1,721,969 research outputs found
Semi-analytical estimates for the chaotic diffusion in the Second Fundamental Model of Resonance. Application to Earth's navigation satellites
No full textArtificial Intelligence for the Characterization of the 2024 May Superstorm: Active Region Classification, Flare Forecasting, and Geomagnetic Storm Prediction
No full textSpace weather, driven by solar flares and coronal mass ejections, poses significant risks to technological systems. Accurately forecasting these events and their impact on Earth’s magnetosphere remains a challenge because of the complexity of solar-terrestrial interactions. This study focuses on the solar and geomagnetic extreme events associated with the 2024 May superstorm and shows that artificial intelligence (AI) tools are able to characterize this storm at three different levels. First, using magnetogram cutouts, a vision transformer was able to classify the morphological evolution of NOAA Active Region 13644 (primarily involved in storm generation), and then a video-based deep learning method predicted the occurrence of the associated solar flares, and a data-driven method exploited in situ measurements to raise 1-hour in advance alerts of the geomagnetic storm during the entire event. These AI models outperformed traditional methods in predicting solar flare occurrences, onset, and recovery phases of the geomagnetic storm. These findings highlight the impressive potential of AI for space weather forecasting as a tool to mitigate the impact of extreme solar events on critical infrastructure
Perfluorobutanesulfonic acids and other fluorine-free acid as Bronsted catalyst in alkylation reactions
No full textPerfluorobutanesulfonic acid and other fluorine-free Bronsted acids (phosphoric acid, methanesulfonic acid, p-toluensulfonic acid) are compared with regard their catalytic performance in alkylation reactions involving oxygen-containing alkylating agents (alcohols, ethers, esters, ketones). Reactions were carried out using benzene or toluene as an aromatic substrate; in any case experimental data emphasized the very high catalyzing ability of perfluorobutanesulfonic acid in comparison with the other experimented acids.
In particular, in the alkylation of benzene iso-butyl ether, iso-butyl alcohol, n-butyl alcohol and tert-butyl alcohol were used as alkylating agents; tert-butylbenzene was always the main reaction product. Perfluorobutanesulfonic acid only, compared with phosphoric acid and methanesulfonic acid, gives complete conversion of the alkylating agent and shows the highest reaction rate.
In the alkylation of toluene di-iso-butyl and methyl-tert-butyl ether, iso-propyl alcohol, ethyl acetate and acetone were used as alkylating agents. Either reactivity or yield were always higher with perfluorobutanesulfonic acid in comparison with the other used acids. Acetone experimented as an alkylating agent led to a surprising result because of the formation of unexpected p-tert-butyltoluene as a sole reaction product.
Experimental results are described
Deep Learning for Active Region Classification: A Systematic Study from Convolutional Neural Networks to Vision Transformers
No full textSolar active regions can significantly disrupt the Sun-Earth space environment, leading to severe space weather events such as solar flares or coronal mass ejections. Consequently, the automatic classification of active region groups is a crucial starting point for accurately and promptly predicting solar activity. This study presents our application of deep learning techniques to classify active region cutouts based on the Mount Wilson classification scheme. We explore the latest advantages in image classification architectures, ranging from convolutional neural networks to vision transformers, alongside modern training procedures, including on-the-fly data augmentations and transfer learning. We aim at evaluating the respective strengths and limitations of different neural network architectures in classifying solar active region cutouts. We observed that combining magnetogram and continuum image types enhances model performance by leveraging complementary features from diverse inputs. When considering only magnetograms, data-efficient image transformers achieve the best performance, suggesting that these models can better capture the spatial complexity of magnetograms. Models trained exclusively on continuum images exhibit overall lower performance, suggesting that continuum images, due to their more homogeneous nature, offer less spatial variability
Perfluorobutanesulfonic acid and other fluorine-free acids as Bronsted catalyst in alkalation reactions. I. Preliminary experiments and alkylation of benzene
No full textA new procedure is proposed for the synthetizing hypofluorite by fluorination of fluorozcyl fluorides at low temperature, in presence of a suitable catalyst which allows the controlling of the fluorine addition to the carbon-oxygen double bond
Perfluorobutanesulfonic acid and other fluorine-free acids as Bronsted catalyst in alkalation reactions. II. Alkalation of toluene
No full textLiquid-phase fluorination of aromatic compounds by elemental fluorine
No full textThe fluorination of aromatic compounds (benzene, toluene, phenol and benzoic acid) by elemental fluorine diluted with nitrogen has been investigated in various solvents (Freon 11, chloroform, methanol, trifluoroacetic acid, 2,2,2-trifluoroethanol, water) in order to define the influence of the experimental conditions on the reaction. Experiments have been carried out by varying the temperature, the substrate concentration in solution, the molar ratio of fluorine to substrate, and the concentration of fluorine in the fluorine/nitrogen mixture. In all cases, the effects on the yield of fluorinated products were studied. Monofluorinated compounds were mainly found in the reaction mixture, the isomers formed being in accord with the mechanism for electrophilic substitution. The highest yield of monofluorinated products was obtained with polar solvents and the following order was found: CFCl3 < CHCl3 < CH3OH < CF3CH2OH < CF3COOH. Interesting results were also found using particular additives (for instance, KOH or C4F9SO3Na in methanol) or water as the solvent. A direct relationship was observed between the yield of monofluorinated compounds and the molar ratio of fluorine to substrate, which has to be less than one in order to obtain high yields. In contrast, low selectivity, expressed as the yield ratio of ortho to para (or metal isomers, was found
Secular dynamics and the lifetimes of lunar artificial satellites under natural force-driven orbital evolution
No full textIn this paper, we study the long-term (time scale of several years) orbital evolution of lunar satellites under the sole action of natural forces. In particular, we focus on secular resonances, caused either by the influence of the multipole moments of the lunar potential and/or by the Earth's and Sun's third-body effect on the satellite's long-term orbital evolution. Our study is based on a simplified secular model obtained in ‘closed form’, i.e., without expansions in the satellite's orbital eccentricity. Contrary to the case of artificial Earth satellites, in which many secular resonances compete in dynamical impact, we give numerical evidence that for lunar satellites only the 2g−resonance (ω̇=0) affects significantly the orbits at secular timescales. We interpret this as a consequence of the strong effect of lunar mascons. We show that the lifetime of lunar satellites is, in particular, nearly exclusively dictated by the 2g resonance. By deriving a simple analytic model, we propose a theoretical framework which allows for both qualitative and quantitative interpretation of the structures seen in numerically obtained lifetime maps. This involves explaining the main mechanisms driving eccentricity growth in the orbits of lunar satellites. In fact, we argue that the re-entry process for lunar satellites is not necessarily a chaotic process (as is the case for Earth satellites), but rather due to a sequence of bifurcations leading to a dramatic variation in the structure of the separatrices in the 2g resonance's phase portrait, as we move from the lowest to the highest limit in inclination (at each altitude) where the 2g resonance is manifested
A detailed dynamical model for inclination-only dependent lunisolar resonances. Effect on the “eccentricity growth” mechanism
No full textThe focus of this paper is on inclination-only dependent lunisolar resonances, which shape the dynamics of a MEO (Medium Earth Orbit) object over secular time scales (i.e. several decades). Following the formalism of Daquin et al. (2022), we discuss an analytical model yielding the correct form of the separatrices of each one of the major lunisolar resonances in the “action” space (i,e) (inclination, eccentricity) for any given semi-major axis a. We then highlight how our method is able to predict and explain the main structures found numerically in Fast Lyapunov Indicator (FLI) cartography. We focus on explaining the dependence of the FLI maps from the initial phase of the argument of perigee ω and of the longitude of the ascending node Ω of the object and of the moon ΩL. In addition, on the basis of our model, we discuss the role played by the Ω-ΩL and the 2Ω-ΩL resonances, which overlap with the inclination-only dependent ones as they sweep the region for increasing values of a, generating large domains of chaotic motion. Our results provide a framework useful in designing low-cost satellite deployment or space debris mitigation strategies, exploiting the natural dynamics of lunisolar resonances that increase an object's eccentricity up until it reaches a domain where friction leads to atmospheric re-entry
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