Eastward Wave Number Research Articles

Reducing the Ionospheric Contamination Effects on the Column O/N2 Ratio and Its Application to the Identification of Non‐Migrating Tides

AbstractPrior investigations have attempted to characterize the longitudinal variability of the column number density ratio of atomic oxygen to molecular nitrogen (O/N2) in the context of non‐migrating tides. The retrieval of thermospheric O/N2 from far ultra‐violet (FUV) emissions assumes production is due to photoelectron impact excitation on O and N2. Consequently, efforts to characterize the tidal variability in O/N2 have been limited by ionospheric contamination from O+ + e radiative recombination at afternoon local times (LT) around the equatorial ionization anomaly. The retrieval of O/N2 from FUV observations by the Ionospheric Connection Explorer (ICON) provides an opportunity to address this limitation. In this work, we derive modified O/N2 datasets to delineate the response of thermospheric composition to non‐migrating tides as a function of LT in the absence of ionospheric contamination. We assess estimates of the ionospheric contribution to 135.6 nm emission intensities based on either Global Ionospheric Specification (GIS) electron density, International Reference Ionosphere (IRI) model output, or observations from the Extreme Ultra‐Violet imager (EUV) onboard ICON during March and September equinox conditions in 2020. Our approach accounts for any biases between the ionospheric and airglow datasets. We found that the ICON‐FUV data set, corrected for ionospheric contamination based on GIS, uncovered a previously obscured diurnal eastward wavenumber 2 tide in a longitudinal wavenumber 3 pattern at March equinox in 2020. This finding demonstrates not only the necessity of correcting for ionospheric contamination of the FUV signals but also the utility of using GIS for the correction.

Migrating and nonmigrating tidal signatures in sporadic E layer occurrence rates

Abstract. We analyse sporadic E (ES) layer occurrence rates (OR) obtained from ionospheric GPS radio occultation measurements by the FORMOSAT-3/COSMIC constellation. Maximum OR are seen at 95–105 km altitude. Midlatitude ES layers are mainly due to wind shear in the presence of tides, and the strongest signals are the migrating diurnal and semidiurnal components. Especially in the Southern Hemisphere, nonmigrating components such as a diurnal westward wave 2 and a semidiurnal westward wave 1 are also visible, especially at higher latitudes. Near the equator, a strong diurnal eastward wavenumber 3 component and a semidiurnal eastward wavenumber 2 component occur in summer and autumn. Terdiurnal and quarterdiurnal components are weaker than the diurnal and semidiurnal ones.

The Characteristics of Traveling Planetary Waves in the Troposphere and Mesosphere on Mars

AbstractUsing the temperature provided by the Mars Global Circulation Model (MGCM) reanalysis data from Mars Year 30–32, we investigate the global characteristics of eastward traveling planetary waves (TPWs) with different periods in the Martian atmosphere. Based on the wavenumbers and frequencies, the TPWs in the Martian atmosphere are mainly divided into three modes: quasi‐3‐sol waves with eastward wavenumber 2, quasi‐6.5‐sol waves with eastward wavenumber 1, and quasi‐20‐sol waves with eastward wavenumber 1. The temporal, latitudinal, and altitudinal distributions of TPWs are revealed. The amplitudes of these TPWs are all stronger in the Northern Hemisphere than those in the Southern Hemisphere and are annually enhanced during autumn and winter. Quasi‐3‐sol waves are strongest in the lower troposphere from 40°N to 70°N. Quasi‐6.5‐sol waves dominate in the upper troposphere at the latitude from 55°N to 80°N. Quasi‐20‐sol waves exhibit a double‐peak structure in the upper troposphere and the mesosphere from 55°N to 82.5°N in autumn. Additionally, the features of solsticial pause are observed in the quasi‐3‐sol waves at the surface, in the quasi‐6.5‐sol waves from the surface to the middle troposphere, and in the quasi‐20‐sol waves in the lower troposphere. Our analysis suggests that the appearance of the solsticial pause is mainly related to the lack of strong wave sinks or wave sources during the winter solstice.

Investigation on the MLT tidal variability during September 2019 minor sudden stratospheric warming

Tidal variability in the mesosphere and lower thermosphere (MLT) during September 2019 Southern hemisphere minor sudden stratospheric warming (SSW) is investigated utilizing ground-based meteor radar wind observations from the equatorial, extratropical, middle, and high latitude stations and global reanalysis dataset. The polar warming is found to move from the mesosphere to the stratosphere until the peak warming day (PWD) of the SSW. The diurnal and semidiurnal tides at individual observational sites do not exhibit any consistent response during the observational interval, but a notable and consistent variability in some specific zonal wavenumber components, i. e., DW1 (migrating diurnal tide), DE3 (nonmigrating eastward wavenumber 3 diurnal tide), and SW2 (migrating semidiurnal tide) is found in the global reanalysis dataset. Incidentally, the warming event occurs during Spring equinox when a dominant seasonal change in the tidal activities generally takes place and hence seasonal variability is also looked into while identifying the SSW impact during the observational interval. It is found that the seasonal broad changes in the DW1, DE3, and SW2 amplitudes can be explained by the variability in the tidal sources, i.e., water vapor, convective activity, ozone, etc during the observational period. However, the extracted short-term variability in the global tidal modes on removing seasonal trend reveals noticeable response in connection with the warming event. The deseasoned amplitude of the DW1 significantly enhances around the PWD at most of the present latitudes. The deseasoned DE3 amplitude responds significantly in the middle atmosphere at low latitudes during the warming phase. The deseasoned SW2 exhibit clear enhancement around the PWD at all the latitudes. However, the deseasoned tidal features do not seem to correlate well with that of the source species unlike the seasonal ones that imply involvement of complex processes during the warming event, seeking further future investigations in this regard.

El Ni\xf1o\u2013Southern Oscillation effect on quasi-biennial oscillations of temperature diurnal tides in the mesosphere and lower thermosphere

In this study, we evaluate the El Niño–Southern Oscillation (ENSO) signals in the two dominant temperature diurnal tides, diurnal westward wavenumber 1 (DW1) and diurnal eastward wavenumber 3 (DE3) on the quasi-biennial oscillation (QBO) scale (18–34 months) from 50 to 100 km altitudes. The tides are derived from the 21-year (January 1996–February 2017) Ground-to-Topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) temperature simulations and 15-year (February 2002–February 2017) Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED)/Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature observations. The results show that ENSO warm phases shorten the period (~ 2 years) of the QBO in DW1 amplitude near the equator and DE3 amplitude at low latitudes of the Northern Hemisphere. In contrast, the QBO period lengthens (~ 2.5 years) during the ENSO neutral and cold phases. Correlation analysis shows the long-lasting effect of ENSO on the tidal QBO in the mesosphere and lower thermosphere.

Does the MJO Have a Westward Group Velocity?

AbstractCurrent theoretical studies have a debate on whether the Madden–Julian oscillation (MJO) has a zero or westward group velocity. A recent analysis of the observed Hovmöller diagram of MJO signals suggested that the MJO has a significant westward group velocity. Here it is shown that the observed MJO has a negligibly small group velocity, which is manifested in two aspects. First, on the wavenumber–frequency spectra diagram the precipitation spectra indicate quasi independence of the MJO frequency on wavenumber, suggesting a nearly vanishing group velocity. Second, on the Hovmöller diagram of the regressed intraseasonal daily precipitation, the MJO group velocity is defined by the propagation of the wave envelopes of the precipitation and is shown to be negligibly small for the eastward propagating signals. The causes of the discrepancy between this study and the recent study mentioned above are the calculating method and the data filtering process. The group velocity in the recent study is calculated by the propagation of local convection extrema, which does not necessarily indicate the propagation of the wave envelopes. More importantly, the westward propagation of the local convection extrema is an artifact of the data filtering. The Hovmöller diagram in the recent study was constructed by using only the eastward propagating wavenumber-1–5 signals. This truncation of data onto the planetary scales of the eastward wavenumber domain fails to resolve the Maritime Continent “barrier effect,” causing significant artificial westward propagation of local convection extrema.

SAMI3/SD‐WACCM‐X simulations of ionospheric variability during northern winter 2009

AbstractWe have performed simulations using the Naval Research Laboratory's physics‐based model of the ionosphere, Sami3 is A Model of the Ionosphere (SAMI3), to illustrate how neutral wind dynamics is responsible for day‐to‐day variability of the ionosphere. We have used neutral winds specified from the extended version of the specified dynamics Whole Atmosphere Community Climate Model (SD‐WACCM‐X), in which meteorology below 92 km is constrained by atmospheric specifications from an operational weather forecast model and reanalysis. To assess the realism of the simulations against observations, we have carried out a case study during January–February 2009, a dynamically disturbed time characterized by a sudden stratospheric warming (SSW) commencing 24 January 2009. Model results are compared with total electron content (TEC) from Jet Propulsion Laboratory global ionospheric maps. We show that SAMI3/SD‐WACCM‐X captures longitudinal variability in the equatorial ionization anomaly associated with nonmigrating tides, with strongest contributions coming from the diurnal eastward wave number 2 (DE2) and DE3. Both migrating and nonmigrating tides contribute to significant day‐to‐day variability, with TEC varying up to 16%. Our simulation during the SSW period reveals that at the Jicamarca longitude (285°E) on 27 January 2009 nonmigrating tides contribute to an enhancement of the electron density in the morning followed by a decrease in the afternoon. An enhancement of the semidiurnal eastward wave number 2 (SE2) and SE3 nonmigrating tides, likely associated with the appearance of the SSW, suggests that these tides increase the longitudinal variability of the SSW impact on the ionosphere. The conclusion is that realistic meteorology propagating upward from the lower atmosphere influences the dynamo region and reproduces aspects of the observed variability in the ionosphere.

Structure and origins of the Weddell Sea Anomaly from tidal and planetary wave signatures in FORMOSAT‐3/COSMIC observations and GAIA GCM simulations

AbstractThe Weddell Sea Anomaly (WSA) is a recurrent feature of the austral summer midlatitude ionosphere where electron densities are observed to maximize during the local nighttime. In this study, tidal decomposition is applied to FORMOSAT‐3 (Formosa Satellite)/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) total electron content (TEC) and electron density observations between 2007 and 2012 to quantify the components dominating local time and spatial variation in the WSA region. Our results present some of the first three‐dimensional spaceborne analyses of the WSA from a tidal perspective over multiple years. We find that the features of the WSA can be reconstructed as the result of superposition between the dominant diurnal standing (D0), eastward wave number 1 (DE1), westward wave number 2 (DW2), and stationary planetary wave 1 (SPW1) components in TECs, producing the characteristic midnight WSA peak. The D0, DE1, DW2, and SPW1 components are found to be an interannually recurring feature of the southern midlatitude to high‐latitude ionosphere during the summer, manifesting as enhancements in electron density around 300 km altitude of the summer middle to high magnetic latitudes. The phases of the aforementioned nonmigrating diurnal signatures in electron density in this region are near evanescent, suggesting in situ generation, rather than upward propagation from below. However, the SPW1 signature shows some signs of an eastward tilt with altitude, suggesting possible downward propagation. The relation of these components to possible generation via in situ photoionization or plasma transport along magnetic field lines is also discussed using results from the Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) general circulation model (GCM), connecting the tidal interpretation of the WSA to previously examined generation mechanisms.

Nonmigrating tidal modulation of the equatorial thermosphere and ionosphere anomaly

AbstractThe modulation of nonmigrating tides on both the ionospheric equatorial ionization anomaly (EIA) and the equatorial thermosphere anomaly (ETA) is investigated on the basis of simulations from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME‐GCM). Our simulations demonstrate the distinct features of the EIA and ETA seen in observations after the inclusion of field‐aligned ion drag in the model. Both the EIA and the ETA in the constant local time frame display an obvious zonal wave‐4 structure associated with the modulation of nonmigrating tides. However, the modeled EIA and ETA show a primary zonal wave‐1 structure when only the migrating tides are specified at the model lower boundary. Our simulations reveal that the zonal wave‐4 structure of the ETA under both low and high solar activity conditions is mainly caused by the direct response of the upper thermosphere to the diurnal eastward wave number 3 and semidiurnal eastward wave number 2 nonmigrating tides from the lower atmosphere. There is a minor contribution from the ion‐neutral coupling. The zonal wave‐4 structure of the EIA is also caused by these nonmigrating tides but through the modulation of the neutral wind dynamo.

Wave-driven variability in the ionosphere-thermosphere-mesosphere system from TIMED observations: What contributes to the “wave 4”?

[1] Numerous observations and model studies made during the past 5 years have unequivocally revealed that the ionosphere and thermosphere owe a considerable amount of their longitudinal, local time, seasonal latitudinal and day-to-day variability to waves originating in the lower part of the atmosphere. The most prominent pattern is the four-peaked (“wave 4”) longitudinal structure frequently observed by (quasi-) Sun-synchronous satellites in a variety of ionospheric and thermospheric parameters. The “wave 4” has often been attributed to the diurnal, eastward, wave number 3 (DE3), nonmigrating tide alone. A more detailed analysis of TIMED observations, supported by physics-based empirical modeling and data from the CHAMP satellite, now indicates that this interpretation needs to be revised. Secondary wave generation due the nonlinear interaction between the migrating diurnal tide and the DE3 leads to a large stationary planetary wave 4 (SPW4) and a large semidiurnal, eastward wave number 2 (SE2) tide in the equatorial zonal wind at E-region heights. Combined amplitudes can equal those of the DE3. SE2 penetrates into the upper thermosphere with transequatorial wind speeds in excess of 10 m/s. This paper discusses the resulting implications for electric field generation in the E-region and tidal-ionosphere coupling in the F-region and provides observational constraints for future modeling efforts.

Strong longitudinal variations in the OH nightglow

Airglow from the hydroxyl Meinel bands, originating from about 87 km, gives a signature of the atmosphere that can be observed remotely. Analysis of long term global observations of the 2.0 μm OH Meinel brightness observed by the TIMED/SABER satellite instrument presents some striking patterns that appear in the Meinel airglow. The analysis shows that migrating and non‐migrating tides have large effects on the nighttime OH airglow emission in the upper mesosphere. The OH airglow emission rate is positively correlated with temperature below 94 km and negatively correlated above. Variations with longitudinal wavenumbers 1 and 4 are shown to result from the impacts of the stationary (D0), westward wavenumber 2 (DW2), and eastward wavenumber 3 (DE3) nonmigrating diurnal tides.

Correlation between the ionospheric WN4 signature and the upper atmospheric DE3 tide

The present work studies the correlation relationship between the longitudinal ionospheric structure of wave number 4 (WN4) and the upper atmospheric tide of nonmigrating tidal mode DE3 (diurnal eastward wave number 3). Global ionospheric maps produced by the Jet Propulsion Laboratory were used to deduce the latitudinal integration of total electron content in the low‐latitude ionosphere, and TIDI/TIMED observations were used to retrieve the atmospheric zonal and meridional winds. By applying Fourier filtering and fitting techniques, the WN4 wave and DE3 tidal components are derived from the ionospheric and upper atmospheric observations, respectively. We found that the observed WN4 wave and DE3 zonal wind components experience very similar annual and interannual variations, but the DE3 meridional wind component behaves in a quite different manner. Both WN4 and DE3 zonal winds are very intense during northern summer and autumn; they also appear in the later spring, but tend to vanish in winter. Their amplitudes increase as the solar activity decreases, and both are stronger in the quasi‐biennial oscillation (QBO) eastward wind phase than in the westward phase. At the same time, the DE3 meridional wind likes to occur only in winter and seems not change with solar activity and QBO phase. We further studied the correlation between the WN4 wave and the two wind components of the DE3 tide. We found that the cross‐correlation coefficient between the WN4 wave and the DE3 zonal wind is much larger, while that between the WN4 wave and the DE3 meridional wind is relatively smaller. Such different correlations are attributed to the different latitudinal symmetry of different DE3 wind components. The DE3 zonal wind is likely in latitudinally symmetric tidal mode; hence, it can efficiently affect the F region ion drifts. In contrast, the meridional wind is mainly in antisymmetric mode and thus seldom affects the ionospheric drifts. The present results support the suggestion that the longitudinal WN4 structure in the ionospheric F region originates from the symmetric modes, mainly the zonal wind component, of the upper atmospheric nonmigrating tidal mode DE3 in the ionospheric E region.

Thermospheric nitric oxide variability induced by nonmigrating tides

The large influence of nonmigrating tides on nitric oxide variability in the lower thermosphere is revealed for the first time using data from the TIMED (tides) and SNOE (nitric oxide) satellites. Between March and October, a wave‐4 like longitude variation in nitric oxide density residuals is observed with peak‐to‐peak values of 40% between 30°S–30°N and 100–135 km. This variation becomes wave‐3 like in December and January with wave‐4/wave‐3 transitions in November and February. It is quantitatively shown that the nitric oxide longitude variations are caused by nonmigrating tides in neutral gas density. The DE3 (diurnal eastward wavenumber 3) tidal component causes the observed wave‐4 variation. The combination of DE2 (diurnal eastward wavenumber 2) and SE1 (semidiurnal eastward wavenumber 1) causes the observed wave‐3 variation. These results suggest that similar effects will be found in other constituents and airglow emissions important to the aeronomy of the mesosphere and lower thermosphere.

Comparative study of short‐term diurnal tidal variability

The wind and temperature measurements from an unusually long period operation of the sodium lidar at Colorado State University (41°N, 105°W) around September equinox 2003 showed significant short‐term tidal variability. Coincident with the large tidal changes, a strong temperature inversion layer was also observed above 90 km. Examination of the simultaneous temperature measurement from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument, on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite, not only confirms the existence of the inversion layer but also reveals the global nature of the inversion, suggesting the presence of a transient planetary wave in the mesosphere. The large tidal variability, therefore, is probably a consequence of the interaction between the transient planetary wave and tides. This possibility is investigated by using the NCAR thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) and by comparing model results with the lidar, SABER, and TIMED Doppler Interferometer (TIDI) measurements. With a large transient planetary wave specified at the model lower boundary, the model is able to produce strong diurnal tidal variability comparable to that from the lidar observation, and the modeled temperature inversion is similar to that from the SABER measurement. The model results suggest that the planetary/tidal wave interaction excites nonmigrating tides and modulates the gravity modes and/or the rotational modes of the diurnal migrating tide. Among the nonmigrating tides, the diurnal zonally symmetric (S = 0) component is the strongest, and its interaction with the planetary wave leads to a strong diurnal eastward wave number 1 component.