Observations of the reappearance of polar coronal holes and the reversal of the polar magnetic field     

D. F. Webb  J. M. Davis  P. S. McIntosh 《Solar physics》1984,92(1-2):109-132

We examine observations relating to the evolution of the polar magnetic field around sunspot maximum, when the net polar flux reverses polarity and coronal holes redevelop around the poles. Coronal hole observations during the last two solar maxima are examined in detail. Long-term averages of the latitudinal dependence of the photospheric magnetic field and the evolutionary pattern of the polar crown filaments are used to trace the poleward motion of the reversal of the large-scale surface field, and are compared to the redevelopment of the polar holes. The polar holes evolve from small, mid-latitude holes of new-cycle polarity which expand poleward until they join and cover the pole. We find that the appearance of these mid-latitude holes, the peak of flux emergence at low latitudes, and the polar polarity reversal all occur within a few solar rotations. Lagging 6 months to 1 1/2 yr after this time, the polar crown disappears and the polar holes redevelop.These results are examined in the context of phenomenological models of the solar cycle. We believe the following results in particular must be accounted for in successful models of the solar cycle: (1) The process of polarity reversal and redevelopment of the polar holes is discontinuous, occurring in 2 or 3 longitude bands, with surges of flux of old-cycle polarity interrupting the poleward migration of new-cycle flux. There is a persistent asymmetry in these processes between the two hemispheres; the polarity reversal in the two hemispheres is offset by 6 months to 1 1/2 yr. (2) Contrary to the Babcock hypothesis, the polar crown disappears months after the magnetic polar reversal. We suggest one possible scenario to explain this effect. (3) Our observations support suggestions of a poleward meridional flow around solar maximum that cannot be accounted for by Leighton-type diffusion.  相似文献   

Sediment yield and runoff frequency of small drainage basins in the Mojave Desert, U.S.A.     

Peter G. Griffiths  Richard Hereford  Robert H. Webb   《Geomorphology》2006,74(1-4):232-244

Sediment yield from small arid basins, particularly in the Mojave Desert, is largely unknown owing to the ephemeral nature of these fluvial systems and long recurrence interval of flow events. We examined 27 reservoirs in the northern and eastern Mojave Desert that trapped sediment from small (< 1 km²) drainage basins on alluvial fans over the past 100 yr, calculated annual sediment yield, and estimated the average recurrence interval (RI) of sediment-depositing flow events. These reservoirs formed where railbeds crossed and blocked channels, causing sediment to be trapped and stored upslope. Deposits are temporally constrained by the date of railway construction (1906–1910), the presence of ¹³⁷Cs in the reservoir profile (post-1952 sediment), and either 1993, when some basins breached during regional flooding, or 2000–2001, when stratigraphic analyses were performed. Reservoir deposits are well stratified at most sites and have distinct fining-upward couplets indicative of discrete episodes of sediment-bearing runoff. Average RI of runoff events for these basins ranges from 2.6 to 7.3 yr and reflects the incidence of either intense or prolonged rainfall; more than half the runoff events occurred before 1963. A period of above-normal precipitation, from 1905 to 1941, may have increased runoff frequency in these basins. Mean sediment yield (9 to 48 tons km^− 2 yr^− 1) is an order of magnitude smaller than sediment yields calculated elsewhere and may be limited by reduced storm intensity, the presence of desert pavement, and shallow gradient of fan surfaces. Sediment yield decreases as drainage area increases, a trend typical of much larger drainage basins where sediment-transport processes constrain sediment yield. Coarse substrate and low-angle slopes of these alluvial fan surfaces likely limit sediment transport capacity through transmission losses and channel storage.  相似文献   

Laboratory testing of trace metals removal from mine drainage and arsenic removal from groundwater in the Black Hills of South Dakota     

Arden D. Davis  Cathleen J. Webb  Jenifer L. Sorensen  David J. Dixon  Haile Betemariam 《Environmental Earth Sciences》2014,72(2):355-361

The Gilt Edge Superfund Site is a former heap-leach gold mine that currently is being remediated in the Black Hills of South Dakota. Mine runoff water is treated before release from the site. The field pH, before treatment, is about 3; the water contains arsenic at low levels and some trace metals at elevated levels, in addition to total dissolved solids concentrations of more than 1,900 mg/L. In the Keystone area of the Black Hills, naturally occurring arsenic has been detected at elevated concentrations in groundwater samples from wells. The City of Keystone’s Roy Street Well, which is not used currently, showed arsenic concentrations of 36 parts per billion and total dissolved solids of 320 mg/L. With field samples of water from the Gilt Edge site, a limestone-based method was successful in reducing trace metals concentrations to about 0.001 mg/L or less; at the Keystone site, the limestone method reduced arsenic levels to about 0.006 mg/L. The results are significant because previous research with the limestone-based method mainly had involved samples prepared with distilled water in the laboratory, in which interference of other ions such as sulfate did not occur. The research indicates the potential for broader applications of the limestone-based removal method, including scale-up work at field sites for water treatment.  相似文献   

Fish populations of the Avon‐Heathcote Estuary     

B. F. Webb 《新西兰海洋与淡水研究杂志》2013,47(1-2):45-66

Breeding seasons and sizes at first maturity for nine species of fish in the Avon‐Heathcote Estuary, Christchurch, New Zealand, were: sand flounder, Rhombosolea plebeia, mid‐winter to spring at 200 mm+; yellow‐bellied flounder, Rhombosolea leporina, winter and spring at 260 mm+; common sole, Peltorhamphus novaeieelandiae, did not breed in the estuary but reached first maturity at 220 mm +; yellow‐eyed mullet, Aldrichetta forsteri, spawned twice a year, winter and summer, at 220 mm +; kahawai, Arripis trutta, bred outside the estuary at 520–540 mm +; spotty, Pseudolabrus celidotus, spring to autumn at 200 mm for males and 160 mm for females; cockabully, Tripterygion nigripenne, spring and early summer at 52–57 mm +; common bully, Gobiomorphus basalis, spring and early summer at 45–50 mm+; and globefish, Speroides richei, spring to autumn at 147 mm + for males and 120 mm + for females. The successive stages in gonad development are described for these species.

In pelagic species, e.g., flatfish and kahawai, x² analyses showed that for most months females are more numerous than males, and that migration is of major importance. In (littoral species, e.g., common bully and cockabully, there is a 1 : 1 sex ratio in the breeding season, but this ratio subsequently breaks down.

The Avon‐Heathcote estuary was used mainly as a nursery area by juvenile fish, although some species, such as sand flounder, yellow‐bellied flounder, yellow‐eyed mullet, and cockabully spawned in the estuary.  相似文献   

Fish populations of the Avon‐heathcote estuary     

B. F. Webb 《新西兰海洋与淡水研究杂志》2013,47(3):223-234

Details are given of gut‐content analyses for nine fish species from the Avon‐Heathcote Estuary, Christchurch, New Zealand: sand flounder, Rhombosolea plebeia (Richardson); yellow‐bellied flounder, R. leporina (Hutton); common sole, Peltorhamphus novaezeelandiae (Gunther); yellow‐eyed mullet, Aldrichetta forsteri (Cuvier & Valenciennes); kahawai, Arripis trutta (Bloch & Schneider); spotty, Pseudolabrus celidotus (Bloch and Schneider); cockabully, Tripterygion nigripenne (Cuvier & Valenciennes); common bully, Gobiomorphus basalis (Gray); and globefish, Spheroides richei (Freminville). The percent occurrences of each food type recorded over Ihe sampling period (April 1965‐April 1966) for each species are compared. Monthly food tables are given for those species of which suitably large samples were obtained (sand flounder, yellow‐bellied flounder, common sole, yellow‐eyed mullet, and globefish). Where possible, the dietary occurrence of different food types is related to environmental and other factors observed or considered likely to influence food selection.  相似文献   

Shear modulus, heat capacity, viscosity and structural relaxation time of Na2O–Al2O3–SiO2 and Na2O–Fe2O3–Al2O3–SiO2 melts     

Katarzyna Falenty  Sharon L. Webb 《Physics and Chemistry of Minerals》2010,37(9):613-634

The configurational heat capacity, shear modulus and shear viscosity of a series of Na₂O–Fe₂O₃–Al₂O₃–SiO₂ melts have been determined as a function of composition. A change in composition dependence of each of the physical properties is observed as Na₂O/(Na₂O + Al₂O₃) is decreased, and the peralkaline melts become peraluminous and a new charge-balanced Al-structure appears in the melts. Of special interest are the frequency dependent (1 mHz–1 Hz) measurements of the shear modulus. These forced oscillation measurements determine the lifetimes of Si–O bonds and Na–O bonds in the melt. The lifetime of the Al–O bonds could not, however, be resolved from the mechanical spectrum. Therefore, it appears that the lifetime of Al–O bonds in these melts is similar to that of Si–O bonds with the Al–O relaxation peak being subsumed by the Si–O relaxation peak. The appearance of a new Al-structure in the peraluminous melts also cannot be resolved from the mechanical spectra, although a change in elastic shear modulus is determined as a function of composition. The structural shear-relaxation time of some of these melts is not that which is predicted by the Maxwell equation, but up to 1.5 orders of magnitude faster. Although the configurational heat capacity, density and shear modulus of the melts show a change in trend as a function of composition at the boundary between peralkaline and peraluminous, the deviation in relaxation time from the Maxwell equation occurs in the peralkaline regime. The measured relaxation times for both the very peralkaline melts and the peraluminous melts are identical with the calculated Maxwell relaxation time. As the Maxwell equation was created to describe the timescale of flow of a mono-structure material, a deviation from the prediction would indicate that the structure of the melt is too complex to be described by this simple flow equation. One possibility is that Al-rich channels form and then disappear with decreasing Si/Al, and that the flow is dominated by the lifetime of Si–O bonds in the Al-poor peralkaline melts, and by the lifetime of Al–O bonds in the relatively Si-poor peralkaline and peraluminous melts with a complex flow mechanism occurring in the mid-compositions. This anomalous deviation from the calculated relaxation time appears to be independent of the change in structure expected to occur at the peralkaline/peraluminous boundary due to the lack of charge-balancing cations for the Al-tetrahedra.  相似文献   

Understanding sediment in river systems—Building on a 50-year contribution     

Bruce W. Webb  Ian D. L. Foster  James P. McNamara 《水文研究》2023,37(1):e14792

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