The Morphology of the Topside Martian Ionosphere: Implications on Bulk Ion Flow

Prior to the Mars Atmosphere and Volatile Evolution mission, the only information on the composition of the Martian ionosphere came from the Viking Retarding Potential Analyzer data, revealing the presence of substantial ion outflow on the dayside of Mars. Extensive measurements made by the Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer allow us to examine the morphology of the Martian ionosphere not only in unprecedented detail but also on both the dayside and the nightside of the planet. Above 300km, various ionospheric species present a roughly constant density scale height around 100km on the dayside and 180km on the nightside. An evaluation of the ion force balance, appropriate for regions with near-horizontal magnetic field lines, suggests the presence of supersonic ion outflow predominantly driven by the ambient magnetic pressure, with characteristic dayside and nightside flow velocities of 4 and 20km/s, respectively, both referred to an altitude of 500km. The corresponding total ion outflow rates are estimated to be 5x10(25)s(-1) on the dayside and 1x10(25)s(-1) on the nightside. The data also indicate a prominent variation with magnetic field orientation in that the ion distribution over regions with near-vertical field lines tends to be more extended on the dayside but more concentrated on the nightside, as compared to regions with near-horizontal field lines. These observations should have important implications on the pattern of ion dynamics in the vicinity of Mars. Plain Language Summary Prior to the Mars Atmosphere and Volatile Evolution mission, the only information on the composition of the Martian ionosphere came from the Viking Retarding Potential Analyzer data acquired on the dayside of Mars. Recently, extensive measurements made by the Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer allow us to examine the Martian ionosphere not only in unprecedented detail but also on both the dayside and the nightside of the planet. By analyzing these data, we find that on each side, many of the detected ion species share a common density structure at altitudes above 300km. Meanwhile, such a structure is clearly influenced by the ambient magnetic fields, which are well known to be inhomogeneous on Mars and cluster over the Southern Hemisphere. Near strong magnetic fields, the Martian ionosphere tends to be more extended on the dayside but more concentrated on the nightside. These findings reveal the presence of supersonic ion outflow on Mars. Such an ion outflow makes a significant contribution to plasma escape, which influences the long-term evolution of the planet.