Study of solar energetic particles (SEPs) provides important knowledge to understand their acceleration and propagation in the interplanetary space. In most cases the maximum energy of SEPs is several tens of MeV/nucleon, but in some cases it exceeds 100 MeV/nucleon or even reaches several GeV/nucleon. In that case the energy is sufficient to generate an extensive air shower in the Earth’s atmosphere, whose secondary particles reach the ground, subsequently registered by ground based detectors, specifically neutron monitors (NMs). This particular class of events is known as ground level enhancements (GLEs). Historically the strongest GLE #5 was registered on 23 February 1956, with peak increase of the count rate of NMs above 5000 %. The solar cycle 23 provided several strong GLEs, including the second largest event in the observational history observed on 20 January 2005 (GLE #69). Here, we derive the spectral and angular features of the strongest GLEs using data from the world-wide NM network. We model the global NM network
response including the particle propagation in the Earth’s magnetosphere and atmosphere. The method includes several consecutive steps: detailed computation of asymptotic cones and rigidity cut-off of each NM station used in the analysis, an initial guess of the inverse problem by
assuming the apparent source position location in a convenient way, application of the NM yield function for detector response modelling and optimization procedure in order to derive spectral and angular characteristics of SEPs. The SEP spectra and pitch angle distributions are obtained in their dynamical development throughout the events.