Compressional ULF wave modulation of energetic particles in the inner magnetosphere

We present Van Allen Probes observations of modulations in the flux of very energetic electrons up to a few MeV and protons between 1200 and 1400UT on 19 February 2014. During this event the spacecraft were in the dayside magnetosphere at L approximate to 5.5. The modulations extended across a wide range of particle energies, from 79.80keV to 2.85MeV for electrons and from 82.85keV to 636.18keV for protons. The fluxes of /2 pitch angle particles were observed to attain maximum values simultaneously with the ULF compressional magnetic field component reaching a minimum. We use peak-to-valley ratios to quantify the strength of the modulation effect, finding that the modulation is larger at higher energies than at lower energies. It is shown that the compressional wave modulation of the particle distribution is due to the mirror effect, which can trap relativistic electrons efficiently for energies up to 2.85MeV and trap protons up to approximate to 600keV. Larger peak-to-valley ratios at higher energies also attributed to the mirror effect. Finally, we suggest that protons with energies higher than 636.18keV cannot be trapped by the compressional ULF wave efficiently due to the finite Larmor radius effect.National Natural Science Foundation of China [41421003]; Major Project of Chinese National Programs for Fundamental Research and Development [2012CB825603]; NSERC; Canadian Space AgencySCI(E)ARTICLEqgzong@pku.edu.cn76262-627612