2,487 research outputs found
Observation of outer planets at Lyman alpha
A triple planetary observation in one IUE shift was conducted to measure the Lyman alpha reflectivity of Jupiter, Saturn and Uranus. The exposures were planned to take account of the light travel times, Sun to planet and planet to Earth, in order to assess the response of the three atmospheres to essentially the same incident solar flux. All additional uncertainties that were introduced when different instruments were used for such a comparative measurement were automatically eliminated
The Bonn University lidar at the Esrange: technical description and capabilities for atmospheric research
International audienceThe Bonn University operates a Rayleigh/Mie/Raman backscatter lidar at the Esrange near the Swedish city of Kiruna, north of the Arctic circle. The lidar system covers the atmosphere from about 4 km to 100 km altitude and it is capable of measuring aerosols in the tropo-, strato-, and mesosphere, as well as of determining temperature profiles in the aerosol-free part of the atmosphere (i.e. above 30-km altitude). Density tuned fixed-spacer etalons provide daylight capability and thus increased sensitivity to noctilucent clouds during polar summer. Polarisation measurements allow liquid and solid phase discrimination for aerosol and cloud particles in the tropo- and stratosphere. The derived temperature profiles can be used for the detection and analysis of atmospheric gravity waves. Although several lidar experiments are situated in polar latitudes, a comprehensive instrument which covers the troposphere, stratosphere, and mesosphere, is daylight capable, and observes temperature profiles, as well as aerosols, is exceptional. In this article a technical description, in particular, of the optical configuration of this experiment is given, as well as an overview of achievable geophysical parameters. The potential for geophysical analyses is shown. Keywords. Atmospheric composition and structure (Instruments and techniques; Aerosols and particles; Pressure, density and temperature) ? Meteorology and atmospheric dynamics (Middle atmosphere dynamics
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World-models for bitrate streaming
Adaptive bitrate (ABR) algorithms optimize the quality of streaming experiences for users in client-side video players, especially in unreliable or slow mobile networks. Several rule-based heuristic algorithms can achieve stable performance, but they sometimes fail to properly adapt to changing network conditions. Fluctuating bandwidth may cause algorithms to default to behavior that creates a negative experience for the user. ABR algorithms can be generated with reinforcement learning, a decision-making paradigm in which an agent learns to make optimal choices through interactions with an environment. Training reinforcement learning algorithms for bitrate streaming requires building a simulator for an agent to experience interactions quickly; training an agent in the real environment is infeasible due to the long step times in real environments. This project explores using supervised learning to construct a world-model, or a learned simulator, from recorded interactions. A reinforcement learning agent that is trained inside of the learned model, rather than a simulator, can outperform rule-based heuristics. Furthermore, agents that are trained inside the learned world-model can outperform model-free agents in low sample regimes. This work highlights the potential for world-models to quickly learn simulators, and to be used for generating optimal policies.</jats:p
The South African coelacanths — an account of what is known after three submersible expeditions
Using the manned submersible Jago, the habits, distribution and number of coelacanths within all main submarine canyons of the Greater St Lucia Wetland Park were studied during 47 survey dives, with a total bottom time of 166 hours at depths ranging from 46 to 359 m, between 2002 and 2004. Twenty-four individuals were positively identified from three of the canyons, primarily from inside caves at or close to the canyon edges at depths of 96–133 m with water temperatures between 16 and 22.5°C. The population size of coelacanths within the canyons is assumed to be relatively small; coelacanths are resident but not widespread nor abundant within the park
Fishes of the deep demersal habitat at Ngazidja (Grand Comoro) Island, Western Indian Ocean
Underwater observations of the coelacanth, Latimeria chalumnae Smith, 1939, from a research submersible provided opportunities to study the deep demersal fish fauna at the Comoro Islands. The demersal habitat in depths of 150–400 m at the volcanic island of Ngazidja is low in fish diversity and biomass, compared with the shallow-water coral reef habitat of Ngazidja or the deep demersal habitats of other localities in the Indo-Pacific region. The resident deep demersal fish fauna at Ngazidja is dominated by the coelacanth, an ancient predator that is specially adapted for this low-energy environment. Other large fish predators are scarce in this environment, because of the heavy fishing pressure from local fishermen. Eighty-nine fish taxa (including 65 recognizable species) were recorded from videotapes, photographs, visual observations, fishermen’s catches and ancillary attempts to sample the fish fauna with baited fish traps, gill nets, and hook and line. Although no coelacanth feeding events were seen, seven fish species are known from coelacanth stomach contents, and 64 other fish species in this habitat are considered potential prey of this dominant predator
The population biology of the living coelacanth studied over 21Â years
Between 1986 and 2009 nine submersible and
remote-operated vehicle expeditions were carried out to
study the population biology of the coelacanth Latimeria
chalumnae in the Comoro Islands, located in the western
Indian Ocean. Latimeria live in large overlapping home
ranges that can be occupied for as long as 21 years. Most
individuals are confined to relatively small home ranges,
resting in the same caves during the day. One hundred and
forty five coelacanths are individually known, and we
estimate the total population size of Grande Comore as
approximately 300–400 adult individuals. The local population
inhabiting a census area along an 8-km section of
coastline remained stable for at least 18 years. Using
LASER-assisted observations, we recorded length frequencies
between 100 and 200 cm total length and did not
encounter smaller-bodied individuals (\100 cm total
length). It appears that coelacanth recruitment in the
observation areas occur mainly by immigrating adults. We
estimate that the mean numbers of deaths and newcomers
are 3–4 individuals per year, suggesting that longevity may
exceed 100 years. The domestic fishery represents a threat
to the long-term survival of coelacanths in the study area.
Recent changes in the local fishery include a decrease in
the abundance of the un-motorized canoes associated with
exploitation of coelacanths and an increase in motorized
canoes. Exploitation rates have fallen in recent years, and
by 2000, had fallen to lowest ever reported. Finally, future
fishery developments are discussed
First observation of one noctilucent cloud by a twin lidar in two different directions
International audienceIn the early morning hours of 14 July 1999, a noctilucent cloud (NLC) was observed simultaneously by the two branches of a twin lidar system located at the ALOMAR observatory in northern Norway (69° N). The telescopes of the two lidars were pointing vertical (L^) and off the zenith by 30° (L30°). The two lidars detected an enhancement in the altitude profile of backscattered light (relative to the molecular background) for more than 5 h, starting approximately at 01:00 UT. These measurements constitute the detection of one NLC by two lidars under different directions and allow for a detailed study of the morphology of the NLC layer. A cross-correlation analysis of the NLC signals demonstrates that the main structures seen by both lidars are practically identical. This implies that a temporal evolution of the microphysics within the NLC during its drift from one lidar beam to the other is negligible. From the time delay of the NLC structures, a drift velocity of 55?65 m/s is derived which agrees nicely with radar wind measurements. During the observation period, the mean NLC altitude decreases by ~0.5 km/h (=14 cm/s) at both observation volumes. Further-more, the NLC is consistently observed approximately 500 m lower in altitude at L30° compared to L^. Supplementing these data by observations from rocket-borne and ground-based instruments, we show that the general downward progression of the NLC layer through the night, as seen by both lidars, is caused by a combination of particle sedimentation by 4?5 cm/s and a downward directed vertical wind by 9?10 cm/s, whereas a tilt of the layer in drift direction can be excluded
Simultaneous lidar observations of temperatures and waves in the polar middle atmosphere on the east and west side of the Scandinavian mountains: a case study on 19/20 January 2003
Atmospheric gravity waves have been the subject of intense research for several decades because of their extensive effects on the atmospheric circulation and the temperature structure. The U. Bonn lidar at the Esrange and the ALOMAR RMR lidar at the Andøya Rocket Range are located in northern Scandinavia 250 km apart on the east and west side of the Scandinavian mountain ridge. During January and February 2003 both lidar systems conducted measurements and retrieved atmospheric temperatures. On 19/20 January 2003 simultaneous measurements for more than 7 h were possible. Although during most of the campaign time the atmosphere was not transparent for the propagation of orographically induced gravity waves, they were nevertheless observed at both lidar stations with considerable amplitudes during these simultaneous measurements. And while the source of the observed waves cannot be determined unambiguously, the observations show many characteristics of orographically excited gravity waves. The wave patterns at ALOMAR show a random distribution with time whereas at the Esrange a persistency in the wave patterns is observable. This persistency can also be found in the distribution of the most powerful vertical wavelengths. The mode values are both at about 5 km vertical wavelength, however the distributions are quite different, narrow at the Esrange with values from λ<i><sub>z</sub></i>=2–6 km and broad at ALOMAR, covering λ<i><sub>z</sub></i>=1–12 km vertical wavelength. In particular the difference between the observations at ALOMAR and at the Esrange can be understood by different orographic conditions while the propagation conditions were quite similar. At both stations the waves deposit energy in the atmosphere with increasing altitude, which leads to a decrease of the observed gravity wave potential energy density with altitude. The meteorological situation during these measurements was different from common winter situations. The ground winds were mostly northerlies, changed in the upper troposphere and lower stratosphere to westerlies and returned to northerlies in the middle stratosphere
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