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form he sh e tra ining ess i Stör is sufficient for many applications. However, for precise experimental measurements, cutoff rigidities determined by the trajectory-tracing method must be utilized. netosphere. Charged particles traversing a magnetic field undergo a vector force that results in a curved dt mc r r speed digital computers and modern descriptions of the earths magnetic field, cutoff rigidities can be calcu-tions are required to obtain a world-wide set of geomag-netic cutoff rigidities. The computational task is so formidable that the vertical cutoff rigidity is often the quantity calculated, and approximations are used to extrapolate to other directions or nearby locations