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Solar Energetic Particles

Theory & Computation
Selected Publications

D. Ruffolo, P. Tooprakai, M. Rujiwarodom, T. Khumlumlert, M. Wechakama, J. W. Bieber, P. Evenson, and R. Pyle, "Relativistic Solar Protons on 1989 October 22: Injection and Transport along Both Legs of a Closed Interplanetary Magnetic Loop", Astrophys. J. 639, 1186 (2006).

Solar Energetic Particles (SEPs) are high-energy particles coming from the Sun which had been first observed in the early 1940s. They consist of protons, electrons and heavy ions with energy ranging from a few tens of keV to GeV (the fastest particles can reach speed up to 80% of the speed of light). They are of particular interest and importance because they can endanger life in outer space (especially particles above 40 MeV). SEPs can originate from two processes: energetization at a solar flare site or by shock waves associated with Coronal Mass Ejection (CMEs). However, only about 1% of the CMEs produce strong SEP events.

One of the most significant developments of the past decade has been the recognition of the importance of coronal mass ejections (CMEs) in driving geoeffective heliospheric disturbances. Many questions remain, however, about the initiation and evolution of CMEs—about the role of magnetic reconnection, for example, and about the origin of the magnetic flux rope structure observed in a number of CMEs. Similarly, the past decade has seen notable advances in our understanding of particle acceleration at the Sun and in the heliosphere; of particular importance is the recognition of the role of both flares and CME-driven shocks in the acceleration of SEPs. However, much remains to be learned about the spatial and temporal evolution of the SEP sources and about the basic SEP acceleration and transport processes.

The propagation of solar energetic particles and galactic cosmic rays depends on effects at the boundaries and also on small-scale turbulence. Thus the problems of cosmic ray modulation and energetic particle transport are particularly challenging, as they require a systemic understanding of heliospheric properties.

The key topics of SEP research are:

  • What is the structure of the interplanetary magnetic field at very large distances from the Sun and as a function of the solar cycle?
  • How are plasma, neutrals, heavy ions, turbulent fluctuations, solar energetic particles, and galactic cosmic rays distributed throughout the entire heliospheric volume.