Wave‐particle resonant interactions are the main driver of radiation belt dynamics. The electron motion in the strong dipole field consists of three types of essentially periodic components and possesses three corresponding adiabatic invariants. Consequently, three different types of waves contribute to violation of these invariants and associated electron acceleration, scattering, and radial transport. For the two fastest types of periodic motion, gyromotion and bounce motion, theoretical models of adiabatic invariant violation have covered two main regimes—the quasi‐linear regime of invariant diffusion and the nonlinear regime, characterized by large‐amplitude jumps of invariants. Until very recently, the slowest type of particle motion, the azimuthal drift, was included in the global simulations of the radiation belt dynamics only through quasi‐linear models of electron diffusion by ultra‐low‐frequency (ULF) waves. However, the growing number of spacecraft observations of narrow‐band intense ULF waves requires a formulation of a new theoretical framework accounting for the electron nonlinear resonant interactions with such waves. In this commentary, we review the recent progress in the development of this framework, most of which was originally reported in four recent papers (Li et al., 2018, 2020, 2021, 2024, https://doi.org/10.1029/2018gl079038 , https://doi.org/10.1029/2020ja027787 , https://doi.org/10.1029/2020ja028842 , https://doi.org/10.1029/2024ja032742 ). We also discuss the potential importance of nonlinear electron resonances with ULF waves for explaining observational features in electron flux dynamics.

Electron resonant interaction with ultra-low-frequency (ULF) waves is the one of the main drivers of the electron radial transport in the Earth's inner magnetosphere. Recent spacecraft observations reported a possibility for electrons to resonate nonlinearly with intense coherent ULF waves, way beyond traditional approach of a slow diffusive scattering of electrons by a broad-band ULF spectrum. In this study we propose a theoretical model describing key elements of such nonlinear resonant interactions. We adapted the Hamiltonian approach to describe equatorial electron motion in the dipole magnetic field and electric ULF wave field. We demonstrated the presence of two main regimes for ULF-electron interaction: the phase bunching and phase trapping. We discuss possible applications of the proposed theoretical approach and the importance of ULF-electron nonlinear resonance.

Recent studies have demonstrated that the physical properties of water treated with non-thermal plasma, or plasma-activated water (PAW), significantly differ from those of distilled water. For example, contrary to expectation, the viscosity of PAW becomes lower than that of distilled water at certain temperatures. This study developed a model to explain these differences by combining the two-state model of ordinary water, which describes water as a combination of nano-crystalline clusters and amorphous, free-floating molecules, using the Debye–Huckel theory for a fluid containing ions. A model for the viscosity of PAW was then developed from the general model. It explains how PAW has a lower viscosity than distilled water as the temperature decreases and why this effect is stronger than the colligative effect for ideal solutions. Finally, the viscosity model is compared to the experimental measurements of PAW treated with gliding arc plasma, showing that the data match the predicted values quite well. The model of PAW developed here can be used to understand other physical properties beyond viscosity, such as the surface tension, contact angle, electric conductivity, heat capacity, isothermal compressibility, and density, potentially facilitating new applications of PAW.

The purpose of this study is to establish the correlation between the boundary layer over the subgrade and the aerodynamic loads acting on the train model in conventional wind tunnel tests. Firstly, flow characteristics around the subgrade with different leading-edge angles (15◦, 30◦, and 45◦) are investigated through PIV experimental test method. Then, wind tunnel tests of the aerodynamic performance of a high-speed train are carried out. The results are compared with previous experimental data obtained by moving model tests. Results show that, due to the presence of boundary layer, the pressure acting on the lower part of the train head decreases, while on other location is not significantly affected. This is the reason for the reduction of the aerodynamic drag and lift on the train. In addition, the reduction effects become more obviously when the thickness of boundary layer increasing. The experimental results obtained could serve as a calibration of aerodynamic forces for wind tunnel tests on high-speed trains.

The near‐Earth plasma sheet region is the main source of energetic (tens to hundreds keV) ion and electron populations transported by convection and injections into the inner magnetosphere. Energetic ions from the plasma sheet contribute to the ring current, whereas energetic electrons contribute to the radiation belt seed population for further acceleration to relativistic energies. Near‐Earth plasma sheet energetic fluxes have been traditionally used to set boundary conditions for radiation belt and ring current models. This study provides an empirical parametrization for ∼75 keV flux intensity as a function of the geomagnetic activity index auroral electrojet and the equatorial magnetic field B z . Such parametrization includes the dynamic magnetic field configuration in the near‐Earth plasma sheet and may be merged with empirical magnetic field models. We also provide models extending this parametrization to the [20, 300] keV of electron energy range and [75, 300] keV of ion energy range. The parametrization is developed based on THEMIS and Geostationary Operational Environmental Satellite measurements, and verified by comparison with MMS measurements in the near‐Earth plasma sheet. This parametrization incorporates meso‐scale transient flux variations associated with B z perturbations into ring current and radiation belt simulations.

The wave-particle resonant interaction is a key process controlling energetic electron flux dynamics in the Earth’s radiation belts. All existing radiation belt codes are Fokker-Planck models relying on the quasi-linear diffusion theory to describe the impact of wave-particle interactions. However, in the outer radiation belt, spacecraft often detect waves sufficiently intense to interact resonantly with electrons in the nonlinear regime.We propose an approach to (1) estimate the contribution of such nonlinear resonant interactions, and (2) include them into diffusion-based radiation belt models. Using statistics of chorus wave-packet amplitudes and sizes (number of wave periods within one packet), we provide a rescaling factor for the quasi-linear diffusion rates to account for the contribution of nonlinear interactions in long-term electron flux dynamics. Such nonlinear effects may speed up 0.1-1 MeV electron diffusive acceleration by a factor of x2-3 during disturbed periods.

Washing fresh produce using Plasma-activated water recently became a promising eco-friendly alternative to using chemical additives such as Chlorine. We discuss the produce-washing experiments that illustrate that addition of plasma to washing water is a multi-faced phenomena. Not only it increases the sterilization ability of water by killing pathogens, but it also has improved washibility: the ability to remove pathogens from the cleaning surface. We propose an explanation of these features based on the recently discoveries that many physical and chemical properties of water change their temperature dependence between about 35 and 60 degrees Celsius. In particular, heat conductance, light absorption, and surface tension all change their temperature dependence. These drastic changes were associated with water gradually changing its mesoscopic structure: while at the higher temperatures water is a uniform media (amorphous state), at the temperatures below transition it consists of many nano-to-micro-scale clusters (crystalline state). This transition is similar to the second order phase transition. In the present paper we propose that treating water with non-thermal plasma (adding plasma-created active compounds) can lower the temperature of the transition and thus cause a significant change in such physical quantities as surface tension, viscosity, freezing rate, and wettability and washability.

Recently is was discovered in various applications that many physical and chemical properties of water change their temperature dependence between about 35 and 60 degrees Celsius. In particular, heat conductance, light absorption, and surface tension all change their temperature dependence. These drastic changes were associated with water gradually changing its mesoscopic structure: while at the higher temperatures water is a uniform media (amorphous state), at the temperatures below transition it consists of many nano-to-micro-scale clusters (crystalline state). This transition is similar to the second order phase transition. In the present paper we show that treating water with non-thermal plasma (adding plasma-created active compounds) can lower the temperature of the transition and thus cause a significant change in such physical quantities as surface tension, viscosity, freezing rate, and wettability and washability. We present analytical estimates of the transition temperature shift based on the Debya-Huckel theory. We discuss the produce-washing experiments that illustrate the predicted effects.

It has been observed that the condensing air flux through the air-conditioning (AC) unit on a running high-speed train significantly decreases due to the shear stream flowing over the train roof. Insufficient condensing air may put the AC under a risk of overheating. In this study, four shapes of flow deflectors that are able to alleviate the flux drop are designed and investigated using experimentally validated Delayed Detached Eddy Simulation (DDES). Numerical results show that the air flux drop mainly depends on the cross-sectional shape of deflectors, which changes the flow conditions above AC condensing outlets, including the properties of the negative pressure region and the incident angle of the high-speed shear flow produced by the train operation. Meanwhile, the extra aerodynamic drag brought by the installation of deflectors is computed and analyzed. The results presented in this paper can be applied to the design of the AC system, as well as other equipment requiring active convective cooling, on higher speed trains.

Key elements of space weather models are energetic electron fluxes in the inner magnetosphere and the outer radiation belt. Flux depletion is driven by various loss processes: scattering into atmosphere, magnetopause shadowing. Flux enhancement is driven by various acceleration processes: local wave‐particle interactions, radial transport, plasma sheet injections. Many of these processes operate on ∼ hour timescales. Such mesoscale flux variations are not well traced by equatorial spacecraft with much longer orbits. Energetic electron detectors onboard the Global Positioning System (GPS) constellation provide a unique opportunity for probing such ∼ hour‐scale flux variations. Measurements from up to 23 identically instrumented GPS satellites cover a wide energy and L ‐shell range with a subhour time resolution. However, their orbits are inclined and thus all measurements at L ‐shell >4.3 are off‐equatorial. In this report, we present a comparison of equatorial THEMIS and nonequatorial GPS measurements of omnidirectional ≤600 keV electron fluxes. Such a comparison allows us to derive coefficients for using off‐equatorial GPS fluxes to infer the equatorial values. These coefficients depend on particle energy and L ‐shell. We demonstrate a new data set derived from GPS measurements and discuss how it can be used to investigate mesoscale dynamics of energetic electron fluxes in the inner magnetosphere.

Solar wind plasma at the Earth's orbit carries transient magnetic field structures including discontinuities. Their interaction with the Earth's bow shock can significantly alter discontinuity configuration and stability. We investigate such an interaction for the most widespread type of solar wind discontinuities - rotational discontinuities (RDs). We use a set of in situ multispacecraft observations and perform kinetic hybrid simulations. We focus on the RD current density amplification that may lead to magnetic reconnection. We show that the amplification can be as high as two orders of magnitude and is mainly governed by three processes: the transverse magnetic field compression, global thinning of RD, and interaction of RD with low-frequency electromagnetic waves in the magnetosheath, downstream of the bow shock. The first factor is found to substantially exceed simple hydrodynamic predictions in most observed cases, the second effect has a rather moderate impact, while the third causes strong oscillations of the current density. We show that the presence of accelerated particles in the bow shock precursor highly boosts the current density amplification, making the postshock magnetic reconnection more probable. The pool of accelerated particles strongly affects the interaction of RDs with the Earth's bow shock, as it is demonstrated by observational data analysis and hybrid code simulations. Thus, shocks should be distinguished not by the inclination angle, but rather by the presence of foreshocks populated with shock reflected particles. Plasma processes in the RD-shock interaction affect magnetic structures and turbulence in the Earth's magnetosphere and may have implications for the processes in astrophysics.

In this study we consider the Hamiltonian approach for the construction of a map for a system with nonlinear resonant interaction, including phase trapping and phase bunching effects. We derive basic equations for a single resonant trajectory analysis and then generalize them into a map in the energy/pitch-angle space. The main advances of this approach are the possibility of considering effects of many resonances and to simulate the evolution of the resonant particle ensemble on long time ranges. For illustrative purposes we consider the system with resonant relativistic electrons and field-aligned whistler-mode waves. The simulation results show that the electron phase space density within the resonant region is flattened with reduction of gradients. This evolution is much faster than the predictions of quasi-linear theory. We discuss further applications of the proposed approach and possible ways for its generalization.

We combined in-situ solar wind observations by ARTEMIS and MMS missions with kinetic hybrid simulations to study the interaction of solar wind rotational discontinuities (RDs) with the foreshock of the Earth’s bow shock. We found that whistler modes excited by diffuse energetic particles were strongly coupled with RDs and lead to their temporary dissociation. At the same time, RDs trigger the steepening of whistler waves and the generation of ’shocklets’ - small localised shock-like structures, capable of trapping energetic particles and growing up by absorbing the particles energy.

Accurate and controlled movement of small, untethered objects within soft tissues has many potential applications in medical robotics. While medium reaction forces due to slow movement of solid objects in viscoelastic fluids are well-known, such forces have received much less attention in soft media and tissues where the movement is accompanied by highly non-linear and history dependent phenomena. This paper develops a model of such forces for spherical solids. The reaction forces are investigated experimentally in the limit when the spherical solid moves at only a small fraction of its diameter per second. A mathematical model consistent with observations is proposed. The key element of the model is the history-dependent nature of the medium reaction force. A method of the model parameter identification is described, and its experimental implementation is demonstrated in gels that simulate soft tissues. In the experiments, known magnetic forces are employed as the external forces to drag a permanent magnet sphere inside Agarose gel phantom, and video tracking assisted by template matching calculations is used to accurately track the sphere translation. Numerical simulations of the model illustrate results that are consistent with observations.

Synchronized motion and pattern formation of magnetic particle aggregates in variable magnetic field is of growing interest from the point of view of both fundamental science and its applications. In this article, the role of size differences in synchronized motion of magnetic particles under external rotating magnetic field at low Reynolds number flow regimes is considered. Several different motion regimes are observed in simulations as a function of particle size differences and field rotation rate. Simulation indicates phase-like transitions between different motion regimes when particle size differences become significant. It is also shown that particle size differences permit the possibility of locomotion for particle aggregates which cannot be observed for uniform particle sizes.

In this paper, we report a qualitative model of operation and energy release in pulsed dielectric barrier discharges (DBDs). We demonstrate that pulsed DBDs operate according to the “maximum power principle” and explain the relevant physical processes. Compared to experimental data, the proposed model allows an accurate estimation of the discharge pulse energy as a function of dielectric properties, electrode size, and pulse parameters (shape and voltage amplitude).

It is well known that resonance phenomena can destroy adiabatic invariance and cause chaos and mixing. In the present Rapid Communication, we show that a nonlinear wave-particle resonant interaction may do the opposite-generate large-scale coherent structures in phase space. The combined action of the drift due to nonlinear scattering on resonance and trapping (capture) into resonance creates a convection cell-like structure, where the areas of particle acceleration and deceleration are macroscopically separated. At the same time, nonlinear scattering also creates a diffusion that cause mixing on and between the energy levels.