No abstract

Insight and other observations of the Martian surface at different locations have recorded the diurnal variation in surface pressure (Ps) with two rapid fluctuations that occur at dawn and dusk (around LT0800 and LT2000). These short-period surface pressure perturbations at specific local times are typically observed near Martian equinox. Similar phase-locked surface pressure fluctuations over most areas of the middle and low latitudes are simulated by the Martian General Circulation Model at the Dynamic Meteorology Laboratory (LMD). This phenomenon is thus likely to be global rather than local. By reconstructing the surface pressure variation from the horizontal mass flux, the pressure fluctuations in a sol can be attributed to the diurnal variation in the horizontal wind divergence and convergence in the Martain tropical troposphere in the GCM simulations. The background diurnal variation in Ps is related to the diurnal migrating tidal wind, while the enhanced convergence due to the overlap of the 4-hour and 6-hour tides before LT0800 and LT2000 is responsible for the Ps peaks occurring at dawn and twilgith. Although the amplitudes of the 4-hour and 6-hour tides are smaller than those of diurnal tides, the phases of these tides remain similar in the Martain troposphere, which suggests that the convergences and divergences due to 4 h/6 h tidal winds at different altitudes are in phase and together create a mass flux comparable to that induced by diurnal/semidiurnal components and lead to rapid pressure fluctuations.

Multilanders and rovers on Mars have observed the diurnal variation in surface pressure with two significant peaks at dawn and twilight (around 8:00 a.m. and 8:00 p.m.) at different locations near the equinox. The Dynamic Meteorology Laboratory Martian General Circulation Model (GCM) simulates similar phase‐locked surface pressure fluctuations. The surface pressure peaks at a certain local time occur at most mid‐ and low‐latitude locations, as GCM simulations suggested. By reconstructing the surface pressure variation from the horizontal mass flux obtained in the GCM simulations, we find that the pressure fluctuations are due to the diurnal variation in the horizontal wind divergence and convergence in the Martian tropical troposphere. The diurnal variation in surface pressure corresponds to the migrating diurnal tidal wind, with the enhanced convergence due to the overlap of the 4‐hr and 6‐hr tides before 8:00 a.m. and 8:00 p.m. Although the amplitudes of the 4‐hr and 6‐hr tides are smaller than those of diurnal tides, the convergences/divergences caused by the 4‐hr/6‐hr tidal winds at different altitudes are in phase and added up together to create a mass flux comparable to that induced by diurnal/semidiurnal components and lead to rapid pressure fluctuations. The 4‐hr/6‐hr tidal amplitudes and the two pressure peaks at dawn and twilight are the strongest near the equinox.

The seasonal variations in gravity wave (GW) vertical fluxes of zonal momentum and heat in the mesopause region are calculated using nearly 10 years (from 2012 to 2021) of temperature and wind observations by a narrowband sodium lidar located in Hefei, China (32°N, 117°E). Except in summer, the GW zonal momentum fluxes are mostly westward, with ∼80% contributed by the short‐period (10 min–1 hr) GWs, and anticorrelate with the eastward zonal wind. The mean westward zonal momentum fluxes in spring and winter are −2.55  and −1.78 m2/s2, respectively, which are much stronger than −0.98 m2/s2 in fall. The momentum flux in summer is primarily eastward, dominated by long‐period (1–16 hr) GWs. These seasonal variations in zonal momentum fluxes are mainly due to the filtering of GWs by the mean background wind at lower altitudes. The annual mean zonal momentum flux is westward with a value of −1.08 m2/s2. The calculated acceleration from flux divergence is westward below 93 km, opposite to the eastward mean zonal wind. The heat flux below 94 km is downward throughout the year, with no apparent seasonal variation. The annual mean heat flux is primarily downward, with a cooling rate of ∼−35 K/day between 87 and 95 km.