Revealing the EMIC Wave Frequency Differences in the Ionosphere via Coordinated Observations: A Case Study

We study electromagnetic ion cyclotron (EMIC) waves based on observations from the ionosphere, magnetosphere, and ground during a geomagnetic storm recovery phase on 28 August 2018. In this case, multiple ducting EMIC waves in the ionosphere show higher frequencies in the post‐midnight than those in the pre‐midnight. Ionospheric EMIC wave frequency differences in magnetic local time (MLT) are consistent with MLT frequency differences in the equatorial magnetosphere, which are mainly caused by different background magnetic field at different L‐shells. Moreover, we report the first observation of frequency range selections in ionospheric ducting EMIC waves and find that frequency selections depend on the magnetic field intensity in the main part of the ionospheric waveguide, with higher frequency corresponding to larger magnetic field. This study reveals the important role of background magnetic field in regulating ducting EMIC wave frequencies in the ionosphere. Plain Language Summary Electromagnetic ion cyclotron (EMIC) waves are in the frequency range of 0.2–5 Hz, and they may occur in three different bands, which are named as H+, He+, and O+ bands from high to low frequencies. In this paper, He+ band EMIC waves in the nightside during a geomagnetic storm are observed by satellites in the ionosphere, and their frequencies are different, that is, frequencies are higher in the post‐midnight than those in the pre‐midnight. Ionospheric EMIC wave frequency differences in local time have been noticed before but have not yet been explained. We discover that ionospheric EMIC wave frequency differences in local time agree well with frequency differences in the magnetosphere. Besides frequency discrepancies in local time, in the post‐midnight, ionospheric EMIC waves tend to select higher or lower frequency range relative to frequencies in the magnetosphere. We find that these frequency selections mainly depend on the magnetic field intensity in the ionosphere. The frequency selections in ionospheric EMIC waves have not been identified before, and it is important to understand variations of EMIC waves in the ionosphere. Key Points Ionospheric EMIC wave frequency differences in magnetic local time (MLT) are consistent with MLT frequency differences in the equatorial magnetosphere Higher/lower wave frequencies in the equatorial magnetosphere are at lower/higher L‐shells with larger/smaller background magnetic field Frequency selections of ducting EMIC waves depend on