Nonstorm time scattering of ring current protons by electromagnetic ion cyclotron waves

We report correlated observation of enhanced electromagnetic ion cyclotron (EMIC) waves and dynamic evolution of ring current proton flux collected by Cluster satellite near the location L = 4.5 during March 26-27, 2003, a nonstorm period (D-st > -10). Energetic (5-30 keV) proton fluxes are found to drop rapidly (e.g., a half hour) at lower pitch angles, corresponding to intensified EMIC wave activities. By adopting a Gaussian fit to the observed spectra of EMIC waves, we present two-dimensional (2D) numerical simulations which demonstrate that EMIC wave can yield such decrements in proton flux within 30 minutes, consistent with the observational data. The current result provides a further understanding of ring current dynamics driven by wave-particle interaction under different geomagnetic activities.