We have solved the long-standing problem of the mechanism of terahertz (THz) generation by a two-color filament in air and found that both neutrals and plasma contribute to the radiation. We reveal ...that the contribution from neutrals by four-wave mixing is much weaker and higher in frequency than the distinctive plasma lower-frequency contribution. The former is in the forward direction while the latter is in a cone and reveals an abrupt down-shift to the plasma frequency. Ring-shaped spatial distributions of the THz radiation are shown to be of universal nature and they occur in both collimated and focusing propagation geometries. Experimental measurements of the frequency-angular spectrum generated by 130-fs laser pulses agree with numerical simulations based on a unidirectional pulse propagation model.
Thin Current Sheet Behind the Dipolarization Front Nakamura, R.; Baumjohann, W.; Nakamura, T. K. M. ...
Journal of geophysical research. Space physics,
October 2021, Letnik:
126, Številka:
10
Journal Article
Recenzirano
Odprti dostop
We report a unique conjugate observation of fast flows and associated current sheet disturbances in the near‐Earth magnetotail by MMS (Magnetospheric Multiscale) and Cluster preceding a positive bay ...onset of a small substorm at ∼14:10 UT, September 8, 2018. MMS and Cluster were located both at X ∼ −14 RE. A dipolarization front (DF) of a localized fast flow was detected by Cluster and MMS, separated in the dawn‐dusk direction by ∼4 RE, almost simultaneously. Adiabatic electron acceleration signatures revealed from the comparison of the energy spectra confirm that both spacecraft encounter the same DF. We analyzed the change in the current sheet structure based on multi‐scale multi‐point data analysis. The current sheet thickened during the passage of DF, yet, temporally thinned subsequently associated with another flow enhancement centered more on the dawnward side of the initial flow. MMS and Cluster observed intense perpendicular and parallel current in the off‐equatorial region mainly during this interval of the current sheet thinning. Maximum field‐aligned currents both at MMS and Cluster are directed tailward. Detailed analysis of MMS data showed that the intense field‐aligned currents consisted of multiple small‐scale intense current layers accompanied by enhanced Hall‐currents in the dawn‐dusk flow‐shear region. We suggest that the current sheet thinning is related to the flow bouncing process and/or to the expansion/activation of reconnection. Based on these mesoscale and small‐scale multipoint observations, 3D evolution of the flow and current‐sheet disturbances was inferred preceding the development of a substorm current wedge.
Key Points
Evolution of localized fast flows and dipolarization front is obtained from multi‐scale multi‐point observations in near‐Earth magnetotail
Current sheet thinning accompanied by intense field‐aligned currents is detected following the passage of the dipolarization front
From signatures of adiabatic electron acceleration it is confirmed that the same flow front was detected by the multi‐point measurements
The reasons for the creation of the Japanese–American Military–Political Alliance and the factors that were conductive for its existence after the conclusion in 1951 of the first Treaty on Security ...Guarantees between Japan and the Unites States are analyzed. The present successor of this treaty, the Treaty on Mutual Cooperation and Security between Japan and the Unites States, which was concluded in 1960, constitutes the foundation of alliance relations between Tokyo and Washington. The content of these treaties, the obligations of the participants according the articles of the treaties, and the evolution of the interpretations of these mutual obligations in accordance with changes in the international situation and in relations in the Asia-Pacific region are considered. Finally, the growing possibilities for Japan to play a more independent role in this bilateral alliance and the reasons why Tokyo has not taken steps to explore them are evaluated.
•β-solidifying TiAl-based alloy subjected to N ion implantation with low (40 keV) energy.•TiN, Ti2N, Ti3AlN, and Ti3Al2N2 particles nitrides formed in the modified layer.•Annealing at 550 or 750 °C ...resulted in dissolution of the Ti2N phase.•N ion implantation significantly enhanced oxidation resistance of the alloy.•Thin, continuous, and stable oxide layer is formed during oxidation tests.
Oxidation behavior and thermal stability of a β-solidifying γ-TiAl alloy after plasma source nitrogen ion implantation were studied. Nano-sized TiN, Ti2N, Ti3AlN, and Ti3Al2N2 particles were found in the modified layer. The microhardness of the modified layer was 2.5 times higher than that of the as-cast condition. Nitrogen ion implantation increased the oxidation resistance significantly in comparison to the as-cast condition. Thin inner and outer TiO2, dense protective Al2O3 and fine-grained mixed Al2O3+TiO2 layers were found on the surface after oxidation. The effect of nitrogen ion implantation on phase stability and oxidation mechanisms is analyzed.
The paper reveals the possibility for obtaining excellent strength-ductility combination under both tension and compression in a fully lamellar β-solidified γ-TiAl based alloy by controlling the ...interlamellar spacing. Ingots of the alloy were subjected to multi-axial forging, quenching and further aging at 800–1050 °C to produce microstructures with the colony size of 25 μm and interlamellar spacing (λ) in the range from 10 to 1500 nm. A good balance between ductility and strength was found at λ ∼100 nm under both tension (elongation to fracture of 2.9%; ultimate strength - 875 MPa) and compression (peak strain and stress of 39% and 2700 MPa, respectively) due to a strain transfer by transverse twin/slip bands that improved strain hardening capacity. Both coarser and thinner lamellae caused pronounced strain localization and early crack nucleation resulting in failure. Meanwhile, the Hall-Petch relationship was observed between the yield strength and interlamellar spacing above λ ∼20–50 nm. At a lower interlamellar spacing the yield strength stabilized at the values of 820 and 1150 MPa upon tensile and compression loading, respectively. The dependences of deformation and fracture mechanisms on the interlamellar spacing and loading scheme are discussed.
•Interlamellar spacing (λ) of Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B alloy varies from 10 to 1500 nm.•Tensile-compression asymmetry is derived.•Maximum of ductility and ultimate strength is observed at λ∼100 nm under both tension and compression.•Yield strength and interlamellar spacing obey to the Hall-Petch relationship above the critical interlamellar spacing.•Yield strength stabilized below the critical interlamellar spacing.
We construct a supercharacter theory for the finite groups of triangular type. Its special case is the supercharacter theory for algebra groups of Diaconis and Isaacs. The supercharacter analog of ...the Kirillov formula for irreducible characters of unipotent groups is proved.
We examine a Dipolarization Front (DF) event with an embedded electron diffusion region (EDR), observed by the Magnetospheric Multiscale (MMS) spacecraft on 08 September 2018 at 14:51:30 UT in the ...Earth's magnetotail by applying multi‐scale multipoint analysis methods. In order to study the large‐scale context of this DF, we use conjunction observations of the Cluster spacecraft together with MMS. A polynomial magnetic field reconstruction technique is applied to MMS data to characterize the embedded electron current sheet including its velocity and the X‐line exhaust opening angle. Our results show that the MMS and Cluster spacecraft were located in two counter‐rotating vortex flows, and such flows may distort a flux tube in a way that the local magnetic shear angle is increased and localized magnetic reconnection may be triggered. Using multi‐point data from MMS we further show that the local normalized reconnection rate is in the range of R ∼ 0.16 to 0.18. We find a highly asymmetric electron in‐ and outflow structure, consistent with previous simulations on strong guide‐field reconnection events. This study shows that magnetic reconnection may not only take place at large‐scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets, produced as a consequence of the interaction between the fast Earthward flows and the Earth's dipole field.
Plain Language Summary
Magnetic Reconnection is a key energy conversion process, where magnetic energy is converted into kinetic energy of plasma particles. During this process the magnetic field topology changes and the plasma particles decouple from the magnetic field in the so‐called diffusion region and get accelerated, forming a fast outflow jet. Over the last decades, hints arise that reconnection can take place at many different places in the magnetosphere and also very locally and intermittently. Fast plasma flows in the Magnetotail, moving toward the Earth, are assumed to be a consequence of magnetic reconnection, and are often accompanied by dipolar‐shaped magnetic flux bundles, embedded into them. The leading edges of such flux bundles are called dipolarization fronts (DF). In this work, we investigate a DF event, which hosts a diffusion region. First, we study the large‐scale characteristics of the DF, by utilizing data from both the Magnetospheric Multiscale (MMS) and the Cluster mission, that observe different regions of the event almost simultaneously. Second, we performed a 3D magnetic field reconstruction technique and compared the results to MMS data, to investigate the event on small scales.
Key Points
A thin current sheet inside a dipolarization front, embedded in a diverging flow is analyzed using a polynomial reconstruction technique
Transient reconnection event is detected in a high magnetic shear region, where the magnetic field is deflected due to duskward fast plasma flow
The reconstructed current sheet has a guide field of ∼1.8 the reconnecting component with normalized reconnection rate between 0.16 and 0.18
We construct two supercharacter theories (in the sense of P. Diaconis and I.M. Isaacs) for the parabolic subgroups in orthogonal and symplectic groups. For each supercharacter theory, we obtain a ...supercharacter analog of the A.A. Kirillov formula for irreducible characters of finite unipotent groups.
The article deals with the actual problem of risk optimization for complex transport systems. Optimization models for risk aspects are proposed and developed: "probability-damage"; "quality-price"; ..."variation-damage"; acceptability of the measurement system; degradation monitoring.
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