Abstract It is generally accepted that a fraction of H+ pickup ions originating from the Martian hydrogen (H) exosphere can be reflected and energized at the bow shock (BS), but the underlying reflection and acceleration mechanisms have not been explicitly identified. Using in situ observations from the MAVEN spacecraft, we present evidence for both shock surfing acceleration (SSA) and shock drift acceleration (SDA) of exospheric H+ pickup ions at the Martian quasi-perpendicular BS. On 12 August 2016, MAVEN detected a reflected H+ ion beam with energies of ~1.3-7.5 keV, propagating nearly along the solar wind convection electric field. The low-energy component is consistent with single reflections via SSA, while the high-energy component exceeds the maximum energy achievable from single reflections, indicating multiple reflections at the BS. These high-energy ions likely experienced either direct SDA or sequential acceleration involving SSA followed by SDA, depending on their incident normal velocities. The middle-energy component can be explained by either single or multiple reflections via SDA alone, or by a combination of SSA and subsequent SDA at the BS.
Chen et al. (Wed,) studied this question.