2,828 research outputs found
Evidence for Partial Taylor Relaxation from Changes in Magnetic Geometry and Energy during a Solar Flare
Solar flares are powered by energy stored in the coronal magnetic field, a
portion of which is released when the field reconfigures into a lower energy
state. Investigation of sunspot magnetic field topology during flare activity
is useful to improve our understanding of flaring processes. Here we
investigate the deviation of the non-linear field configuration from that of
the linear and potential configurations, and study the free energy available
leading up to and after a flare. The evolution of the magnetic field in NOAA
region 10953 was examined using data from Hinode/SOT-SP, over a period of 12
hours leading up to and after a GOES B1.0 flare. Previous work on this region
found pre- and post-flare changes in photospheric vector magnetic field
parameters of flux elements outside the primary sunspot. 3D geometry was thus
investigated using potential, linear force-free, and non-linear force-free
field extrapolations in order to fully understand the evolution of the field
lines. Traced field line geometrical and footpoint orientation differences show
that the field does not completely relax to a fully potential or linear
force-free state after the flare. Magnetic and free magnetic energies increase
significantly ~ 6.5-2.5 hours before the flare by ~ 10^31 erg. After the flare,
the non-linear force-free magnetic energy and free magnetic energies decrease
but do not return to pre-flare 'quiet' values. The post-flare non-linear
force-free field configuration is closer (but not equal) to that of the linear
force-free field configuration than a potential one. However, the small degree
of similarity suggests that partial Taylor relaxation has occurred over a time
scale of ~ 3-4 hours.Comment: Accepted for Publication in Astronomy & Astrophysics. 11 pages, 11
figure
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Integrity static analysis of COTS/SOUP
This paper describes the integrity static analysis approach developed to support the justification of commercial off-the-shelf software (COTS) used in a safety-related system. The static analysis was part of an overall software qualification programme, which also included the work reported in our paper presented at Safecomp 2002. Integrity static analysis focuses on unsafe language constructs and “covert” flows, where one thread can affect the data or control flow of another thread. The analysis addressed two main aspects: the internal integrity of the code (especially for the more critical functions), and the intra-component integrity, checking for covert channels. The analysis process was supported by an aggregation of tools, combined and engineered to support the checks done and to scale as necessary. Integrity static analysis is feasible for industrial scale software, did not require unreasonable resources and we provide data that illustrates its contribution to the software qualification programme
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Stochastic modelling of the effects of interdependencies between critical infrastructure
An approach to Quantitative Interdependency Analysis, in the context of Large Complex Critical Infrastructures, is presented in this paper. A Discrete state–space, Continuous–time, Stochastic Process models the operation of critical infrastructure, taking interdependencies into account. Of primary interest are the implications of both model detail (that is, level of model abstraction) and model parameterisation for the study of dependencies. Both of these factors are observed to affect the distribution of cascade–sizes within and across infrastructure
Validating performance of automotive materials at high strain rate for improved crash design
This paper investigates sources of performance variability in high velocity testing of automotive crash structures. Sources of variability, or so called noise factors, present in a testing environment, arise from uncertainty in structural properties, joints, boundary conditions and measurement system. A box structure, which is representative of a crash component, is designed and fabricated from a high strength Dual Phase sheet steel. Crush tests are conducted at low and high speed. Such tests intend to validate a component model and material strain rate sensitivity data determined from high speed tensile testing. To support experimental investigations, stochastic modeling is used to investigate the effect of noise factors on crash structure performance variability, and to identify suitable performance measures to validate a component model and material strain rate sensitivity data. The results of the project will enable the measurement of more reliable strain rate sensitivity data for improved crashworthiness predictions of automotive structures
Performance of Major Flare Watches from the Max Millennium Program (2001-2010)
The physical processes that trigger solar flares are not well understood and
significant debate remains around processes governing particle acceleration,
energy partition, and particle and energy transport. Observations at high
resolution in energy, time, and space are required in multiple energy ranges
over the whole course of many flares in order to build an understanding of
these processes. Obtaining high-quality, co-temporal data from ground- and
space- based instruments is crucial to achieving this goal and was the primary
motivation for starting the Max Millennium program and Major Flare Watch (MFW)
alerts, aimed at coordinating observations of all flares >X1 GOES X-ray
classification (including those partially occulted by the limb). We present a
review of the performance of MFWs from 1 February 2001 to 31 May 2010,
inclusive, that finds: (1) 220 MFWs were issued in 3,407 days considered (6.5%
duty cycle), with these occurring in 32 uninterrupted periods that typically
last 2-8 days; (2) 56% of flares >X1 were caught, occurring in 19% of MFW days;
(3) MFW periods ended at suitable times, but substantial gain could have been
achieved in percentage of flares caught if periods had started 24 h earlier;
(4) MFWs successfully forecast X-class flares with a true skill statistic (TSS)
verification metric score of 0.500, that is comparable to a categorical
flare/no-flare interpretation of the NOAA Space Weather Prediction Centre
probabilistic forecasts (TSS = 0.488).Comment: 19 pages, 2 figures, accepted for publication in Solar Physic
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The impact of "difficulty" variation on the probability of coincident failure of diverse systems
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