The geomagnetic condition dependence of the spatial distributions of EMIC waves observed by the Van Allen Probes

Abstract:

Electromagnetic ion cyclotron (EMIC) waves play an important role in the overall dynamics of the Earth’s magnetosphere, including the energization and loss of particles. We perform a statistical study of EMIC waves detected by the Van Allen Probes mission to investigate their spatial distributions under different geomagnetic activity levels, gauged by the AE index and dynamic pressure. Magnetic field measurements from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) onboard the Van Allen Probes are used to identify EMIC wave events from the first 22 months of the mission operation. EMIC waves are examined in H⁺-, He⁺- and O⁺-bands. Throughout the first 22 months of the Van Allen Probes mission, majority of the data coverage has been during quiet (AE ≤ 100) conditions. Results show that as AE increases, the spatial distributions of EMIC waves converges to the afternoon sector. Disturbed conditions yielded stronger H⁺- and He⁺-band EMIC waves. The afternoon sector featured higher occurrence rates for He⁺-band EMIC waves. Provided are also breakdowns by spectral properties versus L* under the different geomagnetic conditions and wave power thresholds. Examining solar wind dynamic pressure shows that most events occurred with a pressure < 4 nPa. A wave power versus pressure analysis under the different wave bands has also been conducted.