Comparison of EMIC wave observation and modeling under different geomagnetic activities

Electromagnetic (EMIC) waves are known to be excited by the cyclotron instability associated with hot and anisotropic ion distributions in the equatorial region of the magnetosphere during geomagnetic storms and substorms. Over the years, many studies have investigated EMIC waves and their relationship with geomagnetic storms, focusing on when, where, and under what conditions EMIC waves are detected. However, previous ground based studies have primarily focused only on when EMIC waves are detected. We address here both where and under what conditions EMIC waves are observed at the ground. We present space-ground conjunction events that show EMIC wave occurrence peaks around 12 MLT at the ground while peaking 16 MLT at geosynchronous orbit. It is also shown that EMIC waves were less likely to propagate into the ionosphere with higher Kp indices. We compare these observations with wave full solutions employing a finite element code developed at the Princeton Plasma Physics Laboratory. The code describes a three-dimensional wave structure including mode conversion when ULF, EMIC, and whistler waves are launched in a two-dimensional axisymmetric background plasma with general magnetic field topology. By adopting a realistic magnetospheric and ionospheric density structure, we examine the spatial and temporal features of EMIC waves in the inner magnetosphere. Using the model, we examine how EMIC waves propagate in the magnetosphere and reach the ionosphere after the waves are launched in space where they are typically observed. We also investigate how changes in the He+ and/or O+ density that result from the ion outflows would affect the accessibility of the waves to the ground.