Turbulent Origins of Particle Intensity Dropout in Gradual Solar Energetic Particle Events During Solar Cycle 23

Dropout is a low-energy particle phenomenon in which the particle intensity drops sharply and then rises rapidly during an impulsive or gradual solar energetic particle (SEP) event. We investigated dropouts in gradual SEP events during solar cycle 23, in which we identified 77 dropout periods with an average duration of approximately 1 hr. During most of the dropout periods, we observe large angles between the mean magnetic field and the solar wind velocity, implying that the slab turbulent component dominates. We therefore explore the origin of particle intensity dropout in slab turbulent environments. At a wave frequency of 1 Hz in the spacecraft frame, we observed a significant positive correlation between the turbulent power spectral density (PSD) and its spectral index, both in the ion dissipation range. As the input PSD decreases, the correlation can amplify the reduction factor for pitch-angle scatterings, quickly suppressing particle scattering through the 90° pitch angle. Hence, particle dropout may occur due to lack of spatial diffusion of particles.