SEP propagation in interplanetary space

For Solar Eneregetic Particles to reach Earth, they first have to propagate in the turbulent interplanetary space. This makes understanding SEP observations very complicated.

I am currently working on improving our understanding on how the SEPs propagate in the interplanetary space, to arrive from the Sun to Earth and beyond. As charged particles, the SEPs are affected by electromagnetic forces, and thus, in the turbulent plasmas of the interplanetary space, they do not propagate along a direct path from the Sun. The follow the large-scale spiral structure, the "Parker Spiral", of the heliospheric magnetic field, and scatter off the magnetic field irregularities. This propagation is often described as a combination of diffusion and convection of the particles.

Because of the scattering, we must understand the SEP propagation in the interplanetary space in order to be able to understand how the SEP acceleration and release at the Sun is related to other phenomena during a Solar eruption. Unfortunately, our understanding on how to model the spread of particles into the heliosphere is still far from complete.

In particular, the spread of particles across the mean Parker Spiral magnetic field is not properly understood. This is crucial for both understanding the SEP acceleration mechanisms, and for the forecasting of SEP intensities at Earth, as currently we cannot reliably determine whether the width of an SEP event is due to the width of the acceleration region close to the Sun, or due to the spreading of the particles in the interplanetary space.

Our contribution to understanding of the cross-field propagation goes beyond the diffusion description of the particles, as we follow the complete orbit of the particle in the turbulent magnetic fields. The turbulent fields are described as superposition of waves, and the crucial question is: how to make the modelling of turbulence as realistic as possible. We have studied in particular the effect of structuredness of the turbulence using envelopes to "break" the turbulence into non-coherent packets, as the turbulence is a nonlinear phenomenon. Our models can be used to study the rate the particles spread in space, and how the spreading compares to other, more simple models.