Vertical plasma drifts in magnetically controlled ionosphere of Mars 

Abstract

<p>Our focus in this paper is to explain the electron density (N<sub>e</sub>) profiles measured over the Martian high-latitude region by the radio occultation experiments performed onboard the Mars Express (MEX) and Mars Atmosphere and Volatile Evolution (MAVEN) orbiters. <sub>These</sub> N<sub>e</sub> measurements reveal that the Martian ionospheric structure is more complicated than previously thought.  The northern hemispheric N<sub>e</sub> profiles show wide and narrow shapes of the main ionospheric peaks, sudden enhancements and bite-outs in the topside plasma densities, and multiple topside N<sub>e</sub> scale heights. While the crustal magnetic field over the northern hemisphere is mostly horizontal, the resulting topside ionospheric loss is likely to be caused by diverging horizontal fluxes of ions. However, the situation at the measured N<sub>e</sub> locations in the southern hemisphere (near longitudes of 180<sup>o</sup>E) is different where the crustal field lines are nearly vertical and open to the access of solar wind plasma through magnetic reconnection with the interplanetary magnetic field.  This can lead to the acceleration of electrons and ions during the daytime ionosphere. The downward accelerated electrons with energies >200 eV penetrate deep into the Martian upper ionosphere along vertical magnetic field lines and cause heating, excitation, and ionization of the background atmosphere. The upward acceleration of ions resulting from energy input by precipitating electrons can lead to enhance ion escape rate and modify scale heights of the topside ionosphere. We use our 1-D chemical diffusive model coupled with the Mars - Global Ionosphere Thermosphere Model (M-GITM) to quantify the physical processes necessary to interpret the most dynamic dayside southern hemispheric N<sub>e</sub> profiles. Our model is a coupled finite difference primitive equation model which solves for plasma densities and vertical ion fluxes. The model assumes photochemical equilibrium condition for each ion at lower boundary located at 100 km altitude, while a fixed velocity boundary condition is assumed at upper boundary (400 km altitude) to simulate plasma loss from the Martian ionosphere. While the primary source of ionization in the model is due to solar EUV radiation, an extra ionization source due to precipitating electrons of 0.25 keV, peaking near an altitude of 145 km is added in the model.<sup>  </sup>We find that the photochemical control of the Martian ionosphere is ended at the height well above the ionospheric peak.  To interpret the measured ionospheric structure at altitudes where plasma transport dominates, we find it is necessary to impose field-aligned vertical plasma drifts caused by the motion of neutral winds.  The most interesting finding of this study is that both upward (between 110 ms<sup>-1</sup> and 150 ms<sup>-1</sup>) and downward (between -55 ms<sup>-1</sup> and -120 ms<sup>-1</sup>) drifts are required to maintain the topside N<sub>e</sub> distribution comparable with the measured distribution.  We also find that a fixed velocity boundary condition at the upper boundary with a sizeable upward ion velocity is needed to encounter any unexpected ion accumulation in the topside ionosphere to limit the Martian ion outflow.  It is determined that the variation of the topside N<sub>e</sub> scale heights is sensitive to the magnitudes of vertical drifts and the outward flow of plasma (10<sup>7</sup> ions cm<sup>-2</sup> s<sup>-1 </sup>for both O<sub>2</sub><sup>+</sup> and O<sup>+</sup>). We find that the impact of vertical drifts on the plasma transport is gradually weakened in the region below the transition height (the height between photochemistry and transport) as the thermospheric winds slowed down.  The Martian ionosphere in this region is found to be in photochemical equilibrium to maintain the model densities close to the measured densities.  The model results will be presented in comparison with N<sub>e</sub> measurements.  A discussion is also presented on estimates of ion escape rates from the upper ionosphere of Mars.  We acknowledge support for this work from the Research Office of the American University of Sharjah, UAE.</p>