The synthesis of travelling ionospheric disturbance (TID) signatures in HF radar observations using ray tracing   总被引：1，自引：0，他引：1  

A. J. Stocker  N. F. Arnold  T. B. Jones 《Annales Geophysicae》1999,18(1):56-64

Characteristic signatures are often observed in HF radar range-time-intensity plots when travelling ionospheric disturbances (TIDs) are present. These signatures, in particular the variation of the F-region skip distance, have been synthesised using a ray tracing model. The magnitude of the skip variation is found to be a function of the peak electron density perturbation associated with the TID and radar frequency. Examination of experimental observations leads to an estimate of the peak electron density perturbation amplitude of around 25% for those TIDs observed by the CUTLASS radar system. The advantage of using the skip variation over the radar return amplitude as an indicator of density perturbation is also discussed. An example of a dual radar frequency experiment has been given. The investigation of the effect of radar frequency on the observations will aid the optimisation of future experiments.  相似文献   

The role of bromine and chlorine chemistry for arctic ozone depletion events in Ny-Ålesund and comparison with model calculations     

M. Martinez  T. Arnold  D. Perner 《Annales Geophysicae》1999,17(7):941-956

During the Arctic Tropospheric Ozone Chemistry (ARCTOC) campaigns at Ny-Ålesund, Spitsbergen, the role of halogens in the depletion of boundary layer ozone was investigated. In spring 1995 and 1996 up to 30 ppt bromine monoxide were found whenever ozone decreased from normal levels of about 40 ppb. Those main trace gases and others were specifically followed in the UV-VIS spectral region by differential optical absorption spectroscopy (DOAS) along light paths running between 20 and 475 m a.s.l. The daily variation of peroxy radicals closely followed the ozone photolysis rate J(O₃(O¹D)) in the absence of ozone depletion most of the time. However, during low ozone events this close correlation was no longer found because the measurement of radicals by chemical amplification (CA) turned out to be sensitive to peroxy radicals and ClO_x. Large CA signals at night can sometimes definitely be assigned to ClO_x and reached up to 2 ppt. Total bromine and iodine were both stripped quantitatively from air by active charcoal traps and measured after neutron activation of the samples. Total bromine increased from background levels of about 15 ppt to a maximum of 90 ppt during an event of complete ozone depletion. For the spring season a strong source of bromine is identified in the pack ice region according to back trajectories. Though biogenic emission sources cannot be completely ruled out, a primary activation of halogenides by various oxidants seems to initiate an efficient autocatalytic process, mainly driven by ozone and light, on ice and perhaps on aerosols. Halogenides residing on pack ice surfaces are continuously oxidised by hypohalogenous acids releasing bromine and chlorine into the air. During transport and especially above open water this air mixes with upper layer pristine air. As large quantities of bromine, often in the form of BrO, have been observed at polar sunrise also around Antarctica, its release seems to be a natural phenomenon. The source strength of bromine from halogen activation on the pack ice, as based on the measured inorganic bromine levels, averages about 10¹² Br-atoms m⁻² s⁻¹ during sunlit periods in Arctic spring. The total source strength of inorganic bromine from sunlit polar regions may therefore amount to 30 kt y⁻¹.  相似文献   

Historical environmental changes at phalasarna harbor,West Crete     

P. A. Pirazzoli  J. Ausseil-Badie  P. Giresse  E. Hadjidaki  M. Arnold 《Geoarchaeology》1992,7(4):371-392

The ongoing archaeological excavations that started in 1986 on the site of the harbor of Phalasarna have confirmed that the ancient port described as a closed harbor by ancient geographers is, due to the regional uplift of west Crete, now situated on dry land, about 6.6 m above sea level. In this paper, after summarizing the geological background, the main historical sources and recent archaeological results, new stratigraphical data obtained from the sediments filling the harbor basin are presented and discussed. The harbor was fortified in the second part of the 4th century B.C. It was a military port, probably a base for pirates, and was destroyed and abandoned in the second part of the first century B.C. After that time the harbor basin was rapidly silted by marine then terrestrial sedimentation. Deposits corresponding to two tsunami waves have been identified and ascribed to events occurring in 66 A.D. and 365 A.D., respectively. About 1530 ± 40 yr B.P., probably in 365 A.D., when west Crete was suddenly uplifted by 6–9 m, Phalasarna harbor was removed permanently from marine influence. Radiometric dating confirms that the harbor could not have been in use in Roman times, in contradiction with ancient Periploi, which continue to mention Phalasarna port until at least the second or third century A.D.  相似文献   

Long-term changes in zooplankton and the climate of the North Atlantic   总被引：13，自引：3，他引：10  

Planque  Benjamin; Taylor  Arnold H. 《ICES Journal of Marine Science》1998,55(4):644-654

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Rebuilding viable spawner patches of the overfished Spisula solida (Mollusca: Bivalvia): a preliminary contribution to fishery sustainability     

Joaquim  Sandra; Gaspar  Miguel B.; Matias  Domitilia; Ben-Hamadou  Radhouan; Arnold  William S. 《ICES Journal of Marine Science》2008,65(1):60-64

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Deep-sea dune-like features     

Arnold H. Bouma  T.K. Treadwell 《Marine Geology》1975,19(5):M53-M59

Three different types of dune-like features in depths between 2,500 and 4,000 m were observed on what is supposed to be the outer Magdalena Fan, Colombian Basin. Each feature has its own characteristic height and length pattern as well as its location with regard to the local slope of the sea bottom. They have in common, however, that internal reflectors all dip in a westerly direction. Reflectivity changes on 3.5-kHz records may indicate the presence of textural variations.Since the area is generally considered to be void of any continuous, deep currents, it is suggested that these large bodies are fan lobes emplaced by high-viscosity turbidity currents or debris flows. Temporal, locally induced currents, moving at least partly in easterly direction, may have caused a winnowing effect resulting in textural variations.  相似文献   

San Quirce (Palencia,Spain): new chronologies for the Lower to Middle Palaeolithic transition of south-west Europe     

Marcos Terradillos-Bernal  Martina Demuro  Lee J. Arnold  Jesús F. Jordá-Pardo  Ignacio Clemente-Conte  Alfonso Benito-Calvo  J. Carlos Díez Fernández-Lomana 《第四纪科学杂志》2023,38(1):21-37

San Quirce is an open-air archaeological site situated on a fluvial terrace in the Duero basin (Palencia, northern Iberia). This paper presents new and consistent chronologies obtained for the sedimentary sequence using post-infrared infrared stimulated luminescence (pIR-IR) dating of K-feldspars and single-grain thermally transferred optically stimulated luminescence (TT-OSL) dating of quartz. The new dating results indicate that the sequence is older than ~200 000 years and place San Quirce Level III within marine isotope stages (MIS) 8 and 7, between 274 ± 13 ka and 238 ± 13 ka. The main lithic assemblage at San Quirce comes from Level III. The predominant tool types found in this level are hammerstones, manuports and flakes, with a small proportion of cores and a significant presence of denticulates. Adaptation to local environmental conditions resulted in distinctive cultural habits, which were embedded in the cultural tradition of hominins occupying the site during the final third of the Middle Pleistocene. San Quirce preserves a simple cultural tradition that was employed by local hominins to engage in a diverse array of activities, and highlights the cultural diversity that appears to have been a characteristic feature of the Lower to Middle Palaeolithic transition 300–200 ka.  相似文献   

On the date of construction of Lechaion,western harbor of ancient Corinth,Greece     

S. Stiros  P. Pirazzoli  R. Rothaus  S. Papageorgiou  J. Laborel  M. Arnold 《Geoarchaeology》1996,11(3):251-263

Lechaion, the western harbor of ancient Corinth, was an artificial harbor excavated in a marshy area and connected to the open sea through a channel with revetment walls. It was probably the most important harbor of this type in antiquity, and one of the most important harbors in Greece for more than one millennium. Yet, the date of construction of this harbor is a matter of debate; 600 B.C., ca. 44 B.C., and A.D. 350 are the most probable dates. Geomorphological and biological investigations, in combination with AMS radiocarbon dating of exposed marine shells found in the walls of the channel leading to the inner basin of the harbor indicate that the channel was open to the sea before a seismic land uplift that probably occurred sometime in the fifth to third century B.C. These data indicate that the construction of the harbor began in ca. 600 B.C., the period of Corinthian expansion to the Ionian Sea and southern Italy. © 1996 John Wiley & Sons, Inc.  相似文献