Low-frequency lower E-region wind and reflection height measurements as sensor for climate variability

[1] A detailed overview of long-term secular trends in temperature of the mesosphere and lower thermosphere considered to be induced by increase in greenhouse gases has been provided by Beig et al. (2003). Since then, quite a few new results have been emerged as some of the data series have become sufficiently large enough to provide results with improved confidence. Our understanding on the nature of temperature trends in the mesosphere/lower thermosphere (MLT) region is relatively better now. In the mesosphere, some of the results confirmed the earlier findings, and some new results obtained by satellite and lidar data over the tropical region have indicated a relatively weaker cooling trend as compared to the past but nevertheless strengthened the conclusion about the cooling trends. However, in the mesopause region, some of the new results now indicate a break in trend and tendency of negative signal where earlier no trend feature was noticed. This slice of no trend feature in between two cooling regimes was puzzling the modeling community, who were in search of a convincing explanation. This paper briefly outlines the progress made over the recent past in the field of MLT region secular temperature trends attributed mainly to growth of greenhouse gases near the Earth's surface.

Investigation of long-term measurements of winter mesosphere/lower thermosphere (MLT) region zonal winds has shown a connection of northern hemisphere tropospheric parameters and MLT conditions. These are on one hand due to a connection of the stratospheric polar vortex and the North Atlantic Oscillation (NAO) and on the other hand forced by a coupling of the troposphere/lower stratosphere and the MLT region through planetary wave propagation and wave-mean flow interaction. The connection between European and Asian winter climatological parameters and the NAO leads to the fact, that the signal of Eurasian winter conditions can be found in the interannual variability of MLT winds. Some of these winter tropospheric parameters influence the strength of the Indian monsoon in the following summer, and this leads to a correlation between winter MLT winds and summer Indian rainfall, so that the midlatitude MLT winter dynamics can be used as a precursory signal of the strength of the Indian monsoon.

Connection between winter mesopause region temperatures and diurnal LF reflection height variations measured at Collm

On the night of 21/22 May 2018, clear-sky conditions enabled a 12-hour-long temperature measurement of the Advanced Mesospheric Temperature Mapper (AMTM) in the mesosphere-lower thermosphere (MLT) region over Río Grande, Argentina. Given a westerly forcing over Patagonia, we observe North-South-oriented phase lines in the AMTM temperature maps exclusively during the westerly phase of the semi-diurnal tide, indicating the deep propagation of mountain waves (MWs) with horizontal wavelengths between 20 km and 40 km. After a wind reversal in the MLT, we observe two large-scale gravity waves (GWs) propagating rapidly in a south-eastward direction. We use one- and two-dimensional wavelet analysis to characterize the observed GWs and find that their wavelengths and phase speeds are consistent with secondary GW theory. Ray tracing results suggest a possible source region for these 2GWs located north-westward, near the Chilean Torres del Paine region. In addition, co-located temperature and wind measurements from the Compact Rayleigh Autonomous Lidar (CORAL) and the Southern Argentine Agile Meteor Radar (SAAMER), in combination with a Monte Carlo approach, allow for the accurate determination of both the GW momentum flux and its uncertainty. Although we exclude a direct cause-and-effect relationship within our field of view, we find that, on average, the observed MWs carry momentum fluxes an order of magnitude larger than those of the 2GWs.

This is an introductory review of modern meteor radar and its application to the measurement of the dynamical parameters of the Mesosphere Lower Thermosphere (MLT) Region within the altitude range of around 70 to 110 km, which is where most meteors are detected. We take a historical approach, following the development of meteor radar for studies of the MLT from the time of their development after the Second World War until the present. The application of the meteor radar technique is closely aligned with their ability to make contributions to Meteor Astronomy in that they can determine meteor radiants, and measure meteoroid velocities and orbits, and so these aspects are noted when required. Meteor radar capabilities now extend to measurements of temperature and density in the MLT region and show potential to be extended to ionospheric studies. New meteor radar networks are commencing operation, and this heralds a new area of investigation as the horizontal spatial variation of the upper-atmosphere wind over an extended area is becoming available for the first time.

Abstract. Accurate knowledge of the rate as well as the mechanism of excitation of the bending mode of CO2 is necessary for reliable modeling of the mesosphere–lower thermosphere (MLT) region of the atmosphere. Assuming the excitation mechanism to be thermal collisions with atomic oxygen, the rate coefficient derived from the observed 15 μm emission by space-based experiments (kATM = 6.0 × 10−12 cm3s−1) differs from the laboratory measurements (kLAB =(1.5-2.5) × 10−12 cm3s−1) by a factor of 2–4. The general circulation models (GCMs) of Earth, Venus, and Mars have chosen to use a median value of kGCM = 3.0 × 10−12 cm3s−1 for this rate coefficient. As a first step to resolve the discrepancies between the three rate coefficients, we attempt to find the source of disagreement between the first two. It is pointed out that a large magnitude of the difference between these two rate coefficients (kx ≡ kATM - kLAB) requires that the unknown mechanism involve one or both major species: N2, O. Because of the rapidly decreasing volume mixing ratio (VMR) of CO2 with altitude, the exciting partner must be long lived and transfer energy efficiently. It is shown that thermal collisions with N2, mediated by a near-resonant rotation-to-vibration (RV) energy transfer process, while giving a reasonable rate coefficient kVR for de-excitation of the bending mode of CO2, lead to vibration-to-translation kVT rate coefficients in the terrestrial atmosphere that are 1–2 orders of magnitude larger than those observed in the laboratory. It is pointed out that the efficient near-resonant rotation-to-vibration (RV) energy transfer process has a chance of being the unknown mechanism if very high rotational levels of N2, produced by the reaction of N and NO and other collisional processes, have a super-thermal population and are long lived. Since atomic oxygen plays a critical role in the mechanisms discussed here, it suggested that its density be determined experimentally by ground- and space-based Raman lidars proposed earlier.

Tides forced by large‐scale weather systems in the tropical troposphere introduce significant longitudinal and local time variability in the upper atmosphere. This paper presents variability of tidal harmonics of the latent heating from 2002 to 2011 associated with the El Niño–Southern Oscillation (ENSO) and the tidal wind response in the mesosphere/lower thermosphere (MLT) region. Emphasis is on the strong ENSO cycle 2009–2011. Latent tidal heating rates are computed from TRMM satellite precipitation data, with added radiative heating from MERRA reanalysis, as functions of time, latitude, and altitude. The heating rates for the two most affected nonmigrating tides (DE3 and DE2) are examined and compared with MLT tidal wind variability from TIDI/TIMED. Principal component analysis (PCA) is used to identify the tidal modes most affected by ENSO. Our results indicate that the tidal response to ENSO is largest during winter for both of the tides, with the largest response occurring in the DE3 tidal winds during the La Niña phase, with an increase of roughly 70% for the winter months of 2010/2011, and negligible response during the El Niño phase. The ENSO effect in the tidal forcing closely resembles the first symmetric and antisymmetric Hough modes of DE3 and DE2, thus being an efficient mechanism to transmit the ENSO signal into the MLT tides.

Abstract. Absolute profiles of atomic oxygen number densities with high vertical resolution have been determined in the mesosphere–lower thermosphere (MLT) region from in situ measurements by several rocket-borne solid electrolyte sensors. The amperometric sensors were operated in both controlled and uncontrolled modes and with various orientations on the foredeck and aft deck of the payload. Calibration was based on mass spectrometry in a molecular beam containing atomic oxygen produced in a microwave discharge. The sensor signal is proportional to the number flux onto the electrodes, and the mass flow rate in the molecular beam was additionally measured to derive this quantity from the spectrometer reading. Numerical simulations provided aerodynamic correction factors to derive the atmospheric number density of atomic oxygen from the sensor data. The flight results indicate a preferable orientation of the electrode surface perpendicular to the rocket axis. While unstable during the upleg, the density profiles measured by these sensors show an excellent agreement with the atmospheric models and photometer results during the downleg of the trajectory. The high spatial resolution of the measurements allows for the identification of small-scale variations in the atomic oxygen concentration.

When solar flares occur, as results of increases in X-rays and extreme ultraviolet (EUV) radiations the electron density in the ionosphere increases. The increase in the electron density can cause problems with satellite attitude control, satellite communication and GPS positioning accuracy. To validate the impacts of solar flares on satellites, it is necessary to quantify the increase in ionospheric plasma density caused by solar flares, although this process has not yet been precisely clarified. It is widely known that soft X-rays (0.1-0.8nm) generated during solar flares ionize the D-region ionosphere (60–90 km) and affect the propagation path of VLF (very low frequency, 3–30 kHz)/LF (low frequency, 30–300 kHz) radio waves (especially the reflection height). Lyman-alpha radiation (121.6 nm) is the major component of ultraviolet radiation, and is known to affect the D-region. However, the response of the D-region ionosphere to Lyman-alpha during solar flares remains unclear. In this study, we used the LF transmitter signals to investigate D-region ionospheric variations for the X5.89class solar flare that occurred at 01:10 UT on May 11, 2024. This was the largest solar flare among the consecutive solar flares that occurred in May 2024. The LF transmitter signals used in this study were OCTAVE/AVON data operated by our research group around the world. The LF waves were transmitted from JJY60 (60.0 kHz) in Japan and BPC (68.5 kHz) in China, and received at Pontianak (PTK), Indonesia and Rikubetsu (RKB), Japan. When a solar flare occurs the LF phase normally exhibits time variations similar to those of X-ray flux. However, during the X5.89-class solar flare, unusual oscillations in phase and amplitude were observed. Based on wavelet analysis of these oscillations, the periods were 549 s for the phase of the JJY60-PTK path and 388 s for the phase of the BPC-PTK path. To investigate the cause of the LF oscillations, we compared between the LF waves and EUV data observed by GOES-16 and SDO satellites. A part of LF oscillations shows similar time variations with that of the summation of EUV in the 20–140 nm range. A few peaks of the LF oscillation coincided with peaks in the composited EUV emissions. Coherence analysis was conducted to examine the relationship with EUV emissions at wavelengths of 117.5 nm, 133.5 nm, and 140.5 nm. The analysis revealed statistically significant coherence between the LF phase and EUV emissions within the 300–1000 s period range across all wavelengths, with particularly prominent peaks observed around 400–800 s. These results strongly suggest that the periodic variations in EUV emissions associated with the solar flare affected the electron density in the D-region ionosphere.

The study of the Earth’s atmosphere and the space environment is important because of the role played by the medium in the activities that affect the Earth and its inhabitants directly or indirectly. A robust capability to monitor, model and predict the happenings in the atmospheric space through deployment of both space- and ground-based observational systems for data acquisition, is key to result-oriented scientific research in atmospheric and space sciences. In this paper, we highlight the importance of regional deployment of observational facilities for data acquisition to complement current observational tools. We briefly review the capabilities of very low frequency (VLF) radio waves (in monitoring and studying changes in the atmosphere and ionosphere), and also present data obtained from our newly deployed VLF radio waves receiver at Anchor University, Lagos (AUL). We show that the diurnal signature characterised by VLF radio signal reflected in the data of three of four propagation paths (AUL-HWU, AUL-JJI, AUL-NWC and AUL-VTX) received at AUL. This outcome shows that operational condition of the VLF radio wave receiver is good. We anticipate interesting findings as we exploit this institution-based dataset to probe ionospheric irregularities in our region for the first time. In the light of some identified ionospheric changes in the equatorial region, we anticipate that our research effort using this regional VLF dataset will yield new results that will be resourceful for better understanding and characterisation of the dynamics of the equatorial D-layer ionosphere, since our region of reception (Lagos) lies close to the equator.

Continuous medium frequency radar observations of mesosphere lower thermosphere (MLT) region winds during February 2004–May 2005 (486 days) over Tirunelveli (8.7°N, 77.8°E) revealed intraseasonal oscillations (ISOs) in the 82‐ to 94‐km height region. Two distinct oscillations with periods 50–70 and 20–40 days are noticed predominantly in zonal winds. As it is well established that these oscillations are nonstationary and localized in time, wavelet analysis has been employed to study the time evolution of these oscillations. The analysis showed that 50‐ to 70‐day oscillation peaks during June‐October and 20‐ to 40‐day oscillation peaks during January‐March in the MLT region. To trace back the origin of these oscillations, the tropospheric ISO has been studied for the same period using outgoing long‐wave radiation (OLR) observations. The OLR, which is the proxy for convective activity in the lower atmosphere, around the radar site 5–10°N, 70–80°E is used for the present analysis. The wavelet analysis of OLR showed the 50‐ to 70‐day oscillation peaking at the same time as in the MLT region. The shorter period oscillation (20–40 days), which showed its peak during January‐March in the MLT region, is not observed in the OLR data during these months. However, the analysis of water vapor, which is the prime candidate for excitation of tides, showed the 20‐ to 40‐day oscillation during the same time as in the MLT region. In the present study, the lower atmospheric convective activity through gravity wave excitation and water vapor through tidal forcing are accounted for the observed ISO in the MLT region. The significance of present results lies in showing the highly coherent oscillations in OLR and MLT region zonal winds. The coupling between the lower and middle atmospheric ISO is extensively discussed in the light of existing mechanisms.

Waves with period near 3 days in the equatorial mesosphere and lower thermosphere over Ascension Island

Empirical model of the dynamics in the mesosphere and lower thermosphere region over the UK, including solar and geomagnetic activity

Anthropogenic effects on the distribution of minor chemical constituents in the mesosphere/lower thermosphere – A model study

Wind measurements from continuous meteor radar observations during June 2004–May 2007 are used to estimate the gravity wave momentum fluxes in the Mesosphere Lower Thermosphere (MLT) region over Trivandrum (8.5°N, 76.9°E), a low‐latitude station in India. The radial velocity variances in the 82–98 km height region, which are mainly caused by gravity waves, measured by the meteor radar are used to determine the gravity wave momentum fluxes. Using a novel method proposed by Hocking (2005), altitude profiles of momentum fluxes of short period (less than 2–3 h) gravity waves are estimated for three continuous years. Seasonal variation in the gravity wave momentum fluxes showed semi annual variation with equinoctial maximum and solstitial minimum. By using estimated gravity wave momentum fluxes, an attempt is made to quantify their contribution in driving the Mesospheric Semi Annual Oscillation (MSAO). The mean flow acceleration estimated from the divergence of gravity wave momentum fluxes is compared with the observed mean flow acceleration computed from monthly mean zonal winds during six MSAO cycles over three years. This comparison reveled that the gravity wave contribution toward the westerly phase of MSAO varies from ∼20–60% while that toward the easterly phase varies from ∼30–70%. Variations are observed from cycle‐to‐cycle in the gravity wave forcing toward both phases of MSAO. The significance of the present study lies in estimating the gravity wave momentum fluxes in the MLT region and quantifying their contribution toward the generation of MSAO over the low‐latitude for the first time.