Coordinate system for use with high-latitude energetic-particle phenomena

The coordinate system generally used to describe the structure of solar-particle profiles over the polar caps is based on invariant latitude and magnetic local time. By using trajectory integrations in a model geomagnetic field a new coordinate system is developed that is not based on internal geomagnetic-field components alone but also takes into account the currents flowing in the magnetopause and the neutral sheet. It is shown that the new coordinate system is more relevant for the interpretation of high-latitude energetic-particle data than the old system and that its use makes it possible to calculate reconnection rates and scattering mean free paths in the tail. In a companion paper the results of the calculations performed here are applied to observations first to decide which magnetosphere model (open or diffusive) is appropriate and second to use the solar-particle data for making measurements of important magnetospheric parameters.