Martian Low-Altitude Magnetic Topology Deduced from MAVEN/SWEA Observations

The Mars Atmosphere and Volatile Evolution (MAVEN) mission is the first to make comprehensive plasma and magnetic field measurements down to 150 km altitude over wide ranges of solar zenith angle, local time, longitude, and latitude. The Magnetometer (MAG) and the Solar Wind Electron Analyzer (SWEA) measure the magnetic field vector and the energy-angle distribution of superthermal (3-4600 eV) electrons along the spacecraft trajectory. This study presents pitch angle-resolved electron energy spectra, which we use to infer the plasma source regions sampled by the magnetic field line at large distances from the spacecraft, and in particular whether one or both ends of the magnetic field line have access to the day-side ionosphere. This is a key piece of information for deducing Martian magnetic topology. In the northern hemisphere at altitudes below 400 km, we find that magnetic field lines typically have both ends embedded in the collisional ionosphere, forming loops that connect distant regions on both the day and night hemispheres. This implies that this low-altitude region is dominated by closed crustal magnetic field lines, as opposed to the draped interplanetary magnetic field (IMF), which is prevalent at higher altitudes. Closed loops straddling the terminator allow transport of superthermal photoelectrons to the night hemisphere, which provides a source of ionization to support Mars' patchy night-side ionosphere. This study can also identify "open" field lines, with one end embedded in the ionosphere and the other end connected to the solar wind. This topology provides a conduit for ion outflow from the day-side ionosphere as well as precipitation of (possibly energized) solar wind electrons onto the atmosphere, which can also produce ionospheric patches and possibly auroral emissions.