Terdiurnal Tide Research Articles

Thermal tides in the middle atmosphere at mid-latitudes measured with a ground-based microwave radiometer

Abstract. In recent decades, theoretical studies and numerical models of thermal tides have gained attention. It has been recognized that tides have a significant influence on the dynamics of the middle and upper atmosphere; as they grow in amplitude and propagate upward, they transport energy and momentum from the lower to the upper atmosphere, contributing to the vertical coupling between atmospheric layers. The superposition of tides with other atmospheric waves leads to non-linear wave–wave interactions. However, direct measurements of thermal tides in the middle atmosphere are challenging and are often limited to satellite measurements in the tropics and at low latitudes. Due to orbit geometry, such observations provide only a reduced insight into the short-term variability in atmospheric tides. In this paper, we present tidal analysis from 5 years of continuous observations of middle-atmospheric temperatures. The measurements were performed with the ground-based temperature radiometer TEMPERA (TEMPErature RAdiometer), which was developed at the University of Bern in 2013 and was located in Bern (46.95° N, 7.45° E) and Payerne (46.82° N, 6.94° E). TEMPERA achieves a temporal resolution of 1–3 h and covers the altitude range between 25–50 km. Using an adaptive spectral filter with a vertical regularization (ASF2D) for the tidal analysis, we found maximum amplitudes for the diurnal tide of approximately 2.4 K, accompanied by seasonal variability. The maximum amplitude was reached on average at an altitude of 43 km, which also reflected some seasonal characteristics. We demonstrate that TEMPERA is suitable for providing continuous temperature soundings in the stratosphere and lower mesosphere with a sufficient cadence to infer tidal amplitudes and phases for the dominating tidal modes. Furthermore, our measurements exhibit a dominating diurnal tide and smaller amplitudes for the semidiurnal and terdiurnal tides in the stratosphere.

On the seasonal dependence of intermediate descending layer in low latitudes

Understanding the variable behavior of the IDL/Es system contributes significantly to our ability in characterizing and modeling the critical ionospheric dynamics across the globe. In this work we aim to improve the understanding of the IDL/Es system by analyzing the diurnal and seasonal variations in the altitude descent with reference to local time of the day, season, and solar activity. We apply the well-known height-time-intensity analysis method on measurements obtained from the Sharjah ionosonde station, located at the Arabian Peninsula (Sharjah: 25.28°N, 55.46°E) near the northern crest of the equatorial ionization anomaly. The measurements cover three years of the increasing phase of the solar cycle 25 from 2020 to 2022. Considering an oversimplified wind system based on windshear theory, we attempt to establish a relationship between IDL/Es periodicities with diurnal, semidiurnal, and terdiurnal tides without digging into the underlying mechanism. Results suggest a strong relation between semidiurnal and terdiurnal tides and the formation and transport of the IDL, particularly impacting lower E-region and sporadic-E layers during the afternoon and nighttime hours. The results of manual scaling of over eleven thousand ionograms is presented to quantify the behavior of the IDL/Es system's transport. Additionally, a connection between solar activity, descent rate, and initial descending height, with semidiurnal patterns consistently present across seasons and solar activity has been discussed. For different frequency bins used in this work, e.g. 3 MHz, 4 MHz, and 5 MHz, seasonal variations significantly influence descent patterns.

Seasonal and Interannual Variations of Global Tides in the Mesosphere and Lower Thermosphere Neutral Winds: II. Semidiurnal and Terdiurnal Tides

AbstractIn the mesosphere and lower thermosphere (MLT) region, semidiurnal and terdiurnal tides are dominant at middle and high latitudes and are important for the dynamics and structures. Using the global neutral horizontal wind data in the MLT region observed by TIMED Doppler interferometer from 2002 to 2021, the seasonal and interannual variations of six semidiurnal and terdiurnal tidal components (SW2, SW3, SW4, TW3, TW2 and TW4) are investigated. Particularly, the responses of these tidal components to the stratospheric quasi‐biennial oscillation (SQBO) and the solar cycle are presented. The results indicate that: (a) seasonal and interannual variations of these tidal components are pronounced in their peak regions. The peak region is defined as the region where the ratio of the annual mean amplitude to the maximum annual mean amplitude is larger than 0.8. (b) In their peak regions, migrating tidal components exhibit strong seasonal variations at annual (annual oscillations (AO)), semiannual (semiannual oscillations (SAO)) and terannual (terannual oscillations (TAO)) periods, whereas nonmigrating tidal components only exhibit prominent AO and SAO. (c) In their peak regions, the responses of the annual mean amplitudes of these tidal components to SQBO are negative; the responses of migrating tidal components to solar cycle are negative whereas those of nonmigrating tidal components are positive. (d) For these tidal components, the responses of the amplitudes of seasonal variations to SQBO and solar cycle are weaker than their annual mean amplitudes, and the latitude × altitude response patterns are not always consistent with those of the annual mean amplitudes.

Characteristics of IDL and Es layers and the impact of increasing solar activity on their descent at the Arabian Peninsula

Intermediate descending layers (IDLs) are often referred to enhanced plasma regions that are developed near lower-F/upper-E layers in the ionosphere. These layers often move down to the lower E-layers and merge with sporadic-E (Es) layers within a time span of several minutes to hours. The current study is focused on the investigation of the variability of IDL and Es layers’ diurnally and seasonally at a low-mid latitude region (Sharjah: 25.28oN, 55.46oE) close to the northern crest of equatorial ionization anomaly. We have used the very well-known technique of height-time-intensity (HTI) based on raw ionosonde data. It seems conclusive from our data that the diurnal, semidiurnal, and terdiurnal tides impact Es layer indirectly through the formation of IDLs in the upper-E region and their subsequent descent in the lower-E region. Also, the presence of descending IDL and Es layers follow a triannual pattern; the first one is a single layer diurnal pattern of descending early morning IDL during winter solstice and continued to be visible until spring equinox. The second one is the semidiurnal pattern prevails in autumnal equinox and the third one is a terdiurnal pattern in summer solstice. An important observation is the demonstration of the presence of terdiurnal tides being more prominent during lower solar activity period compared to higher solar activity.

Long-Term Observations of the Thermospheric 6 h Oscillation Revealed by an Incoherent Scatter Radar over Arecibo

We present an analysis of 6 h oscillations in the thermosphere ranging from 150 km to 400 km. The analysis applies 134 days of data from an incoherent scatter radar located at Arecibo Observatory (18.3°N, 66.7°W) from 1984 to 2015. To our knowledge, the climatological and seasonal characteristics of the 6 h oscillations in the thermosphere were investigated for the first time over Arecibo. The climatological mean amplitude of the 6 h oscillation in the thermosphere is about 11 m/s, and it increases slowly with altitude above 225 km. The climatological mean amplitude of the 6 h oscillation is comparable with semidiurnal and terdiurnal tides at Arecibo above 250 km. The climatological mean phase exhibits limited vertical variation. The 6 h oscillation is the most prominent in autumn, with amplitudes reaching around 20 m/s compared to approximately 10 m/s in other seasons. The phase structure in all seasons exhibits weak vertical variations. The responses of the thermospheric 6 h oscillation to solar and geomagnetic activities are also analyzed in this study. Our results indicate that at low latitude, solar activities have a small impact on the variation in the thermospheric 6 h oscillation, while it appears that the amplitude of the 6 h oscillation increases with increasing geomagnetic activity. Above 250 km, the amplitude of the 6 h oscillation reaches ~20 m/s during strong geomagnetic activity, which is almost twice of that occurring during weak geomagnetic activity.

Effects of the terdiurnal tide on the sporadic E (Es) layer development at low latitudes over the Brazilian sector

Abstract. Sporadic E (Es) layers are patches of high ionization observed at around 100–140 km height in the E region. Their formation at low latitudes is primarily associated with the diurnal and semidiurnal components of the tidal winds via the ion convergence driven by the wind shear mechanism. However, recent studies have shown the influence of other tidal modes, such as the terdiurnal tide. Therefore, this work investigates the effect of terdiurnal tide-like oscillations on the occurrence and formation of the Es layers observed over Palmas (10.17∘ S, 48.33∘ W; dip lat. −7.31∘), a low-latitude station in Brazil. The analysis was conducted from December 2008 to November 2009 by using data collected from CADI (Canadian Advanced Digital Ionosonde). Additionally, the E Region Ionospheric Model (MIRE) was used to simulate the terdiurnal tidal component in the Es layer development. The results show modulations of 8 h periods on the occurrence rates of the Es layers during all seasonal periods. In general, we see three well-defined peaks in a superimposed summation of the Es layer types per hour in summer and autumn. We also observed that the amplitude modulation of the terdiurnal tide on the Es occurrence rates minimizes in December in comparison to the other months of the summer season. Other relevant aspects of the observations, with complementary statistical and periodogram analysis, are highlighted and discussed.

Terdiurnal Radiational Tides

Abstract Terdiurnal atmospheric tides induce an S3 radiational ocean tide, similar to radiational tides S1 and S2 in the diurnal and semidiurnal bands. Although of small amplitude, the terdiurnal tide has some intriguing properties. The tide has an unusually pronounced seasonal variation, manifested by annual sidelines here denoted R3 and T3, which causes the tide to nearly vanish during times near an equinox. Forcing is generally largest in the winter hemisphere. Complicating matters, the two sideline frequencies coincide with those of nonlinear compound tides SK3 and SP3. Whether radiational tides or nonlinear tides (or both) are appearing at any given tide gauge can usually be determined by the relative amplitudes and phase differences of the two sidelines. The amplitudes of R3 and T3 are generally comparable; the amplitudes of SK3 and SP3 are not. Proper identification can lead to a small improvement in tidal prediction, but more importantly can lead to improved physical interpretation. An example from recent measurements under the Ross Ice Shelf bears on the role of nonlinearity in interactions between the ocean tide and the floating ice shelf.

Studies of atmospheric waves by ground-based observations of OH(3–1) emission and rotational temperature using PRL airglow InfraRed spectrograph (PAIRS)

We present a grating based spectrograph, Physical Research laboratory (PRL) Airglow InfraRed Spectrograph (PAIRS) which provides rotationally resolved OH(3–1) band nightglow emissions in the 1.5–1.6 μm spectral region at a resolution of 0.001 μm @ 1.532 μm that enables estimation of corresponding OH rotational temperatures from Thaltej Campus, Ahmedabad (23.0° N, 72.6° E) in India. The data cadence of PAIRS is 100 s providing an opportunity to study small timescale oscillations present at the OH emission altitude. In a first case study we present a detailed characterization of terdiurnal, large- and small-timescale gravity wave (GW) induced oscillations applying Krassovsky's method to nocturnal variations of the OH(3–1) brightness and its rotational temperatures measured with PAIRS. The amplitudes, phases, Krassovsky parameters, and vertical wavelengths of the terdiurnal tide and GWs are calculated using a least square fitting and wavelet analysis techniques. We also demonstrate capabilities of PAIRS in estimating the characteristics of GWs with periodicities smaller than 30 min, which showed upward wave propagation. The PAIRS derived rotational temperatures for 28 nights of observations for the month of December 2020 show good correspondence with the temperatures obtained from SABER for the passes close to the observational station. PAIRS has been automated for carrying out observations in an unattended mode and data analysis schemes have been developed for the derivation of mesospheric temperatures. Thus, PAIRS provides an opportunity for the detailed investigation of tides and GWs, in general, and short timescale nocturnal MLT dynamics, in particular.

A climatology of the nighttime thermospheric winds over Sutherland, South Africa

We present a climatology of nighttime thermospheric neutral winds between February 2018 and January 2019 measured by a Fabry-Perot interferometer (FPI) located in Sutherland, South Africa (32.2°S, 20.48°E; geomagnetic latitude: 40.7°S). This FPI measures the nighttime oxygen airglow emission at 630.0 nm, which has a peak intensity at an altitude of roughly 250 km. The annual meridional and zonal winds at this location vary between −100 and 120 m/s and show typical midlatitude nocturnal and seasonal variations. During local summer months (December-February), the meridional wind is predominantly equatorward from dusk to predawn. However, during the winter months, the meridional wind is poleward from dusk, turns equatorward around midnight, and either remains in this direction for the rest of the night (June) or turns poleward again just before dawn (July and August). The zonal wind velocity is generally eastward during the evening until just before midnight, changing westward post-midnight. The zonal wind peaks at higher velocities during the winter months compared to the summer months. The eastward-to-westward transition occurs later during the winter months compared to the summer months. We compare results from HWM14 with the FPI measurements and find a better agreement between FPI measured winds and HWM14 modeled winds for the meridional component compared to the zonal component. In addition, the HWM14 zonal wind consistently peaked several hours ( ∼ 3 h) prior to the measured wind. Furthermore, the HWM14-modeled eastward-to-westward transitions times were a couple of hours ( ∼ 2 h) earlier during the months of November to January creating what looks like a phase shift compared to the measured wind. This phase shift was apparent in all months and we suggest this can be attributed to a phase shift of the terdiurnal tide between the model and measurements.

Strong Quarterdiurnal Tides in the Mesosphere and Lower Thermosphere During the 2019 Arctic Sudden Stratospheric Warming Over Mohe, China

AbstractWe present an analysis of a quarterdiurnal tide (QDT) during the 2019 Arctic sudden stratospheric warming (SSW) event. The analysis is based on data obtained from a meteor radar located at Mohe (MH, 53.5°N, 122.3°E), China. The enhancement of the QDT amplitude is observed during the SSW and the maximum QDT amplitude is about 10 m/s in both neutral wind components. Using the meter radar measurements at MH in the period from August 2011 to April 2019, a probability density function of the QDT amplitude is estimated. The probability of the QDT amplitude larger than 8 m/s is less than 1% at MH. A clear seasonal variability of the QDT amplitude is apparent with the maximum amplitudes that appeared in winter. Based on our analysis, the seasonal variability of the QDT is not the main reason that amplified the QDT. According to the bispectral and bicoherence analyses, nonlinear interaction between tidal waves may contribute to the enhancement of the ODTs. The nonlinear interaction between two semidiurnal tides (SDTs) is important, while the effect of the nonlinear interaction between the diurnal tide and the terdiurnal tide is secondary. It may be possible that the effect of the 2019 SSW amplifies the SDT first and then the QDT is generated via nonlinear interaction of SDTs.

Unexpected Decrease in TW3 Amplitude During Antarctic Sudden Stratospheric Warming Events as Revealed by SD‐WACCM‐X

AbstractThe specified dynamics Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (SD‐WACCM‐X) was utilized to explore migrating tidal variabilities that occurred during the 2002, 2010, and 2019 Antarctic sudden stratospheric warming (SSW) events. All migrating tides show prominent day‐to‐day variations in the mesosphere and lower thermosphere regions, which can be mainly attributed to each Antarctic SSW event. The westward propagating diurnal tide with zonal wavenumber 1 (DW1) and the westward propagating semidiurnal tide with zonal wavenumber 2 (SW2) show a distinct reduction and an increase during each Antarctic SSW event, respectively. Specifically, the westward propagating terdiurnal tide with zonal wavenumber 3 (TW3) shows an unexpected decrease, which is opposite to the behaviors of TW3 reported during Arctic SSW events. We conclude that the unexpected decrease in TW3 may be mainly caused by the nonlinear interaction between DW1 and SW2, in which DW1 may play a major role. In the ionosphere, the TW3 amplitudes also show a decrease during each Antarctic SSW event, which is in consistency with decreasing tidal amplitudes in the neutral atmosphere. Our study demonstrates that TW3 is also a considerable source for short‐term ionospheric variability during Antarctic SSW events.

Ionospheric Signatures of Secondary Waves From Quasi‐6‐Day Wave and Tide Interactions

AbstractA sudden stratospheric waring occurred in the southern hemisphere during September 2019, accompanied by an exceptionally strong quasi‐6‐day wave (Q6DW). We examine the ionospheric response using global total electron content (TEC) maps, with a focus on the short‐period variability (5–48 h). A Fourier analysis of the TEC data reveals ionospheric variations associated with the secondary waves due to the non‐linear interaction between the Q6DW and atmospheric tides. The largest signatures among them are related to the ∼29‐h standing oscillation, which is attributable to the Q6DW interaction with the migrating diurnal tide, with the maximum amplitude ∼8% of the zonal mean. Also detected are the signatures associated with the westward‐propagating ∼13‐h oscillation with the zonal wavenumber 1 (∼4%) and westward‐propagating ∼11‐h oscillation with the zonal wavenumber 3 (∼3%), both of which can be attributed to the Q6DW interaction with the migrating semidiurnal tide. The signatures related to the Q6DW interaction with the migrating terdiurnal tide and some non‐migrating tides are also observed. This is the first time that secondary wave signatures of the Q6DW‐tidal interaction are identified in ionospheric observations with predicted zonal wavenumbers and periods. The oscillations are symmetric about the magnetic equator with amplitude peaks at ±20° magnetic latitudes, suggesting that the oscillations are generated by the modulation of the equatorial plasma fountain.

A brief review on the Atmosphere-Ionosphere coupling during Stratospheric sudden warming over the tropical region

Objective: A brief review on the effects of Sudden Stratospheric Warming (SSW) over the equatorial upper atmosphere (80-500 km) obtained using both ground and space-based measurements are presented. The emphasis is given to understand the role of semi-diurnal tidal activity during the SSW. Methods: In order to address this aspect, zonal wind data obtained using meteor wind radar over a tropical station; Trivandrum in India (8.50 N, 77.0 0 E) has been used. Wavelet analysis has been carried out to investigate the role of semi-diurnal and terdiurnal tides during the SSW event of January 2008. For characterizing the event, polar stratospheric temperature (PST) at 10 hPa, as obtained using National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) reanalysis data has been used. In order to characterize the normal behavior, the aforementioned database during December 2007 has been also looked into. Findings: It has been understood that the tropical upper atmosphere exhibits substantial changes in connection with SSW with an increase in the amplitudes of semi diurnal tides of the order of 10-15 m/s in comparison with the normal period. Over the equatorial ionosphere, the changes were observed right from the E region, extending upto the topside F region. It was noticed that the imprints of SSW were not only restricted to the neutral atmospheric parameters such as temperature, density, wind, but also found to have distinct signatures in the electrodynamic processes such as Equatorial electrojet (EEJ), Equatorial Ionization Anomaly (EIA), Equatorial Spread-F (ESF) as well. Novelty: The study, probably for the first time, has conclusively shown that it is not the terdiurnal tides, but the semi-diurnal tides, which are responsible for the SSW induced effects due to its non-linear interaction with planetary waves. Keywords: Stratospheric sudden warming; Equatorial ionosphere; waves and tides

Climatology and seasonal variation of the thermospheric tides and their response to solar activities over Arecibo

A long-term statistical analysis of thermospheric tides in an altitude range from 150 to 400 km is presented. The analysis is based on a dataset with 31 multi-day experiments conducted from an incoherent scatter radar at Arecibo Observatory between 1984 and 2015. This is the first time that the climatological mean and seasonal variations of the thermospheric tides and their response to solar activities are reported using an extensive dataset. The climatological mean amplitude of diurnal tide (DT) is dominant while the amplitudes of the semidiurnal tide (SDT) and terdiurnal tide (TDT) are comparable. Below 250 km, the SDT and TDT phases show downward propagation with vertical wavelengths of 300 and 240 km, respectively. Above 250 km, the DT is the most prominent component except in autumn while the SDT dominates below 250 km except in winter. Above 250 km, the DT is the strongest in winter and its amplitude varies around 28 m/s. Below 250 km, the SDT is prominent in autumn and fluctuates around 35 m/s. The TDT is the most important in winter compared to the other three seasons and its amplitude is slightly less than 20 m/s. The DT and SDT amplitudes show the opposite response to solar activity. Above 250 km, the former increases with increasing solar activity while the latter is the opposite. The enhanced DT amplitude and its phase structure under the high solar activity indicates that the in-situ EUV radiation plays a major role in generating the thermospheric DT over Arecibo.

A Comparison of Meteor Radar Observation over China Region with Horizontal Wind Model (HWM14)

This paper compares the wind fields measured by the meteor radar at Mohe, Beijing, Wuhan, and Sanya stations and horizontal wind model (HWM14) predictions. HWM14 appears to successfully reproduce the height-time distribution of the monthly mean zonal winds, although large discrepancies occur in wind speed between the model and measurement, especially in the summer and winter months. For meridional wind, the consistency between model prediction and radar observation is worse than that of zonal wind. The consistency between radar measurements and model prediction at Sanya station is worse than other sites located at higher latitudes. Harmonic analysis reveals large discrepancies in diurnal, semidiurnal, and terdiurnal tides extracted from meteor radar observations and HWM14 predictions.

Migrating tide climatologies measured by a high-latitude array of SuperDARN HF radars

Abstract. This study uses hourly meteor wind measurements from a longitudinal array of 10 high-latitude SuperDARN high-frequency (HF) radars to isolate the migrating diurnal, semidiurnal, and terdiurnal tides at mesosphere–lower-thermosphere (MLT) altitudes. The planetary-scale array of radars covers 180∘ of longitude, with 8 out of 10 radars being in near-continuous operation since the year 2000. Time series spanning 16 years of tidal amplitudes and phases in both zonal and meridional wind are presented, along with their respective annual climatologies. The method to isolate the migrating tides from SuperDARN meteor winds is validated using 2 years of winds from a high-altitude meteorological analysis system. The validation steps demonstrate that, given the geographical spread of the radar stations, the derived tidal modes are most closely representative of the migrating tides at 60∘ N. Some of the main characteristics of the observed migrating tides are that the semidiurnal tide shows sharp phase jumps around the equinoxes and peak amplitudes during early fall and that the terdiurnal tide shows a pronounced secondary amplitude peak around day of year (DOY) 265. In addition, the diurnal tide is found to show a bi-modal circular polarization phase relation between summer and winter.

Diurnal mesospheric tidal winds observed simultaneously by meteor radars in Costa Rica (10° N, 86° W) and Brazil (7° S, 37° W)

Abstract. This paper presents a study of diurnal tidal winds observed simultaneously by two meteor radars located on each side of the Equator in the equatorial region. The radars were located in Santa Cruz, Costa Rica (10.3∘ N, 85.6∘ W) (hereafter CR) and São João do Cariri, Brazil (7.4∘ S, 36.5∘ W) (hereafter CA). The distance between the sites is 5800 km. Harmonic analysis has been used to obtain amplitudes and phases (hour of peak amplitude) for diurnal, semidiurnal and terdiurnal tides between 82 and 98 km altitude, but in this work we concentrate on the diurnal component. The period of observation was from April 2005 to January 2006. The results were compared to the Global Scale Waves Model (GSWM-09). Magnitudes of zonal and meridional amplitudes from November to January for CR were quite different from the predictions of the model. Concerning phases, the agreement between model and radar meridional tidal phases at each site was good, and a vertical wavelength of 24 km for the diurnal tide was observed practically every month, although on some occasions determination of the vertical wavelength was difficult, especially for the zonal component, due to nonlinear phase variations with height. For the diurnal zonal amplitude, there were notable differences between the two sites. We attribute this site-to-site difference of the diurnal zonal amplitude to the nonmigrating component of the tide and propose that an anomaly was present in the troposphere in the winter (Northern Hemisphere) of 2005–2006 which produced substantial longitudinal variation.

Variability of Gravity Wave Effects on the Zonal Mean Circulation and Migrating Terdiurnal Tide as Studied With the Middle and Upper Atmosphere Model (MUAM2019) Using a Nonlinear Gravity Wave Scheme

Implementing a nonlinear gravity wave (GW) parameterization into a mechanistic middle and upper atmosphere model, which extends to the lower thermosphere (160 km), we study the response of the atmosphere in terms of the circulation patterns, temperature distribution, and migrating terdiurnal solar tide activity to the upward propagating small-scale internal GWs originating in the lower atmosphere. We perform three test simulations for the Northern Hemisphere winter conditions in order to assess the effects of variations in the initial GW spectrum on the climatology and tidal patterns of the mesosphere and lower thermosphere. We find that the overall strength of the source level momentum flux has a relatively small impact on the zonal mean climatology. The tails of the GW source level spectrum, however, are crucial for the lower thermosphere climatology. With respect to the terdiurnal tide, we find a strong dependence of tidal amplitude on the induced GW drag, generally being larger when GW drag is increased.

The characteristics of terdiurnal tides in the ionosphere

Studying the characteristics of terdiurnal tides in the ionosphere provides an important pathway to understand the dynamic coupling between the low atmosphere and the ionosphere. Based on the TEC data from the Madrigal database at the Massachusetts Institute of Technology Haystack Observatory, a decomposition and nonlinear fitting method is used to derive ionospheric terdiurnal tides. Statistical analysis is then carried out to study the variability of ionospheric terdiurnal tides. Both the absolute and relative amplitudes of terdiurnal tides are large near the equator and in the equatorial ionization anomaly (EIA) region. The relative amplitude of terdiurnal tides in the Northern Hemisphere is larger than that in the Southern Hemisphere at low latitude, exhibiting hemispheric asymmetry. The amplitudes of terdiurnal tides at magnetic middle latitudes (∼35 °N MLat) at the east hemisphere (142 °E), west hemisphere (90 °W) and zero degree longitude sites all have annual, semiannual and 4-month cycles. Furthermore, our analysis shows that there are longitudinal variations of terdiurnal tides and these variations appear to be dependent on solar activity. Terdiurnal tides show two peaks near the equinoxes, one in March and April, and the other between October and November. The terdiurnal tides with characteristics of large amplitudes in the equator and EIA regions, and their correlation with the diurnal and semidiurnal components suggest that terdiurnal tides and their intra-annual variations are likely related to the nonlinear interaction between diurnal and semidiurnal tides, which also show strong intra-annual variations.

Signature of a 120-day oscillation in the MLT winds and tides over São João do Cariri (7.4oS, 36.5oW)

A conspicuous and intermittent signature of a 120-day oscillation is found in the winds and tides in the mesosphere and lower thermosphere (MLT) over a Southern hemispheric equatorial station, São João do Cariri (7.4oS, 36.5oW) for the first time using meteor radar observations during the interval 2004–2008. The intermittent 120-day modulation can be as high as 10 m/s in the MLT winds. However, the seasonal component of the concerned periodicity, i.e. terannual oscillation is relatively smaller in amplitude (<4 m/s) than the instantaneous component. The oscillation feature is also investigated for the dominant tides. The 120-day modulation amplitude is higher (maximum ~ 5 m/s) in the diurnal and semidiurnal tides and relatively smaller in the terdiurnal tide (peak ~ 2 m/s). The intermittent behavior of the particular oscillation in wind and absence in temperature may imply it to be a feature of the MLT wind circulation. Proximity of period may also indicate a possible contribution from the lower atmospheric intraseasonal oscillation to some extent at times. Furthermore, existence of the same periodicity in the solar UV flux may indicate a plausible indirect solar connection with the MLT circulation although direct solar influence cannot be ascertained. The evident imprint of the oscillation in the MLT wind and tides accentuates the need of further extensive studies to lend valuable insights of the underlying physical processes/mechanisms associated to their relationship hitherto remained unexplored.