We directly estimate the in situ current density of the Earth’s ring current (RC) using the curlometer method and investigate its morphology using the small spatial separations and high accuracy of the Magnetospheric Multiscale mission (MMS). Through statistical analysis of data from September 2015 to the end of 2016, covering the region of 2-8 RE (Earth radius, 6371 km), we reveal an almost complete near-equatorial (within ) RC morphology in terms of radial distance and local time (MLT) which complements and extends that found from previous studies. We found no evidence of RC enhancement on the dusk-side during geomagnetic active periods, but details of local time (MLT) asymmetries in, and the boundary between, the inner (eastward) and outer (westward) currents are revealed. We propose that part of the asymmetry demonstrated here suggests that in addition to the overall persistence of the westward RC, two large banana-like currents are directly observed, one which could arise from a peak of plasma pressure near ~4.8 RE on the noon side and the other from a valley of plasma pressure which could arise near ~4.8 RE on the night side.

The Earth's ring current (RC) plays an important role in the inner magnetosphere as it has a dominant effect on the Earth's geomagnetic field. It is therefore very important to study its distribution to better understand the inner magnetospheric current system and its dynamics. The RC is primarily formed by charged particles with energies from ∼1 keV to hundreds of keV, captured by the Earth's magnetic field, which have gyration and drift motion caused by magnetic gradients and curvature (Parker, 1957). The RC typically lies between 2 and 8 R E and previously has been found to be reasonably well ordered within ±30° latitude. During magnetic storms, ionospheric particles may inject into the RC, dominating its morphology and dynamics (Daglis et al., 1999).Preceding work has shown a pronounced asymmetry and other effects through the analysis of a variety of observation data, often related to the effects of the RC in adjacent regions. For example, Le and Russell (1998) pointed out that residual magnetic field measurements from low-altitude polar orbiting spacecraft are potentially useful as monitors of RC strength when they cross the polar cap. Le et al. (2011) studied its asymmetry by analyzing the influence on the distribution of the residual normal components of the magnetic field at LEO (low Earth orbit). In situ magnetic data, covering many years and a number of earlier missions, were used by Le et al. (2004); Jorgensen (2004) estimated the RC distribution for different levels of geomagnetic activity. These studies, however, have generally used only single spacecraft to extract components of the current density, J (so that assumptions have to be made to obtain J from magnetic residuals) and indeed discrepancies exist between the morphology implied from low altitudes (to which other influences may contribute, such as field-aligned currents, FACs) and that seen in situ. The phased multispacecraft arrays of Cluster and THEMIS allowed direct estimates

<p>The Earth’s ring current forms a complex current system at the boundary of the inner magnetosphere. It is highly dynamic because of the interaction between the solar wind with the Earth's magnetosphere (the influence of space weather), while its morphology depends on the nature of the magnetospheric-ionospheric (M-I) coupling, generating field-aligned currents (FACs). Its behaviour can therefore have a huge impact on the terrestrial environment. According to Ampere's law, these currents can be directly measured by perturbations in the magnetic field using multi-spacecraft observation techniques. We have analyzed the magnetic field data from the four MMS spacecraft in their small-sale configuration to obtain the in-situ current density and have carried out statistical analysis from several years of data. The form of the current density distribution and its changing nature has been investigated. Our results show that the current density exhibits a three-dimensional layered structure in the ring current region. The significant westward current on the day side flows to higher magnetic latitudes and complete closure there rather than to the magnetic equator. There are some differences between geomagnetic quiet period and storm period on current density, but the basic spatial structure remains similar and compares well with previous space mission data. Comparison with Swarm data at low Earth altitudes, we found that the stratification is consistent with the distribution of the R2 field-aligned currents seen both adjacent to the ring current and at ionospheric altitudes (at Swarm). In addition, significant continuous eastward currents exist in some latitudes and some regions, indicating the complexity of the ring current. Some of them can be explained by the formation of banana currents.</p>

<p>We review the range of applications and use of the curlometer, initially developed to analyze electric current density using Cluster multi-spacecraft magnetic field data; but more recently adapted to other arrays of spacecraft flying in formation, such as MMS small-scale, 4-spacecraft configurations; THEMIS close constellations of 3-5 spacecraft, and Swarm 2-3 spacecraft configurations. The method (and associated methods based on spatial gradients) has been shown to be easily adaptable to other multi-point and multi-scale arrays. Although magnetic gradients require knowledge of spacecraft separations and the magnetic field, the structure of the electric current density (for example, its relative spatial scale), and any temporal evolution, limits measurement accuracy. Nevertheless, in many magnetospheric regions the curlometer is reliable (within certain limits), particularly under conditions of time stationarity, or with supporting information on morphology (for example, when the geometry of the large scale structure is expected). A number of large-scale regions have been investigated directly, such as: the cross-tail current sheet, ring current, the current layer at the magnetopause and field-aligned currents. In addition, the analysis can support investigations of transient and smaller scale current structures (e.g. reconnected flux tubes, boundary layer sub-structure, or dipolarisation fronts) and energy transfer processes. The method is able to provide estimates of single components of the vector current density, even if there are only two or three satellites flying in formation, within the current region, as can be the case when there is a highly irregular spacecraft configuration. The computation of magnetic field gradients and topology in general includes magnetic rotation analysis and various least squares approaches, as well as the curlometer, and indeed the combination with plasma measurements and the extension to larger arrays of spacecraft have recently been considered. We touch on these extensions and on new methodology accessing the properties of the underlying formulism.</p>

<p>The ring current and field-aligned currents have a dominant effect in the inner magnetospheric current system, where the coupling mechanism to the ionosphere has a significant impact on space weather and magnetic activity on the ground. We use FGM data from the four spacecraft MMS mission to estimate <em>in situ</em> current density and study the distribution of the ring current, by statistically analyzing the azimuth component of current density. We also compare the statistical results of field-aligned component of current density from MMS with the statistical results of dual-FAC data of Swarm in the same time range to explore the large-scale coupling of the ring current and field-aligned currents. The joint observational results show that: on the one hand (on the dawn side), due to the injection of Region 2 field-aligned currents, the westward ring current increases continuously from noon to midnight; while (on the dusk side, from midnight to noon), due to the injection of Region 1 field-aligned current near midnight and the outflow of Region 2 field-aligned currents, the westward ring current increases, then decreases until it turns eastward and increases in magnitude. On the other hand, due to the existence of a mesoscale, clockwise banana current centered at about 4.8Re (Earth Radii) between noon and dusk, the eastward ring current is significantly enhanced in the inner ring and enhanced westward in the outer ring. Similarly, due to the existence of a mesoscale counterclockwise current centered at about 4.8Re between midnight and dawn, the westward ring current is slightly enhanced in the inner ring and enhanced eastward in the outer ring. The influence of these two factors on the distribution of the ring current is of the same order of magnitude. Work to clarify the operation of these effects relative to activity levels is in progress.</p>