Upper limit for fourth harmonic of cosmic ray solar daily variation: 1963-2015

In 1970s we analyzed the neutron monitor (NM) and muon telescope (MT) data from the global network for 1966-1970 to determine the amplitude and phase of the significant harmonics constituting the galactic cosmic ray (GCR) daily anisotropy in solar time. The median rigidity of response (Rm) of detectors to GCR spectrum covered a wide range (16-134 GV). The results were reported at the international cosmic ray conferences (ICRCs) held at Denver [1973], Munich [1975] and Paris [1981]. It was inferred that GCR solar daily anisotropy consists of only three harmonics, namely the diurnal (1 cpd, first harmonic), the semidiurnal (2 cpd, second harmonic) and the tridiurnal (3 cpd, third harmonic), with power spectral densities in the ratio 800:20:1 for the Deep River NM data (Rm = 16 GV) for 1962-1971. The fourth harmonic (4 cpd) was absent in these analyses. Since then the volume of data from the global network of NMs and MTs has increased significantly; particularly the multidirectional MT network, with larger Rm values. This motivates us to ascertain whether 4 cpd peak is absent in the cosmic ray solar daily variation data at the global sites, over a longer time period (1963-2015). Our informed conjecture is that 4 cpd peak is insignificant at 2σ-level of experimental error for a range of Rm values (11-60 GV). The discovery of a physically significant fourth harmonic would challenge the theorists to come up with a model for GCR transport in the heliosphere revealing hitherto unknown feature(s) of solar modulation.