Vertical Geomagnetic Cutoff Rigidities for Epoch 2015

Earth's geomagnetic field is evolving rapidly (in geological time), and as a consequence the amount of geomagnetic shielding at a specific location is also changing. Geomagnetic cutoff rigidities derived from the International Geomagnetic Reference Fields (IGRF) are a basic quantity necessary to compute the cosmic radiation exposure at locations around the world. The most recent generation of the IGRF describes Earth's magnetic field to previously unattainable precision. The use of the Epoch 2015 geomagnetic field coefficients in the trajectory-tracing technique gives a more precise set of vertical geomagnetic cutoff rigidity values than previous internal field models. For example, an increase in cutoff rigidity values in the Western North Atlantic Ocean area and eastern North America is the result of the geomagnetic "westward drift". On a world-wide basis the cutoff rigidity values have decreased consistent with a decrease in the magnitude of the geomagnetic dipole term. While requiring an extremely stable neutron monitor over a long time period, neutron monitors in areas of the world with rapidly decreasing cutoff rigidity values should observe an increase in galactic cosmic radiation such as was observed at Huancayo, Peru. Examples of the change in vertical cutoff rigidity values for specific locations including cosmic ray stations are illustrated.