Solar wind-magnetosphere coupling efficiency for solar wind pressure impulses

We investigate the solar wind - magnetosphere coupling efficiency in response to solar wind dynamic pressure impulses. We carry out a superposed epoch analysis of 236 pressure impulses from the years 1998 2002 detected by the ACE/SWEPAM instrument. For the coupling efficiency, we use four definitions based on: the polar cap potential from SuperDARN radars, the northern polar cap index (PCN), the available magnetospheric potential, and the interplanetary electric field (IEF). All definitions show consistent results: the coupling efficiency depends on the internal structure of the impulse. The coupling efficiency increases ( decreases) for events mimicking slow ( fast) MHD shocks. The coupling energy estimated from the IMAGE magnetometer chain is larger for the "fast-type'' events and stronger drivers. Hence, our results indicate that the magnetosphere uses the energy from the weaker driver more geoeffectively, while the energy associated with stronger drivers is partly transmitted through the system.