Latitude local‐time dependence of low‐energy cosmic‐ray cutoffs in a realistic geomagnetic field

A numerical program for integrating the equation of motion of charged particles in the Taylor‐Hones model of the geomagnetic field is developed and used to calculate a large number of trajectories of 1.2‐Mev protons arriving from infinity (solar cosmic rays). Only those particles that approach the earth to an altitude of ≲2000 km are considered. The results show that for such particles: ( a ) field lines attached to the earth at a latitude less than ∼65° are inaccessible to particles of any pitch angle (0–90°) at 2000 km and for all local times; ( b ) on the day side of earth, field lines between ∼65° and ∼75° are accessible only to particles having large pitch angles at the altitudes of ∼2000 km (i.e. moving approximately orthogonal to the magnetic field vector); and ( c ) the ‘polar plateau,’ an irregularly shaped region which is probably accessible to particles of all pitch angles 0–90°, extends from the pole to ∼65° on the midnight meridian, to ∼75° on the noon and dawn meridians, and to ∼70° on the dusk meridian. A comparison with the meager, observed data of Stone and of Harding gives some support to the validity of the calculation.