What is Causing the Deficit of High-Energy Solar Particles in Solar Cycle 24?

Abstract

The number of large Solar Energetic Particle (SEP) events in solar cycle 24 is reduced by a factor of about 2 compared to cycle 23. In the first 8 years of this cycle there have been only 38 "GOES" proton events compared to 79 at this point of cycle 23. What is less well known is that the fluence of protons and heavier ions is reduced by even greater factors (by 6 times for greater than 10 MeV protons, and by 9 times for greater than 100 MeV protons). Indeed the spectral breaks for H, O, and Fe are all occurring about 3 times lower in energy/nucleon in cycle 24. This talk will investigate the reduced acceleration efficiency in cycle 24 by simulating SEP acceleration using the Particle Acceleration and Transport in the Heliosphere (PATH) model, and an improved version known as iPATH, which simulates SEP acceleration at a CME-driven shock in two dimensions. Specifically, we will investigate how SEP fluences and energy spectra depend on variables that include the interplanetary magnetic field strength and turbulence level, and the density and spectrum of suprathermal seed particles.

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