Geospace III: Auroras and Magnetosphere-Ionosphere coupling

We explore the capability of Swarm-Cluster coordination for probing the behavior of the field aligned currents (FAC) adjacent to the ring current (RC) at medium and low orbits and show statistical analysis of the local time variation of R1/R2 FACs. The RC and connecting R2 FACs influence the geomagnetic field at low Earth orbit (LEO) and are sampled in situ by the four Cluster spacecraft. Coordination of the configuration of three Swarm spacecraft configurations with the constellation of the four Cluster spacecraft is possible; providing a set of distributed, multi-point measurements covering this region. Particular events showing close coordination of all spacecraft are considered during the start of the Swarm operations. We report here preliminary results of joint signatures of R1 and R2 FACs and demonstrate the use and application of new analysis techniques derived from the calculation of curl B and magnetic gradients to compare estimates of the current distributions. Multi-spacecraft analysis can access perpendicular currents associated with the FAC signatures at the Swam locations. For context, we identify the associated auroral boundaries determine from FAC intensity gradients in order to help interpret and resolve the R1 and R2 FACs. We also show preliminary results of an extended survey of the ring current crossings for different years, using estimates of the local current density, field curvature and total current

Prominent features of the Earth's auroral oval are large-scale sheets of magnetic field-aligned currents (FACs). The imbalance between upward and downward FAC has to be fed by horizontal Hall currents in the ionosphere and by interhemispheric currents. FAC imbalance is thus an indicator of ionospheric electrodynamics and global current systems. This study aims at an automatic approach to estimate the imbalance of upward and downward FACs from Swarm and CHAMP magnetic measurements. For every satellite crossing of the auroral oval, minimum variance analysis is applied to yield the incidence angle and a current sheet planarity measure derived from eigenvalue ratios. Auroral oval crossings with stable magnetic fields before inbound and after outbound are selected to study the magnetic component in the maximum variance direction. The differences of this component between inbound and outbound are in the range of a few tens of nT, corresponding to typical sheet current densities of few tens of A/km. Statistical studies are carried out to analyze the dependence on local time and seasonal variations.

The Electric Field Instrument (EFI) on each SWARM satellite consists of a pair of thermal ion imagers and a pair of Langmuir probes that provide information about ion drifts, ion and electron temperatures and electron density along the satellites’ high-inclination orbits at about 500 km altitude. In an effort to validate these measurements, we select cases when the Swarm satellites pass close to low and mid-latitude incoherent scatter radar sites and compare the EFI parameters with their corresponding ground-based observations. In addition, in order to improve the accuracy of the EFI data calibrations, we study the applicability of physics-based modeling that enables us to combine various in situ measured quantities and obtain estimates of thermospheric and ionospheric parameters.

Using coordinated magnetic conjunctions between the CARSIMA ground-based magnetometer array and the Swarm satellites we examine the role of high frequency (0.1-5 Hz) ultra-low frequency (ULF) waves in magnetosphere-ionosphere coupling.  Exploiting the unique capabilities of the Swarm  data, we investigate the impacts of both electromagnetic ion cyclotron (EMIC) waves and waves trapped in the ionospheric Alfven resonator (IAR) on magnetosphere-ionosphere coupling. Significantly, we utilise the multi-point capabilities of subsequent close passes of Swarm A and C to investigate the spatio-temporal structure of the magnetic perturbations, examining their relationship to field-aligned currents (FACs) and the dynamical exchange of Alfven waves between the magnetosphere and ionosphere.  In addition to this scientific focus, we also investigate the impacts of these disturbances and on FAC data products from the Swarm mission. Based on the results, our ultimate goal is to also investigate concepts for formulation(s) of potential new Swarm FAC data products which address the challenges associated with spatio-temporal ambiguity, including those arising from ULF waves, in the frame of the Swarm satellites.

The European Space Agency Swarm satellite mission began with all three Swarm satellites in similar, noon-to-midnight polar orbits. The pearls-on-a-string orientation of the satellites during the early part of the mission (December 2013) provide multiple measurements of similar volumes of space taken minutes apart, providing confirmation of observed features, and reducing the spatio-temporal ambiguity intrinsic to in-situ measurements. We will be presenting electric field, magnetic field, electron density, electron temperature, and ion temperature measurements from the Swarm satellites from December 2013. The time period for this set of data is characterized by quiet geomagnetic conditions. Swarm measurements combined with ground-based optical and magnetometer measurements provide a consistent description of ionospheric and field-aligned currents near midnight during quiet geomagnetic conditions. Relationships between the Birkland currents, the electrojets, auroral precipitation and narrow regions of enhanced electric fields have all been studied pairwise previously. We present a synthesis interpretation of Swarm and ground-based measurements to describe a consistent picture of auroral current structure near midnight.

This work is supported by a grant from the Canadian Space Agency