Why is solar cycle 24 an inefficient producer of high-energy particle events?

¹ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
e-mail: rami.vainio@utu.fi
² Emeritus, NASA Goddard Spaceflight Center, Greenbelt, MD 20771, USA
³ Consultant, Prospect Heights, IL 60070, USA

Received: 2 February 2017
Accepted: 14 June 2017

Abstract

Aims. The aim of the study is to investigate the reason for the low productivity of high-energy SEPs in the present solar cycle.

Methods. 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.

Results. 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.