Quasi-diurnal Normal Mode Kelvin Wave Observed in the Atmosphere of Mars by the Pressure Sensor on the InSight Lander

Zonal wavenumber one (s = 1) Kelvin mode (K1) is one of the atmospheric normal modes. In the Martian atmosphere, the theoretical K1 wave period is ~22.8 Mars hours, which is quite close to a diurnal period. Thus, previous studies suggested that K1 becomes near resonance with s = 1 eastward nonmigrating tide (DE1). However, there have been few studies that focus on K1 and directly detect it in observational data, although numerous studies have investigated DE1.

The purpose of this study is to extract K1 using InSight surface pressure measurements and examine its seasonal variations in terms of amplitudes and frequency. The InSight data used in this study span more than one Martian year from late in Mars Year (MY) 34 to the beginning of MY36 including the C dust event in MY34 (C34), which impacted considerably the global meteorology. To distinguish K1 from diurnal signals, Singular Spectrum Analysis (SSA) was used. SSA is commonly used for the analysis of non-stationary and quasi-periodic signals. This method allows us to separate the trend, quasi-periodic signal components, and noise.

In addition to the strong diurnal signals (S1), SSA decomposition captured quasi-diurnal signals whose period is ~22 Mars hours. Although we cannot separate the propagation direction using only one point observation, K1 is the only mode that corresponds to a period slightly shorter than 24 Mars hours. Thus, we regarded this signal with the period of ~22 Mars hours as K1. During C34, a pronounced K1 signal was detected. The amplitudes of K1 and S1 reached their maximum simultaneously, with peak values of ~8 Pa and ~39 Pa, respectively. Near the peak time, K1 and S1 were in phase, and interestingly the S1 period shortened to ~23.5 Mars hours. Afterward, K1 rapidly decayed and the S1 period recovered to ~24 Mars hours. Judging from the phase relation between K1 and S1 and the S1 period shortening, it is considered that a resonance occurred between DE1 and K1. It is also found that the temporal evolution of the S1 and K1 amplitudes appears to correlate with the zonally non-uniform distribution of dust. DE1 is considered to be generated through the interaction of diurnal forcing with a lower-boundary inhomogeneity, especially the s = 2 topography. During the early stage of the dust storm, the dust loading is zonally non-uniform, which provides strong zonally asymmetric thermal forcing similar to the effect of topography. It is inferred that this transient forcing enhances DE1 and consequently amplifies K1.