Study of supergranules     

Haimin Wang  Harold Zirin 《Solar physics》1989,120(1):1-17

Results of a detailed study on supergranule lifetime and velocity fields are presented. We show the correlation between the observed downdraft velocity and the network magnetic flux elements on the quiet sun. After excluding areas with magnetic flux density 25 G, we find that the upper limit of the supergranule vertical speed is 0.1 km s^–1 for both downdraft and updraft, and the r.m.s. speed is 0.03 km s^–1. By observing the evolution of individual supergranules, we find that the average lifetime of supergranules might be 50 hours. We describe different ways of formation and decay of supergranular cells. New cells usually form in an area containing no pre-existing supergranule velocity fields. Cells may disappear in two ways: fragmentation and fading away.  相似文献   

Comparison of COSPEC and two miniature ultraviolet spectrometer systems for SO2 measurements using scattered sunlight     

Tamar Elias  A. Jeff Sutton  Clive Oppenheimer  Keith A. Horton  Harold Garbeil  Vitchko Tsanev  Andrew J. S. McGonigle  Glyn Williams-Jones 《Bulletin of Volcanology》2006,68(4):313-322

The correlation spectrometer (COSPEC), the principal tool for remote measurements of volcanic SO₂, is rapidly being replaced by low-cost, miniature, ultraviolet (UV) spectrometers. We compared two of these new systems with a COSPEC by measuring SO₂ column amounts at Kīlauea Volcano, Hawaii. The two systems, one calibrated using in-situ SO₂ cells, and the other using a calibrated laboratory reference spectrum, employ similar spectrometer hardware, but different foreoptics and spectral retrieval algorithms. Accuracy, signal-to-noise, retrieval parameters, and precision were investigated for the two configurations of new miniature spectrometer. Measurements included traverses beneath the plumes from the summit and east rift zone of Kīlauea, and testing with calibration cells of known SO₂ concentration. The results obtained from the different methods were consistent with each other, with <8% difference in estimated SO₂ column amounts up to 800 ppm m. A further comparison between the COSPEC and one of the miniature spectrometer configurations, the ‘FLYSPEC’, spans an eight month period and showed agreement of measured emission rates to within 10% for SO₂ column amounts up to 1,600 ppm m. The topic of measuring high SO₂ burdens accurately is addressed for the Kīlauea measurements. In comparing the foreoptics, retrieval methods, and resultant implications for data quality, we aim to consolidate the various experiences to date, and improve the application and development of miniature spectrometer systems.  相似文献   

American geography and one non-American geographer     

Harold Brookfield 《GeoJournal》2004,59(1):39-41

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On type B CAI formation: experimental constraints on fO₂ variations in spinel minor element partitioning and reequilibration effects     

Harold C Connolly Jr.  D.S Burnett 《Geochimica et cosmochimica acta》2003,67(22):4429-4434

We report data from a series of dynamic crystallization experiments that focus on determining the partition coefficients (D’s) for V and Ti in the spinel + liquid system of an average type B1 CAI bulk composition for three different fO₂ conditions. Partitioning data for Ca and Si are also obtained. We show that the D’s for V and Ti are fO₂ dependent with D_Ti decreasing at low oxygen fugacity due to the presence of Ti³⁺. D_V is essentially 0 in air, rises to 2.2 at the Fe-FeO buffer and drops to 1.4 at the C-CO buffer. This indicates that V³⁺ is highly compatible in spinel and that higher and lower valence states are much less compatible. We also report data from isothermal experiments that determine diffusion times for V and Ti in same system at a temperature close to the T_max for type B1 CAIs. Diffusion of these elements between spinel and liquid is surprisingly rapid, with essentially total equilibration of Ti and V between spinel and liquid in 90 h run duration. Lack of equilibration of Cr, Si, and Ca shows that the Ti and V equilibration mechanism was diffusion and not dissolution and reprecipitation. Our experimental run durations set an upper limit of a few tens of hours on the time that type B1 CAIs were at their maximum temperature. Based on our data we argue that subsolidus reequilibration between spinel inclusion and host-silicate phases within type B CAIs likely explains the observed range of V and Ti concentrations in spinels which are inclusions in clinopyroxene.  相似文献   

Thick lava flows of Karisimbi Volcano, Rwanda: insights from SIR-C interferometric topography     

Mary E. MacKay  Scott K. Rowland  Peter J. Mouginis-Mark  Harold Garbeil 《Bulletin of Volcanology》1998,60(4):239-251

 We use a digital elevation model (DEM) derived from interferometrically processed SIR-C radar data to estimate the thickness of massive trachyte lava flows on the east flank of Karisimbi Volcano, Rwanda. The flows are as long as 12 km and average 40–60 m (up to >140 m) in thickness. By calculating and subtracting a reference surface from the DEM, we derived a map of flow thickness, which we used to calculate the volume (up to 1 km³ for an individual flow, and 1.8 km³ for all the identified flows) and yield strength of several flows (23–124 kPa). Using the DEM we estimated apparent viscosity based on the spacing of large folds (1.2×10¹² to 5.5×10¹² Pa s for surface viscosity, and 7.5×10¹⁰ to 5.2×10¹¹ Pa s for interior viscosity, for a strain interval of 24 h). We use shaded-relief images of the DEM to map basic flow structures such as channels, shear zones, and surface folds, as well as flow boundaries. The flow thickness map also proves invaluable in mapping flows where flow boundaries are indistinct and poorly expressed in the radar backscatter and shaded-relief images. Received: 6 September 1997 / Accepted: 15 May 1998  相似文献   

Swimming speeds of oyster larvaeCrassostrea virginica in different salinities and temperatures     

Herbert Hidu  Harold H. Haskin 《Estuaries and Coasts》1978,1(4):252-255

Swimming speeds of oyster larvaeCrassostrea virginica were determined in constant and increasing salinities to learn more about the oyster larval “salinity response”. “Normal” non-directed swimming speeds ranged from less than 1 cm/min for early veligers to 5 cm/min for “eyed” veligers with temperature an important variable. When subjected to hourly salinity increases of 0.5‰, most larvae swam upward or downward at approximately 3 times the above speeds. Larvae with values closed in response to traces of formalin sank at speeds of 5 to 50 cm/min depending on larval stage. The results may explain the differential vertical position of larval stages in estuaries and suggest the presence of a taxic component to the salinity response.  相似文献   

Subdivision of the Precambrian — A brief review and a report on recent decisions by the subcommission on Precambrian Stratigraphy   总被引：1，自引：0，他引：1  

Harold L. James 《Precambrian Research》1978,7(3):193-204

In contrast to the Phanerozoic, for which there exists an internationally accepted, detailed time scale, the Precambrian, representing more than four-fifths of all recorded Earth history, is completely without an accepted scheme for time classification. Inspection of the many published proposals for subdivision of the Precambrian reveals much diversity in underlying concept, means of definition, and nomenclature. Most schemes are based on orogenic or magmatic-tectonic cycles, but others are keyed in principle to sedimentary sequences, to total Earth history, or to combinations of various concepts. Several are reviewed as examples. Considered in terms of basic criteria, few if any of the published proposals are entirely satisfactory, and none appears a likely candidate for international acceptance.The Subcommission on Precambrian Stratigraphy of the International Union of Geological Sciences, at its meeting in Cape Town, 11.7.1977–15.7.1977, has taken the initial step toward formulation of a complete time scale by recommending acceptance of a division of the Precambrian into Proterozoic and Archean. These time units have the status of eons, equivalent to the Phanerozoic, though of much greater duration. The age of the time boundary between Proterozoic and Archean is defined as 2500 Ma.  相似文献   

Vegetation and associated environments during the past 14,000 years near Moulton Pond,Maine     

Ronald B. Davis  Theodore E. Bradstreet  Robert Stuckenrath  Harold W. Borns 《Quaternary Research》1975,5(3):435-465

Pollen influx and percentage diagrams were prepared from an 11.4 m core from Moulton Pond, Maine. The pond basin was deglaciated about 14,000 y. a., after which it was located on an island in a sea of subarctic character until about 12,400 y. a. when the surrounding area emerged from the sea. The terrestrial vegetation was tundra until about 10,000 y. a. A change in the tundra vegetation is synchronous with the emergence from the sea, but synchroneity with the Pineo Ridge glacial readvance, which reached its maximum 50 km to the east of the pond about 12,700 y. a., is also possible because of imprecision in the dating. Comparisons of the Moulton Pond results with late-glacial pollen sequences elsewhere in eastern United States and adjacent Canada reveal a lack of synchroneity in vegetational changes casting doubt on claims of major broad-scale climatic shifts over the entire area.The tundra period at Moulton Pond ended with a transition of a few hundred years to partly open, relatively xeric forests of low diversity dominated by white pine, oak, and birch trees. There was no intervening boreal forest. In the postglacial period the vegetation was continually changing, including in the early portion a series of immigrations of temperate tree taxa which later became important in the forests. The transient nature of these assemblages is further indicated by their differences from the closest modern analogs. From about 7100 y. a. until settlement by Europeans 200 y. a., the forests were closed. A major decline of conifers centering about 4700 y. a. was followed by maxima of mesic hardwoods about a thousand years later. In the most recent 2000 yr, the pollen record suggests greater environmental severity, evidenced by increasing spruce. But for the entire postglacial period, the closest modern vegetational analogs are all in the conifer-hardwood region. Much of the postglacial pollen sequence is inexplicable in climatic terms, as evidenced by nonsynchronous behavior of hemlock and beech.The pollen influx diagram is useful for distinguishing tundra from forest, but for the postglacial period it is difficult to interpret. Pollen influx data are strongly affected by shifts in the pattern of sedimentation in lakes. We propose that such shifts account for the major changes in influx in mid- and late-postglacial time at Moulton Pond and at Rogers Lake, Connecticut. This complicates the interpretation of influx data which otherwise are superior to percentage data.  相似文献   

Seasonal phytoplankton composition in the lower Chesapeake Bay and old plantation Creek,Cape Charles,Virginia     

Harold G. Marshall 《Estuaries and Coasts》1980,3(3):207-216

During a 14-month phytoplankton study in the lower Chesapeake Bay, there was a bi-modal pattern of population peaks with fall and spring maxima. The phytoplankton was dominated bySkeletonema costatum and other diatoms similar to major dominants found on the continental shelf. The composition in an inlet adjacent to the Bay was similar throughout most of the period, but differed from Bay populations during the summer months when larger concentrations and diversity of phytoflagellates and small sized diatoms occurred. Seasonal phytoplankton assemblages characteristic for the lower and entire Chesapeake Bay are given with the seasonal appearances noted for 219 phytoplankters. The importance of nanophytoplankters, both diatoms and the phytoflagellates, to the total phytoplankton composition is also emphasized.  相似文献   

A study of the infiltration characteristics of undisturbed soil under simulated rainfall     

Harold T. Johnston  Elsayed M. Elsawy  Stanley R. Cochrane 《地球表面变化过程与地形》1980,5(2):159-174

Infiltration is the single most important parameter in deriving the net quick response rainfall which contributes to stream flood discharges. Rainfall simulation is used to study the infiltration characteristics in a typical catchment, the Six Mile Water in N. Ireland. The design of the simulator was such that it could be easily moved from one test area to another within the catchment to examine the effect of soil and slope variation. The simulator was first calibrated in controlled laboratory conditions and later the calibration was checked in the field. The simulator was mounted over an undisturbed plot of 37 m² and the surface runoff from the area measured by means of a collecting channel located along a lower edge of the plot. Soil moisture variations were monitored using a soil moisture neutron probe. Soil classification tests and gravimetric moisture contents were carried out on each plot. The field tests were carried out with variations in rainfall intensity, initial conditions, changing seasons, and for different plots within the catchment area. The results obtained are unique in that they present data obtained under field conditions for undisturbed soil within a natural catchment. The infiltration behaviour was found to depend upon rainfall intensity, initial conditions of the plot under consideration, seasonal temperature, and a slope of the plot. The data showed that while a classical Horton type equation for infiltration was suitable for the later stages of each test result when significant surface runoff was taking place, the model failed to represent early response adequately due to storage effects being omitted in the equation. A modified form of Horton equation is proposed, which models more accurately the infiltration characteristics of the full period of each test run.  相似文献