Field Aligned Currents and Auroral Precipitation During the Terrestrial Alfven Wing State

Abstract

When sub-Alfv\'enic (Alfv\'en Mach number MA < 1) plasmas impact Earth, Alfv\'en wings (AWs) develop. A Multiscale Atmosphere Geospace Environment (MAGE) simulation of the April 2023 storm, validated by Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) data, reveals the field-aligned-current (FAC) generation mechanism and predicts auroral precipitation for Earth's AWs. Simulation and observations show northern hemisphere planetward flowing electrons are predominantly at magnetic local times (MLTs) 8-13. Before the AWs formed, solar wind conditions were similar and MA ~ 1.4, yet the same FAC system extended from 9-18 MLT. Flow vorticity drives FACs at the boundary of the AWs and unshocked solar wind. The AW shape presents a different obstacle to the solar wind compared to typical lobe fluxes, producing the unique FAC distribution. New insights about AW FACs and precipitating electron energy flux will help understand auroral features for exoplanets inside their host star's Alfv\'en zone.