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The features of electron pitch angle distributions (PADs) often imply different physical mechanisms in planetary magnetospheres. We report a simultaneous equatorial electrostatic electron cyclotron harmonic (ECH) wave event with butterfly PADs of electrons at L ∼ 7.6–9 observed by the Cassini spacecraft. Via calculating the bounce‐averaged electron diffusion rates, we found that Saturnian ECH waves can resonate with ∼10 eV to several keV electrons at <60° pitch angles at time scales from ∼10−8 to 10−4 s−1. Simulations show that the formation of ∼100 eV to ∼1 keV electron butterfly PADs are mainly caused by the pitch angle scattering of electrons at low pitch angles (<30°) and the momentum scattering at intermediate pitch angles of ∼30°–45°, though previous studies suggested the adiabatic transport is the dominated mechanism. Additionally, our results successfully reproduce the variation of peak pitch angles (αp) and phase space density ratios between α90° and αp with L‐shell. Plain Language Summary: Charged particles present different pitch angle distributions (PADs) due to different physical mechanisms in Saturn's magnetosphere. Butterfly PADs have local minimum at ∼90° pitch angle, which are commonly observed in Saturn's magnetosphere for electrons over a wide energy range from ∼100 eV to several MeV. Previous studies proposed that the adiabatic transport dominates the transformation from the electron field‐aligned PADs of ∼100 eV–several keV to the butterfly shapes in Saturn's magnetosphere. In our study, we report a simultaneous electrostatic electron cyclotron harmonic (ECH) wave event with the butterfly PADs of electrons at L ∼ 7.6–9 observed by the Cassini spacecraft. Via calculating the bounce‐averaged electron diffusion rates of ECH waves and simulating the evolution of electron phase space densities (PSDs), we find that the formation of ∼100 eV to ∼1 keV electron butterfly PADs is mainly caused by the scattering of electrons induced by ECH waves. Our simulation results successfully reproduce the variation of peak pitch angle (αp) and PSD ratios between α90° and αp with L‐shell. These results would improve the current understanding of the electron dynamics in Saturn's magnetosphere and shed light on understanding the plasma environment of the outer planets and exoplanets. Key Points: We report a simultaneous equatorial electrostatic electron cyclotron harmonic (ECH) wave event with the butterfly PADs of electronsThe Saturnian ECH waves can resonate with ∼10 eV to several keV electrons at <60° pitch anglesThe formation of ∼100 eV to ∼1 keV electron butterfly PADs is attributed to the scattering of electrons induced by ECH waves [ABSTRACT FROM AUTHOR]

By constructing an empirical model of the spectral and latitudinal distribution of ion cyclotron waves on the basis of Cassini datasets, we investigate the resonant interactions between ion cyclotron waves and radiation belt electrons at Saturn. Calculations based on quasi‐linear bounce‐averaged diffusion coefficients show that at Saturn ion cyclotron waves can efficiently pitch angle scatter >∼1 MeV to tens of MeV electrons into the loss cone thereby inducing precipitation loss, while the mixed and momentum scattering effects are typically negligible. The resultant electron loss timescales range from a few to tens of minutes, which in fact decrease significantly with increasing L‐shell at L = 4–6. We also find that the kinetic effects introduced by pick‐up ring particles cause distinct changes in pitch angle scattering efficiency for lower energy electrons (<3 MeV at L = 5). Our results demonstrate that ion cyclotron waves play a significant role in the dynamics of Saturn's radiation belt electrons. Plain Language Summary: Ion cyclotron waves are a common electromagnetic wave mode in the planetary magnetospheres. At Saturn, ion cyclotron waves are usually observed with wave frequencies near the gyro‐frequency of water‐group ions (e.g., O+, OH+, and H2O+). They are known to be excited by a ring distribution of the pick‐up water‐group ions which are extracted from the extended neutral clouds. In this paper, we investigate the resonant interactions between ion cyclotron waves and radiation belt electrons at Saturn. By constructing an empirical model of the spectral and latitudinal distribution of ion cyclotron waves based on Cassini observations, we calculate the bounce‐averaged electron diffusion coefficients and resultant electron loss timescales. Our results suggest that Saturn's ion cyclotron waves can cause efficient precipitation loss of radiation belt electrons by scattering them into the loss cone. The corresponding loss timescales range from a few to tens of minutes, decreasing with increasing radial distance from Saturn. Our results confirm the important role of ion cyclotron waves in the dynamics of Saturnian radiation belt electrons. Key Points: The resonant interactions between ion cyclotron waves and radiation belt electrons at Saturn are investigatedIon cyclotron waves can efficiently pitch angle scatter >∼1 MeV to tens of MeV electrons into the loss cone for precipitation lossThe resultant electron loss timescales range from a few to tens of minutes, which decrease significantly with increasing L‐shell over L = 4–6 [ABSTRACT FROM AUTHOR]