Global Hybrid Simulation Research Articles

Characteristics of Escaping Magnetospheric Ions Associated With Magnetic Field Fluctuations

AbstractThe four Magnetospheric Multiscale (MMS) spacecraft observed energetic ( keV) ion bursts exhibiting an inverse dispersion in the magnetosheath on 28 December 2015. We consider the possibility that these ions originate from the magnetosphere. The ion composition ratios, flux levels, and the spectral slopes of the energetic ion energy spectra observed in the foreshock and in the magnetosheath resemble those in the outer magnetosphere but differ significantly from those seen further upstream from the bow shock at ACE. The particle gyrocenters lie earthward from the spacecraft, indicating that the maximum ion fluxes come from close to the magnetosphere. We provide important evidence that argues against an explanation of the particle source in terms of hot flow anomaly acceleration. A three‐dimensional global hybrid simulation shows that escaping magnetospheric ions can be scattered and transported across the magnetosheath. Ground magnetometer observations suggest that a solar wind pressure increase accelerates escaping magnetospheric ions via betatron acceleration, resulting in an inverse energy dispersion in the magnetosheath. However, there are no pressure changes detected on the MMS and ACE spacecraft and the ground magnetic field strength does not appear to be large enough to be consistent with the large magnetospheric compression needed to account for a betatron acceleration. Therefore, we suggest that the inverse energy dispersion event can be explained by a magnetic field rotation that connects MMS to the subsolar magnetosphere, enabling high‐energy particles from deep within the inner magnetosphere gain access to the magnetopause and magnetosheath.

Foreshock Bubbles at Venus: Hybrid Simulations and VEX Observations

AbstractIn the absence of a global magnetic field at Venus, its ionosphere is the obstacle to the flow of the solar wind resulting in the formation of a smaller bow shock and foreshock. Spacecraft observations and global hybrid (kinetic ions, fluid electrons) simulations have demonstrated that despite its smaller size, the foreshock at Venus has properties similar to those seen at its much larger terrestrial counterpart. This study employs global hybrid simulations and Venus Express observations to demonstrate the formation of foreshock bubbles (FBs) at Venus and examine their impacts on the Venusian ionosphere. Discovered at Earth, FBs form in response to backstreaming ion beams in the foreshock interacting with solar wind rotational or tangential discontinuities. They exhibit a hot, tenuous, and low magnetic field core bounded by a fast magnetosonic shock wave and the associated plasma sheath. Their sizes scale with the width of the foreshock and they are carried by the solar wind toward the bow shock. The results of the hybrid simulations show that despite the smaller size of the Venusian foreshock, FBs should form at this planet. A preliminary examination of the Venus Express data has shown the presence of a number of FB candidates, one of which is discussed here. Using the results of hybrid simulations we examine the impacts of FBs on the ionosphere and demonstrate their global consequences which exceed those of hot flow anomalies also formed as a result of the interaction between solar wind discontinuities and the Venusian bow shock.

Kinetic Alfvén Waves From Magnetotail to the Ionosphere in Global Hybrid Simulation Associated With Fast Flows

AbstractWe have used the Auburn Global Hybrid Code in 3‐D to study the generation, dynamics, and global structure of kinetic Alfvén waves (KAWs) from the magnetotail to the ionosphere. Our results show that KAWs are generated in magnetic reconnection in the plasma sheet, located around fast flows, and carrying transverse electromagnetic perturbations, parallel Poynting fluxes, parallel currents, and parallel electric field. Overall, shear Alfvénic turbulent spectrum is found in the plasma sheet. The KAWs are shear Alfvén waves possessing short perpendicular wavelength with , where is the perpendicular wave number and the ion Larmor radius. The KAWs are identified by their dispersion relation and polarizations. The structures of these KAWs embedded in the plasma sheet are also revealed by placing a virtual satellite in the tail. In order to understand whether the Poynting fluxes carried by the shear Alfvén waves/KAWs in the plasma sheet can be carried directly along field lines to the ionosphere, we have tracked the wave propagation from the plasma sheet to the ionosphere. It is found that in front of the flow‐braking region, the structure and strength of the shear Alfvén waves are significantly altered due to interaction with the dipole‐like field, mainly by the flow shear associated with the azimuthal convection. Also in front of the dipole‐like field region, ion kinetic effects (Hall effects) lead to the generation of additional pairs of KAWs. As such, the generation and transport of the shear Alfvén waves/KAWs to the ionosphere are illustrated for the first time in a comprehensive manner on the global scale.

Turbulence‐Driven Magnetic Reconnection in the Magnetosheath Downstream of a Quasi‐Parallel Shock: A Three‐Dimensional Global Hybrid Simulation

AbstractSatellite observations with high‐resolution measurements have demonstrated the existence of intermittent current sheets and occurrence of magnetic reconnection in a quasi‐parallel magnetosheath behind the terrestrial bow shock. In this letter, by performing a three‐dimensional global hybrid simulation, we investigated the characteristics of the quasi‐parallel magnetosheath of the bow shock, which is formed due to the interaction of the solar wind with the Earth's magnetosphere. Current sheets with widths of several ion inertial lengths are found to be produced in the magnetosheath after the upstream large‐amplitude electromagnetic waves penetrate through the shock and are then compressed in the downstream. Magnetic reconnection consequently occurs in these current sheets, where high‐speed ion flow jets are identified in the outflow region. Simultaneously, flux ropes with the extension (along the y direction) of about several Earth's radii are also observed. Our simulation shed new insight on the mechanism for the occurrence of magnetic reconnection in the quasi‐parallel shocked magnetosheath.

Dissipation of Earthward Propagating Flux Rope Through Re‐reconnection with Geomagnetic Field: An MMS Case Study

AbstractThree‐dimensional global hybrid simulations and observations have shown that earthward‐moving flux ropes (FRs) can undergo magnetic reconnection (or re‐reconnection) with the near‐Earth dipole field to create dipolarization front (DF)‐like signatures that are immediately preceded by brief intervals of negative BZ. The simultaneous erosion of the southward BZ field at the leading edge of the FR and continuous reconnection of lobe magnetic flux at the X‐line tailward of the FR result in the asymmetric south‐north BZ signature in many earthward‐moving FRs and possibly DFs with negative BZ dips prior to their observation. In this study, we analyzed Magnetospheric MultiScale (MMS) observation of fields and plasma signatures associated with the encounter of an ion diffusion region ahead of an earthward‐moving FR on 3 August 2017. The signatures of this re‐reconnection event were (i) +/− BZ reversal, (ii) −/+ bipolar‐type quadrupolar Hall magnetic fields, (iii) northward super‐Alfvénic electron outflow jet of ~1,000–1,500 km/s, (iv) Hall electric field of ~15 mV/m, (v) intense currents of ~40–100 nA/m2, and (vi) J·E′ ~0.11 nW/m3. Our analysis suggests that the MMS spacecraft encounters the ion and electron diffusion regions but misses the X‐line. Our results are in good agreement with particle‐in‐cell simulations of Lu et al. (2016, https://doi.org/10.1002/2016JA022815). We computed a dimensionless reconnection rate of ~0.09 for this re‐reconnection event and through modeling, estimating that the FR would fully dissipate by −16.58 RE. We demonstrated pertubations in the high‐latitude ionospheric currents at the same time of the dissipation of earthward‐moving FRs using ground‐ and space‐based measurements.

Planetary magnetic field control of ion escape from weakly magnetized planets

ABSTRACT Intrinsic magnetic fields have long been thought to shield planets from atmospheric erosion via stellar winds; however, the influence of the plasma environment on atmospheric escape is complex. Here we study the influence of a weak intrinsic dipolar planetary magnetic field on the plasma environment and subsequent ion escape from a Mars-sized planet in a global three-dimensional hybrid simulation. We find that increasing the strength of a planet’s magnetic field enhances ion escape until the magnetic dipole’s standoff distance reaches the induced magnetosphere boundary. After this point increasing the planetary magnetic field begins to inhibit ion escape. This reflects a balance between shielding of the Southern hemisphere from ‘misaligned’ ion pickup forces and trapping of escaping ions by an equatorial plasmasphere. Thus, the planetary magnetic field associated with the peak ion escape rate is critically dependent on the stellar wind pressure. Where possible we have fit power laws for the variation of fundamental parameters (escape rate, escape power, polar cap opening angle, and effective interaction area) with magnetic field, and assessed upper and lower limits for the relationships.

Magnetosheath Reconnection Before Magnetopause Reconnection Driven by Interplanetary Tangential Discontinuity: A Three‐Dimensional Global Hybrid Simulation With Oblique Interplanetary Magnetic Field

AbstractTerrestrial dayside dynamics associated with a southward turning, oblique interplanetary magnetic field (IMF) carried by an interplanetary tangential discontinuity (TD) is investigated by performing a three‐dimensional global‐scale hybrid simulation systematically for cases in which the incoming solar wind TD possesses various magnetic field rotation angles ΔΦ = 90° to 180° and half widths w = 2di0 to w = 30di0, where di0 is the ion inertial length in the solar wind. Overall, the TD is compressed while being transmitted into the magnetosheath, with different compression processes downstream of the quasi‐parallel (Q‐||) and quasi‐perpendicular (Q‐⊥) shocks. It is found that magnetosheath reconnection may take place downstream of both the Q‐|| and Q‐⊥ shocks due to interaction of the directional TD with the bow shock and magnetopause, but the existence of magnetosheath reconnection depends on w and ΔΦ. Magnetosheath flux ropes are formed through three‐dimensional patchy reconnection in the thinned current sheet, with a longer rope lengths under a larger ΔΦ. There exists a dawn‐dusk asymmetry in the spatial extent of the flux ropes, which becomes more significant as ΔΦ decreases. When ΔΦ decreases to 90°, no reconnection flux ropes are found. Magnetopause reconnection is initiated when the magnetic fluxes with a southward turning IMF (on the sunward side) reach the magnetopause, and the magnetopause flux ropes can be mixed with the magnetosheath ones. Our simulation demonstrates that the effects of a southward turning of the IMF may not be a simple field direction change that leads to reconnection only at the magnetopause.

Oxygen Ion Energization at Mars: Comparison of MAVEN and Mars Express Observations to Global Hybrid Simulation

AbstractWe study oxygen ion energization in the Mars‐solar wind interaction by comparing particle and magnetic field observations on the Mars Atmosphere and Volatile EvolutioN (MAVEN) and Mars Express missions to a global hybrid simulation. We find that large‐scale structures of the Martian‐induced magnetosphere and plasma environment as well as the Mars heavy ion plume as seen by multispacecraft observations are reproduced by the model. Using the simulation, we estimate the dynamics of escaping oxygen ions by analyzing their distance and time of flight as a function of the gained kinetic energy along spacecraft trajectories. In the upstream region the heavy ion energization resembles single‐particle solar wind ion pickup acceleration as expected, while within the induced magnetosphere the energization displays other features including the heavy ion plume from the ionosphere. Oxygen ions take up to 80 s and travel the distance of 20,000 km after their emission from the ionosphere to the induced magnetosphere or photoionization from the neutral exosphere before they have reached energies of 10 keV in the plume along the analyzed spacecraft orbits. Lower oxygen ion energies of 100 eV are reached faster in 10–20 s over the distance of 100–200 km in the plume. Our finding suggests that oxygen ions are typically observed within the first half of their gyrophase if the spacecraft periapsis is on the hemisphere where the solar wind convection electric field points away from Mars.

Mode structure symmetry breaking of energetic particle driven beta-induced Alfvén eigenmode

The mode structure symmetry breaking of energetic particle driven Beta-induced Alfvén Eigenmode (BAE) is studied based on global theory and simulation. The weak coupling formula gives a reasonable estimate of the local eigenvalue compared with global hybrid simulation using XHMGC. The non-perturbative effect of energetic particles on global mode structure symmetry breaking in radial and parallel (along B) directions is demonstrated. With the contribution from energetic particles, two dimensional (radial and poloidal) BAE mode structures with symmetric/asymmetric tails are produced using an analytical model. It is demonstrated that the symmetry breaking in radial and parallel directions is intimately connected. The effects of mode structure symmetry breaking on nonlinear physics, energetic particle transport, and the possible insight for experimental studies are discussed.

Ion Acceleration Inside Foreshock Transients

AbstractRecent observations upstream of Earth's bow shock have revealed that foreshock transients can not only accelerate solar wind ions by reflection at their upstream boundaries but may also accelerate ions inside them. Evidence for the latter comes from comparisons of ion spectra inside and outside the cores, and from evidence of leakage of suprathermal ions from the cores. However, definite evidence for, and the physics of, ion acceleration in the foreshock transients are still open questions. Using case studies of foreshock transients from Time History of Events and Macroscale Interactions during Substorms observations, we reveal an ion acceleration mechanism in foreshock transients that is applicable to ~25% of the transients. The ion energy flux is enhanced between several keV to tens of keV in the cores. We show that these energetic ions are reflected at the earthward moving boundary of foreshock transients, are accelerated through partial gyration along the convection electric field, and can leak out both upstream and downstream of the foreshock transients. Using ions moving self‐consistently with a generic 3‐D global hybrid simulation of a foreshock transient, we confirm this physical picture of ion acceleration and leakage. These accelerated ions could be further accelerated at the local bow shock and repopulate the foreshock, increasing the efficacy of solar wind‐magnetosphere interactions.

The ion temperature gradient: An intrinsic property of Earth's magnetotail

AbstractAlthough the ion temperature gradient along (XGSM) and across (ZGSM) the Earth's magnetotail, which plays a key role in generating the cross‐tail current and establishing pressure balance with the lobes, has been extensively observed by spacecraft, the mechanism responsible for its formation is still unknown. We use multispacecraft observations and three‐dimensional (3‐D) global hybrid simulations to reveal this mechanism. Using THEMIS (Time History of Events and Macroscale Interactions during Substorms), Geotail, and ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun) observations during individual, near‐simultaneous plasma sheet crossings from 10 to 60 RE, we demonstrate that the ion temperature ZGSM profile is bell‐shaped at different geocentric distances. This ZGSM profile is also prevalent in statistics of ~200 THEMIS current sheet crossings in the near‐Earth region. Using 3‐D global hybrid simulations, we show that mapping of the XGSM gradient of ion temperature along magnetic field lines produces such a bell‐shaped profile. The ion temperature mapping along magnetic field lines in the magnetotail enables construction of two‐dimensional distributions of these quantities from vertical (north‐south) spacecraft crossings. Our findings suggest that the ion temperature gradient is an intrinsic property of the magnetotail that should be considered in kinetic descriptions of the magnetotail current sheet. Toward this goal, we use theoretical approaches to incorporate the temperature gradient into kinetic current sheet models, making them more realistic.

Global kinetic hybrid simulation for radially expanding solar wind

AbstractWe present the results of a 1‐D global kinetic simulation of the solar wind in spherical coordinates without a magnetic field in the region from the Sun to the Earth's orbit. Protons are considered as particles while electrons are considered as a massless fluid, with a constant temperature, in order to study the relation between the hybrid and hydrodynamic solutions. It is shown that the strong electric field in the hybrid model accelerates the protons. Since the electric field in the model is related to electron pressure, each proton in the initial Maxwellian velocity distribution function moves under the same forces as in the classical Parker Solar wind model. The study shows that the hybrid model results in very similar velocity and number density distributions along the radial distance as in the Parker model. In the hybrid simulations, the proton temperature is decreased with distance in 1 order of magnitude. The effective polytropic index of the proton population slightly exceeds 1 at larger distances with the maximum value ∼1.15 in the region near the Sun. A highly non‐Maxwellian type of distribution function is initially formed. Further from the Sun, a narrow beam of the escaping protons is created which does not change much in later expansion. The results of our study indicates that already a nonmagnetized global hybrid model is capable of reproducing some fundamental features of the expanding solar wind shown in the Parker model and additional kinetic effects in the solar wind.

Formation and transport of entropy structures in the magnetotail simulated with a 3‐D global hybrid code

AbstractGlobal structure and evolution of flux tube entropy S, integrated over closed field lines, associated with magnetic reconnection in the magnetotail are investigated using the AuburN Global hybrId codE in three dimensions (3‐D), ANGIE3D. Flux tubes with decreased entropy, or “bubbles,” are found to be generated due to the sudden change of flux tube topology and thus volume in reconnection. By tracking the propagation of the entropy‐depleted flux tubes, the roles of the entropy structure in plasma transport to the inner magnetosphere is examined with a self‐consistent global hybrid simulation for the first time. The value of S first decreases due to the shortening of flux tubes and then increases due to local ion heating as the bubbles are injected earthward by interchange‐ballooning instability, finally oscillating around an equilibrium radial distance where S is nearly the same as the ambient value. The pressure remains anisotropic and not constant along the flux tubes during their propagation with a nonzero heat flux along the field line throughout the duration of the simulation. The correlation of these bubbles with earthward fast flows and specific entropy s is also studied.

Electron acceleration in the near-Earth magnetotail: test particle calculations in electromagnetic fields from two-dimensional hybrid simulations

Electron acceleration in the near-Earth magnetotail during the substorm period is still an unresolved question. In this paper, by tracing electron trajectories in the dynamically evolving electromagnetic fields obtained from a two-dimensional (2D) global hybrid simulation, we investigate electron acceleration in the near-Earth magnetotail during dipolarization. In our simulation, electrons with energies above several keV can gain energy in the plasma sheet due to the adiabatic acceleration mechanism when these electrons propagate earthward. In the near-Earth magnetotail (about 9–15 $R_{E}$ from the Earth), these electrons can be accelerated by betatron acceleration which is due to the compression of magnetic field associated with dipolarization of magnetotail. Additionally, in the middle and high latitudes of the near-Earth magnetotail, the parallel electric field carrying by kinetic Alfven waves can also accelerate electrons when these electrons bounce between the mirror points. The combination effects of these three acceleration mechanisms can accelerate electrons from several keV to about one hundred keV. Our results indicate that both the large-scale structure and wave-particle interactions need to be taken into account for electron acceleration in the near-Earth magnetotail.

Hall effect control of magnetotail dawn‐dusk asymmetry: A three‐dimensional global hybrid simulation

AbstractMagnetotail reconnection and related phenomena (e.g., flux ropes, dipolarizing flux bundles, flow bursts, and particle injections) occur more frequently on the duskside than on the dawnside. Because this asymmetry can directly result in dawn‐dusk asymmetric space weather effects, uncovering its physical origin is important for better understanding, modeling, and prediction of the space weather phenomena. However, the cause of this pervasive asymmetry is unclear. Using three‐dimensional global hybrid simulations, we demonstrate that the Hall physics in the magnetotail current sheet is responsible for the asymmetry. The current sheet thins progressively under enhanced global convection; when its thickness reaches ion kinetic scales, some ions are decoupled from the magnetized electrons (the Hall effect). The resultant Hall electric field Ez is directed toward the neutral plane. The Hall effect is stronger (grows faster) on the duskside; i.e., more ions become unmagnetized there and do not comove with the magnetized dawnward Ez × Bx drifting electrons, thus creating a larger additional cross‐tail current intensity jy (in addition to the diamagnetic current) on the duskside, compared to the dawnside. The stronger Hall effect strength on the duskside is controlled by the higher ion temperature, thinner current sheet, and smaller normal magnetic field Bz there. These asymmetric current sheet properties are in turn controlled by two competing processes that correspond to the Hall effect: (1) the dawnward E × B drift of the magnetic flux and magnetized ions and electrons and (2) the transient motion of the unmagnetized ions which do not execute E × B drift.

Impacts of spontaneous hot flow anomalies on the magnetosheath and magnetopause

AbstractSpacecraft observations and global hybrid (kinetic ions and fluid electrons) simulations have demonstrated that ion dissipation processes at the quasi‐parallel bow shock are associated with the formation of structures called spontaneous hot flow anomalies (SHFAs). Previous simulations and recent spacecraft observations have also established that SHFAs result in the formation of magnetosheath filamentary structures (MFS). In this paper we demonstrate that in addition to MFS, SHFAs also result in the formation of magnetosheath cavities that are associated with decreases in density, velocity, and magnetic field and enhancements in temperature. We use the results of a global MHD run to determine the change in the magnetosheath properties associated with cavities due to ion kinetic effects. The results also show the formation of regions of high flow speed called magnetosheath jets whose properties as a function of solar wind Mach number are described in this study. Comparing the properties of the simulated magnetosheath cavities and jets to past spacecraft observations provides good agreement in both cases. We also demonstrate that pressure variations associated with cavities and SHFAs in the sheath result in a continuous sunward and antisunward magnetopause motion. This result is consistent with previous suggestions that SHFAs may be responsible for the generation of ion cyclotron waves and precipitation of ring current protons in the outer magnetosphere.

Generation of electromagnetic ion cyclotron waves in the near-Earth magnetotail during dipolarization: Two-dimensional global hybrid simulation

We employ two-dimensional global hybrid simulations to study the generation, propagation, and polarization of electromagnetic ion cyclotron (EMIC) waves in the near-Earth magnetotail (around x=−10RE) during dipolarization. In our simulation, EMIC waves with left-hand polarized signals originate in the low-latitude magnetotail as a result of the ion temperature anisotropy which is due to ion heating by Alfvén waves. Subsequently, EMIC waves can propagate along the ambient magnetic field toward high-latitudes. Our work provides one possible mechanism for the generation of EMIC waves observed in the near-Earth magnetotail.

Ion heating by Alfvén waves associated with dipolarization in the magnetotail: Two‐dimensional global hybrid simulation

AbstractIn this paper, ion heating by Alfvén waves associated with dipolarization in the near‐Earth magnetotail is investigated by performing a two‐dimensional (2‐D) global‐scale hybrid simulation. In our simulation, the earthward propagating plasma flow is initialized by the E × B drift near the equatorial plane due to the existence of the dawn‐dusk convection electric field. When the earthward flow reaches the strong dipole field region, it is braked by the geomagnetic field and simultaneously leads to the pileup of the magnetic flux. This continuous pileup finally results in the formation of the large‐scale dipolarization. Dipolarization first appears around (x, z) = (−10.5, 0.3)RE (where RE is the radius of Earth) and subsequently spreads tailward. In the dipolarization region, Alfvén waves are excited and cause the scattering and heating of ions. The heating is mainly on the perpendicular direction. Therefore, the ion temperature anisotropy can be formed in the dipolarization region. Our work provides one possible mechanism for the ion heating and anisotropic distributions observed near the dipolarization region.

Dynamics of planetary ions in the induced magnetospheres of Venus and Mars

We compare dynamics of planetary ions in the induced magnetospheres of Venus and Mars in a global hybrid simulation to study factors controlling the ion escape at unmagnetized planets. In the simulation we find that the finite Larmor radius (FLR) effects of escaping heavy ions are stronger at Mars than Venus under nominal solar wind conditions. But, varying upstream conditions, especially the IMF, affects the strength of the FLR effects. We classify three basic types of planetary ion dynamics in an induced magnetosphere. First, light ions such as hydrogen follow the E×B drift, and escape in the wake in the hemisphere where the solar wind convection electric field is pointing towards the planet. Second, heavy ions like oxygen undergo FLR effects, and escape mainly outside of the wake in the hemisphere where the solar wind convection electric field is pointing away from the planet. Third, ion species between light and heavy ions can have both the E×B and FLR type dynamics in the same time.

Dipolarization fronts as earthward propagating flux ropes: A three‐dimensional global hybrid simulation

AbstractDipolarization fronts (DFs) as earthward propagating flux ropes (FRs) in the Earth's magnetotail are presented and investigated with a three‐dimensional (3‐D) global hybrid simulation for the first time. In the simulation, several small‐scale earthward propagating FRs are found to be formed by multiple X line reconnection in the near tail. During their earthward propagation, the magnetic field Bz of the FRs becomes highly asymmetric due to the imbalance of the reconnection rates between the multiple X lines. At the later stage, when the FRs approach the near‐Earth dipole‐like region, the antireconnection between the southward/negative Bz of the FRs and the northward geomagnetic field leads to the erosion of the southward magnetic flux of the FRs, which further aggravates the Bz asymmetry. Eventually, the FRs merge into the near‐Earth region through the antireconnection. These earthward propagating FRs can fully reproduce the observational features of the DFs, e.g., a sharp enhancement of Bz preceded by a smaller amplitude Bz dip, an earthward flow enhancement, the presence of the electric field components in the normal and dawn‐dusk directions, and ion energization. Our results show that the earthward propagating FRs can be used to explain the DFs observed in the magnetotail. The thickness of the DFs is on the order of several ion inertial lengths, and the electric field normal to the front is found to be dominated by the Hall physics. During the earthward propagation from the near‐tail to the near‐Earth region, the speed of the FR/DFs increases from ~150 km/s to ~1000 km/s. The FR/DFs can be tilted in the GSM (x, y) plane with respect to the y (dawn‐dusk) axis and only extend several Earth radii in this direction. Moreover, the structure and evolution of the FRs/DFs are nonuniform in the dawn‐dusk direction, which indicates that the DFs are essentially 3‐D.