Unusually Low-frequency Whistler-mode Waves and Their Association with High-energy Protons Observed Upstream of Martian Bow Shock

Whistler-mode waves upstream of planetary bow shock are often referred to as "1-Hz waves" due to the center of their observed frequency range being at similar to 1 Hz. A series of whistler-mode waves were observed upstream of the Martian bow shock by MAVEN on 2015 August 14, with unusually low frequencies centered at similar to 0.4 Hz. These waves were accompanied (though not synchronized) by the significant flux enhancement of high-energy protons up to similar to 10 keV. By analyzing the wave dispersion property and the wave-particle interaction condition, we find that the observed whistler-mode waves have the potential of resonating with protons of similar to 1 keV with large pitch angles up to nearly perpendicular to the background magnetic field, thereby providing a feasible means of accounting for proton acceleration. Our results indicate the possible origin of energized protons in the Martian environment through the interaction with whistler-mode waves, and their potential relationship with the unique upstream conditions.