Traveling planetary‐scale waves in the lower thermosphere: Effects on neutral density and composition during solar minimum conditions

Abstract

The effects of breaking of traveling, planetary scale Rossby waves (TPWs) in the lower thermosphere are investigated with respect to the mixing of neutral constituents. We use numerical simulations of the Whole Atmosphere Community Climate Model, eXtended version, whose meteorology below 92 km is constrained by atmospheric specifications obtained from operational weather forecast/data assimilation system. The Fourier spectra show that the amplitude of TPWs with periods between 3 and 10 days are statistically significant in some years; the amplitude and phase of the band‐pass filtered behavior is consistent with the behavior of the 5 day wave. A wavelet analysis using the S‐transform shows that large variations with periods between 3 and 10 days can occur in relatively narrow temporal windows (20–30 days) during boreal winter. The momentum flux entering the lower thermosphere during the times of TPW amplification is shown to be large, and the amplifications of the TPWs in the thermosphere are not always associated with stratospheric sudden warming. The subtropical zonal accelerations are consistent with Rossby wave encountering a surf zone at low latitudes, resulting in wave breaking. The zonal acceleration is shown to be associated with a meridional diffusion, which is largest in the lower thermosphere where the wave activity and the wave breaking are also large. The ultimate effect on neutral density and composition is a meridional, down‐gradient mixing; although this horizontal diffusion is largest below 110 km, the effects on the composition are amplified with increasing altitude, due to the diffusive separation of the thermosphere.

Variability and predictability of the space environment as related to lower atmosphere forcing

“…And they are also similar in magnitude to the changes caused by tidal waves [e.g., Yamazaki and Richmond, 2013]. Recent numerical studies using WACCM-X by Sassi et al [2016] found that during periods of strong TPWs events (with periods between 3 and 10days), the dissipation of TPWs in the lower thermosphere reinforces the mean meridional circulation with stronger upwelling in the tropics and downwelling at high latitudes. This, along with fast molecular diffusion, reduces the O/N 2 and neutral density at high latitudes while slightly increasing these quantities at low latitudes.…”

“…TPWs in the winter hemisphere can propagate equatorward and break at subtropical surf zone in the lower thermosphere, as shown by Sassi et al []. In addition to causing mean flow acceleration, the wave breaking also results in a meridional, down‐gradient mixing of constituents.…”

Day‐to‐day variability in the thermosphere and its impact on plasmasphere refilling

“…The global zonal winds shown in Figure 4a typically range from 0 to 10 m/s eastward (i.e., superrotating); the large spike in January is the result of a sudden stratospheric warming. Figure 4b shows that the root variance of the zonal winds ranges from 0 to 25 m/s, with the lower values concentrated at high southern latitudes (compare with the greater traveling planetary wave activity in the Northern Hemisphere during this period reported by Sassi et al [2016]). Winds at this altitude are significant because the conductivity peaks near 100-120 km, allowing these winds to couple to the global wind-driven dynamo.…”

Impact of semidiurnal tidal variability during SSWs on the mean state of the ionosphere and thermosphere

“…However, the impact of these changes on the ionosphere and upper thermosphere remains unclear. It is additionally unknown whether these changes are induced by the SW2 or by gravity wave [Miyoshi et al, 2015] or planetary wave [Sassi et al, 2016] variability during SSWs.…”