Field-Aligned Electrostatic Potentials Above the Martian Exobase From MGS Electron Reflectometry: Structure and Variability

Field-aligned electrostatic potentials in the Martian ionosphere play potentially important roles in maintaining current systems, driving atmospheric escape and producing aurora. The strength and polarity of the potential difference between the observation altitude and the exobase (180km) determine the energy dependence of electron pitch angle distributions (PADs) measured on open magnetic field lines (i.e. those connected both to the collisional atmosphere and to the interplanetary magnetic field). Here we derive and examine a data set of 3.6 million measurements of the potential between 185km and 400km altitude from PADs measured by the Mars Global Surveyor Magnetometer/Electron Reflectometer experiment at 2 A.M./2 P.M. local time from May 1999 to November 2006. Potentials display significant variability, consistent with expected variable positive and negative divergences of the convection electric field in the highly variable and dynamic Martian plasma environment. However, superimposed on this variability are persistent patterns whereby potential magnitudes depend positively on crustal magnetic field strength, being close to zero where crustal fields are weak or nonexistent. Average potentials are typically positive near the center of topologically open crustal field regions where field lines are steeper, and negative near the edges of such regions where fields are shallower, near the boundaries with closed fields. This structure is less pronounced for higher solar wind pressures and (on the dayside) higher solar EUV irradiance. Its causes are uncertain at present but may be due to differential motion of electrons and ions in Mars's substantial but (compared to Earth) weak magnetic fields. Plain Language Summary The Aurora Borealis, or northern lights, occur when high-energy electrons from space collide violently with our upper atmosphere. These electrons receive their energy from strong electric fields that accelerate them downward along the force lines of the earth's global magnetic field. On Mars, accelerated electrons are also responsible for the aurora that has been observed by both the Mars Express and MAVEN spacecraft. In this study we use the observed properties of upward- and downward-traveling electrons measured by the Mars Global Surveyor spacecraft to deduce the strength of these important electric fields. We find that they are highly variable, as is to be expected from the dynamic interaction of the solar wind (a stream of charged particles constantly ejected by the sun) with the Mars upper atmosphere. However, these electric fields also show persistent patterns in regions where Mars' crustal rocks are most strongly and coherently magnetized. Here, the electric fields are predominantly downward where the magnetic field lines are more horizontal and predominantly upward where they are more vertical. Detailed modeling and further observations will be required to fully understand the plasma dynamics behind this phenomenon.