Magnetohydrodynamic Description Research Articles

Characterizing the Solar Wind‐Magnetosphere Viscous Interaction at Uranus and Neptune

AbstractThe solar wind interaction with planetary magnetospheres dictates the mechanism through which energy is transported across planetary systems. The magnetohydrodynamic plasma description suggests that solar wind conditions in the outer solar system encourage the magnetopause boundaries at Uranus and Neptune to be more Kelvin‐Helmholtz unstable, however, no quantitative assessment has been performed. To characterize the viscous solar wind interaction at Uranus and Neptune, we create an analytical model to determine where Kelvin‐Helmholtz Instabilities (KHIs) may form along their magnetopauses by searching for regions where the minimum condition for KHI formation is satisfied. We run the model at solstice and equinox for a range of Interplanetary Magnetic Field (IMF) strengths, and rotation phases. We find minimal seasonal variation for low IMF strengths (B = 0.01 nT), with ∼70% of the magnetopause surface at Uranus and ∼80% at Neptune, enabling KHI formation. For periods of stronger IMF strength (B > 0.3 nT), KHIs were significantly suppressed. While KHIs depend on both the conditions inside the magnetopause boundary and the shocked solar wind IMF strength, we find that the IMF strength is the most significant criterion in determining whether or not KHIs are allowed to form at the magnetopause boundaries.

Anisotropy and paramagnetism of QCD matter with an anomalous magnetic moment

We employ the Polyakov-loop enhanced Nambu–Jona-Lasinio model incorporating the quark anomalous magnetic moment to investigate the anisotropy structure and the renormalized magnetization of magnetized quark matter at finite temperature. The ultraviolet divergences and nonphysical oscillatory behavior are eliminated by the vacuum magnetic regularization scheme. With a parametrization of the anomalous magnetic moment that is proportional to the square of the chiral condensate, the renormalized magnetization is enlarged by the strong magnetic field so that the anisotropy becomes more apparent. The inflection point of the renormalized magnetization indicates the pseudocritical temperature for the chiral crossover. We find that the results with the anomalous magnetic moment are closer to the lattice quantum chromodynamics data. The connection between the paramagnetism and the chiral transition provides new insight into a magnetohydrodynamics description of hot and dense QCD matter produced in heavy-ion collisions.

Hybrid-Vlasov simulation of soft X-ray emissions at the Earth’s dayside magnetospheric boundaries

Solar wind charge exchange produces emissions in the soft X-ray energy range which can enable the study of near-Earth space regions such as the magnetopause, the magnetosheath and the polar cusps by remote sensing techniques. The Solar wind–Magnetosphere–Ionosphere Link Explorer (SMILE) and Lunar Environment heliospheric X-ray Imager (LEXI) missions aim to obtain soft X-ray images of near-Earth space thanks to their Soft X-ray Imager (SXI) instruments. While earlier modeling works have already simulated soft X-ray images as might be obtained by SMILE SXI during its mission, the numerical models used so far are all based on the magnetohydrodynamics description of the space plasma.To investigate the possible signatures of ion-kinetic-scale processes in soft X-ray images,we use for the first time a global hybrid-Vlasov simulation of the geospace from the Vlasiator model. The simulation is driven by fast and tenuous solar wind conditions and purely southward interplanetary magnetic field. We first produce global X-ray images of the dayside near-Earth space by placing a virtual imaging satellite at two different locations,providing meridional and equatorial views. We then analyze regional features present in the images and show that they correspond to signatures in soft X-ray emissions of mirror mode wave structures in the magnetosheath and flux transfer events (FTEs) at the magnetopause. Our results suggest that, although the time scales associated with the motion of those transient phenomena will likely be significantly smaller than the integration time of the SMILE and LEXI imagers, mirror-mode structures and FTEs can cumulatively produce detectable signatures in the soft X-ray images. For instance, a local increase by 30% in the proton density at the dayside magnetopause resulting from the transit of multiple FTEs leads to a 12% enhancement in the line-of-sight- and timeintegrated soft X-ray emissivity originating from this region. Likewise, a proton density increase by 14% in the magnetosheath associated with mirror-mode structures can result in an enhancement in the soft X-ray signal by 4%. These are likely conservative estimates, given that the solar wind conditions used in the Vlasiator run can be expected to generate weaker soft X-ray emissions than the more common denser solar wind. These results will contribute to the preparatory work for the SMILE and LEXI missions by providing the community with quantitative estimates of the effects of small-scale, transient phenomena occurring on the dayside.

Linear Mode Decomposition in Magnetohydrodynamics Revisited

Small-amplitude fluctuations in the magnetized solar wind are measured typically by a single spacecraft. In the magnetohydrodynamics (MHD) description, fluctuations are typically expressed in terms of the fundamental modes admitted by the system. An important question is how to resolve an observed set of fluctuations, typically plasma moments such as the density, velocity, pressure, and magnetic field fluctuations, into their constituent fundamental MHD modal components. Despite its importance in understanding the basic elements of waves and turbulence in the solar wind, this problem has not yet been fully resolved. Here, we introduce a new method that identifies between wave modes and advected structures such as magnetic islands or entropy modes and computes the phase information associated with the eligible MHD modes. The mode-decomposition method developed here identifies the admissible modes in an MHD plasma from a set of plasma and magnetic field fluctuations measured by a single spacecraft at a specific frequency and an inferred wavenumber k m . We present data from three typical intervals measured by the Wind and Solar Orbiter spacecraft at ∼1 au and show how the new method identifies both propagating (wave) and nonpropagating (structures) modes, including entropy and magnetic island modes. This allows us to identify and characterize the separate MHD modes in an observed plasma parcel and to derive wavenumber spectra of entropic density, fast and slow magnetosonic, Alfvénic, and magnetic island fluctuations for the first time. These results help identify the fundamental building blocks of turbulence in the magnetized solar wind.

Efficiency of Magnetohydrodynamic Wave Generation in Weakly Ionized Atmospheres

The generation of Alfvén and slow magnetoacoustic waves in weakly ionized atmospheres by excitation of the charges-only component of the two-fluid (charge and neutral) plasma is shown to be more or less efficient depending on the energy fraction initially allocated to the three stationary flow differential modes, which characterize the inter-species drift. This is explained via detailed analysis of the full 10-dimensional spectral description of two-fluid linear magnetohydrodynamics. Excitation via the velocity of the charges only is found to be very inefficient, in accord with previous results, while excitation via the magnetic field perturbation alone is highly efficient. All 10 eigenvalues and eigenvectors are presented analytically in the high collision frequency regime.

Wave modes and instabilities in gravitating magnetized polytropic quantum plasmas including viscosity tensor and FLR corrections

ABSTRACT The present analytical study extends the problems of pressure anisotropy-driven instabilities and gravitational instability in space plasmas to mixed quantum polytropic gas in the interior of dense stars accounting for the effects of viscosity, finite Larmor radius (FLR) and self-gravitational effects. The generalized polytrope pressure laws are considered as adiabatic equations in which the pressure components depend upon the plasma density, magnitude of the magnetic field, and the polytrope indices. The modified properties of waves and instabilities in gravitating quantum plasmas have been analysed using the quantum magnetohydrodynamic (QMHD) fluid description in the magnetohydrodynamic (MHD) and Chew–Goldberger–Low (CGL) limits. In the parallel propagation, the Jeans instability modified by quantum diffraction parameters and firehose mode modified by FLR parameter is obtained separately. The Jeans instability condition depends upon the quantum diffraction term and polytrope index β, and it remains unaffected due to viscosity and ion Larmor frequency. The growth rate of the Jeans instability decreases due to viscosity and quantum diffraction parameters, while the growth rate of the firehose instability increases due to FLR corrections. In the transverse mode, a similar nature is observed in the growth rates; however, the instability region decreases significantly due to polytrope indices and different dispersion properties of MHD and CGL viscous quantum plasmas. The analytical results have been applied in dense degenerate stars to measure the characteristic parameters and understand the MHD wave propagation, pressure anisotropy-driven, and gravitationally driven instabilities.

Holography and magnetohydrodynamics with dynamical gauge fields

Within the framework of holography, the Einstein-Maxwell action with Dirichlet boundary conditions corresponds to a dual conformal field theory in presence of an external gauge field. Nevertheless, in many real-world applications, e.g., magnetohydrodynamics, plasma physics, superconductors, etc. dynamical gauge fields and Coulomb interactions are fundamental. In this work, we consider bottom-up holographic models at finite magnetic field and (free) charge density in presence of dynamical boundary gauge fields which are introduced using mixed boundary conditions. We numerically study the spectrum of the lowest quasi-normal modes and successfully compare the obtained results to magnetohydrodynamics theory in 2 + 1 dimensions. Surprisingly, as far as the electromagnetic coupling is small enough, we find perfect agreement even in the large magnetic field limit. Our results prove that a holographic description of magnetohydrodynamics does not necessarily need higher-form bulk fields but can be consistently derived using mixed boundary conditions for standard gauge fields.

Legolas: magnetohydrodynamic spectroscopy with viscosity and Hall current

Many linear stability aspects in plasmas are heavily influenced by non-ideal effects beyond the basic ideal magnetohydrodynamics (MHD) description. Here, the extension of the modern open-source MHD spectroscopy code Legolas with viscosity and the Hall current is highlighted and benchmarked on a stringent set of historic and recent findings. The viscosity extension is demonstrated in a cylindrical set-up featuring Taylor–Couette flow and in a viscoresistive plasma slab with a tearing mode. For the Hall extension, we show how the full eigenmode spectrum relates to the analytic dispersion relation in an infinite homogeneous medium. We quantify the Hall term influence on the resistive tearing mode in a Harris current sheet, including the effect of compressibility, which is absent in earlier studies. Furthermore, we illustrate how Legolas mimics the incompressible limit easily to compare with literature results. Going beyond published findings, we emphasise the importance of computing the full eigenmode spectrum, and how elements of the spectrum are modified by compressibility. These extensions allow for future stability studies with Legolas that are relevant to ongoing dynamo experiments, protoplanetary disks or magnetic reconnection.

On Modeling ICME Cross-Sections as Static MHD Columns

Solar coronal mass ejections are well known to expand as they propagate through the heliosphere. Despite this, their cross-sections are usually modeled as static plasma columns within the magnetohydrodynamics (MHD) framework. We test the validity of this approach using in-situ plasma data from 151 magnetic clouds (MCs) observed by the WIND spacecraft and 45 observed by the Helios spacecrafts. We find that the most probable cross-section expansion speeds for the WIND events are only $\approx 0.06$ times the Alfv\'en speed inside the MCs while the most probable cross-section expansion speeds for the Helios events is $\approx 0.03$. MC cross-sections can thus be considered to be nearly static over an Alfv\'en crossing timescale. Using estimates of electrical conductivity arising from Coulomb collisions, we find that the Lundquist number inside MCs is high ($\approx 10^{13}$), suggesting that the MHD description is well justified. The Joule heating rates using our conductivity estimates are several orders of magnitude lower than the requirement for plasma heating inside MCs near the Earth. While the (low) heating rates we compute are consistent with the MHD description, the discrepancy with the heating requirement points to possible departures from MHD and the need for a better understanding of plasma heating in MCs.

Magnetohydrodynamic with Adaptively Embedded Particle-in-Cell model: MHD-AEPIC

Space plasma simulations have seen an increase in the use of magnetohydrodynamic (MHD) with embedded Particle-in-Cell (PIC) models. This combined MHD-EPIC algorithm simulates some regions of interest using the kinetic PIC method while employing the MHD description in the rest of the domain. The MHD models are highly efficient and their fluid descriptions are valid for most part of the computational domain, thus making large-scale global simulations feasible.However, in practical applications, the regions where the kinetic effects are critical can be changing, appearing, disappearing and moving in the computational domain. If a static PIC region is used, this requires a much larger PIC domain than actually needed, which can increase the computational cost dramatically.To address the problem, we have developed a new method that is able to dynamically change the region of the computational domain where a PIC model is applied. We have implemented this new MHD with Adaptively Embedded PIC (MHD-AEPIC) algorithm using the BATS-R-US Hall MHD and the Adaptive Mesh Particle Simulator (AMPS) as the semi-implicit PIC models. We describe the algorithm and present a test case of two merging flux ropes to demonstrate its accuracy. The implementation uses dynamic allocation/deallocation of memory and load balancing for efficient parallel execution. We evaluate the performance of MHD-AEPIC compared to MHD-EPIC and the scaling properties of the model to large number of computational cores.

Modeling of Processes in Plasma of Radio-Frequency Ion Injector with an Antenna Placed inside the Volume of Discharge Chamber

The work is devoted to the theoretical assessment of the efficiency increase possibility for the radio-frequency ion injector, which is designed for the contactless removal of space debris from near-earth orbit by using an antenna located inside the discharge chamber. Four internal antenna configurations and two external ones—end and side—are considered. Expected characteristics were estimated using an engineering mathematical model built in COMSOL Multiphysics using an approximate magnetohydrodynamic description of the charged particle behavior. According to the simulation results, the best characteristics can be obtained with an internal antenna with a conical arrangement of turns. Calculations showed that in some operating modes, such an antenna configuration makes it possible to halve the radio-frequency power consumption compared to the classical antenna located on the discharge chamber side surface. The performed theoretical study showed that the internal antenna can significantly increase the ion injector efficiency. In the future, verification of the obtained results by test is planned.

Implicit highly-coupled single-ion Hall-MHD formulation for hybrid particle-in-cell codes

The rudiments of a particle-based single-fluid two-temperature magnetohydrodynamic (MHD) algorithm have been outlined in Thoma et al. (2013). The extension of this algorithm to include the effect of Hall physics is described in this paper. An implicit leapfrog version of the algorithm, which allows timesteps large compared to the resistive decay time and other relevant timescales, has recently been added to a hybrid particle-in-cell code. In standard MHD the Hall term in the generalized Ohm’s law can often be neglected when the Hall parameter is small. This term must, however, be retained in regimes where it is non-negligible. The retention of displacement current in Maxwell’s equations avoids the numerical difficulties associated with the whistler mode, which are encountered in standard explicit Hall-MHD codes, and allows the algorithm to be incorporated into hybrid particle-in-cell codes, for which particles may migrate from a kinetic to fluid to MHD description based upon local ambient plasma conditions. A highly-coupled implicit Hall-MHD formalism is presented, in which displacement current can either be retained or neglected. Even when displacement current is neglected, the highly-coupled implicit formalism avoids the restrictive timesteps for the whistler mode in explicit Hall-MHD codes. A comparison of numerical and analytic dispersion analysis demonstrates the feasibility of this approach and establishes relevant constraints to assure numerical stability. The implementation of the algorithm is described, and test simulation results in 1D and 2D in both linear and nonlinear regimes are presented.

Electron-Only Reconnection in Plasma Turbulence

Hybrid-Vlasov–Maxwell simulations of magnetized plasma turbulence including non-linear electron-inertia effects in a generalized Ohm's law are presented. When fluctuation energy is injected on scales sufficiently close to ion-kinetic scales, the ions efficiently become de-magnetized and electron-scale current sheets largely dominate the distribution of the emerging current structures, in contrast to the usual picture, where a full hierarchy of structure sizes is generally observed. These current sheets are shown to be the sites of electron-only reconnection (e-rec), in which the usual electron exhausts are unaccompanied by ion outflows and which are in qualitative agreement with those recently observed by MMS in the Earth's turbulent magnetosheath, downstream of the bow shock. Some features of the e-rec phenomenology are shown to be consistent with an electron magnetohydrodynamic description. Simulations suggest that this regime of collisionless reconnection may be found in turbulent systems where plasma processes, such as micro-instabilities and/or shocks, overpower the more customary turbulent cascade by directly injecting energy close to the ion-kinetic scales.

Critical Balance and the Physics of Magnetohydrodynamic Turbulence

Abstract A discussion of the advantages and limitations of the concept of critical balance (CB), as employed in turbulence phenomenologies, is presented. The incompressible magnetohydrodynamic (MHD) case is a particular focus. The discussion emphasizes the status of the original Goldreich & Sridhar CB conjecture relative to related theoretical issues and models in an MHD description of plasma turbulence. Issues examined include variance and spectral anisotropy, influence of a mean magnetic field, local and nonlocal effects, and the potential for effects of external driving. Related models such as Reduced MHD provide a valuable context in the considerations. Some new results concerning spectral features and timescales are presented in the course of the discussion. Also mentioned briefly are some adaptations and variations of CB.

Discrete Alfvén eigenmodes excited by energetic particles in China Fusion Engineering Test Reactor

In burning plasmas, discrete Alfven eigenmodes ( α TAEs, where α denotes a parameter of plasma pressure gradient) can be readily destabilized by energetic particles under the wave-particle resonance condition. In this study, we theoretically analyze the physical features of α TAEs in the China Fusion Engineering Test Reactor (CFETR), which is viewed as the next generation device in the Chinese magnetic confinement fusion program. One of the scientific objectives of the device is to achieve steady-state operation. There are three reference schemes for baseline steady-state operation, namely Case 1: NB+EC, Case 2: NB+EC+LH, and Case 3: EC+LH, where NB, EC, and LH represent neutral beam, electron cyclotron wave, and lower hybrid wave, respectively. Within the ideal magnetohydrodynamic (MHD) description, we focus on the physical characteristics of α TAEs and correlation between the radial profile of total current density and α TAEs for the three scenarios of CFETR operation with the help of a MHD eigenvalue code. The numerical results indicated that multiple branches of α TAEs can be trapped in potential wells of the ballooning drive under the CFETR operation condition. In the inner region, the bootstrap current density is large, and the total current density profile, defined by the peak of the radial profile of total plasma current density is relatively flat. Therefore, there are abundant α TAEs in this region. However, in the outer region, the bootstrap current is relatively small, and the total current density decreases gradually. So, α TAEs are hardly seen in this region. The α TAEs in the inner region are quasi-marginally stable within the ideal MHD description. In the CFETR operations, the energetic particles generated can destablize the MHD α TAEs under the wave-particle resonance conditions. We study the stability features of α TAEs interacting with energetic particles by employing linear gyrokinetic-MHD hybrid eigenvalue codes. The multiple branches of α TAEs are excited under the various resonance conditions. Moreover, the multiple branches of α TAEs can also be excited by energetic particles with virial energy. Furthermore, higher-frequency α TAEs may be destabilized by energetic particles with higher energy. These α TAEs could change the distribution of energetic particles in phase space and cause the loss of a large number of particles, which may affect the plasma confinement.

A Fully Self-consistent Model for Solar Flares

Abstract The “standard solar flare model” collects all physical ingredients identified by multiwavelength observations of our Sun: magnetic reconnection, fast particle acceleration, and the resulting emission at various wavelengths, especially in soft to hard X-ray channels. Its cartoon representation is found throughout textbooks on solar and plasma astrophysics and guides interpretations of unresolved energetic flaring events on other stars, accretion disks, and jets. To date, a fully self-consistent model that reproduces the standard scenario in all its facets is lacking, since this requires the combination of a large-scale, multidimensional magnetohydrodynamic (MHD) plasma description with a realistic fast electron treatment. Here we demonstrate such a novel combination, where MHD combines with an analytic fast electron model, adjusted to handle time-evolving, reconnecting magnetic fields and particle trapping. This allows us to study (1) the role of fast electron deposition in the triggering of chromospheric evaporation flows, (2) the physical mechanisms that generate various hard X-ray sources at chromospheric footpoints or looptops, and (3) the relationship between soft X-ray and hard X-ray fluxes throughout the entire flare loop evolution. For the first time, this self-consistent solar flare model demonstrates the observationally suggested relationship between flux swept out by the hard X-ray footpoint regions and the actual reconnection rate at the X-point, which is a major unknown in flaring scenarios. We also demonstrate that a looptop hard X-ray source can result from fast electron trapping.

Multiscale MHD‐Kinetic PIC Study of Energy Fluxes Caused by Reconnection

AbstractWe present an analysis of the energy partitioning in the magnetotail during a substorm at 03:58:00 UT on 7 February 2009. The analysis employs a multiscale approach where we use a state from a global magnetohydrodynamics (MHD) model to spawn a kinetic particle‐in‐cell (PIC) simulation of a large portion of the tail. We directly investigate the energy fluxes resulting from magnetic reconnection. The kinetic run provides information on the additional processes absent in the MHD description. The ion bulk energy and enthalpy fluxes carry the greatest energy, but the Poynting flux and electron enthalpy flux also carry a significant portion. The other fluxes (e.g., heat flux) are relatively small but are especially important because they allow us to identify the extra processes present only in the kinetic description. The energy fluxes present in the MHD approximation (Poynting flux, enthalpy flux, and bulk energy flux) are quantitatively accurate, and the kinetic correction does not greatly alter the MHD picture. However, there are two unique effects resulting from the kinetic physics. First, the formation of a rarefaction of the plasma flow into the reconnection site leads to a progressive decline in time of the particle energy fluxes with respect to the Poynting flux. Second, we observe that the instabilities developing in the kinetic reconnection outflows form structures absent from the MHD description. These structures reveal themselves as fluctuations within the energy fluxes. Especially notable are regions of inverted heat flux, where the heat flux is in the opposite direction to the total energy and mass flow.

Generation mechanism of 100 MG magnetic fields in the interaction of ultra-intense laser pulse with nanostructured target

Experimental and simulation data [Moreau et al., Plasma Phys. Control. Fusion 62, 014013 (2019); Kaymak et al., Phys. Rev. Lett. 117, 035004 (2016)] indicate that self-generated magnetic fields play an important role in enhancing the flux and energy of relativistic electrons accelerated by ultra-intense laser pulse irradiation with nanostructured arrays. A fully relativistic analytical model for the generation of the magnetic field based on electron magneto-hydrodynamic description is presented here. The analytical model shows that this self-generated magnetic field originates in the nonparallel density gradient and fast electron current at the interfaces of a nanolayered target. A general formula for the self-generated magnetic field is found, which closely agrees with the simulation scaling over the relevant intensity range. The result is beneficial to the experimental designs for the interaction of the laser pulse with the nanostructured arrays to improve laser-to-electron energy coupling and the quality of forward hot electrons.

A resistive extension for ideal magnetohydrodynamics

ABSTRACT We present an extension to the special relativistic, ideal magnetohydrodynamic (MHD) equations, designed to capture effects due to resistivity. The extension takes the simple form of an additional source term that, when implemented numerically, is shown to emulate the behaviour produced by a fully resistive MHD description for a range of initial data. The extension is developed from first principles arguments, and thus requires no fine-tuning of parameters, meaning it can be applied to a wide range of dynamical systems. Furthermore, our extension does not suffer from the same stiffness issues arising in resistive MHD, and thus can be evolved quickly using explicit methods, with performance benefits of roughly an order of magnitude compared to current methods.

Turbulence Dynamo in the Stratified Medium of Galaxy Clusters

The existence of microgauss magnetic fields in galaxy clusters have been established through observations of synchrotron radiation and Faraday rotation. They are conjectured to be generated via small-scale dynamo by turbulent flow motions in the intracluster medium (ICM). Some of giant radio relics, on the other hand, show the structures of synchrotron polarization vectors, organized over the scales of $\sim$ Mpc, challenging the turbulence origin of cluster magnetic fields. Unlike turbulence in the interstellar medium, turbulence in the ICM is subsonic. And it is driven sporadically in highly stratified backgrounds, when major mergers occur during the hierarchical formation of clusters. To investigate quantitatively the characteristics of turbulence dynamo in such ICM environment, we performed a set of turbulence simulations using a high-order-accurate, magnetohydrodynamic (MHD) code. We find that turbulence dynamo could generate the cluster magnetic fields up to the observed level from the primordial seed fields of $10^{-15}$ G or so within the age of the universe, if the MHD description of the ICM could be extended down to $\sim$ kpc scales. However, highly organized structures of polarization vectors, such as those observed in the Sausage relic, are difficult to be reproduced by the shock compression of turbulence-generated magnetic fields. This implies that the modeling of giant radio relics may require the pre-existing magnetic fields organized over $\sim$ Mpc scales.