A survey of natural electromagnetic noise in the frequency range f = 1–10 kHz at Halley station, Antarctica: 1. Radio atmospherics from lightning

first_imgThis paper presents results from the first systematic survey of VLF wave activity at Halley, Antarctica (76 °S, 27 °W, L = 4.3). Beginning in 1971, the peak, average and minimum (P, A, M) signal levels observed in four frequency bands centred on 0.75 kHz, 1.25 kHz, 3.2 kHz and 9.6 kHz have been recorded every 5 min. At these frequencies the observed radio noise is largely natural, the waves being generated either in the magnetosphere (e.g. chorus, hiss, etc.) or near the ground, the latter principally from lightning discharges (radio atmospherics, or “spherics”) which reach the receiver after propagating some distance in the Earth-ionosphere waveguide (lightning does not occur in the immediate vicinity of Halley). Here we analyse the observations for 1984, the first complete year for which we have data in digital form, in terms of thunderstorm regions, as a benchmark for more extended studies of possible long-term change in global lightning activity. The data are presented in compressed colour graphic format which facilitates the identification of periodic (diurnal and annual) and aperiodic variations. At 3.2 kHz, attenuation in the Earth-ionosphere waveguide is severe, and only relatively few spherics, from close lighting source regions, are observed. Thus, whilst the 3.2 M channel is insensitive to lightning, and responds mostly to magnetospheric emissions, the 3.2 P channel is dominated by spherics. The 3.2 P data show a marked diurnal and seasonal variation symmetrical about Halley local noon and about the solstices, consistent with nearby sources and attenuation rates for subionospheric propagation which are much greater during the day than at night. At 9.6 kHz, waveguide attenuation is much lower (and there is less difference between day and night), and the minimum channel is dominated by a continuum of spheric noise originating from globally distributed distant source regions, notably those in the tropics. Consequently, there is no control by the local dawn-dusk terminator; the diurnal and seasonal variation is not symmetrical about Halley local noon and the solstices but consists of a quasi-sinusoidal diurnal variation, in which the phases of the minimum and maximum vary during the year: ~07 LT (LT∼-UT−2h at Halley) and ~17 LT in December (summer) and ~10 LT and ~21 LT in June (winter). Agreement between the observations and the CCIR (1983) empirical model is poor. A somewhat better fit is given by a simple model in which thunderstorm regions consist of point sources having radiated powers which vary with local time and season, the total effect at Halley being modelled as the sum of contributions from these sources.last_img read more

Latitudinal extent of the January 2005 solar proton event in the Northern Hemisphere from satellite observations of hydroxyl

first_imgWe utilise hydroxyl observations from the MLS/Aura satellite instrument to study the latitudinal extent of particle forcing in the northern polar region during the January 2005 solar proton event. MLS is the first satellite instrument to observe HOx changes during such an event. We also predict the hydroxyl changes with respect to the magnetic latitude by the Sodankyla Ion and Neutral Chemistry model, estimating the variable magnetic cutoff energies for protons using a parameterisation based on magnetosphere modelling and the planetary magnetic index K-p. In the middle and lower mesosphere, HOx species are good indicators of the changes in the atmosphere during solar proton events, because they respond rapidly to both increases and decreases in proton forcing. Also, atmospheric transport has a negligible effect on HOx because of its short chemical lifetime. The observations indicate the boundary of the proton forcing and a transition region, from none to the ‘full’ effect, which ranges from about 57 to 64 degrees of magnetic latitude. When saturating the rigidity cutoff K-p at 6 in the model, as suggested by earlier studies using observations of cosmic radio noise absorption, the equatorward boundary of the transition region is offset by approximate to 2 degrees polewards compared with the data, thus the latitudinal extent of the proton forcing in the atmosphere is underestimated. However, the model predictions are in reasonable agreement with the MLS measurements when the K-p index is allowed to vary within its nominal range, i.e., from 1 to 9 in the cutoff calculation.last_img read more

Mite dispersal among the Southern Ocean Islands and Antarctica before the last glacial maximum

first_imgIt has long been maintained that the majority of terrestrial Antarctic species are relatively recent, post last glacial maximum, arrivals with perhaps a few microbial or protozoan taxa being substantially older. Recent studies have questioned this ‘recolonization hypothesis’, though the range of taxa examined has been limited. Here, we present the first large-scale study for mites, one of two dominant terrestrial arthropod groups in the region. Specifically, we provide a broad-scale molecular phylogeny of a biologically significant group of ameronothroid mites from across the maritime and sub-Antarctic regions. Applying different dating approaches, we show that divergences among the ameronothroid mite genera Podacarus, Alaskozetes and Halozetes significantly predate the Pleistocene and provide evidence of independent dispersals across the Antarctic Polar Front. Our data add to a growing body of evidence demonstrating that many taxa have survived glaciation of the Antarctic continent and the sub-Antarctic islands. Moreover, they also provide evidence of a relatively uncommon trend of dispersals from islands to continental mainlands. Within the ameronothroid mites, two distinct clades with specific habitat preferences ( marine intertidal versus terrestrial/supralittoral) exist, supporting a model of within-habitat speciation rather than colonization from marine refugia to terrestrial habitats. The present results provide additional impetus for a search for terrestrial refugia in an area previously thought to have lacked ice-free ground during glacial maxima.last_img read more

On the nature of a short-period mesospheric gravity wave propagation over Halley, Antarctica

first_imgAs part of a collaborative program between British Antarctic Survey and Utah State University, measurements were made using an all-sky airglow imager located at the U.K. Halley Station (76°S, 27°W) during the 2000 and 2001 austral winter seasons from April through to early September. A co-located imaging Doppler interferometer was utilized to obtain coincident wind measurements for a total of 171 wave events. This study comprises the first detailed climatological investigation of the propagation nature (freely propagating, Doppler ducted, or evanescent) of individual quasi-monochromatic, short-period wave events at a high southern latitude. Distributions of the derived vertical wavelength exhibit an interquartile range from ∼16–48 km with a median vertical wavelength of 21 km. The majority of the wave events were found to be freely propagating waves, with only ∼5% exhibiting a clear Doppler ducted signature, while 15% of the waves were found to be evanescent in nature. Although no coincident temperature measurements were available, subsequent SABER temperature measurements suggest that up to ∼28% of the measured temperature profiles are capable of providing a ducted environment for the observed wave field. This is in sharp contrast to findings at mid- and low latitudes where these waves have been shown to be prone to Doppler ducted motion. It is suggested that the relatively weak wind field and associated tidal wind amplitudes over Halley are not capable of forming a significant Doppler ducted region to sustain a substantial amount of ducted waves belonging to the detectable spectrum of the airglow imager. As these wind fields are comparable to wind fields found at other polar latitudes, we hypothesize that the majority of short-period gravity waves observed in the polar mesosphere are freely propagating and thus an important source of energy transfer into the MLT region.last_img read more

A statistical approach to determining energetic outer radiation-belt electron precipitation fluxes

first_imgSub-ionospheric radio-wave data from an AARDDVARK receiver located in Churchill, Canada, is analysed to determine the characteristics of electron precipitation into the atmosphere over the range 3 < L 30 keV precipitation flux determined by the AARDDVARK technique was found to be ±10%. Peak >30 keV precipitation fluxes of AARDDVARK-derived precipitation flux during the main- and recovery-phase of the largest geomagnetic storm, that started on 04 August 2010, were >105 el. cm-2 s-1 sr-1. The largest fluxes observed by AARDDVARK occurred on the dayside, and were delayed by several days from the start of the geomagnetic disturbance. During the main phase of the disturbances nightside fluxes were dominant. Significant differences in flux estimates between POES, AARDDVARK and the riometer were found after the main phase of the largest disturbance, with evidence provided to suggest that >700 keV electron precipitation was occurring. Currently the presence of such relativistic electron precipitation introduces some uncertainty in the analysis of AARDDVARK data, given the assumption of a power-law electron precipitation spectrum.last_img read more

Sensitivity of Southern Ocean circulation to wind stress changes: Role of relative wind stress

first_imgThe influence of different wind stress bulk formulae on the response of the Southern Ocean circulation to wind stress changes is investigated using an idealised channel model. Surface/mixed layer properties are found to be sensitive to the use of the relative wind stress formulation, where the wind stress depends on the difference between the ocean and atmosphere velocities. Previous work has highlighted the surface eddy damping effect of this formulation, which we find leads to increased circumpolar transport. Nevertheless the transport due to thermal wind shear does lose sensitivity to wind stress changes at sufficiently high wind stress. In contrast, the sensitivity of the meridional overturning circulation is broadly the same regardless of the bulk formula used due to the adiabatic nature of the relative wind stress damping. This is a consequence of the steepening of isopycnals offsetting the reduction in eddy diffusivity in their contribution to the eddy bolus overturning, as predicted using a residual mean framework.last_img read more

A horizon scan of global conservation issues for 2016

first_imgThis paper presents the results of our seventh annual horizon scan, in which we aimed to identify issues that could have substantial effects on global biological diversity in the future, but are not currently widely well known or understood within the conservation community. Fifteen issues were identified by a team that included researchers, practitioners, professional horizon scanners, and journalists. The topics include use of managed bees as transporters of biological control agents, artificial superintelligence, electric pulse trawling, testosterone in the aquatic environment, building artificial oceanic islands, and the incorporation of ecological civilization principles into government policies in Chinalast_img read more

New geophysical compilations link crustal block motion to Jurassic extension and strike-slip faulting in the Weddell Sea Rift System of West Antarctica

first_imgGondwana breakup changed the global continental configuration, leading to opening of major oceanic gateways, shifts in the climate system and significant impacts on the biosphere, hydrosphere and cryosphere. Although of global importance, the earliest stages of the supercontinental fragmentation are poorly understood. Reconstructing the processes driving Gondwana breakup within the ice-covered Weddell Sea Rift System (WSRS) has proven particularly challenging. Paleomagnetic data and tectonic reconstructions of the WSRS region indicate that major Jurassic translation and rotation of microcontinental blocks were a key precursor to Gondwana breakup by seafloor spreading. However, geophysical interpretations have provided little support for major motion of crustal blocks during Jurassic extension in the WSRS. Here we present new compilations of airborne magnetic and airborne gravity data, together with digital enhancements and 2D models, enabling us to re-evaluate the crustal architecture of the WSRS and its tectonic and kinematic evolution. Two provinces are identified within the WSRS, a northern E/W trending province and a southern N/S trending province. A simple extensional or transtensional model including ~ 500 km of crustal extension and Jurassic magmatism accounts for the observed geophysical patterns. Magmatism is linked with rifting between South Africa and East Antarctica in the north, and associated with back-arc extension in the south. Our tectonic model implies ~ 30 degrees of Jurassic block rotation and juxtaposes the magnetically similar Haag Block and Shackleton Range, despite differences in both Precambrian and Pan African-age surface geology. Although geophysically favoured our new model cannot easily be reconciled with geological and paleomagnetic interpretations that require ~ 1500 km of motion and 90 degrees anticlockwise rotation of the Haag-Ellsworth Whitmore block from a pre-rift position adjacent to the Maud Belt. However, our model provides a simpler view of the WSRS as a broad Jurassic extensional/transtensional province within a distributed plate boundary between East and West Antarctica.last_img read more

Modification of deep waters in Marguerite Bay, western Antarctic Peninsula, caused by topographic overflows

first_imgCircumpolar Deep Water (CDW) intrudes from the mid-layers of the Antarctic Circumpolar Current onto the shelf of the western Antarctic Peninsula, providing a source of heat and nutrients to the regional ocean. It is well known that CDW is modified as it flows across the shelf, but the mechanisms responsible for this are not fully known. Here, data from underwater gliders with high spatial resolution are used to demonstrate the importance of detailed bathymetry in inducing multiple local mixing events. Clear evidence for overflows is observed in the glider data as water flows along a deep channel with multiple transverse ridges. The ridges block the densest waters, with overflowing water descending several hundred metres to fill subsequent basins. This vertical flow leads to entrainment of overlying colder and fresher water in localised mixing events. Initially this process leads to an increase in bottom temperatures due to the temperature maximum waters descending to greater depths. After several ridges, however, the mixing is sufficient to remove the temperature maximum completely and the entrainment of colder thermocline waters to depth reduces the bottom temperature, to approximately the same as in the source region of Marguerite Trough. Similarly, it is shown that deep waters of Palmer Deep are warmer than at the same depth at the shelf break. The exact details of the transformations observed are heavily dependent on the local bathymetry and water column structure, but glacially-carved troughs and shallow sills are a common feature of the bathymetry of polar shelves, and these types of processes may be a factor in determining the hydrographic conditions close to the coast across a wider area.last_img read more

Dynamics of marine debris ingestion by profitable fishes along the estuarine ecocline

first_imgThe dynamics of microfilament (<5 mm) ingestion were evaluated in three species of snooks. The ingestion of different colours and sizes of microfilaments were strongly associated with the spatio-temporal estuarine use and ontogenetic shifts of snooks. Their feeding ecology was also analysed to assess dietary relationships with patterns of contamination. All species were highly contaminated with microfilaments. The highest ingestion of microfilaments occurred in the adults, when fishes became the main prey item and also during the peak of fishing activities, in the rainy season. This suggests that trophic transfer, in addition to periods of high availability of microfilaments are important pathways for contamination. The ingestion of microfilaments of different colours and sizes was likely influenced by input sources. Blue microfilaments were frequently ingested, and appear to have both riverine and estuarine inputs, since they were ingested in all seasons and habitats. Purple and red microfilaments were more frequently ingested in the lower estuarine habitats. The length of microfilaments was also associated with environmental variability. Longer microfilaments were ingested in habitats with greater riverine influence, the opposite was observed for shorter microfilaments. Therefore, microfilament contamination in snooks are a consequence of their ecological patterns of estuarine uses through different seasons and life history stages.last_img read more