1 Introduction: Solar Terminator Waves in the Terrestrial Thermosphere

Every night, the evening solar terminator (ST) sweeps across Earth, dividing daylight from shadow and interrupting the solar radiation which plays a key role in atmospheric heating and ionospheric plasma production. This generates abrupt gradients in atmospheric temperature and pressure, which can launch disturbances in the mesosphere and thermosphere (Somsikov, 2011). Broadly termed solar terminator waves (STWs), these disturbances form near and propagate with the advancing ST (Miyoshi et al., 2009). Although they theoretically recur every night, STWs' morphology and occurrence patterns remain poorly characterized, and their specific generation mechanisms are still debated.

STW generation theory first emerged when, inspired by Chimonas and Hines (1970)'s anticipation of gravity waves excited by time-variable heating during solar eclipses, Beer (1973) proposed a similar effect from the daily motion of the ST. Subsequent research delved deeper into the theoretical underpinnings of STWs, generally confirming that the moving ST can generate gravity waves, but the scarcity of observations hampered further advancement of this work (Beer, 1978; Cot & Teitelbaum, 1980; Somsikov, 1987; Somsikov & Ganguly, 1995).

Only three studies have reported observations of STWs in the thermosphere. Using the CHAMP satellite's tri-axial accelerometer, Forbes et al. (2008) identified an STW in thermospheric neutral densities. These had a ∼3,000 km horizontal wavelength, had phase fronts inclined ∼30° with respect to the ST, and were more pronounced during solstices than equinoxes. Subsequently, H. Liu et al. (2009) confirmed the density STW and also detected an STW in CHAMP's thermospheric cross-track (i.e., mainly zonal) winds. The zonal wind STW had comparable wavelength and inclination to the ST as the density STW, with zonal wind magnitudes ranging from 5 to 15 m/s, constituting 5%–20% of the mean zonal wind velocity at those local times. Both studies concluded that the STW was more prominent at dusk than at dawn, with most wave structures appearing on the nightside, only extending into the sunlit region around solstices. These results correlated well with General Circulation Model (GCM) simulations conducted by Forbes et al. (2008) and Miyoshi et al. (2009). In a third study, Bespalova et al. (2016) examined in-situ neutral density perturbations detected by the Atmospheric Explorer-E satellite, finding density perturbations with amplitudes of 2%–4% associated with the ST passage.

Recent modeling by Chou et al. (2022) and Vadas et al. (2023) suggests that STWs in neutral winds could have a more significant impact on equatorial thermospheric dynamics than previously considered. Using a Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (SD-WACCM-X) simulation from October 2020, Chou et al. (2022) identified a large-amplitude evening STW with phase fronts aligned from northwest to southeast, the same orientation as winter solstice STWs observed with CHAMP (Forbes et al., 2008; H. Liu et al., 2009). Chou et al. (2022) proposed that evening STWs play an underrecognized role in driving equatorial electrodynamic phenomena such as equatorial plasma bubbles (EPBs). Additionally, Vadas et al. (2023) identified STWs with horizontal wind magnitudes of 50–100 m/s in a HIgh Altitude Mechanistic general Circulation Model (HIAMCM) simulation of 15 January 2022. Although their primary focus was simulating the primary and secondary gravity waves triggered by the Hunga Tonga-Hunga Ha'apai volcanic eruption, the STW was surprisingly prominent in the simulation results and interacted non-linearly with the eruption-induced gravity waves. While both studies report STWs with significant neutral wind amplitudes and emphasize their potential influence on thermospheric and ionospheric dynamics, these conclusions remain to be confirmed with observational evidence.

This study presents the first remotely sensed measurements of evening STWs in thermospheric neutral winds, including the first STW meridional wind observations. By analyzing ∼1.5 years of data from NASA's Ionospheric Connection Explorer (ICON) satellite, we investigate seasonal variation in STWs, and compare these findings to simulations from several models. We also present the first observed altitude profiles of thermospheric evening STWs, comparing our observations with HIAMCM simulation results. This work confirms that STWs are prominent features in the terrestrial thermosphere, suggesting the necessity of future modeling and observational studies which will further enhance our understanding of STW drivers and effects.

2 Methods: Observations and Modeling

3 Results

Figure 1a presents NH winter MIGHTI meridional winds at ∼283 km, binned and averaged as a function of latitude and SLT. This altitude was chosen to minimize terminator-related data artifacts. Gray shading marks nighttime regions (for which the sun is below the horizon) at this altitude. Note that, although none of the tides have been removed in Figure 1a, the evening STW is a prominent feature in the meridional winds, appearing as a northward wind enhancement near the evening ST (the transition between illumination and shadow near 20 SLT). At most MIGHTI-sampled latitudes, the STW-related enhancement corresponds to the largest total meridional wind amplitude with respect to SLT, suggesting its potentially significant role in thermospheric and ionospheric dynamics, including field-aligned ion drag and interhemispheric transport (Heelis et al., 2022).

The second row of Figure 1 shows the tidal decomposition of Figure 1a into its diurnal (Figure 1b), semidiurnal (Figure 1c), and terdiurnal (Figure 1d) components. These components were obtained by fitting sinusoids with 24-hr, 12-hr, and 8-hr periods, respectively, as a function of SLT at each latitude. The amplitude of the colorbar is reduced for each successive tidal component, reflecting the diminishing power in each subsequent component. The final row (Figures 1e–1g) displays the residuals in the data after removing each successive tidal component.

Figure 2 shows the corresponding tidal decomposition of the observed zonal winds. In this case, the diurnal tide is more dominant than in the meridional winds, with the binned wind measurements in Figure 2a showing a clear pattern of westward winds during the day and eastward winds at night. However, with the removal of the diurnal tidal component (Figure 2e), an enhanced eastward wind feature emerges which has a similar amplitude and proximity to the evening ST as the northward meridional wind enhancement.

With the successive removal of the migrating diurnal, semidiurnal, and terdiurnal tidal components, the STW amplitude is reduced, although a distinct signature persists. This suggests the STW has power in multiple tidal components and is not attributable to any individual tide. Therefore, in the subsequent analysis, we characterize the STW after removing only the migrating diurnal tidal component.

Figure 3 displays binned MIGHTI meridional and zonal winds at ∼283 km for NH winter, combined equinox, and NH summer after removing the migrating diurnal tide. Black dotted lines identify the STW feature in the solstice cases. The NH winter STW is characterized by ∼50 m/s northward winds (Figure 3a) and ∼50 m/s eastward winds on the nightside, although the zonal wind component diminishes on the dayside (Figure 3b). In both meridional and zonal wind components, it has a ∼3,400 km scale size (calculated as described in Section 2.1), intersecting the ST between ∼15°–20° latitude (∼18.8 SLT) with a ∼31° phase front inclination compared to the ST.

For the NH summer case, MIGHTI's latitude sampling does not reach father south than −12°, where we might expect the NH summer STW to intersect the evening ST. However, we observe a ∼40 m/s southward wind enhancement (Figure 3e) and a ∼25 m/s eastward wind enhancement (Figure 3f), whose phase fronts, when extrapolated down to lower latitudes, intersect the evening ST between ∼−20° to −25° latitude (∼18.6 SLT). The scale size of this feature is ∼2,700 km, comparable to the NH winter STW, and its phase front is inclined ∼41° relative to the ST at the intersection point. Although there is also a strong northward wind component close to the evening ST and therefore associated with the evening STW during NH summer, a data artifact near the ST prevents us from characterizing it fully.

In combined equinox, the STW is less evident, even after removing diurnal tides (Figures 3c and 3d), consistent with Forbes et al. (2008)'s findings of seasonal asymmetry.

The NH winter and NH summer STWs mirror each other. The NH winter STW wavefront stretches from northwest to southeast with winds blowing northeast, while the NH summer STW wavefront extends from northeast to southwest with winds blowing southeast. Both have phase fronts inclined relative to the ST and intersect it ∼20° off the equator in the winter hemisphere, near 18.7 SLT. The persistent presence of the STW in solstice winds, even with long-term averaging, shows that it is a robust feature. While there is likely day-to-day variability, the large average magnitude of the STW implies that it is a dominant feature in thermospheric winds, at least under solstice solar quiet conditions.

While it is tempting to quantitatively compare our estimated amplitudes to previous studies, each study used a different filtering method, so it is necessary to use caution. Miyoshi et al. (2009) removed diurnal, semidiurnal, and terdiurnal tidal components from their simulation results, Forbes et al. (2008) applied high-pass filtering with a 4,800 km wavelength cutoff to CHAMP neutral density data, and H. Liu et al. (2009) subtracted a third order polynomial from CHAMP densities and zonal winds along each satellite track. This makes an analogous ICON analysis impossible due to the difference in orbital inclinations. Both Vadas et al. (2023) and Chou et al. (2022) presented unfiltered simulation results. The figures we show are binned and averaged to remove the non-migrating tidal features, and we report STW amplitudes after having removed the diurnal tide.

Analyzing simulation outputs alongside MIGHTI observations provides insight into the origins of STWs. Figure 4 displays NH winter meridional winds simulated by four different models, all with diurnal tides removed. In the TIEGCM run without HMEs (Figure 4a), there is no clear STW signature, though a weak (∼25 m/s) signature appears when HMEs are included (Figure 4b). In contrast, both SD-WACCM-X (Figure 4c) and HIAMCM simulations (Figure 4d) exhibit a distinct STW signature. Both models overestimate the STW amplitude relative to observations, with SD-WACCM-X producing ∼60 m/s northward winds and HIAMCM producing ∼100 m/s northward winds. It is important to be cautious when interpreting the HIAMCM's STW amplitude, however, as it is based on a single day and is not averaged like the other models and observations. It is possible that the amplitude is high in this case simply because the STW was particularly strong on this day and that an average, considering more daily variability, would reduce the amplitude. The slight amplitude overestimation in SD-WACCM-X is real since the SD-WACCM-X simulations are sampled identically to MIGHTI. Both models generally capture the STW's scale size and phase front inclination with respect to the ST.

The same figure, but showing the modeled zonal winds, is presented in Figure 5. Again, the TIEGCM run without HMEs (Figure 5a) shows little evidence of an evening STW. The case with HMEs does show an eastward wind enhancement near the evening ST, but the phase front is not comparably inclined with respect to the ST as the observed STW and does not intersect the ST at the sampled latitudes. Therefore, even with HMEs driving the lower boundary, the TIEGCM does not appear to accurately reproduce the observed evening STW. Both the SD-WACCM-X and HIAMCM simulations capture the STW signature in the zonal winds, although the signal does not diminish on the dayside as much as it does in the observations. Further discussion on the implications of STW appearance or absence in the various models is found in Section 4.

Figures 6a and 6b display the altitude structure of the NH winter STW in MIGHTI meridional winds and zonal winds, respectively. The data have been averaged data between 10° and 20° latitude, where the STW intersects the ST, and diurnal tides have been removed. Although MIGHTI data is available between 109 and 200 km during the day, the nighttime gap precluded the removal of diurnal tides at these altitudes, so we do not report any data in this altitude range.

Above 200 km, where nighttime MIGHTI data is available, the STW has a vertical wavelength greater than 200 km. Below 115 km, the STW is not distinguishable, although it may be masked by the large-amplitude tides at these altitudes. The SD-WACCM-X (Figures 6c and 6d) and HIAMCM (Figures 6e and 6f) simulations similarly suggest a nearly constant phase with altitude above 200 km. In their simulations, Miyoshi et al. (2009) similarly reported a nearly constant phase line with altitude above 250 km, descending with local time below. Below 200 km, both simulations show a descent of the phase line with local time, possibly indicating upward wave propagation. The variation with altitude for the NH summer case for MIGHTI observations and SD-WACCM-X simulations are presented in Figure 7.

4 Discussion

While STWs are believed to arise from traveling atmospheric pressure and temperature gradients, precisely where they originate in the atmosphere remains uncertain. Bespalova et al. (2016) suggested that neutral density perturbations observed following the ST might result from gravity waves generated in situ in the thermosphere by solar extreme ultra violet (EUV) heating gradients. In contrast, based on modeling results with and without lower atmospheric effects, Miyoshi et al. (2009) suggested that STWs may propagate up from the lower atmosphere, possibly in line with Chimonas and Hines (1970)'s initial proposal of STW generation initiated by heating due to UV absorption by middle atmospheric ozone.

Our analysis reveals a significant STW signature in solstice neutral wind observations. Both SD-WACCM-X and HIAMCM simulations capture the STW scale size and inclination relative to the ST, although the simulated STWs exceed the observed STW amplitudes. In contrast, TIEGCM simulations lack the STW signature, although introducing HMEs at the lower boundary leads to the emergence of a weak (∼25 m/s) signature in the meridional wind component.

Both SD-WACCM-X and HIAMCM simulate the atmosphere down to Earth's surface (Becker & Vadas, 2020; H.-L. Liu et al., 2018), whereas the TIEGCM cannot self-consistently resolve atmospheric processes below its 97 km lower boundary (Qian et al., 2014). Including ICON HMEs into the TIEGCM partially accounts for lower atmospheric effects by including data-informed diurnal and semidirunal tidal propagation up from the lower atmosphere (Maute et al., 2023), suggesting that global-scale waves from the lower atmosphere may play a role in STW generation. The presence of STW signatures in models with the lower atmosphere but their absence in those without suggests that the lower atmosphere plays an important role in STW generation. This aligns with Miyoshi et al. (2009), who found that excluding atmospheric dynamics below 80 km in their simulations resulted in the disappearance of STW signatures. Further, the weak STW signature in the TIEGCM simulations with ICON HMEs implies that diurnal and semidiurnal tides from the lower and middle atmosphere contribute to, but cannot fully explain, the STW. Although Miyoshi et al. (2009) found that upward propagating migrating tides contributed to STW formation, they suggested that STWs mainly arise from a superposition of these tides with zonal wavenumbers 4 to 6, while our results suggest that lower-order tides also play an important role.

Differences in how the models account for gravity wave effects may also affect their ability to reproduce STWs. STWs could be generated in part by large-scale gravity waves which either propagate directly from the lower/middle atmosphere to the thermosphere, or which are indirectly generated in the thermosphere through the dissipation of upward-propagating gravity waves (Heale et al., 2014; Lund & Fritts, 2012; Vadas, 2007). This latter “indirect” mechanism would arise because gravity wave dissipation by molecular viscosity depends critically on the background temperature, resulting in larger amplitude force/heating at lower altitudes on the nightside of the ST (Vadas, 2007). The resulting “jump” in the force/heating across the ST from gravity wave dissipation could then generate large-scale secondary gravity waves (Vadas, 2013).

Large-scale gravity waves arising from the ST passage would be captured by SD-WACCM-X and the HIAMCM, which resolve gravity waves from below, but not by the TIEGCM. Although direct EUV heating can also generate gravity waves (Chimonas & Hines, 1970; Vadas, 2013), the absence of STWs in the TIEGCM simulations suggests this mechanism is less significant. Notably, the amplitudes of stratospheric gravity waves have been found to be larger during solstice than equinox (Figure 6 of Cullens et al., 2022; Hoffmann et al., 2013), consistent with our finding of larger STW amplitudes during solstices, further supporting their potential connection to gravity waves. Furthermore, previous modeling and observations indicate that large (hundreds of kilometers) vertical wavelengths, like those we have observed for STWs, are consistent with a spectrum of gravity waves (Nicolls et al., 2014; Vadas, 2007; Vadas & Nicolls, 2009).

Future modeling studies will investigate these mechanisms, as well as possible non-linear tidal interactions, as the source of the STWs. Furthermore, the reason for the evening STW's inclination with respect to the ST remains an open question which future modeling should address.

Although we reported significant evening STWs, we do not observe any comparable signature near the morning ST. Both Forbes et al. (2008) and H. Liu et al. (2009) also noted this asymmetry, finding morning STWs to be less well-defined than their evening counterparts. H. Liu et al. (2009) postulated that larger neutral temperature gradients near the evening ST, as suggested by modeled neutral temperatures at 400 km, may make wave generation more efficient in the evening. Some authors suggested the opposite, claiming that the morning heating process is more efficient than evening cooling, resulting in a sharper sunrise gradient which produces smaller scale STWs (Somsikov & Ganguly, 1995).

Indeed, both Chou et al. (2022) and Vadas et al. (2023) report a smaller scale, weaker amplitude morning STW in their simulation results. Ionospheric studies have also shown evidence of morning STWs (Afraimovich, 2008; Ding et al., 2014; Galushko et al., 1998; Song et al., 2013). For example, Zhang et al. (2021) measured post-sunrise electron density perturbations using the Millstone Hill Incoherent Scatter Radar (ISR), identifying traveling ionospheric disturbances (TIDs) with zonal wavelengths of ∼445 km. If similarly sized thermospheric disturbances accompany these TIDs, it is unlikely that MIGHTI would be able to resolve them due to its horizontal resolution.

The thermospheric evening STW may play a currently under-recognized role in driving ionospheric dynamics. The large-amplitude winds reported in this study could influence ionospheric circulation through ion drag or dynamo effects. The meridional STW winds can push plasma along magnetic field lines, contributing to the summer to winter hemisphere redistribution of plasma (Heelis et al., 2022) and affecting the plasma density altitude distribution. Additionally, the F-Region zonal STW winds, when blowing across the westward conductivity gradient caused by changing solar input, may influence the upward plasma drifts of the prereversal enhancement (PRE) (Eccles et al., 2015; H.-L. Liu, 2020; Richmond et al., 2015). Variability in STWs may thus affect the PRE, which, in turn, is closely linked to equatorial plasma bubble (EPB) variability (Fejer et al., 1999).

5 Conclusion

Leveraging ∼1.5 years of MIGHTI data, this study reported the first remotely sensed observations of evening STWs, revealing them as one of the most prominent recurring features in the neutral winds above 200 km during solstices. The STW meridional wind component, reported for the first time, has a similar (and sometimes larger) magnitude compared to the zonal component, indicating that STW winds blow predominantly northeastward during NH winter and southeastward during NH summer. Furthermore, we provided the first observational altitude profile of a STW, revealing vertical wavelengths longer than several hundred kilometers above 200 km. Model comparisons suggested that STW generation is strongly influenced by the lower atmosphere and may result from large-scale gravity waves or their interactions with atmospheric tides.

Given their substantial and persistent presence, STWs hold intrinsic scientific significance, potentially serving as key drivers of thermospheric and ionospheric processes. Future research endeavors, including modeling and observations, are crucial for unraveling the origins and daily variability of these waves, fostering a deeper understanding of their impact on Earth's upper atmosphere.