Embedded Regions 1 and 2 Field‐Aligned Currents: Newly Recognized From Low‐Altitude Spacecraft Observations

Abstract

AbstractRegions 1 and 2 (R1 and R2) field‐aligned currents (FACs), manifestations of large‐scale convection in Earth's magnetosphere‐ionosphere (M‐I) system, often contain intense FAC layers of mesoscale latitudinal width near the R1/R2 interface. We refer to such layers as “embedded” R1 and R2 FACs. Likely resulting from enhanced magnetosphere‐ionosphere (M‐I) convection, these FACs may indicate M‐I configuration change and contribute significantly to substorm current wedges. We present several events in which embedded FACs were observed by low‐altitude spacecraft in the ionosphere. All the events occurred during active geomagnetic conditions or a substorm growth phase, and most map to an equatorial location on the nightside. When an embedded FAC is upward, it coincides with inverted‐V electron precipitation and a discrete auroral arc. If an upward embedded FAC is in the postmidnight‐to‐dawn sector, a dawnside auroral polarization stream appears immediately poleward of it, so it may be important for ionospheric heating, M‐I convection, and instabilities. Our results establish embedded FACs as a frequently appearing, fundamental phenomenon for understanding and modeling the magnetosphere‐ionosphere system.