Present state of knowledge:
SEPs associated with CME-driven shocks have been long known to often arrive at Earth orbit hours later than would be expected based on their velocities. There are two alternate processes that might cause this. (1) The acceleration may require significant time to energize the particles since they must repeatedly collide with the shock to gain energy in many small steps, so the process may continue for many hours as the shock moves well into the inner solar system. Or (2) the particle intensities near the shock may create strong turbulence that traps the particles in the vicinity of the shock, and their intensity observed at Earth orbit depends on the physics of the particles escaping from the trapping region. Once free of the vicinity of the shock, SEPs may spiral relatively freely on their way to earth orbit, or more usually will be scattered repeatedly from kinks in the IMF, delaying their arrival further. The amount of scattering in the interplanetary space varies depending on other activity such as recent passage of other shocks or solar wind stream interactions. By the time the particles reach Earth orbit, they are so thoroughly mixed that these effects cannot be untangled (Gopalswamy et al. 2006; Cohen et al. 2007).
Particles accelerated on magnetic loops can reach very high energies in seconds after the onset of flaring activity, and then collide with the solar surface where they emit gamma radiation. There is a poor correlation between the intensity of the gamma radiation and the SEP intensities observed at Earth orbit, so most particles from this powerful acceleration process do not escape. Much more common are flare events observed in UV and X-rays that produce a sudden acceleration of electrons. The electrons can escape from the corona, producing nonthermal radio emission as they interact with the local plasma. Moving from higher to lower frequencies as the local plasma density decreases with altitude, the (type III) radio emission makes it possible to track the energetic electron burst into interplanetary space where it may pass by the observer. Energetic ions, greatly enriched in 3He and heavy nuclei, accompany these electron bursts (Lin 2006; Mason 2007).
Key open questions in shock associated events are whether particle arrival delays at 1 AU are due to the length of time needed to accelerate the particles, or due to trapping in the turbulence near an accelerating shock or a combination of both? For particles accelerated on loops, are the electrons and ions accelerated from sites low in the corona or at higher altitudes, and how are they related to the X- and gamma-ray signatures?
How Solar Orbiter will address this question:
Solar Orbiter will revolutionize our understanding of SEP acceleration associated with CME-driven shocks by probing the inner heliospheric sites where particle acceleration and release take place. Solar Orbiter will observe how shocks evolve, and whether they are still accelerating particles as they pass by the spacecraft. If particle arrivals are controlled by the time it takes the shock to accelerate them, then the highest energy particles will be delayed since they require many more interactions with the shock. If trapping and release control the timing, then as the shock moves by the faster and slower particles will have similar intensity changes. Since Solar Orbiter will simultaneously measure the turbulence properties in the shock acceleration region, it will be possible to construct a complete theory and models of the acceleration process, and its radial dependence in the inner heliosphere.
For SEPs accelerated on loops or in reconnection regions, Solar Orbiter will see the coronal location from UV and X-rays, and then trace the progress of released electrons by radio emission that will drift to the plasma frequency at the spacecraft for those bursts that pass by. This unambiguously establishes that the magnetic field line at Solar Orbiter connects to the coronal UV and X-ray emission site. Since Solar Orbiter can be connected to active regions for periods of days, this will provide multiple tracings between the heliospheric magnetic field and its origin in the corona. The corotation phase of Solar Orbiter will considerably lengthen the periods of connection to active regions, greatly increasing the number of field line origin sites that can be determined from a single active region. X-ray emission from the flaring sites can be used to derive the energetic electron spectrum at the flare site, which in turn can be compared with the escaping population to see if most of the accelerated electrons are released (usually most do not escape). Thanks to the 1/r^2 intensity advantage, Solar Orbiter will observe thousands of these cases and thereby permit detailed mapping of coronal sources and the trapping properties of the acceleration sites.
Detailed sub-objectives: