Dipolarization Fronts Research Articles

Auroral and Magnetotail Dynamics During Quiet‐Time STEVE and SAID

AbstractAlthough Strong Thermal Emission Velocity Enhancement (STEVE) and subauroral ion drifts (SAID) are often considered in the context of geomagnetically disturbed times, we found that STEVE and SAID can occur even during quiet times. Quiet‐time STEVE has the same properties as substorm‐time STEVE, including its purple/mauve color and occurrence near the equatorward boundary of the pre‐midnight auroral oval. Quiet‐time STEVE and SAID emerged during a non‐substorm auroral intensification at or near the poleward boundary of the auroral oval followed by a streamer. Quiet‐time STEVE only lasted a few minutes but can reappear multiple times, and its latitude was much higher than substorm‐time STEVE due to the contracted auroral oval. The THEMIS satellites in the plasma sheet detected dipolarization fronts and fast flows associated with the auroral intensification, indicating that the transient energy release in the magnetotail was the source of quiet‐time STEVE and SAID. Particle injection was weaker and electron temperature was lower than the events without quiet‐time STEVE. The plasmapause extended beyond the geosynchronous orbit, and the ring current and tail current were weak. The interplanetary magnetic field (IMF) Bz was close to zero, while the IMF Bx was dominant. We suggest that the small energy release in the quiet magnetosphere can significantly impact the flow and field‐aligned current system.

Temporal Evolution of Electron Accelerations behind the Dipolarization Front in the Earth's Magnetotail

We report the temporal evolution of electron pitch angle distributions behind the dipolarization front (DF) in the Earth's magnetotail with observations of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft. Taking advantage of multipoint observations from the THEMIS mission lined up in space, we study pitch angle distributions of energetic electrons behind the DF during two typical events. Pancake, rolling-pin, and cigar distributions are observed sequentially during the acceleration process. Based on Liouville's theorem, it is revealed that pancake distribution is dominantly formed by betatron acceleration in the early stage, and rolling-pin distribution is generated by both dominant Fermi and weak betatron acceleration in the transition stage, while cigar distribution is formed by Fermi acceleration finally. Our results provide comprehensive in situ observational evidence of the temporal evolution of electron acceleration behind the DF during propagation.

Local Transition of Electron Pitch Angle Distribution within Flux Pileup Region behind Dipolarization Front

Dipolarization fronts (DFs), earthward-propagating magnetic transients with a strong magnetic field, are important regions favorable for energetic electron acceleration in the magnetotail. The DF-driven electron acceleration usually generates coherent pitch angle distributions (PADs) inside flux pileup regions (FPRs), i.e., strong magnetic field regions behind the DFs, such as pancake, butterfly, and cigar distributions, which dominate at different tail regions and often occur separately. Here we present unique observations of electron PAD evolution inside the FPR, showing that electron PAD underwent local transition from cigar distribution, to butterfly distribution, then toward pancake distribution, forming a U-shaped distribution. During the local transition, electron perpendicular flux (relative to the local magnetic field) is anticorrelated with magnetic field strength, contrary to traditional expectation. The unexpected feature of the electron U-shaped distribution is associated with multiple physical processes at different scales, including local expansion of flux tubes and pitch angle variation near the neutral sheet. These atypical observations can advance our current understanding of electron acceleration and transport in the magnetosphere.

Energy Conversion in the Dip Region Preceding Dipolarization Front

AbstractDipolarization fronts (DFs), characterized by sharp increases in the northward magnetic field and usually preceded by magnetic dips, are suggested to play an important role in energy conversion and transport in the magnetotail. It has been documented that strong energy conversion typically develops right at the fronts. Here we present spacecraft observations of electron‐scale energy conversion (EEC) developed inside the dip region ahead of a DF, by using high‐cadence data from the Magnetospheric Multiscale Mission. The EEC, with magnitude comparable to that at the front, is primarily driven by ion current and electron‐scale electric field. The electric field inside the dip is provided by electrostatic waves fed by lower hybrid drift instability, which experiences temporal decaying. Such decaying leads to nonhomogeneity of EEC along the dawn‐dusk direction. These results, uncovering a new channel for DF‐driven energy conversion, can provide important insights into understanding energy transport in the magnetotail.

Extended Energy Conversion and Electron Acceleration Behind Dipolarization Front

AbstractEnergy transfer and transport in the terrestrial magnetotails are primarily driven by dipolarization fronts (DFs) embedded inside plasma jets. The DF‐driven energy transfer has hitherto been believed to occur locally at the fronts. Different from the traditional knowledge, here we present the first observation of persistent energy conversion extended far behind a DF. The persistent energy conversion, which was dominated by energy loads and mainly contributed by electron currents, developed inside a turbulent, decaying flux pileup region (FPR), nearly 10 dDF (DF’s thickness) behind the DF. The energy transfer chain may be initiated by interaction between the ion flow and ambient plasmas and closed by electron dynamics, leading to electron acceleration perpendicular to magnetic field. These results highlight that electron physics in turbulent FPRs plays a crucial role in the energy transport in the planetary magnetospheres.

Conceptual design of the tail research experiment at the Space Plasma Environment Research Facility (SPERF-TREX)

The Space Plasma Environment Research Facility (SPERF) for ground simulation of the space plasma environment is a key component of the Space Environment Simulation Research Infrastructure (SESRI), a major national science and technology infrastructure for fundamental research. It is designed to investigate outstanding issues in the space plasma environment, such as energetic particle acceleration, transport, and interaction with electromagnetic waves, as well as magnetic reconnection processes, in magnetospheric plasmas. The Tail-Research EXperiment (TREX) is part of the SPERF for laboratory studies of space physics relevant to magnetic reconnection, dipolarization and hydromagnetic wave excitation in the magnetotail. SPERF-TREX is designed to carry out three types of experiments: the tail plasmoid for magnetic reconnection, dipolarization front formation, and magnetohydrodynamic waves excited by high-speed plasma jets. In this paper, the scientific goals and three scenarios of SPERF-TREX for typical processes in space plasmas are presented, and experimental plans for SPERF-TREX are also reviewed, together with the plasma sources applied to generate the plasma with the desired parameters and various magnetic configurations.

In Situ Observations of Magnetic Reconnection Caused by the Interactions of Two Dipolarization Fronts

AbstractUsing high‐resolution data from the Magnetospheric Multiscale mission, an electron‐only reconnection current sheet is found between two successive dipolarization fronts (DFs). The electron‐only reconnection occurs between the northward component of the magnetic field of the flux pileup region (FPR) of the first DF (DF1) and the southward component of the magnetic dip of the second DF (DF2). The faster DF2 compresses the FPR of DF1, which constitutes an anti‐parallel topology and reduces the thickness of the current sheet. Further analysis shows that the current sheet is unstable to the electron tearing instability, which may power the onset of the reconnection. Our results suggest that these two DFs may merge into one by the reconnection, which sheds light on the evolution of DFs during their earthward propagation.

The influence of boundary conditions on the distribution of energetic electrons during collisionless magnetic reconnection

We conducted 2-D particle-in-cell simulations to investigate the impact of boundary conditions on the evolution of magnetic reconnection. The results demonstrate that the boundary conditions are crucial to this evolution. Specifically, in the cases of traditional periodic boundary (PB) and fully-opened boundary (OB) conditions, the evolutions are quite similar before the system achieves the fastest reconnection rate. However, differences emerge between the two cases afterward. In the PB case, the reconnection electric field experiences a rapid decline and even becomes negative, indicating a reversal of the reconnection process. In contrast, the system maintains a fast reconnection stage in the OB case. Suprathermal electrons are generated near the separatrix and in the exhaust region of both simulation cases. In the electron density depletion layer and the dipolarization front region, a larger proportion of suprathermal electrons are produced in the OB case. Medium-energy electrons are mainly located in the vicinity of the X-line and downstream of the reconnection site in both cases. However, in the OB case, they can also be generated in the electron holes along the separatrix. Before the reverse reconnection stage, no high-energy electrons are present in the PB case. In contrast, about 20% of the electrons in the thin and elongated electron current layer are high-energy in the OB case.

Observations of Energy Conversion Caused by Magnetic Reconnection at a Dipolarization Front

AbstractDipolarization fronts (DFs) are widely believed to host energy conversion processes. However, which mechanism is responsible for the energy conversion is still obscure. Using data from the Magnetospheric Multiscale mission, a current sheet is observed at a DF. This current sheet is caused by interchange instability bending the edge of the DF. Inside the current sheet, Hall electromagnetic field, super Alfvénic electron jets, demagnetization of ions and electrons, strong energy conversion, and steady ion flow and temperature are observed, indicating an electron‐only reconnection at the DF. The duskward plasma flow, which may be deflected by the DF, compresses the bent edges of the DF. As a result, the width of the current sheet between two adjacent bent edges of the DF reduces, and then reconnection begins. Our observations give direct evidence that magnetic reconnection results in energy conversion at a DF.

Magnetic Hump Associated with Electron Vortex at Dipolarization Front

As the plasma boundary between two distinct plasma populations, dipolarization fronts (DFs) host abundant kinetic-scale substructures that change their normal directions and thus cause their deformation. However, studies on such deformation caused by an electron vortex have been lacking. Here, we present novel observations of a subion-scale magnetic hump (MHu) associated with an oblique electron vortex at a DF through strengthening three components of the magnetic field. A radial electric field in the MHu, showing bipolar variation, is also associated with the electron vortex as it is mainly ascribed to the electron convection term. There is apparent energy conversion ( J→·E→ ∼−0.3 nw m−3) from the particles to the electromagnetic field in the MHu’s leading part, which is accompanied by inflow and outflow of electromagnetic energy (nonzero ∇·S→ ). The other regions of the DF host opposite energy conversion ( J→·E→ > 0). Broadband parallel electrostatic waves are also observed in the MHu. Our study provides insights into the kinetic-scale processes at DFs.

Reconnection Inside a Dipolarization Front of a Diverging Earthward Fast Flow

AbstractWe examine a Dipolarization Front (DF) event with an embedded electron diffusion region (EDR), observed by the Magnetospheric Multiscale (MMS) spacecraft on 08 September 2018 at 14:51:30 UT in the Earth's magnetotail by applying multi‐scale multipoint analysis methods. In order to study the large‐scale context of this DF, we use conjunction observations of the Cluster spacecraft together with MMS. A polynomial magnetic field reconstruction technique is applied to MMS data to characterize the embedded electron current sheet including its velocity and the X‐line exhaust opening angle. Our results show that the MMS and Cluster spacecraft were located in two counter‐rotating vortex flows, and such flows may distort a flux tube in a way that the local magnetic shear angle is increased and localized magnetic reconnection may be triggered. Using multi‐point data from MMS we further show that the local normalized reconnection rate is in the range of R ∼ 0.16 to 0.18. We find a highly asymmetric electron in‐ and outflow structure, consistent with previous simulations on strong guide‐field reconnection events. This study shows that magnetic reconnection may not only take place at large‐scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets, produced as a consequence of the interaction between the fast Earthward flows and the Earth's dipole field.

First Observation of Kinetic Alfvén Waves behind Reconnection Front in Terrestrial Magnetotail

Reconnection fronts, also known as dipolarization fronts (DFs), and kinetic Alfvén waves (KAWs) are two of the widely observed structures and waves in the terrestrial magnetotail, but their correlation remains elusive in previous spacecraft measurements. Using high-resolution Magnetospheric Multiscale data, here we provide the first observational identification of KAWs behind the DF. These low-frequency dispersive KAWs propagate quasi-perpendicular to the magnetic field at phase speeds slower than local Alfvén speed, and are highly correlated with high-frequency whistlers propagating obliquely to the magnetic field. Specifically, whistlers appear at local magnetic field minima (wave troughs of KAWs) and disappear at local magnetic field maxima (wave crests of KAWs), highly consistent with the source region of whistler waves previously reported near equator and dayside magnetopause. Our study also suggests that the KAWs behind DFs could originate from the diffusion region of magnetic reconnection. These findings improve our understanding of the relationship between DFs, KAWs, and whistler waves in terrestrial magnetotail.

Testing adiabatic models of energetic electron acceleration at dipolarization fronts

Betatron acceleration is commonly cited as a primary accelerator of energetic electrons at dipolarization fronts, and many case studies compare observed energetic electrons measurements to a betatron model. In this work, we extend this to a statistical study. We identified 168 dipolarizations with an enhanced flux of energetic electrons at Magnetospheric Multiscale (MMS). We compared the observed flux of energetic electrons above 1 keV to a betatron acceleration model assuming a source population similar to the population in the quiet plasma sheet and found that, on average, the model slightly overestimated the observation, but there was a wide spread of errors. We then tested characteristics such as position, change in and strength of magnetic field, and wave power to determine if any of these characteristics affected the accuracy of the model; the only clear correlations were that the model was less accurate when the initial total magnetic field was smaller and when there was a higher Ey during the dipolarization. Since the betatron model did not explain our observations very well, we repeated with a full adiabatic model that included a Fermi acceleration component as well. We found that the adiabatic model slightly underestimated the observations, but with a smaller error than the betatron model under the same assumptions. Testing the same parameters, we found that the adiabatic model also did not strongly rely on any of the parameters except the initial magnetic field, and the anti-correlation with Ey was no longer present. The fact that neither model was generally applicable means that either adiabatic processes alone are not enough to explain electron acceleration at dipolarization fronts in general, or the common assumption we used, that the source population has the same phase space density as the cold pre-existing population, is not valid.

Magnetosphere–Ionosphere Drivers of Transient‐Large‐Amplitude Geomagnetic Disturbances: Statistical Analysis and Event Study

AbstractWe present a comprehensive statistical analysis of high‐frequency transient‐large‐amplitude (TLA) magnetic perturbation events that occurred at 12 high‐latitude ground magnetometer stations throughout Solar Cycle 24 from 2009 to 2019. TLA signatures are defined as one or more second‐timescale dB/dt interval with magnitude ≥6 nT/s within an hour event window. This study characterizes high‐frequency TLA events based on their spatial and temporal behavior, relation to ring current activity, auroral substorms, and nighttime geomagnetic disturbance (GMD) events. We show that TLA events occur primarily at night, solely in the high‐latitude region above 60° geomagnetic latitude, and commonly within 30 min of substorm onsets. The largest TLA events occurred more often in the declining phase of the solar cycle when ring current activity was lower and solar wind velocity was higher, suggesting association to high‐speed streams caused by coronal holes and subsequent corotating interaction regions reaching Earth. TLA perturbations often occurred preceding or within the most extreme nighttime GMD events that have 5–10 min timescales, but the TLA intervals were often even more localized than the ∼300 km effective scale size of GMDs. We provide evidence that shows TLA‐related GMD events are associated with dipolarization fronts in the magnetotail and fast flows toward Earth and are closely temporally associated with poleward boundary intensifications (PBIs) and auroral streamers. The highly localized behavior and connection to the most extreme GMD events suggests that TLA intervals are a ground manifestation of features within rapid and complex ionospheric structures that can drive geomagnetically induced currents.

Turbulent dipolarization regions in the Earth’s magnetotail: ion fluxes and magnetic field changes

In this work, we consider the dynamics of ion fluxes and magnetic field changes in turbulent regions of magnetotail dipolarizations. The data from the Cluster-II mission (magnetic field measurements from fluxgate magnetometers and energetic charged particle observations from RAPID spectrometers) were used for the analysis. We study individual events and investigate statistically the changes of charged particle fluxes during magnetic field dipolarizations observed during 2001–2015. Received changes in the spectral index indicate that CNO+ ions undergo stronger acceleration during dipolarization than protons and helium ions. Before dipolarization front monotonic growth the ions flux is observed (the maximum of flux is observed at 1–1,5 min after the start of dipolarization) in the range of ∼ 92–374 keV for proton; in the energy range ∼ 138–235 keV for He+ and in the energy range of 414–638 keV for CNO+ ions. Flux increase before arriving dipolarization front may result from the reflection of plasma sheet ions at the dipolarization front and the result of the resonant interactions of ions with low-frequency electromagnetic waves.

Interplay of Three‐Dimensional Instabilities and Magnetic Reconnection in the Explosive Onset of Magnetospheric Substorms

AbstractMagnetospheric substorms are preceded by a slow growth phase of magnetic flux loading and current sheet thinning in the tail. Extensive data sets have provided evidence of the triggering of instabilities at substorm onset, including magnetic reconnection and ballooning instabilities. Using an exact kinetic magnetotail equilibrium we present particle‐in‐cell simulations which capture the explosive nature of substorms through a disruption of the dipolarization front by the ballooning instability. We use self‐consistent particle tracking to determine the nonthermal particle acceleration mechanisms.

Two Classes of Equatorial Magnetotail Dipolarization Fronts Observed by Magnetospheric Multiscale Mission: A Statistical Overview

AbstractWe carried out a statistical study of equatorial dipolarization fronts (DFs) detected by the Magnetospheric Multiscale mission during the full 2017 Earth's magnetotail season. We found that two DF classes are distinguished: class I (74.4%) corresponds to the standard DF properties and energy dissipation and a new class II (25.6%). This new class includes the six DF discussed in Alqeeq et al. (2022, https://doi.org/10.1063/5.0069432) and corresponds to a bump of the magnetic field associated with a minimum in the ion and electron pressures and a reversal of the energy conversion process. The possible origin of this second class is discussed. Both DF classes show that the energy conversion process in the spacecraft frame is driven by the diamagnetic current dominated by the ion pressure gradient. In the fluid frame, it is driven by the electron pressure gradient. In addition, we have shown that the energy conversion processes are not homogeneous at the electron scale mostly due to the variations of the electric fields for both DF classes.

Electron Rolling‐Pin Distributions Preceding Dipolarization Fronts

AbstractElectron rolling‐pin distribution (RPDs), characterized by triple peaks at pitch angles 0°, 90°, and 180°, have recently been discovered in the terrestrial magnetosphere. Since the RPDs' formation is typically attributed to local betatron acceleration, RPDs have been believed to appear primarily inside strong magnetic regions, such as flux pileup regions (FPRs) behind dipolarization fronts (DFs). Different from such expectation, in this study we present unique observations of RPDs made by Cluster spacecraft in the terrestrial magnetotail, showing that RPDs are present inside weak magnetic field regions ahead of the DFs but absent in the strong magnetic field region behind them. The presence of RPD prior to the fronts may be atttributed to the combined effect of global betatron acceleration, global Fermi acceleration, and frontward transport driven by magnetic gradient drift. The atypical features of RPDs are important for fully understanding electron acceleration and transport in the magnetosphere.

Successive Energy Conversion at a Stepwise Dipolarization Front

Dipolarization fronts (DFs), ion-scale magnetic structures characterized by sharp enhancement of northward magnetic field developed within plasma jets, have been suggested to play a crucial role in the energy transfer chain in the terrestrial magnetotail. Here we present the first observation of successive energy conversion driven by multiple current layers at a stepwise DF, using high-cadence measurements from NASA’s Magnetospheric Multiscale mission. The multiple current layers are adjacent and cause gradual variations of particles and electromagnetic fields, leading to an intense, successive increase of energy conversion rates at the DF in the satellite frame, with electromagnetic field energy being transformed into particle energy. The energy conversion is contributed by both ion and electron currents. The motional electric field drives the energy conversion, while the wave electric field fed by lower hybrid drift instability modulates the energy partition. These results provide new insights into understanding energy transfer in the terrestrial magnetotail.

Formation of Electron Butterfly Distribution by a Contracting Dipolarization Front

AbstractThe electron butterfly distribution, characterized by pitch angles (PA) primarily at 45° and 135°, was rarely observed in Earth's magnetotail. Here using the high‐resolution measurements from Magnetospheric Multiscale mission, we present the observation of electron butterfly distribution in a contracting dipolarization front (DF), and propose a new physical mechanism to explain its formation. Specifically, we discover that the electron butterfly distribution only exhibited in the locally contracted DF and was observed above 1.7 keV. We infer that local contraction of the DF transformed its configuration from a magnetic bottle to an hourglass‐shaped magnetic structure, and the butterfly distribution was formed by the magnetic mirror effect of this magnetic hourglass. Additionally, the theoretically estimated loss cone of the magnetic hourglass fits well with the observations of electrons, validating our inference about the formation mechanism. These findings can improve our understanding of electron dynamics in Earth's magnetosphere.