Summary
On 20 January 2005 the sun produced the highest intensity of relativistic solar particles observed at Earth in 50 years. The sea-level increase at Terre Adelie (~46 X in 1 minute data) was slightly less than in 1956 (Leeds ~47 X in 15 minute data). A second event on 15 April 2001 also produced relativistic particles at Earth. We modelled the ground-level response to these events using a technique which deduces the spectrum, arrival direction and anisotropy of the high-energy protons [1, 2]. To investigate acceleration processes we employed theoretical shock [3] and stochastic acceleration [4] spectral forms in our fits to neutron monitor data. For the 20 January event we find that, at the peak and decline phases (06:55 UT, 07:10 UT), the spectrum between 400 MeV and 10 GeV is best fitted by a stochastic acceleration spectral form. This spectral form incorporates an injection function using a pre-acceleration step via DC electric fields in a reconnecting neutral current sheet [4]. This implies that initially protons were accelerated to relativistic energies by processes associated with magnetic reconnection. For the 15 April event we find that, at the rise and peak phases of the event (14:20 UT, 14:30 UT respectively), the spectrum between 400 MeV and 10 GeV is best fitted by a shock acceleration spectral form. This result suggests that protons were accelerated to relativistic energies by either a coronal shock or at the bow shock of the associated coronal mass ejection. However, during the decline phase (14:45 UT) it is difficult to differentiate between the various spectral forms suggesting that either shock or stochastic acceleration processes could be responsible for relativistic proton production.