The cosmic ray spectral modulation above 2 GV during the descending phase of solar cycle No. 19. II: A study of large Forbush decreases

This is the second of a series of three papers meant to study both the short-and long-term spectral modulation of primary cosmic rays above 2 GV during the time span from the IGY to the IQSY, through a large part of solar cyc1e no. 19. In this Part II the dai1y sea-level latitude curves obtained by the revision and normalisation procedure of the neutron monitor dai1y data described in Part I, are used to derive the isotropic primary spectral variation on the occasion of 14 large Forbush decreases occurring from July 1957 to December 1965. The criteria adopted for reaching a satisfactory accuracy in the modulation amplitude and for avoiding the spurious or anisotropic effects are reported. Due to the high precision and good rigidity coverage of the data used, detailed information on the modulation function in the studied rigidity region is reached for the first time. The Forbush decrease modulation shows a fairly constant behaviour through the solar cyc1e, with an indication, present in the individual events and emerging clearly in the average function, of a change in slope in the 5-1O GV region.