Quasi‐Periodic EMIC Waves and Pulsating Ionospheric Perturbations Related to ULF Waves

Pulsating proton auroras are often attributed to periodic proton precipitation. However, how the proton precipitation is periodically generated in the magnetosphere remains an open issue. Utilizing multi‐point space‐borne and ground‐based observations, this study proposed a potential mechanism responsible for pulsating proton precipitation and intermittent ionospheric electron density disturbances. On 8 September 2017, Pc4 ULF waves and electromagnetic ion cyclotron (EMIC) wave packets were simultaneously observed by Van Allen Probes (RBSP) in the inner magnetosphere. The EMIC wave packets were quasi‐periodically excited at the same frequency as the ULF waves, which resulted in 30–100 keV proton precipitation detected by Low‐Earth‐Orbit (LEO) POES satellites. Meanwhile, conjugate European Incoherent Scatter (EISCAT) radar on the ground observed E‐region electron density enhancements that intermittently appeared nearly at the same frequency as the EMIC wave packets in space. These observations together suggest that ULF waves in the magnetosphere are the ultimate driver that modulates quasi‐periodic EMIC waves to induce proton precipitation and pulsating disturbances in the ionosphere.