Wavelike structures in the Martian topside ionosphere observed by Mars Global Surveyor

[1] Seven hundred and forty three Martian ionospheric electron density altitude profiles, obtained by the radio occultation experiment onboard Mars Global Surveyor, have been analyzed for the purpose of determining possible spatial density fluctuations. In about 80% of these profiles the high altitude region, above similar to145 km, displays significant wavelike structures with spatial scales of tens kilometers. These structures appear as spatial wave activity superposed on background electron density distributions that decrease exponentially with altitude. The amplitudes of the density variations are mostly limited to be less than +/-50% of the background densities. Spectrum analysis is applied to the ionospheric profiles above 145 km, and it is found that a spatial wavelength'' of similar to40 km is prevailing in the observations. It is demonstrated that the observed fluctuations are consistent with the occurrence of local density fluctuations. The prevailing scale of the wavelike structure agrees very well with the wavelength of hydrodynamic waves predicted by Wang and Nielsen [2002a, 2002b, 2003b]. If hydrodynamic waves is indeed the cause of these electron density fluctuations, then the similar to145 km altitude should be considered the lowest altitude at which the effects of external dynamic disturbances of the Martian ionosphere can reach, and should therefore be considered the lower boundary of the topside ionosphere.