Lower Thermosphere Region Research Articles

Vertical Structure of Terdiurnal Tides in the Antarctic MLT Region: 15‐Year Observation Over Syowa (69°S, 39°E)

AbstractThe 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.

Climatology of the Quasi‐6‐Day Wave in the Mesopause Region and Its Modulations on Total Electron Content During 2003–2017

AbstractThe westward quasi‐6‐day planetary wave (Q6DW) with zonal wave number 1 is a prominent oscillation in the mesosphere and lower thermosphere region, which causes significant variabilities in the ionosphere through E region wind dynamo. In this paper, the neutral wind observations from the Thermosphere Ionosphere and Mesosphere Electric Dynamics Doppler Imager experiment, the kinetic temperature observations from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument, and the total electron content (TEC) maps from globally distributed Global Positioning System (GPS) receivers during 2003–2017 are utilized to statistically study the Q6DW in both the neutral atmosphere and the ionosphere. Our statistical results show that (1) the Q6DW is most likely to occur during May and September, exhibiting a quasi‐semiannual oscillation. (2) The periods of the Q6DW show two statistical crests at ~6 and ~7 days separately. (3) The ionospheric oscillations due to the Q6DW are positively related to both the 10.7‐cm solar flux index and the neutral wave amplitudes in the mesosphere and lower thermosphere region. The stronger oscillations in TEC under higher solar activity conditions are due to the larger ionization rate and thus larger background TEC. A stronger Q6DW results in stronger wind dynamos in the E region, which leads to stronger F region responses indirectly. (4) The interhemispheric asymmetries of the ionospheric responses to Q6DW show weak negative correlations with both solar activities and neutral wave perturbations, the mechanism of which needs our further investigation.

On the Responses of Mesosphere and Lower Thermosphere Temperatures to Geomagnetic Storms at Low and Middle Latitudes

AbstractObservations from lidars and satellites have shown that large neutral temperature increases and decreases occur in the middle and low latitudes of the mesosphere and lower thermosphere region during geomagnetic storms. Here we undertake first‐principles simulations of mesosphere and lower thermosphere temperature responses to storms using the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model to elucidate the nature and causes of these changes. Temperature variations were not uniform; instead, nighttime temperatures changed earlier than daytime temperatures, and temperatures changed earlier at high latitudes than at low ones. Furthermore, temperatures increased in some places/times and decreased in others. As the simulation behaves similar to observations, it provides an opportunity to understand physical processes that drive the observed changes. Our analysis has shown that they were produced mainly by adiabatic heating/cooling that was associated with vertical winds resulting from general circulation changes, with additional contributions from vertical heat advection.

An empirical model of nitric oxide in the upper mesosphere and lower thermosphere based on 12 years of Odin SMR measurements

Abstract. Nitric oxide (NO) is produced by solar photolysis and auroral activity in the upper mesosphere and lower thermosphere region and can, via transport processes, eventually impact the ozone layer in the stratosphere. This work uses measurements of NO taken between 2004 and 2016 by the Odin sub-millimeter radiometer (SMR) to build an empirical model that links the prevailing solar and auroral conditions with the measured number density of NO. The measurement data are averaged daily and sorted into altitude and magnetic latitude bins. For each bin, a multivariate linear fit with five inputs, the planetary K index, solar declination, and the F10.7 cm flux, as well as two newly devised indices that take the planetary K index and the solar declination as inputs in order to take NO created on previous days into account, constitutes the link between environmental conditions and measured NO. This results in a new empirical model, SANOMA, which only requires the three indices to estimate NO between 85 and 115 km and between 80∘ S and 80∘ N in magnetic latitude. Furthermore, this work compares the NO calculated with SANOMA and an older model, NOEM, with measurements of the original SMR dataset, as well as measurements from four other instruments: ACE, MIPAS, SCIAMACHY, and SOFIE. The results suggest that SANOMA can capture roughly 31 %–70 % of the variance of the measured datasets near the magnetic poles, and between 16 % and 73 % near the magnetic equator. The corresponding values for NOEM are 12 %–38 % and 7 %–40 %, indicating that SANOMA captures more of the variance of the measured datasets than NOEM. The simulated NO for these regions was on average 20 % larger for SANOMA, and 78 % larger for NOEM, than the measured NO. Two main reasons for SANOMA outperforming NOEM are identified. Firstly, the input data (Odin SMR NO) for SANOMA span over 12 years, while the input data for NOEM from the Student Nitric Oxide Experiment (SNOE) only cover 1998–2000. Additionally, some of the improvement can be accredited to the introduction of the two new indices, since they include information of auroral activity on prior days that can significantly enhance the number density of NO in the MLT during winter in the absence of sunlight. As a next step, SANOMA could be used as input in chemical models, as a priori information for the retrieval of NO from measurements, or as a tool to compare Odin SMR NO with other instruments. SANOMA and accompanying scripts are available on http://odin.rss.chalmers.se (last access: 15 September 2018).

Study of Mean Wind Variations and Gravity Wave Forcing Via a Meteor Radar Chain and Comparison with HWM‐07 Results

AbstractWe present an analysis of long‐term observation of the mean winds in the mesosphere and lower thermosphere region. The neutral wind data are collected over the period of January 2009 to November 2017 from a chain of meteor radars along the 120°E meridian in the Northern Hemisphere which consists of four stations at Mohe (MH; 53.5°N, 122.3°E), Beijing (BJ; 40.3°N, 116.2°E), Wuhan (WH; 30.5°N, 114.6°E), and Sanya (SY; 18.3°N, 109.6°E). The annual oscillation dominates the neutral wind pattern at midlatitudes while both the annual oscillation and semiannual oscillations are important at low latitudes. The mean zonal wind pattern in midlatitudes exhibits two eastward jets. One is in the period from April to October above 85 km and the other is from November to March below 90 km. A reversal of the mean zonal winds from eastward to westward is observed in midlatitudes around the spring equinox. Gravity wave forcing during this season shows a rapid change from westward to eastward, which is consistent with the reversals. A three‐cell southward wind pattern is observed over WH and SY. A strong vertical wind shear with northward flow below ~90 km and southward flow above is observed over SY from September to April. Composite‐year comparisons between the observations and the Horizontal Wind Model‐07 show large discrepancies during winter time. HWM‐07 predictions agree with the meteor radar observations better in the zonal component than in the meridional component. The model predictions at low latitude are not as accurate as the middle latitudes.

Study of the Quasi‐5‐Day Wave in the MLT Region by a Meteor Radar Chain

AbstractAn extensive analysis of quasi 5‐day waves (5DWs) in the mesosphere and lower thermosphere and their responses to a major sudden stratospheric warming (SSW) event (January 2013) are presented. The analysis is based on data conducted from a meteor radar chain in the period from December 2008 to November 2017 and Aura Microwave Limb Sounder satellite. The radar chain includes three stations located at Mohe (MH, 53.5°N, 122.3°E), Beijing (BJ, 40.3°N, 116.2°E), and Wuhan (WH, 30.5°N, 114.6°E). The 5DWs present clear interannual and seasonal variations, which are observed by both the radar wind data and Aura‐temperature data. Interestingly, the ter‐annual oscillation is found to be as important as the commonly recognized annual oscillation and semiannual oscillation at the three stations in both neutral wind components. The 5DWs are strong mainly during the August/September in the meridional component, while they are primarily enhanced in the period of January, April/May, and late summer in the zonal component. An enhancement of 5DWs is observed during the 2013 SSW event in the mesosphere and lower thermosphere region at the three stations. The amplitudes during the SSW are more than 2 times larger than the January average. The strength of the amplification is most prominent at MH and reduces as latitude decreases. According to the results obtained from the radar wind and Aura temperature, the enhanced 5DW modes are W1 at MH, and W2 at BJ and WH. Our results indicate that the amplification of the 5DWs is very likely associated with the 2013 SSW.

Solar Cycle Response of CO2 Over the Austral Winter Mesosphere and Lower Thermosphere Region

AbstractThis work uses Sounding of the Atmosphere using Broadband Emission Radiometry CO2 data from 2002 to 2015 and Specified Dynamics‐Whole Atmosphere Community Climate Model (SD‐WACCM) outputs from 1979 to 2014 to show, for the first time, the solar cycle response of CO2 in the Austral winter mesosphere and lower thermosphere region. Both Sounding of the Atmosphere using Broadband Emission Radiometry and SD‐WACCM show that CO2 experiences a decrease during solar maximum throughout the Austral winter mesosphere and lower thermosphere region. This work highlights the regions where CO2 experiences its strongest and weakest solar cycle responses as modeled by SD‐WACCM. The region with the strongest solar cycle response experiences around 5% reduction in CO2 between solar maximum and solar minimum. The region with weakest solar cycle response experiences less than 1% reduction in CO2 between solar maximum and solar minimum. It is shown that the region of the strongest CO2 response is driven by photodissociation, downwelling, and reduced eddy diffusion. On the other hand, the region of the weakest CO2 response is driven by the opposing effects of photodissociation and enhanced eddy diffusion. This is the first work to show that the solar cycle could affect the Austral winter lower thermosphere circulation and eddy diffusion processes. These anomalies in the lower thermospheric circulation and eddy diffusion are found to be related to the solar cycle response in the Austral winter mesosphere wave‐mean flow dynamics. This work therefore concludes that the solar cycle affects lower thermospheric CO2 via modulations of the lower thermospheric circulation and eddy diffusion processes.

Multiyear Observations of Gravity Wave Momentum Fluxes in the Midlatitude Mesosphere and Lower Thermosphere Region by Meteor Radar

AbstractMultiyear high‐frequency gravity wave (GW) momentum fluxes and variances in the mesosphere and lower thermosphere region are revealed using four meteor radars along 120°E longitude at Northern Hemisphere midlatitudes for the first time, which are located at Mohe (53.5°N, 122.3°E), Beijing (40.3°N, 116.2°E), Mengcheng (33.3°N, 116.5°E), and Wuhan (30.5°N, 114.2°E), respectively. The seasonal and latitudinal variations of GW momentum fluxes in the midlatitude are discussed. The directions of the monthly mean zonal momentum fluxes are mostly against the background mean zonal winds, which agree well with the selective filtering mechanism. The seasonal variations of meridional momentum fluxes have similar trends over all four stations. The latitudinal variations in the seasonal variation of GW momentum fluxes are mainly due to the latitudinal variations of background winds and GW sources. The unexpected eastward momentum fluxes in winter over Beijing are likely caused by the secondary GWs propagating eastward from the source region over the Tibetan Plateau (25°–40°N, 70–100°E). The GW variances show a V‐shaped structure indicating annual and semiannual variations over four stations in zonal component. A quasi‐4‐month oscillation was observed over Mohe, Mengcheng, and Wuhan in meridional component. The background winds play decisive roles in these GW variance structures.

New Approach to Estimate Tidal Climatology From Ground‐ and Space‐Based Observations

AbstractThis paper proposes a new approach to obtain seasonal variations of global tides for the horizontal winds in the mesosphere and lower thermosphere region, based on daily tides derived from the global coverage of the TIMED Doppler Interferometer (TIDI) and the local‐time coverage of a meteor radar chain. The daily variations of different mesosphere and lower thermosphere tides are obtained taking advantage of empirical tidal mode (ETM), which is first derived from Global Scale Wave Model using empirical orthogonal function analysis. ETM displays latitudinal and vertical features of each tidal component in a realistic background atmosphere with dissipation effects. After fitting the observations by ETM day by day, the monthly mean of daily fitted results is then used to describe tidal monthly features. Seasonal variations of three major tidal components were found: DW1 in both zonal and meridional winds usually has two maxima around equinoxes, DE3 in zonal winds achieves their maximum in September, while that in meridional winds becomes strongest in February and November. SW2 in zonal winds reaches largest amplitudes in May in the southern hemisphere, and meridional winds has minor peaks in February and November in the northern hemisphere. At last, we superpose each tidal component with the same frequency to reconstruct the global diurnal and semidiurnal tidal winds. Comparing them with both of TIDI 60‐day average winds and outputs of Global Scale Wave Model model, similar characteristics and magnitudes indicate their consistency and confirm the reliability and feasibility of this new approach.

The Morphology of the 6‐Day Wave in Both the Neutral Atmosphere and F Region Ionosphere Under Solar Minimum Conditions

AbstractCombined Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED)/TIMED Doppler interferometer horizontal winds, Global Positioning System Total Electron Content maps, and Constellation Observing System for Meteorology, Ionosphere, and Climate electron density profiles are utilized to study the westward zonal wavenumber 1 quasi‐6‐day planetary wave in both the neutral atmosphere and F region ionosphere under solar minimum conditions in 2009. It is found that the zonal (meridional) wind perturbations maximize (minimize) in the geographic equatorial mesosphere and lower thermosphere region and are in phase (out of phase) between hemispheres, which agrees well with both the Upper Atmosphere Research Satellite/high‐resolution Doppler imager wind observations and (1, 1) Rossby normal mode. Analysis of the Total Electron Content maps and Constellation Observing System for Meteorology, Ionosphere, and Climate electron density profiles shows that the F region ionosphere exhibits wave spectra and temporal variations that are consistent with those of the neutral atmosphere. Specifically, the 6‐day oscillation in electron density is the highest during 1200–1800 LT in the equatorial ionospheric anomaly region (~20° geomagnetic latitude), with a minimum amplitude at the equator, which agrees well with the F region fountain effect. At 300 km, the amplitude of the electron density oscillation reaches ~1.2 × 104 cm−3, which accounts for ~14% of the background value. In addition, the F2 layer electron peak density and peak height also exhibit corresponding oscillations with amplitudes of ~9 × 104 cm−3 and ~12 km, respectively. The current work provides clear evidence for F region ionospheric responses to planetary waves in the mesosphere and lower thermosphere region.

Ionospheric Variability Due to Tides and Quasi‐Two Day Wave Interactions

AbstractThe ionospheric variabilities due to planetary waves (PWs) are complicated due to their nonlinear interaction with tides (migrating diurnal and semidiurnal tides [DW1 and SW2]) in the mesosphere and lower thermosphere region. The quasi‐two day wave (QTDW) is the most dominant oscillation during austral summer period except tides. This paper quantitatively studies the contribution of W3 QTDW, secondary PWs generated by the QTDW‐tide interaction, and the change of tides to the ionospheric vertical drift variabilities. It is found that the secondary PWs generated by the QTDW‐DW1 interaction (16 hr W4 and 48 hr E2) contribute more to the ionospheric variability than those generated by the QTDW‐SW2 interaction (9.6 hr W5 and 16 hr E1). The latitudinal distribution of the vertical drift induced by tides and PWs varies due to their different wind structures. The vertical drift induced by DW1 is weakened at all latitudes due to the existence of W3 QTDW, whereas the SW2‐induced vertical drift is enhanced at low latitudes and decreased at middle latitudes. The W3 QTDW induces weaker vertical drift in the equatorial region than that induced by 16 hr W4, 48 hr E2, and the change of tides. At middle latitudes, the vertical drift induced by W3 QTDW is slightly stronger than that induced by 48 hr E2 and the change of DW1 but is comparable to that induced by 16 hr W4 and the change of SW2. Our simulations show that the changes of tidal amplitudes and secondary PWs are as important as the QTDW itself to the day‐to‐day ionospheric variabilities.

Optimizing hydroxyl airglow retrievals from long-slit astronomical spectroscopic observations

Abstract. Astronomical spectroscopic observations from ground-based telescopes contain background emission lines from the terrestrial atmosphere's airglow. In the near infrared, this background is composed mainly of emission from Meinel bands of hydroxyl (OH), which is produced in highly excited vibrational states by reduction of ozone near 90 km. This emission contains a wealth of information on the chemical and dynamical state of the Earth's atmosphere. However, observation strategies and data reduction processes are usually optimized to minimize the influence of these features on the astronomical spectrum. Here we discuss a measurement technique to optimize the extraction of the OH airglow signal itself from routine J-, H-, and K-band long-slit astronomical spectroscopic observations. As an example, we use data recorded from a point-source observation by the Nordic Optical Telescope's intermediate-resolution spectrograph, which has a spatial resolution of approximately 100 m at the airglow layer. Emission spectra from the OH vibrational manifold from v′ = 9 down to v′ = 3, with signal-to-noise ratios up to 280, have been extracted from 10.8 s integrations. Rotational temperatures representative of the background atmospheric temperature near 90 km, the mesosphere and lower thermosphere region, can be fitted to the OH rotational lines with an accuracy of around 0.7 K. Using this measurement and analysis technique, we derive a rotational temperature distribution with v′ that agrees with atmospheric model conditions and the preponderance of previous work. We discuss the derived rotational temperatures from the different vibrational bands and highlight the potential for both the archived and future observations, which are at unprecedented spatial and temporal resolutions, to contribute toward the resolution of long-standing problems in atmospheric physics.

Semidiurnal tidal activity of the middle atmosphere at mid-latitudes derived from O2 atmospheric and OH(6-2) airglow SATI observations

In this paper, we investigate the tidal activity in the mesosphere and lower thermosphere region at 370N using OH Meinel and O2 atmospheric airglow observations from 1998 to 2015. The observations were taken with a Spectral Airglow Temperature Imager (SATI) installed at Sierra Nevada Observatory (SNO) (37.060N, 3.380W) 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 O2 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 O2 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 O2 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.

Modeling study of the ionospheric responses to the quasi-biennial oscillations of the sun and stratosphere

The Quasi-biennial Oscillation (QBO) is a persistent oscillation in the zonal mean zonal winds of the low latitude middle atmosphere that is driven by breaking planetary and gravity waves with a period near two years. The atmospheric tides that dominate the dynamics of the mesosphere and lower thermosphere region (MLT, between heights of 70–120 km) are excited in the troposphere and stratosphere, and propagate through QBO-modulated zonal mean zonal wind fields. This allows the MLT tidal response to also be modulated by the QBO, with implications for ionospheric/thermospheric variability. Interannual oscillations in solar radiation can also directly drive the variations in the ionosphere with similar periodicities through the photoionization. Many studies have observed the connection between the solar activity and QBO signal in ionospheric features such as total electron content (TEC).In this research, we develop an empirical model to isolate stratospheric QBO-related tidal variability in the MLT diurnal and semidiurnal tides using values from assimilated TIMED satellite data. Migrating tidal fields corresponding to stratospheric QBO eastward and westward phases, as well as with the quasi-biennial variations in solar activity isolated by the Multi-dimensional Ensemble Empirical Mode Decomposition (MEEMD) analysis from Hilbert-Huang Transform (HHT), are then used to drive the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM).The numerical experiment results indicate that the ionospheric QBO is mainly driven by the solar quasi-biennial variations during the solar maximum, since the solar quasi-biennial variation amplitude is directly proportionate to the solar cycle. The ionospheric QBO in the model is sensitive to both the stratospheric QBO and solar quasi-biennial variations during the solar minimum, with solar effects still playing a stronger role.

Characteristics of convective structures of sodium layer in lower thermosphere (105–120 km) at Haikou (19.99°N, 110.34°E), China

The atmospheric sodium layer normally occurs in the mesopause (80–105 km) region, but rarely in the lower thermosphere region (>105 km) at low latitude. We observed a kind of peculiar sodium layer in lower thermosphere at Haikou (19.99°N, 110.34°E)—the thermospheric convective sodium layer (TCSL) in a lidargram. The TCSL's sodium density unstably developed over time and appeared as several discontinuous convective shapes vertically. It is the first time convective sodium layer observed in the lower thermosphere region (105–120 km). Based on Haikou lidar data, we obtained 14 TCSL events during 180 nights from March 2010 to August 2012. Most of the apogees of the TCSL events are higher than 108 km. A TCSL event lasts several hours and is composed of several convective structures, with each veitical shape lasting ∼5–30 min. All TCSL events occurred during spring and summer, and generally appear near midnight (22:00–00:00 LT). The TCSL has potential regional feature and appears to be related to the thermospheric sporadic E (Es) layers, winds, and field-aligned ionospheric irregularities (FAI).

Impacts of SABER CO2‐based eddy diffusion coefficients in the lower thermosphere on the ionosphere/thermosphere

AbstractThis work estimates global‐mean Kzz using Sounding of the Atmosphere using Broadband Emission Radiometry/Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics monthly global‐mean CO2 profiles and a one‐dimensional transport model. It is then specified as a lower boundary into the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM). Results first show that global‐mean CO2 in the mesosphere and lower thermosphere region has annual and semiannual oscillations (AO and SAO) with maxima during solstice seasons along with a primary maximum in boreal summer. Our calculated AO and SAO in global‐mean CO2 are then modeled by AO and SAO in global‐mean Kzz. It is then shown that our estimated global‐mean Kzz is lower in magnitude than the suggested global‐mean Kzz from Qian et al. (2009) that can model the observed AO and SAO in the ionosphere/thermosphere (IT) region. However, our estimated global‐mean Kzz is similar in magnitude with recent suggestions of global‐mean Kzz in models with explicit gravity wave parameterization. Our work therefore concludes that global‐mean Kzz from global‐mean CO2 profiles cannot model the observed AO and SAO in the IT region because our estimated global‐mean Kzz may only be representing eddy diffusion due to gravity wave breaking. The difference between our estimated global‐mean Kzz and the global‐mean Kzz from Qian et al. (2009) thus represents diffusion and mixing from other nongravity wave sources not directly accounted for in the TIE‐GCM lower boundary conditions. These other sources may well be the more dominant lower atmospheric forcing behind the AO and SAO in the IT region.

Seasonal, inter-annual and solar cycle variability of the quasi two day wave in the low-latitude mesosphere and lower thermosphere

We analyzed 17 years (1993–2009) of horizontal winds measured by the medium frequency (MF) radar located at Tirunelveli (8.7°N, 77.8°E) and 10 years (2005–2014) of horizontal winds measured by a meteor radar located at Thumba (8.5°N, 77°E) to examine the seasonal, inter-annual, and solar cycle variability of the Quasi-Two Day Wave (QTDW) in the mesosphere and lower thermosphere region. These two radars are nearly co-located, but differ in their measurement technique. Comparison of the estimated QTDW amplitudes by the two radars shows that the amplitudes are larger in the meteor radar than those in the MF radar. The difference between the amplitudes is larger in May in the zonal component and in April and September in the meridional one. Furthermore, the differences are larger in the meridional component. The QTDWs in both the radars show a strong semi-annual oscillation (SAO). In addition, the meridional QTDW amplitudes of both the MF and meteor radars show a distinct enhancement in the month of October. While the whole spectra of QTDWs contribute to the SAO amplitudes, only 45–50h waves contribute to the October enhancement. The amplitudes of the QTDWs, in general, show large inter-annual variability. The QTDW amplitudes from both the radars show modulation at period of quasi-biennial oscillation. The QTDWs of the MF radar show a small negative correlation with solar activity while those of meteor radar do not show any correlation. The above aspects are discussed in the light of current understanding of the QTDWs.

Observational evidence for new instabilities in the midlatitude <i>E</i> and <i>F</i> region

Abstract. Radar observations of the E- and F-region ionosphere from the Arecibo Observatory made during moderately disturbed conditions are presented. The observations indicate the presence of patchy sporadic E (Es) layers, medium-scale traveling ionospheric disturbances (MSTIDs), and depletion plumes associated with spread F conditions. New analysis techniques are applied to the dataset to infer the vector plasma drifts in the F region as well as vector neutral wind and temperature profiles in the E region. Instability mechanisms in both regions are evaluated. The mesosphere–lower-thermosphere (MLT) region is found to meet the conditions for neutral dynamic instability in the vicinity of the patchy Es layers even though the wind shear was relatively modest. An inversion in the MLT temperature profile contributed significantly to instability in the vicinity of one patchy layer. Of particular interest is the evidence for the conditions required for neutral convective instability in the lower-thermosphere region (which is usually associated with highly stable conditions) due to the rapid increase in temperature with altitude. A localized F-region plasma density enhancement associated with a sudden ascent up the magnetic field is shown to create the conditions necessary for convective plasma instability leading to the depletion plume and spread F. The growth time for the instability is short compared to the one described by [Perkins(1973)]. This instability does not offer a simple analytic solution but is clearly present in numerical simulations. The instability mode has not been described previously but appears to be more viable than the various mechanisms that have been suggested previously as an explanation for the occurrence of midlatitude spread F.

Numerical simulation of the 6 day wave effects on the ionosphere: Dynamo modulation

AbstractThe Thermosphere‐Ionosphere‐Mesosphere Electrodynamics General Circulation Model (TIME‐GCM) is used to theoretically study the 6 day wave effects on the ionosphere. By introducing a 6 day perturbation with zonal wave number 1 at the model lower boundary, the TIME‐GCM reasonably reproduces the 6 day wave in temperature and horizontal winds in the mesosphere and lower thermosphere region during the vernal equinox. The E region wind dynamo exhibits a prominent 6 day oscillation that is directly modulated by the 6 day wave. Meanwhile, significant local time variability (diurnal and semidiurnal) is also seen in wind dynamo as a result of altered tides due to the nonlinear interaction between the 6 day wave and migrating tides. More importantly, the perturbations in the E region neutral winds (both the 6 day oscillation and tidal‐induced short‐term variability) modulate the polarization electric fields, thus leading to the perturbations in vertical ion drifts and ionospheric F2 region peak electron density (NmF2). Our modeling work shows that the 6 day wave couples with the ionosphere via both the direct neutral wind modulation and the interaction with atmospheric tides.

MIPAS observations of longitudinal oscillations in the mesosphere and the lower thermosphere: climatology of odd-parity daily frequency modes

Abstract. MIPAS global Sun-synchronous observations are almost fixed in local time. Subtraction of the descending and ascending node measurements at each longitude only includes the longitudinal oscillations with odd daily frequencies nodd from the Sun's perspective at 10:00. Contributions from the background atmosphere, daily-invariant zonal oscillations and tidal modes with even-parity daily frequencies vanish. We have determined longitudinal oscillations in MIPAS temperature with nodd and wavenumber k = 0–4 from the stratosphere to 150 km from April 2007 to March 2012. To our knowledge, this is the first time zonal oscillations in temperature have been derived pole to pole in this altitude range from a single instrument. The major findings are the detection of (1) migrating tides at northern and southern high latitudes; (2) significant k = 1 activity at extratropical and high latitudes, particularly in the Southern Hemisphere; (3) k = 3 and k = 4 eastward-propagating waves that penetrate the lower thermosphere with a significantly larger vertical wavelength than in the mesosphere; and (4) a migrating tide quasi-biennial oscillation in the stratosphere, mesosphere and lower thermosphere. MIPAS global measurements of longitudinal oscillations are useful for testing tide modeling in the mesosphere and lower thermosphere region and as a lower boundary for models extending higher up in the atmosphere.