The Characteristics of EMIC Waves in the Magnetosphere Based on the Van Allen Probes and Arase Observations

We performed a comprehensive statistical study of electromagnetic ion cyclotron (EMIC) waves observed by the Van Allen Probes and Exploration of energization and Radiation in Geospace satellite (ERG/Arase). From 2017 to 2018, we identified and categorized EMIC wave events with respect to wavebands (H+ and He+ EMIC waves) and relative locations from the plasmasphere (inside and outside the plasmasphere). We found that H+ EMIC waves in the morning sector at L > 8 are predominantly observed with a mixture of linear and right‐handed polarity and higher wave normal angles during quiet geomagnetic conditions. Both H+ and He+ EMIC waves observed in the noon sector at L ∼ 4–6 have left‐handed polarity and lower wave normal angles at |MLAT| 8 during geomagnetic quiet conditions. Based on distinct characteristics at different EMIC wave occurrence regions, we suggest that EMIC waves in the magnetosphere can be generated by different free energy sources. Possible sources include the freshly injected particles from the plasma sheet, adiabatic heating by dayside magnetospheric compressions, suprathermal proton heating by magnetosonic waves, and off‐equatorial sources. Key Points Electromagnetic ion cyclotron (EMIC) waves in the magnetosphere have four major occurrence regions excited by possibly different generation processes For He‐EMIC waves in the afternoon sector, injected particles and off‐equatorial sources are the major driver at L ∼ 6 and L > 8, respectively H‐EMIC waves are generated by enhancing dynamic pressure in the noon sector at L 8