1,096 research outputs found
Laplace's rule of succession in information geometry
Laplace's "add-one" rule of succession modifies the observed frequencies in a
sequence of heads and tails by adding one to the observed counts. This improves
prediction by avoiding zero probabilities and corresponds to a uniform Bayesian
prior on the parameter. The canonical Jeffreys prior corresponds to the
"add-one-half" rule. We prove that, for exponential families of distributions,
such Bayesian predictors can be approximated by taking the average of the
maximum likelihood predictor and the \emph{sequential normalized maximum
likelihood} predictor from information theory. Thus in this case it is possible
to approximate Bayesian predictors without the cost of integrating or sampling
in parameter space
Parametric Fokker-Planck equation
We derive the Fokker-Planck equation on the parametric space. It is the
Wasserstein gradient flow of relative entropy on the statistical manifold. We
pull back the PDE to a finite dimensional ODE on parameter space. Some
analytical example and numerical examples are presented
Tests of Dynamical Flux Emergence as a Mechanism for CME Initiation
Current coronal mass ejection (CME) models set their lower boundary to be in
the lower corona. They do not calculate accurately the transfer of free
magnetic energy from the convection zone to the magnetically dominated corona
because they model the effects of flux emergence using kinematic boundary
conditions or simply assume the appearance of flux at these heights. We test
the importance of including dynamical flux emergence in CME modeling by
simulating, in 2.5D, the emergence of sub-surface flux tubes into different
coronal magnetic field configurations. We investigate how much free magnetic
energy, in the form of shear magnetic field, is transported from the convection
zone to the corona, and whether dynamical flux emergence can drive CMEs. We
find that multiple coronal flux ropes can be formed during flux emergence, and
although they carry some shear field into the corona, the majority of shear
field is confined to the lower atmosphere. Less than 10% of the magnetic energy
in the corona is in the shear field, and this, combined with the fact that the
coronal flux ropes bring up significant dense material, means that they do not
erupt. Our results have significant implications for all CME models which rely
on the transfer of free magnetic energy from the lower atmosphere into the
corona but which do not explicitly model this transfer. Such studies of flux
emergence and CMEs are timely, as we have new capabilities to observe this with
Hinode and SDO, and therefore to test the models against observations
SIMS chemical analysis of extended impacts on the leading and trailing edges of LDEF experiment AO187-2
Numerous 'extended impacts' found in both leading and trailing edge capture cells were successfully analyzed for the chemical composition of projectile residues by secondary ion mass spectrometry (SIMS). Most data were obtained from the trailing edge cells where 45 of 58 impacts were classified as 'probably natural' and the remainder as 'possibly man-made debris.' This is in striking contrast to leading edge cells where 9 of 11 impacts so far measured are definitely classified as orbital debris. Although all the leading edge cells had lost their plastic entrance foils during flight, the rate of foil failure was similar to that of the trailing edge cells, 10 percent of which were recovered intact. Ultraviolet embrittlement is suspected as the major cause of failure on both leading and trailing edges. The major impediment to the accurate determination of projectile chemistry is the fractionation of volatile and refractory elements in the hypervelocity impact and redeposition processes. This effect had been noted in a simulation experiment but is more pronounced in the LDEF capture cells, probably due to the higher average velocities of the space impacts. Surface contamination of the pure Ge surfaces with a substance rich in Si, but also containing Mg and Al, provides an additional problem for the accurate determination of impactor chemistry. The effect is variable, being much larger on surfaces that were exposed to space than in those cells that remained intact. Future work will concentrate on the analyses of more leading edge impacts and the development of new SIMS techniques for the measurement of elemental abundances in extended impacts
Observational Evidence for Coronal Twisted Flux Rope
Multi-instrument data sets of NOAA AR10938 on Jan. 16, 2007, (e.g.,
{\emph{Hinode}}, {\it{STEREO}}, {\it{GOES}}, {\it{MLSO}} and {\it{ISOON}}
H) are utilized to study the fine structure and evolution of a magnetic
loop system exhibiting multiple crossing threads, whose arrangement and
individual shapes are very suggestive of individual field lines in a flux rope.
The footpoints of the magnetic threads are closely rooted into pores and plage
areas. A C-class flare recorded by {\it{GOES}} at approximately 2:35 UT near
one of the footpoints of the multi-thread system (along with a wisp of loop
material shown by EUV data) led to the brightening of the magnetic structure
revealing its fine structure with several threads that indicate a high degree
of linking (suggesting a left-handed helical pattern as shown by the filament
structure formed later-on). EUV observations by {\emph{Hinode}}/EIS of hot
spectral lines at 2:46 UT show a complex structure of coronal loops. The same
features were observed about 20 minutes later in X-ray images from
{\emph{Hinode}}/XRT and about 30 minutes further in EUV images of
{\it{STEREO}}/SECCHI/EUVI with much better resolution. H and 304 {\AA}
images revealed the presence of several filament fibrils in the same area. They
evolved a few hours later into a denser structure seemingly showing helical
structure, which persistently lasted for several days forming a segment of a
larger scale filament. The present observations provide an important indication
for a flux robe as a precursor of a solar filament.Comment: 13 pages, 4 figure
Transient dynamics for sequence processing neural networks
An exact solution of the transient dynamics for a sequential associative
memory model is discussed through both the path-integral method and the
statistical neurodynamics. Although the path-integral method has the ability to
give an exact solution of the transient dynamics, only stationary properties
have been discussed for the sequential associative memory. We have succeeded in
deriving an exact macroscopic description of the transient dynamics by
analyzing the correlation of crosstalk noise. Surprisingly, the order parameter
equations of this exact solution are completely equivalent to those of the
statistical neurodynamics, which is an approximation theory that assumes
crosstalk noise to obey the Gaussian distribution. In order to examine our
theoretical findings, we numerically obtain cumulants of the crosstalk noise.
We verify that the third- and fourth-order cumulants are equal to zero, and
that the crosstalk noise is normally distributed even in the non-retrieval
case. We show that the results obtained by our theory agree with those obtained
by computer simulations. We have also found that the macroscopic unstable state
completely coincides with the separatrix.Comment: 21 pages, 4 figure
STEREO quadrature observations of coronal dimming at the onset of mini-CMEs
Context: Using unique quadrature observations with the two STEREO spacecraft,
we investigate coronal dimmings at the onset of small-scale eruptions. In CMEs
they are believed to indicate the opening up of the coronal magnetic fields at
the start of the eruption. Aims: It is to determine whether coronal dimming
seen in small-scale eruptions starts before or after chromospheric plasma
ejection. Methods: One STEREO spacecraft obtained high cadence, 75 s, images in
the He II 304A channel, and the other simultaneous images in the Fe IX/FeX 171A
channel. We concentrate on two well-positioned chromospheric eruptions that
occurred at disk center in the 171A images, and on the limb in 304A. One was in
the quiet Sun and the other was in an equatorial coronal hole. We compare the
timing of chromospheric eruption seen in the 304A limb images with the
brightenings and dimmings seen on disk in the 171A images. Further we use
off-limb images of the low frequency 171A power to infer the coronal structure
near the eruptions. Results: In both the quiet Sun and the coronal hole
eruption, on disk 171A dimming was seen before the chromospheric eruption, and
in both cases it extends beyond the site of the chromospheric eruption. The
quiet Sun eruption occurred on the outer edge of the enclosing magnetic field
of a prominence and may be related to a small disruption of the prominence just
before the 171A dimming. Conclusions: These small-scale chromospheric eruptions
started with a dimming in coronal emission just like their larger counterparts.
We therefore suggest that a fundamental step in triggering them was the removal
of overlying coronal field.Comment: 4 pages, 8 figures. To appear A&A Letters. Movies accompanying this
Letter are at http://www.mps.mpg.de/data/outgoing/innes/dims
From neurons to epidemics: How trophic coherence affects spreading processes
Trophic coherence, a measure of the extent to which the nodes of a directed
network are organised in levels, has recently been shown to be closely related
to many structural and dynamical aspects of complex systems, including graph
eigenspectra, the prevalence or absence of feed-back cycles, and linear
stability. Furthermore, non-trivial trophic structures have been observed in
networks of neurons, species, genes, metabolites, cellular signalling,
concatenated words, P2P users, and world trade. Here we consider two simple yet
apparently quite different dynamical models -- one a
Susceptible-Infected-Susceptible (SIS) epidemic model adapted to include
complex contagion, the other an Amari-Hopfield neural network -- and show that
in both cases the related spreading processes are modulated in similar ways by
the trophic coherence of the underlying networks. To do this, we propose a
network assembly model which can generate structures with tunable trophic
coherence, limiting in either perfectly stratified networks or random graphs.
We find that trophic coherence can exert a qualitative change in spreading
behaviour, determining whether a pulse of activity will percolate through the
entire network or remain confined to a subset of nodes, and whether such
activity will quickly die out or endure indefinitely. These results could be
important for our understanding of phenomena such as epidemics, rumours, shocks
to ecosystems, neuronal avalanches, and many other spreading processes
Nonlinear force-free models for the solar corona I. Two active regions with very different structure
With the development of new instrumentation providing measurements of solar
photospheric vector magnetic fields, we need to develop our understanding of
the effects of current density on coronal magnetic field configurations. The
object is to understand the diverse and complex nature of coronal magnetic
fields in active regions using a nonlinear force-free model. From the observed
photospheric magnetic field we derive the photospheric current density for two
active regions: one is a decaying active region with strong currents (AR8151),
and the other is a newly emerged active region with weak currents (AR8210). We
compare the three-dimensional structure of the magnetic fields for both active
region when they are assumed to be either potential or nonlinear force-free.
The latter is computed using a Grad-Rubin vector-potential-like numerical
scheme. A quantitative comparison is performed in terms of the geometry, the
connectivity of field lines, the magnetic energy and the magnetic helicity
content. For the old decaying active region the connectivity and geometry of
the nonlinear force-free model include strong twist and strong shear and are
very different from the potential model. The twisted flux bundles store
magnetic energy and magnetic helicity high in the corona (about 50 Mm). The
newly emerged active region has a complex topology and the departure from a
potential field is small, but the excess magnetic energy is stored in the low
corona and is enough to trigger powerful flares.Comment: 11 pages, 11 figure
An Information-Geometric Reconstruction of Quantum Theory, I: The Abstract Quantum Formalism
In this paper and a companion paper, we show how the framework of information
geometry, a geometry of discrete probability distributions, can form the basis
of a derivation of the quantum formalism. The derivation rests upon a few
elementary features of quantum phenomena, such as the statistical nature of
measurements, complementarity, and global gauge invariance. It is shown that
these features can be traced to experimental observations characteristic of
quantum phenomena and to general theoretical principles, and thus can
reasonably be taken as a starting point of the derivation. When appropriately
formulated within an information geometric framework, these features lead to
(i) the abstract quantum formalism for finite-dimensional quantum systems, (ii)
the result of Wigner's theorem, and (iii) the fundamental correspondence rules
of quantum theory, such as the canonical commutation relationships. The
formalism also comes naturally equipped with a metric (and associated measure)
over the space of pure states which is unitarily- and anti-unitarily invariant.
The derivation suggests that the information geometric framework is directly or
indirectly responsible for many of the central structural features of the
quantum formalism, such as the importance of square-roots of probability and
the occurrence of sinusoidal functions of phases in a pure quantum state.
Global gauge invariance is seen to play a crucial role in the emergence of the
formalism in its complex form.Comment: 26 page
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