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Ground magnetic field variations have been used to investigate ionospheric dynamics for more than a century. They are usually explained in terms of an electric circuit in the ionosphere driven by an electric field, but this is insufficient to explain how magnetic field disturbances are dynamically established. Here we explain and simulate how the ionosphere dynamically responds to magnetospheric forcing and how it leads to magnetic field deformation via Faraday's law. Our approach underscores the causal relationships, treating the magnetic field and velocity as primary variables (the B, vparadigm), whereas the electric field and current are derived, in contrast to the E, jparadigm commonly used in ionospheric physics. The simulation approach presented here could be used as an alternative to existing circuit‐based numerical models of magnetosphere‐ionosphere coupling. Ground magnetic field variations have been used to investigate ionospheric dynamics for more than a century. They are usually explained in terms of an electric circuit in the ionosphere driven by an electric field; similar to how we would explain the magnetic field of a current‐carrying wire. However, this approach oversimplifies the complex reality in space plasmas. Here, the relationship between currents and magnetic fields is reversed: currents should be understood as a consequence of the magnetic field, and not the other way around. In this paper we explain and simulate how the ionosphere dynamically responds to forcing from above, and how it leads to magnetic field deformation that corresponds to an electrical current. We present a model and simulation of how the magnetic field in the ionosphere dynamically changes in response to external forcingOur model emphasizes the causal relationships, treating Band vas primary instead of Eand j, as is commonly done in ionospheric physicsSimulation results highlighting the self‐consistent ionospheric response in the B, vparadigm are presented We present a model and simulation of how the magnetic field in the ionosphere dynamically changes in response to external forcing Our model emphasizes the causal relationships, treating Band vas primary instead of Eand j, as is commonly done in ionospheric physics Simulation results highlighting the self‐consistent ionospheric response in the B, vparadigm are presented

Acquired cancer therapy resistance evolves under selection pressure of immune surveillance and favors mechanisms that promote drug resistance through cell survival and immune evasion. AXL receptor tyrosine kinase is a mediator of cancer cell phenotypic plasticity and suppression of tumor immunity, and AXL expression is associated with drug resistance and diminished long-term survival in a wide range of malignancies, including NSCLC.

In this study we use high‐quality convection data from the Electron Drift Instrument on board Cluster to investigate how near‐Earth tail activity affects the average convection pattern in the magnetotail lobes when the interplanetary magnetic field has a dominating east‐west (By) component. Two different proxies have been used to represent different levels of reconnection in the near‐Earth tail: The value of the AL index and the substorm phases identified by the Substorm Onsets and Phases from Indices of the Electrojet algorithm. We find that the convection changes from a dominantly YGSMdirection, but opposite in the two hemispheres, to a flow oriented more toward the plasma sheet, as the north‐south component of the convection increases when reconnection enhances in the near Earth tail. This result is consistent with recent observations of the convection in the ionosphere, which suggest that the nightside convection pattern becomes more north‐south symmetric when tail reconnection increases. This is also supported by simultaneous auroral observations from the two hemispheres, which shows that conjugate auroral features become more symmetric during substorm expansion phase. The reduced asymmetry implies that the asymmetric pressure balance in the lobes is altered during periods with strong reconnection in the near‐Earth tail. Average lobe convection when IMF By≠0 is more north‐south aligned for high |AL| compared to low |AL|Average lobe convection when IMF By≠0 is more north‐south aligned during substorms compared to nonsubstorm intervalsThe more north‐south aligned convection implies that enhanced near‐Earth tail reconnection alters the average lobe convection

The warm plasma cloak is a source of magnetospheric plasma that contain significant O+. When the O+density in the magnetosphere near the magnetopause is >0.2 cm‐3and the H+density is <1.5 cm‐3, then O+dominates the magnetospheric ion mass density by more than a factor of 2. A survey is conducted of such O+‐rich warm plasma cloak intervals and their effect on reconnection at the Earth's magnetopause. The survey uses data from the Magnetospheric Multiscale mission (MMS) and the results are compared and combined with a previous survey of the warm plasma cloak. Overall, the warm plasma cloak and the O+‐rich warm plasma cloak reduce the magnetopause reconnection rate by >20% due to mass‐loading only about 2% to 4% of the time. However, during geomagnetic storms, O+dominates the mass density of the warm plasma cloak and these mass densities are very high. Therefore, a separate study is conducted to determine the effect of the warm plasma cloak on magnetopause reconnection during geomagnetically disturbed times. This study shows that the warm plasma cloak reduces the reconnection rate significantly about 25% of the time during disturbed conditions. The magnetospheric warm plasma cloak is O+‐rich during geomagnetically active timesThe warm plasma cloak reduces the magnetic reconnection rate at the magnetopause ~2‐4% of the timeDuring geomagnetic storms, the O+‐rich warm plasma cloak reduces the reconnection rate by >20% sometime during 25% of the storms

We use Geotail, Cluster, and Time History of Events and Macroscale Interactions during Substorms data over 15 years (1995–2009) to statistically investigate convective ion flows (V⊥xy<200 km/s) in the magnetotail plasma sheet under the influence of a clearly nonzero dawn‐dusk interplanetary magnetic field (IMF By). We find that IMF Bycauses an interhemispheric asymmetry in the flows, which depends on the direction of IMF By. On the average, one magnetic hemisphere is dominated by a dawn‐dusk flow component, which is oppositely directed compared to that in the other hemisphere. This asymmetry is observed for both earthward and tailward flows. A comparison to tail Byreveals that the region where the asymmetry in the average flows appears agrees with the appearance of the tail Bydirection collinear to IMF By. The results imply that IMF Byhas a major influence on the direction of the magnetic flux transport in the magnetotail. IMF Bycauses interhemispheric asymmetry in the tail plasma sheet flows perpendicular to the magnetic fieldOpposite dawn‐dusk flow components can appear over large regions in the two magnetic hemispheres under clearly nonzero IMF ByThe asymmetry is statistically seen for both earthward and tailward average flow

We present Magnetospheric Multiscale mission (MMS) observations of Hall physics in the magnetotail, which compared to dayside Hall physics is a relatively unexplored topic. The plasma consists of electrons, moderately cold ions (T∼1.5 keV) and hot ions (T∼20 keV). MMS can differentiate between the cold ion demagnetization region and hot ion demagnetization regions, which suggests that MMS was observing multiscale Hall physics. The observed Hall electric field is compared with a generalized Ohm's law, accounting for multiple ion populations. The cold ion population, despite its relatively high initial temperature, has a significant impact on the Hall electric field. These results show that multiscale Hall physics is relevant over a much larger temperature range than previously observed and is relevant for the whole magnetosphere as well as for other astrophysical plasma. MMS multispacecraft observation of Hall electric fields during a deep partial plasma sheet crossingMixing of hot (20 keV) and moderately cold (1.5 keV) ions generate multiscale Hall electric fieldsElectron and cold/hot ion demagnetization boundaries are observed near their predicted locations

We investigate the role of a background oxygen population in magnetic reconnection, using particle‐in‐cell simulations. We run several simulations, with different initial oxygen temperatures and densities, to understand how the reconnection rate is influenced, as oxygen is captured by the reconnection process. The oxygen remains approximately demagnetized on the relevant time and spatial scales and therefore has little direct(i.e., immediate mass loading) effect on the reconnection process itself. The reconnection rate is independent of the initial oxygen temperature but clearly dependent on the density. The reduced reconnection rate is twice as fast as predicted by mass loading. We describe a mechanism where the oxygen population (and the associated electrons) acts as an energy sink on the system, altering the energy partitioning. Based on a scaling analysis, we derive an estimate of the reconnection electric field that scales as (1+no/np)−1, where noand npis the oxygen and proton densities, respectively. The presence of O+(or other heavy ions) significantly decreases the reconnection rateDemagnetized O+(and the associated electrons) acts as energy sink on the finite energy reservoir at the expense of protonsThe reduction in the reconnection rate follows a simple scaling that depends on the ratio of oxygen to proton concentration

When the interplanetary magnetic field is northward for a period of time, O+from the high‐latitude ionosphere escapes along reconnected magnetic field lines into the dayside magnetopause boundary layer. Dual‐lobe reconnection closes these field lines, which traps O+and mass loads the boundary layer. This O+is an additional source of magnetospheric plasma that interacts with magnetosheath plasma through magnetic reconnection. This mass loading and interaction is illustrated through analysis of a magnetopause crossing by the Magnetospheric Multiscale spacecraft. While in the O+‐rich boundary layer, the interplanetary magnetic field turns southward. As the Magnetospheric Multiscale spacecraft cross the high‐shear magnetopause, reconnection signatures are observed. While the reconnection rate is likely reduced by the mass loading, reconnection is not suppressed at the magnetopause. The high‐latitude dayside ionosphere is therefore a source of magnetospheric ions that contributes often to transient reduction in the reconnection rate at the dayside magnetopause. During sustained periods of northward IMF, O+from the dayside high‐latitude ionosphere mass loads the boundary layer adjacent to the magnetopauseFor the example shown, this mass loading is substantial and does have an effect on reconnection at the magnetopause when the IMF turns southwardThis mass loading causes a transient reduction in the reconnection rate but does not suppress reconnection at the magnetopause

Oxygen of ionospheric origin is a ubiquitous particle species in magnetospheric dynamics. We investigate how an oxygen population influences magnetic reconnection and how it is energized, using particle‐in‐cell simulations. The oxygen population is inserted initially in the inflow region, at two equal distances above and below the current sheet, and as time evolves it is captured by the reconnection process. Three simulations with different initial oxygen temperature are investigated. As the oxygen gets involved, layers of high‐oxygen density forms in a region bounded by the Hall electric field. These density striations consist of a quasi‐steady horizontal layer and a dynamic, inclined, wavefront‐like layer. The acceleration of the oxygen population is dominated by electric forces, as the oxygen remains approximately demagnetized for the relevant timescales of the simulation. We describe two mechanisms of oxygen acceleration that lead to these two different structures. Oxygen density striations forming as oxygen population in inflow is captured by reconnection processQuasi‐steady accelerated oxygen is trapped in potential well, and develops into layers of high densityWavefront‐like striations (oxygen density wave) form as a result of Hall electric field developing into the inflow region

Principal component analysis is performed on Birkeland or field‐aligned current (FAC) measurements from the Active Magnetosphere and Planetary Electrodynamics Response Experiment, to determine the response of dayside and nightside FACs to reversals in the orientation of the interplanetary magnetic field (IMF) and the occurrence of substorms. Dayside FACs respond promptly to changes in IMF BY, but the nightside response is delayed by up to an hour and can take up to 4 hr to develop fully, especially during northward IMF. Nightside FAC asymmetries grow during substorm growth phase when the IMF has a significant BYcomponent, and also promptly at substorm onset. Our findings suggest that magnetotail twisting and/or BYpenetration into the magnetotail, due to subsolar reconnection with east‐west orientated IMF, are the main cause of these nightside FAC asymmetries and that asymmetries also arise due to magnetotail reconnection of these twisted field lines. Principal component analysis is performed separately on the dayside and nightside portions of field‐aligned current maps measured by AMPEREWe determine the timescales of dayside and nightside response to changes in the polarity of IMF BZand By and the occurrence of substormsNightside responses are delayed, interpreted in terms of the substorm cycle and the induction of a BYcomponent in the magnetotail

It is known that the magnetic field of the Earth's closed magnetosphere can be highly displaced from the quiet‐day configuration when interacting with the interplanetary magnetic field (IMF), an asymmetry largely controlled by the dawn‐dusk component of the IMF. The corresponding ionospheric convection has revealed that footprints in one hemisphere tend to move faster to reduce the displacement, a process we refer to as the restoring of symmetry. Although the influence on the return flow convection from the process of restoring symmetry has been shown to be strongly controlled by the IMF, the influence from internal magnetospheric processes has been less investigated. We use 14 years of line‐of‐sight measurements of the ionospheric plasma convection from the Super Dual Auroral Radar Network to produce high‐latitude convection maps sorted by season, IMF, and geomagnetic activity. We find that the restoring symmetry flows dominate the average convection pattern in the nightside ionosphere during low levels of magnetotail activity. For increasing magnetotail activity, signatures of the restoring symmetry process become less and less pronounced in the global average convection maps. We suggest that tail reconnection acts to reduce the asymmetric state of the closed magnetosphere by removing the asymmetric pressure distribution in the tail set up by the IMF Byinteraction. During active periods the nightside magnetosphere will therefore reach a more symmetric configuration on a global scale. These results are relevant for better understanding the dynamics of flux tubes in the asymmetric geospace, which is the most common state of the system. In this study we use observations of plasma drift from the Earth's ionosphere to study the symmetry of the Earth's magnetosphere on a large scale. On this global scale we say that the magnetic field is asymmetric when the field lines connecting the two hemispheres are displaced from their usual location. This can happen when the magnetosphere interact with the interplanetary magnetic field, especially when the latter has a significant magnitude in the east‐west direction. The major discovery of this study is that geomagnetic activity related to processes within the magnetosphere (tail reconnection) also seem to influence the degree of global asymmetry in the system. We find that the magnetosphere can become very asymmetric during periods of low geomagnetic activity, while it is more symmetric during times with higher activity. These results give us a better understanding of the processes leading to an asymmetric magnetosphere, which is needed to better understand the complex near‐Earth space system that is becoming increasingly important for our society. Tail activity reduces asymmetries in convection speed toward the dayside between the dawn and dusk cellsTail reconnection reduces any asymmetric pressure distribution in the tail, leading to a more symmetric magnetosphereNightside convection patternbecomes more symmetric for increasing levels of tail activity

Statistical analyses have shown that the sunward component of the interplanetary magnetic field, Bx(Geocentric Solar Magnetospheric), moderately but significantly affects the auroral intensity. These observations have been interpreted as signatures of a similar interplanetary magnetic field Bxcontrol on Birkeland currents yet to be observed directly. Such a control, attributed to differences in magnetic tension on newly opened magnetic field lines, would lead to stronger region 1 (R1) Birkeland currents for Bxnegative (positive) conditions in the Northern (Southern) Hemispheres than when Bxis positive (negative). In this paper we perform a detailed investigation of three different sets of magnetic field measurements, from the Challenging Minisatellite Payload and Swarmlow Earth orbit satellites, from the Active Magnetosphere and Planetary Electrodynamics Response Experiment products derived from the Iridium satellite constellation, and from the SuperMAG ground magnetometer network, each analyzed using different techniques, to test these predictions. The results show that a change in sign of Bxchanges the Birkeland currents by no more than ≈10%. The current patterns show little support for an interhemispheric asymmetry of the kind proposed to explain auroral observations. Instead, we propose an alternative interpretation, which is consistent with most of the auroral observations and with the current observations in the present paper, except for those based on Active Magnetosphere and Planetary Electrodynamics Response Experiment: The solar wind‐magnetosphere coupling is more efficient when the dipole tilt angle and Bxhave the same sign than when they are different. We suggest that the higher coupling is because the dayside reconnection region is closer to the subsolar point when the dipole tilt angle and Bxhave the same sign. The energy that powers geomagnetic activity and auroras originate in the solar wind. The most important parameters controlling the energy transfer is the solar wind velocity and the orientation of the magnetic field which is being carried by the solar wind. More energy will be transferred when this magnetic field is southward than when it is northward. Its component in the Earth‐Sun direction is most often neglected, although some studies have shown that it has a modest influence on the intensity of auroras. In this paper we search for corresponding effects in electric currents in near‐Earth space. We find that the sunward component of the magnetic field does seem to influence the currents, although not in the way suggested to explain the aurora observations. Instead, we propose that the sunward component of the interplanetary magnetic field modulates the rate at which energy is being transferred from the solar wind to the magnetosphere in a way which is different for different orientations of the Earth's magnetic field with respect to the Sun. Three different observational tests are carried out in search of Bxeffects on currentsOnly small differences are observed between different signs of Bxwhen Bz<0Observations indicate stronger SW‐M coupling when Bxand dipole tilt have the same sign

We use the Lyon‐Fedder‐Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) Bycomponent on the coupling between the solar wind and magnetosphere‐ionosphere system when IMF Bz>0. We describe the evolution of how a magnetospheric Bycomponent is induced on closed field lines during these conditions. Starting from dayside lobe reconnection, the magnetic tension on newly reconnected field lines redistribute the open flux asymmetrically between the two hemispheres. This results in asymmetric magnetic energy density in the lobes. Shear flows are induced to restore equilibrium, and these flows are what effectively induces a local Bycomponent. We show the radial dependence of the induced Byand compare the results to the induced Byduring southward IMF conditions. We also show the response and reconfiguration time of the inner magnetosphere to IMF Byreversals during northward IMF Bz. A superposed epoch analysis of magnetic field measurements from seven Geostationary Operational Environmental Satellite spacecraft at different local times both for negative‐to‐positive and positive‐to‐negative IMF Byreversals is presented. We find that the induced Byresponds within 16 min of the arrival of IMF Byat the bow shock, and it completely reconfigures within 47 min. We present a dynamic description of how Byis induced during northward directed IMFAsymmetries in magnetic energy density drives shear flowsThe magnetosphere response and reconfiguration time are comparable to southward IMF

During southward interplanetary magnetic field, dayside reconnection will drive the Dungey cycle in the magnetosphere, which is manifested as a two‐cell convection pattern in the ionosphere. We address the response of the ionospheric convection to changes in the dayside reconnection rate by examining magnetic field perturbations at 780 km altitude. The Active Magnetosphere and Planetary Electrodynamics Response Experiment data products derived from the Iridium constellation provide global maps of the magnetic field perturbations. Cluster data just upstream of the Earth's bow shock have been used to estimate the dayside reconnection rate. By using a statistical model where the magnetic field can respond on several time scales, we confirm previous reports of an almost immediate response both near noon and near midnight combined with a 10–20 min reconfiguration time of the two‐cell convection pattern. The response of the ionospheric convection has been associated with the expansion of the polar cap boundary in the Cowley‐Lockwood paradigm. In the original formulation of this paradigm the expansion spreads from noon to midnight in 15–20 min. However, also an immediate global response has been shown to be consistent with the paradigm when the previous dayside reconnection history is considered. In this paper we present a new explanation for how the immediate response can be accommodated in the Cowley‐Lockwood paradigm. The new explanation is based on how MHD waves propagate in the magnetospheric lobes when newly reconnected open flux tubes are added to the lobes, and the magnetopause flaring angle increases. We confirm previous reports of an almost simultaneous ionospheric response to dayside reconnection in all MLT sectorsUsing new methodology, we confirm that the two‐cell ionospheric convection pattern reconfigures in about 10 minThe observations are consistent with MHD waves in the lobes excited by the increasing magnetopause flaring angle

The interaction between the interplanetary magnetic field (IMF) and the geomagnetic field at the dayside magnetopause leads to transfer of momentum and energy which changes the magnetospheric configuration, but only after a certain time. In this study we quantify this time, to advance our understanding of the causes for the delayed response of the magnetosphere. We study the response and reconfiguration time of the inner magnetosphere to IMF Byreversals. A superposed epoch analysis of magnetic field measurements from four Geostationary Operational Environmental Satellite spacecraft at different local times both for negative to positive IMF Byreversals and for positive to negative reversals is presented. The magnetospheric response time at geosynchronous orbit to the sudden change of IMF Byis less than 15 (∼10) min from the bow shock (magnetopause) arrival time, while the reconfiguration time is less than 46 (∼41) min. These results are consistent with a Bycomponent induced on closed magnetic field lines due to the asymmetric loading of flux following asymmetric dayside reconnection when IMF By≠0. Our results also confirm our earlier studies that nightside reconnection is not required for generating a Bycomponent on closed field lines. The magnetosphere responds to a change in IMF Byin less than 15 min in all local times from the presumed arrival at the bow shockAt geosynchronous distances, the reconfiguration time is less than 45 min in all local time sectorsThe reconfiguration time and magnitude of the induced Bycomponent depend on the solar wind velocity

In this paper, we present observations of cold (0–70 eV) plasma density in the magnetotail lobes. The observations and results are based on 16 years of Cluster observation of spacecraft potential measurements converted into local plasma densities. Measurements from all four Cluster spacecraft have been used, and the survey indicates a persistent asymmetry in lobe density, with consistently higher cold plasma densities in the northern lobe. External influences, such as daily and seasonal variations in the Earth's tilt angle, can introduce temporary north‐south asymmetries through asymmetric ionization of the two hemispheres. Likewise, external drivers, such as the orientation of the interplanetary magnetic field can set up additional spatial asymmetries in outflow and lobe filling. The persistent asymmetry reported in this paper is also influenced by these external factors but is mainly caused by differences in magnetic field configuration in the Northern and Southern Hemisphere ionospheres. A comprehensive study of the plasma density in the magnetotail lobes is presentedThe lobe density is asymmetric; the density is higher in the northern lobeNorth‐south differences in ion outflow are the most probable explanation for the asymmetry

We investigate the onset of magnetic reconnection, utilizing a fully kinetic Particle‐In‐Cell (PIC) simulation. Characteristic features of the electron phase‐space distributions immediately before reconnection onset are identified. These include signatures of pressure non‐gyrotropy in the velocity distributions, and lemon shaped distributions in the in‐plane velocity directions. Further, we explain how these features form through particle energization by the out‐of‐plane electric field. Identification of these features in the distributions can aid in analysis of data where clear signatures of ongoing reconnection are not yet present. In any environment where magnetic fields and charged particles interact, magnetic reconnection will occur if the conditions are favorable. Magnetic reconnection can be described as magnetic explosions, since it releases stored magnetic energy and converts it into heat and movement of the plasma particles. In this paper, we use numerical simulations to take a closer look at how the reconnection process initiates, a fundamental question still not fully understood. Signatures indicating reconnection onset can be identified in the electron distributionsOnset signatures persist over extended spatial and temporal scalesThe particle distributions immediately preceding onset are characterized by features of non‐gyrotropy and acceleration Signatures indicating reconnection onset can be identified in the electron distributions Onset signatures persist over extended spatial and temporal scales The particle distributions immediately preceding onset are characterized by features of non‐gyrotropy and acceleration

Here we present an event where simultaneous global imaging of the aurora from both hemispheres reveals a large longitudinal shift of the nightside aurora of about 3 h, being the largest relative shift reported on from conjugate auroral imaging. This is interpreted as evidence of closed field lines having very asymmetric footpoints associated with the persistent positive ycomponent of the interplanetary magnetic field before and during the event. At the same time, the Super Dual Auroral Radar Network observes the ionospheric nightside convection throat region in both hemispheres. The radar data indicate faster convection toward the dayside in the dusk cell in the Southern Hemisphere compared to its conjugate region. We interpret this as a signature of a process acting to restore symmetry of the displaced closed magnetic field lines resulting in flux tubes moving faster along the banana cell than the conjugate orange cell. The event is analyzed with emphasis on Birkeland currents (BC) associated with this restoring process, as recently described by Tenfjord et al. (2015). Using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) during the same conditions as the presented event, the large‐scale BC pattern associated with the event is presented. It shows the expected influence of the process of restoring symmetry on BCs. We therefore suggest that these observations should be recognized as being a result of the dynamic effects of restoring footpoint symmetry on closed field lines in the nightside. Observed asymmetric convection and FAC on closed field lines consistent with effect of asymmetric stress release during IMF By conditionsThe event experiences a large (3 h) MLT displacement of the nightside aurora between the two hemispheresThe restoring symmetry process can be an important mechanism in creating asymmetric FACs and convection on closed field lines

We used the Lyon‐Fedder‐Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) Bycomponent on the coupling between the solar wind and magnetosphere‐ionosphere system. When the IMF reconnects with the terrestrial magnetic field with IMF By≠0, flux transport is asymmetrically distributed between the two hemispheres. We describe how Byis induced in the closed magnetosphere on both the dayside and nightside and present the governing equations. The magnetosphere imposes asymmetric forces on the ionosphere, and the effects on the ionospheric flow are characterized by distorted convection cell patterns, often referred to as “banana” and “orange” cell patterns. The flux asymmetrically added to the lobes results in a nonuniform induced Byin the closed magnetosphere. By including the dynamics of the system, we introduce a mechanism that predicts asymmetric Birkeland currents at conjugate foot points. Asymmetric Birkeland currents are created as a consequence of ydirected tension contained in the return flow. Associated with these currents, we expect fast localized ionospheric azimuthal flows present in one hemisphere but not necessarily in the other. We also present current density measurements from Active Magnetosphere and Planetary Electrodynamics Response Experiment that are consistent with this picture. We argue that the induced Byproduces asymmetrical Birkeland currents as a consequence of asymmetric stress balance between the hemispheres. Such an asymmetry will also lead to asymmetrical foot points and asymmetries in the azimuthal flow in the ionosphere. These phenomena should therefore be treated in a unified way. The asymmetrically added flux to the lobes induces Byin the closed magnetosphereAsymmetric FACs are created as a consequence of ydirected tension contained in the return flowIMF Byresults in asymmetries in both currents, zonal flows, and auroras in the ionosphere

Magnetic perturbations on ground at high latitudes are directly associated only with the divergence‐free component of the height‐integrated horizontal ionospheric current, J⊥,df. Here we show how J⊥,dfcan be expressed as the total horizontal current J⊥minus its curl‐free component, the latter being completely determined by the global Birkeland current pattern. Thus, in regions where J⊥=0, the global Birkeland current distribution alone determines the local magnetic perturbation. We show with observations from ground and space that in the polar cap, the ground magnetic field perturbations tend to align with the Birkeland current contribution in darkness but not in sunlight. We also show that in sunlight, the magnetic perturbations are typically such that the equivalent overhead current is antiparallel to the convection, indicating that the Hall current system dominates. Thus, the ground magnetic field in the polar cap relates to different current systems in sunlight and in darkness. Polar cap magnetic field perturbations relate to different current systems in sunlight and darknessPolar cap magnetic field perturbations relate to Birkeland currents in darknessPolar cap magnetic field perturbations relate to Hall currents in sunlight

In the exploration of global‐scale features of the Earth's aurora, little attention has been given to the radial component of the Interplanetary Magnetic Field (IMF). This study investigates the global auroral response in both hemispheres when the IMF is southward and lies in the xzplane. We present a statistical study of the average auroral response in the 12–24 magnetic local time (MLT) sector to an xcomponent in the IMF. Maps of auroral intensity in both hemispheres for two IMF Bxdominated conditions (± IMF Bx) are shown during periods of negative IMF Bz, small IMF By, and local winter. This is obtained by using global imaging from the Wideband Imaging Camera on the IMAGE satellite. The analysis indicates a significant asymmetry between the two IMF Bxdominated conditions in both hemispheres. In the Northern Hemisphere the aurora is brighter in the 15–19 MLT region during negative IMF Bx. In the Southern Hemisphere the aurora is brighter in the 16–20 MLT sector during positive IMF Bx. We interpret the results in the context of a more efficient solar wind dynamo in one hemisphere. Both the intensity asymmetry and its location are consistent with this idea. This has earlier been suggested from case studies of simultaneous observations of the aurora in both hemispheres, but hitherto never been observed to have a general impact on global auroral brightness in both hemispheres from a statistical study. The observed asymmetries between the two IMF Bxcases are not large; however, the difference is significant with a 95% confidence level. As the solar wind conditions examined in the study are rather common (37% of the time) the accumulative effect of this small influence may be important for the total energy budget. Auroral intensity affected differently by IMF Bx in the two hemispheresObservations are consistent with increased SW dynamo efficiency in one hemisphereAccumulative effect might be important in total energy budget

Using a resistive MagnetoHydroDynamic (MHD) simulation, we study how the magnitude and shape of diffusion influence magnetic reconnection. Specifically, we investigate how and why the reconnection rate is influenced by variations in the diffusion distribution and magnitude. By running multiple MHD simulations where we vary the localized resistivity, we find that the properties of the diffusion region greatly influence the rate of reconnection. Increasing the magnitude of the imposed resistivity results in a higher reconnection rate, but the rate saturates at approximately 0.2. We show how a redistribution of the current density, leading to a bifurcated current sheet, play a major role in this limitation. In addition, we investigate the impact of different shapes of resistive region. The shape of the diffusion region also plays a major role in how efficient the reconnection energy conversion can operate. The highest reconnection rate, approximately 0.25, is achieved for an optimal opening angle. Our results imply that reconnection has a speed limit that may depend on properties outside the diffusion region. Magnetic reconnection is a fundamental plasma process, which can explosively convert magnetic energy to particle energy. When reconnection operates, it releases almost all of the energy stored in the magnetic field to plasma acceleration and heating. The consequences of reconnection depend on the magnetic energy available and the process' ability to rapidly release the energy. Thus, the effectiveness of reconnection, which can be quantified by the rate at which energy is converted, is a key factor in understanding the consequences and implications of this universal process. This paper investigates how the reconnection rate depends on the resistivity in the system. In our fluid‐based scheme, resistivity determines the plasma's ability to diffuse across the magnetic field ‐ allowing new magnetic topologies to form. We show that, even when inserting very strong resistive spots with varying shapes, there appears to be a maximum rate of reconnection the system can support. In addition, we find that a sub‐optimal choice of resistivity magnitude or shape of the resistive spot leads to lower overall reconnection rates. These results imply that the reconnection rate depends significantly on properties of the diffusion region, even if the size of that region is much smaller than the system. The reconnection rate depends significantly on the properties of the diffusion regionEven for very strong resistivities and varying shapes of the resistive region there exists a maximum rate that the system can supportWe show how the properties of the diffusion region are altered for various resistive spot magnitudes and shapes The reconnection rate depends significantly on the properties of the diffusion region Even for very strong resistivities and varying shapes of the resistive region there exists a maximum rate that the system can support We show how the properties of the diffusion region are altered for various resistive spot magnitudes and shapes

Using fully kinetic particle‐in‐cell modeling, we investigate how magnetic reconnection responds to a varying guide field in one of the inflow regions. We find that the reconnection rate varies significantly when the orientation of the magnetic field changes between being strictly antiparallel and having a guide field. These variations are fairly consistent with the scaling relation for asymmetric reconnection developed by Cassak and Shay (2007). However, the rate is also found to be nonlinearly modulated by changes in the ion inflow velocity. The spatio‐temporal change in the inflow velocity arises as the magnetic forces reconfigure to regions of different magnetic field strengths. The variations in the inflow magnetic field configuration allow for different gradients in the magnetic field, leading to asymmetries in the magnetic tension force. By momentum conservation, this facilitates asymmetries in the inflow velocity, which in turn affects the flux transport into the reconnection site. The outflow is found to be less laminar when the inflow varies, and various signatures of the inflow variations are identified in the outflow. Magnetic reconnection can be described as magnetic explosions, where energy stored in magnetic fields is converted into heat and movement of particles. It can happen in all environments where magnetic fields and charged particles interact, such as in the Sun, in planetary magnetospheres, and in fusion reactors on Earth. In this article, using numerical simulations, we present new insight into how magnetic reconnection behaves when the magnetic fields vary during the reconnection process. Spatio‐temporal effects in the inflow conditions causes modulations in the reconnection rate and introduces time‐dependent effectsThe asymmetrically varying guide field alters the force balance between the current sheet and inflow regionThe outflow regions show nonlaminar exhaust structures induced by the changing inflow Spatio‐temporal effects in the inflow conditions causes modulations in the reconnection rate and introduces time‐dependent effects The asymmetrically varying guide field alters the force balance between the current sheet and inflow region The outflow regions show nonlaminar exhaust structures induced by the changing inflow

Heavier plasma species such as oxygen ions can have a large impact on the magnetic reconnection process. It has been hypothesized that the acceleration of demagnetized oxygen ions by the Hall electric field will lead to the formation of an oxygen wave that expands into the exhaust. By comparing data from NASA's Magnetospheric Multiscale mission to a fully kinetic particle‐in‐cell simulation, we can for the first time provide observational evidence of such an expanding oxygen wave. The wave is characterized by an oxygen jet consisting of cold ions directed toward the neutral sheet associated with a density cavity. This density cavity forms as the O+are subject to collective acceleration by the Hall electric field leaving behind a region of low‐density oxygen ions. Our results are important for the understanding of the role and effect of oxygen ions in magnetic reconnection. Magnetic reconnection is one of the most important energy release and transport processes in plasmas. In the case of the Earth's magnetosphere, magnetic reconnection is the primary mechanism responsible for the transport of energy, mass, momentum, and magnetic flux into Earth's magnetic cavity. On the night side of Earth in a region called the magnetotail, magnetic flux is transported from the north and south to meet in a current layer that extends across the width of the magnetosphere. While the most common plasma species in the magnetotail are protons and electrons, during high geomagnetic activity heavier ion species, such as oxygen, can sometimes dominate during high geomagnetic activity. Using data collected by NASA's Magnetospheric Multiscale mission together with numerical simulation, we investigate how oxygen ions contribute to the reconnection process. Observation of Hall electric field accelerated O+inside separatrix density cavityObservational evidence of predicted oxygen waveObserved oxygen dynamics and corresponding velocity distribution functions consistent with simulation predictions Observation of Hall electric field accelerated O+inside separatrix density cavity Observational evidence of predicted oxygen wave Observed oxygen dynamics and corresponding velocity distribution functions consistent with simulation predictions

Ionospheric ions (mainly H+, He+, and O+) escape from the ionosphere and populate the Earth's magnetosphere. Their thermal energies are usually low when they first escape the ionosphere, typically a few electron volt to tens of electron volt, but they are energized in their journey through the magnetosphere. The ionospheric population is variable, and it makes significant contributions to the magnetospheric mass density in key regions where magnetic reconnection is at work. Solar wind—magnetosphere coupling occurs primarily via magnetic reconnection, a key plasma process that enables transfer of mass and energy into the near‐Earth space environment. Reconnection leads to the triggering of magnetospheric storms, auroras, energetic particle precipitation and a host of other magnetospheric phenomena. Several works in the last decades have attempted to statistically quantify the amount of ionospheric plasma supplied to the magnetosphere, including the two key regions where magnetic reconnection occurs: the dayside magnetopause and the magnetotail. Recent in situ observations by the Magnetospheric Multiscale spacecraft and associated modeling have advanced our current understanding of how ionospheric ions alter the magnetic reconnection process, including its onset and efficiency. This article compiles the current understanding of the ionospheric plasma supply to the magnetosphere. It reviews both the quantification of these sources and their effects on the process of magnetic reconnection. It also provides a global description of how the ionospheric ion contribution modifies the way the solar wind couples to the Earth's magnetosphere and how these ions modify the global dynamics of the near‐Earth space environment. Above the neutral atmosphere, space is filled with charged particles, which are tied to the Earth's magnetic field. The particles come from two sources, the solar wind and the Earth's upper atmosphere. Most of the solar wind particles are deflected by the Earth´s magnetic field, but some can penetrate into near‐Earth space. The ionized layer of the upper atmosphere is continuously ejecting particles into space, which have low energies and are difficult to measure. We investigate the relative importance of the two charged particle sources for the dynamics of plasma processes in near‐Earth space. In particular, we consider the effects of these sources in magnetic reconnection. Magnetic reconnection allows initially separated plasma regions to become magnetically connected and mix, and converts magnetic energy to kinetic energy of charged particles. Magnetic reconnection is the main driver of geomagnetic activity in the near‐Earth space, and is responsible for the release of energy that drives a variety of space weather effects. We highlight the fact that plasma from the ionized upper atmosphere contributes a significant part of the density in the key regions where magnetic reconnection is at work, and that this contribution is larger when the geomagnetic activity is high. Ionospheric plasma contributes a significant part of the magnetospheric density in the regions where magnetic reconnection is most frequentCold and heavy ions of ionospheric origin reduce magnetic reconnection efficiency and modify energy conversion mechanismsThe presence of ionospheric ions and their effects on reconnection and magnetospheric dynamics are enhanced during geomagnetic storms Ionospheric plasma contributes a significant part of the magnetospheric density in the regions where magnetic reconnection is most frequent Cold and heavy ions of ionospheric origin reduce magnetic reconnection efficiency and modify energy conversion mechanisms The presence of ionospheric ions and their effects on reconnection and magnetospheric dynamics are enhanced during geomagnetic storms

We investigate how magnetic reconnection is influenced by an inflow of a dense cold ion population. We compare two 2.5D Particle‐In‐Cell simulations, one containing the cold population and one without. We find that the cold population influences the reconnection process on both global and kinetic scales, and that the dominant contribution can be explained through mass‐loading. We provide an analysis of how these multiscale changes are related through kinetic processes in the ion diffusion region, the so‐called micro‐macro coupling of mass‐loading. The inertia of the cold ion population is found to be the significant link that connects the changes on different scales. The cold and warm populations exhibit counter streaming behavior when and after the ion diffusion region reorganizes itself in response to the arrival of the cold population. This signature of the cold population should be observable by spacecraft observatories such as MMS. We investigate how magnetic reconnection is influenced by a dense inflow of cold ions. We find that the cold ions influence the reconnection process on both large and small scales, and that these are connected through the cold ion inertia. We also see cold and warm ions moving in opposite directions close to the reconnection site, which should be observable by spacecraft observatories. Cold ions in the inflow region affect reconnection on both micro‐ and macro‐scalesThe micro‐macro coupling of mass‐loading is mediated by the cold ion inertiaCold and warm ions are counter streaming close to the ion diffusion region boundary Cold ions in the inflow region affect reconnection on both micro‐ and macro‐scales The micro‐macro coupling of mass‐loading is mediated by the cold ion inertia Cold and warm ions are counter streaming close to the ion diffusion region boundary

The process of magnetic reconnection in a flux tube can occur in a time‐stationary fashion as slippage reconnection or in a time‐dependent manner based on, for example, the tearing instability. However, it is not well known under which conditions a system can sustain slippage reconnection. Likewise, it is unclear whether systems can exhibit slippage reconnection and time‐dependent reconnection simultaneously. In order to investigate these questions, we employ a set of 3D magnetohydrodynamic simulation. Using these simulations, we model a twisted flux tube with a spatially localized resistive region in the center of the simulation box, and an applied driver on one end plane of the simulation box. As a result, the Poynting flux injected at the boundary propagates to the resistive region and dissipates there. If the driver is nearly circular, we find a predominance of slippage reconnection, which decouples the applied driver entirely from one half of the flux tube. Second, a tearing‐like instability occurs with slippage reconnection in the resistive region when the resistivity is strong enough and the cross section of the flux tube, as defined by the shape of the applied velocity perturbation, is elliptical enough. In this case we find the surprising feature that magnetic field perturbation generated by the tearing‐like instability radiates away from the resistive region to influence the entire flux tube shape. Magnetic reconnection in a twisted magnetic flux tube can be time‐stationary, leading to slippage reconnectionIn case the flux tube profile is elliptical enough, tearing‐like reconnection can occur simultaneously with slippageThese results pertain to a variety of plasma systems involving dissipation in twisted flux tubes Magnetic reconnection in a twisted magnetic flux tube can be time‐stationary, leading to slippage reconnection In case the flux tube profile is elliptical enough, tearing‐like reconnection can occur simultaneously with slippage These results pertain to a variety of plasma systems involving dissipation in twisted flux tubes

North‐south asymmetries arise in the magnetosphere‐ionosphere system when a significant east‐west (By) component is present in the interplanetary magnetic field (IMF). During such conditions, a Bycomponent with the same sign as the IMF Bycomponent is induced in the magnetosphere, and the locations of conjugate magnetic footpoints are displaced between the two hemispheres. It has been suggested that these asymmetries are introduced into the closed magnetosphere by tail reconnection. However, recent studies instead suggest that asymmetric lobe pressure induces the asymmetries, which are then reduced during periods of enhanced tail reconnection. To address this, we use the Lyon‐Fedder‐Mobarry (LFM) model and initiate a loading‐unloading cycle in multiple runs by changing the IMF. Asymmetries are induced during the loading phase and reduced during the unloading phase. The model results thus suggest that asymmetries arise during periods with low tail reconnection and are reduced during periods with enhanced tail reconnection. The aurora is a bright and beautiful manifestation of Earth's connection to space, and can light up the night sky in the high latitude regions in both hemispheres. However, auroral features do not always occur at the expected magnetic location in the two hemispheres, but are often displaced in the longitudinal direction between the Northern and Southern Hemisphere. The displacement is related to the orientation of the magnetic field in the solar wind, but the exact mechanisms responsible for causing and removing this displacement are debated. Previous simulations of the plasma environment around the Earth suggest that when this magnetic field intensifies in the dawn‐dusk direction, magnetic pressure builds up asymmetrically in each hemisphere, causing the displacement. Here we present new simulation results showing that the longitudinal displacement is reduced when magnetic reconnection happens in the Earth’s magnetotail. The evolution of the asymmetric state of the magnetosphere is examined by using the Lyon‐Fedder‐Mobarry model with non‐zero interplanetary magnetic field ByThe simulation shows a clear reduction of north‐south asymmetries associated with enhanced tail reconnectionThe modeling results are consistent with the evolution seen in interhemispheric observations of aurora The evolution of the asymmetric state of the magnetosphere is examined by using the Lyon‐Fedder‐Mobarry model with non‐zero interplanetary magnetic field By The simulation shows a clear reduction of north‐south asymmetries associated with enhanced tail reconnection The modeling results are consistent with the evolution seen in interhemispheric observations of aurora

The decay of magnetospheric activity when driving is turned off (to a minimum) was measured in total field‐aligned current estimates from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) project. Events of distinct northward turnings of the interplanetary magnetic field were identified, with prolonged periods of stable southward driving conditions followed by northward interplanetary magnetic field conditions. All but 4 of 43 identified events exhibit a well‐defined exponential decay in the total hemispheric field‐aligned current following the northward turning. A superposed epoch analysis yields a generic decay constant of 0.9, corresponding to an e‐folding time of 1.1 hr. A statistical analysis of the ensemble of events also reveals a seasonal variation in the decay parameter with faster decay observed in the winter than in the summer hemisphere. This result can be understood in terms of stronger/weaker line tying of the ionospheric foot points of magnetospheric field lines for higher/lower conductivity. Energy and circulation in the Earth's magnetosphere and ionosphere are largely determined by conditions in the solar wind and interplanetary magnetic field. When the driving from the solar wind is turned off, we expect the activity to die down but how this happens is not known. With newly available data from a large constellation of satellites we can get a measure of the global activity level. These observations show that the activity decays exponentially with a generic decay time of 1.1 hr. This finding indicates a fundamental behavior of the magnetosphere‐ionosphere system, which is crucial to fully understand its dynamics. Magnetospheric activity exhibits approximate exponential decay when solar wind driving is turned off (minimized)An average exponential time constant of 1.1 hr is measured in total hemispheric field‐aligned current estimatesLonger decay time is observed in the summer compared to the winter hemisphere, consistent with expectations for ionospheric line tying

A new look at the structure of the electron diffusion region in collision less magnetic reconnection is presented. The research is based on a particle‐in‐cell simulation of asymmetric magnetic reconnection, which includes a temperature gradient across the current layer in addition to density and magnetic field gradient. We find that none of X‐point, flow stagnation point, and local current density peak coincide. Current and energy balance analyses around the flow stagnation point and current density peak show consistently that current dissipation is associated with the divergence of nongyrotropic electron pressure. Furthermore, the same pressure terms, when combined with shear‐type gradients of the electron flow velocity, also serve to maintain local thermal energy against convective losses. These effects are similar to those found also in symmetric magnetic reconnection. In addition, we find here significant effects related to the convection of current, which we can relate to a generalized diamagnetic drift by the nongyrotropic pressure divergence. Therefore, only part of the pressure force serves to dissipate the current density. However, the prior conclusion that the role of the reconnection electric field is to maintain the current density, which was obtained for a symmetric system, applies here as well. Finally, we discuss related features of electron distribution function in the electron diffusion region (EDR). Specifically, we analyze both new crescent substructures as well as outer, higher energy crescents generated by accelerated magnetospheric particles. Magnetic reconnection is arguably the most important mechanism to release energy stored in magnetic fields explosively. Magnetic reconnection is believed to be the driver between as diverse a set of phenomena as solar eruptions, astrophysical radiation bursts, magnetic storms in near‐Earth space, and the aurora. Quite amazingly, magnetic reconnection facilitates energy conversion over huge regions of space with size of many Earth radii by means of a tiny core region, the so‐called diffusion region, with dimensions of a few to a few hundreds of kilometers. The delicate interaction between charged particles and electromagnetic fields in this central region enables the large‐scale conversion of magnetic energy to particle energy to proceed. This paper presents a new look at the inner workings of this region for a fairly generic case of magnetic reconnection, which, among others, occurs at the interface between the Earth's magnetic field and the particle streams originating at the Sun. Flow stagnation point and current density maximum are not necessarily collocated in asymmetric magnetic reconnectionThe reconnection electric field sustains the electron current density and pressure also in asymmetric magnetic reconnectionElectron crescent distributions feature complex substructures related to different electron inflow regions Flow stagnation point and current density maximum are not necessarily collocated in asymmetric magnetic reconnection The reconnection electric field sustains the electron current density and pressure also in asymmetric magnetic reconnection Electron crescent distributions feature complex substructures related to different electron inflow regions

Observations from the Magnetospheric Multiscale (MMS) mission are used to quantify the maximum effect of magnetospheric H+and He+on dayside magnetopause reconnection. A data base of current‐sheet crossings from the first 2 years of the MMS mission is used to identify magnetopause crossings with the highest He+concentrations. While all of these magnetopause crossings exhibit evidence of plasmaspheric plume material, only half of the crossings are directly associated with plasmaspheric plumes. The He+density varies dramatically within the magnetosphere adjacent to the magnetopause, with density variations of an order of magnitude on timescales as short as 10 s, the time resolution of the composition instrument on MMS. Plasma wave observations are used to determine the total electron density, and composition measurements are used to determine the mass density in the magnetosheath and magnetosphere. These mass densities are then used with the magnetic field observations to determine the theoretical reduction in the reconnection rate at the magnetopause. The presence of high‐density plasmaspheric plume material at the magnetopause causes transient reductions in the reconnection rate of up to ∼40%. As the solar wind propagates from the Sun to the Earth, it encounters two boundaries that limit its access to the near‐Earth environment. The first is a bow shock that heats, slows, and deflects the solar wind. The second is the Earth's magnetopause, where the shocked solar wind is deflected around the Earth's magnetic field region called the magnetosphere. Magnetic reconnection at the magnetopause creates an interconnection between the magnetic field of the shocked solar wind and the Earth's magnetic field. The rate at which these magnetic fields interconnect, the reconnection rate, depends on the amount of plasma (ions and electrons) on either side of the magnetopause. This research uses observations from the Magnetospheric Multiscale mission to look at this rate for times when there is substantial plasma on the magnetospheric side of the magnetopause. These instances are rare; however, when they do occur, they reduce the reconnection rate by a substantial amount (up to 40%). An Magnetospheric Multiscale data base of current‐sheet crossings was used to identify magnetopause crossings with the highest He+densitiesAll of the high He+density crossings show evidence of plasmaspheric plume material, but only half are directly associated with a plasmaspheric plumeThe He+density varies dramatically in the magnetosphere, with order of magnitude variations as short as the time resolution of the composition instrument (10 s)The presence of high‐density plasmaspheric plume material at the magnetopause produces transient reconnection rate reductions of up to ∼40% An Magnetospheric Multiscale data base of current‐sheet crossings was used to identify magnetopause crossings with the highest He+densities All of the high He+density crossings show evidence of plasmaspheric plume material, but only half are directly associated with a plasmaspheric plume The He+density varies dramatically in the magnetosphere, with order of magnitude variations as short as the time resolution of the composition instrument (10 s) The presence of high‐density plasmaspheric plume material at the magnetopause produces transient reconnection rate reductions of up to ∼40%

Using 2.5‐D Particle‐In‐Cell (PIC) simulations, we investigate how magnetotail reconnection is affected by a cold, streaming, oxygen plasma population, attributed to an ionospheric source, in the inflow region. As the tailward streaming oxygen reaches the current layer, a tailward motion of the reconnection site is induced. Due to the much longer cyclotron period of the oxygen ions, oxygen cannot couple as directly into the reconnection dynamics as protons. We find that the oxygen ions couple indirectly by means of impacting the electron dynamics. Therefore, a demagnetized species can, in fact, alter the dynamics of the reconnection site. We see further that the reconnection rate remains unchanged relative to a nonstreaming run. Our results may prove useful for understanding the development and dynamics of magnetospheric substorms and storms. Tailward motion of the X‐point is generated even though the oxygen is demagnetizedThe oxygen couples to the magnetic flux through electrostatic coupling with the electronsThe momentum of the streaming oxygen does not affect the reconnection rate

We employ a 2.5D particle‐in‐cell simulation to study a scenario where the reconnection process captures cold streaming protons. As soon as the tailward streaming protons become involved, they contribute to the overall momentum balance, altering the initially symmetric dynamics. Adding tailward‐directed momentum to the reconnection process results in a tailward propagation of the reconnection site. We investigate how the reconnection process reorganizes itself due to the changing momentum conditions on the kinetic scale and how the reconnection rate is affected. We find that adding tailward momentum does not result in a significantly different reconnection rate compared to the case without cold streaming protons, when scaled to the total Alfvén velocity. This implies that the effect of changing inflow conditions due to the motion of the reconnection site appears to be minimal. The dynamics of the particles are, however, significantly different depending on whether they enter on the tailward or Earthward side of the reconnection site. On the Earthward side they are reflected and thermalized, while on the tailward side they are picked up and accelerated. The cold proton density and Ezon the Earthward side are turbulent, while the tailward side has laminar cold proton density striations and an embedded Ezlayer. Also, since the initial plasma sheet population is swept up on one side and flushed out on the other, asymmetries in the densities and strength of Hall fields emerge. Our results are important for understanding the development and dynamics of magnetospheric substorms and storms. Cold streaming protons flowing aligned with the upstream magnetic field cause the Xline to move at the total mass velocityThe reconnection rate is unaffected by the streaming velocityOn the tailward side Ezremains laminar, whereas the Ezstructure broadens and protons become more thermalized at the Earthward side

In this study we use the Magnetospheric Multiscale mission to investigate the electron acceleration and thermalization occurring along the magnetic reconnection separatrices in the magnetotail. We find that initially cold electron lobe populations are accelerated toward the X line forming beams with energies up to a few kiloelectron volts, corresponding to a substantial fraction of the electron thermal energy inside the exhaust. The accelerated electron populations are unstable to the formation of electrostatic waves which develop into nonlinear electrostatic solitary waves. The waves' amplitudes are large enough to interact efficiently with a large part of the electron population, including the electron beam. The wave‐particle interaction gradually thermalizes the beam, transforming directed drift energy to thermal energy. Electrons are accelerated toward the X line at the separatrices, forming beams with energies up to several kiloelectron voltsThe beam is unstable and leads to the formation of electrostatic solitary wavesThe electrostatic waves interact strongly with the electron beams, thermalizing them

We have used a large number of magnetopause crossings by the Magnetospheric Multiscale (MMS) mission to investigate macroscopic properties of this current sheet, with emphasis on the flanks of the magnetopause. Macroscopic features such as thickness, location, and motion of the magnetopause were calculated as a function of local time sector. The results show that the flanks of the magnetopause are significantly thicker than the dayside magnetopause. Thicknesses vary from about 650 km near noon to over 1,000 km near the terminator. Current densities vary in a similar manner, with average current densities around noon almost twice as high as near the terminator. We also find a dawn‐dusk asymmetry in many of the macroscopic parameters; the dawn magnetopause is thicker than at dusk, while the dusk flank is more dynamic, with a higher average normal velocity. The magnetopause flanks are characterized using MMS observationsMagnetopause flanks are thicker and have lower current density than the daysideThe dawn magnetopause is marginally thicker than at dusk

Combined with the magnetic field, the distribution of charged particles in the inflow region is expected to control the rate of magnetic reconnection. This paper investigates how the reconnection process is altered by a cold, asymmetrically distributed, oxygen population, which is initially located away from the current layer in the inflow regions. A particle‐in‐cell simulation is used to gain further insight into the dynamics of the system. The time evolution of the reconnection process proceeds rapidly compared to the cyclotron period of O +. Therefore, the oxygen remains, to a good approximation, demagnetized. Thus, Alfvén scaling is not an adequate description of the reconnection rate. A scaling relation for the reconnection rate for an asymmetrically distributed, demagnetized species has been developed. Additionally, we find that an asymmetric density configuration leads to a distinct motion of the reconnection site and generates an asymmetry of the diffusion region and the Hall electric field. Alfvén scaling does not result in an adequate description of the reconnection rate due to the O +being demagnetizedScaling relation of reconnection rate for an asymmetrically distributed demagnetized species involves the average of the inflowing populationsSignificant asymmetry of the Hall electric field and the diffusion region

It is well established that a transverse (y) component in the interplanetary magnetic field (IMF) induces a Bycomponent in the closed magnetosphere through asymmetric loading and/or redistribution of magnetic flux. Simultaneous images of the aurora in the two hemispheres have revealed that conjugate auroral features are displaced longitudinally during such conditions. Although the direction and magnitude of this displacement show correlations with IMF clock angle and dipole tilt, single events show large temporal and spatial variability of this displacement. For instance, we know little about how the displacement changes during a substorm. A previous case study demonstrated that displaced auroral forms, associated with the prevailing IMF orientation, returned to a more symmetric configuration during the expansion phase of two substorms. Using the far ultraviolet cameras on board the Imager for Magnetopause‐to‐Aurora Global Exploration and Polar satellites, we have identified multiple events where conjugate auroral images are available during periods with substorm activity and IMF By≠0. We identify conjugate auroral features and investigate how the asymmetry evolves during the expansion phase. We find that the system returns to a more symmetric state in the events with a clear increase in the nightside reconnection rate and that the displacement remains unchanged in the events with little or no net closure of open magnetic flux. The return to a more symmetric state can therefore be interpreted as the result of increased reconnection rate in the magnetotail during the expansion phase, which reduces the asymmetric lobe pressure. Longitudinally displaced auroral features become more symmetric during substorm expansion phaseThere is a relationship between the change in asymmetry and the nightside reconnection rateThe observed reduction in asymmetry is consistent with Bybeing introduced into the closed magnetosphere by asymmetric pressure gradients