This thesis employs mutual radio occultation (RO) to investigate the dynamics of the Martian ionosphere and upper atmosphere. This method, which is sometimes referred to as crosslink RO or inter-satellite RO, involves transmitting a radio signal between a transmitter and receiver. As the radio waves propagate, their trajectories are slightly bent due to refractivity gradients in the planetary atmosphere. These refractivity changes are linked to temperature and pressure in the neutral atmosphere, and electron density in the ionosphere.
In 2019, work commenced to enable mutual RO operations between ESA’s Mars Express (MEX) and ExoMars Trace Gas Orbiter (TGO). This configuration eliminates previous geometric limitations, enabling observations across the entire Martian day. This is the first instance of such an approach being routinely applied to a planet other than Earth. This thesis presents 71 RO profiles obtained using this method, including 34 from previously inaccessible midday regions.
The novel dataset enables new investigations into Martian aeronomy, particularly the diurnal variation of ionospheric layer altitudes and electron densities. The inclusion of midday profiles revealed that electron density in these regions varies far less than previously assumed. This analysis also provides conclusive evidence that the lower M1 ionospheric layer persists during midday and reveals a counterintuitive trend where thermospheric neutral temperatures increase with angle from the sun.
Finally, mutual RO is used to examine the Martian ionospheric response to solar weather events. A fortuitous measurement taken just 10 minutes after the impact of a major X3-class solar flare captured the largest M1 layer on record, with a 278% enhancement. This corresponded to a threefold increase in local X-ray irradiance. Contrary to prior assumptions that electron density scales with the square of solar intensity, this result suggests a more linear relationship between flare intensity and ionospheric response.