Short-term variations of the galactic cosmic ray intensity: 1964–1967     

G.D. Parker 《Planetary and Space Science》1977,25(7):681-687

Variations of the ground-level nucleonic intensity and of indices of solar photospheric, chromospheric, and coronal activity at time scales of less than forty days show strong 27-day recurrent structure. Correlation functions for these time series suggest an origin for short-term modulation in impulsive solar activity. Flare-associated shocks originating in long-lived active zones fixed in solar longitude produce recurrent cosmic ray variations during 1964–1967, and corotating solar wind disturbances are of secondary importance to short-term modulation.  相似文献   

δ13C food web analysis of a Texas sand dune community     

Brian Fry  Richard S. Scalan  Patrick L. Parker  John Baccus 《Geochimica et cosmochimica acta》1978,42(8):1299-1302

Stable carbon isotope ratios (δ¹³C) were determined for 31 plant and 16 animal species from a sand dune community on the south Texas coast. Plants and insects had similar δ¹³C values, forming two distinct groups, while herbivorous rodents formed a third intermediate group. This suggests that insects at the study site are specialists which feed on C₃ or C₄ plants, while the rodents are generalists, feeding on plants of both groups.  相似文献   

Waveform modelling of teleseismic S, Sp, SsPmP, and shear-coupled PL waves for crust- and upper-mantle velocity structure beneath Africa     

Abhijit Gangopadhyay  Jay Pulliam  Mrinal K. Sen 《Geophysical Journal International》2007,170(3):1210-1226

We describe a waveform modelling technique and demonstrate its application to determine the crust- and upper-mantle velocity structure beneath Africa. Our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a Very Fast Simulated Annealing (VFSA). We match the S , Sp, SsPmP and shear-coupled PL phases in seismograms of deep (200–800 km), moderate-to-large magnitude (5.5–7.0) earthquakes recorded teleseismically at permanent broad-band seismic stations in Africa. Using our technique we produce P - and S -wave velocity models of crust and upper mantle beneath Africa. Additionally, our use of the shear-coupled PL phase, wherever observed, improves the constraints for lower crust- and upper-mantle velocity structure beneath the corresponding seismic stations. Our technique retains the advantages of receiver function methods, uses a different part of the seismogram, is sensitive to both P - and S -wave velocities directly, and obtains helpful constraints in model parameters in the vicinity of the Moho. The resulting range of crustal thicknesses beneath Africa (21–46 km) indicates that the crust is thicker in south Africa, thinner in east Africa and intermediate in north and west Africa. Crustal P - (4.7–8 km s⁻¹) and S -wave velocities (2.5–4.7  km s⁻¹) obtained in this study show that in some parts of the models, these are slower in east Africa and faster in north, west and south Africa. Anomalous crustal low-velocity zones are also observed in the models for seismic stations in the cratonic regions of north, west and south Africa. Overall, the results of our study are consistent with earlier models and regional tectonics of Africa.  相似文献   

“Whither Geography?” Concluded: Is Geography Graduate Education Missing One of Its Targets?*     

Jay R. Harman 《The Professional geographer》2005,57(1):130-139

As geographers working in a professional capacity, many of us engage in scholarly research. Unless it is self‐supporting and the results are kept confidential, this activity requires that we strike relationships with various “third parties,” constituencies who lie outside of the usual two‐party, researcher–subject relationship but are unavoidably drawn in because they help support our work, use our findings, or bear their practical consequences, good or bad. How (and why) should we relate to them? One option is to maximize all (positive) types of third‐party impacts by intentionally pursuing scholarship that benefits the greatest range of constituencies. If this approach is desirable from the perspective of the discipline, science, and society, we have good reason to promote it, but how? Doing so with artificial (extrinsic) incentives or imposed mandates raises questions about academic freedom. Alternatively, recasting questions about which scholarly line to investigate into ethical terms that emphasize such third‐party issues might, in and of itself, promote greater prior reflection and perhaps broader outcomes. Such outcomes seem even more likely when reflection on the objective question of third‐party interests is accompanied by a nurtured compassion for parties who are less well off. The pedagogical implications of these conclusions will be explored.  相似文献   

Stable isotope variations in modern tropical speleothems: Evaluating equilibrium vs. kinetic isotope effects     

Patrick J. Mickler  Jay L. Banner  Yemane Asmerom  Emi Ito 《Geochimica et cosmochimica acta》2004,68(21):4381-4393

Applications of speleothem calcite geochemistry in climate change studies require the evaluation of the accuracy and sensitivity of speleothem proxies to correctly infer paleoclimatic information. The present study of Harrison’s Cave, Barbados, uses the analysis of the modern climatology and groundwater system to evaluate controls on the C and O isotopic composition of modern speleothems. This new approach directly compares the δ¹⁸O and δ¹³C values of modern speleothems with the values for their corresponding drip waters in order to assess the degree to which isotopic equilibrium is achieved during calcite precipitation. If modern speleothems can be demonstrated to precipitate in isotopic equilibrium, then ancient speleothems, suitable for paleoclimatic studies, from the same cave environment may also have been precipitated in isotopic equilibrium. If modern speleothems are precipitated out of isotopic equilibrium, then the magnitude and direction of the C and O isotopic offsets may allow specific kinetic and/or equilibrium isotopic fractionation mechanisms to be identified.Carbon isotope values for the majority of modern speleothem samples from Harrison’s Cave fall within the range of equilibrium values predicted from the combined use of (1) calcite-water fractionation factors from the literature, (2) measured temperatures, and (3) measured δ¹³C values of the dissolved inorganic carbon of drip waters. Calcite samples range from ∼0.8‰ higher to ∼1.1‰ lower than predicted values. The ¹³C depletions are likely caused by kinetically driven departures in the fractionation between HCO₃⁻ (aq) and CaCO₃ from equilibrium conditions, caused by rapid calcite growth. ¹³C enrichments can be accounted for by Rayleigh distillation of the HCO₃⁻ (aq) reservoir during degassing of ¹³C-depleted CO₂.Modern speleothems from Harrison’s Cave are not in O isotopic equilibrium with their corresponding drip waters and are 0.2‰ to 2.3‰ enriched in ¹⁸O relative to equilibrium values. δ¹⁸O variations in modern calcite are likely controlled by kinetically driven changes in the fractionation between HCO₃⁻ (aq) and CaCO₃ from equilibrium conditions to nonequilibrium conditions, consistent with rapid calcite growth. In contrast to δ¹³C, δ¹⁸O values of modern calcite may not be affected by Rayleigh distillation during degassing because CO₂ hydration and hydroxylation reactions will buffer the O isotopic composition of the HCO₃⁻ (aq) reservoir. If the effects of Rayleigh distillation manifest themselves in the O isotopic system, they will result in ¹⁸O enrichment in the HCO₃⁻ (aq) reservoir and ultimately in the precipitated CaCO₃.  相似文献   

A decade of sedimentation in ice-contact, proglacial lakes, Bering Glacier, AK     

P. Jay Fleisher  Palmer K. Bailey  Donald H. Cadwell 《Sedimentary Geology》2003,160(4):309-324

Bathymetric surveys during the 1991–2000 decade in two ice-contact, proglacial lakes on the eastern sector of Bering piedmont lobe captured the buildup effects of the 1993–1995 surge. Following ice-front advance of 1.0–1.5 km into Tsivat and Tsiu Lakes, the basins were significantly altered by surge-related sedimentation including the impact of a subglacial outburst into Tsivat Lake. The subsequent changes in basin shape, size, and morphology were monitored by six bathymetric surveys. Measured changes in water depth serve as a proxy for determining increments of sediment accumulation.

Upwelling, ice-front vents fed by subglacial tunnels transported suspended fine sediment directly into the lake system. The rate of suspension settling within both lakes varied from 0.6 to 1.2 m year⁻¹ prior to the surge. Suspended load during surge years increased sixfold from 1.7 to 13.9 g l⁻¹, accompanied by increased sediment accumulation of 2.2–3.1 m year⁻¹. Vent-related aggradation and subsequent filling of Tsivat Lake caused sediment bypassing to Tsiu Lake, where encroachment by delta growth contributed to a postsurge rate of bottomset accumulation of 3.0 m year⁻¹.

The total sediment influx from subglacial sources is represented by the sum of bathymetrically determined accumulation, plus an estimated volume of sediment that remained suspended, thus passing through the lake system. Total sediment flux along the eastern Bering piedmont lobe from 1991 to 2000 is approximately 227 million cubic meters.  相似文献   

Mechanism and regulation of land degradation in Yulin district     

LIU Yansui  ZHANG Xiaoping  LI Xianwen  Jay Gao 《地理学报》2003,13(2):217-224

Yulin district is located in the transitional zone between Mu Us Desert and Loess Plateau of northern Shaanxi Province, thus it is particularly vulnerable to degradation due to its fragile ecosystem and intense human activities there. The purpose of this study is to explore the mechanism, process and driving force of land degradation in area with vulnerable eco-environment within the context of increasing population and intensifying human economic activities, and then find out the patterns and countermeasures of how to control them using the economic and technological ways. In detail, this study includes three main sections: the first section analyzes the mechanism, causes and characteristics of land degradation, which can be achieved by the typical field investigations and systematical analysis within the regional natural, social and economic context. Based on the technologies of remote sensing and GIS, and combined with the modeling methods, the second section reveals the change characteristics of land use and its driving force from 1990 to 2000; As to the third section, feasible countermeasures of how to prevent the degradation and rehabilitate the regional ecology are proposed, which are studied from the perspective of harmony between nature and economy, and the conception of regional sustainable development.  相似文献   

Seismic anisotropy in the core–mantle transition zone     

Jay Pulliam  & Mrinal K. Sen 《Geophysical Journal International》1998,135(1):113-128

Split S waves observed at Hockley, Texas from events in the Tonga–Fiji region of the southwest Pacific show predominantly vertically polarized shear-wave ( SV  ) energy arriving earlier than horizontally polarized ( SH ) energy for rays propagating horizontally through D" . After corrections are made for the effects of upper-mantle anisotropy beneath Hockley, a time lag of 1.5 to 2.0  s remains for the furthest events (93.9°–100.6° ), while the time lags of the nearer observations (90.5°–92.9° ) nearly disappear. At closer distances, the S waves from these same events do not penetrate as deeply into the lower mantle, and are not split. These observations suggest that a patch of D" beneath the central Pacific is anisotropic, while the mantle immediately above the patch is isotropic. The thickness of the anisotropic zone appears to be of the order of 100–200  km.
  Observations of shear-wave splitting have previously been made for paths that traverse D" under the Caribbean and under Alaska. SH leads SV , the reverse of the Hockley observations, but in these areas the fact that SV  leads SH in the HKT data shown here suggests a different sort of anisotropy under the central Pacific from that under Alaska and the Caribbean. The case of SH travelling faster than SV  is consistent with transverse isotropy with a vertical axis of symmetry (VTI) and does not require variations with azimuth. The case of SV  leading SH is consistent with transverse isotropy with a horizontal axis of symmetry (HTI), an azimuthally anisotropic medium, and with a VTI medium formed by a hexagonal crystal. Given that (Mg,Fe)SiO₃ perovskite appears unlikely to form anisotropic fabrics on a large scale, the presence of anisotropy may point to chemical heterogeneity in the lowermost mantle, possibly due to mantle–core interactions.  相似文献   

Modeling Solar UV Variations Using Mount Wilson Observatory Indices     

D. G. Parker  R. K. Ulrich  J. M. Pap 《Solar physics》1998,177(1-2):229-241

Understanding the magnitude and temporal structure of variations in solar ultraviolet and extreme ultraviolet irradiance is critical to understanding solar forcing of the Earth's upper and middle atmosphere and hence to assessing the relative impact of natural and anthropogenic influences on Earth's atmospheric environment. Satellite based measurements of such variations are limited to recent times, are short in duration and subject to gaps making necessary ground-based surrogates with longer and more continuous coverage. Using indices derived from synoptic solar magnetograms taken at the Mount Wilson 150-foot solar tower, we have constructed models of several UV and near EUV lines and fluxes which correlate strongly (r > 0.90) with satellite data. These lines and fluxes include the Mgii h and k core-to-wing ratio, the Lα line and the 200–205 nm flux.  相似文献   

β Cephei type variability in the ultraviolet spectrum and radial velocity of PHL 346     

P. L. Dufton  F. P. Keenan  D. Kilkenny  D. O'Donoghue  Q. A. Parker  F. van Wyk  & F. van Leeuwen 《Monthly notices of the Royal Astronomical Society》1998,297(2):565-569

International Ultraviolet Explorer low-resolution and optical moderate-resolution spectra are presented for the high galactic latitude β Cephei type star, PHL 346. Variability is identified in both the ultraviolet flux and the radial velocity with periods and phases consistent with those previously deduced from optical photometry. The similarity of both the flux and the radial velocity amplitude to those previously reported for the β Cephei variable, γ Pegasus, is striking and provides evidence for PHL 346 being a young core hydrogen burning star. A distance estimate for PHL 346 of approximately 7 kpc (corresponding to a z -distance of approximately 6 kpc) is obtained by scaling the distance of γ Pegasus obtained from Hipparcos observations.  相似文献