Existing theories do not explain large parallel electric fields that are associated with keV electron precipitation in auroral arcs. The MHD electron response results in an electrical conductivity in the low altitude magnetosphere that is two orders of magnitude greater than is required. We suggest a new mechanism for forming parallel electric fields based on a nonlocal electron response to standing shear Alfv¿n waves on dipole magnetic fields. Electron trapping is the primary cause of a significant reduction in the collisionless electron conductivity and consequent enhancement of parallel electric fields in the 1--4 mHz frequency range. ¿ 1999 American Geophysical Union