MAVEN observations of ion accelerations within the Martian magnetotail current sheets

Mars lacks a global intrinsic magnetic field; therefore, the interplanetary magnetic field embedded in the solar wind plays a significant role in the formation of the Martian induced magnetosphere by draping around the planet's ionosphere. Localized crustal magnetic fields are nonuniformly distributed over the Martian surface, resulting in a complex electromagnetic field configuration that can significantly affect the draping pattern. Some magnetic field lines in the tail are connected to the ionosphere while others have passed through the ionosphere, so the Martian magnetotail is an important region where planetary ions are accelerated, leading to the atmospheric escape into the interplanetary space.

As the MAVEN spacecraft traversed the Martian magnetotail current sheets characterized by the Bx reversal in the MSO coordinates, it often detected planetary ions including H⁺, O⁺, O₂⁺, (and/or CO₂⁺) accelerated up to at least several hundreds of eV, presumably owing to the jxB force (as well as the resultant Hall electric fields). Previous studies reported that these energized ions within the current sheets have different peak energies, i.e., more heavy ions gain higher energies but slower velocities than lighter ions. Moreover, they also pointed out that the energy ratio of ion species tends to be approximately proportional to the square root of their masses. These results might provide us with some clues to understand the ion acceleration mechanisms; however, it remains poorly understood yet.

Here we investigate characteristics of ions observed within the Martian magnetotail current sheets, based on comprehensive plasma and field measurements from MAVEN. We performed a statistical survey of the current sheet crossings in the Martian magnetotail, and identified more than 500 events from November 2014 until May 2018. We will present number densities, bulk flow velocities, and peak energies of protons and heavy ions, and use this information to infer possible ion acceleration mechanisms within the Martian magnetotail current sheets.