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To selectively control the electrical resistance, thin films of TiO2 were irradiated with a YVO4 third‐harmonic generation pulse laser in various power conditions. It was observed that when the laser power was less than 0.26 W, the electrical resistance of thin films decreased with the increase in laser power, whereas when the laser power was more than 0.26 W, the electrical resistance increased as the laser power increases. The minimum electrical resistance of the irradiated thin film was found to be four orders of magnitude lower than that of the unirradiated thin film. X‐ray absorption fine structure analysis revealed that the valence of Ti ions in the thin films was reduced after laser irradiation, indicating the formation of oxygen vacancies with electron doping. The cross‐sectional observation by a scanning electron microscope revealed that high laser power irradiation caused the thin films to become porous, and the percolation theory explains the origin of the increase in electrical resistance with developing porous structure. We propose a phase diagram to completely explain the relationship between electrical resistance and laser power, which supply a guideline on laser modification in TiO2 electrical resistance. [ABSTRACT FROM AUTHOR]

Development of a novel synthetic method for medium‐sized trans‐cycloalkenes (TCAs) is described. Functionalized TCAs are readily prepared from simple cycloalkanones in a few steps, namely, enol silyl ether formation, [2+2] cycloaddition, and domino 4π electrocyclic ring opening/alkylation (conjugate addition). The first example of central‐to‐planar chirality transfer from enantiomerically enriched cyclobutenes to TCAs is also described. [ABSTRACT FROM AUTHOR]

Development of a novel synthetic method for medium‐sized trans‐cycloalkenes (TCAs) is described. Functionalized TCAs are readily prepared from simple cycloalkanones in a few steps, namely, enol silyl ether formation, [2+2] cycloaddition, and domino 4π electrocyclic ring opening/alkylation (conjugate addition). The first example of central‐to‐planar chirality transfer from enantiomerically enriched cyclobutenes to TCAs is also described. [ABSTRACT FROM AUTHOR]

Development of a novel synthetic method for medium‐sized trans‐cycloalkenes (TCAs) is described. Functionalized TCAs are readily prepared from simple cycloalkanones in a few steps, namely, enol silyl ether formation, [2+2] cycloaddition, and domino 4π electrocyclic ring opening/alkylation (conjugate addition). The first example of central‐to‐planar chirality transfer from enantiomerically enriched cyclobutenes to TCAs is also described. [ABSTRACT FROM AUTHOR]

The terdiurnal tide (TDT) in the Antarctic mesosphere and lower thermosphere region is poorly known. This study examines TDT using neutral wind observations at Syowa during years of 2004–2018. TDT is found to be a significant tidal component with distinct vertical structures and seasonal evolution. (1) It shows a prominent height‐dependent seasonal variation with phase reversal at 94 km. (2) The vertical wavelength in summer is ∼40 km shorter than in winter. These features differ largely from those in the Arctic, indicating hemispheric asymmetry. The phase structure reveals a dominant upward propagating mode in local summer but superposition of more than one mode in other seasons. A downward propagating mode above 94 km in winter suggests Joule heating/ion drag as additional tidal sources to lower atmosphere ones. These results provide new constrains and benchmarks for model simulations that seek to understand terdiurnal tidal forcing mechanisms in polar regions. Plain Language Summary: Terdiurnal tides in the Antarctica mesosphere and lower thermosphere region are poorly known. In this study, we examine its characteristics using long‐term neutral wind observations at Syowa (69°S, 39°E) between January 2004 and July 2018. The analysis reveals terdiurnal tide being a significant tidal component in the Antarctica around solstices with distinct vertical structures. (1) Tides above 94 km has opposite climatological variation to that below 94 km. (2) The zonal and meridional wind components are 90° phase shifted, with similar amplitude in most seasons. (3) The vertical wavelength is shorter in summer than in winter. Most of these features differ from those reported in the Arctic, indicating hemispheric asymmetry. Furthermore, the phase structure reveals a dominant upward propagating mode in local summer but superposition of more than one mode in other seasons. A downward propagating mode in winter above 95 km suggests Joule heating/ion drag as likely tidal sources from above, in addition to those from lower atmosphere. The nearly 15‐year Syowa observations provide new constrains and benchmark for models that seek to understand terdiurnal tidal forcing mechanisms in polar regions. Key Points: Terdiurnal tide is a significant tidal component around solsticesUpward propagating mode dominates in summer, with shorter vertical wavelength than in winterDownward propagating mode above 94 km in winter suggests Joule heating/ion drag as likely tidal sources from above [ABSTRACT FROM AUTHOR]

The PANSY radar is the first Mesosphere-Stratosphere-Troposphere/Incoherent Scatter (MST/IS) radar in the Antarctic region. It is a large VHF monostatic pulse Doppler radar operating at 47MHz, consisting of an active phased array of 1045 Yagi antennas and an equivalent number of transmit-receive (TR) modules with a total peak output power of 500kW. The first stage of the radar was installed at Syowa Station (69°00′S, 39°35′E) in early 2011, and is currently operating with 228 antennas and modules. This paper reports the project's scientific objectives, technical descriptions, and the preliminary results of observations made to date. The radar is designed to clarify the role of atmospheric gravity waves at high latitudes in the momentum budget of the global circulation in the troposphere, stratosphere and mesosphere, and to explore the dynamical aspects of unique polar phenomena such as polar mesospheric clouds (PMC) and polar stratospheric clouds (PSC). The katabatic winds as a branch of Antarctic tropospheric circulation and as an important source of gravity waves are also of special interest. Moreover, strong and sporadic energy inputs from the magnetosphere by energetic particles and field-aligned currents can be quantitatively assessed by the broad height coverage of the radar which extends from the lower troposphere to the upper ionosphere. From engineering points of view, the radar had to overcome restrictions related to the severe environments of Antarctic research, such as very strong winds, limited power availability, short construction periods, and limited manpower availability. We resolved these problems through the adoption of specially designed class-E amplifiers, light weight and tough antenna elements, and versatile antenna arrangements. Although the radar is currently operating with only about a quarter of its full designed system components, we have already obtained interesting results on the Antarctic troposphere, stratosphere and mesosphere, such as gravity waves, multiple tropopauses associated with a severe snow storm in the troposphere and stratosphere, and polar mesosphere summer echoes (PMSE). [ABSTRACT FROM AUTHOR]

The Saura radar is designed and used to measure winds and electron densities at polar latitudes (69∘N) within the D region, namely between 50 and 100 km altitude. A relatively narrow radar beam can be generated and steered into distinct pointing directions as a rather large antenna array is used. From the observed radial velocities of the individual pointing directions, the horizontal and vertical wind fields can be obtained using the Doppler beam swinging (DBS) method. With recent upgrades to the radar, the interferometric capabilities are largely improved allowing simultaneous application of different wind estimation techniques now, and also echo localization. In recent studies, Saura DBS winds assuming isotropic scattering were found to be underestimated in comparison with highly reliable winds observed with the MAARSY MST radar in the presence of polar mesospheric summer echoes (PMSE). This underestimation has been investigated by analyzing the scattering positions as well as applying the imaging Doppler interferometry technique. Besides this, Saura winds derived with the classical DBS method seem to be error prone at altitudes above 90 km and even below this altitude for periods of enhanced ionization, e.g., particle precipitations. Various methods taking into account the scattering positions have been used to correct the wind underestimation. These winds are compared to MST radar winds during PMSE, and an optimal combination of these methods for the Saura radar is presented. This combined wind data appears to be reliable; it shows reasonable amplitudes as well as tidal structures for the entire altitude region. [ABSTRACT FROM AUTHOR]

Atmospheric gravity waves and traveling ionospheric disturbances can be observed in the neutral atmosphere and the ionosphere at a wide range of spatial and temporal scales. Especially at medium scales, these oscillations are often not resolved in general circulation models and are parameterized. We show that ionospheric disturbances forced by upward-propagating atmospheric gravity waves can be simultaneously observed with the EISCAT very high frequency incoherent scatter radar and the Nordic Meteor Radar Cluster. From combined multi-static measurements, both vertical and horizontal wave parameters can be determined by applying a specially developed Fourier filter analysis method. This method is demonstrated using the example of a strongly pronounced wave mode that occurred during the EISCAT experiment on 7 July 2020. Leveraging the developed technique, we show that the wave characteristics of traveling ionospheric disturbances are notably impacted by the fall transition of the mesosphere and lower thermosphere. We also demonstrate the application of using the determined wave parameters to infer the thermospheric neutral wind velocities. Applying the dissipative anelastic gravity wave dispersion relation, we obtain vertical wind profiles in the lower thermosphere. [ABSTRACT FROM AUTHOR]

This paper presents a novel method for the automatic determination of the diagonal-loading level for robust adaptive beamforming on radar wind profilers. This method balances the degradation of the signal-to-interference ratio with that of the signal-to-noise ratio to maximize the detectability of the backscattered signals. Because radial wind velocities are usually estimated from the first moment of the spectrum of backscattered echoes, both the residual ground clutter and any increase in noise level degrade the detectability of atmospheric echoes. The proposed algorithm evaluates the power spectral density of the residual clutter and increased noise to determine the optimal diagonal-loading level by balancing these two factors. The results of numerical simulation show that, without the need to specify any user parameters, the proposed algorithm is stable and more effective at maximizing the signal-to-interference ratio than the conventional norm-constrained diagonal-loading approach. The stability and clutter suppression capability of the proposed algorithm are examined using data from the Program of the Antarctic Syowa Mesosphere-Stratosphere-Troposphere/Incoherent Scatter Radar. [ABSTRACT FROM AUTHOR]

The Hunga Tonga–Hunga Ha′apai volcano eruption was a unique event that caused many atmospheric phenomena around the globe. In this study, we investigate the atmospheric gravity waves in the mesosphere/lower-thermosphere (MLT) launched by the volcanic explosion in the Pacific, leveraging multistatic meteor radar observations from the Chilean Observation Network De Meteor Radars (CONDOR) and the Nordic Meteor Radar Cluster in Fennoscandia. MLT winds are computed using a recently developed 3DVAR+DIV algorithm. We found eastward- and westward-traveling gravity waves in the CONDOR zonal and meridional wind measurements, which arrived 12 and 48 h after the eruption, and we found one in the Nordic Meteor Radar Cluster that arrived 27.5 h after the volcanic detonation. We obtained observed phase speeds for the eastward great circle path at both locations of about 250 m s -1 , and they were 170–150 m s -1 for the opposite propagation direction. The intrinsic phase speed was estimated to be 200–212 m s -1. Furthermore, we identified a potential lamb wave signature in the MLT winds using 5 min resolved 3DVAR+DIV retrievals. [ABSTRACT FROM AUTHOR]

Meteor radars have become widely used instruments to study atmospheric dynamics, particularly in the 70 to 110 km altitude region. These systems have been proven to provide reliable and continuous measurements of horizontal winds in the mesosphere and lower thermosphere. Recently, there have been many attempts to utilize specular and/or transverse scatter meteor measurements to estimate vertical winds and vertical wind variability. In this study we investigate potential biases in vertical wind estimation that are intrinsic to the meteor radar observation geometry and scattering mechanism, and we introduce a mathematical debiasing process to mitigate them. This process makes use of a spatiotemporal Laplace filter, which is based on a generalized Tikhonov regularization. Vertical winds obtained from this retrieval algorithm are compared to UA-ICON model data. This comparison reveals good agreement in the statistical moments of the vertical velocity distributions. Furthermore, we present the first observational indications of a forward scatter wind bias. It appears to be caused by the scattering center's apparent motion along the meteor trajectory when the meteoric plasma column is drifted by the wind. The hypothesis is tested by a radiant mapping of two meteor showers. Finally, we introduce a new retrieval algorithm providing a physically and mathematically sound solution to derive vertical winds and wind variability from multistatic meteor radar networks such as the Nordic Meteor Radar Cluster (NORDIC) and the Chilean Observation Network De meteOr Radars (CONDOR). The new retrieval is called 3DVAR+DIV and includes additional diagnostics such as the horizontal divergence and relative vorticity to ensure a physically consistent solution for all 3D winds in spatially resolved domains. Based on this new algorithm we obtained vertical velocities in the range of w = ± 1–2 ms-1 for most of the analyzed data during 2 years of collection, which is consistent with the values reported from general circulation models (GCMs) for this timescale and spatial resolution. [ABSTRACT FROM AUTHOR]

Strong meteor trail echoes are interferences in the wind velocity estimates made from mesosphere radar observations. Contaminated spectra are detected by their discontinuity and are removed at the risk of greater fluctuations of spectra, leading to a severe reduction of the signal-to-noise ratio (SNR) and inaccurate wind estimates for weak atmospheric echoes. This paper presents an adaptive signal processing technique for the suppression of spectral contaminations by meteor trail echoes. The method is based on the norm-constrained and directionally constrained minimization of power (NC-DCMP), which balances the capability of canceling the clutter and the robustness of beam shaping, at the cost of a slight decrease in the SNR, which can be determined in advance. Simulation results show that with a 3-dB decrease of the SNR being allowed, the method improves the signal-to-interference ratio (SIR) by 15 dB, giving wind estimates that are about 8 m s−1 better in terms of root-mean-square error and providing 4 times as wide an observable range when compared with the results of the ordinary nonadaptive beamforming method. The results for an actual observation show that the improvement of both the SIR and the observable range are achieved as in the simulations, which implies that the method should provide the simulated accuracy for the estimation of wind velocity from actual observations. [ABSTRACT FROM AUTHOR]

Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (∼ 54 ∘ N) and northern Norway (∼ 69 ∘ N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower-thermosphere summer length (MLT-SL) using SMR and PRR winds and (2) the mesosphere summer length (M-SL) using the PRR and MLS. Under both definitions, the summer begins around April and ends around middle September. The largest year-to-year variability is found in the summer beginning in both definitions, particularly at high latitudes, possibly due to the influence of the polar vortex. At high latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity as well as large-scale atmospheric influences (e.g., quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at middle latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO. [ABSTRACT FROM AUTHOR]

Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (∼ 54 ∘ N) and northern Norway (∼ 69 ∘ N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower-thermosphere summer length (MLT-SL) using SMR and PRR winds and (2) the mesosphere summer length (M-SL) using the PRR and MLS. Under both definitions, the summer begins around April and ends around middle September. The largest year-to-year variability is found in the summer beginning in both definitions, particularly at high latitudes, possibly due to the influence of the polar vortex. At high latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity as well as large-scale atmospheric influences (e.g., quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at middle latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO. [ABSTRACT FROM AUTHOR]

Ground-based remote sensing of atmospheric parameters is often limited to single station observations by vertical profiles at a certain geographic location. This is a limiting factor for investigating gravity wave dynamics as the spatial information is often missing, e.g., horizontal wavelength, propagation direction or intrinsic frequency. In this study, we present a new retrieval algorithm for multistatic meteor radar networks to obtain tomographic 3-D wind fields within a pre-defined domain area. The algorithm is part of the Agile Software for Gravity wAve Regional Dynamics (ASGARD) and called 3D-Var, and based on the optimal estimation technique and Bayesian statistics. The performance of the 3D-Var retrieval is demonstrated using two meteor radar networks: the Nordic Meteor Radar Cluster and the Chilean Observation Network De Meteor Radars (CONDOR). The optimal estimation implementation provide statistically sound solutions and diagnostics from the averaging kernels and measurement response. We present initial scientific results such as body forces of breaking gravity waves leading to two counter-rotating vortices and horizontal wavelength spectra indicating a transition between the rotational k-3 and divergent k-5/3 mode at scales of 80–120 km. In addition, we performed a keogram analysis over extended periods to reflect the latitudinal and temporal impact of a minor sudden stratospheric warming in December 2019. Finally, we demonstrate the applicability of the 3D-Var algorithm to perform large-scale retrievals to derive meteorological wind maps covering a latitude region from Svalbard, north of the European Arctic mainland, to central Norway. [ABSTRACT FROM AUTHOR]

A 31 MHz meteor radar located in Svalbard was used to observe polar mesospheric echoes (PMSEs) during summer 2020. Data from 19 July were selected for detailed analysis, with a focus on extracting additional information to characterize the atmosphere in the PMSE region. The use of an all-sky meteor radar adds an additional use to data collected for meteor observations and enables the detection of PMSE layers across a wide field of view. Comparison with data from a 53.5 MHz narrow-beam mesosphere–stratosphere–troposphere (MST) radar shows good agreement in the morphology of the layer as detected between the two systems. Doppler spectra of PMSE layers reveal fine structure, including regions of enhanced return that move across the radar's field of view. Examination of the relationship between range and Doppler shift of off-zenith portions of the layer enables the estimation of wind speeds with high temporal resolution during PMSE conditions. Trials demonstrate good agreement between wind speeds obtained from PMSE Doppler spectra and those calculated from specular meteor trail radial velocities. Combined with the antenna polar diagram of the radar, this same relationship was used to infer the aspect sensitivity of observed PMSE backscatter, yielding a mean backscatter angular width of 6.8±3.3∘. A comparison of underdense meteor radar echo decay times during and outside of PMSE conditions did not demonstrate a strong correlation between the presence of PMSEs and shortened underdense meteor radar echo durations. [ABSTRACT FROM AUTHOR]

Ground-based remote sensing of atmospheric parameters is often limited to single station observations of vertical profiles at a certain geographic location. This can be a limiting factor to investigating gravity wave dynamics. In this study we present a new retrieval algorithm for multi-static meteor radar networks to obtain tomographic 3D wind fields within a pre-defined domain area. The algorithm is part of the Agile Software for Gravity wAve Regional Dynamics (ASGARD) called 3DVAR, and based on the optimal estimation technique and Bayesian statistics. The performance of the 3DVAR retrieval is demonstrated using two meteor radar networks, the Nordic Meteor Radar Cluster and the Chilean Observation Network De MeteOr Radars (CONDOR). The optimal estimation implementation provides a statistically sound solution and additional diagnostics from the averaging kernels and measurement response. We present initial scientific results such as body forces of breaking gravity waves leading to two counter-rotating vortices and horizontal wavelength spectra indicating a transition between the vortical κ-3 and divergent κ-5/3 mode at scales of 80-120 km. In addition, we have performed a keogram analysis over extended periods to reflect the latitudinal and temporal impact of a minor sudden stratospheric warming in December 2019. Finally, we demonstrate the applicability of the 3DVAR algorithm to perform large-scale retrievals to derive meteorological wind maps covering a latitude region from Svalbard, north of the European Arctic mainland, to mid-Norway. [ABSTRACT FROM AUTHOR]

A 31 MHz meteor radar located in Svalbard has been used to observe polar mesospheric echoes (PMSE) during summer 2020. Data from 19 July was selected for detailed analysis, with a focus on extracting additional information to characterize the atmosphere in the PMSE region. The use of an all-sky meteor radar adds an additional use to data collected for meteor observations and enables the detection of PMSE layers across a wide field of view. Comparison with data from a 53.5 MHz narrow-beam MST radar shows good agreement in the morphology of the layer as detected between the two systems. Doppler spectra of PMSE layers reveal fine structure, including regions of enhanced return that move across the radar's field of view. The relationship between range and Doppler shift of off-zenith portions of the layer enable the estimation of wind speeds with high temporal resolution during PMSE conditions. Trials demonstrate good agreement between wind speeds obtained from PMSE Doppler spectra and those calculated from specular meteor trail radial velocities. Combined with the antenna polar diagram of the radar, this same relationship was used to infer the aspect sensitivity of observed PMSE backscatter, yielding a mean backscatter angular width of 6.6 ± 2.8°. A comparison of underdense meteor radar echo decay times during and outside of PMSE conditions did not demonstrate a strong correlation between the presence of PMSE and shortened underdense meteor radar echo durations. [ABSTRACT FROM AUTHOR]

The Program of the Antarctic Syowa Mesosphere–Stratosphere–Troposphere/Incoherent Scatter (PANSY) radar is a large atmospheric radar located at the Antarctic Syowa Station (69.01°S, 39.59°E). The PANSY radar performed the first incoherent scatter (IS) measurements in the Antarctic region in 2015. Several specific observations were undertaken in 2017 including a 24-h observation of the ionosphere using a peripheral antenna array to suppress interference from the field-aligned irregularities (FAIs). This paper presents the preliminary results derived from the IS measurements using the PANSY radar and the adaptive signal processing techniques to suppress FAIs. The norm-constrained and directionally constrained minimization of power (NC-DCMP) algorithm was applied to the 24-h ionosphere observations by the PANSY radar with a weighting applied to the directional constraint based on the gain differences of the subarrays. When compared with the conventional nonadaptive approach, the number of usable power profiles was increased by about 24% by the gain-weighted NC-DCMP algorithm, suggesting its effectiveness for FAI clutter suppression in ionosphere observations. Furthermore, detection of FAIs using the dedicated antenna array was found valuable in assessing the reliability of estimations of electron density based on VHF-band IS radar data. [ABSTRACT FROM AUTHOR]

Abstract: Nightly mean potential energy of gravity waves (GWs) per unit mass ( Ep) over Syowa Station (69 °S, 40 °E) was calculated from temperature profiles observed by the Rayleigh/Raman lidar from 2011 to 2015. The Ep values in the upper stratosphere and lower mesosphere were significantly enhanced on 8–21 August 2014, except on 12 August. A ray‐tracing analysis showed that large‐scale GWs emitted from various latitudes could be refracted and forced to converge above Syowa due to the poleward tilting of the polar night jet with altitude. It should be noted that Ep on 12 August was smaller than the other values during the enhancement, despite similar polar night jet conditions. A synoptic‐scale disturbance, which passed on 12 August, could have blocked the GWs from propagating upward through critical level filtering. These results suggest that convergence of the wave should be considered as a part of the intermittency of the GWs. [ABSTRACT FROM AUTHOR]

Abstract: We report the first observations of a high‐ and middle‐latitude neutral mesospheric density response to geomagnetic storms. Interhemisphere mesospheric densities are estimated using data from meteor radars at Davis Station (68.6°S, 77.9°E), Svalbard (78.3°N, 16°E) and Tromsø (69.6°N, 19.2°E), which are located under the auroral zone; the Mohe (53.5°N, 122.3°E), and Beijing (40.3°N, 116.2°E) meteor radars, located in northern midlatitudes, and the Microwave Limb Sounder (MLS) on the Aura satellite. Both case studies and a superposed epoch analysis indicate that geomagnetic storms can significantly influence mesospheric density, causing a greater than ~10% decrease in the auroral zones and a ~5% decrease at higher midlatitudes. With such large changes, it is reasonable to suspect that geomagnetic storms influence the dynamics of the high and middle latitudes mesosphere. [ABSTRACT FROM AUTHOR]

Multiple tropopauses (MTs) defined by the World Meteorological Organization are frequently detected from autumn to spring at Syowa Station (69.0°S, 39.6°E). The dynamical mechanism of MT events was examined by observations of the first mesosphere-stratosphere-troposphere (MST) radar in the Antarctic, the Program of the Antarctic Syowa MST/Incoherent Scatter (IS) Radar (PANSY), and of radiosondes on 8-11 April 2013. The MT structure above the first tropopause is composed of strong temperature fluctuations. By a detailed analysis of observed three-dimensional wind and temperature fluctuation components, it is shown that the phase and amplitude relations between these components are consistent with the theoretical characteristics of linear inertia-gravity waves (IGWs). Numerical simulations were performed by using a nonhydrostatic model. The simulated MT structures and IGW parameters agree well with the observation. In the analysis using the numerical simulation data, it is seen that IGWs were generated around 65°S, 15°E and around 70°S, 15°E, propagated eastward, and reached the region above Syowa Station when the MT event was observed. These IGWs were likely radiated spontaneously from the upper-tropospheric flow around 65°S, 15°E and were forced by strong southerly surface winds over steep topography (70°S, 15°E). The MT occurrence is attributable to strong IGWs and the low mean static stability in the polar winter lower stratosphere. It is also shown that nonorographic gravity waves associated with the tropopause folding event contribute to 40% of the momentum fluxes, as shown by a gravity wave-resolving general circulation model in the lower stratosphere around 65°S. This result indicates that they are one of the key components for solving the cold-bias problem found in most climate models. [ABSTRACT FROM AUTHOR]

Characteristically strong vertical wind disturbances (VWDs) with magnitudes larger than 1 m s−1 were observed in the Antarctic troposphere using a new mesosphere-stratosphere-troposphere (MST) radar called the Program of the Antarctic Syowa MST/incoherent scatter (IS) Radar (PANSY) during 15-19 June 2012 at Syowa Station (69.0°S, 39.6°E). In the same period, two synoptic-scale cyclones approached Syowa Station and caused a strong wind event (SWE) at the surface. The VWDs observed during the SWE at Syowa Station had a nearly standing (i.e., no phase tilt with height) phase structure up to the tropopause and a power spectrum proportional to the − power of frequency. On the other hand, the observed VWDs were not associated with systematic horizontal momentum fluxes. Meteorological fields around Syowa Station during the SWE were successfully simulated using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). A strong VWD was also simulated at the model grid of 70.0°S, 40.0°E in NICAM, which had a standing phase structure similar to the observed ones. An analysis based on the Froude number showed that the simulated VWD was likely due to a hydraulic jump leeward of the coastal mountain ridge. The Scorer parameter analysis indicated that the observed VWDs at Syowa Station during 16-17 June 2012 were likely due to the hydraulic jump similar to that in NICAM. On the other hand, a possibility of lee waves was also suggested for the VWD observed on 18 June 2012. [ABSTRACT FROM AUTHOR]

Transient mesospheric echo in the VHF range was detected at an altitude of 65-70 km during the auroral breakup that occurred from 2220 to 2226 UT on June 30, 2017. During this event, the footprint of the Arase satellite was located within the field of view of the all-sky imagers at Syowa Station in the Antarctic. Auroral observations at Syowa Station revealed the dominant precipitation of relatively soft electrons during the auroral breakup. A corresponding spike in cosmic noise absorption was also observed at Syowa Station, while the Arase satellite observed a flux enhancement of > 100 keV electrons and a broadband noise without detecting chorus waves or electromagnetic ion cyclotron waves. A general-purpose Monte Carlo particle transport simulation code was used to quantitatively evaluate the ionization in the middle atmosphere. Results of this study indicate that the precipitation of energetic electrons of > 100 keV, rather than X-rays from the auroral electrons, played a dominant role in the transient and deep (65-70 km) mesospheric ionization during the observed auroral breakup. [ABSTRACT FROM AUTHOR]

Do nonmigrating tides SW1 and SW3 enhance during sudden stratospheric warming events?Maosheng He(1), Jorge Luis Chau(1), Gunter Stober(1), Guozhu Li(2) Chris M. Hall(3), Masaki Tsutsumi(4), Chris Meek(5), and Peter Hoffmann(1) 1 Leibniz-Institute of Atmospheric Physics at the Rostock University, Kuehlungsborn, Germany 2 Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China 3 Tromso Geophysical Observatory, UiT - The Arctic University of Norway, Tromso, Norway 4 National Institute of Polar Research, Tokyo, Japan 5 Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatchewan, CanadaIn the last decades, tides have been investigated intensively with observations from both ground-based radars and satellites. Extensively reported are enhancements of solar nonmigrating tides SW1 and SW3 (SWx represents semidiurnal westward mode with zonal wave number x) during sudden stratospheric warming events (SSWs) in a broad latitudinal range in almost all upper atmospheric layers, from the stratosphere to the ionosphere. In the current work, we jointly analyze ground-based radar observations from different longitudinal sectors in both case and statistical studies, to investigate the mesospheric oscillation at the period T=12±0.5hr. Contrary to extensive previous literature, our results suggest that SW1 and SW3 do NOT enhance during SSWs. Instead, enhancements are observed at neighboring periods,12.4hr and 11.6hr, which could be explained as the lower and upper sidebands of nonlinear modulation of the 16-day wave on SW2, respectively. Meanwhile, the enhancements are accompanied with a decrease in SW2 which was argued to be the main energy supplier of the sidebands in previous literature. Our analyses suggested that the sidebands might have been broadly detected at low-frequency resolutions and misinterpreted as SW1 and SW3 enhancements. [ABSTRACT FROM AUTHOR]

Enhanced nonmigrating tides SW1 (SWx represents semidiurnal westward mode with zonal wave number x) and SW3 during sudden stratospheric warming (SSW) were traditionally attributed to nonlinear interactions of quasi‐stationary planetary waves with the migrating tide SW2. Recent studies specified hypothetically that responsible for the interactions is the 16‐day wave, instead of the broadly accepted quasi‐stationary planetary waves. It is suspected that the 16‐day‐wave‐triggered secondary waves, at periods ∼12.4 and ∼11.6 hr, were detected at low‐frequency resolutions and misinterpreted as SW1 and SW3, respectively. While He et al. (2018, https://doi.org/10.1002/2018JD028400) associated the 11.6‐hr oscillation conclusively to the SW3‐like signature during SSW 2013 by diagnosing its wave number, the SW1‐like 12.4hr wave, however, has never been explicitly resolved, given its proximity to the period of an active lunar tide. Here, using the coherency in the mesospheric wind between two longitudinal sectors during SSW 2009, we identify a 12.4‐hr oscillation dominated by wave number 1 and therefore associate it to the SW1‐like signature. Plain Language Summary: The winter upper atmosphere is populated by global‐scale oscillations among which the most radical one appears nearby the period of 12 hr. The existing studies suggest that the near‐12‐hr oscillation consists of at least six waves, namely, the migrating lunar and solar tides, two nonmigrating tides, and two secondary waves of the nonlinear interactions between the migrating solar tide and traveling planetary waves. The current work, together with a counterpart work (He et al., 2018), disentangles the near‐12‐hr oscillation into high‐frequency‐resolved isolated spectral peaks and investigates the longitudinal variation in the phase of the isolated peaks through jointly studying the mesospheric wind collected by longitudinally separated radars in early 2009 and early 2013. Results suggest that the nonmigrating tides did not enhance in both cases and demonstrate that the occurrence of secondary waves might have been misinterpreted as the nonmigrating tides in studies at low‐frequency resolutions. Key Points: We report an upper atmospheric pseudo‐SW1, a 12.4‐hr oscillation characterized by westward traveling wave number 1 structureThe nonlinear interaction between the 16‐day wave and the semidiurnal migrating tide SW2 triggers the pseudo‐SW1The broadly reported SW1‐like enhancements during sudden stratospheric warming events are conceptually not tides [ABSTRACT FROM AUTHOR]