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We develop a general 3-D relativistic test particle (TP) simulation code to examine the trajectories and pitch angle variations of test electrons. We investigate the conditions when the electrons transit broadband, parallel propagating H+ band electromagnetic ion cyclotron (EMIC) waves to find whether the classical quasilinear theory (QLT) can be applied to interactions between H+ band EMIC waves and radiation belt relativistic (and ultra-relativistic) electrons. We find that to reach good consistency between TP scattering coefficients and QLT calculations, the wave-particle resonant interaction time (which indicates the time of particles traveling in the wave fields and being resonant with the wave fields) must be carefully calculated to ensure the validity of the linear diffusion process. This wave-particle resonant interaction time depends largely on the wave amplitude, the bandwidth, and the initial parameters of electrons for TP simulations; once the time of electrons traveling in the wave fields becomes large enough when the pitch angles of electrons scattered by wave fields reach the maximal value limited by the resonance condition and cannot be diffused any longer, the linear diffusion process of particles defined by QLT will not be valid any longer. When the resonant electrons start to travel through the wave field, they undergo the linear diffusion process resulting from the pitch angle scattering by the H+ band EMIC wave field. As the time of traveling in the wave fields increases, the test electrons will suffer effective pitch angle scattering and the pitch angles of these electrons will reach the maximum value limited by the resonance condition, which is determined by the wave and electron parameters. These electrons are then reflected by the wave fields and the linear wave-particle interactions defined by QLT will not be valid anymore. This kind of wave-particle interaction subsequently introduces considerable deviation of TP diffusion coefficients from those predicted by QLT calculations. Our results demonstrate that the general validity of QLT in describing how broadband EMIC waves affect radiation belt relativistic electrons is highly related to the time of electron traveling in the wave fields, which tends to increase for smaller wave power and broader wave frequency spectra. [ABSTRACT FROM AUTHOR]

By investigating the resonant energy and latitudinal resonance region of Landau resonance between H+ band electromagnetic ion cyclotron (EMIC) waves and radiation belt electrons and performing calculations of quasi‐linear diffusion coefficients, we find that Landau resonance with EMIC waves, characterized by a very broad range of resonant energies from below 1 eV to above 10 MeV and the strikingly small extent of the latitudinal resonance region well below 0.1°, can be a feasible candidate accounting for the pitch angle scattering of near‐equatorially mirroring electrons. Compared to the dominant pitch angle scattering caused by the cyclotron resonance for greater than ~2‐MeV electrons at equatorial pitch angles less than ~80° with the rates well above 10−4 s−1, Landau resonance with H+ band EMIC waves has the potential to drive more efficient pitch angle scattering of radiation belt electrons from ~10 MeV down to tens of kiloelectron volts at equatorial pitch angles greater than ~ 85°. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are thought to be very important for pitch angle scattering of radiation belt electrons, which may play a crucial role for the rapid dropout of relativistic electrons fluxes in the outer radiation belt during a geomagnetic storm, or for the intense relativistic electron precipitation. Although cyclotron resonance with EMIC waves acts as an important loss process during geomagnetic storms, this mechanism is believed to solely affect trapped electrons with small pitch angles well away from 90°. In the present work, we introduce the Landau resonance between H+ band EMIC waves and radiation belt electrons. The crucial role for pitch angle scattering near‐equatorially mirroring electrons by Landau resonance is investigated by Quasilinear diffusion theory (QLT) calculations in our work. Key Points: H+ band EMIC waves drive the Landau resonance with radiation belt electrons over a very broad energy range from >10 MeV to tenths of an eVLandau resonance between H+ band EMIC waves and electrons at > ~60 degrees occurs in a very limited rangeEMIC scattering electrons due to the cyclotron resonance and Landau resonance occur at two distinct ranges of equatorial pitch angle [ABSTRACT FROM AUTHOR]