Field-aligned currents are strongly modulated by corotating solar wind high-speed streams

Birkeland field-aligned currents (FACs) are associated with solar wind-magnetosphere energy coupling leading to substorms/convection events. FAC intensity enhancements have been studied during geomagnetic storms driven by interplanetary coronal mass ejections and corotating interaction regions. Here we present an in-depth analysis of long-term FAC variation, strongly modulated by solar wind high-speed streams (HSSs) emanated from solar coronal holes. Variations of both dayside and nightside FAC intensities during June 2016 through June 2017 are characterized by prominent periodicities of ~14, ~26, and ~29 days (in descending order of amplitude), highly correlated to empirically estimated rate of magnetic flux opening at the dayside magnetopause. The periodicities represent the ~27-day solar rotation period and its harmonic, resulting from combined impacts of recurring HSSs emanated from multiple long-lived coronal holes, corotating with the Sun. Modulation of the long-term FAC intensity variation by recurring HSSs is confirmed by statistical regression, cross-wavelet and coherence analyses. These results may be useful in developing FAC prediction models.