The Mars Global Surveyor (MGS) Radio Science (RS) experiment permits retrieval of electron density profiles versus height (~90--200 km) from occultation measurements. An initial set of electron profiles is examined spanning high northern latitudes, early morning solar local times and high solar zenith angles (78 to 81¿) near aphelion. Sampling for these 32-profiles is well distributed over longitude. The height of the photochemically driven ionospheric peak is observed to respond to the background neutral density structure, with a mean height during this season at this location of ~134.4 km. Strong wave-3 oscillations about this mean are clearly observed as a function of longitude, and correspond to neutral density variations measured by the MGS Accelerometer (ACC) experiment. The wave-3 tidal pattern implicated by both the RS and ACC datasets is consistent with a semi-diurnal wave frequency. Clearly, the height of the martian dayside ionospheric peak is a sensitive indicator of the state of the underlying Mars atmosphere. This ionospheric peak height can be used as a proxy of the longitude specific non-migrating tidal variations present in the Mars lower thermosphere. ¿ 2001 American Geophysical Union