mass in the Asteroid Belt 总被引:1,自引:0,他引:1
The total mass of the asteroid belt is estimated from an analysis of the motions of the major planets by processing high precision measurements of ranging to the landers Viking-1, Viking-2, and Pathfinder (1976-1997). Modeling of the perturbing accelerations of the major planets accounts for individual contributions of 300 minor planets; the total contribution of all remaining small asteroids is modeled as an acceleration caused by a solid ring in the ecliptic plane. Mass Mring of the ring and its radius R are considered as solve-for parameters. Masses of the 300 perturbing asteroids have been derived from their published radii based mainly on measured fluxes of radiation, making use of the corresponding densities. This set of asteroids is grouped into three classes in accordance with physical properties and then corrections to the mean density for each class are estimated in the process of treating the observations. In this way an improved system of masses of the perturbing asteroids has been derived.The estimate Mring≈(5±1)×10−10M⊙ is obtained (M⊙ is the solar mass) whose value is about one mass of Ceres. For the mean radius of the ring we have R≈2.80 AU with 3% uncertainty. Then the total mass Mbelt of the main asteroid belt (including the 300 asteroids mentioned above) may be derived: Mbelt≈(18±2)×10−10M⊙. The value Mbelt includes masses of the asteroids which are already discovered, and the total mass of a large number of small asteroids—most of which cannot be observed from the Earth. The second component Mring is the hidden mass in the asteroid belt as evaluated from its dynamical impact onto the motion of the major planets.Two parameters of a theoretical distribution of the number of asteroids over their masses are evaluated by fitting to the improved set of masses of the 300 asteroids (assuming that there is no observational selection effect in this set). This distribution is extrapolated to the whole interval of asteroid masses and as a result the independent estimate Mbelt≈18×10−10M⊙ is obtained which is in excellent agreement with the dynamical finding given above.These results make it possible to predict the total number of minor planets in any unit interval of absolute magnitude H. Such predictions are compared with the observed distribution; the comparison shows that at present only about 10% of the asteroids with absolute magnitude H<14 have been discovered (according to the derived distribution, about 130,000 such asteroids are expected to exist). 相似文献
972.
We present an analysis of OH, CN, and C2 jets observed in thecoma of Comet Hale–Bopp on UT April 22, 23, and 25, 1997. Monte Carlomodels designed to simulate the
gas jets were employed to analyze thenuclear active areas responsible for the observed coma gas jets. Ourresults indicate
that four active areas are necessary to reproduce theCN and C2 jets while five active areas are required to simulatethe OH jets. The additional OH active area must produce significantlevels
of OH, but cannot emit measurable quantities of either carbonradical. This difference suggests that the nucleus of Comet Hale–Boppis
chemically heterogeneous. 相似文献
973.
Solar Physics - On 25 January, 2000, we observed active region NOAA&;nbsp;8844 with the Flare Genesis Experiment (FGE), a balloon-borne observatory with an 80-cm solar telescope. FGE was... 相似文献
974.
Brandt J. C. Snow M. Yi Y. Larson S. M. Mikuz H. Petersen C. C. Liller W. 《Earth, Moon, and Planets》2002,90(1-4):15-33
The plasma tails of comets clearly show the demarcation of the solar wind into distinct equatorial and polar regions (Brandt
and Snow (2000), Icarus
148, 52–64).The boundary is determined by the maximum extent in latitude of the heliospheric current sheet (HCS). The observational
record contains many well-observed equatorial comets, but observations of comets in the polar region are relatively rare.
In addition to its size and brightness, comet Hale–Bopp had an orbital inclination of 89.4° and was well observed for months
in the polar region. We document the comet's large-scale appearance throughout the apparition, including the polar region
and its transition into the equatorial region. The bright dust tail hampered observations of the plasma tail, particularly
near the head, but images taken with a CO+ filter show a very large disconnection event (DE) on May 7 and May 8, 1997. The time of disconnection is estimated at approximately
May 4.0. This DE is associated with a crossing of the HCS. The model calculations of the HCS indicate that other crossings
might have occurred in late April, but given the uncertainty in the calculation, the comet might have missed the HCS. Sparse
observational coverage and the bright dust tail prevent further investigation of the potential earlier HCS crossings. The
plasma tail shows anomalous orientations at the highest latitudes and possible explanations are discussed. 相似文献
975.
976.
Multiple magnetic clouds in interplanetary space 总被引:7,自引:0,他引:7
An interplanetary magnetic cloud (MC) is usually considered the byproduct of a coronal mass ejection (CME). Due to the frequent occurrence of CMEs, multiple magnetic clouds (multi-MCs), in which one MC catches up with another, should be a relatively common phenomenon. A simple flux rope model is used to get the primary magnetic field features of multi-MCs. Results indicate that the magnetic field configuration of multi-MCs mainly depends on the magnetic field characteristics of each member of multi-MCs. It may be entirely different in another situation. Moreover, we fit the data from the Wind spacecraft by using this model. Comparing the model with the observations, we verify the existence of multi-MCs, and propose some suggestions for further work. 相似文献
977.
It is the purpose of this paper to present evidence concerning the appearance of a sunspot in A.D. 939 observed by Arabs from
the Iberian Peninsula. The sunspot record appears in the Arab source known as al-Muqtabis V. 相似文献
978.
Vincent M. Woolf C. Simon Jeffery Donald L. Pollacco 《Monthly notices of the Royal Astronomical Society》2002,329(3):497-501
We present an analysis of high-speed spectroscopy of the pulsating subdwarf B star PG 1605+072. Periodic radial motions are detected at frequencies similar to those reported for photometric variations in the star, with amplitudes of up to 6 km s−1 . Differences between relative strengths for given frequency peaks for our velocity data and previously measured photometry are probably a result of shifting of power between modes over time. Small differences in the detected frequencies may also indicate mode-shifting. We report the detection of line-shape variations using the moments of the cross-correlation function profiles. It may be possible to use the moments to identify the pulsation modes of the star. 相似文献
979.
980.
E. Knobloch S. M. Tobias & N. O. Weiss 《Monthly notices of the Royal Astronomical Society》1998,297(4):1123-1138
Stellar dynamos are governed by non-linear partial differential equations (PDEs) which admit solutions with dipole, quadrupole or mixed symmetry (i.e. with different parities). These PDEs possess periodic solutions that describe magnetic cycles, and numerical studies reveal two different types of modulation. For modulations of Type 1 there are parity changes without significant changes of amplitude, while for Type 2 there are amplitude changes without significant changes in parity. In stars like the Sun, cyclic magnetic activity is interrupted by grand minima that correspond to Type 2 modulation. Although the Sun's magnetic field has maintained dipole symmetry for almost 300 yr, there was a significant parity change at the end of the Maunder Minimum. We infer that the solar field may have flipped from dipole to quadrupole polarity (and back) after deep minima in the past and may do so again in the future. Other stars, with different masses or rotation rates, may exhibit cyclic activity with dipole, quadrupole or mixed parity. The origins of such behaviour can be understood by relating the PDE results to solutions of appropriate low-order systems of ordinary differential equations (ODEs). Type 1 modulation is reproduced in a fourth-order system while Type 2 modulation occurs in a third-order system. Here we construct a new sixth-order system that describes both types of modulation and clarifies the interactions between symmetry-breaking and modulation of activity. Solutions of these non-linear ODEs reproduce the qualitative behaviour found for the PDEs, including flipping of polarity after a prolonged grand minimum. Thus we can be confident that these patterns of behaviour are robust, and will apply to stars that are similar to the Sun. 相似文献