Effects of the El Chichon volcanic cloud in the stratosphere on the polarization of light from the sky

A dense volcanic cloud from the El Chichon volcanic eruption has been observed in the stratosphere over Hawaii since it was first discovered at the Mauna Loa Observatory 9 Apr. 1982. Lidar observations have shown the cloud to have been dense and highly layered in its early stages, but as the cloud matured it became more homogeneous and the top portion underwent considerable enhancement. Measurements of the degree of polarization of skylight at the zenith and across the sky in the sun's vertical show that the polarization field is strongly modified by the effects of the cloud and that the modifications are of a different nature from those produced by high turbidity in the lower layers of the atmosphere. The degree of polarization at the zenith during twilight shows a secondary maximum at a solar depression D = 4.8-5 degrees, a secondary minimum at D = 4 degrees, a primary maximum at D = 1-2 degrees, and a rapid decrease to values generally <10% in the immediate sunrise period. The positions of the neutral points are strongly affected by the cloud, the Arago point being shifted from its normal position by as much as 15-20 degrees and the Babinet point being shifted even farther. Multiple Babinet points were observed on some occasions. The measurements indicate the polarization field to be modified more by the El Chichon cloud than it was by the clouds from previous eruptions which have occurred during this century.     

Transporting QoS adaptive flows

Distributed audio and video applications need to adapt to fluctuations in delivered quality of service (QoS). By trading off temporal and spatial quality to available bandwidth, or manipulating the playout time of continuous media in response to variation in delay, audio and video flows can be made to adapt to fluctuating QoS with minimal perceptual distortion. In this paper, we extend our previous work on a QoS Architecture (QoS-A) by populating the QoS management planes of our architecture with a framework for the control and management of multilayer coded flows operating in heterogeneous multimedia networking environments. Two key techniques are proposed: i) an end-to-end rate-shaping scheme which adapts the rate of MPEG-coded flows to the available network resources while minimizing the distortion observed at the receiver; and ii) an adaptive network service, which offers “hard” guarantees to the base layer of multilayer coded flows and “fairness” guarantees to the enhancement layers based on a bandwidth allocation technique called Weighted Fair Sharing.     

Solar radiation incident upon slopes of different orientations

By means of a pyranometer oriented at various tilt angles and azimuth angles with respect to the Sun, measurements of the direct and diffuse solar flux incident on slopes of various orientations have been made for clear sky conditions. The resulting data have been used to evaluate the validity of the solar flux model of Robinson[1] and to introduce certain modifications of that model to achieve better agreement of computations with measurements. It is concluded that the modified model yields significantly better results than the original for slopes oriented in the azimuth of the Sun, but results are comparable for other azimuthal orientations of the slope. The accuracy attainable by this modeling approach is considered sufficient for many practical applications.     

Statistics of differential rain attenuation on adjacent earth-space propagation paths

Forty hours of observations of the three-dimensional radar reflectivity structure of rain in the Montreal area were used to simulate the centimetre-wavelength attenuations occurring simultaneously over a multitude of earth-space propagation paths. The same data base had been used in a previous study which confirmed that the radar estimates of attenuation statistics are in good agreement with attenuation distributions measured independently by passive radiometers. Differences between the attenuations occurring simultaneously over pairs of paths to a common point at the ground were tabulated as a function of angular path separation, viewing direction, radio frequency, and system margin. These data are required for evaluating the possibility of interference at an earth station caused by a signal from an adjacent satellite. In this study, values of angular separation from 2 to 9 degrees, radio frequencies from 11 to 30 GHz, elevation angles from 5 to 30 degrees, and system margins from 2 to 10 dB were considered. It was found that the cumulative distributions of differential attenuation could be characterized effectively by the attenuation difference occurring during 1 % of the time that the carrier signal suffered some attenuation, but not an attenuation exceeding the system margin. It was also found that this characteristic differential attenuation could be expressed to good approximation as a simple analytical function of the system parameters.     

Nurturing health-related online support groups: Exploring the experiences of patient moderators

The aim of this study was to examine the views of moderators across a diverse and geographically broad range of online support groups about their moderator experiences and to explore both the personal benefits as well as challenges involved. Thirty-three patient moderators completed an online questionnaire which included a series of open-ended questions. Thematic analysis identified three themes: emergence, empowerment, nurturing. Several moderators declared their own diagnosis and for some, being able to share personal insights motivated them to establish the group and in turn offered validation. They felt empowered by helping others and learned more about the condition through accessing the “communal brain”. Some felt the group aided patients’ access to health services and their ability to communicate with health professionals while others worried about them becoming over-dependent. Moderators described needing to nurture their group to ensure it offered a safe space for members. Clear rules of engagement, trust, organisation skills, compassion and kindness were considered essential. Patient moderated online support groups can be successfully developed and facilitated and can be empowering for both the group member and moderator alike.     

Free riding on Gnutella revisited: the bell tolls?

A new analysis of free riding on the Gnutella network updates data from 2000 and points to an increasing downgrade in the network's overall performance and the emergence of a "metatragedy" of the commons. Individuals who use peer-to-peer (P2P) file-sharing networks such as Gnutella face a social dilemma. They must decide whether to contribute to the common good by sharing files or maximize their personal experience by free riding, downloading files while not contributing any to the network. Individuals gain no personal benefits from uploading files (in fact, it's inconvenient), so it's "rational" for users to free ride. However, significant numbers of free riders degrade the entire system's utility, creating a "tragedy of the digital commons."     

Right Hemisphere Sensitivity to Word- and Sentence-Level Context: Evidence From Event-Related Brain Potentials.

Researchers using lateralized stimuli have suggested that the left hemisphere is sensitive to sentence-level context, whereas the right hemisphere (RH) primarily processes word-level meaning. The authors investigated this message-blind RH model by measuring associative priming with event-related brain potentials (ERPs). For word pairs in isolation, associated words elicited more positive ERPs than unassociated words with similar magnitudes and onset latencies in both visual fields. Embedded in sentences, these same pairs showed large sentential context effects in both fields. Small effects of association were observed, confined to incongruous sentences after right visual hemifield presentation but present for both congruous and incongruous sentences after left visual hemifield presentation. Results do not support the message-blind RH model but do suggest hemispheric asymmetries in the use of word and sentence context during real-time processing. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Excess delay estimation from time-varying mobile radio channelimpulse response measurements

This letter proposes a method for estimating the instantaneous excess delay in time-varying mobile radio channel impulse response measurement data using image processing techniques. The method is shown by example to produce more accurate mean power delay profiles than those produced assuming either constant or linearly varying excess delay     

Standards and platforms for open distributed processing

There has recently been considerable research and development in the field of open distributed processing. The goal of this work is to enable access to distributed services without concern for the underlying environment. A number of standards and platforms have emerged to achieve this goal. This paper provides a tutorial introduction to these initiatives. In particular, the paper examines the ISO standard for a reference model for open distributed processing, OMG's CORBA and OSF's DCE. The paper concludes that these initiatives are different in scope and should be viewed as complementary. The paper also speculates on a path to convergence between the three initiatives although a number of issues remain to be solved before such a convergence can become a reality