Numerical surface of section techniques and analytic methods are used to study phase space structure and chaos near a neutral line. We find that, for -(2h)1/2≤Py≤(2h)1/2, the ratio of fields, bn, rather than &kgr;, is the appropriate chaos parameter, where Py and h are the normalized canonical momentum and energy. We find that the action integral adiabatic constant is approximately conserved and can be used to describe particle motion as long as bn≪1 or bn≫1. ¿American Geophysical Union 1991