Atmospheric tides and associated dynamics during two major boreal sudden stratospheric warmings (SSWs) have been investigated. The evolutionary Lomb Scargle and wavelet spectral analysis of specular ...meteor radar (SMR)‐derived hourly winds reveal evidence of non‐linear interactions between the semidiurnal solar tide and the quasi‐20‐day wave (Q20dw) during SSWs. The zonal wavenumber (ZWN) diagnosis indicates possible non‐linear interaction between the dominant semidiurnal migrating tide (SW2) and zonally symmetric 20‐day wave (20dw0) component, producing the secondary waves. The non‐linear interaction between the ZWN 2 component of stationary planetary wave (SPW2) and westward propagating 20‐day wave (20dwW2) in the stratosphere seems crucial to produce the 20dw0. As observed in the SMR‐derived wind spectra, the excited 20dw0 possibly interacts non‐linearly with SW2 to generate secondary waves. Therefore, the present study provides the first observational evidence of a two‐step non‐linear interaction associated with zonally symmetric planetary waves during major SSWs.
Plain Language Summary
The sun‐synchronous semidiurnal tide (SW2) is a major wave in the middle and high latitude mesosphere and lower thermosphere (MLT). Sudden stratospheric warming (SSW) is a polar winter hemispheric event characterized by enhanced planetary wave (PW) activity. Non‐linear interaction between the two waves produces secondary waves whose frequencies are sum and difference of the primary waves. Further, the secondary waves, having a frequency closely spaced to the tidal frequency, beat with the tide, resulting in modulation of the tidal amplitude by the PW's period due to the non‐linear interaction. The spectral analysis of specular meteor radar‐derived hourly winds supports this notion, and hence provides evidence for non‐linear interactions in the MLT. The dominant PW involved in the interaction is found to be zonally symmetric. The non‐linear interaction between the stationary PW and propagating PW in the stratosphere plays an important role in forcing the zonally symmetric component, that can reach MLT altitudes. Furthermore, non‐linear interaction between SW2 and the zonally symmetric PW produces the observed secondary waves in the MLT in the form of side bands in radar spectra. Overall, the present study provides the first observational evidence of a two‐step non‐linear interaction during SSWs.
Key Points
Spectral analysis on meteor radar winds provides evidence of non‐linear interaction between semidiurnal tide and quasi‐20‐day wave
Possible role of zonal wavenumber 2 stationary planetary wave in forcing zonally symmetric 20‐day wave in the stratosphere
First observational evidence of two‐step non‐linear interaction associated with zonally symmetric planetary waves during major sudden stratospheric warmings
An observational evidence of medium‐scale traveling ionospheric disturbances (MSTIDs) reaching to magnetic latitude as low as ~3.5° over the Indian sector is provided for the first time based on OI ...630‐nm airglow imaging observation from a low‐latitude station, Gadanki (13.5°N, 79.2°E; 6.6° magnetic latitude), on 12 January 2016. The horizontal wavelength, horizontal phase velocity, and period of the MSTID are found to be 160 ± 6 km, 138 ± 14 m/s, and 19.5 ± 3 min, respectively. These phase fronts are observed to move toward southwest with a propagation angle of ~235° ± 1° with respect to north. In addition to the MSTID, a strong quasiperiodic southward moving wave (QPSMW) from the evening to midnight interval and a small‐scale southward moving wave structure with wavelength and periodicity different from the QPSMW are also detected on the same night. Horizontal wavelength, horizontal phase velocity, and period of the QPSMW are estimated to be 367 ± 14 km, 131 ± 18 m/s, and 46.7 ± 13 min, respectively, and those of the small‐scale southward moving wave are found to be 157 ± 4 km, 121 ± 17.8 m/s, and 21.7 ± 3.4 min, respectively. Global Positioning System‐total electron content maps suggest that the weak and asymmetric equatorial ionization anomaly helped deep ingression of the MSTID on this night. The descent of the F layer seems to have caused the dissipation of the MSTID and QPSMW closer to the dip equator on this night. Therefore, the present investigation shows that the midlatitude MSTIDs can influence the F region plasma processes even over very low latitudes under favorable background conditions.
Plain Language Summary
Medium‐scale traveling ionospheric disturbances (MSTIDs) generated in the midlatitude are found to propagate toward the equator. While crossing the equatorial ionization anomaly crest region most of these structures dissipate due to ion drag. A deep ingression of the MSTID to a very low geomagnetic latitude (near to the geomagnetic equator) over the Indian sector is noted during the descending phase of the 24th solar cycle. Weak and asymmetric equatorial ionization anomaly formation is believed to support the deep ingression processes. On the observational night quasiperiodic southward moving waves and a small‐scale southward moving wave signature are also observed in addition with the MSTID. The dissipation of all these features is possibly due to existence of the midnight pressure bulge. Present study provides an observational evidence of the midlatitude MSTID propagation into the low latitudes and their interaction with the low‐latitude ionospheric/thermospheric processes, for example, midnight pressure bulge.
Key Points
A deep ingression of the MSTID to a very low geomagnetic latitude is noted during the descending phase of the 24th solar cycle
Quasiperiodic southward moving waves (QPSMW) and a small‐scale southward moving wave signature are also observed in addition with the MSTID
The dissipation of all these features is possibly due to existence of the midnight pressure bulge (MPB)
Mesospheric winds have been measured by meteor radar at Cachoeira Paulista (22.7°S; 45°W) since April 1999. The tidal components were analyzed over about 21 years of available data exhibiting an ...annual and semi‐annual variation. Amplitudes of meridional diurnal (semidiurnal) tide are on averaged 30% (28%) and the zonal ones are 14% (20%) stronger at solar minimum than at solar maximum. The anticorrelation between F10.7 cm solar flux and amplitudes of the semiannual oscillation of diurnal and semidiurnal tides is presented. Additionally, the sporadic E (Es) layers occurrence has an anti‐correlation with solar activity due to the tidal wind variation. A discussion about the physical mechanism is performed in terms of the particle precipitation during High‐Speed Stream (HSS) events according to the solar cycle. Finally, a superposed epoch analysis of the tidal amplitudes during the HSS events in 2018 is presented. And a slight increase in all tidal components is seen when the structure reached the Earth and in the following days showing that indeed the electron precipitation during HSS events affect the tidal amplitudes.
Key Points
Anticorrelation between tidal winds and solar cycle activity
Anticorrelation between sporadic E (Es) layer occurrence and solar cycle
Evidence of solar wind high speed streams influence in the mesosphere lower thermosphere (MLT) region winds
The relationship between the quasi-two-day wave (QTDW) and solar variability during summer in the MLT is studied using long-term meteor wind observations from an extratropical station, Cachoeira ...Paulista (22.7°S, 45°W) in the Southern hemisphere. Overall, the seasonal (summer) mean and monthly mean zonal amplitude of the QTDW show a negative correlation and the meridional amplitude exhibits a positive correlation with the solar F10.7 flux in the MLT. Although the seasonal mean (summer) wave period shows positive correlation with the solar cycle, both positive and negative correlations are found in the monthly mean period in certain summer months at the present location. Additionally, both amplitude and period of the QTDW show slightly higher values in solar minimum and lower values in solar maximum within the limit of standard deviation indicating a weak, but measurable response to the solar cycle. The features of the present study bearing similarity as well as disagreement with the findings of the past investigators are also discussed in the perspective of current understanding.
Using meteor radar observations of four years in the mesosphere and lower thermosphere (MLT) over a subtropical Brazilian station, Cachoeira Paulista (22.7°S, 45°W), the temporal variability ...characteristics of the diurnal tide have been studied. In addition to the semiannual, annual and interannual variations, the diurnal tide amplitude exhibits clear intermittent modulation at periods of planetary waves. The tidal amplitude exhibits clear seasonal pattern with largest amplitude in fall equinox. The dominant periods of modulation of the diurnal tide are found to be greater than 10 days in the MLT. The diurnal tide, as detected in the power spectra of the horizontal winds, shows a spread in period around the central period (24h) which is an indication of nonlinear interactions between the diurnal tide and planetary waves. A bispectral analysis reveals prominent triplets (two primary waves and a secondary wave) confirming the interaction of the diurnal tide with planetary waves persistent over a broad spectral range. Also there is an indication of coupling of the diurnal tide with the intraseasonal oscillations at various times of the year.
•Temporal variability of the diurnal tide amplitude is studied in the MLT.•Possible coupling between the diurnal tide and planetary waves is observed.•Bispectral analysis showed triplets of interacting diurnal tide and planetary waves.
Extensive study of middle atmospheric gravity waves (GWs) has been carried out to characterize seasonal variability of wave associated potential energy and vertical component of horizontal momentum ...flux with long term database (1998–2008) over Gadanki (13.5°N, 79.2°E), India. GWs are observed to transport significant energy and momentum flux with considerable variability throughout the seasons. Wave dissipation is found to be quite large (>30%) at upper mesospheric altitude. Dominant annual oscillation (AO) is observed in the energy and momentum flux pattern in the middle atmosphere. Power spectral density of the vertical wavenumber spectra of normalized temperature derived for the upper stratosphere and mesosphere exhibit resemblance in the saturated region. Logarithmic slope of vertical wavenumber spectra reveals more negative value and near to the model estimation in the upper stratosphere (∼−2.83) than the mesosphere (∼−2.43), which implies linear wave theory can explain stratospheric wave characteristics in a better way than mesospheric characteristics.
► GWs are observed to transport significant energy and momentum flux throughout the seasons. ► Wave dissipation is found to be quite large (>30%) at upper mesospheric altitude. ► Dominant annual oscillation (AO) is observed in the energy and momentum flux pattern.
An exclusive study has been carried out with long term meteor wind data (2000–2014) to characterize the quasi-2-day wave (QTDW) in the mesosphere and lower thermosphere (MLT) and its interactions ...with the longer period planetary waves at Cachoeira Paulista (22.7°S, 45°W). The QTDW is observed to be dominant during late summer (January–February) all the years under consideration except 2013. All the wave parameters exhibit significant interannual variability. The maximum wave amplitude comes out to be 39m/s, which is significantly higher than the reported northern hemispheric findings. The mean MLT period exhibits a wide range of variability (36–70h) indicating the presence of multiple Rossby normal modes with varying zonal wave numbers. Modulations of the QTDW amplitude by the planetary waves with longer periodicities (>9 days) are evident, especially during summer. The nonlinear interactions between the 2-day wave and longer period waves are believed to give rise to a host of secondary waves with frequencies lying close to 2-day. The strong QTDW activity, as observed at this location, has potential to cause significant effect on the overlying ionosphere and hence the atmosphere-ionosphere dynamical coupling.
•Variability and interaction of the quasi-2-day wave is studied in the MLT.•Wave coupling is found to be strongest in summer.•Wave parameters exhibit seasonal and interannual variability.
Systematic measurements using sodium (Na) airglow photometers were carried out from Mt. Abu (24.6°N, 72.7°E) and Gadanki (13.5°N, 79.2°E) during cloudless and moonless nights of winter and spring ...equinoctial months during 2006–2009. Na airglow intensity fluctuations with periodicities ranging from 5min to 3h are analyzed to determine the systematic pattern, if any, in the gravity wave modes that affect Na airglow emission. The power spectra derived from Na airglow intensity variation from Mt. Abu and Gadanki reveal common occurrence of 15–30min periodicities although spectral powers maximize at longer periodicities over these stations. Further, a few case studies were performed using mesospheric temperature and horizontal wind obtained from the SABER and TIDI onboard TIMED satellite nearly over both places. This investigation provides an experimental indication on the possible relationship between the gravity wave periodicities and the occurrence of neutral instabilities in mesosphere. Based on the case studies, it is shown that whenever the neutral instabilities occur within the Na airglow emission layer, the periodicities of gravity waves tend to become shorter in comparison with the periodicities that are observed when the neutral instabilities occur beyond Na airglow emission altitude or do not occur.
► We present systematic measurements of Na airglow from Indian low latitude stations. ► Power spectra indicate occurrence of short period waves in Na airglow region. ► Impact of neutral instability processes on the Na airglow intensity is discussed.
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