1. the mechanisms by which the magnetosphere stores and rids itself of energy accumulated in the magnetotail and
2. the way that energy is put in from the solar wind.

If we wish to understand the effect of one factor then it is desirable to keep the other factor fixed. This was the philosophy adopted in a study of substorm recurrence during the passage of a magnetic cloud by the earth. Magnetic clouds are giant interplanetary flux ropes which realise very stable solar wind power input to the magnetosphere over substorm time scales. Multi-instrument analysis revealed that substorm onsets recurred on average every 50 min during an 18 hour interval of continuous southward IMF, regardless of its very slowly diminishing magnitude. Yet the substorms were not exactly periodic, varying between 25 min and 100 min.

In another study, the substorm behaviour during the magnetic cloud has been used to constrain a simple substorm model that is then driven by realistic, variable solar wind power inputs to try to understand the overall substorm recurrence during more typical IMF conditions. This study revealed that much of the variability in substorm recurrence can be readily attributable to solar wind variability and that the intrinsic magnetospheric behaviour could be surprisingly simple.