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Theoretical bounce resonance diffusion coefficient for waves generated near the equatorial plane with arbitrary wave normal angle distributions is derived. Previous studies either assumed waves to cover the whole bounce trajectory or to have only one wave normal angle for a given frequency. In this work, we theoretically derive a new bounce resonance diffusion coefficient without these limitations. We demonstrate that the pitch angle diffusion from bounce resonance is significant for near‐equatorially mirroring particles using a published magnetosonic wave model. Our results suggest that the bounce resonance diffusion by magnetosonic waves could be an important mechanism for pitch angle scattering of near‐equatorially mirroring particles; therefore, it might be important to include bounce resonance with magnetosonic waves in global radiation belt modeling.

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An efficient and positivity‐preserving layer method is introduced to solve the radiation belt diffusion equation and is applied to study the bounce resonance interaction between relativistic electrons and magnetosonic waves. The layer method with linear interpolation, denoted by LM‐L (layer method‐linear), requires the use of a large number of grid points to ensure accurate solutions. We introduce a monotonicity‐ and positivity‐preserving cubic interpolation method to be used with the Milstein‐Tretyakov layer method. The resulting method, called LM‐MC (layer method‐monotone cubic), can be used to solve the radiation belt diffusion equation with a much smaller number of grid points than LM‐L while still being able to preserve the positivity of the solution. We suggest that LM‐MC can be used to study long‐term dynamics of radiation belts. We then develop a 2‐D LM‐MC code and use it to investigate the bounce resonance diffusion of radiation belt electrons by magnetosonic waves. Using a previously published magnetosonic wave model, we demonstrate that bounce resonance with magnetosonic waves is as important as gyroresonance; both can cause several orders of magnitude increase of MeV electron fluxes within 1 day. We conclude that bounce resonance with magnetosonic waves should be taken into consideration together with gyroresonance.

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