Light ion trough measurements near midnight made by the Bennett RF ion mass spectrometer on Ogo 4 operating in the high-resolution mode reveal the existence of irregular structure on the low-latitude side of the mid-latitude trough. By using two different relations between the equatorial convection electric field, assumed to be spatially invariant and directed from dawn to dusk, and Kp (one based on plasmapause measurements and the other on polar cap E field measurements) a model development was made of the outer plasmasphere. The model calculations produced multiple plasma tails that compare favorably with the observed thermal proton irregularities. Owing to magnetic local time differences between the northern amd southern hemisphere protons of the Ogo orbit, the time dependent irregularity structure observed is not symmetrical about the equator. The model development produces an outer plasmasphere boundary location that varies similarly to the observed miniumum density point of the light ion trough. However, the measurements are not extensive enough to yield conclusive proof one of the electric field models is better than the other.