At quasi-parallel shocks, the diffusive shock acceleration mechanism operates when particles repeatedly cross the shock front and gain energy by scattering off magnetic irregularities carried by the plasma flow. One of the essential elements of this theory is that protons streaming upstream from the shock can amplify anti-sunward propagating Alfvén waves in the solar wind. The amplified waves can then resonantly scatter subsequent ions escaping upstream, trapping them near the shock and increasing the efficiency of the shock acceleration process (Lee 1983). Although the existence of these waves is a cornerstone of the theory (also invoked for galactic cosmic ray acceleration by supernovae shocks), the proton beams and the amplified waves that are required for diffusive shock acceleration of SEPs to high energies (>~10-100MeV) at a CME shock have never been observed.
The waves responsible for accelerating MeV ions at shocks are seldom observed at 1 AU, where they are difficult to observe against the background turbulence (e.g., Bamert et al. 2004), but in the inner heliosphere, wave growth is expected to proceed more rapidly, and the wave intensities are predicted to be significantly greater (Ng, et al, 2003) and should be easily detectable at 0.35 AU. SolO observations of the amplified waves by the magnetometer (MAG) and RPW search coil (for electrons), in association with the proton (and electron) beams and simultaneous accelerated particle spectra (by EPD), would be a crucial test of this fundamental theory.