SH33D-03
Why is the Sun No Longer Accelerating Particles to High Energy in Solar Cycle 24?

Wednesday, 16 December 2015: 14:12
2011 (Moscone West)
Richard A Mewaldt1, Christina MS Cohen1, Gang Li2, Glenn M Mason3, Charles William Smith4, Tycho T von Rosenvinge5 and Angelos Vourlidas6, (1)California Institute of Technology, Pasadena, CA, United States, (2)University of Alabama in Huntsville, Huntsville, AL, United States, (3)JHU / APL, Laurel, MD, United States, (4)University of New Hampshire Main Campus, Durham, NH, United States, (5)NASA Goddard SFC, Greenbelt, MD, United States, (6)Applied Physics Laboratory Johns Hopkins, Space Department, Laurel, MD, United States
Abstract:
Why is the Sun No Longer Accelerating Particles to High Energy in Solar Cycle 24?

Measurements by ACE, STEREO, and GOES show that the number of large Solar Energetic Particle (SEP) events in solar cycle 24 is reduced by a factor of ~2 compared to this point of solar cycle 23, while the fluences of >10 MeV/nuc ions from H to Fe are reduced by factors ranging from ~4 to ~10. Compared to solar Cycle 22 and 23, the fluence of >100 MeV protons is reduced by factors of ~7 to ~10 in the current cycle. A common element of these observations is that the observed Cycle-24 energy spectra have "breaks" that suddenly steepen 2 to 4 times lower in energy/nucleon than in Cycle 23. We investigate the origin of these cycle-to-cycle spectral differences by evaluating possible factors that control the maximum energy of CME-shock-accelerated particles in the two cycles, including seed-particle densities of suprathermal ions, the interplanetary magnetic field strength and turbulence level, and properties of the associated CMEs. The effect of these conditions will be evaluated in the context of existing SEP acceleration models by comparing SEP data with simulations and with analytic evaluations of the maximum kinetic energy to which CME shocks can accelerate solar energetic ions from H to Fe. Understanding the properties that control the maximum kinetic energy of CME-shock accelerated particles has important implications for predicting future solar activity.

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