236 research outputs found
The Specific Acceleration Rate in Loop-structured Solar Flares -- Implications for Electron Acceleration Models
We analyze electron flux maps based on RHESSI hard X-ray imaging spectroscopy
data for a number of extended coronal loop flare events. For each event, we
determine the variation of the characteristic loop length with electron
energy , and we fit this observed behavior with models that incorporate an
extended acceleration region and an exterior "propagation" region, and which
may include collisional modification of the accelerated electron spectrum
inside the acceleration region. The models are characterized by two parameters:
the plasma density in, and the longitudinal extent of, the
acceleration region. Determination of the best-fit values of these parameters
permits inference of the volume that encompasses the acceleration region and of
the total number of particles within it. It is then straightforward to compute
values for the emission filling factor and for the {\it specific acceleration
rate} (electrons s per ambient electron above a chosen reference
energy). For the 24 events studied, the range of inferred filling factors is
consistent with a value of unity. The inferred mean value of the specific
acceleration rate above keV is s, with a
1 spread of about a half-order-of-magnitude above and below this value.
We compare these values with the predictions of several models, including
acceleration by large-scale, weak (sub-Dreicer) fields, by strong
(super-Dreicer) electric fields in a reconnecting current sheet, and by
stochastic acceleration processes
A self-learning algorithm for biased molecular dynamics
A new self-learning algorithm for accelerated dynamics, reconnaissance
metadynamics, is proposed that is able to work with a very large number of
collective coordinates. Acceleration of the dynamics is achieved by
constructing a bias potential in terms of a patchwork of one-dimensional,
locally valid collective coordinates. These collective coordinates are obtained
from trajectory analyses so that they adapt to any new features encountered
during the simulation. We show how this methodology can be used to enhance
sampling in real chemical systems citing examples both from the physics of
clusters and from the biological sciences.Comment: 6 pages, 5 figures + 9 pages of supplementary informatio
A hybrid supervised/unsupervised machine learning approach to solar flare prediction
We introduce a hybrid approach to solar flare prediction, whereby a
supervised regularization method is used to realize feature importance and an
unsupervised clustering method is used to realize the binary flare/no-flare
decision. The approach is validated against NOAA SWPC data
Expectation Maximization for Hard X-ray Count Modulation Profiles
This paper is concerned with the image reconstruction problem when the
measured data are solar hard X-ray modulation profiles obtained from the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI)} instrument. Our goal is
to demonstrate that a statistical iterative method classically applied to the
image deconvolution problem is very effective when utilized for the analysis of
count modulation profiles in solar hard X-ray imaging based on Rotating
Modulation Collimators. The algorithm described in this paper solves the
maximum likelihood problem iteratively and encoding a positivity constraint
into the iterative optimization scheme. The result is therefore a classical
Expectation Maximization method this time applied not to an image deconvolution
problem but to image reconstruction from count modulation profiles. The
technical reason that makes our implementation particularly effective in this
application is the use of a very reliable stopping rule which is able to
regularize the solution providing, at the same time, a very satisfactory
Cash-statistic (C-statistic). The method is applied to both reproduce synthetic
flaring configurations and reconstruct images from experimental data
corresponding to three real events. In this second case, the performance of
Expectation Maximization, when compared to Pixon image reconstruction, shows a
comparable accuracy and a notably reduced computational burden; when compared
to CLEAN, shows a better fidelity with respect to the measurements with a
comparable computational effectiveness. If optimally stopped, Expectation
Maximization represents a very reliable method for image reconstruction in the
RHESSI context when count modulation profiles are used as input data
Inverse diffraction for the Atmospheric Imaging Assembly in the Solar Dynamics Observatory
The Atmospheric Imaging Assembly in the Solar Dynamics Observatory provides
full Sun images every 1 seconds in each of 7 Extreme Ultraviolet passbands.
However, for a significant amount of these images, saturation affects their
most intense core, preventing scientists from a full exploitation of their
physical meaning. In this paper we describe a mathematical and automatic
procedure for the recovery of information in the primary saturation region
based on a correlation/inversion analysis of the diffraction pattern associated
to the telescope observations. Further, we suggest an interpolation-based
method for determining the image background that allows the recovery of
information also in the region of secondary saturation (blooming)
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