An examination of the origin of substorm periodicities in CIR- and CME-driven storms

The differences in magnetospheric response between storms driven by ejecta (CMEs) and coronal hole outflow (CIRs and HSSs) are a topic of current discussion in the literature. CMEs are well known to cause periodic injection signatures in geosynchronous particle data - so called sawtooth events. A number of authors have concluded that these are phenomenologically indistinguishable from isolated substorms, except for their magnitude and local time extent. What determines the timing and periodicity (~2-3h) of these substorms is widely debated. CIRs have also been reported to drive periodic substorms and again the origin of the timing and periodicity (~4h) of these substorms is unresolved. Cited origins include reductions in the motional electric field (northward IMF turnings) and solar wind pressure variations. Using the Minimal Substorm Model (MSM), we model periodic substorm events driven by both CIR (and attendant high-speed stream) and ejecta. We present a possible explanation for the lengthened inter-substorm period under HSS driving, and show that the periodicities and timings of the onsets are consistent with a simple loading- unloading model. Further, the extent to which a simple model such as the MSM can explain substorm magnitudes is discussed, as are the factors that may limit its predictive power.