Statistical Study of EMIC Wave-Related Electron Precipitation at Subauroral Latitude

Electromagnetic ion cyclotron (EMIC) waves are excited near the equatorial plane by anisotropic ring current ions. They mainly propagate along magnetic field lines and are observed on the ground as Pc1 geomagnetic pulsations. Pitch angle scattering of relativistic electrons by interaction with EMIC waves has been considered as one of the mechanisms to cause the loss of the outer radiation belt electrons. However, there are still questions left about the contribution of EMIC waves to the overall loss of radiation belt electrons and possible conditions favorable to pitch angle scattering. Here, we performed a statistical study about EMIC waves and associated high-energy electron precipitation from November 2016 to June 2018. We used a ground-based magnetometer and receiver for man-made VLF radio waves installed at Athabasca, Canada. VLF radio waves propagate from transmitters to receivers in the earth-ionosphere wave guide. They are sensitive to ionization changes in the night time lower ionosphere due to precipitating electrons with energies higher than 100 keV. Using this method, we identified electron precipitation that occurred simultaneously with the EMIC wave activity. In the period of analysis, simultaneous observation of the magnetometer and VLF radio waves were available for 286 days, and we identified 162 EMIC wave events. We found that 19 of 162 EMIC wave events were clearly associated with high-energy electron precipitation (12%). Although occurrence of EMIC waves on the duskside (83 events) is almost the same as on the dawnside (79 events), electron precipitation occurred more often on the duskside (15 events) than dawnside (4 events). We also found that the preferential condition for electron precipitation occurs during the main phase of geomagnetic storms.