Drift Resonance Research Articles

Giant pulsations on the afternoonside: Geostationary satellite and ground observations

AbstractGiant pulsations (Pgs) are a special class of oscillations recognized in ground magnetometer records as exhibiting highly regular sinusoidal waveforms in the east‐west component with periods around 100s. Previous statistical studies showed that Pgs occur almost exclusively on the morningside with peak occurrence in the postmidnight sector. In this paper, we present observations of Pgs extending to the afternoonside, using data from the GOES13 and 15 geostationary satellites and multiple ground magnetometers located in North America. For a long‐lasting event on 29 February 2012, which spanned ∼08–18h magnetic local time, we show that basic Pg properties did not change with the local time, although the period of the pulsations was longer at later local time due to increasing mass loading. There is evidence that the Pgs resulted from fundamental poloidal mode standing Alfvén waves, both on the morning and afternoonsides. Oscillations of energetic particles associated with the field oscillations exhibited an energy‐dependent phase, which has previously been reported and explained by drift resonance. A statistical analysis of the ground magnetic field data (L = 3.8–7.4) covering 2008–2013 confirms that afternoon Pgs are not unusual. We identified a total of 105 Pg events (about 70% (30%) of the events occurred during non‐storm (late storm recovery) periods), 31 of which occurred on the afternoonside. The afternoon Pgs occur under solar wind and geomagnetic conditions that are similar to the morning Pgs, but the afternoon Pgs tend to have short durations and occur frequently in winter instead of around spring and fall equinoxes that are favored by the morning Pgs.

A novel non-linear elastic wave acoustic spectroscopy (NEWS) non-destructive inspection (NDI) method for aeronautic and spacecraft materials and components

Current non-destructive inspection (NDI) methods have limitations in their ability to identify certain flaws in complex materials and structures where a defect has not been fully formed, for example a kissing de-bond early-stage defect. Non-linear elastic wave acoustic spectroscopy (NEWS) methods have shown potential to detect early-stage flaws in metals and composites. The objectives of the ESA study reported here were to review the state of the art, then to design and build a breadboard demonstrator “NEWS Imaging” (NEWSI) non-linear acoustics instrument, and finally to demonstrate its capability using well-characterised reference defect artefacts (RDAs). A multi-mode NEWSI instrument was produced with the following modes of operation: (a) single and dual frequency, (b) pulse inversion, and (c) resonance drift modes. The first two are suitable for C-scan style imaging by contact testing or with the NACE (air-coupled detection) mode. A set of RDAs were sourced or manufactured, the most important of which was the development of successful kissing de-bonds; areas of significantly reduced bond strength, created in the bond-line between the composite laminates. All the RDAs were characterised by a wide range of complementary NDI methods, and the best results in terms of detection of all the known flaws was the scanning acoustic microscopy ultrasonic C-scan method. This technique detected all of the RDA defects except for kissing de-bonds. The NEWSI instrument was shown to be capable of detecting, with a certain degree of repeatability under controlled conditions, early-stage defects (kissing de-bonds) in composite materials. No other NDI method is known to detect these defect types. The results so far are encouraging and have potentially a lot of interest for ESA and the wider industry, if the unique capabilities of the NEWSI instrument demonstrated on the kissing de-bonds could be developed into a “routine practical test instrument”. The target of the next stage of work is to bring the technology readiness level of the instrument up to pre-production levels of development.

Effects of parallel sound wave damping and drift kinetic damping on the resistive wall mode stability with various plasma rotation profiles

The effect of a parallel viscous force induced damping and the magnetic precessional drift resonance induced damping on the stability of the resistive wall mode (RWM) is numerically investigated for one of the advanced steady-state scenarios in international thermonuclear experimental reactor (ITER). The key element of the investigation is to study how different plasma rotation profiles affect the stability prediction. The single-fluid, toroidal magnetohydrodynamic (MHD) code MARS-F (Liu et al., Phys. Plasmas, vol. 7, 2000, p. 3681) and the MHD–kinetic hybrid code MARS-K (Liu et al., Phys. Plasmas, vol. 15, 2008, 112503) are used for this purpose. Three extreme rotation profiles are considered: (a) a uniform profile with no shear, (b) a profile with negative flow shear at the $q=2$ rational surface ($q$ is the equilibrium safety factor), and (c) a profile with positive shear at $q=2$. The parallel viscous force is found to be effective for the mode stabilization at high plasma flow speed (about a few percent of the Alfven speed) for the no shear flow profile and the negative shear flow profile, but the stable domain does not appear with the positive shear flow profile. The predicted eigenmode structure is different with different rotation profiles. With a self-consistent inclusion of the magnetic precession drift resonance of thermal particles in MARS-K computations, a lower critical flow speed, i.e. the minimum speed needed for full suppression of the mode, is obtained. Likewise the eigenmode structure is also modified by different rotation profiles in the kinetic results.

Imprints of impulse‐excited hydromagnetic waves on electrons in the Van Allen radiation belts

AbstractUltralow frequency electromagnetic oscillations, interpreted as standing hydromagnetic waves in the magnetosphere, are a major energy source that accelerates electrons to relativistic energies in the Van Allen radiation belt. Electrons can rapidly gain energy from the waves when they resonate via a process called drift resonance, which is observationally characterized by energy‐dependent phase differences between electron flux and electromagnetic oscillations. Such dependence has been recently observed and interpreted as spacecraft identifications of drift resonance electron acceleration. Here we show that in the initial wave cycles, the observed phase relationship differs from that characteristic of well‐developed drift resonance. We further examine the differences and find that they are imprints of impulse‐excited, coupled fast‐Alfvén waves before they transform into more typical standing waves. Our identification of such imprints provides a new understanding of how energy couples in the inner magnetosphere, and a new diagnostic for the generation and growth of magnetospheric hydromagnetic pulsations.

Stabilization of resistive wall modes in tokamaks by drift kinetic effects combined with magnetic feedback

The synergetic effects of drift kinetic resonances, the resistive layer dissipation, the magnetic feedback, and the toroidal plasma flow on the stability of the resistive wall mode are numerically investigated using a full toroidal resistive magnetohydrodynamic-kinetic hybrid stability code MARS-K (Liu et al 2008 Phys. Plasmas 15 112503). It is found that the plasma resistivity, coupled with the favourable average curvature effect, can enlarge the stable domain predicted by the drift kinetic model. A synergy between the precessional drift resonance damping, the magnetic feedback and the plasma flow helps open two stability windows. The width of the inner stability window increases with the feedback gain, but decreases with the flow speed. In addition, optimization of the toroidal phase difference of the feedback gains between the upper and lower active coils can lead to a full suppression of the mode.

Collisionless microtearing modes in hot tokamaks: Effect of trapped electrons

Collisionless microtearing modes have recently been found linearly unstable in sharp temperature gradient regions of large aspect ratio tokamaks. The magnetic drift resonance of passing electrons has been found to be sufficient to destabilise these modes above a threshold plasma β. A global gyrokinetic study, including both passing electrons as well as trapped electrons, shows that the non-adiabatic contribution of the trapped electrons provides a resonant destabilization, especially at large toroidal mode numbers, for a given aspect ratio. The global 2D mode structures show important changes to the destabilising electrostatic potential. The β threshold for the onset of the instability is found to be generally downshifted by the inclusion of trapped electrons. A scan in the aspect ratio of the tokamak configuration, from medium to large but finite values, clearly indicates a significant destabilizing contribution from trapped electrons at small aspect ratio, with a diminishing role at larger aspect ratios.

Overview of gyrokinetic studies of finite-β microturbulence

Recent results on electromagnetic turbulence from gyrokinetic studies in different magnetic configurations are overviewed, detailing the physics of electromagnetic turbulence and transport, and the effect of equilibrium magnetic field scale lengths. Ion temperature gradient (ITG) turbulence is shown to produce magnetic stochasticity through nonlinear excitation of linearly stable tearing-parity modes. The excitation, which is catalyzed by the zonal flow, produces an electron heat flux proportional to β2 that deviates markedly from quasilinear theory. Above a critical beta known as the non-zonal transition (NZT), the magnetic fluctuations disable zonal flows by allowing electron streaming that shorts zonal potential between flux surfaces. This leads to a regime of very high transport levels. Kinetic ballooning mode (KBM) saturation is described. For tokamaks saturation involves twisted structures arising from magnetic shear; for helical plasmas oppositely inclined convection cells interact by mutual shearing. Microtearing modes are unstable in the magnetic geometry of tokamaks and the reversed field pinch (RFP). In NSTX instability requires finite collisionality, large beta, and is favored by increasing magnetic shear and decreasing safety factor. In the RFP, a new branch of microtearing with finite growth rate at vanishing collisionality is shown from analytic theory to require the electron grad-B/curvature drift resonance. However, gyrokinetic modeling of experimental MST RFP discharges at finite beta reveals turbulence that is electrostatic, has large zonal flows, and a large Dimits shift. Analysis shows that the shorter equilibrium magnetic field scale lengths increase the critical gradients associated with the instability of trapped electron modes, ITG and microtearing, while increasing beta thresholds for KBM instability and the NZT.

Controllable near-field intensity and spot size of hybrid terahertz metamaterial.

We report controllable near fields around split-ring resonator (SRR) gaps of an active terahertz metamaterial. As extension of parallel-plate capacitors, patterned VO2 is integrated into the metallic SRRs to manipulate the near-field intensity and hot spot size through its metal-insulator transition. This design enhances the device reliability by preventing VO2 dielectric breakdown at a strongly enhanced near field. The near-field intensity and spot size are tunable in broad ranges, and the device is demonstrated to be capable of compensating resonant frequency drift arisen from different interactions due to near-field coupling. It provides an effective method to actively manipulate the light-matter interaction through the strongly enhanced and tunable near fields.

Mechanism of runaway electron beam formation during plasma disruptions in tokamaks

A new physical mechanism of formation of runaway electron beams during plasma disruptions in tokamaks is proposed. The plasma disruption is caused by a strong stochastic magnetic field formed due to nonlinearly excited low-mode number magnetohydrodynamic (MHD) modes. It is conjectured that the runaway electron beam is formed in the central plasma region confined inside the intact magnetic surface located between q = 1 and the closest low–order rational magnetic surfaces [q = 5/4 or q = 4/3,…]. It results in that runaway electron beam current has a helical nature with a predominant m/n = 1/1 component. The thermal quench and current quench times are estimated using the collisional models for electron diffusion and ambipolar particle transport in a stochastic magnetic field, respectively. Possible mechanisms for the decay of the runaway electron current owing to an outward drift electron orbits and resonance interaction of high–energy electrons with the m/n = 1/1 MHD mode are discussed.

Transfer efficiency analysis of wireless power transfer system under frequency drift

Magnetic resonant wireless power transfer (WPT) is an emerging technology that may create new applications for wireless power charging. However, low efficiency resulting from resonant frequency drift is a main obstructing factor for promoting this technology. In this paper, the system efficiency and input impedance are obtained by solving the system equivalent equations and based on that a method of adjusting the operating frequency and load resistor is proposed. When resonant frequency drift occurs, the WPT system can now operate at quasi-resonant state and the efficiency can be improved by using the proposed method. The WPT system via magnetic resonance coupling is designed. Simulation and experimental results validating the proposed method are given.

Shock‐induced prompt relativistic electron acceleration in the inner magnetosphere

AbstractWe present twin Van Allen Probes spacecraft observations of the effects of a solar wind shock impacting the magnetosphere on 8 October 2013. The event provides details both of the accelerating electric fields associated with the shock and the response of inner magnetosphere electron populations across a broad range of energies. During this period, the two Van Allen Probes observed shock effects from the vantage point of the dayside magnetosphere at radial positions of L = 3 and L = 5, at the location where shock‐induced acceleration of relativistic electrons occurs. The extended (~1 min) duration of the accelerating electric field across a broad extent of the dayside magnetosphere, coupled with energy‐dependent relativistic electron gradient drift velocities, selects a preferred range of energies (3–4 MeV) for the initial enhancement. Those electrons—whose drift velocity closely matches the azimuthal phase velocity of the shock‐induced pulse—stayed in the accelerating wave as it propagated tailward and received the largest increase in energy. Drift resonance with subsequent strong ULF waves further accentuated this range of electron energies. Phase space density and positional considerations permit the identification of the source population of the energized electrons. Observations detail the promptness (<20 min), energy range (1.5–4.5 MeV), energy increase (~500 keV), and spatial extent (L* ~3.5–4.0) of the enhancement of the relativistic electrons. Prompt acceleration by impulsive shock‐induced electric fields and subsequent ULF wave processes therefore comprises a significant mechanism for the acceleration of highly relativistic electrons deep inside the outer radiation belt as shown clearly by this event.

Coreless, Contactless Power Supply System with DSP Controlled Active Compensation of Parameter Changes

Contactless power supply systems are of high interest in small power and large power applications. Many high efficiency contactless systems use the resonant operation mode. A parallel compensation system is presented enlarging overall system safety and allowing for resonant system parameter change while maintaining zero current switching (ZCS) operation. Basics of digital control unit construction and operation are presented. Simulation and experimental results are given for control system operation. Systems performance allows for large resonant circuit parameter drift. Modern DSP control unit minimizes the necessary external circuitry and time delays in case of most important operations. DOI: http://dx.doi.org/10.5755/j01.eee.20.10.4989

Asymptotic dynamics of reflecting spiral waves.

Resonantly forced spiral waves in excitable media drift in straight-line paths, their rotation centers behaving as pointlike objects moving along trajectories with a constant velocity. Interaction with medium boundaries alters this velocity and may often result in a reflection of the drift trajectory. Such reflections have diverse characteristics and are known to be highly nonspecular in general. In this context we apply the theory of response functions, which via numerically computable integrals, reduces the reaction-diffusion equations governing the whole excitable medium to the dynamics of just the rotation center and rotation phase of a spiral wave. Spiral reflection trajectories are computed by this method for both small- and large-core spiral waves in the Barkley model. Such calculations provide insight into the process of reflection as well as explanations for differences in trajectories across parameters, including the effects of incidence angle and forcing amplitude. Qualitative aspects of these results are preserved far beyond the asymptotic limit of weak boundary effects and slow resonant drift.

Role of Trapped Electrons on Global Gyrokinetic Linear Stability of Collisionless Microtearing Modes

Unstable collisional MicroTearing Mode (MTMs) have been found in experiments of high-β Spherical Tokamaks and are believed to be driven by drift resonance of trapped electrons. It has been recently shown that at large aspect ratio, the magnetic drift resonance of highly passing electrons is a minimal mechanism to drive the collisionless MTM unstable. In this work, a preliminary study of inclusion of trapped electrons in large aspect ratio tokamaks indicate that for the reference parameters investigated, the collisionless MTM retain their essential mode structures, while growth rates are only moderately affected.

Drift resonance effect on stochastic runaway electron orbit in the presence of low-order magnetic perturbations

During major disruptions, an induced loop voltage accelerates runaway electrons (REs) towards high energy, being in the order of 1–100 MeV in present tokamaks and ITER. The stochastization mechanisms of such high-energy RE drift orbits are investigated by three-dimensional (3D) orbit following in tokamak plasmas. Drift resonance is shown to play an important role in determining the onset of stochastic drift orbits for different electron energies, particularly in cases with low-order perturbations that have radially global eigenfunctions of the scale of the plasma minor radius. The drift resonance due to the coupling between the cross-field drift motion with radially global modes yields a secondary island structure in the RE drift orbit, where the width of the secondary drift islands shows a square-root dependence on the relativistic gamma factor γ. Only for highly relativistic REs (γ ≫ 1), the widths of secondary drift islands are comparable with those of magnetic islands due to the primary resonance, thus the stochastic threshold becoming sensitive to the RE energy. Because of poloidal asymmetry due to toroidicity, the threshold becomes sensitive not only to the relative amplitude but also to the phase difference between the modes. In this paper, some examples of 3D orbit-following calculations are presented for analytic models of magnetic perturbations with multiple toroidal mode numbers, for both possibilities that the drift resonance enhances and suppresses the stochastization being illustrated.

An unusual long‐lived relativistic electron enhancement event excited by sequential CMEs

AbstractAn unusual long‐lived intense relativistic electron enhancement event from July to August 2004 is examined using data from Fengyun‐1, POES, GOES, ACE, the Cluster Mission, and geomagnetic indices. During the initial 6 days of this event, the observed fluxes in the outer zone enhanced continuously, and their maximum increased from 2.1 × 102 cm−2 sr−1 s−1 to 3.5 × 104 cm−2 sr−1 s−1, the region of enhanced fluxes extended from L = 3.5–6.5 to L = 2.5–6.5, and the flux peak location shifted inward from L ~ 4.2 to L ~ 3.3. During the following 7 days, without any locational movement, the flux peak increased slowly and exceeded the prestorm fluxes by about 4 orders of magnitude. Subsequently, the decay rate of relativistic electrons is so slow that the peak remains over 104 cm−2 sr−1 s−1 for about 30 days. The drift resonance between ULF waves, which arose from high‐speed solar wind and frequent impulses of solar wind dynamic pressure, and energetic electrons injected by substorms could be an important acceleration mechanism in this event. The local acceleration by whistler mode chorus could be another mechanism contributing to this enhancement. The plasmaspheric response to the interplanetary disturbances reveals that the enhanced outer zone is divided into two portions by the plasmapause. Accordingly, the slow loss rate in the plasmasphere due to hiss primarily contributed to the long‐lived characteristic of this event. This event reveals that the outer zone population behaviors are dominated by the interplanetary variations together with the responses of geomagnetic field and plasmasphere to these variations.

Modeling radiation belt electron acceleration by ULF fast mode waves, launched by solar wind dynamic pressure fluctuations

AbstractWe investigate the magnetospheric MHD and energetic electron response to a Storm Sudden Commencement (SSC) and subsequent magnetopause buffeting, focusing on an interval following an SSC event on 25 November 2001. We find that the electron flux signatures observed by LANL, Cluster, and GOES spacecraft during this event can largely be reproduced using an advective kinetic model for electron phase space density, using externally prescribed electromagnetic field inputs, (herein described as a “test‐kinetic model”) with electromagnetic field inputs provided by a 2‐D linear ideal MHD model for ULF waves. In particular, we find modulations in electron flux phase shifted by 90° from the local azimuthal ULF wave electric field (Eφ) and a net enhancement in electron flux after 1.5 h for energies between 500 keV and 1.5 MeV near geosynchronous orbit. We also demonstrate that electrons in this energy range satisfy the drift resonance condition for the ULF waves produced by the MHD model. This confirms the conclusions reached by Tan et al. (2011), that the energization process in this case is dominated by drift‐resonant interactions between electrons and MHD fast mode waves, produced by fluctuations in solar wind dynamic pressure.

A ULF wave driver of ring current energization

AbstractULF wave radial diffusion plays an important role in the transport of energetic electrons in the outer radiation belt, yet similar ring current transport is seldom considered even though ions satisfy a nearly identical drift resonance condition albeit without the relativistic correction. By examining the correlation between ULF wave power and the response of the ring current, characterized by Dst, we demonstrate a definite correlation between ULF wave power and Dst. Significantly, the lagged correlation peaks such that ULF waves precede the response of the ring current and Dst. We suggest that this correlation is the result of enhanced radial transport and energization of ring current ions through drift resonance and ULF wave radial diffusion of ring current ions. An analysis and comparison of the ion and electron diffusion coefficients further support this conclusion, ULF waves providing an important missing physical transport process explaining Dst underestimation in ring current models.

Interactions of energetic electrons with ULF waves triggered by interplanetary shock: Van Allen Probes observations in the magnetotail

AbstractWe present in situ observations of a shock‐induced substorm‐like event on 13 April 2013 observed by the newly launched Van Allen twin probes. Substorm‐like electron injections with energy of 30–500 keV were observed in the region from L∼5.2 to 5.5 immediately after the shock arrival (followed by energetic electron drift echoes). Meanwhile, the electron flux was clearly and strongly varying on the ULF wave time scale. It is found that both toroidal and poloidal mode ULF waves with a period of 150 s emerged following the magnetotail magnetic field reconfiguration after the interplanetary (IP) shock passage. The poloidal mode is more intense than the toroidal mode. The 90° phase shift between the poloidal mode Br and Ea suggests the standing poloidal waves in the Northern Hemisphere. Furthermore, the energetic electron flux modulations indicate that the azimuthal wave number is ∼14. Direct evidence of drift resonance between the injected electrons and the excited poloidal ULF wave has been obtained. The resonant energy is estimated to be between 150 keV and 230 keV. Two possible scenaria on ULF wave triggering are discussed: vortex‐like flow structure‐driven field line resonance and ULF wave growth through drift resonance. It is found that the IP shock may trigger intense ULF wave and energetic electron behavior at L∼3 to 6 on the nightside, while the time profile of the wave is different from dayside cases.

Simultaneous measuring of magnetic field and temperature based on photonic crystal fiber loop filled with magnetic fluid

ABSTRACTA novel simultaneous measurement method for magnetic field and temperature is proposed based on the photonic crystal fiber loop mirror selectively filled magnetic fluid. The measuring principle was introduced in detail and verified by experiment. Sensitivity coefficients that the pitch between adjacent resonant valley and the resonant valley drift changed with temperature and magnetic field were achieved theoretically and found to be consistent basically. Therefore, the temperature and magnetic field can be simultaneous detected according to the two‐parameter matrix. The measurement errors and the fitting curves between resonant valley pitch, resonant valley shift, and temperature/magnetic field were given by experiment. The results show that the theoretical calculations and experimental results are basically consistent, and the scheme is feasible. The detection resolution of temperature and magnetic field were 0.07°C and 0.41 Oe respectively for resolution of 0.02 nm spectrometer. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2548–2552, 2014