Excitation of Storm‐Time Pc5 ULF Waves During the 22 July 2009 Storm: Comparison of GOES, Ground Observations, and GEMSIS Coupled Simulation

Abstract Internally driven ULF waves are excited in the premidnight region during the magnetic storm on 22 July 2009. The excitation of ULF waves was investigated based on the Geospace Environment Modeling System for Integrated Studies (GEMSIS) magnetosphere‐ionosphere coupled model, GOES spacecrafts, and ground magnetometers. In order to apply the GEMSIS coupled model to the magnetic storm, we included solar wind conditions and loss term of ring current ions due to charge exchange in the model. During the magnetic storm on 22 July 2009, the SYM‐H index dropped to nearly −90 nT at 06:00 and 09:00 UT. Poloidal ULF waves were observed by GOES satellites and ground stations from 02:50 to 03:40 UT in the premidnight region. Wave frequency of observed ULF waves was in the range of 1.3–1.8 mHz, and the m number was relatively small, ranging from −3 to −6, propagating westward. ULF waves are also generated in the model from 02:50 UT in the premidnight region. Wave frequency and the m number are similar between the model and observations. Simulation results suggest that the waves are generated by the drift resonance with ring current ions at 50–200 keV and inward gradient of ion PSD contributes to the wave growth. These results indicate that the GEMSIS coupled model is capable of providing the excitation mechanism and global distribution of observed ULF waves.