Ionospheric Ambipolar Electric Fields of Mars and Venus: Comparisons Between Theoretical Predictions and Direct Observations of the Electric Potential Drop. Issue 3 (13th February 2019)

• Record Type: Journal Article
• Authors: Collinson, Glyn; Glocer, Alex; Xu, Shaosui; Mitchell, David; Frahm, Rudy A; Grebowsky, Joseph; Andersson, Laila; Jakosky, Bruce
• Is Part Of: Geophysical research letters. Volume 46:Issue 3(2019)
• Page Start: 1168
• Page End: 1176
• Publication Date: 2019-02-13
• Subjects: ambipolar fields -- Mars -- Venus -- comparative planetology -- ionospheric escape; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2018GL080597
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: 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 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. Plain Language Summary: Every planet with an atmosphere generates a weak electric field, called an "ambipolar field, " which plays a critical role in the escape of the ionosphere. Until recently, these fields had never been measured due to their low strength. However, by measuring the subtle shifts in the energies of electrons generated in the ionosphere, the total potential drop associated with this field has recently been measured at both Venus and Mars. These measurements permit us to make the first investigation of the fundamental physics that underpins this field. Specifically, we test the long‐held hypothesis that ambipolar fields are primarily generated by the gradient of electron pressure along magnetic field lines that connect the ionosphere to space. We find the potential drop at Mars is consistent with theory, but Venus' field is 10 times stronger than expected. Key Points: We map the ionospheres of Venus and Mars to investigate whether ambipolar fields are generated by the thermal electron pressure gradient Mars' ambipolar potential drop is consistent with what would be expected from existing theory (∼0.7 V peaking at 220 km) Venus' potential drop (10 V) far exceeds what can be explained by the mean electron pressure gradient (1 V), motivating further research