Spectral index of solar cosmic-ray flux from the analysis of ground-level enhancements

In this work we analyze the ground-level enhancement of the cosmic-ray intensity due to solar energetic particles as observed on 29 September 1989, by using two pairs of standard and lead-free neutron monitors. This enables one to separate spectral and anisotropy effects. This has been done previously by several authors for other events, but in this paper we make use of the large size and long duration of this event, as well as the fact that it is perhaps the best-observed one in the whole data base since 1942. It is shown that the method is more sensitive than the standard method that uses neutron monitors at different locations. The analysis provides a prototype for what can potentially be achieved by a new generation of mini neutron monitors.