Amplitude-Dependent Properties and Excitation Mechanisms of EMIC Waves in the Earth's Inner Magnetosphere

Large-amplitude (B-w > 1 nT) electromagnetic ion cyclotron (EMIC) waves can cause the rapid loss of >1 MeV electrons, greatly impacting radiation belt dynamics. With long-term Van Allen Probe B observations from 2013 to 2018, we conducted a statistical survey to reveal the amplitude-dependent EMIC wave properties and excitation mechanisms in the Earth's inner magnetosphere. Statistical results show that large-amplitude EMIC waves prefer to occur in the afternoon-dusk sector in the northern hemisphere and tend to be more left-hand polarized with smaller wave normal angles. In addition, the high proton beta parallel conditions also favor the generation of larger-amplitude EMIC waves. From the variations of EMIC wave occurrence rate as a function of SuperMAG electrojet (SME) index and solar wind dynamic pressure, we find that the small-amplitude EMIC waves are generally triggered by high solar wind dynamic pressure, while large-amplitude EMIC wave generation is both affected by substorm activity and solar wind dynamic pressure. The normalized magnetic field perturbations during EMIC wave appearance, which enable us to distinguish the relative roles of magnetospheric compression and substorm injection in the excitation of different-amplitude EMIC waves, provide further evidence that as wave amplitude increases, substorm injection plays a more important role in EMIC wave excitation, and magnetospheric compression is also an indispensable trigger.