A study of quasi-periodic ELF-VLF emissions at three Antarctic stations: evidence for off-equatorial generation?

first_imgThe spatial extent and temporal behaviour of quasi-periodic (QP) intensity modulations of 0.5-2 kHz ELF-VLF signals were investigated in a comparative study of data collected at the Antarctic stations of South Pole (L=14), Halley (L=4), and Siple (L=4). Frequently, the waveforms of ELF-VLF signals simultaneously received at each site were identical. Although of similar frequency structure, the waveforms of the accompanying Pc3 magnetic pulsations did not show a one-to-one association. Whereas both are dayside phenomena, QP emissions occur over a smaller range of local times, and have a maximum of occurrence later in the day closer to local noon. QP emissions are identified with the periodic modulation of the electron pitch-angle distribution by the propagation of ULF compressional fast-mode waves through a region. However, contrary to previous ideas, rising-tone emissions do not represent the frequency-time signatures of such waves. In addition to generation close to the equatorial plane, we propose an additional high-latitude source of QP emissions. These emissions are associated with regions of minimum B produced by the dayside compression of the magnetosphere close to the magnetopause. Model magnetic field calculations of these minimum-B regions as a function of magnetic local time and invariant latitude are presented.last_img read more

Considerations of major issues in ecosystem monitoring and management

first_imgThis paper outlines the requirements of an ecosystem approach to the management of Southern Ocean resources and highlights the need for information on harvestedand dependent species, their interactions and the manner in which their populations vary naturally. Large-scale interactions are catered for in the Krill Yield Model (KYM).Smaller-scale interactions centre around three main categories: the availability of krill, variation in vital rates of the dependent species and the overlap between commercial fishing and predator foraging. The CCAMLR Ecosystem Monitoring Program (CEMP) provides a good framework within which to investigate krill availability. Vital rates can be investigated directly and also by means of CEMP. The overlap between fishing and predator foraging is being monitored. A mechanism for bringing these variouscomponents together as an ecosystem approach to management is discussed.last_img read more

On the consistent scaling of terms in the sea-ice dynamics equation

first_imgThe standard way in which the sea-ice dynamics equation is used in models assumes that the wind stress and ocean drag do not depend on the sea-ice concentration. It is demonstrated that this assumption is inconsistent with the free-drift limit, and how great an effect it has in practice is examined. By examining the momentum balance in the free-drift limit, the authors determine the proper area scaling for the forcing terms, thereby obtaining a more accurate solution, particularly in low-ice-concentration regions.last_img

Magnetic local time, substorm, and particle precipitation-related variations in the behaviour of SuperDARN Doppler spectral widths

first_imgSuper Dual Auroral Radar Network (DARN) radars often detect a distinct transition in line-of-sight Doppler velocity spread, or spectral width, from 200 m s(-1) at higher latitude. They also detect a similar boundary, namely the range at which ionospheric scatter with large spectral width suddenly commences (i.e. without preceding scatter with low spectral width). The location and behaviour of the spectral width boundary (SWB) (and scatter boundary) and the open-closed magnetic field line boundary (OCB) are thought to be closely related. The location of the nightside OCB can be inferred from the poleward edge of the auroral oval determined using energy spectra of precipitating particles measured on board Defence Meteorology Satellite Program (DMSP) satellites. Observations made with the Halley SuperDARN radar (75.5degrees S, 26.6degrees W, geographic; -62.0degrees Lambda) and the Tasman International Geospace Environment Radar (TIGER) (43.4degrees S, 147.2degrees E; -54.5degrees Lambda) are used to compare the location of the SWB with the DMSP-inferred OCB during 08:00 to 22:00 UT on 1 April 2000. This study interval was chosen because it includes several moderate substorms, whilst the Halley radar provided almost continuous high-time resolution measurements of the dayside SWB location and shape, and TIGER provided the same in the nightside ionosphere. The behaviour of the day- and nightside SWB can be understood in terms of the expanding/contracting polar cap model of high-latitude convection change, and the behaviour of the nightside SWB can also be organised according to substorm phase. Previous comparisons with DMSP OCBs have proven that the radar SWB is often a reasonable proxy for the OCB from dusk to just past midnight (Chisham et al., 2004). However, the present case study actually suggests that the nightside SWB is often a better proxy for the poleward edge of Pedersen conductance enhanced by hot particle precipitation in the auroral zone. Simple modeling implies that the large spectral widths must be caused by similar to10-km scale velocity fluctuations.last_img read more

Recent ice loss from the Fleming and other glaciers, Wordie Bay, West Antarctic Peninsula

first_imgSatellite radar interferometry data from 1995 to 2004, and airborne ice thickness data from 2002, reveal that the glaciers flowing into former Wordie Ice Shelf, West Antarctic Peninsula, discharge 6.8 ± 0.3 km3/yr of ice, which is 84 ± 30 percent larger than a snow accumulation of 3.7 ± 0.8 km3/yr over a 6,300 km2 drainage basin. Airborne and ICESat laser altimetry elevation data reveal glacier thinning at rates up to 2 m/yr. Fifty km from its ice front, Fleming Glacier flows 50 percent faster than it did in 1974 prior to the main collapse of Wordie Ice Shelf. We conclude that the glaciers accelerated following ice shelf removal, and have been thinning and losing mass to the ocean over the last decade. This and other observations suggest that the mass loss from the northern part of the Peninsula is not negligible at present.last_img read more

A model study of the seasonal and long-term North Atlantic surface pCO2 variability

first_imgA coupled biogeochemical-physical ocean model is used to study the seasonal and long–term variations of surface pCO2 in the North Atlantic Ocean. The model agrees well with recent underway pCO2 observations from the Surface Ocean CO2 Atlas (SOCAT) in various locations in the North Atlantic. Some of the distinct seasonal cycles observed in different parts of the North Atlantic are well reproduced by the model. In most regions except the subpolar domain, recent observed trends in pCO2 and air–sea carbon fluxes are also simulated by the model. Over the longer period between 1960–2008, the primary mode of surface pCO2 variability is dominated by the increasing trend associated with the invasion of anthropogenic CO2 into the ocean. We show that the spatial variability of this dominant increasing trend, to first order, can be explained by the surface ocean circulation and air–sea heat flux patterns. Regions with large surface mass transport and negative air–sea heat flux have the tendency to maintain lower surface pCO2. Regions of surface convergence and mean positive air–sea heat flux such as the subtropical gyre and the western subpolar gyre have a higher long–term surface pCO2 mean. The North Atlantic Oscillation (NAO) plays a major role in controlling the variability occurring at interannual to decadal time scales. The NAO predominantly influences surface pCO2 in the North Atlantic by changing the physical properties of the North Atlantic water masses, particularly by perturbing the temperature and dissolved inorganic carbon in the surface ocean. We show that present underway sea surface pCO2 observations are valuable for both calibrating the model, as well as for improving our understanding of the regionally heterogeneous variability of surface pCO2. In addition, they can be important for detecting any long term change in the regional carbon cycle due to ongoing climate change.last_img read more

Dr Reginald Koettlitz (1860-1916): Arctic and Antarctic explorer

first_imgReginald Koettlitz was born in Ostend but moved to England as a child and qualified at Guy’s Hospital. He was a general practitioner in County Durham for eight years before serving as doctor and geologist to the Jackson-Harmsworth Expedition to Franz-Josef Land in 1894–97. Thereafter he made further expeditions to Somaliland, Abyssinia and the Amazon before joining Captain Scott’s Discovery Expedition to the Antarctic in 1901–04 as surgeon and botanist. After the expedition he emigrated to South Africa, where he worked as a general practitioner, dying in 1916.last_img

Survey of whistler mode chorus intensity at Jupiter

first_imgWhistler mode chorus emission is important in the acceleration of electrons and filling of the radiation belts at Jupiter. In this work chorus magnetic intensity levels (frequency-integrated spectral density, PB) at Jupiter are comprehensively binned and parameterized. The frequency range of chorus under study extends from the lower hybrid frequency, flh, to fceq/2 and fceq/2 < f < 0.8 fceq, where fceq is the cyclotron frequency mapped to the magnetic equator. The goal is to obtain a quantized distribution of magnetic intensity for use in stochastic modeling efforts. Parametric fits of magnetic plasma wave intensity are obtained, including PB versus frequency, latitude, and L shell. The results indicate that Jupiter chorus occurrence probability and intensity are higher than those at Saturn, reaching values observed at Earth. Jovian chorus is observed over most local times, confined primarily to the range 8 < L < 15, outside the high densities of the Io torus. The largest intensity levels are seen on the dayside; however, the sampling of chorus on the nightside is much less than on the dayside. Peak intensities occur near the equator with a weak dependence on magnetic latitude, λ. We conclude that Jovian chorus average intensity levels are approximately an order of magnitude lower than those at Earth. In more isolated regions the intensities are comparable to those observed at Earth. The spatial range of the chorus emissions extends beyond that assumed in previous Jovian global diffusive models of wave-particle electron acceleration.last_img read more

Segregation in space and time explains the coexistence of two sympatric sub-Antarctic petrels

first_imgBiological communities are shaped by competition between and within species. Competition is often reduced by inter- and intraspecific specialization on resources, such as differencet foraging areas or time, allowing similar species to coexist and potentially contributing to reproductive isolation. Here, we examine the simultaneous role of temporal and spatial foraging segregation within and between two sympatric sister species of seabirds, Northern Macronectes halli and Southern Macronectes giganteus Giant Petrels. These species show marked sexual size dimorphism and allochrony (with earlier breeding by Northern Giant Petrels) but this is the first study to test for differences in foraging behaviours and areas across the entire breeding season both between the two species and between the sexes. We tracked males and females of both species in all breeding stages at Bird Island, South Georgia, to test how foraging distribution, behaviour and habitat use vary between and within species in biological time (incubation, brood-guard or post-brood stages) and in absolute time (calendar date). Within each breeding stage, both species took trips of comparable duration to similar areas, but due to breeding allochrony they segregated temporally. Northern Giant Petrels had a somewhat smaller foraging range than Southern Giant Petrels, reflecting their greater exploitation of local carrion and probably contributing to their recent higher population growth. Within species, segregation was spatial, with females generally taking longer, more pelagic trips than males, although both sexes of both species showed unexpectedly plastic foraging behaviour. There was little evidence of interspecific differences in habitat use. Thus, in giant petrels, temporal segregation reduces interspecific competition and sexual segregation reduces intraspecific competition. These results demonstrate how both specialization and dynamic changes in foraging strategies at different scales underpin resource division within a community.last_img read more

Jazz try to slow Harden in Game 2 after Game 1 rout

first_img Tags: Basketball/Houston Rockets/NBA/NBA Playoffs/Utah Jazz FacebookTwitterLinkedInEmailHOUSTON (AP) No one on the Utah Jazz is using the word ”stop” when referring to what they’ll try to do to James Harden in Game 2 of the Western Conference semifinals on Wednesday night after the Houston Rockets’ star scored 41 points in a Game 1 rout.They’re simply searching for ways to slow him down a bit after he’s averaged almost 36 points a game in five meetings this season.”He is a (heck) of a player so it’s going to be tough, but we feel like we can just try to make it a bit tough on him for the whole (time) he’s out there,” Utah’s Joe Ingles said.Harden made seven 3-pointers in Game 1, leaving the Jazz focused on limiting him from long range.”Just make him a driver,” rookie Donovan Mitchell said. ”He loves getting back to that 3. They got comfortable in the first half and they went up 30. So just being able to make them uncomfortable and not let them dictate what we do on defense. Make sure we dictate what they do.”Coach Mike D’Antoni isn’t concerned about anything that the Jazz might throw at Harden on Wednesday and believes the only thing that can keep Harden from a big game is, well, Harden.”There’s no answer. He’s seen it all,” D’Antoni said. ”Now, he might play bad and that’s because he’s human. But there’s nothing that you can conceivably come up with that can stop one of the best offensive players ever.”Harden led the NBA in scoring in the regular season by averaging a career-high 30.4 points a game and ranks second behind LeBron James this postseason with 31 points a game.Houston’s Trevor Ariza, a player known for his defense, has the task of guarding Mitchell in this series. He was asked what he would try if he had to guard Harden. There was a long pause before Ariza let out a loud sigh and said with a laugh: ”I’d figure out a way to slow him down.”But the veteran is happy to be playing with Harden and not against him.”I’m glad I don’t have that problem right now,” he said. ”I wouldn’t tell them what I think. I would let them try to figure it out on their own.”The top-seeded Rockets expect to see a different team than they did in Game 1 after the Jazz had only about 36 hours between the end of Game 6 against the Thunder and the start of this series.”I think a couple days’ rest will help them and we’ll get the best they’ve got,” D’Antoni said.The Jazz refused to use fatigue as an excuse for their poor performance in Game 1, but they did admit that they were feeling better about this game with a little bit of rest. Mitchell said having two days to break down film and work on their game plan has also been beneficial.Despite trailing by double digits for most of Sunday’s game, the Jazz don’t seemed daunted or discouraged entering Game 2.”A lot of people have been hitting me up saying: ”Try to keep your head up. It’s only Game 1,”’ Mitchell said. ”And my head was never down. I don’t think any of our heads were ever down. It’s just one game out of a long series … I think being down 0-1 last series and seeing how we came back and responded definitely gives you a sense of more hope than if this would have been the opening round.”The Jazz will try and even the series without starting point guard Ricky Rubio, who sat out Game 1 and is out indefinitely with a strained left hamstring. The Rockets believe Rubio’s absence changed things for Mitchell. He scored a playoff-high 38 points in Game 6 to lead Utah to the win before scoring 21, which were his fewest of the postseason, on Sunday.”It makes Donovan handle the ball more than they would like,” Ariza said of Rubio’s injury. ”He’s their dominant scorer at this point so when he has to handle the ball and get everybody involved in the game it kind of takes away from him scoring the ball all the time or being aggressive all the time.” Written by Associated Press May 2, 2018 /Sports News – Local Jazz try to slow Harden in Game 2 after Game 1 routlast_img read more