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This article is an editorial that discusses advances in mesosphere and thermosphere dynamics. The mesosphere-thermosphere is a coupled system that is influenced by both lower atmospheric and magnetospheric forces. Various atmospheric waves and disturbances can impact this region, affecting the ionosphere and circulation patterns. The article emphasizes the need for further research and observations to better understand this complex region and its interactions with the Sun-Earth system. The editorial also highlights five articles included in the research topic, covering topics such as atmospheric discharge, ionospheric electrodynamics, and mesosphere and lower thermosphere dynamics. The authors express gratitude to the contributors and reviewers for their valuable work. [Extracted from the article]

A prominent signature of dark bore front in the mesospheric O(1S) airglow emission is observed on a night in the late winter at Hanle (32.7oN, 78.9oE) located in the western Himalaya. The leading front was followed by a series of evident trailing waves and the event lasts for more than two hours. The characteristic features of the bore indicate it to be linear undular type. Instantaneous temperature profile shows presence of stable region through formation of thermal duct in the upper mesosphere possibly supported by chemistry and/or dynamics. With time, the bore fronts became faint in presence of ripples as they reached other side of imager field of view (FOV). During the initial period around 3–4 waves h−1 appear to enter the imager FOV. The observed average phase speed, period and horizontal wavelength are found to be 40 m/s, 12 min and 29 km, respectively. The bore fronts exhibit a clockwise rotation at a rate of around 5° h−1. The front edge perpendicular to the direction of propagation shows small-amplitude undulation indicating nonuniform duct structure. The tropospheric meteorological conditions may indicate plausible contribution from jet stream, weather front (linked with the Himalayan orography), and nonmigrating tides to excite and sustain the mesospheric bore event although further investigations in this direction are being sought to understand the actual underlying physical processes. [ABSTRACT FROM AUTHOR]

Observations of neutral winds from rocket release experiments within the premidnight and postmidnight substorm recovery phase aurora, show very large E-region neutral winds of several hundred m/s, where winds measured on the dusk side are even larger than those on the dawn side. These large winds are also associated with strong shears, and there is evidence that some of the regions below these shears may be unstable. The mechanisms which generate this strong vertical structure are not well understood. It is also not known whether the acceleration conditions in the pre and post midnight sectors of the aurora may produce significantly different neutral responses on the dawn and dusk sides. Simulations have been performed using a three-dimensional high resolution limited area thermosphere model to try to understand the neutral structure within the dawn and dusk side aurora. When simulations are performed using auroral forcing alone, for equivalent conditions within the dawn and dusk sectors, differences are found in the simulated response on each side. When measured values of auroral forcing parameters, and background winds and tides consistent with recent observations, are used as model inputs, some of the main features of the zonal and meridional wind observations are reproduced in the simulations, but the magnitude of the peak zonal wind around 140 km tends to be too small and the maximum meridional wind around 130 km is overestimated. The winds above 120 km altitude are found to be sensitive to changes in electric fields and ion densities, as was the case for the dawn side, but the effects of background winds and tides on the magnitudes of the winds above 120 km are found to be relatively small on the dusk side. The structure below 120 km appears to be related mainly to background winds and tides rather than auroral forcing, as was found in earlier studies on the dawn side, although the peak magnitudes of simulated wind variations in the 100 to 120 km altitude range are smaller than those observed. The source of the strong shears measured around I 10 km altitude on the dusk side is uncertain, but may be related to different kinds of oscillations, such as gravity waves, non migrating semidiurnal tides, or secondary oscillations produced by non linear interactions between waves.

Measurements of the neutral thermosphere within the postmidnight substorm recovery phase diffuse aurora show very large horizontal winds, and strong vertical structure. Rocket, satellite, and ground based observations during the ARIA (Atmospheric Response in Aurora) campaigns, and earlier dawn side rocket observations, indicate neutral winds of up to 200m/s, and a characteristic jet-like wind maximum around 110 to 120-km altitude, with strong shears above and below. The observed wind magnitudes are found to have a dependence on geomagnetic activity level, but recent modeling studies suggest that tides which propagate up from the troposphere and stratosphere may play an important role in generating the strong vertical variations in the neutral winds. The relative importance of auroral and tidal forcing in producing the measured wind structure is not known, however. Simulations have been performed using a three dimensional (3-D) high resolution limited area thermosphere model to understand the processes which generate the observed neutral structure within the postmidnight diffuse aurora. Parameters measured during the ARIA I observational campaign have been used to provide auroral forcing inputs for the model. Global background winds and tides have been provided by the CTIP (Coupled Thermosphere Ionosphere Plasmasphere) model. The sensitivity of the response of the neutral atmosphere to changes in different parameters has been examined. Variations in the amplitudes and phases of the propagating tides in the background winds are found to have significant effects on the neutral structure in the E region, and the wind structure below around I 10 km is found to be mainly produced by tidal forcing. Changes in the electric field and ion density affect the winds above around 120km, and the importance of auroral forcing is found to depend on background winds. Variations in the orientation of the aurora relative to the background field, which may be caused by changes in the interplanetary magnetic field, are also found to modify the wind structure. When both auroral forcing and propagating tides are included, many of the basic characteristics of the wind structure are displayed, although the great strength of the wind shears is not well reproduced. The strength of the shears may be related to a currently unmodeled process, or to different types of waves.

In this paper, we investigate the tidal activity in the mesosphere and lower thermosphere region at 37 0 N using OH Meinel and O 2 atmospheric airglow observations from 1998 to 2015. The observations were taken with a Spectral Airglow Temperature Imager (SATI) installed at Sierra Nevada Observatory (SNO) (37.06 0 N, 3.38 0 W) at 2900 m height. From these observations a seasonal dependence of the amplitudes of the semidiurnal tide is inferred. The maximum tidal amplitude occurs in winter and the minimum in summer. The vertically averaged rotational temperatures and vertically integrated volume emission rate (rotational temperatures and intensities here in after), from the O 2 atmospheric band measurements and the rotational temperature derived from OH Meinel band measurements reach the maximum amplitude about 1–4 h after midnight during almost all the year except in August–September where the maximum is found 2–4 h earlier. The amplitude of the tide in the OH intensity reaches the minimum near midnight in midwinter, then it is progressively delayed until 4:00 LT in August–September, and from there on it moves again forward towards midnight. The mean Krassovsky numbers for OH and O 2 emissions are 5.9 ± 1.8 and 5.6 ± 1.0, respectively, with negative Krassovsky phases for almost all the year, indicating an upward energy transport. The mean vertical wavelengths for the vertical tidal propagation derived from OH and O 2 emissions are 35 ± 20 km and 33 ± 18 km, respectively. The vertical wavelengths together with the phase shift in the temperature derived from both airglow emissions indicate that these airglow emission layers are separated by 7 ± 3 km, on average. [ABSTRACT FROM AUTHOR]

The diurnal tide (DT) and its variability in the lower atmosphere over Chongyang (114.14° E, 29.53° N) were studied based on the newly established Wuhan University (WHU) VHF radar observations with the height intervals of 0.145 km (below 9 km) and 0.58 km (above 9 km) in the whole year of 2012. We find that the DT was the dominant tidal component and showed remarkable height and season variations. A prominent seasonally dependent height variability characteristic is that maximum DT amplitude usually occurs around 6 km in the winter and spring months, which might be due to the tidal wave energy concentration arising from the reflections from the strong eastward tropospheric jet around 13 km and the ground surface. Our results suggest that the background wind is a crucial cause for height variability and seasonal variability of DT. In April 2012, a notable strengthening of DT is observed. Meanwhile, the significant higher harmonics of tides, i.e., the semidiurnal, terdiurnal, and even quarterdiurnal tides, can also be observed, which has seldom been reported. Interestingly, these four tidal components displayed consistent short-term variability, implying that they were excited by the same dramatically varying tidal source. In addition, we identified two symptoms of the coupling of DT and planetary waves (PWs), which can also lead to the short-term DT variability. One is the sum and difference interactions between DT and PWs, causing the tidal amplitude short-term variability as a consequence of the energy exchange among the interacting waves. The other one is the modulation of DT by PWs, leading to that the amplitude of DT varies with the periods of the PWs. [ABSTRACT FROM AUTHOR]

In this study we have used wind observation data from the mesosphere and lower thermosphere (MLT) region, obtained from meteor radar measurements in São João do Cariri (7.4° S, 36.5° W) from July 2004 to December 2008 and in Cachoeira Paulista (22.7° S, 45.0° W) from January 2002 to July 2006 and from September 2007 to November 2008. From the spectral analysis it was possible to identify the presence of planetary-scale oscillations in the hourly winds for the two latitudes and to study their transient character, which allowed elaboration of a climatology of planetary oscillation signatures. Planetary waves with periods near 2-days, 6-7 days, and 16 days were focussed on in this study. The quasi-2-day waves in the meteoric winds showed a seasonal cycle, with intense amplitudes occurring after the austral summer solstice and extending until the end of the season. The vertical wavelengths of the 2- day wave over Cachoeira Paulista were larger than those at São João do Cariri. A possible modulation of the quasi-2- day wave amplitudes by the quasi-biennial oscillation (QBO) has been observed only at São João do Cariri. The 6-7 day oscillations presented more intense amplitudes during August-November but were present with lower amplitudes during March-April at both sites. The 6-7 day vertical wavelengths over São João do Cariri were larger than at Cachoeira Paulista. The 6-7 day amplitudes exhibited intra-seasonal and annual behavior, however, there was no clear evidence of QBO modulation. The 16-day oscillations showed a seasonal cycle at São João do Cariri, with amplifications from austral spring to mid-summer and weaker amplitudes from autumn until early winter, however, there was no clear seasonality over Cachoeira Paulista. The 16-day vertical wavelengths have assumed values of λz ~45-85 km over both sites. 16-day wave amplitudes at the two sites showed different long-term behaviors. [ABSTRACT FROM AUTHOR]

Mesospheric data were analyzed by a composite method combining phase and group velocity tracing technique and the spectra method of Stokes parameter analysis to obtain the propagation parameters of atmospheric gravity waves (AGW) in the height ranges between 63.6 and 99.3 km, observed using the MU radar at Shigaraki in Japan in the months of November and July in the years 1986, 1988 and 1989. The data of waves with downward phase velocity and the data of waves with upward phase velocity were independently treated. First, the vertical phase velocity and vertical group velocity as well as the characteristic wave period for each wave packet were obtained by phase and group velocity tracing technique. Then its horizontal wavelength, intrinsic wave period and horizontal group velocity were obtained by the dispersion relation. The intrinsic frequency and azimuth of wave vector of each wave packet were checked by Stokes parameters analysis. The results showed that the waves with intrinsic periods in the range 30 min-4.5 h had horizontal wavelength ranging from 25 to 240 km, vertical wavelength from 2.5 to 12 km, and horizontal group velocities from 15 to 60ms-1. Both upward moving wave packets and downward moving wave packets had horizontal group velocities mostly directed in the sector between directions NNE (north-north-east) and SEE in the month of November, and mostly in the sector between directions NW and SWS in the month of July. Comparing with mean wind directions, the gravity waves appeared to be more likely to propagate along with mean wind than against it. This apparent prevalence for downstream wave packets was found to be caused by a systematic filtering effect existing in the process of phase and group velocity tracing analysis: A significant portion of upstream wave packets might have been Doppler shifted out ofthe vertical range in phase and group velocity tracing analysis. [ABSTRACT FROM AUTHOR]

Abstract: The effect of Sudden Stratospheric Warming (SSW) event of January 2008 on the Equatorial Ionosphere-Thermosphere System (EITS) over Trivandrum (8.5°N, 77°E, 0.5°N diplat.), a geomagnetic dip equatorial station in India has been investigated. It has been observed that the entire EITS exhibits significant variability during the SSW period. EITS parameters like Equatorial Electrojet (EEJ), F2 layer critical frequency (foF2), F1 layer base height (h''F), Total Electron Content (TEC) and 630nm thermospheric dayglow showed clear cut signatures of the SSW induced effects. It is suggested that dynamical perturbations associated with the SSW and subsequent modifications in the tidal components through wave–tidal interactions are responsible for such changes in the equatorial electrodynamics. The present study comprehensively brings out the quiet time equatorial ionosphere/thermosphere variability, during both night and daytimes, vis-à-vis the SSW event of January 2008. [Copyright &y& Elsevier]

Abstract: This paper reports the variation of the daytime zonal wind and OH temperature at upper mesospheric altitudes during the afternoon equatorial counter electrojet (CEJ) events over a geomagnetic dip equatorial station, Trivandrum (8.5°N, 77°E, 0.5°N diplat.), in India. The OH temperature is estimated using a unique Multiwavelength Dayglow Photometer and the zonal wind is obtained from a co-located Meteor Wind Radar. The CEJ events are identified by taking the difference between the surface magnetic field measurements of Trivandrum and Alibag (18.3°N, 72.6°E, 10.2°N diplat.), obtained using two Proton Precession Magnetometers. Two major features were observed associated with the CEJ events: (i) an eastward increase in the zonal wind at 98km altitudes and (ii) a clear-cut cooling in the mesopause, which in turn is proportional to the extent of the magnetic field reversal. These results are discussed in terms of the role played by the neutral winds, upward propagating gravity waves and the changes in the mesospheric chemistry. [Copyright &y& Elsevier]

Abstract: This paper presents for the first time a full decomposition of tidal signatures in three important ionospheric quantities, the equatorial electrojet (EEJ), vertical plasma drift and the crest-to-trough ratio (CTR) of the equatorial ionization anomaly. Data sources are the EEJM-2 model, ROCSAT-1 data and CHAMP electron density measurements. The analysis is based on data sampled around the solar maximum 23 (2000–2004). Full spectra of the predominant nonmigrating tides were determined. The tidal component DE3 is dominating the spectrum during the months around August in all three quantities. Conversely, DE3 disappears around December solstice everywhere. The August enhancement in EEJ strength is almost 3 times larger than that in plasma drift and CTR. The DE2 tide is strong during solstice months and shows minima around equinoxes. The relative amplitudes of the annual variations are much the same for the three investigated quantities. The EEJ and the zonal wind around 100km altitude exhibit almost identical DE2 and DE3 annual variations. Similarly, the vertical plasma drift and the zonal wind around 400km altitude show much the same DE2 and DE3 annual variations. But their phase values are quite different, making a direct interaction less probable. Clear DE2 and DE3 tidal signature are only found in ionospheric quantities during daylight hours. There is a suite of other nonmigrating tides, which can be explained by the interaction of migrating diurnal and semi-diurnal solar tides with stationary longitudinal structures. These tides are prominent during solstices and generally weak during equinoxes. [Copyright &y& Elsevier]

Because the TIMED satellite orbit precesses slowly in local time, 3 degrees (12 min) per day, thus apparently covering all local times at low and mid latitudes in 2 months when ascending and descending nodes are included, there is a potential for determining migrating and non-migrating tides from temperatures. Many papers have been written on the subject, but none have taken a realistic model atmosphere with high time resolution and tested tidal analysis. The present paper contains results from a tidal analysis of a SABER-sampled global model, the Canadian Middle Atmosphere (data assimilated) Model, CMAM-DAS, which has hourly values, and compares these with analysis using the full model. Fits to 60 d intervals at the equator show reasonable agreement between full and SABERsampled data analysis for mean, diurnal and semidiurnal tides, with or without inclusion of a linear detrend term. At 50° N where there is irregular coverage of local times, there are serious disagreements in all parameters except the background mean, and detrending has a significant effect. [ABSTRACT FROM AUTHOR]

Abstract: Horizontal winds in the mesosphere and lower thermosphere at a height of ∼90–95km have been measured by a VHF meteor radar in Castle Eaton, UK (52.6°N, 2.2°W). The measurements span the 16-year interval from January 1988 to January 2004. The complete dataset has been used to investigate the 8-h (terdiurnal) tide which at this latitude appears as a persistent feature of the atmosphere. Instantaneous amplitudes can be as large as ∼25ms−1. However, the monthly mean values are smaller and range from ∼1 to ∼9ms−1. A clear seasonal behaviour is evident, in which the largest amplitudes are observed in autumn and early winter (September–November) and the smallest amplitudes are observed in early summer (May). In general, the phase difference between the zonal and meridional components is close to 2h, indicating a circularly polarised motion. Conspicuous inter-annual variability is seen in both the amplitude and phase of the tide. On shorter timescales the amplitude of the tide sometimes exhibits a periodic modulation at frequencies associated with planetary waves (particularly the 2-day wave), indicating a degree of non-linear coupling between the tide and planetary waves. Comparisons with observations made by another meteor radar at Esrange (68°N, 21°E) in the Arctic suggest that at higher latitudes the winter time amplitude maximum is absent below heights of ∼95km. [Copyright &y& Elsevier]

Abstract: The tidal variability observed in the equatorial mesosphere–lower thermosphere (MLT) region over Ascension Island at a time when a very large quasi-2-day wave was present (January/February 2003) has been investigated. The results indicate that two different types of variability of the diurnal tide were present: (i) a change of the diurnal tidal mode when the period of the 2-day wave was very close to 48h and the amplitudes were very large, and (ii) a 2-day wave/tidal coupling when the period of the 2-day wave was different from 48h, the phases were stable, and the amplitudes were not so large as in the previous case. A secondary wave analysis was performed to prove the frequency and vertical wavenumber (wavelength) relationship between the primary and the secondary waves. A cross-correlation analysis between the amplitudes of the primary and secondary waves shed some light on the source of energy for the secondary waves. [Copyright &y& Elsevier]

Abstract: A new single-station empirical model of the dynamics of the mesosphere/lower thermosphere (MLT) region has been developed. The data for the model were recorded with a meteor radar located in the UK. The model uses hourly data in a “representative day” for each month, where the representative day for each month is obtained by a superposed epoch analysis. The F10.7 solar radio flux and the planetary geomagnetic Ap-index are included as external parameters. The monthly representative-day winds recorded over the UK near , in the interval January 1988–December 2000 have been used to construct the model. The accuracy of the model is discussed in terms of standard deviation and systematic error. In general, the model describes the zonal wind component more accurately than the meridional component. To assess the contribution of solar and geomagnetic influences, a quantitative comparison is made with a wind model in which these influences were not taken into account. The results show a noticeable contribution to the dynamics of the MLT region from changes in the solar/geomagnetic factors. The major contribution of the solar/geomagnetic influences is found to be evident in the variability of the 12-h tide. The solar/geomagnetic influences are more noticeable in the meridional wind component. The results also indicate tides of 8- and 6-h period, often considered to be insignificant, are present in the MLT region. [Copyright &y& Elsevier]

A network of 15 northern hemisphere radars has been used to measure horizontal winds in the mesosphere and lower thermosphere during the PSMOS campaign of Summer 1999. The radars are sited at latitudes ranging from 21°N to 75°N and longitudes from 142°E to 157°W. The data were examined to investigate the Northern Hemisphere structure of the quasi-2-day planetary wave during the interval June–August. The amplitude of the 2-day wave was found to exhibit great day-to-day variability. In particular, significant periodic fluctuations in amplitude occurred with periods of 8–10 and 14–17 days. These modulations were strongest in July and largely absent in June and August. In July, the wave activity can be resolved into three westward-propagating waves with zonal wave numbers of 2, 3 and 4. The periods associated with these wave numbers were 53–56, 48–50 and 42–43 h, respectively. The simultaneous presence of at least two spectral components with periods close to each other may serve to explain the observed amplitude modulations as a result of a beating between different spectral components. An earlier analysis of the planetary-wave field during this interval has revealed a westward propagating ∼16-day wave with zonal wave number 1 (Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002b) 1865–1896). A non-linear interaction between this ∼16-day planetary wave and the (3,0) Rossby-gravity mode (the 2-day-wave) provides a possible mechanism to generate the above ∼42 h/wavenumber 4 wave and the ∼55 h/wavenumber 2 waves as sum and difference secondary waves. A bispectral analysis was used to further investigate non-linear interactions between members of the planetary-wave field and suggested a number of interactions occur within the planetary-wave field, but that some of the interactions also involve the non-migrating diurnal tide with zonal wavenumber 6. [Copyright &y& Elsevier]

MF radar observations of winds in the mesosphere and lower thermosphere (MLT) at Wakkanai (45.4° N, 141.7° E) have been significantly contributing to the studies of MLT dynamics. The present paper is an attempt to describe the characteristics of the terdiurnal tide at Wakkanai. We have used the continuous data gathered during the 5-year period 1997–2001. Mainly, the analysis focuses upon the seasonal aspects of the terdiurnal tide characteristics. The observed results have been compared with the results reported at London, Ontario, another station located at a similar midlatitude (43° N) sector. It is observed that the terdiurnal tide is a regular feature in the MLT region. There exists strong day-to-day variability in the terdiurnal tide amplitudes. Short-time variability confirms that the terdiurnal oscillation can have an amplitude comparable to that of the diurnal and semidiurnal tides on occasions. Presence of dominant 8-h oscillation is observed in all seasons. However, a strong seasonal dependence is not observed in their amplitude variations. Amplitudes show slightly larger values in the winter season than the corresponding values in the summer season. The terdiurnal amplitudes at Wakkanai are more comparable to the diurnal tide amplitudes than the semidiurnal values. Comparison of the observed characteristics at Wakkanai and London reveals some similarities as well as differences. [Copyright &y& Elsevier]