A document by L. Claire Gasque. Click on the document to view its contents.

Recent evidence has revealed that strong coupling between the lower atmosphere and the thermosphere ($>$100 km) occurs on intra-seasonal (IS) timescales ($\sim$30-90 days). The Madden-Julian Oscillation (MJO), a primary source of IS variability in tropical tropospheric convection and circulation, can influence the generation and propagation characteristics of atmospheric tides and has been proposed as a significant driver of thermospheric IS oscillations (ISOs). Despite this progress, the limited availability of satellite observations in the ‘thermospheric gap’ region (ca. 100-300 km) and the inability of numerical models to accurately characterize this region have hindered a comprehensive understanding of this connection and the fundamental processes involved. In this study, an Ionospheric Connection Explorer (ICON)-adapted version of the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM), incorporating lower boundary tides derived from MIGHTI observations, is utilized to characterize and quantify the impact of the upward-propagating tidal spectrum on thermospheric ISOs and to elucidate connections to the MJO. Thermospheric zonal and diurnal mean zonal winds are shown to exhibit prominent ($\sim$20 m/s) tidally-driven ISOs throughout 2020-2021, largest at low latitudes ($\pm$30$^\circ$) near $\sim$110-150 km altitude. Correlation analyses demonstrate a robust (r$>$0.6) connection between the thermospheric ISOs, tides, and the tropospheric MJO, moreover, Hovm\”oller diagrams indicate eastward tidal propagation consistent with the MJO and concurrent SABER observations. This study demonstrates that vertically propagating tides play a crucial role in linking IS variability from the lower atmosphere to the thermosphere, with the MJO identified as a primary contributor to this significant whole-atmosphere teleconnection.

Recent evidence has revealed that strong coupling between the lower atmosphere and the thermosphere ($>$100 km) occurs on intra-seasonal (IS) timescales (~30-90 days). The Madden-Julian Oscillation (MJO), a key source of IS variability in tropical convection and circulation, influences the generation and propagation of atmospheric tides and is believed to be a significant driver of thermospheric IS oscillations (ISOs). However, limited satellite observations in the ‘thermospheric gap’ (100-300 km) and challenges faced by numerical models in characterizing this region have hindered a comprehensive understanding of this connection. This study utilizes an ICON-adapted version of the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM), incorporating lower boundary tides from Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) observations, to quantify the impact of the upward-propagating tidal spectrum on thermospheric ISOs and elucidate connections to the MJO. Thermospheric zonal and diurnal mean zonal winds exhibit prominent (~20 m/s) tidally-driven ISOs throughout 2020-2021, largest at low latitudes ($\pm$30$^\circ$) near 110-150 km altitude. Correlation analyses (r>0.6) confirm a robust connection between thermospheric ISOs, tides, and the MJO. Additionally, Hovmoller diagrams show eastward tidal propagation consistent with MJO and concurrent Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observations. This study demonstrates that vertically propagating tides play a crucial role in linking IS variability from the lower atmosphere to the thermosphere, with the MJO identified as a primary driver of this whole-atmosphere teleconnection. Understanding these connections is vital for advancing our knowledge in space physics, particularly regarding the dynamics of the upper atmosphere and ionosphere.

The influence of atmospheric planetary waves on the occurrence of irregularities in the low latitude ionosphere is investigated using Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X) simulations and Global Observations of the Limb and Disk (GOLD) observations. GOLD observations of equatorial plasma bubbles (EPBs) exhibit a ~6-8 day periodicity during January-February 2021. Analysis of WACCM-X simulations, which are constrained to reproduce realistic weather variability in the lower atmosphere, reveals that this coincides with an amplification of the westward propagating wavenumber-1 quasi-six day wave (Q6DW) in the mesosphere and lower thermosphere (MLT). The WACCM-X simulated Rayleigh-Taylor (R-T) instability growth rate, considered as a proxy of EPB occurrence, is found to exhibit a ~6-day periodicity that is coincident with the enhanced Q6DW in the MLT. Additional WACCM-X simulations performed with fixed solar and geomagnetic activity demonstrate that the ~6-day periodicity in the R-T instability growth rate is related to the forcing from the lower atmosphere. The simulations suggest that the Q6DW influences the day-to-day formation of EPBs through interaction with the migrating semidiurnal tide. This leads to periodic oscillations in the zonal winds, resulting in periodic variability in the strength of the prereversal enhancement, which influences the R-T instability growth rate and EPBs. The results demonstrate that atmospheric planetary waves, and their interaction with atmospheric tides, can have a significant impact on the day-to-day variability of EPBs.        

A unique phenomenon {\textendash} merging of Equatorial Ionization Anomaly (EIA) crests, leading to an X-pattern (EIA-X) around the magnetic equator {\textendash} has been observed in the night-time ionospheric measurements by the Global-scale Observations of the Limb and Disk (GOLD) mission. A whole atmospheric general circulation model simulation reproduces this pattern. The pattern is also produced in an assimilative ionosphere model that assimilates slant Total Electron Content (slant-TEC) from Global Navigation Satellite System (GNSS) and Constellation Observing System for Meteorology, Ionosphere, and Climate 2 (COSMIC-2). Due to the observed similarity between measurements and simulations, the latter is used to diagnose this heretofore unexplained phenomenon. The simulation shows that the EIA-X occurs in the afternoon to evening sector at a longitude where the vertical drift is negative, which is a necessary but not sufficient condition. The simulation was performed under constant low-solar and quiescent-geomagnetic forcing conditions, therefore we suggest that one of the drivers of this phenomenon is from lower-atmospheric processes.

Observations show that equatorial ionospheric vertical drifts during solar minimum differ from the climatology between late afternoon and midnight. By analyzing WACCM-X simulations, which reproduce this solar cycle dependence, we show that the interplay of the dominant migrating tides, their propagating and in-situ forced components, and their solar cycle dependence impact the F-region wind dynamo. In particular, the amplitude and phase of the propagating migrating semidiurnal tide (SW2) in the F-region plays a key role. Under solar minimum conditions, the SW2 tide propagate to and beyond the F-region in the winter hemisphere, and consequently its zonal wind amplitude in the F-region is much stronger than that under solar maximum conditions. Furthermore, its phase shift leads to a strong eastward wind perturbation near local midnight. This in turn drives a F-region dynamo with an equatorial upward drift between 18-1 hour local times.