Solar Energetic Particle propagation in the 3D heliosphere

Solar Energetic Particle propagation in the 3D heliosphere

diffusion-based model (Laitinen et al 2016) of SEP propagation from the Sun (yellow dot in the middle) as guided by Parker spiral (black curve).

During solar eruptions, charged particles are accelerated to relativistic energies. These solar energetic particles (SEPs) propagate through the interplanetary space to Earth and cause a serious Space Weather hazard to humans and technology in space and for high latitude air travellers. To understand and mitigate these risks, it is crucial to be able to understand the processes behind both the acceleration of these particles, and how they propagate from the Sun to our Near-Earth environment.

Understanding the propagation of SEPs in the interplanetary space is particularly challenging. The medium is turbulent plasma, and the charged particles thus experience a stochastic force field due to fluctuating magnetic fields. The medium is also very inhomogeneous: the magnetic field and turbulence typically vary as powers of radial distance and as a result the transport parameters can vary orders of magnitude during the SEP propagation. A particular problem is caused by the spiral shape of the mean field, the Parker spiral, which is due to the rotating Sun. The large-scale curvature and gradients cause the SEPs to drift, and the coupling between the drifts and the stochastic motion are currently only approximated using diffusive transport models. These are not satisfactory, as we observe the particles typically at times when the diffusion approximation is not yet applicable. We use 3D particle orbit simulations to get a full picture of this link.

depiction of locations where SEPs cross a 1-au sphere in modelled turbulence, with 3D particle simulations. The white square depicts the source region at the Sun (preliminary data).

In this project, we are investigating how the large scale drifts affect the stochastic motion of the particles affect one another. In one branch of the project, we investigate how large-scale drifts influence the stochastic motion of the particles, to ascertain if the asymmetry caused by the drifts.  

The second part of the project uses a novel model to address a major complication which has prevented full investigation of SEP propagation in turbulence that is set about the Parker spiral. A preliminary result in the top right figure shows latitudinal asymmetry, as well as local features.

Due to the complications during the Covid pandemic, the running of simulations and publication of the first results were delayed, with necessity to perform the planned development and analysis of the second part in a shorter timeframe than desired. Both datasets are now available, and will result in several publications in near future.