Longitudinal harmonic components in the Martian upper atmosphere observed by SPICAM and their possible tidal origins

The longitudinal variations of temperature and pressure in the upper atmosphere of Mars provide important observational constraints on atmospheric waves, including thermal tides. This study aims to characterize the longitudinal harmonic components associated with non-migrating thermal tides between 80 and 130 km and to examine their dominant tidal contributions. We analyzed nightside temperature and pressure profiles obtained by the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument on board Mars Express during Mars years 26–30, excluding the global dust storm period. We used simulations from the Mars Planetary Climate Model (PCM) to interpret the observed longitudinal structures. Wave--2 and wave--3 are the dominant longitudinal components in the SPICAM observations. In contrast, wave--1 is generally weak and its longitudinal variability is consistent with that in the PCM, including the diurnal zonal-mean tide (D0), the stationary planetary wave with zonal wavenumber 1 (SPW1), and the westward propagating semidiurnal tide with zonal wavenumber 1 (SW1). Above 110 , we also identify a robust wave--4 component. The mode decomposition further indicates that wave--2 and wave--3 are primarily associated with the eastward--propagating diurnal tide with zonal wavenumber 1 (DE1) and the eastward--propagating semidiurnal tide with zonal wavenumber 2 (DE2), respectively. Wave--4 mainly arises from contributions of the eastward-propagating diurnal tide with zonal wavenumber 3 (DE3) and the eastward--propagating semidiurnal tide with zonal wavenumber 2 (SE2). km These results provide new observational constraints on non-migrating thermal tides in the Martian upper atmosphere and show that higher-order longitudinal components, such as wave--4, become increasingly important above 110 km, particularly during dusty seasons. The combined observational and model-based analysis highlights both the robustness of the dominant tidal structures and the limitations of current models in capturing the full complexity of longitudinal variability in the upper atmosphere.