Geoefficiency and energy partitioning in CIR-driven and CME-driven storms

Magnetic storms due to corotating interaction regions (CIRs) have been shown to elicit different responses in the magnetosphere than those prompted by other types of solar wind driving conditions such as coronal mass ejections (CMEs). In particular, CIRs provoke a much weaker response in ground magnetometer data (Dst), possibly indicative of a weaker ring current. They also last many days longer than the CME events, yet over these longer events they couple a great deal of energy, sometimes comparable to that involved in typically larger-Dst CME events. It may seem at first that the weaker driving of CIR events must result in proportionally weaker magnetospheric response, but that is not always the case. In this work we show that magnetic storms driven by CIRs deposit more energy in the ionosphere and ring current than would be expected from the electromagnetic energy input from the CIRs. They appear to be more geoefficient, in the sense that the ratio of the measured energy deposited (ring current, Joule heating, and auroral precipitation) to energy input is greater than that for CMEs. (C) 2009 Elsevier Ltd. All rights reserved.