Electromagnetic Ion Cyclotron Waves in a Magnetic Reconnection Exhaust at Earth's Magnetopause

Abstract Plasma waves can initiate, regulate, or reflect magnetic reconnection efficiently converting magnetic energy into plasma energy. While waves ranging from below the ion cyclotron frequency to above the electron plasma frequency are commonly observed near reconnection sites, electromagnetic ion cyclotron (EMIC) waves—frequent in other plasma environments—have been rarely observed in the reconnection region. Here, we report the first detection of EMIC waves in a magnetic reconnection exhaust at Earth's magnetopause. The free energy required for EMIC wave growth was supplied by the strong perpendicular‐to‐parallel temperature anisotropy of hot proton beams. This proton temperature anisotropy was generated by magnetopause reconnection, rather than inherited from the magnetosheath. Our findings differ from previous reports of parallel‐preferential proton heating during magnetopause reconnection, calling for revised theoretical frameworks to reconcile observed perpendicular‐preferential heating with established reconnection paradigms.