Local Time and Hemispheric Asymmetries of Ionospheric Electromagnetic Ion Cyclotron Waves During May 2024 Superstorm Periods

Abstract This study investigates the spatiotemporal evolution of Electromagnetic ion cyclotron (EMIC) waves across various local time sectors in both hemispheres during the May 2024 geomagnetic storm, utilizing observations from Swarm A and B satellites. Electromagnetic ion cyclotron waves were detected in the northern dawn and midday sectors during both the growth and late recovery phases, while primarily emerging in the northern dusk/midnight sectors and the Southern Hemisphere during the main and early recovery phases. The poleward expansion of the Equatorial Ionization Anomaly (EIA) crests was especially pronounced in the dusk sector. During the expansion and main recovery phases, waves were predominantly confined to subauroral regions. However, during the main and early recovery phases, wave activity shifted to the low‐latitude ionosphere near or below the EIA crests. Seven O‐band EMIC waves originated from regions below 1,000 km altitude. Defense Meteorological Satellite Program measurements documented a substantial enhancement of O + ion concentrations in the magnetic equatorial region during the storm's main phase, underscoring the significant role of ionospheric O + in EMIC wave generation. Moreover, the extensive propagation in the midday and dusk sectors demonstrated a strong dependency on plasma density structures, with sharp decreases in local electron density effectively halting equatorward wave transmission.