Geomagnetic Cutoffs for Cosmic Ray Protons for Seven Energy Intervals Between 1.2 and 39 MeV

The vertical geomagnetic cutoffs for cosmic-ray protons are presented for seven different energy intervals between 1.2 and 39 Mev. These data, representing approximately 160 passes through the cutoff, were taken during 1967 and 1968, between 408- and 912-km altitude, during times of K_p < 1^+. These passes provide nearly an order of magnitude more data during geomagnetically quiet times than have been previously reported at even one of these energies. In addition, the energy resolution of the instrument was significantly better than that of previous instruments. With these data, we find that the measured invariant latitudes for the cutoffs are 3° to 5° below previous calculations. We were unable to find any correlation of these observations with any physical phenomenon, including DST or the sun-earth-dipole angle. However, these data do indicate that even during ‘quiet’ times there are temporal changes in the geomagnetic field that cause the cutoff to fluctuate by 1° to 2°