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. Plain Language Summary Pulsating proton auroras are a kind of auroral emission with a periodicity of tens of seconds. Although proton aurora has been known to be generally caused by energetic protons (at energies of tens of kiloelectron volts) precipitating from space into the upper atmosphere, the generation mechanism of its periodicity is still not well understood. This study explored a case study using various observations in space and on ground to analyze the magnetospheric source and ionospheric responses. It is found that the ultra‐low‐frequency (ULF) waves in the magnetosphere are able to modulate and trigger quasi‐periodic higher‐frequency waves called electromagnetic ion cyclotron (EMIC) waves. Furthermore, direct evidence is found in the electron density at about 120–140 km altitude, where proton aurora would occur, experiences intermittent disturbances, in good correlation with the quasi‐periodic EMIC waves. Therefore, we have found a causal link from the ULF waves to quasi‐periodic EMIC waves that subsequently induce pulsating ionospheric disturbances, an indication of pulsating proton precipitation and pulsating proton aurora. Key Points We demonstrate quasi‐periodic EMIC waves modulated by ULF waves and subsequent pulsating disturbances in the ionosphere Ionosphe