On Modeling the April 21, 2002 SEP event

Gradual solar energetic particle (SEP) events, where particles are often accelerated to 10's MeV (sometimes even GeV) energies, are often associated with CME-driven shocks. Thus, interpreting the observational data requires understanding the structure of the shock, the acceleration process, and the subsequent transport of energetic particles. Theoretically, the underlying acceleration mechanism is thought to be first-order Fermi acceleration, also known as diffusive shock acceleration. Such a mechanism leads naturally to power-law particle spectra, which are often observed. To connect observations with the underlying theory, a comprehensive model that tracks particle acceleration and transport is necessary. In this work, we describe our model of particle acceleration and transport at CME-driven shocks and compare the model simulation with a particular event - the April 21st, 2002 event. We chose this event for its relatively clean intensity-time profile which is relatively free of contributions from other events. The parent shock is thought to be driven by a CME associated with active region 9906 (S03 W56) and was magnetically well connected to Earth. Velocity dispersion observed during the onset of the event is a signature of particles being accelerated near the Sun and their subsequent transport through the interplanetary medium to 1 AU. In this work, we have modeled the intensity-time profiles and particle energy spectra obtained over a broad range of energies between ~ 0.08~MeV/nucleon through ~ 100~MeV/nucleon from instruments on ACE, Wind, and SAMPEX. We find excellent agreement between our model simulations and the observations, suggesting that this approach may provide an important step toward understanding and tracking the influence of large SEP events in the interplanetary and geospace environments.