Ionospheric troughs in Antarctica

first_imgWe report here the first observations of the dynamics of both the mid-latitude and high-latitude troughs made by the Advanced Ionospheric Sounder (AIS) at Halley Station, Antarctica (76° S, 27° W; invariant lat. 61°). This experiment is part of a major international project1 to study the sub-auroral ionosphere and its associations with the magnetosphere. Our analysis provides an accurate quantitative description of the latitudinal movements of these features and the first results delineating the orientation of the poleward edge of the mid-latitude trough. These results show that the AIS has a much greater potential for monitoring large scale ionospheric structures and for tracking their motions than more conventional radio wave experiments.last_img

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Flow of Rutford Ice Stream and comparison with Carlson Inlet, Antarctica

first_imgResults from movement surveys on Rutford Ice Stream are presented with complementary surface-elevation and ice-thickness measurements. Surface velocities of 300 m a−1 occur at least 130 km up-stream of the grounding line and contrast strongly with the neighbouring Carlson Inlet, where a velocity of 7 m a−1 has been measured. This contrast in velocity is not topographically controlled but appears to be due instead to differences in basal conditions, with Carlson Inlet probably being frozen to its bed. Concentration of lateral shear close to the margins and surface expression of subglacial topography both support a view of significant basal shear stresses in the central part of Rutford Ice Stream. The pattern of principal strain-rate trajectories shows a small number of characteristic features which can be compared with results from future modelling of the glacier’s flow.last_img read more

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Studies of conjugate plasma convection in the vicinity of the Harang discontinuity

first_imgTwo case studies are presented of the large-scale nightside ionospheric plasma convection observed simultaneously in the two polar regions using HF backscatter radars. The case studies occur during geomagnetically quiet conditions, and for one of the two periods the interplanetary magnetic field (IMF) is known to be northwards. For both cases plasma generally convects westward in the evening and eastward in the morning. The times at which the nightside flow reversal (the Harang discontinuity) occurs is observed to differ by several hours between the two hemispheres, and between the two study periods. For the case where IMF data are available, the nightside plasma flow is shown to respond to a step change in the IMF y-component (becoming less negative), with a time delay of about 25 min. In the southern hemisphere, the flow reversal appeared simply to be shifted to later magnetic local times, whilst for the northern hemisphere the evening westward flow was disrupted by the occurrence of eastward flow ahead of the Harang discontinuity. The general features of the observed convection are not consistent with the Heppner and Maynard (1987, J. geophys. Res.92, 4467) convection models, which exclude any significant association between the Harang discontinuity and the state of the IMF. The results are more in agreement with the MHD simulations for northward IMF of Oginoet al. (1985, J. geophys. Res.90, 10835). The latter invoke nightside reconnection cells whose behaviour is dependent upon the IMF y-component in the same sense as for our observations.last_img read more

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Ionospheric convection response to slow, strong variations in a northward interplanetary magnetic field: A case study for January 14, 1988

first_imgWe analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on January 14, 1988, when the interplanetary magnetic field (IMF) rotated slowly in direction and had a large amplitude. Using the assimilative mapping of ionospheric electrodynamics (AMIE) procedure, we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high-latitude electrodynamics, focusing on the period of northward IMF. By combining satellite data with ground-based observations, we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres. We present evidence that a lobe convection cell was embedded within normal merging convection during a period when the IMF By and Bz components were large and positive. As the IMF became predominantly northward, a strong reversed convection pattern (afternoon-to-morning potential drop of around 100 kV) appeared in the southern (summer) polar cap, while convection in the northern (winter) hemisphere became weak and disordered with a dawn-to-dusk potential drop of the order of 30 kV. These patterns persisted for about 3 hours, until the IMF rotated significantly toward the west. We interpret this behavior in terms of a recently proposed merging model for northward IMF under solstice conditions, for which lobe field lines from the hemisphere tilted toward the Sun (summer hemisphere) drape over the dayside magnetosphere, producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap. The positive IMF Bx component present at this time could have contributed to the observed hemispheric asymmetry. Reverse convection in the summer hemisphere broke down rapidly after the ratio |By/Bz| exceeded unity, while convection in the winter hemisphere strengthened. A dominant dawn-to-dusk potential drop was established in both hemispheres when the magnitude of By exceeded that of Bz, with potential drops of the order of 100 kV, even while Bz remained northward. The later transition to southward Bz produced a gradual intensification of the convection, but a greater qualitative change occurred at the transition through |By/Bz| = 1 than at the transition through Bz = 0. The various convection patterns we derive under northward IMF conditions illustrate all possibilities previously discussed in the literature: nearly single-cell and multicell, distorted and symmetric, ordered and unordered, and sunward and antisunward.last_img read more

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Modelling reproductive effort in sub-and maritime Antarctic mosses

first_imgA comparison is made of the reproductive effort (RE), considered as the investment in sporophyte relative to gametophyte biomass, of eight species of moss occurring at sub-and maritime Antarctic sites. Six of the species showed smaller sporophytes and game-tophytes at the climatically more extreme maritime Antaretic sites and one species showed no size difference between regions. The remaining species, although showing no regional difference, showed some evidence of a reverse pattern, with higher altitude samples having greater biomass than lower altitude samples. Spore counts indicated a measure of compensation in maritime Antarctic samples, with no significant decrease in spore output in several species despite smaller sporophyte biomass. The relationship between sporophyte (S) and gametophyte (G) biomass within samples was described by an allometric curve (S=aGb) which gave a better fit than a straight line for six species. This form of model allows comparisons of patterns of RE to be made between samples with non-or partially overlapping size distributions, even when the relationship involves size-dependence. An allometric curve was not appropriate for describing samples of one species (Andreaea regularis), and insufficient data were available to identify any relationship in Polytrichum alpinum. The exponent (b) differed between species, but there were no statistically significant differences between exponents from samples of the same species. Samples of two species could further be described by the same coefficient (a), indicating that they lie on the same curve. However, samples of three species from sub-Antarctic South Georgia gave significantly higher coefficients, indicating increased RE relative to maritime Antarctic populations.last_img read more

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Retrieval of NO2vertical profiles from ground-based UV-visible measurements: Method and validation

first_imgVertical profiles of NO2 are retrieved from ground-based UV-visible slant columns by sequential estimation, using a forward model that consists of a stacked box photochemical model and a radiative transfer model. The retrieval method is characterized, and a rigorous error analysis is presented. The vertical resolution of the retrieved profiles is shown to vary from 5 to 10 km, depending on the retrieval altitude. The retrieved profiles are found to be moderately sensitive to the assumed vertical profiles of ozone and aerosol, to the range of solar zenith angles of the observations, and to the error in the slant column. However, in the altitude range 10 to 35 km they are relatively insensitive to a priori information and agreed well with profiles from simultaneous balloon-borne measurementslast_img read more

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Foraging ranges, diets and feeding locations of gannets Morus bassanus in the North Sea:evidence from satellite telemetry

first_imgWe used satellite telemetry to examine the foraging ranges, feeding locations and travel speeds of 17 chick-rearing gannets Morus bassanus from the Bass Rock, SE Scotland. Regurgitates indicated that birds at the colony exploited a wide range of prey, frequently including 0-group sandeels (<10 cm in length) and mature mackerel and herring (up to 33 cm) in the diet. The maximum foraging range was 540 km, and the mean distance to the furthest point from the colony on any one trip was 232 km. Destinations of foraging trips covered a wide area of the North Sea, with a non-random distribution and a higher than expected proportion of trips to the NE (generally in the vicinity of Buchan Deep and Halibut Bank) and to the SE (mostly between Farne Deep, Dogger Bank and Outer Silver Pit) of the colony. Foraging trips lasted 13 to 84 h, and trip duration explained 94% of the variance in maximum distance from the colony, indicating that distance travelled could be predicted with a high degree of accuracy from time spent at sea. However, the average speed of travel during foraging trips (15 km h-1) was considerably lower than maximum ground speed (~55 km h-1 in most cases). The results of this study suggest that gannets breeding at the Bass Rock utilize a wide range of species and sizes of prey over a large area of the North Sea, and that they focus their activity on bathymetric features that are probably associated with high primary production. Further data are now required to examine the foraging ranges and feeding locations of gannets in different oceanographic regions in order to obtain a broader understanding of how gannets make use of different marine environments.last_img read more

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Activity patterns of giant petrels, Macronectes spp., using different foraging strategies

first_imgWe studied foraging activity of giant petrels during the incubation period, by simultaneously deploying activity recorders and satellite transmitters on northern (Macronectes halli) and southern giant petrels (Macronectes giganteus) at Bird Island (South Georgia, Antarctica) between 29 October and 26 December 1998, Satellite tracking showed two types of trips: (1) coastal trips, all undertaken by male northern giant petrels, to the nearby South Georgia mainland, presumably foraging on seal and penguin carcasses on beaches, and (2) pelagic trips, foraging at sea for marine prey or potentially scavenging on distant archipelagos (e.g. South Sandwich, Falkland or South Orkney Islands). Activity recorder data were consistent with the types of trip defined by the satellite tracking data, with median wet activity (time spent at the sea surface) during pelagic trips being 41%, but only 14% on coastal trips. On pelagic trips, there was a significant negative correlation between the duration of wet periods and the speed of travel between satellite uplinks. Mean travelling speed between uplinks was greater during day than night for both types of trips, suggesting that giant petrels prefer to travel during daylight and are less active at night. The scarcity of wet periods during the night in giant petrels foraging to the South Georgia coast (median = 3%, range = 1-9 %) indicates that such birds spent almost all night on land. Likewise, the scarcity of wet periods at night for three birds foraging 700-1,000km south of Bird Island, where there is no land but abundant icebergs, suggests these birds were resting on the icebergs at night. In addition to the adaptations to scavenging on carrion, pelagic trips by giant petrels contain elements similar to those of albatrosses, indicating a complexity to giant petrel lifestyle hitherto unrecognised.last_img read more

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Modeling ocean processes below Fimbulisen, Antarctica

first_imgModel simulations of circulation and melting beneath Fimbulisen, Antarctica, obtained using an isopycnic coordinate ocean model, are presented. Model results compare well with available observations of currents and hydrography in the open ocean to the north of Fimbulisen and suggest that Warm Deep Water exists above the level of a sub-ice-shelf bedrock sill, the principal pathway for warm waters to enter the sub-ice-shelf cavity. The model shows a southward inflow of Warm Deep Water over this sill and into the cavity, producing a mean cavity temperature close to −1.0°C. This leads to high levels of basal melting (>10 m/a) at the grounding line of Jutulstraumen and an average melting over the ice shelf base close to 1.9 m/a. The southward inflow is a compensating flow caused by the northward outflow of fresh, cold water produced by the basal melting. Results on inflow and melting are difficult to validate since no in situ measurements yet exist in the cavity. If such high melt rates are realistic, the mass balance of Fimbulisen must be significantly negative, and the ice shelves along Dronning Maud Land must contribute about 4.4 mSv of melt water to the Weddell Sea, about 15% of the total Antarctic meltwater input to the Southern Ocean.last_img read more

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Assessment of Southern Ocean mixed-layer depth in CMIP5 models: historical bias and forcing response

first_imgThe development of the deep Southern Ocean winter mixed layer in the climate models participating in the fifth Coupled Models Intercomparison Project (CMIP5) is assessed. The deep winter convection regions are key to the ventilation of the ocean interior, and changes in their properties have been related to climate change in numerous studies. Their simulation in climate models is consistently too shallow, too light and shifted equatorward compared to observations. The shallow bias is mostly associated with an excess annual-mean freshwater input at the sea surface that over-stratifies the surface layer and prevents deep convection from developing in winter. In contrast, modeled future changes are mostly associated with a reduced heat loss in winter that leads to even shallower winter mixed layers. The mixed layers shallow most strongly in the Pacific basin under future scenarios, and this is associated with a reduction of the ventilated water volume in the interior. We find a strong state dependency for the future change of mixed-layer depth, with larger future shallowing being simulated by models with larger historical mixed-layer depths. Given that most models are biased shallow, we expect that most CMIP5 climate models might underestimate the future winter mixed-layer shallowing, with important implications for the sequestration of heat, and gases such as carbon dioxide, and therefore for climate.last_img read more

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