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Abstract Energy dissipation in collisionless plasmas is a long-standing fundamental physics problem. Although it is well known that magnetic reconnection and turbulence are coupled and transport energy from system-size scales to subproton scales, the details of the energy distribution and energy dissipation channels remain poorly understood. Especially, the energy transfer and transport associated with 3D small-scale reconnection that occurs as a consequence of a turbulent cascade is unknown. We use an explicit fully kinetic particle-in-cell code to simulate 3D small-scale magnetic reconnection events forming in anisotropic and decaying Alfvénic turbulence. We identify a highly dynamic and asymmetric reconnection event that involves two reconnecting flux ropes. We use a two-fluid approach based on the Boltzmann equation to study the spatial energy transfer associated with the reconnection event and compare the power density terms in the two-fluid energy equations with standard energy-based damping, heating, and dissipation proxies. Our findings suggest that the electron bulk flow transports thermal energy density more efficiently than kinetic energy density. Moreover, in our turbulent reconnection event, the energy density transfer is dominated by plasma compression. This is consistent with turbulent current sheets and turbulent reconnection events, but not with laminar reconnection.

Abstract Magnetocaloric hydrogen liquefaction could be a ‘game-changer’ for liquid hydrogen industry. Although heavy rare-earth based magnetocaloric materials show strong magnetocaloric effects in the temperature range required by hydrogen liquefaction (77–20 K), the high resource criticality of the heavy rare-earth elements is a major obstacle for upscaling this emerging liquefaction technology. In contrast, the higher abundances of the light rare-earth elements make their alloys highly appealing for magnetocaloric hydrogen liquefaction. Via a mean-field approach, it is demonstrated that tuning the Curie temperature (T C) of an idealized light rare-earth based magnetocaloric material towards lower cryogenic temperatures leads to larger maximum magnetic and adiabatic temperature changes (ΔS T and ΔT ad). Especially in the vicinity of the condensation point of hydrogen (20 K), ΔS T and ΔT ad of the optimized light rare-earth based material are predicted to show significantly large values. Following the mean-field approach and taking the chemical and physical similarities of the light rare-earth elements into consideration, a method of designing light rare-earth intermetallic compounds for hydrogen liquefaction is used: tuning T C of a rare-earth alloy to approach 20 K by mixing light rare-earth elements with different de Gennes factors. By mixing Nd and Pr in Laves phase (Nd, Pr)Al2, and Pr and Ce in Laves phase (Pr, Ce)Al2, a fully light rare-earth intermetallic series with large magnetocaloric effects covering the temperature range required by hydrogen liquefaction is developed, demonstrating a competitive maximum effect compared to the heavy rare-earth compound DyAl2.

Objectifs. Nous étudions la stabilité et les modes des modèles de boucle coronale non isotherme avec différentes valeurs d'intensité du champ magnétique tordu d'équilibre. Méthodes. Nous utilisons un principe énergétique obtenu au moyen d'arguments thermodynamiques de non-équilibre. Le principe est exprimé en termes d'opérateurs hermitiens et nous permet de considérer le système couplé d'équations, l'équation de l'équilibre de l'énergie et l'équation du mouvement, pour obtenir des modes et des modes propres dans un spectre allant des perturbations de courte à longue longueur d'onde sans utiliser de faibles approximations variables des paramètres d'équilibre. Les perturbations de grande longueur d'onde introduisent des difficultés supplémentaires car le caractère inhomogène du milieu produit des perturbations correspondant à des intervalles continus de fréquences propres, qui ne peuvent être considérées comme purement sinusoïdales. Résultats. Nous analysons la modification des périodes, de la structure des modes et de la stabilité lorsque l'hélicité, l'intensité du champ magnétique et le rayon du tube de flux varient. L'efficacité de l'amortissement due au mécanisme d'absorption résonant est analysée en termes de modes qui peuvent soit libérer soit stocker de l'énergie magnétique de manière impulsive. Conclusions. Nous constatons que l'apparition de l'instabilité est associée à une valeur critique de l'hélicité et que le contenu en énergie magnétique a un rôle déterminant sur l'instabilité du système par rapport à l'effet stabilisateur du mécanisme d'absorption résonant. Objetivos. Estudiamos la estabilidad y los modos de los modelos de bucle coronal no isotérmico con diferentes valores de intensidad de campo magnético retorcido de equilibrio. Métodos. Utilizamos un principio de energía obtenido por medio de argumentos termodinámicos de no equilibrio. El principio se expresa en términos de operadores hermíticos y nos permite considerar el sistema acoplado de ecuaciones, la ecuación de equilibrio de energía y la ecuación de movimiento, para obtener modos y modos propios en un espectro que va desde perturbaciones de longitud de onda corta a larga sin utilizar aproximaciones variables débiles de los parámetros de equilibrio. Las perturbaciones de longitud de onda larga introducen dificultades adicionales porque la naturaleza no homogénea del medio produce perturbaciones correspondientes a intervalos continuos de frecuencias propias, que no pueden considerarse puramente sinusoidales. Resultados. Analizamos la modificación de los períodos, la estructura de los modos y la estabilidad cuando se varían la helicidad, la intensidad del campo magnético y el radio del tubo de flujo. La eficiencia de la amortiguación debido al mecanismo de absorción resonante se analiza en términos de modos que pueden liberar o almacenar energía magnética de forma impulsiva. Conclusiones. Encontramos que el inicio de la inestabilidad se asocia con un valor crítico de la helicidad y el contenido de energía magnética tiene un papel determinante en la inestabilidad del sistema con respecto al efecto estabilizador del mecanismo de absorción resonante. Aims. We study the stability and modes of non – isothermal coronal loop models with different intensity values of equilibrium twisted magnetic field. Methods. We use an energy principle obtained by means of non – equilibrium thermodynamic arguments. The principle is expressed in terms of Hermitian operators and enables us to consider the coupled system of equations, the balance of energy equation and the equation of motion, to obtain modes and eigenmodes in a spectrum ranging from short to long-wavelength disturbances without using weak varying approximations of the equilibrium parameters. Long-wavelength perturbations introduce additional difficulties because the inhomogeneous nature of the medium produce disturbances corresponding to continuous intervals of eigenfrequencies, which cannot be considered as purely sinusoidal. Results. We analyze the modification of periods, modes structure, and stability when the helicity, the magnetic field strength, and the radius of the fluxtube are varied. The efficiency of the damping due to the resonant absorption mechanism is analyzed in terms of modes that can either impulsively release or store magnetic energy. Conclusions. We find that the onset of the instability is associated with a critical value of the helicity and the magnetic energy content has a determinant role on the instability of the system with respect to the stabilizing effect of the resonant absorption mechanism. الأهداف. ندرس استقرار وأنماط نماذج الحلقة الإكليلية غير متساوية الحرارة بقيم كثافة مختلفة للمجال المغناطيسي الملتوي المتوازن. الطرق. نستخدم مبدأ الطاقة الذي تم الحصول عليه عن طريق الحجج الديناميكية الحرارية غير المتوازنة. يتم التعبير عن المبدأ من حيث العوامل الهرمسية ويمكّننا من النظر في نظام المعادلات المقترن، وتوازن معادلة الطاقة ومعادلة الحركة، للحصول على الأنماط والأنماط الذاتية في طيف يتراوح من اضطرابات الطول الموجي القصير إلى الطويل دون استخدام تقديرات تقريبية متفاوتة ضعيفة لمعلمات التوازن. تؤدي اضطرابات الطول الموجي الطويل إلى صعوبات إضافية لأن الطبيعة غير المتجانسة للوسيط تنتج اضطرابات مقابلة لفترات مستمرة من الترددات الذاتية، والتي لا يمكن اعتبارها جيبية بحتة. النتائج. نحلل تعديل الفترات، وبنية الأوضاع، والاستقرار عندما تتنوع الحلزونية، وقوة المجال المغناطيسي، ونصف قطر الأنبوب الفلوكسي. يتم تحليل كفاءة التخميد بسبب آلية الامتصاص الرنانة من حيث الأوضاع التي يمكن أن تطلق أو تخزن الطاقة المغناطيسية بشكل اندفاعي. الاستنتاجات. نجد أن بداية عدم الاستقرار ترتبط بقيمة حرجة للحلزونية ومحتوى الطاقة المغناطيسية له دور محدد في عدم استقرار النظام فيما يتعلق بتأثير الاستقرار لآلية الامتصاص الرنانة.

Magnetic reconnection, topological changes in magnetic fields, is a fundamental process in magnetized plasmas. It is associated with energy release in regions of magnetic field annihilation, but this is only one facet of this process. Astrophysical fluid flows normally have very large Reynolds numbers and are expected to be turbulent, in agreement with observations. In strong turbulence, magnetic field lines constantly reconnect everywhere and on all scales, thus making magnetic reconnection an intrinsic part of the turbulent cascade. We note in particular that this is inconsistent with the usual practice of magnetic field lines as persistent dynamical elements. A number of theoretical, numerical, and observational studies starting with the paper done by Lazarian and Vishniac [Astrophys. J. 517, 700–718 (1999)] proposed that 3D turbulence makes magnetic reconnection fast and that magnetic reconnection and turbulence are intrinsically connected. In particular, we discuss the dramatic violation of the textbook concept of magnetic flux-freezing in the presence of turbulence. We demonstrate that in the presence of turbulence, the plasma effects are subdominant to turbulence as far as the magnetic reconnection is concerned. The latter fact justifies a magnetohydrodynamiclike treatment of magnetic reconnection on all scales much larger than the relevant plasma scales. We discuss the numerical and observational evidence supporting the turbulent reconnection model. In particular, we demonstrate that the tearing reconnection is suppressed in 3D, and unlike the 2D settings, 3D reconnection induces turbulence that makes magnetic reconnection independent of resistivity. We show that turbulent reconnection dramatically affects key astrophysical processes, e.g., star formation, turbulent dynamo, and acceleration of cosmic rays. We provide criticism of the concept of “reconnection-mediated turbulence” and explain why turbulent reconnection is very different from enhanced turbulent resistivity and hyper-resistivity and why the latter have fatal conceptual flaws.

The anomalous Nernst effect (ANE) is a thermomagnetic phenomenon with potential applications in thermal energy harvesting. While many recent works studied the approaches to increase the ANE coefficient of materials, relatively little effort was devoted to increasing the power supplied by the effect. Here we demonstrate a nanofabricated device with record power density generated by the ANE. To accomplish this, we fabricate micrometer-sized devices in which the thermal gradient is three orders of magnitude higher than conventional macroscopic devices. In addition, we use Co/Pt multilayers, a system characterized by a high ANE thermopower (~1 microV/K), low electrical resistivity, and perpendicular magnetic anisotropy. These innovations allow us to obtain power densities of around 13 W/cm3. We believe that this design may find uses in harvesting wasted energy in e.g. electronic devices.

Abstract We study the effect of a tangled sub-fG level intergalactic magnetic field (IGMF) on the electrostatic instability of a blazar-induced pair beam. Sufficiently strong IGMF may significantly deflect the TeV pair beams, which would reduce the flux of secondary cascade emission below the observational limits. A similar flux reduction may result from the electrostatic beam–plasma instability, which operates the best in the absence of IGMF. Considering IGMF with correlation lengths smaller than a kiloparsec, we find that weak magnetic fields increase the transverse momentum of the pair-beam particles, which dramatically reduces the linear growth rate of the electrostatic instability and hence the energy-loss rate of the pair beam. We show that the beam–plasma instability is eliminated as an effective energy-loss agent at a field strength three orders of magnitude below that needed to suppress the secondary cascade emission by magnetic deflection. For intermediate-strength IGMF, we do not know a viable process to explain the observed absence of GeV-scale cascade emission.

We report a novel method of determining the average Néel relaxation time and its temperature dependence by calculating derivatives of the measured time dependence of temperature for a frozen ferrofluid exposed to an alternating magnetic field. The ferrofluid, composed of dextran-coated Fe3O4 nanoparticles (diameter 13.7 nm ± 4.7 nm), was synthesized via wet chemical precipitation and characterized by x-ray diffraction and transmission electron microscopy. An alternating magnetic field of constant amplitude (H0=20 kA/m) driven at frequencies of 171 kHz, 232 kHz, and 343 kHz was used to determine the temperature dependent magnetic energy absorption rate in the temperature range from 160 K to 210 K. We found that the specific absorption rate of the ferrofluid decreased monotonically with temperature over this range at the given frequencies. From these measured data, we determined the temperature dependence of the Néel relaxation time and estimate a room-temperature magnetocrystalline anisotropy constant of 40 kJ/m3, in agreement with previously published results.