‘What are the roles of shocks, reconnections, waves, and turbulence in
the acceleration of energetic particles?’
CME-driven shocks play a central role in determining the energetic particle populations in the heliosphere and in driving geospace storms. They are known to accelerate solar energetic particles (SEPs) to high energies (e.g., Reames 1999; Kahler 2001), even GeV energies (Bieber et al. 2004) during the so-called gradual SEP events. Fermi acceleration is the likely acceleration mechanism for quasi-parallel shocks while gradient-drift acceleration operates at quasi-perpendicular shocks (e.g., Lee 2000). The geometry of the shock seems to play a further role in the observed variability of the spectral characteristics and composition of SEPs (Tylka 2005). The shock compression ratio determines the power law index of the SEP spectrum under some simplifying assumptions such as equilibrium conditions. It appears that the particle kinetic energy might be a fairly significant percentage of the CME kinetic energy (Mewaldt et al. 2005). Many of these shock-related parameters (geometry, compression ratio, speed) are available or can be deduced from in-situ measurements at 1 AU. None, however, is actually measured in the low corona where the highest energy particles originate (≤10 Rs, Tylka 2005). Moreover, the large scatter in the correlation between CME speeds and SEP peak intensities suggests a complex interplay among the CME speed, the acceleration mechanism(s) and the ambient environment.
Some works have focused on the role of the variations of the environment through which the CME shocks and particles propagate (Gopalswamy et al. 2004; Kahler and Vourlidas 2005, 2013). The results indicate that SEP-rich CMEs tend to occur during periods of enhanced activity signifying the presence of elevated levels of seed particles. But the coronagraphic observations also show that SEP-rich CMEs tend to have much brighter fronts than A. Vourlidas et al. SEP-poor events. Since bright emission in a coronagraph image may imply a large extent along the LOS, the latter finding suggests that SEP-rich CMEs either attain larger longitudinal and latitudinal extents than SEP-poor CMEs or achieve higher compression ratios. Therefore, the height of formation of the shock, the 3D extent of the CME, and the monitoring of the activity levels (via CMEs, and jets) are necessary observations for a better understanding of the generation and propagation of SEPs.
WISPR will provide these crucial observations for SPP. The telescope will image CMEs and their associated shocks at the coronal heights where the particles originate (≤10 Rs) with high spatial and temporal resolution to resolve the locations of the CME-driven shocks, for all SPP perihelion distances (Table 1). Previous work has shown that CME-driven shocks can be easily detected in coronagraphs (Vourlidas and Ontiveros 2009) and that several physical parameters, such as density compression ratio, speed, and even upstream magnetic field, can be derived.With its higher spatial and sensitivity performance, WISPR will readily observe and characterize the evolution of even the fastest shocks. For example, the synoptic cadence of 5–10 min within 15 Rs (Table 2) will allow 13–26 observations of a 2000 km/s CME in the WISPR FOV providing detailed information on the evolution of the associated shock.