Alfvén Waves Generated Through the Drift‐Bounce Resonant Instability in the Ring Current: A THEMIS Multi‐Spacecraft Case Study

Abstract

AbstractThe satellites THA, THD, THE of THEMIS mission (Time History of Events and Macroscale Interactions during Substorms) have consistently recorded quasi‐monochromatic poloidal oscillations of the Pc4‐5 range in the nighttime magnetosphere. Wave activity was recorded after a substorm in the ring current region, between L‐shells 6 and 7.3. According to the data of the THA satellite, the fundamental and second harmonics of Alfvén wave were simultaneously recorded at frequencies of about 7 and 18 mHz, respectively. The satellites THD and THE recorded only the second harmonic and oscillating modulations of proton fluxes with energies of about 50 keV at the harmonic frequency. For this second harmonic wave, the azimuthal wave number was estimated using the phase delays of the oscillations in the proton fluxes with different gyrophases. It is shown that the wave was in drift‐bounce resonance with 46 keV protons and was generated through resonant instability caused by non‐equilibrium distribution of ring current protons with the significant earthward spatial gradient at the resonance energy.