Physics of Space Plasma Activity

In the Solar System the most spectacular manifestations of space plasma activity are the large-scale solar eruptions, such as coronal mass ejections (CMEs), solar flares and prominence eruptions, as briefly described in Section 2.2. In this chapter we attempt to address the underlying physical processes. The approach leaves aside many details, although they would be exciting from a more morphological point of view. Instead, we are interested in the basic physical mechanisms and concentrate on the models and numerical simulations, which provide an excellent frame for our discussion. Naturally, as in the previous chapter on magnetospheric activity, the focus is on loading and release processes.

As we will see, the building blocks, such as ideal dynamics, magnetic reconnection, formation of thin current layers, plasmoid or flux rope formation are relevant elements also in current modelling of solar activity. However, in most solar activity models their role is different from their magnetospheric role. In other words, the building blocks are put together in a different way.

General aspects

Observations strongly suggest that solar eruption processes are of the loading/release type. The energy flux into the corona from below is considerably smaller than the energy flux that would be required if the eruptions were directly driven by the subphotospheric dynamics. In fact, it has been argued that models based on direct driving have been shown to be grossly inconsistent with observations (e.g., Forbes, 2000a).