16,199 research outputs found
A unified wavelet-based modelling framework for non-linear system identification: the WANARX model structure
A new unified modelling framework based on the superposition of additive submodels, functional components, and
wavelet decompositions is proposed for non-linear system identification. A non-linear model, which is often represented
using a multivariate non-linear function, is initially decomposed into a number of functional components via the wellknown
analysis of variance (ANOVA) expression, which can be viewed as a special form of the NARX (non-linear
autoregressive with exogenous inputs) model for representing dynamic input–output systems. By expanding each functional
component using wavelet decompositions including the regular lattice frame decomposition, wavelet series and
multiresolution wavelet decompositions, the multivariate non-linear model can then be converted into a linear-in-theparameters
problem, which can be solved using least-squares type methods. An efficient model structure determination
approach based upon a forward orthogonal least squares (OLS) algorithm, which involves a stepwise orthogonalization
of the regressors and a forward selection of the relevant model terms based on the error reduction ratio (ERR), is
employed to solve the linear-in-the-parameters problem in the present study. The new modelling structure is referred to
as a wavelet-based ANOVA decomposition of the NARX model or simply WANARX model, and can be applied to
represent high-order and high dimensional non-linear systems
The NWRA Classification Infrastructure: Description and Extension to the Discriminant Analysis Flare Forecasting System (DAFFS)
A classification infrastructure built upon Discriminant Analysis has been
developed at NorthWest Research Associates for examining the statistical
differences between samples of two known populations. Originating to examine
the physical differences between flare-quiet and flare-imminent solar active
regions, we describe herein some details of the infrastructure including:
parametrization of large datasets, schemes for handling "null" and "bad" data
in multi-parameter analysis, application of non-parametric multi-dimensional
Discriminant Analysis, an extension through Bayes' theorem to probabilistic
classification, and methods invoked for evaluating classifier success. The
classifier infrastructure is applicable to a wide range of scientific questions
in solar physics. We demonstrate its application to the question of
distinguishing flare-imminent from flare-quiet solar active regions, updating
results from the original publications that were based on different data and
much smaller sample sizes. Finally, as a demonstration of "Research to
Operations" efforts in the space-weather forecasting context, we present the
Discriminant Analysis Flare Forecasting System (DAFFS), a near-real-time
operationally-running solar flare forecasting tool that was developed from the
research-directed infrastructure.Comment: J. Space Weather Space Climate: Accepted / in press; access
supplementary materials through journal; some figures are less than full
resolution for arXi
Interplanetary Particle Environment. Proceedings of a Conference
A workshop entitled the Interplanetary Charged Particle Environment was held at the Jet Propulsion Laboratory (JPL) on March 16 and 17, 1987. The purpose of the Workshop was to define the environment that will be seen by spacecraft operating in the 1990s. It focused on those particles that are involved in single event upset, latch-up, total dose and displacement damage in spacecraft microelectronic parts. Several problems specific to Magellan were also discussed because of the sensitivity of some electronic parts to single-event phenomena. Scientists and engineers representing over a dozen institutions took part in the meeting. The workshop consisted of two major activities, reviews of the current state of knowledge and the formation of working groups and the drafting of their reports
Parametrizing the time-variation of the "surface term" of stellar p-mode frequencies: application to helioseismic data
The solar-cyle variation of acoustic mode frequencies has a frequency
dependence related to the inverse mode inertia. The discrepancy between model
predictions and measured oscillation frequencies for solar and solar-type
stellar acoustic modes includes a significant frequency-dependent term known as
the surface term that is also related to the inverse mode inertia. We
parametrize both the surface term and the frequency variations for low-degree
solar data from Birmingham Solar-Oscillations Network (BiSON) and medium-degree
data from the Global Oscillations Network Group (GONG) using the mode inertia
together with cubic and inverse frequency terms. We find that for the central
frequency of rotationally split multiplets the cubic term dominates both the
average surface term and the temporal variation, but for the medium-degree case
the inverse term improves the fit to the temporal variation. We also examine
the variation of the even-order splitting coefficients for the medium-degree
data and find that, as for the central frequency, the latitude-dependent
frequency variation, which reflects the changing latitudinal distribution of
magnetic activity over the solar cycle, can be described by the combination of
a cubic and an inverse function of frequency scaled by inverse mode inertia.
The results suggest that this simple parametrization could be used to assess
the activity-related frequency variation in solar-like asteroseismic targets.Comment: 13 pages, 11 figures. Accepted by MNRAS 13 October 201
Magnetic effects on the low-T/|W| instability in differentially rotating neutron stars
Dynamical instabilities in protoneutron stars may produce gravitational waves
whose observation could shed light on the physics of core-collapse supernovae.
When born with sufficient differential rotation, these stars are susceptible to
a shear instability (the "low-T/|W| instability"), but such rotation can also
amplify magnetic fields to strengths where they have a considerable impact on
the dynamics of the stellar matter. Using a new magnetohydrodynamics module for
the Spectral Einstein Code, we have simulated a differentially-rotating neutron
star in full 3D to study the effects of magnetic fields on this instability.
Though strong toroidal fields were predicted to suppress the low-T/|W|
instability, we find that they do so only in a small range of field strengths.
Below 4e13 G, poloidal seed fields do not wind up fast enough to have an effect
before the instability saturates, while above 5e14 G, magnetic instabilities
can actually amplify a global quadrupole mode (this threshold may be even lower
in reality, as small-scale magnetic instabilities remain difficult to resolve
numerically). Thus, the prospects for observing gravitational waves from such
systems are not in fact diminished over most of the magnetic parameter space.
Additionally, we report that the detailed development of the low-T/|W|
instability, including its growth rate, depends strongly on the particular
numerical methods used. The high-order methods we employ suggest that growth
might be considerably slower than found in some previous simulations.Comment: REVTeX 4.1, 21 pages, 18 figures, submitting to Physical Review
Numerical Non-Linear Modelling Algorithm Using Radial Kernels on Local Mesh Support
Estimation problems are frequent in several fields such as engineering, economics, and physics, etc. Linear and non-linear regression are powerful techniques based on optimizing an error defined over a dataset. Although they have a strong theoretical background, the need of supposing an analytical expression sometimes makes them impractical. Consequently, a group of other approaches and methodologies are available, from neural networks to random forest, etc. This work presents a new methodology to increase the number of available numerical techniques and corresponds to a natural evolution of the previous algorithms for regression based on finite elements developed by the authors improving the computational behavior and allowing the study of problems with a greater number of points. It possesses an interesting characteristic: Its direct and clear geometrical meaning. The modelling problem is presented from the point of view of the statistical analysis of the data noise considered as a random field. The goodness of fit of the generated models has been tested and compared with some other methodologies validating the results with some experimental campaigns obtained from bibliography in the engineering field, showing good approximation. In addition, a small variation on the data estimation algorithm allows studying overfitting in a model, that it is a problematic fact when numerical methods are used to model experimental values.This research has been partially funded by the Spanish Ministry of Science, Innovation and Universities, grant number RTI2018-101148-B-I00
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