SEP acceleration

The Solar Energetic Particles are accelerate during solar eruptions, with the acceleration believed to be connected to the flare processes and the vast magnetic clouds that are ejected from the Sun at supersonic speeds into the corona and interplanetary space.

In the recent past, I worked on SEP acceleration with Rami Vainio (University of Turku, Finland). Our studies concentrated on SEP acceleration by the shock waves driven by the mass clouds, and the text below describes that process and our work

In the future, I plan to amend this text to include also something about particle acceleration in flares.

SEP acceleration in shock waves driven by coronal mass ejections

The shock waves in the corona are related to the coronal mass ejections (CMEs), the vast magnetic structures ejected from the Sun during the solar eruptions. These CMEs are hurled from the Sun at velocities faster than the magnetosonic speed, and thus drive a shock ahead of them, as a supersonic jet does.

Such shocks are considered as the primary contributor to SEPs in large events (although this is still debated). The particles are accelerated by first-order Fermi mechanism, as the particles scatter in the upstream and downstream of the shock, gaining energy at each crossing of the shock front.

As the acceleration mechanism requires scattering, it requires turbulent magnetic field. With Rami Vainio, we studied the effect of the accelerated particle streams generating this turbulence themselves. In this manner, the acceleration process can be bootstrapped: the accelerated particles amplify the turbulence, making subsequent acceleration even faster and more efficient.

The acceleration process is, however, highly dynamic, and we have shown that the intensity of the injected particles is an important factor for the attainable SEP energies and intensities. In addition, the acceleration of minor ions depends on the evolution of the proton acceleration in a considerable way, and the geometry of the coronal field lines determines the attainable energies of the particles.