208,205 research outputs found
Method and apparatus for calibrating the ionosphere and application to surveillance of geophysical events
The columnar electron content of the ionosphere between a spacecraft and a receiver is measured in realtime by cross correlating two coherently modulated signals transmitted at different frequencies (L1,L2) from the spacecraft to the receiver using a cross correlator. The time difference of arrival of the modulated signals is proportional to electron content of the ionosphere. A variable delay is adjusted relative to a fixed delay in the respective channels (L1,L2) to produce a maximum at the cross correlator output. The difference in delay required to produce this maximum is a measure of the columnar electron content of the ionosphere. A plurality of monitoring stations and spacecraft (Global Positioning System satellites) are employed to locate any terrestrial event that produces an ionospheric disturbance
The blogosphere as an excitable social medium: Richter's and Omori's Law in media coverage
We study the dynamics of public media attention by monitoring the content of
online blogs. Social and media events can be traced by the propagation of word
frequencies of related keywords. Media events are classified as exogenous -
where blogging activity is triggered by an external news item - or endogenous
where word frequencies build up within a blogging community without external
influences. We show that word occurrences show statistical similarities to
earthquakes. The size distribution of media events follows a Gutenberg-Richter
law, the dynamics of media attention before and after the media event follows
Omori's law. We present further empirical evidence that for media events of
endogenous origin the overall public reception of the event is correlated with
the behavior of word frequencies at the beginning of the event, and is to a
certain degree predictable. These results may imply that the process of opinion
formation in a human society might be related to effects known from excitable
media.Comment: 4 pages, 4 figure
On the phenomenological classification of continuum radio spectra variability patterns of Fermi blazars
The F-GAMMA program is a coordinated effort to investigate the physics of
Active Galactic Nuclei (AGNs) via multi-frequency monitoring of {\em Fermi}
blazars. The current study is concerned with the broad-band radio spectra
composed of measurement at ten frequencies between 2.64 and 142 GHz. It is
shown that any of the 78 sources studied can be classified in terms of their
variability characteristics in merely 5 types of variability. The first four
types are dominated by spectral evolution and can be reproduced by a simple
two-component system made of the quiescent spectrum of a large scale jet
populated with a flaring event evolving according to Marscher & Gear (1985).
The last type is characterized by an achromatic change of the broad-band
spectrum which must be attributed to a completely different mechanism. Here are
presented, the classification, the assumed physical system and the results of
simulations that have been conducted.Comment: 2011 Fermi Symposium proceedings - eConf C11050
LIPIcs
The monitoring of event frequencies can be used to recognize behavioral anomalies, to identify trends, and to deduce or discard hypotheses about the underlying system. For example, the performance of a web server may be monitored based on the ratio of the total count of requests from the least and most active clients. Exact frequency monitoring, however, can be prohibitively expensive; in the above example it would require as many counters as there are clients. In this paper, we propose the efficient probabilistic monitoring of common frequency properties, including the mode (i.e., the most common event) and the median of an event sequence. We define a logic to express composite frequency properties as a combination of atomic frequency properties. Our main contribution is an algorithm that, under suitable probabilistic assumptions, can be used to monitor these important frequency properties with four counters, independent of the number of different events. Our algorithm samples longer and longer subwords of an infinite event sequence. We prove the almost-sure convergence of our algorithm by generalizing ergodic theory from increasing-length prefixes to increasing-length subwords of an infinite sequence. A similar algorithm could be used to learn a connected Markov chain of a given structure from observing its outputs, to arbitrary precision, for a given confidence
Radio jet emission from GeV-emitting narrow-line Seyfert 1 galaxies
We studied the radio emission from four radio-loud and gamma-ray-loud
narrow-line Seyfert 1 galaxies. The goal was to investigate whether a
relativistic jet is operating at the source, and quantify its characteristics.
We relied on the most systematic monitoring of such system in the cm and mm
radio bands which is conducted with the Effelsberg 100 m and IRAM 30 m
telescopes and covers the longest time-baselines and the most radio frequencies
to date. We extract variability parameters and compute variability brightness
temperatures and Doppler factors. The jet powers were computed from the light
curves to estimate the energy output. The dynamics of radio spectral energy
distributions were examined to understand the mechanism causing the
variability. All the sources display intensive variability that occurs at a
pace faster than what is commonly seen in blazars. The flaring events show
intensive spectral evolution indicative of shock evolution. The brightness
temperatures and Doppler factors are moderate, implying a mildly relativistic
jet. The computed jet powers show very energetic flows. The radio polarisation
in one case clearly implies a quiescent jet underlying the recursive flaring
activity. Despite the generally lower flux densities, the sources appear to
show all typical characteristics seen in blazars that are powered by
relativistic jets.Comment: Accepted for publication in 4 - Extragalactic astronomy of Astronomy
and Astrophysic
Human response to vibration in residential environments (NANR209), technical report 3 : calculation of vibration exposure
The Technical Report 3 describes the research undertaken to develop a methodology by which human exposure to vibration in residential environments can be calculated. That work has carried out by the University of Salford supported by the Department of environment food and rural affairs (Defra).
The overall aim of the project is to derive exposure-response relationships for human vibration in residential environments. This document in particular focuses on the methods used to calculate vibration exposure from measured vibration signals due to different sources.
The main objective of this report is to describe the different approaches used for calculating the different source-specific exposure. Reported here are findings obtained and a description of the feasibility of the methods used for evaluating exposure for different sources. In addition, an evaluation of the uncertainty related to the exposure calculation is considered
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