AbstractRecent measurements by Mars Global Surveyor and Mars Express have greatly increased the number of observations of the martian dayside ionosphere available for study. Together with earlier measurements from the Viking era, these datasets have been used to investigate variations in well-known properties of the martian dayside ionosphere and to discover new ionospheric features. The dayside ionosphere includes the main peak, called the M2 layer, and a lower layer, called the M1 layer. In the topside, above the M2 layer, electron densities exponentially decrease with increasing altitude.The following variations in ionospheric properties are addressed. Peak electron densities and altitudes depend on solar zenith angle as predicted by Chapman theory. Electron densities in the M1 layer have a similar dependence on solar zenith angle. Peak electron densities are sensitive to the Sun’s rotation and solar flares, although the quantitative dependence of peak electron densities on solar irradiance is not as strong as theoretically predicted. Peak electron densities are increased in regions of strong and vertical magnetic field, possibly due to a two-stream plasma instability that increases electron temperatures. Peak altitudes follow fixed pressure levels in the neutral atmosphere, rising and sinking in response to thermal tides and dust storms. Electron densities below the M2 layer are highly variable because the relevant portion of the solar spectrum (<20nm) varies significantly on a range of timescales. In addition, electron densities below the M2 layer increase in response to solar flares, solar energetic particle events, and increases in meteoroid flux. Electron densities above the M2 layer are affected by magnetic fields. Abrupt changes in topside electron density with altitude are sometimes observed above strong magnetic fields and topside electron densities are increased in regions of strong and vertical magnetic field. Layering has been observed at a range of altitudes throughout the topside. A bulge in electron densities is a persistent feature at 160–180km, whereas layers above 200km occur sporadically. The upper boundary of the ionosphere is affected by complex interactions with the dynamic solar wind.In summary, external factors, including the solar irradiance, the solar wind, and meteoroid flux, and internal factors, including neutral atmospheric density and composition, solar zenith angle, and crustal magnetic fields, affect the variability of the martian dayside ionosphere.