Authors: Nikolett Biro (University of Michigan), Lulu Zhao (University of Michigan)

Predicting space weather accurately and in a timely manner remains one of the significant challenges of space science. In order to improve upon existing models and forecasting capabilities, it is essential to study and understand the underlying physical processes driving it. One of the manifestations of space weather is Solar Energetic Particle (SEP) events, which result from the acceleration of charged particles by shocks into near-relativistic speeds. These events pose a radiation hazard not only to spacecraft but also for humans onboard aircraft and in space.

SEP events can exhibit time periods where the detected particle flux shows a significant decrease and returns to its previous, elevated levels. This behavior is called a dropout. Current theoretical models attribute it either to the meandering of empty and filled magnetic flux tubes, or to particles becoming trapped within turbulent magnetic structures. While the underlying mechanism is still under debate, both theories indicate that studying dropouts would enhance our knowledge of the topology of the interplanetary magnetic field. The recently launched Solar Orbiter (SolO) spacecraft presents a perfect opportunity to study the dropout features in SEP events, particularly their radial evolution, as most of the previous detections have been made by instruments located around 1 AU.

The main goal of this work is to characterize the radial dependence of the properties of dropout events and to study their evolution throughout the inner heliosphere, which is made possible by SolO providing in-situ measurements at varying radial distances from the Sun. The length scale on which the energetic particle detections return after a dropout is investigated, and compared to particles’ local gyroradius. Our findings will also be compared with values determined from measurements at 1 AU. The results suggest that there is no cross-field diffusion of the particles during a dropout event. Future work will aim at conducting a multi-spacecraft investigation involving other operational spacecraft in the inner heliosphere, such as ACE, Wind, PSP, or STEREO A.