Plasma Waves in the Distant Martian Environment: Implications for Mars' Sphere of Influence

We identify magnetic waves in the initial insertion orbits of the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission spacecraft far upstream of the Martian bow shock. Overall, the majority of the observed magnetic waves are elliptically polarized and more perpendicular than parallel relative to the mean magnetic field. We find and analyze numerous waves near 1 Hz. The handedness of these 1 Hz waves tends toward left-handed as the angle between the solar wind velocity and the magnetic field decrease. Similar waves are not seen in the Juno cruise data when it passed Mars' orbit (while Mars was more than 1 AU away). The presence of these waves out to distances of 14 Mars radii as observed by MAVEN and their absence in the Juno cruise data suggests the 1 Hz waves are amplified and/or created by the Martian environment. We also find a second, separate class of intermittent waves with frequencies near the proton cyclotron gyrofrequency. These waves are primarily right-handed polarized in the spacecraft frame. The spatial distribution of the waves suggests no clear association with the well-known plume of escaping ions that flows in the direction of the solar wind electric field. There are no obvious correlations between the wave amplitudes and the direction of the local interplanetary magnetic field or on their proximity to Mars. These characteristics lead us to conclude that the majority of these near-proton cyclotron gyrofrequency waves are likely whistler-mode waves that are not directly associated with Mars.