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Present state of knowledge:

One of the two major physical mechanisms for energizing particles involves particles interacting with moving or turbulent magnetic fields, gaining small amounts of energy at each step and eventually reaching high energies. Called Fermi or stochastic acceleration, this mechanism is believed to operate in shock waves and in turbulent regions such as those associated with reconnecting magnetic fields or in heated coronal loops. The second major physical mechanism is a magnetic field whose strength or configuration changes in time, producing an electric field which can directly accelerate particles in a single step. At the Sun, such changes occur when large magnetic loop structures reconnect or are explosively rearranged due to the stress from the motion of their footpoints on the solar surface (e.g., Aschwanden 2006; Giacalone and Kota 2006). 

Multiple processes may take place in SEP events, and while it is not possible to cleanly separate them, they can be split into two broad classes, the first being events associated with shock waves. As a CME moves into space, it drives a shock creating turbulence that accelerates SEPs from a seed population of ions filling the interplanetary medium. Mixed into this may be particles from an associated solar flare. CMEs often accelerate particles for hours as they move away from the Sun, and in some cases are still accelerating particles when they pass Earth orbit in a day or two. Since CMEs can be huge, it is easy to see how they can fill a large portion of the heliosphere with SEPs. However, the correlation of the observed radiation intensities with CME properties is poor, indicating that additional aspects of the mechanism such as seed populations or shock geometry must play important roles that are not yet well understood (Gopalswamy 2006; Desai et al. 2006; Mewaldt 2006). 

The second class of events is associated with plasma and magnetic field processes in loops and active regions that accelerate particles. Reconnecting magnetic loops, and emerging magnetic flux regions provide sites for stochastic energetic particle acceleration or acceleration by electric fields. Because these regions are relatively small, the acceleration process is quick: on the order of seconds or minutes, but the resulting event is small and often difficult to observe. Since the energized particles are in the relatively high-density regions of the corona, they collide with coronal plasma, producing ultraviolet (UV) and X-ray signatures that make it possible to locate their acceleration sites and probe the local plasma density. Most of these particles remain trapped in their parent loops, traveling down the legs to the solar surface where they lose their energy to the ambient material, producing X- and gamma-rays. A few escape on magnetic field lines leading to interplanetary space, traceable by their (‘type III’) radio signatures, electrons, and highly fractionated ion abundances where the rare 3He can be enhanced by 1000-10,000 times more than in solar material. 

The energetic particles from these events reach our detectors at Earth orbit after spiraling around the IMF, which is an Archimedes spiral on average. But since the IMF meanders, and has many kinks, the length of the particle’s path has a good deal of uncertainty, and the particles themselves scatter and mix, smearing and blurring signatures of the acceleration at the Sun. Although we can enumerate candidate mechanisms for producing SEPs, a critical question is: what actually happens in nature? Which processes dominate? How can shocks form fast enough to accelerate ions and electrons to relativistic energies in a matter of minutes?


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