Plasma instabilities driven by cold ion outflow in the Mars magnetotail region

The planet Mars does not possess an intrinsic global dipole magnetic field. As a result, the solar wind interacts with the gravitationally bound and electrically conducting ionosphere to form an induced magnetosphere that acts to decelerate and deflect the incident solar wind flow about the planet. An important aspect of such an induced magnetosphere is the “draping” of interplanetary magnetic field lines about the obstacle. This draping can lead to field lines having one end anchored within the planetary ionosphere while the other end connects to the solar wind (then known as “open” field lines). On the nightside of the planet within the magnetotail region, these open field lines can provide a pathway for cold low-energy ionospheric ions to flow tailward and escape to space.

                  The MAVEN spacecraft commonly observes low frequency (<100 Hz) electric field waves in conjunction with this cold ion outflow. In this work, we discuss several characteristics of the observed waves; discuss possibilities for the underlying wave growth mechanism(s), and source(s) of free energy; and evaluate their impact on the outflowing plasma. Candidate instability mechanisms include the two stream instability between outflowing ionospheric protons and heavier O+ and O2+ ions or wave growth due to specific features within the energy and pitch-angle distributions of precipitating sheath electrons: when drawn out of the ionosphere by an ambipolar electric field, for example, the ions will be accelerated through the same electric potential, flowing at the same energy but different velocity, leading to instability.

                  The observed plasma waves coincide with modifications of the ion distribution functions (namely, emergence of energetic tails), suggesting the possibility that the waves act to decelerate protons and accelerate the heavier ions. We evaluate the impact of such a process on heavy ion escape rates. In particular, we evaluate how effective the process can be in shifting the heavy ion distribution function from initially just below escape energy, to just above it.