165 research outputs found
Mining Missing Hyperlinks from Human Navigation Traces: A Case Study of Wikipedia
Hyperlinks are an essential feature of the World Wide Web. They are
especially important for online encyclopedias such as Wikipedia: an article can
often only be understood in the context of related articles, and hyperlinks
make it easy to explore this context. But important links are often missing,
and several methods have been proposed to alleviate this problem by learning a
linking model based on the structure of the existing links. Here we propose a
novel approach to identifying missing links in Wikipedia. We build on the fact
that the ultimate purpose of Wikipedia links is to aid navigation. Rather than
merely suggesting new links that are in tune with the structure of existing
links, our method finds missing links that would immediately enhance
Wikipedia's navigability. We leverage data sets of navigation paths collected
through a Wikipedia-based human-computation game in which users must find a
short path from a start to a target article by only clicking links encountered
along the way. We harness human navigational traces to identify a set of
candidates for missing links and then rank these candidates. Experiments show
that our procedure identifies missing links of high quality
Meteor trail characteristics observed by high time resolution lidar
We report and analyse the characteristics of 1382 meteor trails based on a sodium data set of ∼680 h. The observations were made at Yanqing (115.97° E, 40.47° N), China by a ground-based Na fluorescence lidar. The temporal resolution of the raw profiles is 1.5 s and the altitude resolution is 96 m. We discover some characteristics of meteor trails different from those presented in previous reports. The occurrence heights of the trails follow a double-peak distribution with the peaks at ∼83.5 km and at ∼95.5 km, away from the peak height of the regular Na layer. 4.7% of the trails occur below 80 km, and 3.25% above 100 km. 75% of the trails are observed in only one 1.5 s profile, suggesting that the dwell time in the laser beam is not greater than 1.5 s. The peak density of the trails as a function of height is similar to that of the background sodium layer. The raw occurrence height distribution is corrected taking account of three factors which affect the relative lifetime of a trail as a function of height: the meteoroid velocity (which controls the ratio of Na / Na+ ablated); diffusional spreading of the trail; and chemical removal of Na. As a result, the bi-modal distribution is more pronounced. Modelling results show that the higher peak corresponds to a meteoroid population with speeds between 20 and 30 km s?1, whereas the lower peak should arise from much slower particles in a near-prograde orbit. It is inferred that most meteoroids in this data set have masses of ∼1 mg, in order for ablation to produce sufficient Na atoms to be detected by lidar. Finally, the evolution of longer-duration meteor trails is investigated. Signals at each altitude channel consist of density enhancement bursts with the growth process usually faster than the decay process, and there exists a progressive phase shift among these altitude channels
Measuring the Absolute Height and Profile of the Mesospheric Sodium Layer using a Continuous Wave Laser
We have developed and tested a novel method, based on LIDAR, of measuring the
height and profile of the mesospheric sodium layer using a continuous wave
laser. It is more efficient than classical LIDAR as the laser is on for 50% of
the time, and so can in principle be used during laser guide star adaptive
optics observations. It also has significant advantages over direct imaging
techniques because it does not require a second telescope, is almost
independent of the atmospheric conditions, and avoids triangulation problems in
determining the height. In the long term, regular monitoring using this method
would allow a valuable database of sodium layer profiles, heights, and return
flux measurements to be built up which would enable observatory staff
astronomers to schedule observations optimally. In this paper we describe the
original experiment carried out using the ALFA laser guide star system at Calar
Alto Observatory in Spain. We validate the method by comparing the LIDAR
results with those obtained from simultaneous imaging from an auxiliary
telescope. Models are presented of a similar system to be implemented in the
Very Large Telescope Laser Guide Star Facility, which will enable the initial
focus setting for the adaptive optics systems to be determined with an accuracy
of less than 200 m on a timescale of 1 minute.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 14
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Precipitation regime change in Western North America: The role of Atmospheric Rivers.
Daily precipitation in California has been projected to become less frequent even as precipitation extremes intensify, leading to uncertainty in the overall response to climate warming. Precipitation extremes are historically associated with Atmospheric Rivers (ARs). Sixteen global climate models are evaluated for realism in modeled historical AR behavior and contribution of the resulting daily precipitation to annual total precipitation over Western North America. The five most realistic models display consistent changes in future AR behavior, constraining the spread of the full ensemble. They, moreover, project increasing year-to-year variability of total annual precipitation, particularly over California, where change in total annual precipitation is not projected with confidence. Focusing on three representative river basins along the West Coast, we show that, while the decrease in precipitation frequency is mostly due to non-AR events, the increase in heavy and extreme precipitation is almost entirely due to ARs. This research demonstrates that examining meteorological causes of precipitation regime change can lead to better and more nuanced understanding of climate projections. It highlights the critical role of future changes in ARs to Western water resources, especially over California
Diurnal Variation in Gravity Wave Activity at Low and Middle Latitudes
We employ a modified composite day extension of the Hocking (2005) analysis method to study gravity wave (GW) activity in the mesosphere and lower thermosphere using 4 meteor radars spanning latitudes from 7deg S to 53.6deg S. Diurnal and semidiurnal modulations were observed in GW variances over all sites. Semidiurnal modulation with downward phase propagation was observed at lower latitudes mainly near the equinoxes. Diurnal modulations occur mainly near solstice and, except for the zonal component at Cariri (7deg S), do not exhibit downward phase propagation. At a higher latitude (SAAMER, 53.6deg S) these modulations are only observed in the meridional component where we can observe diurnal variation from March to May, and semidiurnal, during January, February, October (above 88 km) and November. Some of these modulations with downward phase progression correlate well with wind shear. When the wind shear is well correlated with the maximum of the variances the diurnal tide has its largest amplitudes, i.e., near equinox. Correlations exhibiting variations with tidal phases suggest significant GW-tidal interactions that have different characters depending on the tidal components and possible mean wind shears. Modulations that do not exhibit phase variations could be indicative of diurnal variations in GW sources
MultipleWavelength Optical Observations of a Long-lived Meteor Trail
A long‐lived meteor trail has been observed at wavelengths of 572.5 nm, 557.7 nm, 630.0 nm, 865.5 nm and in the near infrared band from 715 to 930 nm. The trail was detected at all these wavelengths, with the possible exception of 865.5 nm, where its identification was marginal. It was seen longest (17 minutes) through the wide band NIR 715‐930 nm filter. The fact that the trail was only marginally visible in the 865.5 nm (0–1) band of molecular oxygen, and was strongest in the wide‐band NIR image, raises serious doubts about an earlier suggestion that the infrared light from long‐lived meteor trails corresponds to emissions from molecular oxygen excited by the Chapman mechanism
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