After SEPs leave their acceleration sites - flares or CME-driven shocks or high coronal structures - they propagate along the open coronal magnetic fields that expand rapidly close to the Sun, and then along the large spatial scale interplanetary magnetic field (IMF) - Parker's spiral for steady flows but highly complex in transients such as ICMEs. The IMF appears to be random and turbulent at smaller spatial scale. Understanding how energetic particles propagate from their sources to the point of observation is not only essential to better understand the acceleration processes on or near the sun, but an important problem in its own right. Propagation of charged particles parallel to the IMF is affected by two competing processes: the adiabatic motion due to the divergence of the large-scale IMF results in “focusing” wherein particle pitch angles decrease on average as they move out; and scattering in pitch angle by small-scale magnetic irregularities at a rate depending on the strength of the turbulent magnetic field.

A crucial parameter to solve the transport equation is the Fokker-Planck coefficient, Duu, which can be related to the particle's mean free path if the power spectrum of the turbulence is known. To understand SEP observations at 1 AU, model calculations of the transport equations are fit to observation, with assumptions on the r-dependence of the turbulent magnetic field along the Parker spiral from the acceleration site to 1 AU, since we can only obtain wave spectra at 1 AU. SolO’s in situ magnetic field and plasma measurements will map the power spectrum of the turbulent magnetic field as a function of heliocentric distance in to 0.28 AU.

Transport of ions and electrons may be quite different because electrons and ions resonate in different regions of the turbulence power spectrum, leading to a rigidity dependence of the particle's mean free path. Whereas high-rigidity particles are suited to probe the geometry of the fluctuations, low-rigidity particles are sensitive to the dissipation range and dynamical and thermal effects. Some models (Dröge, 2003), using power spectra at 1 AU, are able to correctly account for the dependence of the scattering mean free path on the particle's rigidity and have significantly improved the agreement between scattering theory and observations. It is important to examine if a similar rigidity dependence also holds at 0.28 AU.

- 3.2.1.1 Map the power spectrum of the turbulent magnetic field as a function of heliocentric distance in order to provide ground-truth for transport models
- 3.2.1.2 Measurements of SEP events time profiles and anisotropy in order to probe solar wind turbulence
- 3.2.1.3 Identify dropouts and measure scattering of SEPs by turbulence.