Ionospheric Ambipolar Electric Fields of Mars and Venus: Comparisons Between Theoretical Predictions and Direct Observations of the Electric Potential Drop

Abstract

We test the hypothesis that their dominant driver of a planetary ambipolar electric field is the ionospheric electron pressure gradient (∇Pe). The ionospheres of Venus and Mars are mapped using Langmuir probe measurements from NASA's 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 Earth's closest analog.

Ambipolar Electric Field in the Martian Ionosphere: MAVEN Measurements

“…Especial attention has been paid to the topology of the magnetic field (draped around the planet) and the effective location of the exobase as well as the altitude at which the ions transition from unmagnetized to magnetized. We compare and contrast our results with those of Xu et al () and Collinson et al () and provide some clarifying comments on the previous results.…”

“…They concluded on the total ambipolar potential difference of no more than 0.7 V on the dayside which supposedly arises from a <3‐μV/m electric field that is limited to the 100‐ to 300‐km range. A similar total potential difference was obtained by Collinson et al () who utilized the electron density and temperature measurements to estimate the electron pressure and subsequently the ambipolar electric field near dawn/dusk. Considering the escape energy of the major ions in the Martian ionosphere—0.1 eV for H + , 2.1 eV for O + , and 4.2 eV for O —a 0.7‐V ambipolar potential drop may not lead to direct escape of O + and O ions; however, its distribution around the exobase—that is, the critical region with respect to ion escape—could be the key in understanding the upflow and escape of the planetary ions at Mars.…”

“…Here, by adopting an approach similar to that of Collinson et al (), we revisit the ambipolar electric field in the Martian ionosphere with the focus on the dayside. Especial attention has been paid to the topology of the magnetic field (draped around the planet) and the effective location of the exobase as well as the altitude at which the ions transition from unmagnetized to magnetized.…”

Atmospheric Escape Processes and Planetary Atmospheric Evolution

“…Studies by Collinson et al. (2019) tend to indicate that ambipolar diffusion helps the ion escape at Venus, but it remains to be modeled exhaustively. The long‐lived doubly charged ions, observed/predicted in several ionospheres (Gronoff et al., 2007; Lilensten et al., 2005; Simon et al., 2005; Thissen et al., 2011), are easier to lift and therefore to escape through these processes.…”

“…Ion upwelling (Strangeway et al., 2005), which can be linked to the ambipolar electric field, but at levels that do not lead to escape, is one of these processes at Earth. For unmagnetized planets, ions are transported in the upper layers of the ionosphere by ambipolar electric field (Akbari et al., 2019; Collinson et al., 2019). The draping of the IMF could lead to additional induced field with respect to the processes described in section 2.4; this process is called the “snowplow” (Halekas et al., 2016).…”