Cosmic rays are ions that move at relativistic speeds. They generate secondary cosmic rays by successive collisions with atmospheric particles, and then, the secondary particles reach the ground. The secondary particles are mainly neutrons and muons, and the neutrons are observed by the ground neutron monitor. This study compared the diurnal variation in cosmic ray intensity obtained via harmonic analysis and that obtained through the pile-up method, which was examined in a previous study. In addition, we analyzed the maximum phase of the diurnal variation using four neutron monitors with a cutoff rigidity below approximately 6 GV, located at similar longitudes to the Oulu and Rome neutron monitors. Expanding the data of solar cycles 20–24, we examined the time of the maximum cosmic ray intensity, that is, the maximum phase regarding the solar cyclic modulation. During solar cycles 20–24, the maximum phase derived by harmonic analysis showed no significant difference with that derived by the pile-up method. Thus, the pile-up method, a relatively straightforward process to analyze diurnal variation, could replace the complex harmonic analysis. In addition, the maximum phase at six neutron monitors shows the 22-year cyclic variation very clearly. The maximum phase tends to appear earlier and increase the width of the variation in solar cycles as the cutoff rigidity increases.

1. INTRODUCTION
 2. DATA AND METHOD
 3. RESULTS
  3-1 Diurnal Variation for Whole Analyzed Period (1964–2017)
  3-2 Solar Cyclic Modulation of Diurnal Variation
 4. SUMMARY
 REFERENCES

• Eun Ho Park(Department of Earth Science Education, Chonnam National University, Faculty of Science, Gwangju Soongil High School)
• Jongil Jung(Department of Astronomy, Space Science and Geology, Chungnam National University)
• Suyeon Oh(Department of Earth Science Education, Chonnam National University) Corresponding Author
• Paul Evenson(Department of Physics and Astronomy, University of Delaware)