Correlated observations linking loss of energetic protons to EMIC waves

Electromagnetic ion cyclotron (EMIC) emission is an efficient mechanism for scattering loss of energetic protons. Here, we report an event that provides both in-situ observation of energetic proton differential fluxes in the inner magnetosphere and precipitation of protons at ionospheric altitudes. During the 7–8 September 2015 geomagnetic storm the Van Allen Probes observed strong EMIC waves around L = 5 and a distinct decrement in fluxes of tens of keV protons around pitch angles 0°–45°. Meanwhile, precipitating protons at ionospheric altitudes were found to significantly enhanced (by several orders of magnitude), measured by NOAA 18 and 19 when they magnetically linked to the Van Allen Probe-A. By solving the Fokker-Planck diffusion equation, we show that EMIC waves can efficiently produce loss of energetic protons within about 2 h in the pitch angle range of ～ 0°–45°, comparable to the satellite observations.