Collaborative Research: Transport Processes Affecting High-Energy Solar Energetic Particles Observed at Earth

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The Sun occasionally has massive eruptions that are capable of producing very high-energy particulate radiation. These "solar cosmic rays" are created near the Sun, move through space between the Sun and Earth, and impinge on the Earth's upper atmosphere. These particles pose a significant radiation hazard to humans in space, space hardware, and may even increase the radiation dosage received by airline crews. The project teams can measure these particles using Earth-based detectors, known as neutron monitors, which are also used to measure galactic cosmic rays that come from outside our solar system. Relating the size of any given solar storm -- i.e. the amount of energy released or how big it is, etc. -- to the intensity of the particles at Earth requires a careful and thorough study of how these particles move between the Sun and the Earth. This "transport" is not uniform, and the resulting intensity at Earth depends on a number of factors, such as the nature of the magnetic field in space, that must be studied carefully. That is what this three-year collaborative project is about, and it involves numerical modeling and comparing those modeling results with neutron monitor data. The research studies address the general problem of understanding the space radiation environment and space weather especially that from solar flares, leading to more accurate future predictions.This three-year collaborative project will investigate the role of interplanetary transport, in the form of pitch-angle scattering, cross-field diffusion, field-line meandering, and drifts, of high-energy solar particles in affecting the observed intensity and direction at Earth, seen as ground-level enhancements (GLE) in Earth's neutron monitors. GLE events can often show very unusual profiles in terms of the observed intensity versus time and anisotropy. Some show an abrupt, short-lived peak followed by a gradual decay, or even re-enhancement, followed by a decay. The observed profiles provide important information about the nature of the particle transport from the Sun to the Earth, which has not been fully explored. The project teams will perform a large number of numerical simulations which solve the equations of motion of a large number of individual particles moving in a set of kinematically specified interplanetary magnetic fields. In parallel, they will also analyze the neutron monitor data set for GLE events, characterizing the intensity versus time at various stations. Through collaboration, they will identify events to compare directly with the numerical simulations. The numerical simulations will consider a number of important new effects which have not yet been completely explored, including the heliospheric current sheet, which likely has a profound effect on ~GeV-energy particles. In addition, the researchers will consider the importance of field-line meandering, which can lead to a non-uniform intensity of high-energy solar particles seen at Earth. The research and EPO agenda of this project supports the Strategic Goals of the AGS Division in discovery, learning, diversity, and interdisciplinary research.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.