Magnetic Field Fluctuations Research Articles

Influence of ambipolar and Hall effects on vorticity in three-dimensional simulations of magneto-convection: Vorticity with ambipolar and Hall effect

This paper presents the results of the analysis of three-dimensional simulations of solar magneto-convection that include the joint action of the ambipolar diffusion and the Hall effect. Three simulation runs are compared: One including both ambipolar diffusion and the Hall effect; one including only ambipolar diffusion and one without any of these two effects. The magnetic field is amplified from initial field to saturation level by the action of turbulent local dynamo. In each of these cases, we study 2 h of simulated solar time after the local dynamo reaches the saturation regime. We analyse the power spectra of vorticity, of magnetic field fluctuations and of the different components of the magnetic Poynting flux responsible for the transport of vertical or horizontal perturbations. Our preliminary results show that the ambipolar diffusion produces a strong reduction of vorticity in the upper chromospheric layers and that it dissipates the vortical perturbations converting them into thermal energy. The Hall effect acts in the opposite way, strongly enhancing the vorticity. When the Hall effect is included, the magnetic field in the simulations becomes, on average, more vertical and long-lived flux tube-like structures are produced. We trace a single magnetic structure to study its evolution pattern and the magnetic field intensification, and their possible relation to the Hall effect. This article is part of the Theo Murphy meeting issue 'High-resolution wave dynamics in the lower solar atmosphere'.

Ефекти геокосмічної бурі 5-6 серпня 2019 р.

Geospace storms are the synergistically interacting magnetic storms, ionospheric storms, atmospheric storms, and the storms in an electric field of magnetospheric, ionospheric, and atmospheric origins. Geospace storms are very diverse, and no two of them behave exactly the same. Therefore, studying the effects of each new storm becomes an urgent task for us. Such research will reveal both the general laws and individual characteristics of storm processes. The purpose of this paper is to present general information about the geospace storm, the results of the analysis of features of magnetic and ionospheric storms. To analyze the magnetic environment, we used the measurement results of magnetic field fluctuations in the range from 1 s to 1000 s, performed at the Magnetometric Observatory of V. N. Karazin Kharkiv National University, and variations of three components of the geomagnetic field, performed at the Low-frequency observatory of the IRA NASU. We analyzed the ionospheric environment using multi-frequency multi-path measurements performed at Harbin Engineering University (China) and also the data of ionosonde. The main results of the work are as follows. An increase in the main parameters of the solar wind on August 5, 2019, led to a geospace storm, which was mainly observed on August 5 and 6, 2019. The main phase of the magnetic storm took place on August 5, 2019, from 06:00 a.m. to 08:30 a.m. The recovery phase lasted at no less than 4 days. The magnetic storm shows significant variations of all components of the geomagnetic field, and there is an increase by order of magnitude of the oscillations’ level of the geomagnetic field in the range from 400 s to 950 s. During the ionospheric storm, significant disturbances occurred in the F region of the ionosphere. The E-region of the ionosphere remained weakly perturbed. The ionospheric storm has severely affected the Doppler spectra of radio waves in the 5 – 10 MHz frequency range. The Doppler spectra are significantly broadened, and the Doppler frequency shift and its quasi-periodic change with a period of 20–40 minutes and a duration of 120–240 minutes have taken place. The quasi-periodic variations of the Doppler frequency shift are due to quasi-periodic variations in the electron concentration, and the amplitude of their relative perturbations varied from 3% to 16%. On one of these paths, the amplitude of the Doppler frequency shift reached 0.7 Hz. And in this case, the amplitude of the relative perturbations of the electron concentration could reach 80 - 90%. In addition, the ionospheric storm little affected the signal amplitude on most radio paths.

Three-dimensional magnetic reconnection in particle-in-cell simulations of anisotropic plasma turbulence

We use three-dimensional (3-D) fully kinetic particle-in-cell simulations to study the occurrence of magnetic reconnection in a simulation of decaying turbulence created by anisotropic counter-propagating low-frequency Alfvén waves consistent with critical-balance theory. We observe the formation of small-scale current-density structures such as current filaments and current sheets as well as the formation of magnetic flux ropes as part of the turbulent cascade. The large magnetic structures present in the simulation domain retain the initial anisotropy while the small-scale structures produced by the turbulent cascade are less anisotropic. To quantify the occurrence of reconnection in our simulation domain, we develop a new set of indicators based on intensity thresholds to identify reconnection events in which both ions and electrons are heated and accelerated in 3-D particle-in-cell simulations. According to the application of these indicators, we identify the occurrence of reconnection events in the simulation domain and analyse one of these events in detail. The event is related to the reconnection of two flux ropes, and the associated ion and electron exhausts exhibit a complex 3-D structure. We study the profiles of plasma and magnetic-field fluctuations recorded along artificial-spacecraft trajectories passing near and through the reconnection region. Our results suggest the presence of particle heating and acceleration related to small-scale reconnection events within magnetic flux ropes produced by the anisotropic Alfvénic turbulent cascade in the solar wind. These events are related to current structures of the order of a few ion inertial lengths in size.

Heisenberg uncertainty of spatially gated electromagnetic fields.

A Heisenberg uncertainty relation is derived for spatially-gated electric ΔE and magnetic ΔH field fluctuations. The uncertainty increases for small gating sizes, which implies that in confined spaces, the quantum nature of the electromagnetic field must be taken into account. Optimizing the state of light to minimize ΔE at the expense of ΔH and vice versa should be possible. Spatial confinements and quantum fields may alternatively be realized without gating by interaction of the field with a nanostructure. Possible applications include nonlinear spectroscopy of nanostructures and optical cavities and chiral signals.

Mirror-mode Wave Identification Methods and Their Application to Martian Magnetosheath

Mirror-mode waves are structures usually seen in plasma with temperature anisotropy, identifiable through features in magnetic field and particle distribution and fluctuation. Two identification methods are analyzed and compared in this paper. Method A uses magnetic field data only, while Method B combines magnetic field and particle data. Method A is based mainly on features of magnetic field variation such as large amplitude fluctuation along background field direction, using magnitude of magnetic field fluctuation ∆<italic>B</italic>/|<italic>B</italic>| and angles between background field and maximum/minimum variation direction θ_max, θ_min as criteria. Method B is based on features such as wave compression, total pressure balance and zero velocity in plasma frame. Identification using data from MAVEN probe in the Martian magnetosheath shows that Method A can cause misidentification under certain circumstances, <italic>e.g.</italic> magneto-sonic waves. Results of identification using Method A with varying criteria ∆<italic>B</italic>/|<italic>B</italic>| and θ_min/θ_max are studied in the Martian magnetosheath, suggesting threshold values: θ_min> 40º, θ_max< 40º and ∆<italic>B</italic>/|<italic>B</italic>| > 80% can yield satisfying results.

Observational Signatures of Cosmic-Ray Interactions in Molecular Clouds

Abstract We investigate ionization and heating of gas in the dense, shielded clumps/cores of molecular clouds bathed by an influx of energetic, charged cosmic rays (CRs). These molecular clouds have complex structures, with substantial variation in their physical properties over a wide range of length scales. The propagation and distribution of CRs is thus regulated accordingly, in particular, by the magnetic fields threaded through the clouds and into the dense regions within. We have found that a specific heating rate reaching 10−26 erg cm−3 s−1 can be sustained in the dense clumps/cores for Galactic environments, and this rate increases with CR energy density. The propagation of CRs and heating rates in some star-forming filaments identified in IC 5146 are calculated, with the CR diffusion coefficients in these structures determined from magnetic field fluctuations inferred from optical and near-infrared polarizations of starlight, which is presumably a magnetic field tracer. Our calculations indicate that CR heating can vary by nearly three orders of magnitude between different filaments within a cloud due to different levels of CR penetration. The CR ionization rate among these filaments is similar. The equilibrium temperature that could be maintained by CR heating alone is of order 1 K in a Galactic environment, but this value would be higher in strongly star-forming environments, thus causing an increase in the Jeans mass of their molecular clouds.

Multi‐Spacecraft Measurement of Anisotropic Spatial Correlation Functions at Kinetic Range in the Magnetosheath Turbulence

AbstractWe investigate spatial correlation functions (SCFs) at kinetic range in the terrestrial magnetosheath because of wide angular distributions of the magnetic field fluctuations and the high Signal‐to‐Noise‐Ratio. We employ four‐point measurements of magnetic fluctuations from the Cluster and MMS concerning a spatiotemporally varying background magnetic field to constitute the two‐dimensional (2‐D) SCFs between ion and electron scales. It is found that two populations exist in the 2‐D SCFs, with the minority elongated along the perpendicular coordinate (S⊥) and the majority of population elongated along the coordinate parallel to average magnetic field (S||). This kind of phenomenon indicates that the distribution of magnetosheath turbulence in the wavenumber space is dominantly transverse to the background magnetic field, simultaneously partly along the field (i.e., k⊥ &gt;&gt; k||). In addition, we find that magnetosheath turbulence is the dominance of shear Alfvénic‐like fluctuations, and is quasi‐monofractal intermittent turbulence at kinetic range for both the Cluster case and the MMS case, indicating that strong spatially anisotropic fluctuations may be universal in the magnetosheath. Our observations pose new challenges to the theoretical modeling of turbulence and dissipation at kinetic range in the collisionless magnetized plasma.

Quantum lock-in detection of a vector light shift

We demonstrate detection of a vector light shift (VLS) using the quantum lock-in method. The method offers precise and accurate VLS measurement without being affected by real magnetic field fluctuations. We detect a VLS on a Bose--Einstein condensate (BEC) of $^{87}$Rb atoms caused by an optical trap beam with a resolution less than 1 Hz. We also demonstrate elimination of a VLS by controlling the beam polarization to realize a long coherence time of a transversally polarized $F$ = 2 BEC. Quantum lock-in VLS detection should find wide application, including the study of spinor BECs, electric-dipole moment searches, and precise magnetometry.

Kinetic Interaction of Cold and Hot Protons With an Oblique EMIC Wave Near the Dayside Reconnecting Magnetopause

AbstractWe report observations of the ion dynamics inside an Alfvén branch wave that propagates near the reconnecting dayside magnetopause. The measured frequency, wave normal angle and polarization are consistent with the predictions of a dispersion solver. The magnetospheric plasma contains hot protons (keV), cold protons (eV), plus some heavy ions. While the cold protons follow the magnetic field fluctuations and remain frozen‐in, the hot protons are at the limit of magnetization. The cold protons exchange energy back and forth, adiabatically, with the wave fields. The cold proton velocity fluctuations contribute to balance the Hall term fluctuations in Ohm's law, and the waveEfield has small ellipticity and right‐handed polarization. The dispersion solver indicates that increasing the cold proton density facilitates propagation and amplification of these waves at oblique angles, as for the observed wave.

The Influence of Magnetic Turbulence on the Energetic Particle Transport Upstream of Shock Waves

Energetic particles are ubiquitous in the interplanetary space and their transport properties are strongly influenced by the interaction with magnetic field fluctuations. Numerical experiments have shown that transport in both the parallel and perpendicular directions with respect to the background magnetic field is deeply affected by magnetic turbulence spectral properties. Recently, making use of a numerical model with three dimensional isotropic turbulence, the influence of turbulence intermittency and magnetic fluctuations on the energetic particle transport was investigated in the solar wind context. Stimulated by this previous theoretical work, here we analyze the parallel transport of supra-thermal particles upstream of interplanetary shock waves by using in situ particle flux measurements; the aim was to relate particle transport properties to the degree of intermittency of the magnetic field fluctuations and to their relative amplitude at the energetic particle resonant scale measured in the same regions. We selected five quasi-perpendicular and five quasi-parallel shock crossings by the ACE satellite. The analysis clearly shows a tendency to find parallel superdiffusive transport at quasi-perpendicular shocks, with a significantly higher level of the energetic particle fluxes than those observed in the quasi-parallel shocks. Furthermore, the occurrence of anomalous parallel transport is only weakly related to the presence of magnetic field intermittency.

Applying the Horizontal Visibility Graph Method to Study Irreversibility of Electromagnetic Turbulence in Non-Thermal Plasmas.

One of the fundamental open questions in plasma physics is the role of non-thermal particles distributions in poorly collisional plasma environments, a system that is commonly found throughout the Universe, e.g., the solar wind and the Earth’s magnetosphere correspond to natural plasma physics laboratories in which turbulent phenomena can be studied. Our study perspective is born from the method of Horizontal Visibility Graph (HVG) that has been developed in the last years to analyze time series avoiding the tedium and the high computational cost that other methods offer. Here, we build a complex network based on directed HVG technique applied to magnetic field fluctuations time series obtained from Particle In Cell (PIC) simulations of a magnetized collisionless plasma to distinguish the degree distributions and calculate the Kullback–Leibler Divergence (KLD) as a measure of relative entropy of data sets produced by processes that are not in equilibrium. First, we analyze the connectivity probability distribution for the undirected version of HVG finding how the Kappa distribution for low values of tends to be an uncorrelated time series, while the Maxwell–Boltzmann distribution shows a correlated stochastic processes behavior. Subsequently, we investigate the degree of temporary irreversibility of magnetic fluctuations that are self-generated by the plasma, comparing the case of a thermal plasma (described by a Maxwell–Botzmann velocity distribution function) with non-thermal Kappa distributions. We have shown that the KLD associated to the HVG is able to distinguish the level of reversibility that is associated to the thermal equilibrium in the plasma, because the dissipative degree of the system increases as the value of parameter decreases and the distribution function departs from the Maxwell–Boltzmann equilibrium.

Perturbation theory for solitons of the Fokas-Lenells equation: Inverse scattering transform approach

We present perturbation theory based on the inverse scattering transform method for solitons described by an equation with the inverse linear dispersion law ω∼1/k, where ω is the frequency and k is the wave number, and cubic nonlinearity. This equation, first suggested by Davydova and Lashkin for describing dynamics of nonlinear short-wavelength ion-cyclotron waves in plasmas and later known as the Fokas-Lenells equation, arises from the first negative flow of the Kaup-Newell hierarchy. Local and nonlocal integrals of motion, in particular the energy and momentum of nonlinear ion-cyclotron waves, are explicitly expressed in terms of the discrete (solitonic) and continuous (radiative) scattering data. Evolution equations for the scattering data in the presence of a perturbation are presented. Spectral distributions in the wave number domain of the energy emitted by the soliton in the presence of a perturbation are calculated analytically for two cases: (i) linear damping that corresponds to Landau damping of plasma waves, and (ii) multiplicative noise which corresponds to thermodynamic fluctuations of the external magnetic field (thermal noise) and/or the presence of a weak plasma turbulence.

Magnetic Resistance of HTS Maglev Moving Above a Permanent Magnet Guideway with Large Magnetic Field Fluctuations

Unlike the magnetic levitation characteristics that most research focuses on, this article focuses on the magnetic resistance of high-temperature superconductor (HTS) maglev system. By superimposing iron sheets on the permanent magnet (PM) track to create significant magnetic field fluctuations, we studied the magnetic resistance of HTS-PM maglev caused by the fluctuating magnetic field, and used the rotating orbit platform to study the influence of the running speed and the field cooling height (FCH) on the magnetic resistance. The magnetic resistance is found mainly originating from the self-stabilizing characteristic of type II superconductor in an uneven external magnetic field, showing oscillation characteristics near a certain equilibrium value, with nearly constant amplitude at a given FCH and running speed. Spectrum analysis shows that the main frequency of the oscillation is the same as the frequency of the alternating magnetic field received by the maglev vehicle during running above the fluctuating magnetic field, revealing that this is a forced vibration driven by a fluctuating magnetic field. The oscillation amplitude and power density increase steadily with the increase of FCH, and show the characteristics of first increasing and then reaching saturation with the increase of running speed. The research results reveal that there is an inherent correlation between the oscillation of the magnetic resistance and the stability of the system.

Reynolds number dependence of Lyapunov exponents of turbulence and fluid particles.

The Navier-Stokes equations generate an infinite set of generalized Lyapunov exponents defined by different ways of measuring the distance between exponentially diverging perturbed and unperturbed solutions. This set is demonstrated to be similar, yet different, from the generalized Lyapunov exponent that provides moments of distance between two fluid particles below the Kolmogorov scale. We derive rigorous upper bounds on dimensionless Lyapunov exponent of the fluid particles that demonstrate the exponent's decay with Reynolds number Re in accord with previous studies. In contrast, terms of cumulant series for exponents of the moments have power-law growth with Re. We demonstrate as an application that the growth of small fluctuations of magnetic field in ideal conducting turbulence is hyperintermittent, being exponential in both time and Reynolds number. We resolve the existing contradiction between the theory, that predicts slow decrease of dimensionless Lyapunov exponent of turbulence with Re, and observations exhibiting quite fast growth. We demonstrate that it is highly plausible that a pointwise limit for the growth of small perturbations of the Navier-Stokes equations exists.

Wavelet analysis of low frequency magnetic field fluctuations in the Jupiter’s magnetotail

Jupiter’s magnetospheric processes are strongly influenced by the planet fast rotation and by its internal plasma sources. As a consequence, the 10-h rotation period signal is dominant on many datasets. In this work, in order to study low frequency magnetic fluctuations in the Jupiter’s magnetotail, the continuous wavelet transform (CWT) analysis is employed to investigate the magnetic field vector data during six orbits of Galileo spacecraft. Besides the near 10-h Jovian rotation period, the major periods found in this study are 5-h, most likely due to transient disturbances in Jupiter’s magnetosphere, and from 1.5 to 3.5 days, possibly related to global reconfiguration in the magnetosphere.These periodicities may also be related to the mass-loading from the moon Io and to possible modulation by the solar wind. Furthermore, long periods have been also found from 5 to 35 days. The method presented in this study could be very useful for application in other datasets of planetary magnetospheres.

On the Relation between Kappa Distribution Functions and the Plasma Beta Parameter in the Earth’s Magnetosphere: THEMIS Observations

Abstract The Earth’s magnetosphere represents a natural plasma laboratory that allows us to study the behavior of particle distribution functions in the absence of Coulomb collisions, typically described by the kappa distributions. We have investigated the properties of these functions for ions and electrons in different magnetospheric regions, thereby making it possible to reveal the κ-parameters for a wide range of plasma beta (β) values (from 10−3 to 102). This was done using simultaneous ion and electron measurements from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft spanning the years 2008–2018. It was found that for a fixed plasma β, the κ-index and core energy (E c ) of the distribution can be modeled by the power law for both species, and the relation between β, κ, and E c is much more complex than earlier reported: both A and γ exhibit systematic dependencies with β. Our results indicate that β ∼ 0.1–0.3 is a range where the plasma is more dynamic, since it is influenced by both the magnetic field and temperature fluctuations, which suggests that the transition between magnetically and kinetically dominated plasmas occurs at these values of β. For β &gt; 1, both A and γ take nearly constant values, a feature that is especially notable for the electrons and might be related to their demagnetization. The relation between β, κ, and E c that we present is an important result that can be used by theoretical models in the future.

Scale-dependent anisotropy of electric field fluctuations in solar wind turbulence

ABSTRACT We study the variation of average powers and spectral indices of electric field fluctuations with respect to the angle between average flow direction and the mean magnetic field in solar wind turbulence. Cluster spacecraft data from the years 2002 and 2007 are used for the present analysis. We perform a scale-dependent study with respect to the local mean magnetic field using wavelet analysis technique. Prominent anisotropies are found for both the spectral index and power levels of the electric power spectra. Similar to the magnetic field fluctuations, the parallel (or antiparallel) electric fluctuation spectrum is found to be steeper than the perpendicular spectrum. However, the parallel (or antiparallel) electric power is found to be greater than the perpendicular one. Below 0.1 Hz, the slope of the parallel electric power spectra deviates substantially from that of the total magnetic power spectra, supporting the existence of Alfvénic turbulence.

Magnetic field generation from PBH distributions

ABSTRACT We introduce a statistical method for estimating magnetic field fluctuations generated from primordial black hole (PBH) populations. To that end, we consider monochromatic and extended Press–Schechter PBH mass functions, such that each constituent is capable of producing its own magnetic field due to some given physical mechanism. Assuming a linear correlation between magnetic field fluctuations and matter overdensities, our estimates depend on the mass function, the physical field generation mechanism by each PBH constituent, and the characteristic PBH separation. After computing the power spectrum of magnetic field fluctuations, we apply our formalism to study the plausibility that two particular field generation mechanisms could have given rise to the expected seed fields according to current observational constraints. The first mechanism is the Biermann battery and the second one is due to the accretion of magnetic monopoles at PBH formation, constituting magnetic PBHs. Our results show that, for monochromatic distributions, it does not seem to be possible to generate sufficiently intense seed fields in any of the two field generation mechanisms. For extended distributions, it is also not possible to generate the required seed field by only assuming a Biermann battery mechanism. In fact, we report an average seed field by this mechanism of about 10−47 G, at z = 20. For the case of magnetic monopoles, we instead assume that the seed values from the literature are achieved and calculate the necessary number density of monopoles. In this case, we obtain values that are below the upper limits from current constraints.

Inverse cascade and magnetic vortices in kinetic Alfvén-wave turbulence

A Hamiltonian two-field gyrofluid model for kinetic Alfvén waves (KAWs) in a magnetized electron–proton plasma, retaining ion finite-Larmor-radius corrections and parallel magnetic field fluctuations, is used to study the inverse cascades that develop when turbulence is randomly driven at sub-ion scales. In the directions perpendicular to the ambient field, the dynamics of the cascade turns out to be non-local and the ratio $\chi _f$ of the wave period to the characteristic nonlinear time at the driving scale affects some of its properties. For example, at small values of $\chi _f$ , parametric decay instability of the modes driven by the forcing can develop, enhancing for a while inverse transfers. The balanced state, obtained at early time when the two counter-propagating waves are equally driven, also becomes unstable at small $\chi _f$ , leading to an inverse cascade. For $\beta _e$ smaller than a few units, the cascade slows down when reaching the low-dispersion spectral range. For higher $\beta _e$ , the ratio of the KAW to the Alfvén frequencies displays a local minimum. At the corresponding transverse wavenumber, a condensate is formed, and the cascade towards larger scales is then inhibited. Depending on the parameters, a parallel inverse cascade can develop, enhancing the elongation of the ion-scale magnetic vortices that generically form.

Phase relation between rotating phase locked (2, 1) and (3, 1) tearing modes in ASDEX Upgrade

Tearing modes are a major concern for large tokamak devices. Therefore their detection and characterization is of importance for timely countermeasures to avoid significant impact on the discharge or at least prevent possible machine damage. In case of phase-locked tearing modes, the poloidal variation of induced magnetic field fluctuation depends on the amplitudes of and the phase difference between the individual modes. This affects mode detection and identification when not considered appropriately. The phase between phase-locked (2, 1) and (3, 1) tearing modes in ASDEX Upgrade has been determined from local electron temperature and magnetic measurements independently. It is shown that the modes can be in phase at any poloidal position starting from the low field side plasma midplane over the plasma top to the high field side midplane. This observation invalidates the widespread assumption that phase-locked tearing modes are in phase near the low field side midplane. Dependence of the in-phase position on both, the plasma pressure and the plasma rotation velocity, is observed.