Magnetic Field Discontinuities Research Articles

Orientation of IMF Discontinuity Normals Across the Solar Cycles

AbstractTransient events like hot flow anomalies and foreshock bubbles are common in the Earth's foreshock. These foreshock transients may play an important role in the solar wind‐magnetosphere interaction. They typically occur when backstreaming ions accumulate at the intersection of interplanetary magnetic field (IMF) discontinuities with the bow shock. Discontinuity orientations play a key role in determining when and where transients form in the foreshock and strike the magnetopause. They also play a role in determining the amplitude, and significance, of individual transients. We investigate properties of the IMF discontinuity normals across two solar cycles. We compare Advanced Composition Explorer (ACE) IMF discontinuity observations for 2001, 2002, 2012, and 2014 (solar maximum) and 2008, 2009, 2019, and 2020 (solar minimum). We employ both the Minimum variance analysis (MVA) and Cross‐product methods to determine phase front normals. Most discontinuity normals point earthward and dawnward at longitudes ranging from 180° to 250°. A greater range of normal can be seen during solar minimum than during solar maximum. Normals lie further from the Sun‐Earth line when solar wind densities are low and velocities are high. This provides a natural explanation for the observed tendency of transients to occur for low solar wind densities and high velocities in terms of longer discontinuity interaction times with the bow shock.

Evaluation of weak discontinuity in rotating medium with magnetic field, characteristic shock and weak discontinuity interaction

Abstract In this article we investigated the characteristic shock and weak discontinuity wave in a rotating medium of perfect gas in the case of one-dimensional (1-D) adiabatic motion under an axial magnetic field governed by the system of PDEs (partial differential equations). We have obtained some classes of analytical solutions of the system of PDEs that demonstrates the time-space dependency. With change in the values of rotational parameter, adiabatic index and the ratio of initial magnetic pressure to dynamic pressure, effect on the acceleration wave’s amplitude and jump in the flow variables across the characteristic shock is analyzed in detail. We have obtained an expression for the jump in shock acceleration, the amplitudes of transmitted and reflected waves caused by the incident wave on the characteristic shock after the interaction of a weak discontinuity. It is investigated that the jump function across the characteristic shock decay effect, and goes to 0 as time t → ∞, whereas a weak discontinuity wave may culminate into a shock wave, depending on the initial amplitude value. It is also found that the shock formation time reduces due to the consideration of magnetic field or rotating medium.

Ion kinetic effects linked to magnetic field discontinuities in the slow Alfvénic wind observed by Solar Orbiter in the inner heliosphere

Slow solar wind, sharing magnetic and plasma properties typical of fast wind, the so-called slow Alfvénic wind, has been widely observed in the heliosphere. Here, we report an analysis of the turbulent properties of a slow Alfvénic stream observed by Solar Orbiter at 0.64 AU. This solar wind stream is characterized by well distinguishable regions, namely, a main portion, an intermediate region, and a rarefaction region. Each of those intervals have been studied separately, in order to enhance similarities and differences in their turbulence properties. Coherent structures naturally emerge over different time/spatial scales and their characteristics at ion scales have been investigated. The presence of these intermittent events have been found to be closely related to kinetic features in the ion (both proton and alpha particles) velocity distribution functions, suggesting a fundamental role in the kinetic physical processes that mediate the sub-ion turbulence cascade.

Ion Acceleration by Foreshock Bubbles: Magnetospheric Multiscale Observations

AbstractForeshock bubbles (FBs) occur when interplanetary magnetic field discontinuities encounter the Earth's foreshock. These transient (∼1 to 5 min) features exhibit depressed densities and magnetic field strengths, enhanced temperatures, and deflected plasma flows trailed by a region of enhanced plasma density and magnetic field strength. Ions can be accelerated inside the FBs through the Fermi acceleration process. Hybrid simulations and test particle calculations predict that the maximum energies of ions accelerated by FBs reach 5.6 times the solar wind ram energy (Esw). We identify 23 FBs from September 2015 to January 2020 Magnetospheric Multiscale spacecraft observations. Most FBs (17 of 23) occurred upstream of the dusk‐side bow shock and above the ecliptic. The FBs occurred for Alfvn Mach numbers ranging from 5 to 15, with 11 FBs having an Alfvn Mach number near 10. To investigate ion energization inside the cores of the FBs we compare the proton spectra observed by the Hot Plasma Composition Analyzer and Energetic Ion Spectrometer before (upstream), during (core), and after (downstream) the FBs. The proton intensities at energies from Esw (the solar wind ram energy, ) up to about 5.6Esw are greater inside than outside 19 of 23 FBs, confirming that FBs can accelerate particles to these energies. The proton flux intensities at energies between Esw and 5.6Esw in the core of the FBs are consistent with results from global hybrid simulations for ion energization from FBs through second‐order Fermi acceleration.

A Novel Hybrid Approach for Computing Electromagnetic Scattering from Objects with Honeycomb Structures

We propose in this paper a novel hybrid numerical modeling method for computing electromagnetic scattering from inhomogeneous targets containing honeycomb structures. In the proposed approach, the whole honeycomb structure is divided into the inner and outer two subregions. Each thin wall of a unit cell in the outer subregion is replaced by a zero-thickness surface, with the aid of a resistive sheet boundary condition (RSBC) to describe the electric and magnetic field discontinuities across the surface. Each unit cell in the inner subregion is homogenized by using the Hashin–Shtrikman and the Mori–Tanaka formulae. The two subregions are further divided into smaller subdomains by introducing the Robin-type transmission condition to couple subregion interfaces, as well as subdomain interfaces. The whole solution region is then discretized and solved using the nonconformal domain decomposition-based hybrid finite element–boundary integral–multilevel fast multipole algorithm (FE-BI-MLFMA). The numerical results demonstrate that the proposed approach exhibits a high accuracy, efficiency, and flexibility. Solutions of scattering by a wing-like object and a practical unmanned aerial vehicle (UAV) model with honeycomb radar-absorbing structures are presented, showing the superior performance of the proposed algorithm.

Synthesis and Magnetic Properties of FeCo Alloy Nanowires via AC Frequency Conversion Electrodeposition

Binary alloy nanowires of Fe₃CoSUB7/SUB system are synthesized in the highly ordered porous anodic aluminum oxide (PAA) templates by AC frequency (5, 50, 120, and 200 Hz) conversion electrodeposition method. High resolution field emission transmission electron microscope (HRTEM) and Lorenz TEM (LTEM) were used to study the morphology and the magnetic domain structure of the Fe₃CoSUB7/SUB alloy nanowires, respectively. Magnetic measurements showed that the M-H curves presented a discontinuity in a restricted magnetic field, but with high squareness (about 90 %). Quantitative local measurements of the magnetic properties of the nanowires are explored by differential phase contrast (DPC) imaging. The magnetic properties, however, were significantly affected by the AC frequency, which is correlated with the specific magnetic structure formation of Bloch line. We reported herein a detailed investigation on Fe₃CoSUB7/SUB alloy nanowires and their magnetic properties.

Minimization of magnetic forces on stellarator coils

Magnetic confinement devices for nuclear fusion can be large and expensive. Compact stellarators are promising candidates for cost-reduction, but introduce new difficulties: confinement in smaller volumes requires higher magnetic field, which calls for higher coil-currents and ultimately causes higher Laplace forces on the coils—if everything else remains the same. This motivates the inclusion of force reduction in stellarator coil optimization. In the present paper we consider a coil winding surface, we prove that there is a natural and rigorous way to define the Laplace force (despite the magnetic field discontinuity across the current-sheet), we provide examples of cost associated (peak force, surface-integral of the force squared) and discuss easy generalizations to parallel and normal force-components, as these will be subject to different engineering constraints. Such costs can then be easily added to the figure of merit in any multi-objective stellarator coil optimization code. We demonstrate this for a generalization of the REGCOIL code (Landreman 2017 Nucl. Fusion 57 046003), which we rewrote in python, and provide numerical examples for the NCSX (Zarnstorff et al 2001 Plasma Phys. Control. Fusion 43 A237–49) (now QUASAR) design. We present results for various definitions of the cost function, including peak force reductions by up to 40%, and outline future work for further reduction.

Response of the Jovian Magnetosphere‐Ionosphere System to the Interplanetary Magnetic Field Discontinuity: A Simulation Study

AbstractThe response of the magnetosphere‐ionosphere (MI) system at Jupiter to the interplanetary magnetic field (IMF) tangential discontinuity is studied via global Magnetohydrodynamics (MHD) simulations. Our results show that the IMF discontinuity, which has a transition front perpendicular to the Sun‐Jupiter line, has strong effects on the Jovian MI system depending on local times and radial distances to Jupiter. The magnetospheric significant response to the discontinuity is delayed by ∼2–6 hr after the encounter of the discontinuity front with the bow shock, while the delay time for the ionospheric field‐aligned currents (FACs) is ∼3–8 hr. The outer magnetosphere, particularly at dayside, tends to respond slightly quicker than the inner regions. At the ionosphere, the response of downward FACs is found to occur earlier than that of the upward ones by a time ranging from less than 1 hr to several hours, which is also more prominent at dayside sectors. Using a new MI mapping method with Jupiter's JRM09 magnetic field model, we reproduce for the first time the ionospheric FACs consistent with the morphology of the observed ultraviolet (UV) main auroral emissions. This indicates a potential application of the simulated upward FACs on the morphology of the UV main auroral ovals.

Confined chiral dynamo action in torsional dilatonic DWs from CME discontinuities

In this paper, spin polarized domain walls in Einstein–Cartan (EC) gravity have been recently investigated [L. C. Garcia de Andrade, Einstein–Cartan non-supersymmetric dynamo walls, Ann. Phys. 432 (2021).], where the spin polarizes along the direction orthogonal to planar domain wall. In this case, dynamo effects may appear on the wall. Moreover, dilatonic domain walls (DDWs) have shown to be responsible for inflating DWs in EC gravity, where DWs were non-magnetized [L. Garcia de Andrade, Axial and chiral anomalies in Riemann–Cartan spacetime black holes and electrodynamics, Class. Quantum Grav. 16 (1999) 2097–2103.]. In this paper, our goal is to show that when we consider a magnetized DW in the EC–Maxwell cosmology, the dilaton potential in the form of a DDW potential expressed in terms of torsion and the magnetic energy density may drive dynamo effects. Analytical solution of the dynamo equation shows that depending on the handness of DDW metric, the magnetic fields are amplified by dynamo action. From the coupled system of ECM and dilaton field equations, we find a relation between the magnetic energy density of [Formula: see text] and the DDW torsion of [Formula: see text] which leads to a thin DDW thickness of [Formula: see text]. For a galactic magnetic field, energy density of [Formula: see text] leads to a thickness of [Formula: see text], a much thinner DDW. For galactic dynamo amplification, the ratio r between the magnetic energy density and photon energy density is around [Formula: see text]. From this ratio, a relation between photon mass density and torsion is obtained. Chiral Magnetic effect (CME), torsion and magnetic discontinuities along the wall are investigated. Magnetic field discontinuity across the wall allows us to understand why the magnetic fields at DDW must be located along the wall since the orthogonal magnetic field may vanish.

Hadron-quark phase transition at finite density in the presence of a magnetic field: Anisotropic approach

We investigate the hadron-quark phase transition at finite density in the presence of a magnetic field taking into account the anisotropy created by a uniform magnetic field in the system’s equations of state. We find a new anisotropic equilibrium condition that will drive the first-order phase transition along the boundary between the two phases. Fixing the magnetic field in the hadronic phase, the phase transition is realized by increasing the baryonic chemical potential at zero-temperature. It is shown that the magnetic field is mildly boosted after the system transitions from the hadronic to the quark phase. The magnetic-field discontinuity between the two phases is supported by a surface density of magnetic monopoles, which accumulate at the boundary separating the two phases. The mechanism responsible for the monopole charge density generation is discussed. Each phase is found to be paramagnetic with higher magnetic susceptibility in the quark phase. The connection with the physics of neutron stars is highlighted throughout the paper.

On Differentiating Multiple Types of ULF Magnetospheric Waves in Response to Solar Wind Periodic Density Structures

Identifying the nature and source of ultra‐low frequencies (ULF) waves (f ⪅ 4 mHz) at discrete frequencies in the Earth's magnetosphere is a complex task. The challenge comes from the simultaneous occurrence of externally and internally generated waves, and the ability to robustly identify such perturbations. Using a recently developed robust spectral analysis procedure, we study an interval that exhibited in magnetic field measurements at geosynchronous orbit and in‐ground magnetic observatories both internally supported and externally generated ULF waves. The event occurred on 9 November 2002 during the interaction of the magnetosphere with two interplanetary shocks that were followed by a train of 90 min solar wind periodic density structures. Using the Wang‐Sheeley‐Arge model, we mapped the source of this solar wind stream to an active region and a mid‐latitude coronal hole just prior to crossing the Heliospheric current sheet. In both the solar wind density and magnetospheric field fluctuations, we separated broad power increases from enhancements at specific frequencies. For the waves at discrete frequencies, we used the combination of satellite and ground magnetometer observations to identify differences in frequency, polarization, and observed magnetospheric locations. The magnetospheric response was characterized by: (a) forced breathing by periodic solar wind dynamic pressure variations below ≈1 mHz, (b) a combination of directly driven oscillations and wave modes triggered by additional mechanisms (e.g., shock and interplanetary magnetic field discontinuity impact, and substorm activity) between ≈1 and 4 mHz, and (c) largely triggered modes above ≈4 mHz.

SLAU2-HLLD numerical flux with wiggle-sensor for stable low mach Magnetohydrodynamics simulations

SLAU2 (Simple Low-dissipation Advection-Upstream-splitting-method 2) is an approximate Riemann solver belonging to the AUSM family of schemes in gas dynamics. SLAU2 captures shock waves stably in a supersonic range and realizes high resolutions in the low-speed range (Mach number 0.01 or less) by controlling numerical dissipation. In a previous study, this scheme was successfully extended to magnetohydrodynamics (MHD) simulations, such as SLAU2-HLL (SLAU2-Harten–Lax–van Leer). However, the present study reveals that SLAU2-HLL exhibits unphysical oscillations in low-speed MHD tests. This problem has been found to be caused by insufficient numerical dissipation at the magnetic field discontinuity in a low speed range; thus, we recover the numerical dissipation only where necessary by introducing a wiggle sensor function to the pressure flux part of SLAU2-HLL. This change has enabled SLAU2-HLL to calculate low-speed MHD without unphysical oscillations. Furthermore, we constructed SLAU2-HLLD hybridized with HLLD (HLL-Discontinuity), instead of HLL, which can handle Alfvén waves. By combining this change with the wiggle-sensing stabilization method, we have succeeded in improving the stability and resolution for low Mach MHD flows from the original SLAU2-HLL. The effect of the modification is demonstrated using one- and two-dimensional numerical examples.

Dynamic Identification of HTS Maglev Module for Suspended Vehicle by Using a Single-Degree-of-Freedom Generalized Bouc–Wen Hysteresis Model

The energy dissipation phenomenon of a high-temperature superconducting (HTS) magnetic levitation (Maglev) transport system is investigated by using a single degree of freedom generalized Bouc–Wen hysteresis model. The Maglev system under test consists of a bogie with HTS skaters, which interacts with permanent magnets distributed on the guideways; thanks to a passive and self-balanced magnetic interaction, the vehicle is suspended and guided in all phases of the motion, including zero speed. The response of the HTS skate is tested under pseudo-static cyclic load in the vertical and lateral directions. The parameters of the generalized Bouc–Wen (GBW) model are calibrated on the experimental hysteresis loops using an ordinary least squares–based algorithm. The vertical dynamics of the levitating HTS skate is affected by the magnetic field discontinuities of the guideway. The dissipated hysteretic energy is simulated to assess the dependence on the velocity and pattern of the magnetic field. The displacement response is obtained by solving the nonlinear differential equation representative of the moving HTS skater, modelled as a single degree of freedom system, whose interactions with the magnetic guideway are described by the identified GBW hysteresis law.

Quiet Time Structured Pc1 Waves Generated During Transient Foreshock

AbstractWe study two events of structured Pc1 waves (pearls) observed by CARISMA magnetometers during a period of quiet magnetic (Kp ~1) and solar wind conditions on 1 August 2008. Bursts of proton precipitations and electromagnetic ion‐cyclotron waves were simultaneously observed by NOAA/POES and GOES satellites at dayside. Apparent correspondence was found between ground pearl pulsations and magnetospheric compressions observed by GOES and THEMIS satellites. We have found the Pc1 wave splitting into very close subfrequencies (within 0.1 Hz) that can be explained by weak magnetospheric compressions. The compressions are caused by pressure pulses originated from transient foreshock and interplanetary magnetic field discontinuities observed upstream of the bow shock by the THEMIS‐C probe. Thus, the observed pearls are compression‐related Pc1 waves generated under transient foreshock conditions.

Existence and uniqueness of low-energy weak solutions to the compressible 3D magnetohydrodynamics equations

We prove the existence and uniqueness of weak solutions of the three dimensional compressible magnetohydrodynamics (MHD) equations. We first obtain the existence of weak solutions with small L2-norm which may display codimension-one discontinuities in density, pressure, magnetic field and velocity gradient. The weak solutions we consider here exhibit just enough regularity and structure which allow us to develop uniqueness and continuous dependence theory for the compressible MHD equations. Our results generalise and extend those for the intermediate weak solutions of compressible Navier-Stokes equations.

On the Kinetic Nature of Solar Wind Discontinuities

AbstractThe most intense currents in the solar wind are carried by magnetic field discontinuities. The formation of such structures depends on whether they are rotational or tangential discontinuities. To distinguish between these two types, variations of plasma characteristics should be studied. We analyze data from a set of discontinuities observed by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun and Magnetospheric Multiscale missions in the near‐Earth solar wind. We show that these discontinuities have properties characteristic of both tangential and rotational discontinuities: (1) Jumps in the tangential velocity component are well correlated with jumps in the Alfven speed, and (2) Electron density and temperature vary significantly across these discontinuities. Suprathermal electron pitch angle distributions change across discontinuities, revealing the importance of electron kinetics to discontinuity structure. Investigation of the discontinuity formation needs to go beyond magnetohydrodynamics theory and highly likely involves both ion and electron kinetics.

Superpenumbral chromospheric flare

We observed a C-class flare at the outer boundary of the superpenumbra of a sunspot. The flare was triggered by an emerging magnetic bipolar region that was obliquely oriented with respect to the superpenumbral fibrils. The flare started due to the low height magnetic reconnection of emerging magnetic flux with a superpenumbral field resulting in hot multi-temperature plasma flows in the inverse Evershed flow channel and its overlying atmosphere. Inverse Evershed flows in the chromosphere start from the superpenumbra towards the sunspot that end at the outer boundary of the penumbra. The hot plasma flow towards the sunspot in the inverse Evershed channels show about 10 km s−1 higher velocity in Hα wavelengths compared to the plasma emissions at various temperatures as seen in different AIA filters. Even though these velocities are about seven times higher than the typical inverse Evershed flow speeds, the flow is diminished at the outer boundary of the sunspot’s penumbra. This suggests that the superpenumbral field lines that carry inverse Evershed flows are discontinued at the boundary where the penumbral field lines dive into the sun and these two sets of field lines are completely distinct. The discontinuity in the typical magnetic field and plasma properties at the adjoining of these two sets of field lines further leads to discontinuity in the characteristic magnetoacoustic and Alfvén speeds, thereby stopping the plasma flows further on. The multi-temperature plasma in the inverse Evershed channels exhibits possible longitudinal oscillations initially during the onset of the flare, and later flows towards the sunspot. In the multi-temperature view, the different layers above the flare region have a mixture of supersonic as well as subsonic flows.

Dynamics of Intense Currents in the Solar Wind

Abstract Transient currents in the solar wind are carried by various magnetic field discontinuities that contribute significantly to the magnetic field fluctuation spectrum. Internal instabilities and dynamics of these discontinuities are believed to be responsible for magnetic field energy dissipation and corresponding charged particle acceleration and heating. Accurate modeling of these phenomena requires detailed investigation of transient current formation and evolution. By examining such evolution using a unique data set compiled from observations of the same solar wind flow by two spacecraft at Earth’s and Mars’s orbits, we show that it consists of several processes: discontinuity thinning (decrease in thickness normalized by the ion inertial length), intensification of currents normalized to the proton thermal current (i.e., the product of proton charge, density, and thermal velocity), and increase in the compressional component of magnetic field variations across discontinuities. The significant proton temperature variation around most observed discontinuities indicates possible proton heating. Plasma velocity jumps across the discontinuities are well correlated with Alfvén velocity changes. We discuss possible explanations of the observed discontinuity evolution. We also compare the observed evolution with predictions of models describing discontinuity formation due to Alfvén wave steepening. Our results show that discontinuity modeling likely requires taking into account both the effects of nonlinear Alfvén wave dynamics and solar wind expansion.

Interaction of the Interplanetary Shock and IMF Directional Discontinuity in the Solar Wind

AbstractThe solar wind is not stationary and variations of the magnetic field magnitude and/or direction and plasma parameters are frequent. Interactions of fast forward shocks and interplanetary magnetic field discontinuities with the bow shock were analyzed in details theoretically as well as experimentally. However, an oblique interaction between a fast‐forward interplanetary shock and a directional interplanetary magnetic field discontinuity was studied only within the framework of the ideal magnetohydrodynamics. A fortunate position of six spacecraft on 22 June 2015 facilitates detailed observations of an interplanetary shock modification due to the interaction with the directional discontinuity in the solar wind. We present a multispacecraft analysis of the event supported with magnetohydrodynamic (MHD) modeling showing that this interaction can lead to a bifurcation of the velocity jump across the shock.

Investigating the anatomy of magnetosheath jets – MMS observations

Abstract. We use Magnetosphere Multiscale (MMS) mission data to investigate a small number of magnetosheath jets, which are localized and transient increases in dynamic pressure, typically due to a combined increase in plasma velocity and density. For two approximately hour-long intervals in November, 2015 we found six jets, which are of two distinct types. (a) Two of the jets are associated with the magnetic field discontinuities at the boundary between the quasi-parallel and quasi-perpendicular magnetosheath. Straddling the boundary, the leading part of these jets contains an ion population similar to the quasi-parallel magnetosheath, while the trailing part contains ion populations similar to the quasi-perpendicular magnetosheath. Both populations are, however, cooler than the surrounding ion populations. These two jets also have clear increases in plasma density and magnetic field strength, correlated with a velocity increase. (b) Three of the jets are found embedded within the quasi-parallel magnetosheath. They contain ion populations similar to the surrounding quasi-parallel magnetosheath, but with a lower temperature. Out of these three jets, two have a simple structure. For these two jets, the increases in density and magnetic field strength are correlated with the dynamic pressure increases. The other jet has a more complicated structure, and no clear correlations between density, magnetic field strength and dynamic pressure. This jet has likely interacted with the magnetosphere, and contains ions similar to the jets inside the quasi-parallel magnetosheath, but shows signs of adiabatic heating. All jets are associated with emissions of whistler, lower hybrid, and broadband electrostatic waves, as well as approximately 10 s period electromagnetic waves with a compressional component. The latter have a Poynting flux of up to 40 µW m−2 and may be energetically important for the evolution of the jets, depending on the wave excitation mechanism. Only one of the jets is likely to have modified the surrounding magnetic field into a stretched configuration, as has recently been reported in other studies. None of the jets are associated with clear signatures of either magnetic or thermal pressure gradient forces acting on them. The different properties of the two types also point to different generation mechanisms, which are discussed here. Their different properties and origins suggest that the two types of jets need to be separated in future statistical and simulation studies. Keywords. Magnetospheric physics (magnetosheath; plasma waves and instabilities; solar wind–magnetosphere interactions)