The propagation characteristics and Landau damping of Jovian whistlers in the Io torus

first_imgThe propagation characteristics and path-integrated gain of Jovian lightning-generated whistlers are explored using the HOTRAY code. All waves are launched into the magnetosphere from just above the density peak in the ionosphere and followed using a realistic analytical density model based on Voyager and Pioneer data. Over a broader range of input L shells (3.5 ≤ L ≤ 6.5) these unducted waves are strongly guided into the equatorial region near 5.3 ≤ L ≤ 6 by the density maxima in the Io plasma torus. This is consistent with the limited spatial extent of the region where whistlers were detected by Voyager. The path-integrated attenuation of whistlers is relatively weak in the cool inner torus but becomes strong for waves that propagate into the outer warm torus region due to Landau damping by suprathermal electrons (E ≥ 200 eV). This can explain the absence of whistlers observations by Voyager at L > 6. To account for the observed upper frequency cutoff (ƒ ≤ 9 kHz ), our calculations indicate that the lightning source locations should be confined to lower L shells (L ≤ 4). The alternative explanation based on the plasma frequency cutoff requires an unreasonable plasma density modellast_img read more

The metazoan meiofauna in its biogeochemical environment: the case of an Antarctic coastal sediment

first_imgThe metazoan meiobenthos was investigated in an Antarctic coastal sediment (Factory Cove, Signy Island, Antarctica). The fine sands contained much higher abundances compared to major sublittoral sediments worldwide. Classified second after Narrangansett Bay (North Atlantic) they reached numbers of 13 × 106 ind m-2. The meiofauna was highly abundant in the surface layers, but densities decreased sharply below 2 cm. Vertical profiles mirrored steep gradients of microbiota, chloropigments and organic matter and were coincident with chemical stratification. Spatial patchiness manifested especially in the surface layer. Nematodes dominated (up to 90%), and Aponema, Chromctdorita, Diplolaimella, Daptonema, Microlaimus and Neochromadora constituted almost the entire community. Overall, the nematode fauna showed a strong similarity with fine sand communities elsewhere. The dominant trophic strategies were epistrarum and non-selective deposit feeding, but the applied classification for feeding guild structure of the nematodes of Factory Cove is discussed. High standing stock, low diversity and shallow depth distribution may have occurred because of the high nutritive (chlorophyll exceeded lOOOmgm-2 and constituted almost 50% of the organic pool) and reductive character of the benthic environment. These observations must have originated from the substantial input of fresh organic matter from phytoplankton and microphytobenthic production, typical for an Antarctic coastal ecosystem during the austral summer.last_img read more

Genetic population structure in the black-spot sea bream (Pagellus bogaraveo Brünnich, 1768) from the NE Atlantic

first_imgThe depletion of shallow-water fish stocks through overexploitation has led to increasing fishing pressure on deep-sea species. Poor knowledge of the biology of commercially valuable deep-water fish has led to the serial depletion of stocks of several species across the world. Data regarding the genetic structure of deep-sea fish populations is important in determining the impact of overfishing on the overall genetic variability of species and can be used to estimate the likelihood of recolonisation of damaged populations through immigration of individuals from distant localities. Here the genetic structure of the commercially fished deep-water species the blackspot sea bream, Pagellus bogaraveo is investigated in the northeastern Atlantic using partial DNA sequencing of mitochondrial cytochrome b (cyt-b) and D-loop regions and genotyping of microsatellite loci. An absence of variation in cyt-b and low genetic variation in D-loop sequences potentially indicate that P. bogaraveo may have undergone a severe bottleneck in the past. Similar bottlenecks have been detected in other Atlantic species of fish and have possibly originated from the last glaciation. P. bogaraveo may have been particularly vulnerable to the effects of low temperature and a fall in sea level because stages of its life history occur in shallow water and coastal sites. However, there are other explanations of low genetic variability in populations of P. bogaraveo, such as a low population size and the impacts of fishing on population structure. Analysis of population structure using both D-loop and microsatellite analysis indicates low to moderate, but significant, genetic differentiation between populations at a regional level. This study supports studies on other deep-sea fish species that indicate that hydrographic or topographic barriers prevent dispersal of adults and/or larvae between populations at regional and oceanographic scales. The implications for the management and conservation of deep-sea fish populations are discussed.last_img read more

Recent trends in melting conditions on the Antarctic Peninsula and their implications for ice-sheet mass balance and sea level

first_imgLong-term records from meteorological stations on the Antarctic Peninsula show strong rising trends in the annual duration of melting conditions. In each case, the trend is statistically significant and represents a major increase in the potential for melting; for example, between 1950 and 2000 the record from Faraday/Vernadsky Station showed a 74% increase in the number of positive degree-days (PDDs). A simple parameterization of the likely effects of the warming on the rate of snow melt suggests an increase across the Antarctic Peninsula ice sheet from 28 ± 12 Gt a–1 in 1950, to 54 ± 26 Gt a–1 by 2000. Given a similar rate of warming over the next 50 years this may reach 100 ± 46 Gt a–1. The majority of this increased meltwater does not drain into the sea but is refrozen in the ice sheet, and it is difficult to predict the fraction of ablation that will become runoff; however, a calculation based on an established criterion for runoff indicates that the contribution from the Antarctic Peninsula, as a direct and immediate response to climate warming is significant, equivalent to (0.008–0.055) mm a–1 of global sea level rise. Given future warming this could easily treble in the coming 50 years. This contribution due to increased runoff could be augmented by any dynamic imbalance in the glaciers draining the ice sheet. This finding appears to contradict the conclusions of previous assessments, including the Intergovernmental Panel on Climate Change, which considered the contribution of runoff from Antarctica to sea level rise would be insignificant.last_img read more

A climatology of tides and gravity wave variance in the MLT above Rothera, Antarctica obtained by MF radar

first_imgA cumulative total of over 5 years of data from an MF radar situated at Rothera (67°S, 68°W) on the Antarctic Peninsula have been used to derive climatologies of periodic motions in the wind field in the mesosphere and lower thermosphere with periods less than or equal to 1 day. Strong tidal motions are observed at 24, 12 and 8 h and monthly mean climatologies are presented between 74 and 94 km altitude for comparison with the HWM-93 horizontal wind model. The 24 h tide shows a strong seasonal dependence in both the zonal and meridional components with a summertime maximum and wintertime minimum over all altitudes. The monthly mean maximum amplitude is 12(±2) ms−1 at 94 km in January and the minimum is <1 ms−1 around 86 km in early winter. The 12 h wave shows large short-term amplitude variability with a peak in amplitude around late autumn. It reaches a minimum at high altitudes in winter and below 80 km during summer, characteristic of a mixture of migrating and non-migrating modes. The phase of the 12 h wave is relatively constant throughout winter with a minimum mean vertical wavelength of 75 km around equinox. The 8 h wave is predominantly a summertime high altitude phenomenon. It is seen most strongly in the winds above 85 km and reaches monthly mean amplitudes of 6(±2) ms−1 in the zonal winds at 94 km altitude. Finally, a seasonal climatology of gravity wave variances is generated by calculating the daily mean variance in the raw winds after subtracting the fitted tidal components. This index shows a strong seasonal and height dependence in both components with a wintertime peak of 2000 m2s−2 in the zonal component at the highest altitudes. This peak occurs when the stratospheric zonal jets are strongest and therefore the filtering of upward-propagating waves in the stratosphere should be greatest; implying that either a significant part of this wintertime wave activity is generated from a region above the peak stratospheric wind or that there is a strong annual variability in the source or propagation of the gravity wave activity at Rothera.last_img read more

Very strong atmospheric methane growth in the four years 2014 – 2017: Implications for the Paris Agreement

first_imgAtmospheric methane grew very rapidly in 2014 (12.7±0.5 ppb/yr), 2015 (10.1±0.7 ppb/yr), 2016 (7.0± 0.7 ppb/yr) and 2017 (7.7±0.7 ppb/yr), at rates not observed since the 1980s. The increase in the methane burden began in 2007, with the mean global mole fraction in remote surface background air rising from about 1775 ppb in 2006 to 1850 ppb in 2017. Simultaneously the 13C/12C isotopic ratio (expressed as δ13CCH4) has shifted, in a new trend to more negative values that have been observed worldwide for over a decade. The causes of methane’s recent mole fraction increase are therefore either a change in the relative proportions (and totals) of emissions from biogenic and thermogenic and pyrogenic sources, especially in the tropics and sub‐tropics, or a decline in the atmospheric sink of methane, or both. Unfortunately, with limited measurement data sets, it is not currently possible to be more definitive. The climate warming impact of the observed methane increase over the past decade, if continued at >5 ppb/yr in the coming decades, is sufficient to challenge the Paris Agreement, which requires sharp cuts in the atmospheric methane burden. However, anthropogenic methane emissions are relatively very large and thus offer attractive targets for rapid reduction, which are essential if the Paris Agreement aims are to be attained.last_img read more

Magmatism of the Weddell Sea rift system in Antarctica: Implications for the age and mechanism of rifting and early stage Gondwana breakup

first_imgThick (∼800 m) basaltic successions from the eastern Antarctic Peninsula have been dated in the interval 180 – 177 Ma and preserve a transition from a continental margin arc to a back-arc extensional setting. Amygdaloidal basalts from the Black Coast region of the eastern margin of the Antarctic Peninsula represent a rare onshore example of magmatism associated with back-arc extension that defines the early phase of Weddell Sea rifting and magmatism, and Gondwana breakup. The early phase of extension in the Weddell Sea rift system has previously been interpreted to be related to back-arc basin development with associated magnetic anomalies attributed to mafic-intermediate magmatism, but with no clearly defined evidence of back-arc magmatism. The analysis provided here identifies the first geochemical evidence of a transition from arc-like basalts to the development of depleted back-arc basin basalts in the interval 180 – 177 Ma. The exposed Black Coast basaltic successions are interpreted to form a minor component of magmatism that is also defined by onshore sub-ice magnetic anomalies, as well as the extensive magnetic anomalies of the southern Weddell Sea. Back-arc magmatism is also preserved on the Falkland Plateau where intrusions postdating 180 Ma are associated with early phase rifting in the Weddell Sea rift system. Back-arc extension was probably short-lived and had ceased by the time the northern Weddell Sea magmatism was emplaced (<175 Ma) and certainly by 171 Ma, when an episode of silicic magmatism was widespread along the eastern Antarctic Peninsula. Previous attempts to correlate mafic magmatism from the eastern Antarctic Peninsula to the Ferrar large igneous province, or, as part of a bimodal association with the Chon Aike silicic province are both dismissed based on age and geochemical criteria.last_img read more

Zircon O and Hf isotope constraints on the genesis of Permian–Triassic magmatic and metamorphic rocks in the Antarctic Peninsula and correlations with Patagonia

first_imgThe Permian–Triassic is a critical period for interpreting and understanding the development of West Antarctica and its correlations into Patagonia, South America. The Antarctic Peninsula preserves isolated outcrops of Permian–Triassic age magmatic and metamorphic rocks of granodiorite, orthogneiss, paragneiss and migmatites. Outcrops from the eastern margin of the Antarctic Peninsula (Eastern Domain) have the strongest affinity with continental Gondwana, and have igneous zircons with initial εHf values ranging from −2.8 to −21.6, and δ18O from 10.5 to 5.6‰. These values record a strong sedimentary influence on the magma source and imply crustal recycling. This is in concert with the extensive inherited zircon components that record late Mesoproterozoic–early Neoproterozoic ages, Cambrian and older Permian ages. U–Pb zircon ages for a ca. 202 Ma orthogneiss from the western Antarctic Peninsula (Central Domain) record the presence of a Permian protolith with zircon overgrowth at ca. 222 Ma due to partial melting. These zircon rims have initial εHf values of +1.5 to −0.9 and δ18O of 5.3 to 4.2‰, similar to those determined for the cores. This indicates that the new zircon crystallised from dissolution of partly dissolved cores. Our new data indicate strong similarities between the Eastern Domain of the Antarctic Peninsula and the southern tip of Patagonia, supporting a continuation of both areas, without the need for a great degree of geographical overlap during the Permian and Triassic.last_img read more

Scoreboard Roundup 4/30/18

first_img FacebookTwitterLinkedInEmailiStock/Thinkstock(NEW YORK) — Here are the scores from Saturday’s sports events:  INTERLEAGUE Final  Cincinnati   8  Minnesota   2   ——   AMERICAN LEAGUE Final  Boston        4  Tampa Bay       3 Final  Baltimore     5  Detroit         3 Final  Toronto       7  Texas           2 Final  Seattle      10  Cleveland       4 Final  Houston       8  Oakland         4 Final  Kansas City   5  Chi White Sox   4 Final  N-Y Yankees   2  L-A Angels      1   ——   NATIONAL LEAGUE Final  Miami           3  Colorado       0 Final  Atlanta        10  Philadelphia   1 Final  Washington      3  Arizona        1 Final  Pittsburgh      5  St. Louis      0 Final  Chi Cubs        2  Milwaukee      0 Final  N-Y Mets       14  San Diego      2 Final  San Francisco   4  L-A Dodgers    2   ——   NATIONAL BASKETBALL ASSOCIATION PLAYOFFS Final  Cleveland  105  Indiana  101 Final  Houston    110  Utah      96   ——   NATIONAL HOCKEY LEAGUE PLAYOFFS Final  Washington   4  Pittsburgh   1 Final 2OT  Nashville    5  Winnipeg   4Copyright © 2018, ABC Radio. All rights reserved. Beau Lund Written bycenter_img April 30, 2018 /Sports News – National Scoreboard Roundup 4/30/18last_img read more

Bees Salvage Finale In Fresno

first_img Robert Lovell May 16, 2018 /Sports News – Local Bees Salvage Finale In Fresno Tags: Baseball/PCL/Salt Lake Bees Written by FacebookTwitterLinkedInEmail(Fresno, CA)  —  The Bees rallied for a run in the eighth and another in the ninth to edge the Grizzlies 3-2 in Fresno.David Fletcher singled home the go-ahead run as the Bees avoided the four-game sweep.  Adam Hofacket earned the win and Eduardo Paredes picked up his first save.Salt Lake improved to 23-and-17.The Bees return home to open up a four-game set with Sacramento tomorrow at Smith’s Ballpark.  Salt Lake split four games at Sacramento last week.  First pitch for tomorrow’s opener is slated for 6:35 p.m.last_img