Onset of reconnection in the tail requires the current sheet thickness to be of the order of the ion thermal gyroradius or smaller. However, existing isotropic plasma models cannot explain the ...formation of such thin sheets at distances where the X‐lines are typically observed. Here we reproduce such thin and long sheets in particle‐in‐cell simulations using a new model of their equilibria with weakly anisotropic ion species assuming quasi‐adiabatic ion dynamics, which substantially modifies the current density. It is found that anisotropy/agyrotropy contributions to the force balance in such equilibria are comparable to the pressure gradient in spite of weak ion anisotropy. New equilibria whose current distributions are substantially overstretched compared to the magnetic field lines are found to be stable in spite of the fact that they are substantially longer than isotropic sheets with similar thickness.
Plain Language Summary
Ion scale current sheets forming sufficiently far from Earth are necessary to explain its stretched magnetic field reconfiguration on the night side. However, isotropic plasmas form magnetic fields that inflate with distance from Earth and cannot reproduce the observed stretched geometry. We present kinetic simulations of current sheets that inflate more gradually with distance due to slight field‐aligned anisotropy of the ion species. Their formation is provided by a special population of suprathermal ions with figure‐of‐eight orbits. We find that the resulting current sheets are stable over a long time scale and have a thickness comparable to the size of these orbits.
Key Points
Two‐dimensional ion‐scale current sheets stretched way beyond the isotropic limit are reproduced in particle‐in‐cell simulations
Weak ion anisotropy and agyrotropy substantially modify the current density and the isotropic force balance
Ion‐scale current sheets are stable in spite of the fact that they are longer compared to isotropic sheets with similar thickness
This work builds on and extends our previous effort (Tsyganenko et al., 2003) to develop a dynamical model of the storm‐time geomagnetic field in the inner magnetosphere, using space magnetometer ...data taken during 37 major events in 1996–2000 and concurrent observations of the solar wind and interplanetary magnetic field (IMF). The essence of the approach is to derive from the data the temporal variation of all major current systems contributing to the distant geomagnetic field during the entire storm cycle, using a simple model of their growth and decay. Each principal source of the external magnetic field (magnetopause, cross‐tail current sheet, axisymmetric and partial ring currents, and Birkeland current systems) is driven by a separate variable, calculated as a time integral of a combination of geoeffective parameters NλVβBsγ, where N, V, and Bs are the solar wind density, speed, and the magnitude of the southward component of the IMF, respectively. In this approach we assume that each source has its individual relaxation timescale and residual quiet‐time strength, and its partial contribution to the total field depends on the entire history of the external driving of the magnetosphere during a storm. In addition, the magnitudes of the principal field sources were assumed to saturate during extremely large storms with abnormally strong external driving. All the parameters of the model field sources, including their magnitudes, geometrical characteristics, solar wind/IMF driving functions, decay timescales, and saturation thresholds, were treated as free variables, and their values were derived from the data. As an independent consistency test, we calculated the expected Dst variation on the basis of the model output at Earth's surface and compared it with the actual observed Dst. A good agreement (cumulative correlation coefficient R = 0.92) was found, in spite of the fact that ∼90% of the spacecraft data used in the fitting were taken at synchronous orbit and beyond, while only 3.7% of those data came from distances 2.5 ≤ R ≤ 4 RE. The obtained results demonstrate the possibility to develop a truly dynamical model of the magnetic field, based on magnetospheric and interplanetary data and allowing one to reproduce and forecast the entire process of a geomagnetic storm, as it unfolds in time and space.
Dipolarization fronts (DFs), characterized by a strong and steep increase of the tail magnetic field component Bz normal to the neutral plane and preceded by a much less negative dip of Bz, are ...reported in many observations of bursty bulk flows and substorm activations throughout the whole Earth's magnetotail. It is shown that similar structures appear in full‐particle simulations with open boundaries in a transient regime before the steady reconnection in the original Harris current sheet driven out of the equilibrium by the initial X‐line perturbation is established. Being secondary reconnection structures propagating with the Alfvén speed, DFs are different from the magnetic field pileup regions reported in earlier simulations with closed boundaries. They also differ from the secondary plasmoids with bipolar Bz changes reported in earlier fluid simulations and particle simulations with open boundaries. In spite of their transient nature, DFs are found to form when the force balance is already restored in the system, which justifies their interpretation as a nonlinear stage of the tearing instability developing in two magnetotail‐like structures on the left and on the right of the initial central X‐line. Both electrons and ions are magnetized at the front of the dipolarization wave. In contrast, in its trail, ions are unmagnetized and move slower compared to the E × B drift, whereas electrons either follow that drift being completely magnetized or move faster, forming super‐Alfvénic jets. In spite of the different motions of electrons and ions, the growth of the front is not accompanied by the corresponding growth of the electrostatic field and the energy dissipation in fronts is dominated by ions.
Statistical and case studies, as well as data‐mining reconstructions suggest that the magnetotail current in the substorm growth phase has a multiscale structure with a thin ion‐scale current sheet ...embedded into a much thicker sheet. This multiscale structure may be critically important for the tail stability and onset conditions for magnetospheric substorms. The observed thin current sheets are found to be too long to be explained by the models with isotropic plasmas. At the same time, plasma observations reveal only weak field‐aligned anisotropy of the ion species, whereas the anisotropic electron contribution is insufficient to explain the force balance discrepancy. Here we elaborate a self‐consistent equilibrium theory of multiscale current sheets, which differs from conventional isotropic models by weak ion anisotropy outside the sheet and agyrotropy caused by quasi‐adiabatic ion orbits inside the sheet. It is shown that, in spite of weak anisotropy, the current density perturbation may be quite strong and localized on the scale of the figure‐of‐eight ion orbits. The magnetic field, current and plasma density in the limit of weak field‐aligned ion anisotropy and strong current sheet embedding, when the ion scale thin current sheet is nested in a much thicker Harris‐like current sheet, are investigated and presented in an analytical form making it possible to describe the multiscale equilibrium in sharply stretched 2D magnetic field configurations and to use it in kinetic simulations and stability analysis.
Plain Language Summary
Conventional kinetic equilibria with isotropic pressures for ions and electrons aimed to describe the current sheet in Earth's magnetotail cannot reproduce its multiscale structure with the proton gyroradius‐scale current sheet being embedded into a much thicker sheet. They cannot explain either the formation of such thin current sheets sufficiently far from Earth. The embedding effect can be reproduced in case of anisotropic and agyrotropic plasmas because orbits of weakly magnetized ions near the current sheet deviate from the Larmor circle and become more like a figure of eight. However, the corresponding multiscale current sheet models have been studied so far for substantial and strong plasma anisotropy, while observations suggest that the tail plasmas are weakly anisotropic. Here we perform an analysis of a weakly anisotropic current sheet model, which transforms in the isotropic limit into a classical Harris sheet model, and show that the key observed embedding features can be reproduced.
Key Points
Kinetic equilibria of ion‐scale current sheets embedded into a thicker weakly anisotropic Harris‐like current sheet are investigated
The current density increase due to quasi‐adiabatic ion motions may be substantial in spite of weak plasma anisotropy
2D thin current sheets have aspect ratios consistent with observations and controlled by the embedding strength
The onset of reconnection in 2‐D current sheet equilibria that include an X line separating tail‐like regions with magnetized electrons is simulated with a full‐particle code. The onset is driven by ...a finite convection electric field applied outside the current sheet. In the case of tearing stable tails with no accumulated magnetic flux, the convection electric field penetrates the sheet near the X line. In contrast, in multiscale equilibria where the X line is framed by local areas of enhanced flux, the electric field avoids the X line, directly penetrates the areas of increased flux, and ejects them downstream. The ejecta form dipolarization fronts (DFs), sharp magnetic pileups with a thickness on the order of the ion inertial length, much smaller than the mesoscales of the initial flux increase regions. The DFs move with the reconnection outflows in the direction opposite the magnetic field stretching, while behind them new X lines, distinct from the original, form. Simulations with a reduced driving field suggest that DF formation shares properties with the ion tearing instability, which is consistent with its potential destabilization in multiscale equilibria. Weak driving of equilibria with tearing stable tails first forms flux accumulation regions, which then rapidly transform into DFs, making 2‐D equilibria inherently metastable. The results are compared with observations of DFs, the statistical visualization of Earth's magnetotail during substorm onset, and the bubble‐blob pair formation model.
Key Points
Formation of dipolarization fronts is a key feature of reconnection onset
Reconnection onset in 2‐D equilbria is consistent with tearing stability theory
The 2‐D reconnecting current sheet behaves as a metastable system
The sufficient stability criterion of the collisionless ion tearing mode in the magnetotail current sheet, which was first obtained by Lembege and Pellat in 1982, is considered. For many conventional ...2D current sheet equilibria, this criterion is satisfied within the WKB approximation, which is commonly interpreted as stability of those equilibria with respect to tearing. However, this is not necessarily the case for equilibria with more than two characteristic spatial scales. An example for substantial tearing destabilization of an equilibrium with accumulation of the magnetic flux at the tailward end of a thin current sheet is presented. Similar equilibria are reported in Geotail and THEMIS observations prior to onsets of magnetospheric substorms and dipolarization fronts associated with bursty bulk flows.
Magnetospheric substorms represent key explosive processes in the interaction of the Earth's magnetosphere with the solar wind, and their understanding and modeling are critical for space weather ...forecasting. During substorms, the magnetic field on the nightside is first stretched in the antisunward direction and then it rapidly contracts earthward bringing hot plasmas from the distant space regions into the inner magnetosphere, where they contribute to geomagnetic storms and Joule dissipation in the polar ionosphere, causing impressive splashes of aurora. Here we show for the first time that mining millions of spaceborne magnetometer data records from multiple missions allows one to reconstruct the global 3‐D picture of these stretching and dipolarization processes. Stretching results in the formation of a thin (less than the Earth's radius) and strong current sheet, which is diverted into the ionosphere during dipolarization. In the meantime, the dipolarization signal propagates further into the inner magnetosphere resulting in the accumulation of a longer lived current there, giving rise to a protogeomagnetic storm. The global 3‐D structure of the corresponding substorm currents including the substorm current wedge is reconstructed from data.
Plain Language Summary
Using several millions of historical magnetometer records and data mining techniques, we form virtual spacecraft constellations of tens of thousands of spacecraft to reconstruct the global shape of the terrestrial magnetosphere at the moments of its most dramatic reconfigurations responsible for major space weather disturbances.
Key Points
Substorm tail current sheet thinning and dipolarization are reproduced using novel data mining technique
Global 3‐D structure of substorm currents including the substorm current wedge is reconstructed from data
Substorms contribute to an accumulation of a longer‐lived thick current in the innermost part of the magnetosphere
Dipolarization fronts (DFs) are frequently detected in the Earth's magnetotail from XGSM = −30 RE to XGSM = −7 RE. How these DFs are formed is still poorly understood. Three possible mechanisms have ...been suggested in previous simulations: (1) jet braking, (2) transient reconnection, and (3) spontaneous formation. Among these three mechanisms, the first has been verified by using spacecraft observation, while the second and third have not. In this study, we show Cluster observation of DFs inside reconnection diffusion region. This observation provides in situ evidence of the second mechanism: Transient reconnection can produce DFs. We suggest that the DFs detected in the near‐Earth region (XGSM > −10 RE) are primarily attributed to jet braking, while the DFs detected in the mid‐ or far‐tail region (XGSM < −15 RE) are primarily attributed to transient reconnection or spontaneous formation. In the jet‐braking mechanism, the high‐speed flow “pushes” the preexisting plasmas to produce the DF so that there is causality between high‐speed flow and DF. In the transient‐reconnection mechanism, there is no causality between high‐speed flow and DF, because the frozen‐in condition is violated.
Key Points
DFs are observed inside reconnection diffusion region
Three formation mechanisms of DF are compared
Causality between flow and DF is discussed
We report THEMIS observations of a dipolarization front, a sharp, large‐amplitude increase in the Z‐component of the magnetic field. The front was detected in the central plasma sheet sequentially at ...X = −20.1 RE (THEMIS P1 probe), at X = −16.7 RE (P2), and at X = −11.0 RE (P3/P4 pair), suggesting its earthward propagation as a coherent structure over a distance more than 10 RE at a velocity of 300 km/s. The front thickness was found to be as small as the ion inertial length. Comparison with simulations allows us to interpret the front as the leading edge of a plasma fast flow formed by a burst of magnetic reconnection in the midtail.
Substorm‐type evolution of the Earth's magnetosphere is investigated by mining more than two decades (1995–2017) of spaceborne magnetometer data from multiple missions including the first two years ...(2016‐2017) of the Magnetospheric MultiScale mission. This investigation reveals interesting features of plasma evolution distinct from ideal magnetohydrodynamics (MHD) behavior: X‐lines, thin current sheets, and regions with the tailward gradient of the equatorial magnetic field Bz. X‐lines are found to form mainly beyond 20 RE, but for strong driving, with the solar wind electric field exceeding ∼5mV/m, they may come closer. For substorms with weaker driving, X‐lines may be preceded by redistribution of the magnetic flux in the tailward Bz gradient regions, similar to the magnetic flux release instability discovered earlier in PIC and MHD simulations as a precursor mechanism of the reconnection onset. Current sheets in the growth phase may be as thin as 0.2 RE, comparable to the thermal ions gyroradius, and at the same time, as long as 15 RE. Such an aspect ratio is inconsistent with the isotropic force balance for observed magnetic field configurations. These findings can help resolve kinetic mechanisms of substorm dipolarizations and adjust kinetic generalizations of global MHD models of the magnetosphere. They can also guide and complement microscale analysis of nonideal effects.
Plain Language Summary
The sun emits a steam of charged particles called the solar wind that flows past the Earth interacting with the planet's dipole magnetic field. This stretches the dipolar magnetic field away from the sun on the nightside of the planet storing energy in the stretched field. Once every few hours, this stretched configuration suddenly becomes more dipolar bringing particles and magnetic flux closer to the planet and powering aurora in the polar regions. During these processes, termed substorms, the gas of charged particles, protons, and electrons trapped by the dipole and known as plasma, behaves largely as a perfectly conducting fluid. However, only deviations from this ideal conducting plasma behavior can explain the substorm mechanisms. We mine two decades of spacecraft magnetometer data from multiple missions to form swarms of thousands of synthetic probes. They help reveal effects of nonideal plasma evolution during substorms, which cannot be captured by direct in situ observations because of their extreme paucity.
Key Points
X‐lines, including the 11 July 2017 reconnection event, are reconstructed at and beyond 20 RE, but for strong driving they can come closer
Current sheets in the growth phase may be as thin as 0.2 RE, and at the same time, as long as 15 RE, violating isotropic force balance
Bz humps form in the growth phase, and their reconfiguration may precede X‐line formation and substorm onset
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