Electric Mars: The first direct measurement of an upper limit for the Martian “polar wind” electric potential

Abstract

An important mechanism in the generation of polar wind outflow is the ambipolar electric potential which assists ions in overcoming gravity and is a key mechanism for Terrestrial ionospheric escape. At Mars, open field lines are not confined to the poles, and outflow of ionospheric electrons is observed far into the tail. It has thus been hypothesized that a similar electric potential may be present at Mars, contributing to global ionospheric loss. However, no direct measurements of this potential have been made. In this pilot study, we examine photoelectron spectra measured by the Solar Wind Electron Analyzer instrument on the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) Mars Scout to put an initial upper bound on the total potential drop in the ionosphere of Mars of Φ♂0.3em0.3em≾⊥0.3em0.3em2V , with the possibility of a further ≾4.5 V potential drop above this in the magnetotail. If the total potential drop was close to the upper limit, then strong outflows of major ionospheric species (H+, O+, and normalO2+) would be expected. However, if most of the potential drop is confined below the spacecraft, as expected by current theory, then such a potential would not be sufficient on its own to accelerate normalO2+ to escape velocities, but would be sufficient for lighter ions. However, any potential would contribute to atmospheric loss through the enhancement of Jeans escape.

Martian low‐altitude magnetic topology deduced from MAVEN/SWEA observations

“…Open magnetic field lines attached to the dayside ionosphere also provide possible passages for ion escape [e.g., Lillis et al ., ]. For example, cold ions may be accelerated by the ambipolar electric fields to reach the escape velocity [e.g., Collinson et al ., ], resembling the polar wind at Earth [e.g., Ganguli , ; Khazanov et al ., ; Glocer et al ., ]. Harada et al .…”

The Influence of Solar Wind Pressure on Martian Crustal Magnetic Field Topology

“…These electrons enter the atmosphere and impart energy, leading to impact ionization (Fillingim et al, , ; Lillis & Fang, ; Lillis et al, ; Němec et al, ), local heating (Fox & Dalgarno, ; Krymskii et al, ; Sakai et al, ), and the production of aurora (Brain et al, ; Haider et al, ; Leblanc et al, ). At the same time, magnetic fields also guide the outward flow of low‐energy ions, regulating ion escape processes in the Martian ionosphere (Collinson et al, ; Ergun et al, ; Jakosky et al, ).…”

“…These electrons enter the atmosphere and impart energy, leading to impact ionization (Fillingim et al, 2010(Fillingim et al, , 2007Lillis & Fang, 2015;Lillis et al, 2011;Němec et al, 2011), local heating (Fox & Dalgarno, 1979;Krymskii et al, 2004;Sakai et al, 2016), and the production of aurora (Brain et al, 2006;Haider et al, 1992;Leblanc et al, 2008). At the same time, magnetic fields also guide the outward flow of low-energy ions, regulating ion escape processes in the Martian ionosphere (Collinson et al, 2015;Ergun et al, 2016;Jakosky et al, 2018). The Martian plasma processes described above are governed largely by the presence of crustal magnetic fields (Acuna et al, 1998(Acuna et al, , 1999Brain et al, 2003), which interact and reconnect with the interplanetary magnetic field (IMF) to produce a unique and dynamic magnetic environment.…”

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

“…We find the potential drop at Mars is consistent with theory, but Venus' field is 10 times stronger than expected.At Mars, a preliminary pilot study by Collinson et al (2015) shortly after orbital insertion of NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft put an upper limit of less than ±2 V on the 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.…”