Compressional ULF wave modulation of energetic particles in the inner magnetosphere. Issue 7 (9th July 2016) - Liu, H.
Abstract: 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 1400 UT on 19 February 2014. During this event the spacecraft were in the dayside magnetosphere at L ⋆ ≈5.5. The modulations extended across a wide range of particle energies, from 79.80 keV to 2.85 MeV for electrons and from 82.85 keV to 636.18 keV for protons. The fluxes of π /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 2.85 MeV and trap protons up to ≈600 keV. Larger peak‐to‐valley ratios at higher energies also attributed to the mirror effect. Finally, we suggest that protons with energies higher than 636.18 keV cannot be trapped by the compressional ULF wave efficiently due to the finite Larmor radius effect. Key Points: We present an event about compressional ULF wave modulation of energetic particles observed by Van Allen Probes on 19 February 2014 The modulation extends across a wide range of energies, from 79.80 to 2850 keV for electrons and from 82.85 to 636.18 keV for protons The modulation is ascribed to mirror effect, and the peak‐to‐valley ratios of fluxes are determined by shapes of pitch angle distributions