On the Generation Mechanism of Externally Driven EMIC Waves

It has been well known that EMIC waves are excited by free energy sources associated with temperature anisotropy. Recent studies indicate that EMIC waves are found even when theres is no such corresponding sources confirmed along the magnetic flux tube. In this study, we investigate how EMIC waves are generated differentially by external sources in the inhomogeneous magnetosphere by adopting the simulation models [e.g., Kim et al., 2008] as well as their corresponding analytical calculations (IIM: Invariant Imbedding Method) in an exact manner. Our results are as follows: 1) the presence of EMIC waves in the inner magnetosphere is mainly controlled by time-dependent characteristics of the sources, 2) if there are relatively rapid variations comparable to the ion gyro periods, even during the very short interval, it is well confirmed to observe the generation of EMIC waves, 3) if the source variation is assumed to be of arbitrary shapes, we found that the EMIC waves tend to occur more frequently together with MHD waves such as field line resonances rather than the isolated EMIC events, and 4) the EMIC occurrences with relatively large amplitude are strongly associated with the local inhomogeneity profile in the sense that dominant field-aligned EMIC waves often appear at the frequencies corresponding to local multi-ion hybrid resonances near the low-latitude region. We present the details about this significant role of external sources for the EMIC excitation, and suggest how such sources are identified in realistic observations.