Using global neutron monitor network data for GLE analysis : recent results

The solar cycle 23 provided several strong ground level enhancements (GLEs). The first strong GLE event of the cycle was observed on 14 July 2000 (the Bastille day event), while the last was observed on 13 December 2006. In addition, the period of late October - early November 2003 was characterized by strong cosmic ray variability and sequence of three GLEs. Cycle 23 produced also the second largest event in the observational history - 20 January 2005 event. Here we perform a precise analysis of spectral and angular characteristics of solar energetic particle (SEP) events on the basis of neutron monitor (NM) data. We model particle propagation in the Earth’s magnetosphere and atmosphere using a newly computed NM yield function at several altitudes above the sea level. The method includes several consecutive steps: detailed computation of asymptotic cones and rigidity cut-off of each NMstation used in the analysis, making 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 subsequent optimization procedure in order to derive spectral and angular characteristics of SEPs. Here, we present results from analysis of several GLEs. The SEP spectra and pitch angle distributions were obtained in their dynamical development throughout the events. We briefly demonstrate the capability of the method to assess SEP features of sub-GLE events, the details reported in this volum. Several case examples are discussed.