UFKW Propagation in the Dissipative Thermosphere

Abstract

“Ultra‐fast” Kelvin waves (UFKWs) serve as a mechanism for coupling the tropical troposphere with the mesosphere, thermosphere and ionosphere. Herein, solutions to the linearized wave equations in a dissipative thermosphere in the form of “Hough Mode Extensions (HMEs)” are employed to better understand the vertical propagation of the subset of these waves that most effectively penetrate into the thermosphere above about 100 km altitude; namely, UFKWs with periods ≲4 days, vertical wavelengths (λz) ≳30 km, and zonal wavenumber s = −1. Molecular dissipation is found to broaden latitude structures of UFKWs with increasing height while their vertical wavelengths (λz) increase with latitude. Collisions with ions fixed to Earth's magnetic field (“ion drag”) are found to dampen UFKW amplitudes, increasingly so as the densities of those ions increase with increased solar flux. The direct effect of ion drag is to decelerate the zonal wind. This leads to suppression of vertical velocity and the velocity divergence, and related terms in the continuity and thermal energy equations, respectively, that lead to diminished perturbation temperature and density responses. Access is provided to the UFKW HMEs analyzed here in tabular and graphical form, and potential uses for future scientific studies are noted.

Thermosphere UFKW Structures and Ionosphere Coupling as Observed by ICON

“…The present results provide the first experimental evidence supporting the theoretical explanation for this difference posed by Forbes et al. (2023).…”

“…(2020) and Forbes et al. (2023). The present work is the first to experimentally verify these heretofore unnoticed model predictions.…”

“…Note that while the amplitude peaks in Figure 1 occur at 3 days and 3.5 days, the ionospheric responses near 2.5 days can be equal to or greater than those at longer periods. This suggests that the 2.5d UFKW is more electrodynamically efficient than the longer‐period waves, likely due to its longer vertical wavelength, better matching with the conductivity profiles, and ability to penetrate to higher altitudes (see Forbes, He, et al., 2020, 2023 for further insights on this point).…”

Different Response of the Ionospheric TEC and EEJ to Ultra‐Fast Kelvin Waves in the Mesosphere and Lower Thermosphere

“…The second type of UFKWs have longer period than the first type, but the maximum amplitudes are larger and the vertical wavelengths are longer than the first type. According to the dissipation formula in the paper of Forbes et al (2023), the vertical wavelength has more significant influence on dissipation than period. The second type of UFKW is therefore more likely to spread to higher altitudes, which supports our previous speculation that the second type of UFKW produces responses in the EEJ at higher altitudes.…”

Ultra‐Fast Kelvin Wave Packets in Mars' Atmosphere and Their Interactions With Tides as Viewed by MAVEN/NGIMS and MRO/MCS

“…With an analogy to terrestrial observational (e.g., Coy & Hitchman, 1984;Gu et al, 2014;Liu et al, 2015) and modeling (e.g., Lott et al, 2014;Nystrom et al, 2018) studies showing UFKW events to occur over a range of wave periods between about 2 and 6 days, UFKWs in Mars' thermosphere can be considered as a "wave packet" instead of a monochromatic wave. As discussed in previous terrestrial investigations (e.g., Forbes, Maute, & Zhang, 2020;Forbes, Zhang, & Palo, 2023), UFKWs emanate from specific longitudinal regions, necessitating a distribution of zonal wavenumbers to encompass these localizations and the transience in the forcing results in a distribution of wave frequencies. The resultant wave packet manifests as a constructive-destructive interference pattern arising from the superposition of a spectrum of waves encompassing various frequencies and zonal wavenumbers (e.g., Forbes, Maute, & Zhang, 2020).…”