We test the hypothesis that their dominant driver of a planetary ambipolar electric field is the ionospheric electron pressure gradient (∇P e). The ionospheres of Venus and Mars are mapped using Langmuir probe measurements from NASAs Pioneer Venus Orbiter (PVO) and Mars Atmosphere and Volatile Evolution (MAVEN) missions. We then determine the component of the ionospheric potential drop that can be explained by the electron pressure gradient drop along a simple draped field line. At Mars, this calculation is consistent with the mean potential drops measured statistically by MAVEN. However, at Venus, contrary to our current understanding, the thermal electron pressure gradient alone cannot explain Venus strong ambipolar field. These results strongly motivate a return to Venus with a comprehensive plasmas and fields package, similar to that on MAVEN, to investigate the physics of atmospheric escape at Earths closest analog.

We investigate how the Martian dayside ionospheric structure is modified by crustal magnetic field (CMF) strength and upstream solar wind pressure by analyzing electron density data from the Langmuir Probe and Waves instrument onboard the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft. We find that the electron density above the exobase is anticorrelated with the ratio of solar winds normal dynamic pressure ( P SW ⊥ ) to CMF magnetic pressure ( P CMF ). We also analyze the electron density behavior across different magnetic topologies as a function of P SW ⊥ / P CMF . The extremely low electron density in the draped topology relates to ionopause-like structures. The lower electron density in the closed and open topology under higher P SW ⊥ / P CMF may be attributed to a downward force, potentially the J × B force in the case of closed topology. This study highlights the complex interplay between solar wind and CMF in influencing the Martian dayside upper ionosphere.