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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 (B$_{y}$) 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 Y$_{GSM}$direction, 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.

The solar wind influence on geospace can be described as the sum of a directly driven component, or dayside reconnection, and an unloading component, associated with the release of magnetic energy via nightside reconnection. The two processes are poorly correlated on short time scales, but exactly equal when averaged over long time windows. Because of this peculiar property, regression models of ionospheric electrodynamics that are based on solar wind data are time scale specific: Models derived from 1 min resolution data will be different from models derived from hourly, daily, or monthly data. We explain and quantify this effect on simple linear regression models of various geomagnetic indices. We also derive a time scale-dependent correction factor that can be used with the Average Magnetic field and Polar current System model. Finally, we show how absolute estimates of the nightside reconnection rate can be calculated from solar wind measurements and geomagnetic indices.

This paper describes a novel technique that allows separation and quantification of different sources of convection in the high-latitude ionosphere. To represent the ionospheric convection electric field, we use the Spherical Elementary Convection Systems representation. We demonstrate how this technique can separate and quantify the contributions from different magnetospheric source regions to the overall ionospheric convection pattern. The technique is in particular useful for distinguishing the contributions of high-latitude reconnection associated with lobe cells from the low-latitude reconnection associated with Dungey two-cell circulation. The results from the current paper are utilized in a companion paper (Reistad et al., 2019, https://doi.org/10.1029/2019JA026641) to quantify how the dipole tilt angle influences lobe convection cells. We also describe a relation bridging other representations of the ionospheric convection electric field or potential to the Spherical Elementary Convection Systems description, enabling a similar separation of convection sources from existing models.

We use the Lyon-Fedder-Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) $\mathit{B}_{y}$ component on the coupling between the solar wind and magnetosphere-ionosphere system when IMF $\mathit{B}_{z}$ $>$0. We describe the evolution of how a magnetospheric $\mathit{B}_{y}$ component 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 $\mathit{B}_{y}$ component. We show the radial dependence of the induced $\mathit{B}_{y}$ and compare the results to the induced $\mathit{B}_{y}$ during southward IMF conditions. We also show the response and reconfiguration time of the inner magnetosphere to IMF $\mathit{B}_{y}$ reversals during northward IMF $\mathit{B}_{z}$. 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 $\mathit{B}_{y}$ reversals is presented. We find that the induced $\mathit{B}_{y}$ responds within 16 min of the arrival of IMF $\mathit{B}_{y}$ at the bow shock, and it completely reconfigures within 47 min.

Statistical analyses have shown that the sunward component of the interplanetary magnetic field, $\mathit{B}_{x}$ (Geocentric Solar Magnetospheric), moderately but significantly affects the auroral intensity. These observations have been interpreted as signatures of a similar interplanetary magnetic field $\mathit{B}_{x}$ control 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 $\mathit{B}_{x}$ negative (positive) conditions in the Northern (Southern) Hemispheres than when $\mathit{B}_{x}$ is positive (negative). In this paper we perform a detailed investigation of three different sets of magnetic field measurements, from the Challenging Minisatellite Payload and Swarm low 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 $\mathit{B}_{x}$ changes the Birkeland currents by no more than $\approx$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 $\mathit{B}_{x}$ have the same sign than when they are different... (continued on https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2017JA024864)

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 $\mathit{B}_{y}$ interaction. 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.

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.

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.

Electric currents flowing through near-Earth space ($\textit{R}$ $\leq$12 $\mathit{R}_{E}$) can support a highly distorted magnetic field topology, changing particle drift paths and therefore having a nonlinear feedback on the currents themselves. A number of current systems exist in the magnetosphere, most commonly defined as (1) the dayside magnetopause Chapman-Ferraro currents, (2) the Birkeland field-aligned currents with high latitude "region 1" and lower-latitude "region 2" currents connected to the partial ring current, (3) the magnetotail currents, and (4) the symmetric ring current. In the near-Earth nightside region, however, several of these current systems flow in close proximity to each other. Moreover, the existence of other temporal current systems, such as the substorm current wedge or "banana" current, has been reported. It is very difficult to identify a local measurement as belonging to a specific system. Such identification is important, however, because how the current closes and how these loops change in space and time governs the magnetic topology of the magnetosphere and therefore controls the physical processes of geospace. Furthermore, many methods exist for identifying the regions of near-Earth space carrying each type of current. This study presents a robust collection of these definitions of current systems in geospace, particularly in the near-Earth nightside magnetosphere, as viewed from a variety of observational and computational analysis techniques. The influence of definitional choice on the resulting interpretation of physical processes governing geospace dynamics is presented and discussed.

The solar-wind magnetosphere interaction primarily occurs at altitudes where the dipole component of Earth's magnetic field is dominating. The disturbances that are created in this interaction propagate along magnetic field lines and interact with the ionosphere-thermosphere system. At ionospheric altitudes, the Earth's field deviates significantly from a dipole. North-South asymmetries in the magnetic field imply that the magnetosphere ionosphere-thermosphere (M-I-T) coupling is different in the two hemispheres. In this paper we review the primary differences in the magnetic field at polar latitudes, and the consequences that these have for the M-I-T coupling. We focus on two interhemispheric differences which are thought to have the strongest effects: 1) A difference in the offset between magnetic and geographic poles in the Northern and Southern Hemispheres, and 2) differences in the magnetic field strength at magnetically conjugate regions. These asymmetries lead to differences in plasma convection, neutral winds, total electron content, ion outflow, ionospheric currents and auroral precipitation.

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 $\textit{y}$ component 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.

Magnetic perturbations on ground at high latitudes are directly associated only with the divergence-free component of the height-integrated horizontal ionospheric current, $\textbf{J}_{\perp,df}$. Here we show how $\textbf{J}_{\perp,df}$ can be expressed as the total horizontal current $\textbf{J}_\perp$ minus its curl-free component, the latter being completely determined by the global Birkeland current pattern. Thus in regions where $\textbf{J}_\perp = 0$, 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 anti-parallel 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.

The magnetopause is a current sheet forming the boundary between the geomagnetic field on one side and the shocked solar wind on the other side. This paper discusses properties of the low-latitude dawn and dusk flanks of the magnetopause. The reported results are based on a large number of measurements obtained by the Cluster satellites during magnetopause traversals. Using a combination of single-spacecraft and multispacecraft techniques, we calculated macroscopic features such as thickness, location, and motion of the magnetopause. The results show that the typical flank magnetopause is significantly thicker than the dayside magnetopause and also possesses a pronounced and persistent dawn-dusk asymmetry. Thicknesses vary from 150 to 5000 km, with an median thickness of around 1400 km at dawn and around 1150 km at dusk. Current densities are on average higher on dusk, suggesting that the total current at dawn and dusk are similar. Solar wind conditions and the interplanetary magnetic field cannot fully explain the observed dawn-dusk asymmetry. For a number of crossings we were also able to derive detailed current density profiles. The profiles show that the magnetopause often consists of two or more adjacent current sheets, each current sheet typically several ion gyroradii thick and often with different current direction. This demonstrates that the flank magnetopause has a structure that is more complex than the thin, one-dimensional current sheet described by a Chapman-Ferraro layer.

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 $\textit{xz}$ plane. We present a statistical study of the average auroral response in the 12-24 magnetic local time (MLT) sector to an $\textit{x}$ component in the IMF. Maps of auroral intensity in both hemispheres for two IMF $\mathit{B}_{x}$ dominated conditions($ \pm $ IMF $\mathit{B}_{x}$) are shown during periods of negative IMF $\mathit{B}_{z}$, small IMF $\mathit{B}_{y}$, 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 $\mathit{B}_{x}$ dominated conditions in both hemispheres. In the Northern Hemisphere the aurora is brighter in the 15-19 MLT region during negative IMF $\mathit{B}_{x}$. In the Southern Hemisphere the aurora is brighter in the 16-20 MLT sector during positive IMF $\mathit{B}_{x}$. 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 $\mathit{B}_{x}$ cases 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.

We used the Lyon-Fedder-Mobarry global magnetohydrodynamics model to study the effects of the interplanetary magnetic field (IMF) IMF $\mathit{B}_{y}$ component on the coupling between the solar wind and magnetosphere-ionosphere system. When the IMF reconnects with the terrestrial magnetic field with IMF $\mathit{B}_{y}$ $\neq$ 0, flux transport is asymmetrically distributed between the two hemispheres. We describe how $\mathit{B}_{y}$ is 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 non-uniform induced $\mathit{B}_{y}$ in 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 $\textit {y}$ directed 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 $\mathit{B}_{y}$ produces 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.

Ground magnetic field measurements can be mathematically related to an overhead ionospheric equivalent current. In this study we look in detail at how the global equivalent current, calculated using more than 30 years of SuperMAG magnetometer data, changes with sunlight conditions. The calculations are done using spherical harmonic analysis in quasi-dipole coordinates, a technique which leads to improved accuracy compared to previous studies. Sorting the data according to the location of the sunlight terminator and orientation of the interplanetary magnetic field (IMF), we find that the equivalent current resembles ionospheric convection patterns on the sunlit side of the terminator but not on the dark side. On the dark side, with southward IMF, the current is strongly dominated by a dawn cell and the current across the polar cap has a strong dawnward component. The contrast between the sunlit and dark side increases with increasing values of the $\mathit{F}_{10.7}$ index, showing that increasing solar EUV flux changes not only the magnitude but also the morphology of the equivalent current system. The results are consistent with a recent study showing that Birkeland currents indirectly determine the equivalent current in darkness and that Hall currents dominate in sunlight. This has implication for the interpretation of ground magnetic field measurements and suggests that the magnetic disturbances at conjugate points will be asymmetrical when the solar illumination is different.