In common treatment of magnetosphere‐ionosphere coupling at high latitudes, the ionosphere is represented by a thin conducting spherical shell, which closes field‐aligned currents generated in the ...magnetosphere. In this approach, the current continuity yields a Poisson equation for the electrostatic potential associated with the ionospheric convection pattern. Solution of the Poisson equation then provides a means of self‐consistently describing magnetospheric and ionospheric plasma convection with a feedback of one on the other. While the high‐latitude ionospheric convection is driven by the solar wind and magnetosphere interaction, at lower latitudes atmospheric neutral winds start to dominate. The question that arises then is whether and how midlaltitude and low‐latitude ionospheric convection affects high‐latitude ionospheric and magnetospheric convection. In global magnetospheric models, ionospheric convection equatorward of the low‐latitude boundary is excluded from the simulation domain. However, the boundary condition applied at that boundary to the electrostatic potential may be used as a proxy of this convection. In this paper, we explore effects that different idealized low‐latitude boundary conditions have on the magnetospheric configuration simulated by the Lyon‐Fedder‐Mobarry global magnetohydrodynamic model. To this end, we perform a number of idealized simulations different only in the low‐latitude ionospheric boundary condition used. We find that the behavior of the system can be influenced rather significantly by the different boundary conditions, which is expressed by changes in the evolution of the polar cap potential, global magnetospheric convection, and plasma pressure distribution in the magnetotail and on the dayside. The differences in the cross‐polar cap potential can reach up to >10%, dependent on the boundary condition used. In the magnetosphere the low‐latitude ionospheric boundary condition affects the strength and location of the plasma outflow from the distant tail x‐line and the subsequent earthward convection. Changes in the plasma pressure distribution on the nightside are accompanied by noticeable differences in the shape of the magnetotail. We confirm that the changes in the magnetospheric and ionospheric configuration are not just temporal deviations of the system from the same average dynamical state by considering 1 h averages of the magnetospheric flow and pressure distribution. These results verify that the simulated system reaches similar but distinctly different dynamical states dependent on the low‐latitude boundary condition applied.
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
We present results of a global, fully three‐dimensional, high‐resolution magnetohydrodynamic (MHD) simulation of the magnetosphere during steady northward interplanetary magnetic field (IMF) ...conditions. We investigate the stability of the magnetospheric boundary with respect to the growth of the Kelvin‐Helmholtz instability (KHI) driven by the velocity shear between the nearly stagnant magnetospheric plasma and the magnetosheath flow past it. We find the magnetospheric boundary to be globally unstable, including the high‐latitude boundary layer (meridional plane), where magnetic tension is not sufficient to stabilize the growth of oscillations. Roughly beyond the terminator, global modes coupled into the surface modes become most apparent, so that the entire body of the magnetosphere is engaged in an oscillatory motion. The wave vector of the surface oscillations has a component perpendicular to the background flow and tangential to the shear layer (in the equatorial plane, kz component of the wave vector), which is consistent with the generation of field‐aligned currents that flow on closed field lines between the inner portion of the boundary layer and the ionosphere. The distribution of wave power in the equatorial plane is consistent with the existence of a double‐vortex sheet, with vortex trains propagating along the inner and outer edges of the boundary layer. The double‐vortex sheet is most apparent in the simulation past the terminator plane but is transient and appears to be unstable and is most likely a consequence of nonlinear development of the velocity shear layer with a finite width. For the simulation with the solar wind velocity of 600 km/s, we find the width of the layer to be Δ≈1 RE at the terminator and the phase speed there to be similar to half of the total velocity drop across the layer (∼440 km/s), which is expected for a shear layer with uniform background density. We calculate the spatial growth rate for the dominant frequency mode in this region (∼4.4 mHz) to be ∼0.19RE−1, which is in excellent agreement with linear theory. For this mode, we find kΔ≈0.9, where k is the wave number, which corresponds to the fastest growing mode predicted by the linear theory. Finally, we find that the plasma compressibility is a key factor in controlling the growth rate of the KHI at the magnetosphere flanks in our simulation.
Key Points
Global 3D high‐resolution simulations of KHI at the magnetospheric boundary.
The low‐latitude boundary layer has the form of a transient double‐vortex sheet.
Growth rate estimated from the simulation is in agreement with linear theory.
Kinetic 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.
Plain Language Summary
We explore new measures of plasma dissipation in rapidly contracting tubes of magnetic flux and plasma on the nightside of the terrestrial magnetosphere. These contracting tubes make the stretched tail‐like magnetic field more dipolar and have sharp profiles of plasma density and magnetic field at the leading edge. Relaxation of the stretched magnetic field releases the energy, which is spent for plasma acceleration and heating. Since collisions are extremely rare, the energy dissipation processes are different for electrons and ions and hence require special quantitative measures. Here we derive such measures from massively parallel three‐dimensional particle‐in‐cell simulations of the tail plasmas and demonstrate that as expected for measures of dissipation, they are positive on average and different for ions and electrons. The new quantitative measures allow us to reveal specific physical processes responsible for energy dissipation.
Key Points
Newly derived kinetic dissipation parameters are largely positive and different for ions and electrons
Ion dissipation is dominated by ion reflection from fronts
Electron dissipation is dominated by the lower‐hybrid drift instability
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
We present full‐particle simulations of 2‐D magnetotail current sheet equilibria with open boundaries and zero driving. The simulations show that spontaneous formation of dipolarization fronts and ...subsequent formation of magnetic islands are possible in equilibria with an accumulation of magnetic flux at the tailward end of a sufficiently thin current sheet. These results confirm recent findings in the linear stability of the ion tearing mode, including the predicted dependence of the tail current sheet stability on the amount of accumulated magnetic flux expressed in terms of the specific destabilization parameter. The initial phase of reconnection onset associated with the front formation represents a process of slippage of magnetic field lines with frozen‐in electrons relative to the ion plasma species. This non‐MHD process characterized by different motions of ion and electron species generates a substantial charge separation electric field normal to the front.
Key Points
Spontaneous reconnection onset is possible in the magnetotail
Onset conditions are consistent with the tearing stability theory
Magnetotail reconnection starts from the formation of a dipolarization front
Much 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.
Key Points
Energetic protons can be trapped at dipolarization fronts, which enables their transport from the tail to the inner magnetosphere and violates the first invariant
Trapping is important for the buildup of ion pressure in the inner magnetosphere
Acceleration of trapped ions is proportional to ion charge and is independent of mass
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
We study the spatiotemporal characteristics of energetic particle losses from the magnetosphere using test‐particle trajectories in electromagnetic fields from a global magnetosphere ...magnetohydrodynamic (MHD) simulation. We use a dynamically evolving distribution of high‐resolution electromagnetic fields from the Lyon‐Fedder‐Mobarry global MHD model and trace large ensembles of 100 keV hydrogen and oxygen ions as well as electrons from a near‐Earth plasma sheet location through their escape from the magnetosphere. In agreement with recent MMS observations, we demonstrate that both ions and electrons have access to and escape throughout the dayside magnetopause, including magnetically drift‐shadowed regions. Also, in agreement with MMS observations, the depth of penetration and persistence of particles in the magnetosheath has a clear mass dependence, heavier particles penetrating further and lingering longer. We demonstrate both magnetic local time and latitude dependence of particles losses as manifested by their crossings of the open‐closed boundary and relate them to the complex field topology. Finally, we establish a significant role of Kelvin‐Helmholtz instability in facilitating particle losses at the magnetopause flanks.
Key Points
Energetic particles have access to and escape from both sides of the dayside magnetosphere
Salient features of losses for different species are in agreement with recent MMS observations
Kelvin‐Helmholtz instability enhances losses, particularly for smaller gyroradius particles
When the interplanetary magnetic field (IMF) is southward, most of the ionospheric potential is generated by merging between the IMF and the magnetospheric field. Typically, the ionospheric potential ...responds linearly to the magnitude of the southward IMF. However, when the IMF magnitude is large, the ionospheric potential saturates and it becomes relatively insensitive to further increases in the IMF magnitude. We present evidence from simulations that under purely southward IMF conditions, the value of the portion of the potential due to reconnection is controlled by the divergence of the magnetosheath flow, which determines the geoeffective length in the solar wind. Typically, the gradient in the plasma pressure controls the magnetosheath flow, so as the southward IMF increases in magnitude, the change in the magnetosheath force balance is negligible, the geoeffective length in the solar wind does not change, and the reconnection potential increases linearly with the magnitude of the IMF. However, when the IMF magnitude increases to the point where J × B becomes the dominant force in the magnetosheath, further increases in IMF magnitude do affect the overall force balance, diverting more flow away from the merging line, decreasing the geoeffective length, and limiting the global merging rate. Thus magnetosheath force balance can be seen as a single organizing factor that regulates the geoeffective length in the solar wind for the entire range of solar wind parameters.
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