Modelling cosmic ray ground level enhancements and relativistic solar proton acceleration

Three solar cosmic ray ground level enhancements (GLEs) have been analysed to better understand the acceleration of protons to relativistic energies during major solar eruptive events. These GLEs, amongst the largest of solar cycle 23, occurred on 14 July 2000, 15 April 2001 and 20 January 2005. A global analysis technique is used to derive the spectrum, the axis of symmetry of the particle arrival and the anisotropy of relativistic solar protons arriving at Earth. The modelling procedure employs a least squares method to efficiently analyse parameter space for optimum solutions. Theoretical shock and stochastic acceleration models were used to investigate the source mechanisms. For each GLE, fluxes generated from the response of the global neutron monitor network were input to a generalised non-linear least squares program to assess the respective acceleration models. Analyses were restricted to protons of energy >450 MeV to avoid complications arising from transport processes which can delay the arrival of low-energy protons. Each GLE was marked by a strong anisotropic onset. However, for the July 2000 and January 2005 GLEs, the field-aligned component of the pitch angle distribution began to broaden several minutes after their onset and, in addition, local scattering began to increase. For the July 2000 GLE the isotropic component in pitch angle distributions is probably due to scattering effects associated with the interplanetary magnetic field (IMF). For the January 2005 GLE, part of the underlying isotropic component in the pitch angle distributions is attributed to bi-directional flow. Backscattering from a reflecting boundary beyond Earth is a likely cause. In the case of the April 2001 GLE, the comparatively smaller isotropic component in the pitch angle distributions is probably the result of limited local scattering associated with the IMF. During the rising phase of the July 2000 GLE, the spectrum derived from neutron monitor observations is best fitted by a shock acceleration spectral form. In contrast, the spectrum at the peak and declining phases is best fitted by a stochastic acceleration spectral form. These results indicate that at least two processes accelerated protons to relativistic energies: (1) a shock driven by a coronal mass ejection (CME) and (2) a stochastic process associated with magnetic reconnection. For each phase of the 15 April 2001 GLE, the spectrum derived from neutron monitor observations is best fitted by a shock acceleration spectral form. This implies that protons were accelerated to relativistic energies by a CME-driven shock. The 20 January 2005 solar eruption produced the highest intensity of relativistic solar particles since the famous event on 23 February 1956. For each phase of the January 2005 GLE, the spectrum derived from neutron monitor observations is best fitted by a stochastic acceleration spectral form. This result suggests that a stochastic process cannot be ruled out as a mechanism for accelerating protons to relativistic energies for this solar event. In summary, the major finding of this study indicates that, along with CME—driven shocks, sites of magnetic reconnection in the solar corona are a potential source of relativistic protons that give rise to GLEs