Associated Research Papers

ApJ_642_L81.pdf – Sokolov et al., Diffusive Shock Acceleration Theory Revisited

ApJ_622_1225.pdf – Manchester et al., Coronal Mass Ejection Shock and Sheath Structures Relevant to Particle Acceleration

ApJ_616_L171.pdf – Sokolov et al., A New Field Line Advection Model for Solar Particle Acceleration

ApJ_605_L73.pdf – Roussev et al., A Numerical Model of a Coronal Mass Ejection: Shock Development with Implications for the Acceleration of GeV Protons

Research

Production of Solar Energetic Particles

Solar Energetic Particles (SEPs) are high energy protons, electrons, and heavy ions that originate in the Sun's corona and are accelerated into space by flare and CME events. SEPs may reach up to 80% of the speed of light, arriving at the Earth as quickly as a few minutes after an eruption on the solar surface. Understanding SEPs is an important problem because these very energetic particles can pose a radiation threat to astronauts in space or knock out satellites.

While SEPs were first detected in 1942, it is still not completely known how these particles are produced. Observations do support the theory that particles are accelerated during flare and CME events when a process called magnetic reconnection occurs. The magnetic fields inside the sun are constantly twisting and kinking due to various types of rotation in the sun. Sometimes the magnetic field will pierce the surface of the sun, forming a sunspot. Here, the magnetic field is very concentrated and when conditions are right (i.e. when equilibrium is lost), the field can break and then reconnect, releasing a large amount of energy that then forms a flare or ejects material as a CME. SEPs are also a result of this magnetic reconnection. One potential method for energizing SEPs theorizes that the magnetic reconnection event may inject energy into the ions, protons, and electrons themselves during the outburst. SEPs may also be accelerated by shock waves created by the outward traveling material of CMEs.

   

As we can see, a better undstanding of SEPs will only be possible with a better understanding of the processes that lead up to and occur during eruption events. A principal aim of C2H2 is to develop numerical simulations and models that explain the intiation and evolution of solar eruptions. These models must be able to reproduce the general characteristics of an eruption, including shock waves, interaction of the ejecta with the solar wind, and the production of SEPs. Dr. Roussev et al. (2003) have developed a fully three-dimensional numerical model of a solar eruption that incorporates solar magnetogram data and a loss-of-equiplibrium mechanism. The CME model showed that a CME-driven shock wave can develop close to the Sun and is sufficiently strong enough to account for the prompt appearance of high-energy (~1 GeV) solar protons at the Earth.

An SEP study carried out by Sokolov et al. (2004) utilized the CME model of Roussev et al. to model the acceleration of SEPs by shock waves that evolve over time, a process known as diffusive shock acceleration (DSA). This study also performed frequent dynamical coupling between an MHD code, BATS-R-US, and a kinetic code, FLAMPA, to capture time-scales and spatial gradients of dynamical importance for the diffusive shock acceleration of solar protons.

The results of this CME-SEP model demonstrated that it was possible to simulate the acceleration and transport of solar protons during SEP events, however comparison with an actual CME/SEP event observed by the GOES-8 satellite revealed discrepencies in proton fluxes and timing. While not yet perfect, this coupled model is very informative and can be compared to real SEP data. Research at C2H2 continues to strive towards a further understanding of SEP events and the refinement of coupled models that have significantly increased the understanding of the production of SEPs during flare and CME events.