Dipolarization Fronts Research Articles

Acceleration of plasma in current sheet during substorm dipolarizations in the Earth's magnetotail: Comparison of different mechanisms

This work is devoted to the investigation of particle acceleration during magnetospheric dipolarizations. A numerical model is presented taking into account the four scenarios of plasma acceleration that can be realized: (A) total dipolarization with characteristic time scales of ≈3 min; (B) single peak value of the normal magnetic component B z occurring on the time scale of less than 1 min; (C) a sequence of rapid jumps of B z interpreted as the passage of a chain of multiple dipolarization fronts (DFs); and (D) the simultaneous action of mechanism (C) followed by the consequent enhancement of electric and magnetic fluctuations with the small characteristic time scale ≤1 s. In a frame of the model, we have obtained and analyzed the energy spectra of four plasma populations: electrons e−, protons H+, helium He+, and oxygen O+ ions, accelerated by the above-mentioned processes (A)–(D). It is shown that O+ ions can be accelerated mainly due to the mechanism (A); H+ and He+ ions (and to some extent electrons) can be more effectively accelerated due to the mechanism (C) than the single dipolarization (B). It is found that high-frequency electric and magnetic fluctuations accompanying multiple DFs (D) can strongly accelerate electrons e− and really weakly influence other populations of plasma. The results of modeling demonstrated clearly the distinguishable spatial and temporal resonance character of particle acceleration processes. The maximum particle energies depending on the scale of the magnetic acceleration region and the value of the magnetic field are estimated. The shapes of energy spectra are discussed.This work is devoted to the investigation of particle acceleration during magnetospheric dipolarizations. A numerical model is presented taking into account the four scenarios of plasma acceleration that can be realized: (A) total dipolarization with characteristic time scales of ≈3 min; (B) single peak value of the normal magnetic component B z occurring on the time scale of less than 1 min; (C) a sequence of rapid jumps of B z interpreted as the passage of a chain of multiple dipolarization fronts (DFs); and (D) the simultaneous action of mechanism (C) followed by the consequent enhancement of electric and magnetic fluctuations with the small characteristic time scale ≤1 s. In a frame of the model, we have obtained and analyzed the energy spectra of four plasma populations: electrons e−, protons H+, helium He+, and oxygen O+ ions, accelerated by the above-mentioned processes (A)–(D). It is shown that O+ ions can be accelerated mainly due to the mechanism (A); H+ and He+ ions (and to some exten...

Energy Range of Electron Rolling Pin Distribution Behind Dipolarization Front

AbstractThe electron rolling pin distribution, showing electron pitch angles primarily at 0°, 90°, and 180°, has recently been observed behind dipolarization fronts (DFs) and explained using an analytical model. However, the energy range of such distribution has been unknown so far, owing to the low‐resolution data in previous spacecraft missions. Here using the high‐resolution measurements of Magnetospheric Multiscale, we reveal the energy range of electron rolling pin distribution behind DFs for the first time. We find that such distribution appears only above 1.7 keV, falling well into the suprathermal energy range. Below 1.7 keV, electrons exhibit a Maxwell distribution, while above 1.7 keV, they exhibit a power law distribution. In addition, such distribution appears primarily in the growing phase of the flow and disappears quickly in the decaying phase. During the formation of the rolling pin distribution, electrons are gyrotropic. These findings have greatly improved our knowledge of electron dynamics around DFs.

Kinetic Equilibrium and Stability Analysis of Dipolarization Fronts

AbstractDipolarization fronts are typically observed with a density gradient of scale size comparable to an ion gyroradius, which naturally results in an ambipolar electric field in the direction of the gradient. Prevailing models ignore this ambipolar electric field, the separation of ion and electron scale physics, and consequent non‐Maxwellian plasma distributions with strong spatial gradients in velocity, all of which we investigate in this paper. We examine two dipolarization front events observed by the Magnetospheric Multiscale mission (one with low plasma beta, one with high plasma beta), develop a rigorous kinetic equilibrium for dipolarization fronts, analyze the linear stability, and explore the nonlinear evolution and observable signatures with kinetic simulations. There are two major drivers of instability in the lower‐hybrid frequency range: the density gradient (lower‐hybrid drift instability) and the velocity shear (electron‐ion hybrid instability). We argue the electron‐ion hybrid mode is dominant, and consequently a dipolarization front approaches a steady or saturated state through the emission of waves that relax the velocity shear. A key aspect of these shear‐driven waves is a broadband frequency spectrum that is consistent with satellite observation.

Anchor Point of Electron Acceleration around Dipolarization Fronts in Space Plasmas

Abstract An anchor point, which is an energy threshold above which electrons are accelerated and below which electrons are decelerated, has recently been reported within the dipolarizing flux bundles behind dipolarization fronts (DFs) both in observations and simulations. However, what determines this point and how it is formed remain unclear. In this study, we investigate for the first time the formation of this point and the relation between this point and the plasma properties by considering a large amount of DF events measured by Cluster. We find a good correlation between this anchor point and the plasma-sheet density and temperature. We notice that such a point appears primarily in the DF events associated with strong whistlers, suggesting that it is formed due to wave-particle interactions near DFs. Quantitatively, we establish a model for the anchor point, eV, where N and T are the normalized plasma-sheet density and temperature, respectively. With this model, we can predict the electron acceleration features behind DFs, by monitoring plasma properties in the plasma sheet. Such a model can be crucial for understanding electron acceleration regions elsewhere in space, such as reconnection diffusion region and collisionless shocks.

Electron-scale Vertical Current Sheets in a Bursty Bulk Flow in the Terrestrial Magnetotail

Abstract We report Magnetospheric Multiscale observations of multiple vertical current sheets (CSs) in a bursty bulk flow in the near-Earth magnetotail. Two of the CSs were fine structures of a dipolarization front (DF) at the leading edge of the flow. The other CSs were a few Earth radii tailward of the DF; that is, in the wake of the DF. Some of these vertical CSs were a few electron inertial lengths thick and were converting energy from magnetic field to plasma. The currents of the CSs in the DF wake were carried by electrons that formed flow shear layers. These electron-scale CSs were probably formed during the turbulent evolution of the bursty bulk flow and are important for energy conversion associated with fast flows.

Small-scale dipolarization fronts in the Earth′s magnetotail

Previous studies suggest that dipolarization fronts (DFs) are 1 to 3RE (RE is the earth radius) wide in the dawn-dusk direction. Recent kinetic simulations have found that DFs may break up into small-scale structures after they are produced by reconnection. Motivated by this simulation, we revisited the scale size of DFs in the dawn-dusk direction by using Cluster observations during the years when the inter-distance among Cluster spacecraft was between 1000 and 2000 km. We selected the DFs that were detected by more than one spacecraft and estimated the radii of these DFs by a simple geometrical analysis, which is based on comparison of DF normals observed by different spacecraft. We found a few DFs that were only a few ion inertial lengths in the dawn-dusk direction. These results point out the importance of multi-scale coupling during the evolution of DFs.

Statistical Properties of Sub‐Ion Magnetic Holes in the Dipolarized Magnetotail: Formation, Structure, and Dynamics

AbstractMagnetic energy release during magnetic reconnection in the magnetotail leads to fast plasma flows transporting thermal energy toward the inner magnetosphere or deep tail. The interaction of such flows with the ambient plasmas is controlled by forces at the flow's leading edge, manifested as a sharp enhancement of the south‐north component of magnetic field there, which has been called the dipolarization front. In this study, we examine the kinetic plasma structure of equatorial magnetic field perturbations observed behind dipolarization fronts. Using statistical observations of dipolarization fronts in the near‐Earth magnetotail by Time History of Events and Macroscale Interactions during Substorms mission, we find sub‐ion scale (scale is below ion gyroradius) magnetic field depressions (magnetic holes), mostly around the equatorial plane, drifting dawnward. They are populated by hot, transversely anisotropic electrons, likely heated around the front. Combining spacecraft observations, analytical estimates, and particle‐in‐cell simulations, we suggest that these holes result from the ballooning/interchange instability at the dipolarization front. They may represent the nonlinear stage of magnetic field perturbations associated with front instability, which trap hot electrons behind the front. We also discuss the possible role of these holes in scattering and heating electrons and ions in the dipolarized magnetotail.

A Statistical Study on the Properties of Dips Ahead of Dipolarization Fronts Observed by MMS

AbstractMagnetic reconnection in the magnetotail accelerates plasma in short‐duration bursts of fast flows, referred to as bursty bulk flows toward the Earth. These bursty bulk flows are typically accompanied by a sharp increase in the northward magnetic field component, the so‐called dipolarization front (DF). This rapid increase in northward magnetic field is often preceded by a decrease, the so called DF‐dip, where the northward magnetic field component sometimes even changes sign and turns southward. Here we present a statistical study of the DF‐dip of 43 events, using observations from the Magnetospheric Multiscale (MMS) mission during summer 2017, when MMS's apogee was located in the magnetotail around 25 RE. The 43 events are subdivided into two categories according to their DF‐dip: 20 events where the DF‐dip stays northward (positive DF‐dip category) and 23 events where the DF‐dip turns fully southward (negative DF‐dip category). We find that (1) the magnetic field depression ahead of the DF of the positive DF‐dip events correlates mainly with perpendicular currents and diamagnetic currents flowing ahead of the DF and (2) the magnetic field depression ahead of the DF of the negative DF‐dip events correlates mainly with field‐aligned currents and that this type of events might be (a) earthward moving flux rope‐like structures caused by multiple X‐line reconnection and/or (b) are a result of localized, single X‐line reconnection under a guide field.

Rippled Electron‐Scale Structure of a Dipolarization Front

AbstractWe use the Magnetospheric Multiscale mission to investigate electron‐scale structures at a dipolarization front. The four spacecraft are separated by electron scales and observe large differences in plasma and field parameters within the dipolarization front, indicating strong deviation from typically assumed plane or slightly curved front surface. We attribute this to ripples generated by the lower hybrid drift instability (LHDI) with wave number of kρe≃0.4 and maximum wave potential of ∼1 kV ∼kBTe. Power law‐like spectra of E⊥ with slope of −3 indicates the turbulent cascade of LHDI. LHDI is observed together with bursty high‐frequency parallel electric fields, suggesting coupling of LHDI to higher‐frequency electrostatic waves.

Kinetic Equilibrium of Dipolarization Fronts

The unprecedented high-resolution data from the Magnetospheric Multi-Scale (MMS) satellites is revealing the physics of dipolarization fronts created in the aftermath of magnetic reconnection in extraordinary detail. The data shows that the fronts contain structures on small spatial scales beyond the scope of fluid framework. A new kinetic analysis, applied to MMS data here, predicts that global plasma compression produces a unique particle distribution in a narrow boundary layer with separation of electron and ion scale physics. Layer widths on the order of an ion gyro-diameter lead to an ambipolar potential across the magnetic field resulting in strongly sheared flows. Gradients along the magnetic field lines create a potential difference, which can accelerate ions and electrons into beams. These small-scale kinetic effects determine the plasma dynamics in dipolarization fronts, including the origin of the distinctive broadband emissions.

Modeling of Magnetic Dipolarizations and Turbulence in Earth’s Magnetotail as Factors of Plasma Acceleration and Transfer

The paper is devoted to studying processes of plasma particle acceleration in the process of magnetic dipolarizations in a current sheet of Earth’s magnetotail. A numerical model is constructed that allows evaluation of particle acceleration in three possible scenarios: (A) Proper dipolarization; (B) Passage of multiple dipolarization fronts; (C) Passage of fronts followed by high-frequency electromagnetic oscillations. The energy spectra of three types of accelerated particles are obtained: hydrogen H+ and oxygen O+ ions and electrons e–. It is shown that, at different time scales, predominant acceleration of various particle populations occurs in scenarios (A)–(C). Oxygen ions are accelerated most efficiently in single dipolarization process (A), protons (and, to some extent, electrons), in scenario (B), whereas scenario (C) is most efficient for acceleration of electrons. It is shown that accounting for high-frequency electromagnetic fluctuations, accompanying magnetic dipolarization, may explain the appearance of streams of particles with energies on the order of hundreds of keV in Earth’s magnetotail.

Whistler and Electron Firehose Instability Control of Electron Distributions in and Around Dipolarizing Flux Bundles

AbstractElectromagnetic fluctuations associated with various plasma instabilities have been previously identified near dipolarization fronts in Earth's magnetotail. However, the potential effect of these fluctuations on particle distributions and energy conversion is poorly understood. The most important instabilities responsible for electron acceleration and scattering are the whistler instability and the electron firehose instability. Utilizing 10 years of Time History of Events and Macroscale Interactions during Substorms satellite observations, we explore the occurrence probability and intensity of these instabilities near dipolarization fronts, as function of electron anisotropy and plasma beta. Electron temperature anisotropies are well constrained by the marginal stability conditions of the whistler and oblique electron firehose instabilities. In fact, the observed enhancement of magnetic field fluctuations near the instability thresholds provides good evidence for the operation of these instabilities on electrons near fronts. Since the build‐up of electron anisotropy is limited by wave‐particle interactions, we conclude that such interactions are important enough to affect electron dynamics and energetics.

Oxygen Ion Reflection at Earthward Propagating Dipolarization Fronts in the Magnetotail

AbstractA dipolarization front (DF) is known as the leading edge of an earthward high‐speed flow with a sharp enhancement in the northward magnetic field (Bz). Analysis of an event observed by Cluster shows that the behavior of oxygen ions (O+) around the DF is very different from protons (H+). After the crossing of the DF, the O+ density decreases more gradually than H+. The distance between the density minimum of O+ and the DF layer is ~4 times longer than that of H+, which is close to their gyroradii ratio with the same energy. A flux dropout is observed in the O+ energy spectrum, whose energy dependence indicates that ions with higher energies can reach locations farther tailward of the DF. Similar variations are also seen in studies of 22 events in which a common pattern of ion properties is obtained by performing a superposed epoch analysis. Finally, using backward tracing test‐particle simulations, we reproduce the characteristics of the flux dropout and verify that the time dependence of the dropout is highly correlated with the gyromotion of different energy O+ behind the DF. All these results provide a further understanding of ion dynamics associated with DFs and suggest that the observed O+ ions are reflected within a half gyromotion in the central plasma sheet.

Three dimensional analytical model of dipolarizing flux bundles

In many plasma systems, magnetic reconnection plays a crucial role in particle acceleration. In Earth's magnetotail, reconnection releases a significant portion of the stored magnetic energy (which is primarily converted into ion enthalpy) behind intense electromagnetic boundaries called dipolarization fronts. Dipolarizing flux bundles behind these fronts accelerate ambient plasma and transport charged particles from the magnetotail to near-Earth space. Appropriate modeling of such flux bundle-particle interactions requires self-consistent models that describe both the bundle's electromagnetic fields and the magnetotail's static background magnetic field. We develop a 2D solution of the Vlasov-Maxwell equation for a magnetotail with an embedded dipolarization front and generalize this solution to a 3D system. Our 3D model describes both global (within the magnetotail) and local (around the front) electromagnetic field distributions and a consistent distribution of plasma. The magnetic field topology in our model agrees with that deduced from multispacecraft observations. Our model can be used for a test particle tracing and investigation of particle acceleration/transport by dipolarization fronts in planetary magnetotails.

Plasma acceleration on multiscale temporal variations of electric and magnetic fields during substorm dipolarization in the Earth’s magnetotail

Magnetic field dipolarizations are often observed in the magnetotail during substorms. These generally include three temporal scales: (1) actual dipolarization when the normal magnetic field changes during several minutes from minimum to maximum level; (2) sharp Bz bursts (pulses) interpreted as the passage of multiple dipolarization fronts with characteristic time scales < 1 min, and (3) bursts of electric and magnetic fluctuations with frequencies up to electron gyrofrequency occurring at the smallest time scales (≤ 1 s). We present a numerical model where the contributions of the above processes (1)-(3) in particle acceleration are analyzed. It is shown that these processes have a resonant character at different temporal scales. While O+ ions are more likely accelerated due to the mechanism (1), H+ ions (and to some extent electrons) are effectively accelerated due to the second mechanism. High-frequency electric and magnetic fluctuations accompanying magnetic dipolarization as in (3) are also found to efficiently accelerate electrons.

Ion Trapping and Acceleration at Dipolarization Fronts: High‐Resolution MHD and Test‐Particle Simulations

AbstractMuch of plasma heating and transport from the magnetotail into the inner magnetosphere occurs in the form of mesoscale discrete injections associated with sharp dipolarizations of magnetic field (dipolarization fronts). In this paper we investigate the role of magnetic trapping in acceleration and transport of the plasma sheet ions into the ring current. For this purpose we use high‐resolution global magnetohydrodynamic (MHD) and three‐dimensional test‐particle simulations. It is shown that trapping, produced by sharp magnetic field gradients at the interface between dipolarizations and the ambient plasma, affects plasma sheet protons with energies above approximately 10 keV, enabling their transport across more than 10 Earth radii and acceleration by a factor of 10. Our estimates show that trapping is important to the buildup of the ring current plasma pressure of injected particles; depending on the plasma sheet temperature and energy spectrum, trapped protons can contribute between 20% and 60% of the plasma pressure. It is also shown that the acceleration process does not conserve the particle first invariant; on average protons are accelerated to higher energies compared to a purely adiabatic process. We also investigate how trapping and energization vary for deferent ions species and show that in accordance with recent observations, ion acceleration is proportional to the ion charge and is independent of its mass.

Formation of dipolarization fronts after current sheet thinning

Dipolarization front (DF)—a sharp boundary separating hot tenuous plasmas from cold dense plasmas—is a key structure responsible for particle acceleration and energy transport in the magnetotail. How such a structure is formed has been unclear so far. Two possible mechanisms suggested in previous studies are magnetic reconnection and spontaneous formation. Both of them require current sheet thinning as a prerequisite. However, observational evidence of the DF formation associated with current sheet thinning has not been reported. In this study, we present such an observation, showing the DF formation after current sheet thinning. We estimate the half thickness of the current sheet to be ∼1000 km and the rate of current sheet thinning as ∼38 km/s. We find that the DF is likely formed at XGSM ≈ −20 RE. During the current sheet thinning, the plasma becomes cold and dense; during DF formation, many magnetic islands are produced. Although current sheet thinning and DF formation have been individually analyzed in the previous studies, this study, for the first time, links the two transient processes in the magnetotail.

Kinetic Dissipation Around a Dipolarization Front

AbstractKinetic aspects of energy conversion and dissipation near a dipolarization front (DF) in the magnetotail are considered using fully kinetic 3‐D particle‐in‐cell simulations. The energy conversion is described in terms of the pressure dilatation, as well as the double contraction of deviatoric pressure tensor and traceless strain rate tensor, also known as the Pi‐D parameter in turbulence studies. It is shown that in contrast to the fluid dissipation measure, the Joule heating rate, which cannot distinguish between ion and electron dissipation and reveals deep negative dips at the DF, the Pi‐D parameters, as kinetic analogs of the Joule heating rate, are largely positive and drastically different for ions and electrons. Further analysis of these parameters suggests that ions are heated at and ahead of the DF due to their reflection from the front, while electrons are heated at and behind the DF due to the long‐wavelength lower‐hybrid drift instability.

Contrasting dynamics of electrons and protons in the near-Earth plasma sheet during dipolarization

Abstract. The fortunate location of Cluster and the THEMIS P3 probe in the near-Earth plasma sheet (PS) (at X ∼ −7–−9 RE) allowed for the multipoint analysis of properties and spectra of electron and proton injections. The injections were observed during dipolarization and substorm current wedge formation associated with braking of multiple bursty bulk flows (BBFs). In the course of dipolarization, a gradual growth of the BZ magnetic field lasted ∼ 13 min and it was comprised of several BZ pulses or dipolarization fronts (DFs) with duration ≤ 1 min. Multipoint observations have shown that the beginning of the increase in suprathermal (&gt; 50 keV) electron fluxes – the injection boundary – was observed in the PS simultaneously with the dipolarization onset and it propagated dawnward along with the onset-related DF. The subsequent dynamics of the energetic electron flux was similar to the dynamics of the magnetic field during the dipolarization. Namely, a gradual linear growth of the electron flux occurred simultaneously with the gradual growth of the BZ field, and it was comprised of multiple short (∼ few minutes) electron injections associated with the BZ pulses. This behavior can be explained by the combined action of local betatron acceleration at the BZ pulses and subsequent gradient drifts of electrons in the flux pile up region through the numerous braking and diverting DFs. The nonadiabatic features occasionally observed in the electron spectra during the injections can be due to the electron interactions with high-frequency electromagnetic or electrostatic fluctuations transiently observed in the course of dipolarization. On the contrary, proton injections were detected only in the vicinity of the strongest BZ pulses. The front thickness of these pulses was less than a gyroradius of thermal protons that ensured the nonadiabatic acceleration of protons. Indeed, during the injections in the energy spectra of protons the pronounced bulge was clearly observed in a finite energy range ∼ 70–90 keV. This feature can be explained by the nonadiabatic resonant acceleration of protons by the bursts of the dawn–dusk electric field associated with the BZ pulses. Keywords. Magnetospheric physics (Magnetotail; plasma sheet) – Space plasma physics (Transport processes)

MHD Stability of Magnetotail Configurations With a Bz Hump

AbstractUsing two‐dimensional magnetohydrodynamic (MHD) simulations, we explore the stability of magnetotail configurations that include a local enhancement of Bz (a “Bz hump”). We demonstrate that hump configurations can become unstable within the constraints of ideal MHD, even when strict boundary conditions v = 0 are imposed at all boundaries, consistent with the constraints of the ideal MHD energy principle. Necessary conditions for instability are that the boundaries on the earthward and high‐latitude sides are far enough away, that the background pressure, equivalent to the lobe pressure, is small enough, and that the Bz enhancement or entropy reduction is strong enough. The unstable evolution then bears strong similarity to the evolution of observed and simulated dipolarized flux bundles and dipolarization fronts, presumed or simulated to be the consequence of reconnection. The Bz humps, which for sufficiently low background pressure correspond to entropy depletions, move faster for lower background density and penetrate closer toward Earth when they are initiated closer to Earth or when the background pressure is lower, similar to 3‐D low‐entropy flux tubes, which are affected by ballooning‐/interchange‐type modes.