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
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
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
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
Extended periods of northward interplanetary magnetic field (IMF) lead to the formation of a cold, dense plasma sheet due to the entry of solar wind plasma into the magnetosphere. Identifying the ...paths that the solar wind takes to enter the magnetosphere, and their relative importance has remained elusive. Any theoretical model of entry must satisfy observational constraints, such as the overall entry rate and the dawn‐dusk asymmetry observed in the cold, dense plasma sheet. We model, using a combination of global magnetohydrodynamic and test particle simulations, solar wind ion entry into the magnetosphere during northward IMF and compare entry facilitated by the Kelvin‐Helmholtz instability to cusp reconnection. For Kelvin‐Helmholtz entry we reproduce transport rates inferred from observation and kinetic modeling and find that intravortex reconnection creates buoyant flux tubes, which provides, through interchange instability, a mechanism of filling the central plasma sheet with cold magnetosheath plasma. For cusp entry we show that an intrinsic dawn‐dusk asymmetry is created during entry that is the result of alignment of the westward ion drift with the dawnward electric field typically observed during northward IMF. We show that both entry mechanisms provide comparable mass but affect entering plasma differently. The flank‐entering plasma is cold and dawn‐dusk symmetric, whereas the cusp‐entering plasma is accelerated and preferentially deflected toward dawn. The combined effect of these entry mechanisms results in a plasma sheet population that exhibits dawn‐dusk asymmetry in the manner that is seen in nature: a two‐component (hot and cold) dusk flank and hotter, broadly peaked dawn population.
Key Points
Global MHD model with kinetic test particles shows comparable solar wind entry from KH vortices and cusp reconnection under northward IMF
Solar wind entry exhibits intrinsic dawn‐dusk asymmetry due to the preferential energization and dawnward deflection of cusp‐entering ions
Combined flank‐ and cusp‐entering solar wind ions recreates observed two‐component (hot and cold) dusk and broad‐peaked dawn population
The Kelvin‐Helmholtz instability at the magnetospheric boundary plays a crucial role in solar wind‐magnetosphere‐ionosphere coupling, particle entry, and energization. The full extent of its impact ...has remained an open question due, in part, to global models without sufficient resolution to capture waves at higher latitudes. Using global magnetohydrodynamic simulations, we investigate an event when the Magnetospheric Multiscale (MMS) mission observed periodic low‐frequency waves at the dawn‐flank, high‐latitude boundary layer. We show the layer to be unstable, even though the slow solar wind with the draped interplanetary magnetic field is seemingly unfavorable for wave generation. The simulated velocity shear at the boundary is thin (∼0.65RE) and requires commensurately high spatial resolution. These results, together with MMS observations, confirm for the first time in fully three‐dimensional global geometry that KH waves can grow in this region and thus can be an important process for energetic particle acceleration, dynamics, and transport.
Plain Language Summary
The boundary separating magnetospheric plasma and the solar wind can become unstable due to the Kelvin‐Helmholtz instability, forming waves that can facilitate mass, momentum, and energy transfer into the magnetosphere. How prevalent Kelvin‐Helmholtz waves are along the magnetopause is therefore a fundamental question to understanding the magnetospheric response to the solar wind. Determining when and where these waves occur on the boundary has remained a challenge. The lower latitudes have been extensively studied while the Cluster mission has provided observations of KH at high‐latitudes. No global models have been capable of resolving the high‐latitude boundary layer, preventing numerical studies of waves within this region. We present a simulation that captures Kelvin‐Helmholtz waves at the high‐latitude boundary for the first time. The instability formed despite the magnetosphere being immersed in pristine slow wind, which reduced the velocity drop across the shear layer at the equator. The simulated period took advantage of an opportunity when the Magnetospheric Multiscale mission was located at the high‐latitude boundary and observed boundary oscillations. Together with the observations, we confirm that Kelvin‐Helmholtz waves can grow at high‐latitudes and thus be able to contribute to particle entry and energization in this region.
Key Points
Global magnetohydrodynamic simulation of the magnetosphere captures unstable Kelvin‐Helmholtz waves at the high‐latitude boundary layer for the first time
The growth of the surface waves occurs despite the stabilizing slow solar wind and draped magnetic field near the high‐latitude cusp
The fastest growing wave mode resolved by the simulation is consistent with Magnetospheric Multiscale mission observations of the same event
Earth's magnetotail plays a critical role in the dynamics of the magnetosphere, particularly during intervals of geomagnetic activity. To improve our understanding of the ion dynamics in this region, ...energetic neutral atom (ENA) imaging can provide global measurements to place in situ measurements in context and validate simulations. The NASA Two Wide‐angle Imaging Neutral‐atom Spectrometers mission provided near‐continuous observations using ENA imagers. ENA data can be used to calculate maps of equatorial ion temperatures that often show observations of regions of enhanced temperatures associated with phenomena in the magnetotail such as magnetic reconnection and narrow flow channels. We present an algorithm that can be used to search through a collection of these maps to identify intervals with such enhancements for further study. The algorithm results are validated against two sets of related phenomena: (a) a database of dipolarizing flux bundle (DFB) measurements from THEMIS and (b) a list of substorm onsets from SuperMAG. We demonstrate that the algorithm is very good at identifying intervals when there are DFB measurements or substorm onsets as long as there sufficient ENA data. We discuss some potential scientific studies that can result from use of the algorithm. We also show a preliminary application of the algorithm to simulation output to demonstrate the usefulness for other datasets, facilitate comparative studies, and introduce a new method for model validation.
Key Points
An algorithm has been developed to identify regions of increased ion temperatures in maps calculated from Two Wide‐angle Imaging Neutral‐atom Spectrometers energetic neutral atom data
The algorithm is validated against a list of dipolarizing flux bundle measurements and a list of substorm onsets
The algorithm will be useful for future case and statistical studies of mesoscale phenomena as well as model validation
The multifluid Lyon‐Fedder‐Mobarry (MFLFM) global magnetosphere model is used to study the interactions between solar wind and rapidly rotating, internally driven Jupiter magnetosphere. The MFLFM ...model is the first global simulation of Jupiter magnetosphere that captures the Kelvin‐Helmholtz instability (KHI) in the critically important subsolar region. Observations indicate that Kelvin‐Helmholtz vortices are found predominantly in the dusk sector. Our simulations explain that this distribution is driven by the growth of KHI modes in the prenoon and subsolar region (e.g., >10 local time) that are advected by magnetospheric flows to the dusk sector. The period of density fluctuations at the dusk terminator flank (18 magnetic local time, MLT) is roughly 1.4 h compared with 7.2 h at the dawn flank (6 MLT). Although the simulations are only performed using parameters of the Jupiter's magnetosphere, the results may also have implications for solar wind‐magnetosphere interactions at other corotation‐dominated systems such as Saturn. For instance, the simulated average azimuthal speed of magnetosheath flows exhibit significant dawn‐dusk asymmetry, consistent with recent observations at Saturn. The results are particularly relevant for the ongoing Juno mission and the analysis of dawnside magnetopause boundary crossings for other planetary missions.
Key Points
KHI in the subsolar region is critically important for boundary layer dynamics in Jupiter's magnetosphere
Simulations show that duskward moving KH waves originate at around 10 MLT, and stationary KH waves form on the dawnside around 08‐09 MLT
Duskside density fluctuations associated with KHI exhibit much longer periodicity compared to those on the dawnside
An accurate description of the state of the ionosphere is crucial for understanding the physics of Earth's coupling to space, including many potentially hazardous space weather phenomena. To support ...this effort, ground networks of magnetometer stations, optical instruments, and radars have been deployed. However, the spatial coverage of such networks is naturally restricted by the distribution of land mass and access to necessary infrastructure. We present a new technique for local mapping of polar ionospheric electrodynamics, for use in regions with high data density, such as Fennoscandia and North America. The technique is based on spherical elementary current systems (SECS), which were originally developed to map ionospheric currents. We expand their use by linking magnetic field perturbations in space and on ground, convection measurements from space and ground, and conductance measurements, via the ionospheric Ohm's law. The result is a technique that is similar to the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique, but tailored for regional analyses of arbitrary spatial extent and resolution. We demonstrate our technique on synthetic data, and with real data from three different regions. We also discuss limitations of the technique and potential areas for improvement.
Plain Language Summary
The ionosphere, where a small but significant fraction of the atmosphere is ionized, forms the edge of space. At only 100‐km altitude, it is the region in space which is by far best monitored by human instruments. Space scientists routinely use measurements that inform about specific aspects of the dynamics in the ionosphere, but not the whole picture. For example, magnetometers on ground measure one part of the electric current system while magnetometers on satellites measure another part. Radars measure the flow of charged particles in the ionosphere, while optical images and particle measurements can be used to estimate electric conductivity. In this paper, we present a technique that combines all these different types of measurements to give a complete picture of what takes place in the ionosphere. The technique is tailored for use in regions where the data density is high, and the spatial resolution and extent of the analysis region are flexible.
Key Points
We present a technique to use disparate data types to produce local maps of polar ionospheric electrodynamics
ΔB and convection measurements are related via ionospheric Ohm's law and spherical elementary current systems
We demonstrate the technique on real and synthetic data, and discuss limitations and future development
A characteristic feature of the main phase of geomagnetic storms is the dawn‐dusk asymmetric depression of low‐ and mid‐latitude ground magnetic fields, with largest depression in the dusk sector. ...Recent work has shown, using data taken from hundreds of storms, that this dawn‐dusk asymmetry is strongly correlated with enhancements of the dawnside westward electrojet and this has been interpreted as a “dawnside current wedge” (DCW). Its ubiquity suggests it is an important aspect of stormtime magnetosphere‐ionosphere (MI) coupling. In this work we simulate a moderate geomagnetic storm to investigate the mechanisms that give rise to the formation of the DCW. Using synthetic SuperMAG indices we show that the model reproduces the observed phenomenology of the DCW, namely the correlation between asymmetry in the low‐latitude ground perturbation and the dawnside high‐latitude ground perturbation. We further show that these periods are characterized by the penetration of mesoscale bursty bulk flows (BBFs) into the dawnside inner magnetosphere. In the context of this event we find that the development of the asymmetric ring current, which inflates the dusk‐side magnetotail, leads to asymmetric reconnection and dawnward‐biased flow bursts. This results in an eastward expansion and multiscale enhancement of the dawnside electrojet. The electrojet enhancement extends across the dawn quadrant with localized enhancements associated with the wedgelet current systems of the penetrating BBFs. Finally, we connect this work with recent studies that have shown rapid, localized ground variability on the dawnside which can lead to hazardous geomagnetically induced currents.
Plain Language Summary
During geomagnetic storms, electric currents in space can have a dramatic effect on the magnetic field on the ground, causing so‐called geomagnetic disturbances (GMDs). Storm‐time GMDs exhibit a lopsided asymmetry: dusk‐biased near the equator and dawn‐biased at high latitudes where aurora usually occur. This asymmetry has been interpreted as a giant wedge‐like current system, a dawnside current wedge (DCW). Using a high‐resolution supercomputer model, we successfully reproduced the DCW and showed that it occurred during a period of intense, localized flow bursts, akin to bubbles, on the nightside of near‐Earth space. The bubbles' buoyancy propels them from the nightside inwards toward dawn, driving intense currents into the Earth's atmosphere. Our simulations suggest that the causal agent of these dawnside bubbles is magnetic reconnection, typically symmetric but skewed dawnward due to asymmetry in the ring current, a crescent‐shaped population of energetic ions in space which intensifies during geomagnetic storms. Understanding the cause of stormtime GMD asymmetry is not only important to characterize how electric currents bind the magnetosphere and upper atmosphere, but also to mitigate space weather hazards, as intense GMDs can disrupt and damage power systems on Earth.
Key Points
Global model reproduces correlation between ring current asymmetry and dawnside electrojet inferred from hundreds of geomagnetic storms
Analysis of the model reveals a dawnside current wedge mediated by mesoscale flow bursts and driven by an asymmetric substorm‐like process
Model reveals multiscale enhancement of dawnside electrojet with space weather implications due to rapid, localized ground variability
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