Comet P/Holmes, 1892III: A case of duplicity?     

Fred L. Whipple 《Icarus》1984,60(3):522-531

The observations of comet P/Holmes 1892III, exhibiting two 8- to 10-mag bursts, have been carefully analyzed. The phenomena are consistent with the grazing encounter of a small satellite with the nucleus on November 4.6, 1892, and the final encounter on January 16.3, 1893. The grazing encounter produced, besides the first great burst, an active area on the nucleus, which was rotating retrograde with a period of 16.3 hr and inclination of nearly 180°. After the final encounter, the spin period was essentially unchanged, but two areas became active, separated some 164° in longitude on the nucleus. After the first burst the total magnitude fell less than 2 mag from November 7 to 30 (barely naked eye) while the nuclear region remained diffuse or complex, rarely of ever showing a stellar appearance. The fading was much more rapid after the second burst (barely naked eye at maximum) while the nucleus frequently appeared stellar after the first day. It seems reasonable to conclude that the grazing encounter distributed a volume of large chunks in the neighborhood of the nucleus, maintaining activity for weeks. The final encounter activated a new area on the nucleus, the shock and fall back disturbing the area already exposed by the grazing encounter. Several details of this scenario are fitted rather well.  相似文献   

Interplanetary dust particles near Jupiter     

S.Fred Singer  John E. Stanley 《Icarus》1976,27(2):197-205

We calculate the expected counting rate of a flat micrometeoroid detector of finite sensitivity passing in hyperbolic orbit near a planet. We assume that the distribution of particle sizes, s, can be expressed as a power law spectrum of index p, i.e. dN(s) = Cs^?pds, and also that the particles encounter the sphere of influence of the planet with a certain speed v_∞. The results of the calculations are then compared with the results returned by Pioneer 10 in its flyby of Jupiter. The observed increase in impact rate near Jupiter can be completely explained in terms of gravitational “focusing” of particles which are in heliocentric orbits; i.e., they are not in orbit about Jupiter. The absolute concentration of particles near the orbit of Jupiter is of the same order as at 1 AU: the exact ratio being a function of particle speed and spectral index. Data from one flyby are insufficient to determine a unique value for both the spectral index, p, and the particle velocity, v_∞, but limits can be set. For reasonable encounter speeds (corresponding to eccentricities and inclinations of dust particles experienced near the Earth), the particles near Jupiter are characterized by a spectrum of index p ～ 3. The spectral index which best fits the data increases with increasing encounter speeds.  相似文献   

Moonquakes and lunar tectonism   总被引：1，自引：0，他引：1  

Gary Latham  Maurice Ewing  James Dorman  David Lammlein  Frank Press  Naft Toksőz  George Sutton  Fred Duennebier  Yosio Nakamura 《Earth, Moon, and Planets》1972,4(3-4):373-382

With the succesful installation of a geophysical station at Hadley Rille, on July 31, 1971, on the Apollo 15 mission, and the continued operation of stations 12 and 14 approximately 1100 km SW, the Apollo program for the first time achieved a network of seismic stations on the lunar surface. A network of at least three stations is essential for the location of natural events on the Moon. Thus, the establishment of this network was one of the most important milestones in the geophysical exploration of the Moon. The major discoveries that have resulted to date from the analysis of seismic data from this network can be summarized as follows:

1. Lunar seismic signals differ greatly from typical terrestrial seismic signals. It now appears that this can be explained almost entirely by the presence of a thin dry, heterogeneous layer which blankets the Moon to a probable depth of few km with a maximum possible depth of about 20 km. Seismic waves are highly scattered in this zone. Seismic wave propagation within the lunar interior, below the scattering zone, is highly efficient. As a result, it is probable that meteoroid impact signals are being received from the entire lunar surface.
2. The Moon possesses a crust and a mantle, at least in the region of the Apollo 12 and 14 stations. The thickness of the crust is between 55 and 70 km and may consist of two layers. The contrast in elastic properties of the rocks which comprise these major structural units is at least as great as that which exists between the crust and mantle of the earth. (See Toks?zet al., p. 490, for further discussion of seismic evidence of a lunar crust.)
3. Natural lunar events detected by the Apollo seismic network are moonquakes and meteoroid impacts. The average rate of release of seismic energy from moonquakes is far below that of the Earth. Although present data do not permit a completely unambiguous interpretation, the best solution obtainable places the most active moonquake focus at a depth of 800 km; slightly deeper than any known earthquake. These moonquakes occur in monthly cycles; triggered by lunar tides. There are at least 10 zones within which the repeating moonquakes originate.
4. In addition to the repeating moonquakes, moonquake ‘swarms’ have been discovered. During periods of swarm activity, events may occur as frequently as one event every two hours over intervals lasting several days. The source of these swarms is unknown at present. The occurrence of moonquake swarms also appears to be related to lunar tides; although, it is too soon to be certain of this point.

These findings have been discussed in eight previous papers (Lathamet al., 1969, 1970, 1971) The instrument has been described by Lathamet al. (1969) and Sutton and Latham (1964). The locations of the seismic stations are shown in Figure 1.  相似文献   

Synoptic charts of solar 9.1 cm and coronal hole data     

Fred L. Wefer  Michael D. Papagiannis 《Solar physics》1980,67(1):13-21

Synoptic charts for Carrington rotations 1601–1605 (May–August, 1973) were prepared using the central meridian column of the daily 9.1 cm Stanford solar radio maps. These charts were especially contoured to emphasize temperatures near the quiet solar disk level. Synoptic charts of coronal holes from the ATM-Skylab were superimposed on the radio data to investigate the ability of the radio charts to show coronal holes. This brief period is unfortunately the only interval for which both sets of data are available. The conclusion reached is that in spite of certain problems due to active regions, side-lobe effects and a rather large beamwidth, the 9.1 cm synoptic charts can be of substantial value in identifying large coronal holes, especially during periods of low solar activity. Such synoptic charts, therefore, for the years 1962–1973 that Stanford data are available, could enhance significantly the meagre data pool for coronal holes prior to the Skylab mission.  相似文献   

Comparison of single-domain and dual-domain subsurface transport models     

Flach GP  Crisman SA  Molz FJ 《Ground water》2004,42(6-7):815-828

Subgrid modeling of some type is typically used to account for heterogeneity at scales below the grid scale. The single-domain model (SDM), employing field-scale dispersion, and the dual-domain model (DDM), employing local hydrodynamic dispersion and exchange between domains having large hydraulic conductivity contrasts, are well-known examples. In this paper, the two modeling approaches are applied to tritium migration from the H-area seepage basins to a nearby stream--Fourmile Branch--at the Savannah River Site. This location has been monitored since 1955, so an extensive dataset exists for formulating realistic simulations and comparing the results to data. It is concluded that the main parameters of both models are scale-dependent, and methods are discussed for making initial estimates of the DDM parameters, which include mobile/immobile porosities and the mass exchange coefficient. Both models were calibrated to produce the best fit to recorded tritium data. When various attributes of the dataset were considered, including cumulative tritium activity discharged to Fourmile Branch, plume arrival time, and plume attenuation due to closure of the seepage basins in 1988, the DDM produced results superior to the SDM, while causing no unrealistic upgradient dispersion. A sensitivity analysis showed that only the DDM was able to accurately produce both the instantaneous activity discharge and cumulative activity with a single parameter set. This is thought to be due to the advection-dominated nature of transport in natural porous media and the more realistic treatment of this type of transport in the DDM relative to the SDM.  相似文献   

Upwelling signals in radiocarbon from early 20th-century Peruvian bay scallop (Argopecten purpuratus) shells     

Kevin B. Jones  Gregory W.L. Hodgins  Miguel F. Etayo-Cadavid  C. Fred T. Andrus 《Quaternary Research》2009,72(3):452-456

We quantified Δ¹⁴C, δ¹⁸O, and δ¹³C cycles along ontogeny within four bay scallop (Argopecten purpuratus) shells collected from Callao Bay, Salaverry, and Sechura Bay, Peru following the 1907–1908 non-El Niño years and the 1925–1926 El Niño. Δ¹⁴C and δ¹³C generally covary; Δ¹⁴C and δ¹⁸O vary inversely. Simultaneous decreases in Δ¹⁴C and increases in δ¹⁸O in non-El Niño shells are followed by constant Δ¹⁴C and gradually decreasing δ¹⁸O, which we interpret as evidence for discrete marine upwelling events followed by warming of the initially cold upwelled water. Upwelling changes from El Niño events are detectable with difficulty in mollusk shell Δ¹⁴C.  相似文献   

Characterization of sudden and sustained base flow jump hydrologic behaviour in the humid seasonal tropics of the Panama Canal Watershed     

Yanyan Cheng  Fred L. Ogden  Jianting Zhu 《水文研究》2020,34(3):569-582

Base flows are important for tropical regions with pronounced dry seasons, which are facing increasing water demands. Base flow generation, however, is one of the most challenging hydrological processes to characterize in the tropics. In many years during the May–December wet season in the Panama Canal Watershed (PCW), base flows in rivers abruptly increase. This increase persists until the start of the December–April dry season. Understanding this unusual base flow jump (BFJ) behaviour is critical to improve water provisioning in the seasonal tropics, especially during droughts and extended dry seasons. This study developed an integrated approach combining piecewise regression on cumulative average base flow and sensitivity analysis to calculate the timing and magnitude of BFJ. Rainfall, forest cover, mean land surface slope, catchment area, and estimated subsurface storage were tested as predictors for the occurrence and magnitude of the BFJs in seven subcatchments of the PCW. Sensitivity analysis on correlated predictors allowed ranking of predictor contributions due to isolated and cross-correlation effects. Correlations between observed BFJs and BFJs predicted by watershed and rainfall-related predictors were 0.92 and 0.65 for BFJ timing and magnitude, respectively. Forest cover was the second most significant predictor after cumulative rainfall for jump magnitude, owing to larger subsurface storage and groundwater recharge in forests than pastures. Catchments in the mountainous eastern PCW always generated larger jumps due to their higher rainfall and greater forest cover than the western PCW catchments. The cross-correlations between predictors contributed to more than 50% of the jump variances. The results demonstrate the importance of rainfall gradient and catchment characteristics in affecting the sudden and sustained BFJs, which can help inform land management decisions intended to enhance water supplies in the tropics. This study underscores the need for more research to further understand the hydrological processes involved in the BFJ phenomenon, including better BFJ models and field characterizations, to help improve tropical ecosystem services under a changing environment.  相似文献   

The Geology of the District of the Bergen Arches     

Carl Fred Kolderup 《Proceedings of the Geologists' Association. Geologists' Association》1912,23(1):18-IN9

  相似文献   

Scalar-Flux Similarity in the Layer Near the Surface Over Mountainous Terrain     

Eleni Sfyri  Mathias W. Rotach  Ivana Stiperski  Fred C. Bosveld  Manuela Lehner  Friedrich Obleitner 《Boundary-Layer Meteorology》2018,169(1):11-46

The scaled standard deviations of temperature and humidity are investigated in complex terrain. The study area is a steep Alpine valley, with six measurement sites of different slope, orientation and roughness (i-Box experimental site, Inn Valley, Austria). Examined here are several assumptions forming the basis of Monin–Obukhov similarity theory (MOST), including constant turbulence fluxes with height and the degree of self-correlation between the involved turbulence variables. Since the basic assumptions for the applicability of the MOST approach—horizontally homogeneous and flat conditions—are violated, the analysis is performed based on a local similarity hypothesis. The scaled standard deviations as a function of local stability are compared with previous studies from horizontally homogeneous and flat terrain, horizontally inhomogeneous and flat terrain, weakly inhomogeneous and flat terrain, as well as complex terrain. As a reference, similarity relations for unstable and stable conditions are evaluated using turbulence data from the weakly inhomogeneous and flat terrain of the Cabauw experimental site in the Netherlands, and assessed with the same post-processing method as the i-Box data. Significant differences from the reference curve and also among the i-Box sites are noted, especially for data derived from the i-Box sites with steep slopes. These differences concern the slope and the magnitude of the best-fit curves, illustrating the site dependence of any similarity theory.  相似文献   

Advection,Dispersion, and Confusion          下载免费PDF全文

Fred Molz 《Ground water》2015,53(3):348-353

  相似文献