Steepened Ultra-Low Frequency Waves and Foreshock Dynamics in the Upstream Martian Bow Shock Region

Abstract

We presented the analysis of steepened ultra-low frequency (ULF) waves in the region upstream of Mars' bow shock observed by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We surveyed MAVEN measurements for magnetic fields and plasma signatures associated with compressive steepened waves, mainly (a) large-amplitude, quasi-periodic gradual increase followed by sharp decrease in |B| (i.e., shock-like) with corresponding complex rotations in the measured magnetic field, (b) enhanced proton and electron fluxes at energies greater than that of the solar wind, and (c) occasional observation of discrete, quasi-monochromatic higher frequency wave packets at the leading edges of the steepened waves. We identified 16 intervals when MAVEN observed steepened waves, and from these 16 wave intervals, we identified 79 individual steepened wave events (or wave cycles) over 1.5 years of MAVEN data. Our statistical analysis indicates that these steepened waves occur at an average frequency (in the spacecraft frame) of ∼0.5 local proton gyrofrequency (Ω_i), magnetic compression ratio of ∼3.52 and time separation of ∼48 s. Statistical analysis also indicates no obvious preference in the spatial occurrence of these steepened waves, with most events observed near the boundary between quasi-parallel and perpendicular shock regions; there is also no obvious correlation between their spatial occurrence and solar wind conditions (e.g., P_dyn and M_A). Our results support the idea proposed by earlier studies that the steepened wave events presented in this study are likely to be steepened from the ULF waves associated with the photo-ionization of the proton in the extended martian hydrogen corona.