The cosmic ray spectral modulation above 2 GV during the descending phase of solar cycle No. 19. I: A comprehensive treatment of the neutron monitor data from the worldwide station network and latitude surveys

This is the first of a series of three papers meant to study the short-and long-term spectral modulation of primary cosmic rays above 2 GV during the time from the IGY to the IQSY, for a large part of solar cycle no. 19 with the highest acitivity recorded so far, and offering a complex superposition of different kinds of consipicuous solar-induced interplanetary perturbations. In this Part I we discuss the information on the modulation as gained by means of neutron monitor data, provided sufficient accuracy is reached; in fact this information covers a fair primary rigidity range and offers a time and rigidity resolution which is not reached by other experimental data. We also present and apply to the 1957-65 data an indirect calibration procedure of practically the shole network of the near sea level neutron monitors by means of seven latitude surveys performed by ship or by terrestrial veyicle. This procedure provides a set of daily latitude curves suitable for directly deriving the iosotropic variation of the primary differential spectrum.