A Comprehensive Analysis of Multiscale Field-Aligned Currents: Characteristics, Controlling Parameters, and Relationships

We explore the characteristics, controlling parameters, and relationships of multiscale field-aligned currents (FACs) using a rigorous, comprehensive, and cross-platform analysis. Our unique approach combines FAC data from the Swarm satellites and the Advanced Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) to create a database of small-scale (similar to 10-150 km, < 1 degrees latitudinal width), mesoscale (similar to 150-250 km, 1-2 degrees latitudinal width), and large-scale (> 250 km) FACs. We examine these data for the repeatable behavior of FACs across scales (i.e., the characteristics), the dependence on the interplanetary magnetic field orientation, and the degree to which each scale "departs" from nominal large-scale specification. We retrieve new information by utilizing magnetic latitude and local time dependence, correlation analyses, and quantification of the departure of smaller from larger scales. We find that (1) FACs characteristics and dependence on controlling parameters do not map between scales in a straight forward manner, (2) relationships between FAC scales exhibit local time dependence, and (3) the dayside high-latitude region is characterized by remarkably distinct FAC behavior when analyzed at different scales, and the locations of distinction correspond to " anomalous" ionosphere-thermosphere behavior. Comparing with nominal large-scale FACs, we find that differences are characterized by a horseshoe shape, maximizing across dayside local times, and that difference magnitudes increase when smaller-scale observed FACs are considered. We suggest that both new physics and increased resolution of models are required to address the multiscale complexities. We include a summary table of our findings to provide a quick reference for differences between multiscale FACs. Plain LanguageSummary Multiscale processes, or those which contain important features across multiple scales in time and/or space, characterize the near-Earth space environment. Multiscale effects are particularly important to understand interactions between regions, where numerous processes contribute to the dynamics. In the solar wind-magnetosphere-ionosphere system, currents flowing along Earth's magnetic field lines, or field-aligned currents (FACs), provide the dominant form of energy and momentum exchange. FACs are, therefore, a critical component of the space weather environment and are inherently multiscale. Though much progress has been made toward understanding FACs at large spatial scales (> 250 km), smaller-scale FACs and relationships between scales are not well understood. We take advantage of two different satellite missions to explore the repeatable spatial behavior (i.e., the characteristics) of FACs across scales, the dependencies on solar wind conditions, and the degree to which each scale differs from nominal large-scale FACs. We specify differences in the spatial patterns of multiscale FACs and connect particularly strong differences at dayside local times to previously identified irregular space weather activity. We find that the relationships between scales are complex. We provide a summary table of our key findings and suggest areas of future work that will contribute to new multiscale understanding.