Explosive magnetotail activity has long been understood in the context of its auroral manifestations. While global models have been used to interpret and understand many magnetospheric processes, the ...temporal and spatial scales of some auroral forms have been inaccessible to global modeling creating a gulf between observational and theoretical studies of these phenomena. We present here an important step toward bridging this gulf using a newly developed global magnetosphere‐ionosphere model with resolution capturing
≲ 30 km azimuthal scales in the auroral zone. In a global magnetohydrodynamic (MHD) simulation of the growth phase of a synthetic substorm, we find the self‐consistent formation and destabilization of localized magnetic field minima in the near‐Earth magnetotail. We demonstrate that this destabilization is due to ballooning‐interchange instability which drives earthward entropy bubbles with embedded magnetic fronts. Finally, we show that these bubbles create localized field‐aligned current structures that manifest in the ionosphere with properties matching observed auroral beads.
Plain Language Summary
The aurora has long been used as a window onto the magnetosphere. However, auroral observations are inherently limited in trying to reconstruct global magnetospheric dynamics from the “magnetic shadow” they cast on Earth. For this reason modeling has been used in tandem with observations to better contextualize and understand the data. Substorms, the violent reconfiguration of the magnetotail and one of the most dynamic magnetospheric phenomena, have been known to be preceded by the formation of bead‐like structures in the aurora. The processes responsible for auroral beading and their causal versus correlative role with substorm onset have remained an enduring mystery. The vast disparity between the spatial scales of auroral beads and those of the global magnetosphere has greatly complicated the use of modeling in unraveling this mystery. We show here for the first time a demonstration of the self‐consistent formation of a magnetospheric configuration that becomes unstable during the period preceding the substorm onset and that this instability manifests in the ionosphere with similar morphology to auroral beads. The global context of the model shows that the magnetospheric processes responsible for beading are not necessarily causal to onset but a consequence of the slow magnetotail reconfiguration that precedes onset.
Key Points
We present the first global magnetosphere simulation to reveal ballooning‐interchange instability of a narrow Bz minimum in the near‐Earth magnetotail
The instability is prominent during the substorm growth phase and generates earthward entropy bubbles with embedded magnetic fronts
The bubbles drive mesoscale ionospheric field‐aligned currents and auroral structures (beads) with properties matching to those observed
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.
This paper addresses the question of the contribution of azimuthally localized flow channels and magnetic field dipolarizations embedded in them in the global dipolarization of the inner ...magnetosphere during substorms. We employ the high‐resolution Lyon‐Fedder‐Mobarry global magnetosphere magnetohydrodynamic model and simulate an isolated substorm event, which was observed by the geostationary satellites and by the Magnetospheric Multiscale spacecraft. The results of our simulations reveal that plasma sheet flow channels (bursty bulk flows, BBFs) and elementary dipolarizations (dipolarization fronts, DFs) occur in the growth phase of the substorm but are rare and do not penetrate to the geosynchronous orbit. The substorm onset is characterized by an abrupt increase in the occurrence and intensity of BBFs/DFs, which penetrate well earthward of the geosynchronous orbit during the expansion phase. These azimuthally localized structures are solely responsible for the global (in terms of the magnetic local time) dipolarization of the inner magnetosphere toward the end of the substorm expansion. Comparison with the geostationary satellites and Magnetospheric Multiscale data shows that the properties of the BBFs/DFs in the simulation are similar to those observed, which gives credence to the above results. Additionally, the simulation reveals many previously observed signatures of BBFs and DFs, including overshoots and oscillations around their equilibrium position, strong rebounds and vortical tailward flows, and the corresponding plasma sheet expansion and thinning.
Key Points
During substorm expansion all magnetic flux transport into the inner magnetosphere occurs via azimuthally localized earthward flows
Substorm onset is characterized by an abrupt increase in the number of such flows penetrating to the geosynchronous orbit
Properties of simulated bursty bulk flows/dipolarization fronts are similar to those observed including flux tube oscillations and rebounds
Thin Current Sheet Behind the Dipolarization Front Nakamura, R.; Baumjohann, W.; Nakamura, T. K. M. ...
Journal of geophysical research. Space physics,
October 2021, Letnik:
126, Številka:
10
Journal Article
Recenzirano
Odprti dostop
We report a unique conjugate observation of fast flows and associated current sheet disturbances in the near‐Earth magnetotail by MMS (Magnetospheric Multiscale) and Cluster preceding a positive bay ...onset of a small substorm at ∼14:10 UT, September 8, 2018. MMS and Cluster were located both at X ∼ −14 RE. A dipolarization front (DF) of a localized fast flow was detected by Cluster and MMS, separated in the dawn‐dusk direction by ∼4 RE, almost simultaneously. Adiabatic electron acceleration signatures revealed from the comparison of the energy spectra confirm that both spacecraft encounter the same DF. We analyzed the change in the current sheet structure based on multi‐scale multi‐point data analysis. The current sheet thickened during the passage of DF, yet, temporally thinned subsequently associated with another flow enhancement centered more on the dawnward side of the initial flow. MMS and Cluster observed intense perpendicular and parallel current in the off‐equatorial region mainly during this interval of the current sheet thinning. Maximum field‐aligned currents both at MMS and Cluster are directed tailward. Detailed analysis of MMS data showed that the intense field‐aligned currents consisted of multiple small‐scale intense current layers accompanied by enhanced Hall‐currents in the dawn‐dusk flow‐shear region. We suggest that the current sheet thinning is related to the flow bouncing process and/or to the expansion/activation of reconnection. Based on these mesoscale and small‐scale multipoint observations, 3D evolution of the flow and current‐sheet disturbances was inferred preceding the development of a substorm current wedge.
Key Points
Evolution of localized fast flows and dipolarization front is obtained from multi‐scale multi‐point observations in near‐Earth magnetotail
Current sheet thinning accompanied by intense field‐aligned currents is detected following the passage of the dipolarization front
From signatures of adiabatic electron acceleration it is confirmed that the same flow front was detected by the multi‐point measurements
We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) data acquired on 17 March 2008 between 10:22 and 10:32 UT to study the mechanism of transient electron injection ...into the loss cone during oscillatory bursty bulk flow (BBF) braking. During braking, transient regions of piled‐up magnetic fluxes are formed. Perpendicular electron anisotropy observed in these regions (presumably caused by betatron perpendicular electron heating) may be a free‐energy source of coexisting whistler waves. Parallel electrons with energies between 1 and 5 keV disappear inside these regions, and transient auroral forms (both rather discrete arcs and diffuse‐like aurora around the arcs) are observed simultaneously by the ground all‐sky imager at Fort Yukon. We use quasi‐linear theory of electron resonant interaction with whistler waves and also estimate the effectiveness of electron nonlinear capture by strong whistler waves. We suggest that electron injection into the loss cone is caused by: (1) scattering by whistler waves and (2) parallel acceleration of electrons captured by stronger whistler waves.
Key Points
Whistler waves are generated by electron anisotropy at dipolarization fronts.
Electron-whistler interaction provides electron escape into the loss cone.
Whistler waves at dipolarization fronts drive aurora.
The present study is focused on the problem of reconstruction of the magnetic configuration in the magnetic reconnection electron diffusion region (EDR). The problem is addressed in the frame of ...electron magnetohydrodynamics with kept electron inertia term. We introduce the new reconstruction model independent of divergence of the electron pressure tensor and reconnection electric field. The model is tested on the magnetotail reconnection event of July 11, 2017 observed by the Magnetospheric Multiscale (MMS) spacecraft in the course of crossing the very core part of the reconnection region, the internal EDR. This new model demonstrates considerably better accuracy of the longitudinal electron velocity reconstruction due to the lower sensitivity to the configuration deviation from the two‐dimensional time‐independent model adopted in our study. We suggest also a new technique to estimate the guide field, implementing the reconstruction of magnetic potential of the in‐plane magnetic field and relying on symmetric properties of magnetic reconnection.
Plain Language Summary
Magnetic reconnection is a fundamental plasma process responsible for the magnetic field reconfiguration and transforming magnetic energy to kinetic and thermal energy of plasma. In the Earth's magnetosphere, the magnetospheric conditions are monitored by several spacecraft missions. Among them, the NASA Magnetospheric Multiscale (MMS) mission is designed for exploring the process of reconnection. On July 11, 2017 at about 22:34 UT MMS was located in the magnetotail at a very fortunate position, intersecting the reconnection region in its very central part, the so‐called electron diffusion region (EDR).
Since MMS consists of four identical spacecraft, MMS provides an excellent tool for testing analytical models of reconnection. Taking the data of one probe as the boundary condition for the analytical model, one can compare the results of calculations with other probes data. In the present paper we suggest a new model of EDR, and compare it to the existing one using the data of 2017/07/11 event. This comparison has shown that the electron inertia term plays an important role in the EDR physics; the proper handling of this term allows considerable improvement of the EDR reconstruction accuracy.
Key Points
A model for electron diffusion region (EDR) reconstruction in electron magnetohydrodynamics (EMHD) approximation with kept electron inertia is developed
Reconstruction of the out‐of‐plane magnetic field is performed independent of the reconnection electric field and pressure anisotropy
Reconstruction is applied for self‐consistent estimate of the guide field value and local coordinate system orientation
Magnetic flux transfer events (FTEs) are signatures of unsteady magnetic reconnection, often observed at planetary magnetopauses. Their generation mechanism, a key ingredient determining how they ...regulate the transfer of solar wind energy into magnetospheres, is still largely unknown. We report THEMIS spacecraft observations on 2007‐06‐14 of an FTE generated by multiple X‐line reconnection at the dayside magnetopause. The evidence consists of (1) two oppositely‐directed ion jets converging toward the FTE that was slowly moving southward, (2) the cross‐section of the FTE core being elongated along the magnetopause normal, probably squeezed by the oppositely‐directed jets, and (3) bidirectional field‐aligned fluxes of energetic electrons in the magnetosheath, indicating reconnection on both sides of the FTE. The observations agree well with a global magnetohydrodynamic model of the FTE generation under large geomagnetic dipole tilt, which implies the efficiency of magnetic flux transport into the magnetotail being lower for larger dipole tilt.
Using burst mode Magnetospheric Multiscale (MMS) observations in the plasma sheet (PS), we study the dynamics of electron anisotropy and its relation to quasi‐parallel narrowband whistler bursts in ...37 dipolarization fronts (DFs) propagating in the Earth's magnetotail along with fast flows at −25 RE ≤ X ≤ −17 RE. The bursts were observed at the DFs and behind them in the dipolarizing flux bundle (DFB) region with frequencies fpeak ~ (0.1–0.6) fce ( fce is electron gyrofrequency) and durations approximately a few seconds. The majority of the whistler waves were associated with perpendicular electron temperature anisotropy TPER/TPAR > 1, and the value of this anisotropy decreased by the end of the bursts suggesting electron scattering by the waves. We found that the major contribution to the growth rate of whistler waves is made by resonant electrons with energies Wres ~ 1–5 keV and pitch angles αres ~ 40–75° and ~100–135°. In the majority of cases, the largest Wres was observed at the DF and immediately behind it, while in the DFB the Wres decreased. The sources of the majority of whistler bursts were not confined near the neutral plane but could be extended into the PS where the perpendicular anisotropy of the local electron distribution provided the positive growth rate of the whistler waves. We show that the observed whistler waves play a significant role in the dynamics of electron velocity distribution in DFs, leading to energy exchange between various parts of electron population and constraining temperature anisotropy of electron distribution.
Key Points
Electron distribution function is highly variable on time scales of short narrowband quasi‐parallel whistler bursts at and behind DFs
Electrons with energies 1–5 keV and pitch angles ~40–75° and 100–135° make the major contribution to the growth rate of these waves
The source of the wave bursts is spread out in space and not confined near the neutral plane
Pritchett and Coroniti (2011, https://doi.org/10.1029/2011GL047527, 2013, https://doi.org/10.1029/2012JA018143) have predicted that the kinetic ballooning/interchange instability (BICI) can provoke ...reconnection onsets that lead to detached azimuthally thin earthward intrusions (heads) of depleted plasma tubes when
βeq⩽ 100. Such detached BICI heads would be seen as localized earthward‐propagating dipolarization fronts. Using Time History of Events and Macroscale Interactions during Substorms observations in the plasma sheet at XGSM≈−11RE and conjugate All‐Sky Imager and magnetometer networks observations on the ground, we show four examples when prominent dipolarization fronts with moderate earthward flows were observed amidst azimuthally drifting interchange heads and concurrently with the ionospheric current intensifications near Time History of Events and Macroscale Interactions during Substorms footprints and auroral bright spots originating from dimmer azimuthal beads/rays. These events support the idea that some of the BICI heads detach from the region with reversed radial gradient of BZ due to local reconnection. The detached BICI heads propagate earthward‐driving ionospheric pseudo‐breakups.
Plain Language Summary
The Earth's magnetotail periodically accumulates energy in form of the magnetic flux in the tail lobes and dumps the energy as fast earthward and tailward plasma flows, which are produced by magnetic reconnection. Yet there is no consensus on what magnetotail processes may lead to reconnection. Examples of multiprobe space observations are used to reveal the possible process that might be important for azimuthally localized reconnection in the tail that leads to pseudo‐breakups in aurora and local ionospheric current systems. The examples show the appearance of earthward‐propagating reconnection (dipolarization) fronts amidst azimuthally propagating clumps of more dipolar field lines that were produced by an instability which was predicted to lead to localized reconnection by earlier plasma computer simulations. The conjugate ground auroral and magnetic field observations support the reconnection fronts' origin hypothesis.
Key Points
Azimuthally drifting interchange heads in the near‐Earth plasma sheet may detach and propagate earthward
Detached interchange heads are seen as localized dipolarization fronts amidst azimuthally drifting interchange heads
The detachments cause ionospheric pseudo‐breakups with local current system and auroral bright spots originating from azimuthal beads/rays
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