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We determine the primary modes of field‐aligned current (FAC) variability and their hemispheric asymmetry by nonlinear regression analysis of a multiyear global data set of Iridium constellation engineering‐grade magnetometer data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment program. The spatial and temporal FAC variability associated with three major categories of solar wind drivers, (1) slow flow, (2) high‐speed streams (HSS), (3) transient flow related to coronal mass ejections (CMEs), and (4) a combination of these, is characterized as empirical orthogonal functions (EOFs) and their time‐varying amplitude. For the combined solar wind category, the order of the modes of variability are strengthening/weakening of (1) EOF1—all FACs; (2) EOF2—Region 2 (R2) FACs; and (3) EOF3—dayside/nightside FACs. The first two EOFs are associated with solar wind coupling; EOF3 is associated with the ecliptic components of the interplanetary magnetic field (IMF). We also find hemispheric asymmetry in FACs. Northern Hemisphere EOFs show clearer spatial features and higher correlation coefficients with solar wind drivers. The Northern Hemisphere also shows higher correlation coefficients in all seasons except winter. We find transient flow EOFs to be better correlated with solar wind drivers such as IMFB_zand coupling functions, while HSS EOFs are better correlated with solar wind plasma parameters. CME‐related transient flow EOFs also show R2 FAC variabilities that are not found in other separate wind drivers. Application of the EOF analysis to the Iridium magnetometer data shows significant promise for greater understanding of geoeffectiveness of solar wind interactions with geospace.

 

We present examples of high‐latitude field‐aligned current (FAC) and toroidal magnetic potential patterns in both hemispheres reconstructed at a 2‐min cadence using an updated optimal interpolation (OI) method that ingests magnetic perturbation data provided by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) program. A solstice and an equinoctial event are studied to demonstrate the reconstructed patterns and to provide scientific insights into FAC response to different solar wind drivers. For the 14 June 2011 high‐speed stream event with mostly northwardB_zdriving, we found persistently stronger FACs in the Northern Hemisphere. Extreme interhemispheric asymmetry is associated with the interplanetary magnetic field (IMF) direction and large dipole tilt, consistent with earlier studies. FAC asymmetries seen during an isolated substorm can be attributed to dipole tilt. During relatively low geomagnetic activity, the FAC response to IMFB_xchanges is identified. For the 17–18 March 2013 period, we provide global snapshots of rapid FAC changes related to an interplanetary shock passage. We further present comparisons between instantaneous and mean behaviors of FAC for the solar wind sheath passage and interplanetary coronal mass ejection southwardB_zinterval and northwardB_zintervals. We show that (1) sheath passage results in strong FAC and high variation in the dayside polar cap region and pre‐midnight region, different from the typical R1/R2 currents during prolonged southwardB_z; (2) four‐cell reverse patterns appear during northwardB_zbut are not stable; and (3) persistent dawn‐dusk asymmetry is seen throughout the storm, especially during an extreme substorm, likely associated with a dawnside current wedge.

 

Polar ionospheric electrodynamics plays an important role in the Sun–Earth connection chain, acting as one of the major driving forces of the upper atmosphere and providing us with a means to probe physical processes in the distant magnetosphere. Accurate specification of the constantly changing conditions of high-latitude ionospheric electrodynamics has long been of paramount interest to the geospace science community. The Assimilative Mapping of Ionospheric Electrodynamics procedure, developed with an emphasis on inverting ground-based magnetometer observations for historical reasons, has long been used in the geospace science community as a way to obtain complete maps of high-latitude ionospheric electrodynamics by overcoming the limitations of a given geospace monitoring system. This Chapter presents recent technical progress on inverse and data assimilation procedures motivated primarily by availability of regular monitoring of high-latitude electrodynamics by space-borne instruments. The method overview describes how electrodynamic state variables are represented with polar-cap spherical harmonics and how coefficients are estimated from the point of view of the Bayesian inferential framework. Some examples of the recent applications to analysis of SuperDARN plasma drift, Iridium, and DMSP magnetic fields, as well as DMSP auroral particle precipitation data are included to demonstrate the method.