Earthward Propagating Dipolarization Fronts Research Articles

Electron Vorticity at Dipolarization Fronts

Abstract Using magnetospheric multiscale high-resolution measurements, we present the first comprehensive investigation of electron vorticity developing at earthward-propagating dipolarization fronts (DFs). Superposed epoch analysis reveals that electron vorticity increases at and behind the DFs, with its average magnitude slightly larger than lower hybrid frequency, indicating that vorticity is related to Hall dynamics at the DFs. Vorticity at the DFs exhibits clear anisotropy, ω ⊥ > ω ∥, where ω ⊥, ω ∥ are the components perpendicular and parallel to local magnetic field, respectively. Parametric dependence analysis shows that electron vorticity is well anticorrelated with electron density. We find that in a statistical sense, vorticity-induced magnetic field perturbations are not significant, although in a few cases vorticity can generate intense magnetic field perturbations. This study can improve our current understanding of the DF dynamics.

Fast plasma sheet flows and X line motion in the Earth's magnetotail: results from a global hybrid-Vlasov simulation

Abstract. Fast plasma flows produced as outflow jets from reconnection sites or X lines are a key feature of the dynamics in the Earth's magnetosphere. We have used a polar plane simulation of the hybrid-Vlasov model Vlasiator, driven by steady southward interplanetary magnetic field and fast solar wind, to study fast plasma sheet ion flows and related magnetic field structures in the Earth's magnetotail. In the simulation, lobe reconnection starts to produce fast flows after the increasing pressure in the lobes has caused the plasma sheet to thin sufficiently. The characteristics of the earthward and tailward fast flows and embedded magnetic field structures produced by multi-point tail reconnection are in general agreement with spacecraft measurements reported in the literature. The structuring of the flows is caused by internal processes: interactions between major X points determine the earthward or tailward direction of the flow, while interactions between minor X points, associated with leading edges of magnetic islands carried by the flow, induce local minima and maxima in the flow speed. Earthward moving flows are stopped and diverted duskward in an oscillatory (bouncing) manner at the transition region between tail-like and dipolar magnetic fields. Increasing and decreasing dynamic pressure of the flows causes the transition region to shift earthward and tailward, respectively. The leading edge of the train of earthward flow bursts is associated with an earthward propagating dipolarization front, while the leading edge of the train of tailward flow bursts is associated with a tailward propagating plasmoid. The impact of the dipolarization front with the dipole field causes magnetic field variations in the Pi2 range. Major X points can move either earthward or tailward, although tailward motion is more common. They are generally not advected by the ambient flow. Instead, their velocity is better described by local parameters, such that an X point moves in the direction of increasing reconnection electric field strength. Our results indicate that ion kinetics might be sufficient to describe the behavior of plasma sheet bulk ion flows produced by tail reconnection in global near-Earth simulations. Keywords. Magnetospheric physics (magnetospheric configuration and dynamics; plasma sheet) – space plasma physics (numerical simulation studies)

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.

Ion velocity distributions in dipolarization events: Beams in the vicinity of the plasma sheet boundary

AbstractUsing combined MHD/test particle simulations, we further explore characteristic ion velocity distributions in relation to magnetotail reconnection and dipolarization events, focusing on distributions at and near the plasma sheet boundary layer (PSBL). Simulated distributions right at the boundary are characterized by a single earthward beam, as discussed earlier. However, farther inside, the distributions consist of multiple beams parallel and antiparallel to the magnetic field, remarkably similar to recent Magnetospheric Multiscale observations. The simulations provide insight into the mechanisms: the lowest earthward beam results from direct acceleration at an earthward propagating dipolarization front (DF), with a return beam at somewhat higher energy. A higher‐energy earthward beam results from dual acceleration, first near the reconnection site and then at the DF, again with a corresponding return beam resulting from mirroring closer to Earth. Multiple acceleration at the X line or the propagating DF with intermediate bounces may produce even higher‐energy beams. Particles contributing to the lower energy beams are found to originate from the PSBL with thermal source energies, increasing with increasing beam energy. In contrast, the highest‐energy beams consist mostly of particles that have entered the acceleration region via cross‐tail drift with source energies in the suprathermal range.

Ion acceleration and reflection on magnetotail antidipolarization fronts

AbstractAntidipolarization fronts (ADFs), tailward moving structures in the Earth's magnetotail with sharp decreases in their magnetic Bz component, are thought to be mirror images of earthward propagating dipolarization fronts (DFs) generated on the opposite side of the reconnection site. We use ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun) observations and numerical simulations to study the role of ADFs in accelerating and reflecting ambient plasma sheet ions. In both case and statistical observations before ADF arrival, tailward streaming, energy‐dispersed ions are seen first. After about 1 min, the ion fluxes are enhanced significantly with the peak shifted duskward, and then the peak gradually shifts back to the tailward direction until the ADF arrives. All these signatures are reproduced by our simulation model of ion acceleration and reflection on ADFs. We further examine typical ion trajectories before and after ADF reflection, to understand these seemingly complicated ion signatures as well as their similarities with and differences from the DF preceding signatures.

Ion flux dropout observed near dipolarization front

An ion flux dropout near the dipolarization front (DF) at around X GSM = −11 RE in the Earth’s plasma sheet was observed by Time History of Events and Macroscale Interaction during substorms (THEMIS) on March 31, 2009. The ion differential energy fluxes at energies from 450 eV to 150 keV measured by the ESA and SST instruments from THC began to decrease about 2 s before the detection of the DF and reached a local minimum 6 s later. Then, the ion fluxes gradually increased to form a dropout around the DF. The spatial extent of the dropout was about 4,000 km. For energies above 20 keV, the ion fluxes after the dropout are greater than those before it, contrary to the fluxes at energies below 20 keV. The associated ion density variation indicates that the ion flux dropout coincides with the ion density dropout. Taking advantage of multipoint observations, THD, THC, and THE detected the same DF consecutively. Only THC detected an obvious ion flux dropout; THD observed an indistinct one about 2 s before THC; no high-energy (E > 30 keV) ion flux dropout was observed by THE. Our study suggests that the ion flux dropout may evolve with the earthward-propagating DF, and its properties can depend on locations relative to the DF.

Ion dynamics associated with substorm dipolarization fronts

We report where and how ions are accelerated in the proximity of earthward propagating dipolarization fronts (DFs) in the magnetotail during a magnetospheric substorm on February 15, 2008. Two DFs were observed by multiple THEMIS spacecraft in the near-Earth magnetotail (∼−10 Re). We studied the ion dynamics associated with these DFs by comparing observed results with large scale kinetic (LSK) simulation results. The LSK simulation reproduced the sudden ion energy flux enhancement concurrent with the arrival of the DF at the satellite locations. We found that ions can be accelerated to more than 100 keV energy at the DF. These ions were initially non-adiabatically accelerated near magnetic reconnection site and then still non-adiabatically accelerated at the DF structure.

Contrasting electron acceleration processes during two substorms

AbstractAlthough energetic particle injections during substorms have been observed near geosynchronous orbit since the late 1960s, how the injected particles are accelerated during a substorm is still not fully understood. Studies show that injections are associated with earthward propagating dipolarization fronts and suggest that particle acceleration can occur due to a combination of betatron acceleration during earthward convection and acceleration in the vicinity of magnetotail reconnection sites. Since observed energetic particle distributions vary from substorm to substorm, it is important to study acceleration mechanisms as a function of different substorm magnetotail configurations. In this paper, we discuss electron acceleration for two substorm events featuring different magnetotail configurations and particle acceleration processes. The study utilizes in situ observations, global MHD simulations, and large‐scale‐kinetic (LSK) simulations. The two substorms in question occurred on 15 February 2008 and 15 August 2001. The 15 February 2008 event was observed by the THEMIS spacecraft at xGSE ~ −10 RE, while the 15 August 2001 event was observed by Cluster spacecraft at xGSE ~ −20 RE. Earthward propagating dipolarization fronts are found in both the observations and the MHD simulations, which are driven by the solar wind conditions that existed during those events. The two simulations show that very different magnetotail configurations are formed for each event, with contrasting flows, magnetic reconnection configuration, and plasma sheet structure. We find that for the 15 February 2008 event reconnection is localized with a narrow region of high‐speed flow. During this event, the electron velocity distribution functions f(v) are found to be anisotropic with f(v⊥) > f(v||), where || and ⊥ correspond to parallel and perpendicular to magnetic field. In contrast, for the 15 August 2001 event, there is a wide neutral line extending across the tail with low‐speed flows. For this event, the distribution functions are anisotropic with f(v||) > f(v⊥), the opposite sense as the 15 February 2008 event. We are able to relate these differences to the different magnetotail configurations seen during each event. These differences lead to contrasting occurrence rates of nonadiabatic acceleration. The differences between the magnetotail configurations during these two substorms may result from differences in the location of dayside reconnection. On 15 August 2001 the reconnection occurs on the flanks of the magnetopause, while on 15 February 2008 the reconnection occurs nearer the subsolar point.

Wave‐particle interactions during a dipolarization front event

We present Cluster observations of wave‐particle interactions during an earthward propagating dipolarization front (DF) and associated fast plasma bulk flows detected at the central current sheet in Earth's magnetotail. During this period, flux tubes behind the DF frequently contain more energetic or hotter ions than did the preexisting flux tubes ahead of the DF. On the other hand, electrons within the DF flux tubes heat less, or are even colder, than were the preexisting populations and are often accompanied by superposed isolated beams. At the same time, electrostatic emissions are strongly enhanced over a wide range of frequencies (up to several times the electron cyclotron frequency) behind the DFs. This low‐frequency electrostatic wave power is well correlated with ion energization. From linear theory, we find two wave modes: a high‐frequency beam mode and a low‐frequency whistler mode that are associated with the electron beam component. We attribute the generation of whistlers to electron beams that persist for a while before undergoing rapid thermalization. The existence of isolated beam components behind DFs detected during the 4 s Cluster spin period indicates that DFs either provide a continuous source of electron beams or facilitate a physical process that maintains the beams against rapid thermalization. Our analysis suggests that the earthward motion of the DF flux tube, via Fermi acceleration as the magnetic field lines behind the DF shorten, can lead to the persistent electron beams that generate whistler mode waves, which in turn can heat ions. This scenario, by which free energy in electron beams generates waves that then heat ions, accounts for the Cluster observations of different energization behaviors between electrons and ions behind DFs.

Generation of Pi2 pulsations by intermittent earthward propagating dipolarization fronts: An MHD case study

Using a global magnetohydrodynamic (MHD) simulation of the magnetosphere during a disturbed interval on 14 September 2004, we have investigated fluctuations in plasma properties of the magnetotail in the Pi2 range and their relationship to dipolarization fronts (DFs). Results from the MHD simulation indicate that this event is a very active interval with variable convection and disorder in the tail on a range of scales as small as ∼1 RE. DFs are observed in the simulation at the leading edge of fast earthward flows that originate from reconnection regions that form between ∼−15 and −30 RE in the tail. Pi2 period fluctuations are identified in pressure, magnetic field, and velocity components inside −13 REfollowing each burst of DFs in the midnight sector. The fluctuations observed in the pressure appear to be generated by the successive DFs as they approach the interface between stretched tail field lines and dipolar field lines. Fluctuations in the velocity may be the result of interactions between successive DFs and are amplified directly following the passage of the DFs as they propagate earthward. Although the limited azimuthal extent of the pulsations near the plasma sheet, just inside of the braking region, makes it difficult to draw a direct comparison between the ground‐based measurements and the pulsations at −6 RE, the temporal evolution of the simulated DFs and Pi2 pulsations approximately reproduces the timing of the variations observed by satellites and ground‐based instruments. Therefore, we have been able to use the global simulation to track the bursty flows, dipolarization fronts, and associated Pi2 period fluctuations throughout the entire magnetosphere in order to understand the sources of the changes measured in the near‐Earth region.

Rapid acceleration of protons upstream of earthward propagating dipolarization fronts.

[1] Transport and acceleration of ions in the magnetotail largely occurs in the form of discrete impulsive events associated with a steep increase of the tail magnetic field normal to the neutral plane (Bz), which are referred to as dipolarization fronts. The goal of this paper is to investigate how protons initially located upstream of earthward moving fronts are accelerated at their encounter. According to our analytical analysis and simplified two-dimensional test-particle simulations of equatorially mirroring particles, there are two regimes of proton acceleration: trapping and quasi-trapping, which are realized depending on whether the front is preceded by a negative depletion in Bz. We then use three-dimensional test-particle simulations to investigate how these acceleration processes operate in a realistic magnetotail geometry. For this purpose we construct an analytical model of the front which is superimposed onto the ambient field of the magnetotail. According to our numerical simulations, both trapping and quasi-trapping can produce rapid acceleration of protons by more than an order of magnitude. In the case of trapping, the acceleration levels depend on the amount of time particles stay in phase with the front which is controlled by the magnetic field curvature ahead of the front and the front width. Quasi-trapping does not cause particle scattering out of the equatorial plane. Energization levels in this case are limited by the number of encounters particles have with the front before they get magnetized behind it.

Flow bouncing and electron injection observed by Cluster

Characteristics of particles and fields in the flow‐bouncing region are studied based on multipoint observations from Cluster located at 13–15RE downtail during a substorm event around 12:50 UT on 7 September 2007. The Cluster spacecraft were separated by a distance of up to 10,000 km and allowed to determine the mesoscale evolution of the current sheet as well as the development of the dipolarization front. We show that the flow bouncing took place associated with a tailward‐directed j × B force in a disturbed current sheet in addition to an enhanced tailward pressure gradient force. Multiple Earthward propagating dipolarization fronts accompanied by enhanced flux of energetic electrons were observed before the flow bouncing. The sequence of events started with a localized dipolarization front and ended with a large scale (>10RE) dipolarization front accompanied by a major increase in energetic electrons at all spacecraft and immediately followed by flow bouncing. Multiple dipolarization fronts result in the formation of compressed magnetic field with a plasma bulge bounded by thin ion‐scale current layers, a favorable condition for flow bouncing. These observations suggest that to understand the flow bouncing and related acceleration of plasma in the near‐Earth tail, both the large‐scale MHD properties and the transient and small‐scale effect of the plasma interaction with the Earth‐dipole field need to be taken into account.

Adiabatic acceleration of suprathermal electrons associated with dipolarization fronts

Recent observations in the inner magnetotail have shown rapid and significant flux increases (usually an order of magnitude of increase within seconds) of suprathermal electrons (tens of keV to hundreds of keV) associated with earthward moving dipolarization fronts. To explain where and how these suprathermal electrons are produced during substorm intervals, two types of acceleration models have been suggested by previous studies: acceleration that localizes near the reconnection site and acceleration that occurs during earthward transport. We perform an analytical analysis of adiabatic acceleration to show that the slope of source differential fluxes is critical for understanding adiabatic flux enhancements during earthward transport. Observationally, two earthward propagating dipolarization fronts accompanied by energetic electron flux enhancements observed by the THEMIS spacecraft have been analyzed; in each event the properties of dipolarization fronts in the inner magnetosphere (XGSM ≈ −10RE) were well correlated with those further down the tail (XGSM ≈ −15RE or XGSM ≈ −20RE). Coupled with theoretical analysis, this enables us to estimate the relative acceleration that occurred as the electrons propagated earthward between the two spacecraft. During the two events studied, the differential fluxes of supra thermal electrons had steep energy spectra with power law indices of −4 to −6.These spectra were much steeper than those at lower energy, as well as those of the supra thermal electrons observed before the fronts arrived. A compression factor of 1.5 as the electrons propagated earthward induced a flux increase of suprathermal electrons by a factor of 7 to 17. Provided these steep spectra, we demonstrate that adiabatic acceleration from the betatron and Fermi mechanisms simultaneously operating can account for these flux increases. Since both analytical analysis and data from the two events show that adiabatic acceleration during earthward transport does not significantly change the power law indices, the steep spectra were likely to be traced back to their source region, presumably near the reconnection site.

On the nature of precursor flows upstream of advancing dipolarization fronts

[1] Earthward propagating dipolarization fronts, interpreted as thin, vertical current sheets that separate plasmas of different origins in the Earth's magnetotail, are embedded within flow bursts, often near the leading edge of bursty bulk flows. Observations have also shown that bursty bulk flow onset typically precedes dipolarization front arrival by ∼1 min. Ion distribution functions reveal that earthward flows in advance of front arrival are often caused by the appearance of a new ion population atop a preexisting plasma sheet component. Particle simulations suggest that this second population, which contributes most to the plasma velocity, is composed of ions that have been reflected at and accelerated by the approaching front. We propose that in the presence of a finite upstream Bz field, the reflected ions would be confined in a region with a size comparable to the ion thermal gyroradius over the upstream Bz. THEMIS observations confirm that the measured time difference δt between the appearance of earthward plasma flows and the dipolarization front arrival is consistent with the predicted size of the ion accessibility region.

Bursty bulk flows and dipolarization in MHD simulations of magnetotail reconnection

[1] Using three-dimensional MHD simulations of magnetic reconnection in the magnetotail, we investigate the fate of earthward bursty bulk flows (BBFs). The flow bursts are identified as entropy-depleted magnetic flux tubes (“bubbles”) generated by the severance of a plasmoid via magnetic reconnection. The onset of fast reconnection coincides closely with a drastic entropy reduction at the onset of lobe reconnection. The fact that, in the simulation, the Alfvén speed does not change significantly at this time suggests that the destabilization of ballooning/interchange modes is important in driving faster reconnection as well as in providing cross-tail structure. In the initial phase, the BBFs are associated with earthward propagating dipolarization fronts. When the flow is stopped nearer to Earth, the region of dipolarization expands both azimuthally and tailward. Tailward flows are found to be associated with a rebound of the earthward flow and with reversed vortices on the outside of the flow. Earthward and tailward flows are also associated with expansion and contraction of the near plasma sheet. All of these features are consistent with recent satellite observations by Cluster and the Time History of Events and their Macroscopic Interactions during Substorms (THEMIS) mission.

Accelerated ions ahead of earthward propagating dipolarization fronts

We report on the evolving ion distributions associated with the arrival of an earthward propagating dipolarization front in the near‐Earth magnetotail using Time History of Events and Macroscale Interactions during Substorms (THEMIS). Ion distributions exhibit steady duskward anisotropy well before the front arrival, suggesting thin current sheet formation at ∼11 RE, during the growth phase of a moderate geomagnetic substorm. As the dipolarization front moves closer, an additional, earthward streaming ion population appears, resulting in an earthward velocity moment. This population eventually overwhelms the preexisting duskward anisotropy and merges with the earthward convecting bulk flow once the dipolarization front arrives. Test‐particle simulations show that the observed ion evolution is consistent with a picture of ions reflected and accelerated by the approaching front and moving ahead of it.

Auroral signatures of the plasma injection and dipolarization in the inner magnetosphere

Using auroral TV data and particle precipitation data from low‐altitude satellites, we identify the ionospheric signature of magnetotail dipolarizations and substorm injections measured in the near‐Earth near‐equatorial plasma sheet by Time History of Events and Macroscale Interactions during Substorms (THEMIS). Field line mapping exploits a recently developed time‐dependent adaptive model which minimizes the variance to THEMIS in situ magnetotail observations. We present strong evidence that the equatorward edge of the auroral bulge corresponds to the innermost extent of earthward propagating dipolarization fronts in the magnetosphere, whereas individual equatorward moving auroral enhancements correspond to the motion of individual injection fronts reaching at times distances as close to Earth as 5.5 RE. The region of tail dipolarization corresponds to the auroral bulge, a broad spatial region of enhanced but structured auroral emissions, bounded on the poleward side by discrete auroral forms and on the equatorward side by a sharp drop in auroral luminosity and particle precipitation. Particle precipitation within the bulge is enhanced considerably at the energies above 30 keV. Ionospheric protons are isotropic and electrons are anisotropic but with fluctuating fluxes which are below, but on occasion comparable with, trapped levels. The equatorward edge of the bulge, herein termed the “Equatorward edge of Auroral Bulge” propagates during substorm expansion toward lower latitudes, initially fast (corresponding to 100 km/s in space at r ∼ 7 RE) but with decreasing speed after onset. Our adaptive model mapping suggests that equatorial points at near‐geosynchronous altitude can map to ionospheric magnetic latitudes up to 2°–3° off of predictions using standard T96 models. The offsets can be either toward lower latitudes due to field line stretching before auroral breakup or toward higher latitudes after breakup due to the near‐Earth tail dipolarization.