Having considered two major applications of space plasma activity, it seems appropriate to close this part with a discussion of their differences and similarities. We first address the role of magnetic reconnection and then suggest a general eruption scheme that covers both magnetospheric and solar activity.

The reconnection problem

We begin by considering reconnection in solar activity and then bring in magnetospheric reconnection for comparison.

As we have seen, models of solar activity involve magnetic reconnection in one way or the other. In some models reconnection is involved in the eruption process itself. In addition, reconnection is considered generally as a potential release process for a field configuration with a thin current layer below a fast rising object. A CME-associated flare would be the consequence (e.g., Amari et al., 2000).

Here, and more generally in the context of solar activity, the difficult question arises of what are the quantitative criteria for reconnection to occur. Unfortunately, we can only narrow down the problem, a clear-cut answer is not yet available. We limit the discussion to fast reconnection (Chapter 11).

As magnetic reconnection cannot take place under ideal MHD conditions, the first point to address is whether the necessary nonideal process is collisional or collisionless. Using the values of the middle column of Table 9.1 as an example for coronal plasma conditions, we see that the plasma parameter is of the order of 108, which is a first indication that collisions are extremely rare.