Collaborative Research: SHINE: What is Causing the Deficit of High-Energy Solar Particles in Cycle 24?

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This 3-year collaborative SHINE project is aimed at exploring and modeling a key under-appreciated aspect of the shock acceleration mechanism of solar energetic particles (SEPs) in the heliosphere, namely the role of the proton seed population in initiating and amplifying the shock acceleration process. This is one of the most important mechanisms in solar and heliospheric physics. The project will also explore how the maximum energy of SEPs can be limited by pre-existing suprathermal seed-particle densities and by the strength and turbulence level of the interplanetary magnetic field, which has been gradually declining over the past several solar cycles. Knowledge of suprathermal seed particle densities can aid in forecasting the maximum intensity of SEP events and in forecasting all-clear periods, providing a valuable space-weather tool. The PATH acceleration and transport model to be employed during this project will be put online, where it can be used by students and researchers to model SEP spectra, time profiles, maximum energies, and composition. This research project is especially relevant to the Solar Probe Plus and Solar Orbiter missions. The project will also employ a student and involve an under-represented minority. This 3-year SHINE project aims to understand the behavior of our Sun, which has dramatically changed in the last decade. The solar wind ram pressure and the solar magnetic field strength fell to a historic low. While solar cycles 22 and 23 were active with large SEP events, solar cycle 24 brought significantly fewer large events. The difference is particularly striking at the high-energy part (tens of MeV/n and up). This change, which has far reaching implications, puzzles the scientific community. This SHINE project aims at identifying the cause of this deficit. The project teams suggest that the two main causes are the reduced IMF strength and associated turbulence level, and reduced density of suprathermal seed particles. They will use the PATH model to understand how the maximum particle energy depends on: (i) the seed population; (ii) the background magnetic field; (iii) the upstream turbulence level; and, (iv) the shock speed and compression ratio. The investigators will also look at how changes in the seed population from solar cycle 23 to solar cycle 24 affect the accelerated spectra. The project is directly relevant to the NSF's SHINE program, because it will provide important knowledge about the acceleration and transport of SEPs during solar eruptive events. Such knowledge is critical for accurate modeling and prediction of space weather conditions from the solar surface to the Earth and beyond. The research and EPO agenda of this project supports the Strategic Goals of the AGS Division in discovery, learning, diversity, and interdisciplinary research.