Explaining circumsolar Solar Energetic Particle events

Solar Energetic Particles (SEPs) are accelerated during large eruptions at the Sun and travel through the interplanetary medium to reach near-Earth space. They can be detected by instrumentation on spacecraft. SEP data and models are used to understand the physics of particle acceleration and propagation in space. When their flux is very large, SEPs pose a risk to satellites and humans in space: for this reason they are an important component of Space Weather.

A new era of SEP research started with the launch of Parker Solar Probe (PSP, launch 2018) and Solar Orbiter (SolO, launch 2020): together with STEREO A and spacecraft near Earth and other planets, they form a fleet of missions often located at widely separated points in space, providing the opportunity to map the spatial distribution of SEPs.

Since 2021, data from this fleet have shown that circumsolar events, where SEPs fill the entire 360 degrees in longitude around the Sun, are not rare, as previously thought, but a common occurrence.

This project focusses on the question: How are SEPs able to fill wide regions of space, including areas on the opposite side of the Sun with respect to the location of the solar eruption that accelerated them? In classic SEP models, particles are expected to remain tied to the magnetic field of interplanetary space, and propagate inefficiently in the direction perpendicular to the field, making it impossible for them to reach locations on the opposite side of the Sun compared to where they were accelerated.

In this project, models of SEP propagation will be developed that are able to describe a variety of mechanisms for propagation across the magnetic field. These include the effects of turbulence in space, of a wide region called the heliospheric current sheet, where the direction of the magnetic field in the heliosphere reverses, and of the rotation of the Sun.

The possibility that CMEs are accelerated over wide regions of space in association with coronal mass ejections will also be considered. Results from the models will be compared with spacecraft measurements from PSP, SolO and near-Earth spacecraft to test different propagation scenarios.