Troughs in the thermal plasma densities in the top side ionosphere by the ion mass spectrometer on Ogo 6 are frequently observed at high latitudes. The position and characteristics of the high-latitude trough are distinct from those of the mid-latitude trough formation in H+ and He+ which results primarily from the loss of light ions to interplanetary space. The locations of many prominent high-latitude troughs detected along the trajectory of Ogo 6 were compared to the location of the >30-keV trapping boundary observed simultaneously by the energetic electron detector and/or to the polar cap boundary determined from simultaneous dc electric field measurements. The boundary of the polar cap was generally traversed near or in the midst of the trough. The molecular ion NO+ density frequently peaked within the atomic ion troughs in H+ and O+.