One of the most intriguing questions about gradual events is their great variability in intensity: while the CME speed varies by a factor of 4, the SEP intensities of >20 MeV protons vary by 3 to 4 orders of magnitude for a given CME speed (Kahler and Vourlidas 2005). In addition, there are wide variations in the composition of gradual SEP events, including Fe/O ratios that vary by a factor of ~100 (Mewaldt et al. 2006).
Observations from solar cycle 23 suggest that CME-driven shocks accelerate mainly suprathermal ions rather than the bulk solar wind (Mason et al. 1999). The intensity and composition of suprathermal ions with 10 to 100 keV/nuc is known to be much more variable than the solar wind. One reason for this variability is that suprathermal particles are believed to arise from a variety of sources, including previous gradual and impulsive SEP events, ubiquitous micro- and nano-flaring at the Sun, and interplanetary acceleration of solar wind by stochastic processes (Fisk and Gloeckler 2006). In a steady state solution, particles are assumed to be continually injected at the shock and a power law spectrum is achieved when there is enough time. However, at a propagating CME-driven shock, the amount of time available for acceleration is limited. Thus, for the acceleration to successfully operate, it is crucial that there are enough seed particles.