Simultaneous occurrence of four magnetospheric wave modes and the resultant combined scattering effect on radiation belt electrons

We report a representative concurrent event of four wave modes at L approximate to 5.0, including electrostatic electron cyclotron harmonic (ECH) waves, exohiss, magnetosonic (MS) waves, and electromagnetic ion cyclotron (EMIC) waves, based on the observations from Van Allen Probe A on October 15, 2015. The diffusion coefficients induced by these waves are calculated by using both the Full Diffusion Code and test particle simulations. Moreover, the scattering effects of these waves on energetic electrons are simulated by using a two-dimensional Fokker-Planck diffusion model. The results show that ECH waves mainly scatter low-pitch-angle (< 20 degrees) electrons at 0.1-10 keV; exohiss can significantly scatter hundreds of kiloelectron volt electrons to form a reversed energy spectrum; MS waves mainly affect high-pitch-angle electrons (> 60 degrees); and EMIC waves scatter only > 5 MeV electrons. The combined scattering effects of exohiss and MS waves are stronger than those of exohiss alone. The top-hat pitch angle distributions produced by exohiss are relaxed after adding the effect of MS waves. Because the energies of electrons scattered by ECH waves and EMIC waves are much lower and higher than those scattered by exohiss and MS waves, respectively, the combined scattering effects with the addition of ECH and EMIC waves show little difference from the results for the combination of MS waves and exohiss. These results suggest that distinct wave modes can occur simultaneously and scatter electrons in combination or individually, which requires careful consideration in future global simulations of the complex dynamics of radiation belt energetic electrons.