Observations of an enhanced convection channel in the cusp ionosphere

first_imgTransient or patchy magnetic field line merging on the dayside magnetopause, giving rise to flux transfer events (FTEs), is thought to play a significant role in energizing high-latitude ionospheric convection during periods of southward interplanetary magnetic field. Several transient velocity patterns in the cusp ionosphere have been presented as candidate FTE signatures. Instrument limitations, combined with uncertainties about the magnetopause processes causing individual velocity transients, mean that definitive observations of the ionospheric signature of FTEs have yet to be presented. This paper describes combined observations by the PACE HF backscatter radar and the DMSP F9 polar-orbiting satellite of a transient velocity signature in the southern hemisphere ionospheric cusp. The prevailing solar wind conditions suggest that it is the result of enhanced magnetic merging at the magnetopause. The satellite particle precipitation data associated with the transient are typically cusplike in nature. The presence of spatially discrete patches of accelerated ions at the equatorward edge of the cusp is consistent with the ion acceleration that could occur with merging. The combined radar line-of-sight velocity data and the satellite transverse plasma drift data are consistent with a channel of enhanced convection superposed on the ambient cusp plasma flow. This channel is at least 900 km in longitudinal extent but only 100 km wide. It is zonally aligned for most of its extent, except at the western limit where it rotates sharply poleward. Weak return flow is observed outside the channel. These observations are compared with and contrasted to similar events seen by the EISCAT radar and by optical instruments.last_img read more

Tooth growth in male Antarctic fur seals (Arctocephalus gazella) from South Georgia: an indicator of long-term growth history

first_imgGrowth of upper canine teeth of male Antarctic fur seals (Arctocephalus gazella) which died of natural causes at Bird Island, South Georgia, was quantified from measurements of annual layers in longitudinal sections of teeth. Mean age at death was 7.69±0.07 years and this showed a small but significant increase through the period when samples were collected (1972/73–1988/89). There were significant correlations between morphometrics of teeth and those of seals, suggesting that tooth growth provided an indication of body growth. Tooth growth rate was lowest in seals which died early (age 4 years) and increased with age at death. Changes in the growth pattern of teeth suggested that fur seals which became sexually mature early also died early. Tooth growth layers deposited in each calendar year were compared with the expected layer depth based on a linear relationship between layer depth and age at which each layer was deposited. There was significant variation in the depth of tooth growth layers deposited in different years, suggesting that growth was greater in some years than others. No trends in cohort strengths were detected, but particularly poor years for growth were closely related to years in which reproductive performance was also observed to be low. Variations in growth from 1967/68 to 1987/88 were correlated significantly (P < 0.008) with the Southern Oscillation Index of climatic variation.last_img read more

Induced in vitro phagocytosis of the Antarctic starfish Odontaster validus (Koehler 1906) at 0°C

first_imgPhagocytosis was studied in vitro using coelomic fluid of the Antarctic starfish Odontaster validus at 0°C. The number of coelomocytes present was determined and the phagocytic activity of the phagocytic amoebocytes (PA) was quantified with yeast during incubations of 1 and 2 h. The percentage of PA phagocytosing increased significantly from 42.29 ± 10.50% (SD) at 1 h to 52.57 ± 13.96% at 2 h. Numbers of yeast per PA also rose significantly from 2.27 to 2.45 cells per amoebocyte, indicating that phagocytic activity was maintained. In vitro phagocytosis of an Antarctic invertebrate at 0°C is shown for the first time, and the types of amoebocytes involved identified. Rates of phagocytosis were similar to, or higher than, reported data for temperate starfish, although this conclusion must be treated cautiously because of scarcity of data and differences in methods used. However, the data suggest that phagocytosis in O. validus is well adapted to low temperature.last_img read more

Provenance and sex ratio of black-browed albatross, Thalassarche melanophrys, breeding on Campbell Island, New Zealand

first_imgSmall numbers of Black-browed Albatross, Thalassarche melanophrys, breed on Campbell Island, New Zealand. Their dark brown irises had previously been used to distinguish them from the more numerous Campbell Albatross, T. impavida, which has light irises. Blood samples were collected from dark-eyed birds and their partners on Campbell Island to determine their provenance and whether a sex imbalance caused them to breed with Campbell Albatrosses. Analysis of mitochondrial DNA revealed that dark-eyed birds were of three genetically distinct groups: T. melanophrys, Falkland Islands T. melanophrysand T. impavida. The majority of both types of T. melanophrys on Campbell Island were male, hence none was paired with the same taxon, but most of the more widespread form chose dark-eyed mates. Other individuals had a succession of light-eyed partners over several years. Dark-eyed T. impavida may have been hybrid progeny of femaleT. impavida× male T. melanophrys pairings. One of these birds was banded as a chick in 1970, suggesting that hybridisation has occurred on Campbell Island at least as early as that date. Their presence suggests a low rate of interchange between the island groups or recent immigration of T. melanophrys to Campbell Island and neighbouring island groups.last_img read more

Involvement of the Argentine Precordillera terrane in the Famatinian mobile belt: U-Pb SHRIMP and metamorphic evidence from the Sierra de Pie de Palo

first_imgNew data suggest that the eastern margin of the Argentine Precordillera terrane comprises Grenvillian basement and a sedimentary cover derived from it that were together affected by Middle Ordovician deformation and metamorphism during accretion to the Gondwana margin. The basement first underwent low pressure/temperature (P/T) type metamorphism, reaching high-grade migmatitic conditions in places (686 ± 40 MPa, 790 ± 17 °C), comparable to the Grenvillian M2 metamorphism of the supposed Laurentian counterpart of the terrane. The second metamorphism, recognized in the cover sequence, is of Famatinian age and took place under higher P/T conditions, following a clockwise P-T path (baric peak: 1300 ± 100 Mpa, 600 ± 50 °C). Low-U zircon overgrew detrital Grenvillian cores as pressure fell from its peak, and yields U-Pb SHRIMP ages of ca. 460 Ma. This is interpreted as the age of ductile thrusting coincident with early uplift; initial accretion to Gondwana must have occurred before this. The absence of late Neoproterozoic detrital zircons is consistent with a Laurentian origin of the Argentine Precordillera terrane.last_img read more

Antarctic climate change during the last 50 years

first_imgThe Reference Antarctic Data for Environmental Research (READER) project data set of monthly mean Antarctic near-surface temperature, mean sea-level pressure (MSLP) and wind speed has been used to investigate trends in these quantities over the last 50 years for 19 stations with long records. Eleven of these had warming trends and seven had cooling trends in their annual data (one station had too little data to allow an annual trend to be computed), indicating the spatial complexity of change that has occurred across the Antarctic in recent decades. The Antarctic Peninsula has experienced a major warming over the last 50 years, with temperatures at Faraday/Vernadsky station having increased at a rate of 0.56 °C decade-1 over the year and 1.09 °C decade-1 during the winter; both figures are statistically significant at less than the 5% level. Overlapping 30 year trends of annual mean temperatures indicate that, at all but two of the 10 coastal stations for which trends could be computed back to 1961, the warming trend was greater (or the cooling trend less) during the 1961-90 period compared with 1971-2000. All the continental stations for which MSLP data were available show negative trends in the annual mean pressures over the full length of their records, which we attribute to the trend in recent decades towards the Southern Hemisphere annular mode (SAM) being in its high-index state. Except for Halley, where the trends are constant, the MSLP trends for all stations on the Antarctic continent for 1971-2000 were more negative than for 1961-90. All but two of the coastal stations have recorded increasing mean wind speeds over recent decades, which is also consistent with the change in the nature of the SAM. Copyright © 2005 Royal Meteorological Societylast_img read more

Boundary layer characteristics and turbulent exchange mechanisms in highly complex terrain

first_imgThe Mesoscale Alpine Programme’s Riviera project investigated the turbulence structure and related exchange processes in an Alpine valley by combining a detailed experimental campaign with high-resolution numerical modelling. The present contribution reviews published material on the Riviera Valley’s boundary layer structure and discusses new material on the near-surface turbulence structure. The general conclusion of the project is that despite the large spatial variability of turbulence characteristics and the crucial influence of topography at all scales, the physical processes can accurately be understood and modelled. Nevertheless, many of the “text book characteristics” like the interaction between the valley and slope wind systems or the erosion of the nocturnal valley inversion need reconsideration, at least for small non-ideal valleys like the Riviera Valley. The project has identified new areas of research such as post-processing methods for turbulence variables in complex terrain and new approaches for the surface energy balance when advection is non-negligible. The exchange of moisture and heat between the valley atmosphere and the free troposphere is dominated by local “secondary” circulations due to the curvature of the valley axis. Because many curved valleys exist, and operational models still have rather poor resolution, parameterization of these processes may be required.last_img read more

From Scotia to ‘Operation Tabarin’: developing British policy for Antarctica

first_imgThe roots of a British Antarctic policy can be traced, paradoxically, back to the establishment of a meteorological station by the Scottish Antarctic Expedition in the South Orkneys, in 1903, and the indifference of the British Government to its almost immediate transfer to the Argentine Government. It was from that modest physical presence upon Laurie Island that Argentina came increasingly to challenge British claims to the sovereignty of the Falkland Islands Dependencies (FID), first in the late 1920s and then more extensively in the second world war. This challenge shaped British policy for the next forty years, with further complications caused by overlapping territorial claims made by Chile and the possible territorial ambitions of the USA. Britain’s eventual response, at the height of World War II, was to establish permanent occupation of Antarctica from the southern summer of 1943–1944. This occupation was given the military codename Operation Tabarin. However, it was never a military operation as such, although monitoring the activities of enemy surface raiders and submarines provided a convenient cover story, as did scientific research once the operation became public. Whilst successive parties, rich in professional scientists, considerably expanded the pre-war survey and research of the Discovery Investigations Committee, their physical occupancy of the Antarctic islands and Peninsula was essentially a political statement, whereby the Admiralty and Colonial Office (CO) strove to protect British territorial rights, whilst the Foreign Office (FO) endeavoured to minimise disruption to Britain’s long-standing economic and cultural ties with Argentina, and most critically, the shipment of war-time meat supplies. In meeting that immediate need, Tabarin also provided the basis from which Britain’s subsequent post-war leadership in Antarctic affairs developed.last_img read more

Trends in Antarctic Peninsula surface melting conditions from observations and regional climate modelling

first_imgMultidecadal meteorological station records and microwave backscatter time-series from the SeaWinds scatterometer onboard QuikSCAT (QSCAT) were used to calculate temporal and spatial trends in surface melting conditions on the Antarctic Peninsula (AP). Four of six long-term station records showed strongly positive and statistically significant trends in duration of melting conditions, including a 95% increase in the average annual positive degree day sum (PDD) at Faraday/Vernadsky, since 1948. A validated, threshold-based melt detection method was employed to derive detailed melt season onset, extent, and duration climatologies on the AP from enhanced resolution QSCAT data during 1999–2009. Austral summer melt on the AP was linked to regional- and synoptic-scale atmospheric variability by respectively correlating melt season onset and extent with November near-surface air temperatures and the October–January averaged index of the Southern Hemisphere Annular Mode (SAM). The spatial pattern, magnitude, and interannual variability of AP melt from observations was closely reproduced by simulations of the regional model RACMO2. Local discrepancies between observations and model simulations were likely a result of the QSCAT response to, and RACMO2 treatment of, ponded surface water, and the relatively crude representation of coastal climate in the 27 km RACMO2 grid.last_img read more