Residual mass from ablation of meteoroid grains detached during atmospheric flight     

J. D. Dean Fyfe  R. L. Hawkes 《Planetary and Space Science》1986,34(12):1201-1212

Previous studies of the residual masses resulting from ablation of small meteoroid grains have been concerned with the ablation of particles which enter the atmosphere independently. There is widespread evidence that fragmentation is a common occurrence for meteors ranging from bright fireballs to the smallest meteors recorded with optical techniques. According to a widely accepted model, meteoroids can be considered to be a collection of tiny grains, with these grains being detached from the meteoroid during atmospheric flight. This investigation numerically solves the differential equations governing ablation of grains detached at different heights. Initial velocities from 12 to 70km s⁻¹, and initial masses from 10⁻⁵ to 10⁻¹³kg, are considered. The ablation equations allow for thermal heating prior to the onset of intensive evaporation, and thermal reradiation throughout. The atmospheric density profile used is one based on the U.S. Standard Atmosphere (1962, U.S. Government Printing Office, Washington). Calculations were completed for grains detached at 120, 100, 95, 90, 85, 80 and 75km height. For the purposes of the ablation model it is assumed that grains are ejected with an initial temperature of 1300 K, and that intensive grain evaporation begins at 2100 K. These values are consistent with grains emitted according to the model of Hawkes and Jones (1975a, Mon. Not. R. astr. Soc. 173, 339; Mon. Not. R. astr. Soc. 185, 727). For comparison purposes, calculations were also completed for grains entering the atmosphere independently (initial height 140km and beginning temperature 280 K assumed).

It is found that particles ejected at heights of 100km and above behave essentially as independent particles incident from infinity. Hence the results of earlier studies (e.g. Nicol et al., 1985, Planet. Space Sci.33, 315) can be applied. For ejection at lower heights the resultant residual mass is somewhat less than that corresponding to grains of the same initial mass and velocity. The difference is greatest for high velocity, low mass meteors. For initial masses near 10⁻⁵kg, residual mass is almost independent of ejection height, at least down to an ejection height of 75km. The significant finding of Nicol et al. (1985, Planet. Space Sci.33, 315) that residual mass is almost independent of initial mass for a fairly wide range of initial masses is only loosely followed when in-flight ejection of particles at heights below about 95 km is considered.

Typical calculations are presented to show that in-flight fragmentation of dustballs can be an important source of macroscopic ablation product micrometeorites. The astronomical and atmospheric implications of this finding are briefly discussed.  相似文献   

Model Validation and Performance Evaluation for The Multi-Tethered Aerostat Subsystem of the Large Adaptive Reflector     

Meyer Nahon  Casey Lambert  Dean Chalmers  Wen Bo 《Experimental Astronomy》2004,17(1-3):163-175

The Canadian design for the Square Kilometre Array radio telescope includes a large multi-tethered aerostat to support the telescope’s receiver. To validate this design concept, two parallel tracks have been undertaken: a numerical simulation of the multi-tethered aerostat system has been assembled, and a one-third scale prototype of the system has been constructed. This paper describes the experimental facility, presents results from initial tests of the uncontrolled system and compares these results to the predictions of the computer model of the system. Generally, the results compare very favourably. Using the simulation, we arrive at two important design philosophies to be used in the design of the full-scale system: (a) perturbations on the confluence point should be minimized, and (b) the system stiffness should be maximized to ensure minimum response to disturbances.  相似文献   

Social theory and prospects in social geography     

K. G. Dean Dr. 《GeoJournal》1984,9(3):287-299

There is little doubt that a phenomenological approach in the Husserlian mould is valuable in curbing the excessive positivism and naivety that have characterised much of the social geography undertaken in Britain during the past twenty years. However, the operational value of phenomenology in social geography is more controversial. Ley's formulation of a phenomenologically based social geography focussing on the concept of place suffers from certain methodological and theoretical problems that stem, in part, from his adoption of Schutz's conceptualisation of the social world. A phenomenologically inspired but modified approach, involving the concepts of structuration and power and drawing heavily on the work of Giddens, is suggested as superior. This conceptualisation, which connects human action with structural explanation, permits a valuable reformulation of the geography of mental illness and may have similarly worthwhile applications in other areas of interest to social geographers.  相似文献   

Desert margins near the Chinese Loess Plateau during the mid-Holocene and at the Last Glacial Maximum: a model–data intercomparison     

Andrew B. G. Bush  Dean Rokosh  Nathaniel W. Rutter  T. Bryant Moodie 《Global and Planetary Change》2002,32(4):129

Expansion and contraction of desert margins around the globe have been inferred from a variety of proxy data and have since been linked, particularly in northern China and in the sub-Sahel, to changes in freshwater flux, vegetation cover, sea surface temperatures and, perhaps most importantly, monsoon circulations. We present a direct comparison of results from numerical general circulation model experiments for the mid-Holocene and for the Last Glacial Maximum (LGM) with the climatic conditions that have been inferred from loess–paleosol sequences taken from the Chinese Loess Plateau.During the mid-Holocene in northern China, the northwestward migration of the southeast desert margin that has been suggested by grain size analysis is also expressed in the model results. There is a statistically significant wetting of the Plateau region, and increased soil moisture is a consequence of an enhanced summer monsoon whose latent heat release deepens the cyclonic Tibetan low and brings increased low-level convergence and precipitation to the area. North of the desert region, this circulation dries the soil through enhanced atmospheric subsidence, although the northern margin of the desert does not migrate significantly.Expansion of the desert margin toward the southeast at the LGM is small, but there is a statistically significant drying of the Plateau. The local hydrological cycle is reduced, and there is an increase in large-scale atmospheric subsidence over the region that is caused by the presence of the Fennoscandian ice sheet upwind. Model results therefore suggest that, in addition to local micro- and mesoclimatic conditions, regional effects, such as monsoon circulations and distal orography, are also important factors in determining the location of desert margins.  相似文献   

The 2010 European Venus Explorer (EVE) mission proposal     

Colin Frank Wilson  Eric Chassefière  Emmanuel Hinglais  Kevin H. Baines  Tibor S. Balint  Jean-Jacques Berthelier  Jacques Blamont  Georges Durry  Csaba S. Ferencz  Robert E. Grimm  Takeshi Imamura  Jean-Luc Josset  Fran?ois Leblanc  Sebastien Lebonnois  Johannes J. Leitner  Sanjay S. Limaye  Bernard Marty  Ernesto Palomba  Sergei V. Pogrebenko  Scot C. R. Rafkin  Dean L. Talboys  Rainer Wieler  Liudmila V. Zasova  Cyrill Szopa 《Experimental Astronomy》2012,33(2-3):305-335

The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an ‘M-class’ mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55?km, where temperatures and pressures are benign (～25°C and ～0.5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond.  相似文献   

Solar Cycle Predictions (Invited Review)     

Pesnell  W. Dean 《Solar physics》2012,281(1):507-532

Solar Physics - Solar cycle predictions are needed to plan long-term space missions, just as weather predictions are needed to plan the launch. Fleets of satellites circle the Earth collecting many...  相似文献   

Ensemble Randomized Maximum Likelihood Method as an Iterative Ensemble Smoother   总被引：3，自引：1，他引：2  

Yan Chen  Dean S. Oliver 《Mathematical Geosciences》2012,44(1):1-26

  相似文献   

Timing and habitat of koaro (Galaxias brevipinnis) spawning in streams draining MtTaranaki,New Zealand     

Richard M. Allibone  Dean Caskey 《新西兰海洋与淡水研究杂志》2013,47(4):593-595

A spawning site of Galaxias brevipinnis Giinther was located in a New Zealand stream for the first time. It was at the edge of a riffle and only partially submerged. The habitat used for spawning matched that described for G. brevipinnis in Australia. Spawning was estimated to have occurred between late April and early May and the eggs hatched in late May. The species of fish spawning was identified by rearing the eggs through to identifiable juveniles and by DNA sequencing of one individual.  相似文献   

Ocean temperature climate off North‐East New Zealand     

Philip J. H. Sutton  Dean Roemmich 《新西兰海洋与淡水研究杂志》2013,47(3):553-565

The ocean temperature field off the north‐east coast of New Zealand is studied to quantify the annual cycle and reveal the intra‐ and inter‐annual variability. The data used are repeat expendable bathythermograph (XBT) sections between Auckland and either Suva or Honolulu which have been collected quarterly since 1986. These sections give temperature measurements between the surface and 800 m and Auckland and 30°S from 1986 to August 1999. The mean and annual cycle are compared with those from the NOAA World Ocean Atlas (WOA98). The results are similar; however WOA98 lacks the horizontal resolution to fully discern the East Auckland Current and North Cape Eddy, while the XBT analysis lacks the temporal resolution to discern higher frequency intra‐annual signals. The temperature variability in the mixed layer is dominated by the annual cycle, which accounts for 80–90% of the variance. The amplitude of the annual cycle diminishes rapidly with depth, from 2.8°C at the surface, to c. 0.1°C at 180 m. The phase of the annual cycle is retarded with depth, with peak temperatures occurring in February at the surface and in June/July at 180 m. Removing the annual cycle from the time series reveals the more subtle inter‐ and intra‐annual variability. This variability is of the order of 1°C in the upper 50 m, decreasing to 0.3°C at 400–500 m. The surface layer was cold between 1991 and 1994 (c. 0.7°C cooler than average), and 0.7°C warmer than average in 1999. The deeper ocean shows a different signal, being up to 0.3°C cooler in 1990–92, 0.3°C warmer in 1998, and c. 0.2°C warmer than average in 1999. The inter‐annual mixed layer variability is highly correlated with the Southern Oscillation Index and also with inter‐annual terrestrial air temperature and wind measurements from northern New Zealand. In contrast, at higher intra‐annual frequencies, the mixed layer variability is not correlated with air and wind measurements. At these higher frequencies, the air temperature is better correlated with the sea surface temperature (SST) than with the bulk mixed layer temperature.  相似文献   

A multidisciplinary effort to assign realistic source parameters to models of volcanic ash-cloud transport and dispersion during eruptions   总被引：6，自引：1，他引：5  

L.G. Mastin  M. Guffanti  R. Servranckx  P. Webley  S. Barsotti  K. Dean  A. Durant  J.W. Ewert  A. Neri  W.I. Rose  D. Schneider  L. Siebert  B. Stunder  G. Swanson  A. Tupper  A. Volentik  C.F. Waythomas 《Journal of Volcanology and Geothermal Research》2009,186(1-2):10

During volcanic eruptions, volcanic ash transport and dispersion models (VATDs) are used to forecast the location and movement of ash clouds over hours to days in order to define hazards to aircraft and to communities downwind. Those models use input parameters, called “eruption source parameters”, such as plume height H, mass eruption rate Ṁ, duration D, and the mass fraction m₆₃ of erupted debris finer than about 4 or 63 μm, which can remain in the cloud for many hours or days. Observational constraints on the value of such parameters are frequently unavailable in the first minutes or hours after an eruption is detected. Moreover, observed plume height may change during an eruption, requiring rapid assignment of new parameters. This paper reports on a group effort to improve the accuracy of source parameters used by VATDs in the early hours of an eruption. We do so by first compiling a list of eruptions for which these parameters are well constrained, and then using these data to review and update previously studied parameter relationships. We find that the existing scatter in plots of H versus Ṁ yields an uncertainty within the 50% confidence interval of plus or minus a factor of four in eruption rate for a given plume height. This scatter is not clearly attributable to biases in measurement techniques or to well-recognized processes such as elutriation from pyroclastic flows. Sparse data on total grain-size distribution suggest that the mass fraction of fine debris m₆₃ could vary by nearly two orders of magnitude between small basaltic eruptions ( 0.01) and large silicic ones (> 0.5). We classify eleven eruption types; four types each for different sizes of silicic and mafic eruptions; submarine eruptions; “brief” or Vulcanian eruptions; and eruptions that generate co-ignimbrite or co-pyroclastic flow plumes. For each eruption type we assign source parameters. We then assign a characteristic eruption type to each of the world's  1500 Holocene volcanoes. These eruption types and associated parameters can be used for ash-cloud modeling in the event of an eruption, when no observational constraints on these parameters are available.  相似文献