Anti-phase oscillation of energetic electron and proton fluxes: Cluster observations

Simultaneous modulation of energetic electron and proton fluxes in the outer radiation belt with the phase difference between them of 180 degrees were observed by the Cluster satellites during a magnetic storm period on Oct. 31, 2003. The "out-of-phase" event, surprisingly, was observed associated with the toroidal ultra-low frequency (ULF) waves instead of the compressional ones, and therefore could not be caused by the mirror effect (Kivelson and Southwood, 1985). To explain the unexpected 180 degree phase difference, we analyze the behaviors of both the electrons and the protons in the toroidal waves, and highlight the role of the background convection. With the effect of the strong convectional electric field, the anti-phase modulation between the electron and the proton fluxes can be understood and predicted. The convection may also affect the modulation amplitude of the ULF waves, and further accelerate particles with a broader energy range by changing the wave-particle resonance conditions.