MAVEN Observations of Low Frequency Steepened Magnetosonic Waves and Associated Heating of the Martian Nightside Ionosphere

We present Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of low frequency steepened fast magnetosonic waves in the Martian magnetosphere and ionosphere. Solar wind pressure pulses generated in the upstream foreshock region impact the magnetopause and generate the magnetosonic waves within the magnetosphere, in a process analogous to the production of magnetic Pc pulsations in the terrestrial magnetosphere. The draped nature of the IMF about Mars, combined with the near-perpendicular propagation of these waves across the magnetic field, act to channel these waves into the nightside ionosphere, where they are observed in their non-linear steepened form. Coincident-in-time ion observations show that the light (H+) and heavy (O+, O-2(+), CO2+) planetary ion distribution functions possess significant suprathermal energetic tails, arising from wave-particle interactions with the steepened waves. The short gyro period and small gyro radius of the protons, relative to the steepened waves, results in proton heating via adiabatic compression. In contrast, the long gyro period of the heavy ions relative to the wave frequency leads to nonadiabatic heating via wave-trapping processes. The light and heavy ion species are heated above escape energy by these waves, even down close to the exobase. A limited statistical study of 101 neighboring orbits found that similar wave events occurred on 28% of orbits analyzed, suggesting that such wave-heating events may be important drivers of the Mars nightside ionospheric dynamics and energy budget. Our discussion includes placing our results in the context of solar wind energy transfer to the ionospheres of unmagnetized and magnetized bodies in general.