Type II radio bursts show that CME-driven shocks form in the corona, but we do not know when they start accelerating ions. We do know from energetic storm particle (ESP)/shock spike events that shocks accelerate particles at 1 AU, but rarely to >~ a few 10s of MeV. As the shock propagates and evolves, when is it most potent in accelerating particles? How is the acceleration related to the changing magnetic geometry? Comparisons of the kinetic energy of fast CMEs measured by SOHO/LASCO with ACE and GOES measurements of accelerated particle energy spectra indicate that as much as 10% of the CME kinetic energy often goes into accelerated particles (see Figure below). That is about the efficiency needed for the acceleration of galactic cosmic rays by supernovae shocks.
Correlating the SEP/CME efficiency estimates with interplanetary conditions near the Sun will probe why and how the CME acceleration efficiency is so variable. With the wide range of other multi-point imaging and in situ data available, it will be possible to significantly improve on estimates of the CME energy, and of the global energy budget for solar eruptions, including plasma, magnetic field, energetic particle, and photon contributions. (e.g., Emslie et al. 2004).
There is now a well-developed theory of SEP shock acceleration and the capability to perform sophisticated simulations of these events. The models, however, depend on assumptions about conditions in the inner heliosphere, and many predictions of the models cannot be verified using data from 1 AU. By going close to the Sun, within ~1-2 λ, SolO provides a unique opportunity to probe shock acceleration of SEPs to high energies with powerful new in situ and imaging diagnostic measurements. Only by measuring the accelerated particles and the plasma and magnetic field properties close to the acceleration source can we test and improve current models of the SEP acceleration and transport.
We, therefore, need to explore the following questions:
- As the shock propagates and evolves, when is it most potent in accelerating particles?
- When/where does the shock start accelerating ions?
- How is the acceleration related to the changing magnetic geometry?
- Improve SEP kinetic energy estimates (variable CME acceleration efficiency) (Mewaldt et al., 2005)
- As much as 10% of the CME kinetic energy often goes into accelerated particles.
- Show why the efficiency is so variable (by better constraining CME energies?).
- What is the relation between shock acceleration, turbulence properties, and anomalous diffusion mechanisms? (Perri and Zimbardo, 2012)
SEP acceleration efficiency