The sharp decreases of high energy cosmic ray fluxes at the Earth, known as Forbush decreases (e.g. Ifedili, 2004), are caused by the passage of “magnetic barriers” in the solar wind. As well as magnetic clouds, these barriers are often the sheaths of compressed solar wind ahead of ICMEs, or compressed CIRs (Clack et al., 2000). Such compressions lead to “planar magnetic structures” (e.g. Jones et al., 2002: see Figure below) and it appears to be these sheets of magnetic field that efficiently block the particles (Intriligator et al., 2001). However, the efficiency of these barriers, as they develop close to the Sun, is not known. 




At a much smaller scale, “particle channels” of dramatically enhanced or reduced particle flux during solar particle events, lasting only around an hour, as well as comparable duration burst of Jovian electrons observed several AU from the planet, demonstrate the existence of very small scale magnetic connections and disconnections from particle sources. Solar Orbiter will travel much closer to the solar sources of particles and, by nearly co-rotating with the Sun, distinguish temporal variability from spatial structures.