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The guiding center equations are solved numerically to follow 200 ev protons with pitch angles near the loss cone from the dayside magnetosheath on the noon meridian to their mirror height in a model open magnetosphere. We examine the effect of magnetic field line curvature and electric field strength upon proton mirror heights. We find that when the magnetic field has dipole-like curvature along a dayside proton trajectory an increase in the strength of the dawn-dusk electric field will result in an increase of the proton mirror height. When there exists significant nondipole-like magnetic field curvature along a dayside proton trajectory an increase in the dawn-dusk electric field strength can result in lower proton mirror heights. In our model, when the interplanetary magnetic field is directed northward or southward the latitude dependence of changes in proton mirror height resulting from changed dawn-dusk electric field strength is the same along the noon-midnight meridian over the northern and southern polar caps, but when the interplanetary magnetic field vector lies in the noon-midnight meridian plane at an angle to the earth dipole axis, proton mirror height behavior in response to electric field strength changes is different over the northern and southern polar caps. Proton drift perpendicular to the magnetic field but having a component parallel to the electric field can result in kinetic energy changes of up to 25% as a proton moves from the magnetosheath to its mirror height.