323 research outputs found
The August 24, 2002 Coronal Mass Ejection: When a Western Limb Event Connects to Earth
We discuss how some coronal mass ejections (CMEs) originating from the
western limb of the Sun are associated with space weather effects such as solar
energetic particles (SEPs), shock or geo-effective ejecta at Earth. We focus on
the August 24, 2002 coronal mass ejection, a fast (~ 2000 km/s) eruption
originating from W81. Using a three-dimensional magneto-hydrodynamic simulation
of this ejection with the Space Weather Modeling Framework (SWMF), we show how
a realistic initiation mechanism enables us to study the deflection of the CME
in the corona and the heliosphere. Reconnection of the erupting magnetic field
with that of neighboring streamers and active regions modify the solar
connectivity of the field lines connecting to Earth and can also partly explain
the deflection of the eruption during the first tens of minutes. Comparing the
results at 1 AU of our simulation with observations by the ACE spacecraft, we
find that the simulated shock does not reach Earth, but has a maximum angular
span of about 120, and reaches 35 West of Earth in 58 hours. We
find no significant deflection of the CME and its associated shock wave in the
heliosphere, and we discuss the consequences for the shock angular span.Comment: 7 pages, 4 figures, IAU 257 Symposium Proceeding
Assessing the Constrained Harmonic Mean Method for Deriving the Kinematics of ICMEs with a Numerical Simulation
In this study we use a numerical simulation of an artificial coronal mass
ejection (CME) to validate a method for calculating propagation directions and
kinematical profiles of interplanetary CMEs (ICMEs). In this method
observations from heliospheric images are constrained with in-situ plasma and
field data at 1 AU. These data are used to convert measured ICME elongations
into distance by applying the Harmonic Mean approach that assumes a spherical
shape of the ICME front. We use synthetic white-light images, similar as
observed by STEREO-A/HI, for three different separation angles between remote
and in-situ spacecraft, of 30{\deg}, 60{\deg}, and 90{\deg}. To validate the
results of the method they are compared to the apex speed profile of the
modeled ICME, as obtained from a top view. This profile reflects the "true"
apex kinematics since it is not affected by scattering or projection effects.
In this way it is possible to determine the accuracy of the method for
revealing ICME propagation directions and kinematics. We found that the
direction obtained by the constrained Harmonic Mean method is not very
sensitive to the separation angle (30{\deg} sep: \phi = W7; 60{\deg} sep: \phi
= W12; 90{\deg} sep: \phi = W15; true dir.: E0/W0). For all three cases the
derived kinematics are in a relatively good agreement with the real kinematics.
The best consistency is obtained for the 30{\deg} case, while with growing
separation angle the ICME speed at 1 AU is increasingly overestimated (30{\deg}
sep: \Delta V_arr ~-50 km/s, 60{\deg} sep: \Delta V_arr ~+75 km/s, 90{\deg}
sep: \Delta V_arr ~+125 km/s). Especially for future L4/L5 missions the
60{\deg} separation case is highly interesting in order to improve space
weather forecasts.Comment: accepted for publication in Solar Physic
Automatic Annotation of Direct Speech in Written French Narratives
The automatic annotation of direct speech (AADS) in written text has been
often used in computational narrative understanding. Methods based on either
rules or deep neural networks have been explored, in particular for English or
German languages. Yet, for French, our target language, not many works exist.
Our goal is to create a unified framework to design and evaluate AADS models in
French. For this, we consolidated the largest-to-date French narrative dataset
annotated with DS per word; we adapted various baselines for sequence labelling
or from AADS in other languages; and we designed and conducted an extensive
evaluation focused on generalisation. Results show that the task still requires
substantial efforts and emphasise characteristics of each baseline. Although
this framework could be improved, it is a step further to encourage more
research on the topic.Comment: 9 pages, ACL 202
Spin injection in Silicon at zero magnetic field
In this letter, we show efficient electrical spin injection into a SiGe based
\textit{p-i-n} light emitting diode from the remanent state of a
perpendicularly magnetized ferromagnetic contact. Electron spin injection is
carried out through an alumina tunnel barrier from a Co/Pt thin film exhibiting
a strong out-of-plane anisotropy. The electrons spin polarization is then
analysed through the circular polarization of emitted light. All the light
polarization measurements are performed without an external applied magnetic
field \textit{i.e.} in remanent magnetic states. The light polarization as a
function of the magnetic field closely traces the out-of-plane magnetization of
the Co/Pt injector. We could achieve a circular polarization degree of the
emitted light of 3 % at 5 K. Moreover this light polarization remains almost
constant at least up to 200 K.Comment: accepted in AP
Observations of an extreme storm in interplanetary space caused by successive coronal mass ejections
Space weather refers to dynamic conditions on the Sun and in the space
environment of the Earth, which are often driven by solar eruptions and their
subsequent interplanetary disturbances. It has been unclear how an extreme
space weather storm forms and how severe it can be. Here we report and
investigate an extreme event with multi-point remote-sensing and in-situ
observations. The formation of the extreme storm showed striking novel
features. We suggest that the in-transit interaction between two closely
launched coronal mass ejections resulted in the extreme enhancement of the
ejecta magnetic field observed near 1 AU at STEREO A. The fast transit to
STEREO A (in only 18.6 hours), or the unusually weak deceleration of the event,
was caused by the preconditioning of the upstream solar wind by an earlier
solar eruption. These results provide a new view crucial to solar physics and
space weather as to how an extreme space weather event can arise from a
combination of solar eruptions.Comment: 23 pages, 7 figure
Formation of high-order acoustic Bessel beams by spiral diffraction gratings
The formation of high-order Bessel beams by a passive acoustic device consisting of an Archimedes' spiral diffraction grating is theoretically, numerically, and experimentally reported in this paper. These beams are propagation-invariant solutions of the Helmholtz equation and are characterized by an azimuthal variation of the phase along its annular spectrum producing an acoustic vortex in the near field. In our system, the scattering of plane acoustic waves by the spiral grating leads to the formation of the acoustic vortex with zero pressure on axis and the angular phase dislocations characterized by the spiral geometry. The order of the generated Bessel beam and, as a consequence, the size of the generated vortex can be fixed by the number of arms in the spiral diffraction grating. The obtained results allow for obtaining Bessel beams with controllable vorticity by a passive device, which has potential applications in low-cost acoustic tweezers and acoustic radiation force devices.We acknowledge financial support from MINECO of the Spanish Government under Grants No. MTM2012-36740-C02-02, No. FIS2015-65998-C2-1-P, and No. FIS2015-65998-C2-2-P. N.J. acknowledges financial support from PAID-2011 Universitat Politecnica de Valencia.Jimenez, N.; Picó Vila, R.; Sánchez Morcillo, VJ.; Romero García, V.; García-Raffi, LM.; Staliünas, K. (2016). Formation of high-order acoustic Bessel beams by spiral diffraction gratings. Physical Review E. 94(5). doi:10.1103/PhysRevE.94.053004S05300494
Generating stable molecules using imitation and reinforcement learning
Chemical space is routinely explored by machine learning methods to discover interesting molecules, before time-consuming experimental synthesizing is attempted. However, these methods often rely on a graph representation, ignoring 3D information necessary for determining the stability of the molecules. We propose a reinforcement learning (RL) approach for generating molecules in Cartesian coordinates allowing for quantum chemical prediction of the stability. To improve sample-efficiency we learn basic chemical rules from imitation learning (IL) on the GDB-11 database to create an initial model applicable for all stoichiometries. We then deploy multiple copies of the model conditioned on a specific stoichiometry in a RL setting. The models correctly identify low energy molecules in the database and produce novel isomers not found in the training set. Finally, we apply the model to larger molecules to show how RL further refines the IL model in domains far from the training data
Quasi-perfect absorption by sub-wavelength acoustic panels in transmission using accumulation of resonances due to slow sound
[EN] We theoretically and experimentally report sub-wavelength resonant panels for low-frequency quasi-perfect sound absorption including transmission by using the accumulation of cavity resonances due to the slow sound phenomenon. The sub-wavelength panel is composed of periodic horizontal slits loaded by identical Helmholtz resonators (HRs). Due to the presence of the HRs, the propagation inside each slit is strongly dispersive, with near-zero phase velocity close to the resonance of the HRs. In this slow sound regime, the frequencies of the cavity modes inside the slit are down-shifted and the slit behaves as a subwavelength resonator. Moreover, due to strong dispersion, the cavity resonances accumulate at the limit of the bandgap below the resonance frequency of the HRs. Near this accumulation frequency, simultaneously symmetric and antisymmetric quasi-critical coupling can be achieved. In this way, using only monopolar resonators quasi-perfect absorption can be obtained in a material including transmission.This work has been funded by the Metaudible Project No. ANR-13-BS09-0003, cofunded by ANR and FRAE.Jimenez, N.; Romero García, V.; Pagneux, V.; Groby, J. (2017). Quasi-perfect absorption by sub-wavelength acoustic panels in transmission using accumulation of resonances due to slow sound. PHYSICAL REVIEW B-CONDENSED MATTER. 95(1). doi:10.1103/PhysRevB.95.014205S01420595
Variational Approach to Molecular Kinetics
The eigenvalues and eigenvectors of the molecular dynamics propagator (or transfer operator) contain the essential information about the molecular thermodynamics and kinetics. This includes the stationary distribution, the metastable states, and state-to-state transition rates. Here, we present a variational approach for computing these dominant eigenvalues and eigenvectors. This approach is analogous the variational approach used for computing stationary states in quantum mechanics. A corresponding method of linear variation is formulated. It is shown that the matrices needed for the linear variation method are correlation matrices that can be estimated from simple MD simulations for a given basis set. The method proposed here is thus to first define a basis set able to capture the relevant conformational transitions, then compute the respective correlation matrices, and then to compute their dominant eigenvalues and eigenvectors, thus obtaining the key ingredients of the slow kinetics
The mechanism of RNA base fraying: Molecular dynamics simulations analyzed with core-set Markov state models
The process of RNA base fraying (i.e., the transient opening of the termini of a helix) is involved in many aspects of RNA dynamics. We here use molecular dynamics simulations and Markov state models to characterize the kinetics of RNA fraying and its sequence and direction dependence. In particular, we first introduce a method for determining biomolecular dynamics employing core-set Markov state models constructed using an advanced clustering technique. The method is validated on previously reported simulations. We then use the method to analyze extensive trajectories for four different RNA model duplexes. Results obtained using D. E. Shaw research and AMBER force fields are compared and discussed in detail and show a non-trivial interplay between the stability of intermediate states and the overall fraying kinetics
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