Title: Why is solar cycle 24 an inefficient producer of high-energy particle events?,
Abstract: The aim of the study is to investigate the reason for the low productivity of
high-energy SEPs in the present solar cycle. We employ scaling laws derived
from diffusive shock acceleration theory and simulation studies including
proton-generated upstream Alfvén waves to find out how the changes observed
in the long-term average properties of the erupting and ambient coronal and/or
solar wind plasma would affect the ability of shocks to accelerate particles to
the highest energies. Provided that self-generated turbulence dominates
particle transport around coronal shocks, it is found that the most crucial
factors controlling the diffusive shock acceleration process are the number
density of seed particles and the plasma density of the ambient medium.
Assuming that suprathermal populations provide a fraction of the particles
injected to shock acceleration in the corona, we show that the lack of most
energetic particle events as well as the lack of low charge-to-mass ratio ion
species in the present cycle can be understood as a result of the reduction of
average coronal plasma and suprathermal densities in the present cycle over the
previous one.