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

Abstract

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 the 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 the 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.