The Penetration of Draped Magnetic Field Into the Martian Upper Ionosphere and Correlations With Upstream Solar Wind Dynamic Pressure

International audienceOpen and draped magnetic field topologies are important at Mars because they can provide ionospheric particles a path to escape to space. Four years of Mars Atmosphere and Volatile EvolutioN data are analyzed in this study, demonstrating that the altitude at which the ionospheric density drops below 102 cm-3 is essentially coincident with the altitude down to which open and draped magnetic field lines are observed in the ionosphere. During times of enhanced solar wind dynamic pressure, a greater fraction of the magnetic topology was observed as open or draped (as opposed to closed) above densities of 102 cm-3. The altitudes at which the ionospheric density fell below 102 cm-3, and the magnetic field topology transitioned from closed to open or draped, also decreased during higher dynamic pressure conditions. Times of enhanced solar wind dynamic pressure thus appear to drive greater penetration of draped magnetic field into the ionosphere, enhancing the rate of reconnection between draped and crustal magnetic fields and producing more open field. Such observations may have implications for the long-term evolution of the Martian ionosphere; the historic solar wind is thought to have been denser and faster than present-day conditions, and "quiet time" conditions may have been equivalent to extreme dynamic pressure events today. Depending on past atmospheric conditions at Mars, draped topology may have routinely penetrated deep into the ionosphere, and quiet time rates of ionospheric escape to space may thus have been much greater for early Mars than today