Search

The Equatorial Electrojet (EEJ) is one of the important near‐earth space weather phenomena which exhibits significant diurnal, seasonal and solar activity variations. This paper investigates the EEJ variations at diurnal, seasonal and solar cycle time scales from the Indian sector and portrays a new empirical EEJ field model developed using the observations spanning over nearly two solar cycles. The Method of Naturally Orthogonal Components (MNOC), also known as Principal Component Analysis (PCA), was employed to extract the dominant patterns of principal diurnal, semi‐diurnal, and ter‐diurnal components contributing to the EEJ variation. The amplitudes of these diurnal, semi‐diurnal, and ter‐diurnal components in EEJ are found to vary significantly with the season and solar activity. The seasonal and solar activity dependencies of these principal components are modeled using suitable bimodal distribution functions. Finally, the empirical model for EEJ field was built by combining the principal components with their corresponding modeled amplitudes. This model accurately reproduces the diurnal, seasonal and solar activity variations of EEJ. The modeled monthly mean variations of EEJ field at ground exhibit excellent correlation of 0.96 with the observations with the root mean square error <5 nT. It also successfully captures the seasonal and solar activity variations of Counter Electrojet (CEJ). Finally, this model named "Indian Equatorial Electrojet (IEEJ) Model" is made publicly available for interested scientific users (https://iigm.res.in/system/files/IEEJ_model.html). Plain Language Summary: The equatorial electrojet (EEJ) is an intense current jet flowing in the upper atmosphere around 100 km. This is a unique phenomenon that occurs at only a few kilometers in height (e.g., 105–110 km) and with a horizontal (North‐South) width of a few hundred kilometers (e.g., ∼600 km) centered on the geomagnetic equator. This important geophysical phenomenon gives more insights into the equatorial ionospheric electrodynamical processes that have various impacts on satellite orbital dynamics, Global Positioning Systems and other satellite communication links, electrical power grids, etc. This study develops a new empirical model to predict the diurnal, seasonal, and solar activity variations in the intensity of this unique current system over the Indian longitude sector. Key Points: An empirical Equatorial Electrojet (EEJ) field model was developed using nearly two solar cycles of EEJ observations from the Indian sectorThe principal diurnal, semi‐diurnal, and ter‐diurnal components in the EEJ are modeled using suitable analytical functionsThis model can very accurately predict the diurnal, seasonal, and solar activity variations of monthly mean EEJ and CEJ fields at ground [ABSTRACT FROM AUTHOR]

Observational data sets for the high latitude middle atmosphere are key to understand the dynamics over those latitudes and the coupling between the lower and middle atmosphere. Utilizing long‐term data sets from an all‐sky imager at Tromsø, Norway (69.6°N, 19.2°E), the characteristics of 18 mesospheric frontal events in the Arctic winter mesosphere from 2011 to 2015 were studied. These frontal events exhibit horizontal extensions exceeding 500 km and were characterized by a sharp leading front, sometimes followed by a quasi‐monochromatic wave train or a turbulent region. A subset of these frontal gravity wave events has been identified in the past as "bores." While there have been numerous previous reports from low‐ and mid‐latitude sites, and also from southern high latitudes, there have been a few from northern high latitudes. This study focuses on the frontal events in the northern high latitudes and provides new insights into the characteristics of these events. Their horizontal wavelengths primarily ranged from 20 to 40 km, and they exhibited phase speeds in the range 30–80 m/s. Most events were observed before local midnight. No clear link between these events and auroral activity was found. The majority of fronts were found propagating in the north‐west direction, which might be due to the wind filtering effects. Plain Language Summary: To better understand the dynamics of the high latitude middle atmosphere, we used long‐term data from various instruments in Tromsø, Norway. Our focus was on examining mesospheric frontal events that occurred in the Arctic winter mesosphere between 2011 and 2015. These events were notable for their large horizontal extent, often exceeding 500 km, and their distinctive characteristics, such as a sharp leading front, followed at times by a wave train or a region of turbulence. Some of these events were previously referred to as "bores." While similar events have been reported for lower and mid‐latitudes, there has been limited research focusing on northern high latitudes. Our study fills this gap, providing new insights into this phenomenon. We found that such events are less frequent in the northern high latitudes compared to the southern high latitudes. These frontal structures mostly traveled in directions ranging from northwest to north, and also to the southeast. While there was no clear link between these events and auroral activity, the majority of fronts were found to be propagating in the northwest direction, which might be attributed to wind filtering effects. Key Points: The study investigated the characteristics of mesospheric frontal structures observed in airglow emissions during Arctic winters using long‐term data setsThe occurrence of frontal structure at northern high latitude site Tromsø is less frequent than South PoleThe majority of the observed fronts were likely associated with tropospheric meteorological processes, which serve as potential sources [ABSTRACT FROM AUTHOR]

Quasi 16‐day waves (Q16DWs) are a prominent and recurrent phenomenon in the middle atmosphere, typically observed over winter mid and high latitudes. This study investigates the intense Q16DW event during the 2018–2019 Northern Hemisphere (NH) winter, and explores its propagation in the middle atmosphere and its notable influence on the E‐region ionosphere. Long‐term geopotential height estimates of Aura Microwave Limb Sounder (MLS) reveal that the wave activity under consideration exhibited the largest amplitudes in the mesosphere for past 16 years. An analysis of wind data obtained from medium frequency (MF) and meteor radars, as well as from Modern‐Era Retrospective analysis for Research and Applications, Version 2 (MERRA‐2) reanalysis, reveals the presence of a westward‐propagating Q16DW with zonal wavenumber 1 exhibiting notable asymmetry about the equator, with the majority of the wave activity being confined to the NH. The prominently large amplitudes and vertical wavelengths of the wave suggest potential for the wave propagation to extend deep into the E‐region ionosphere. Swarm satellite observations reveal concurrent ∼16‐day oscillations in the eastward component of the geomagnetic field at low latitudes. These oscillations can be attributed to the periodic variations in interhemispheric field‐aligned currents (IHFACs). The ∼16‐day oscillations in the IHFACs are likely a consequence of asymmetric wind‐dynamo action, which is directly or indirectly associated with the Q16DW. These findings suggest that planetary waves originating in the middle atmosphere can cause interhemispheric coupling in the ionosphere. Plain Language Summary: This study examines the strong quasi‐16‐day wave activity that occurred during the winter of 2018–2019 over mid‐to‐high latitudes. A data set consisting of medium frequency (MF) and meteor radar winds, reanalysis data, Aura Microwave Limb Sounder (MLS) and Swarm geomagnetic field measurements is utilized to characterize the wave. The simultaneous occurrence of wave signatures in the middle atmosphere and the ionosphere suggests some atmosphere‐ionosphere coupling. The wave signatures observed in the eastward component of the geomagnetic field are attributed to interhemispheric field‐aligned currents. Key Points: Strong quasi‐16‐day wave signatures in middle atmospheric temperatures and winds during the northern hemispheric winter of 2018–2019Simultaneous occurrence of the wave in the eastward component of the geomagnetic fieldPlanetary wave oscillations of ∼16‐day period observed in the interhemispheric field aligned currents [ABSTRACT FROM AUTHOR]

Rotational temperatures in the Mesosphere‐Lower Thermosphere region are estimated by utilizing the OH(6,2) Meinel band nightglow data obtained with an Ebert‐Fastie spectrometer (EFS) operated at Arecibo Observatory (AO), Puerto Rico (18.35°N, 66.75°W) during February‐April 2005. To validate the estimated rotational temperatures, a comparison with temperatures obtained from a co‐located Potassium Temperature Lidar (K‐Lidar) and overhead passes of the Sounding of the Atmosphere by Broadband Emission Radiometry (SABER) instrument onboard NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite is performed. Two types of weighting functions are applied to the K‐Lidar temperature profiles to compare them with EFS temperatures. The first type has a fixed peak altitude and a fixed full width at half maximum (FWHM) for the whole night. In the second type, the peak altitude and FWHM vary with the local time. SABER measurements are utilized to estimate the OH(6,2) band peak altitudes and FWHMs as a function of local time and considerable temporal variations are observed in both the parameters. The average temperature differences between the EFS and K‐Lidar obtained with both types of weighting functions are comparable with previously published results from different latitude‐longitude sectors. We found that the temperature comparison improves when the time‐varying weighting functions are considered. Comparison between EFS, K‐Lidar, and SABER temperatures reveal that on average, SABER temperatures are lower than the other two instruments and K‐Lidar temperatures compare better with SABER in comparison to EFS. Such a detailed study using the AO EFS data has not been carried out previously. Plain Language Summary: In the Earth's atmosphere, a strong band of near‐infrared (NIR) emissions exists at ∼90 km altitude due to the vibrational‐rotational excitation of the Hydroxyl (OH) molecules. The intensity of these OH emissions depends on the temperatures of their rotational states. Under the assumption of thermodynamic equilibrium, the rotational temperatures should represent the background atmospheric temperatures. A scanning spectrometer of the Ebert‐Fastie configuration (EFS) was operated on the nights of February‐April 2005 at Arecibo Observatory (AO), Puerto Rico (18.35°N, 66.75°W) to record the intensity of the OH(6,2) band which is used to estimate rotational temperatures in this work. During the same period, a co‐located Potassium temperature Lidar (K‐Lidar) utilized resonance scattering from the atmospheric K‐atoms to measure temperatures in the 80–100 km altitude range. By nature, the EFS‐derived temperatures are an altitude‐integrated quantity while the K‐Lidar provides altitude‐resolved temperatures. First, we followed a commonly used methodology to compare the EFS and K‐Lidar temperatures by using the SABER data onboard NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. We found good agreement between our results and previously published studies. Then, we propose a modified methodology with the help of SABER data to show that it provides a better comparison between the EFS and K‐Lidar temperatures. Key Points: Good inter‐comparison is obtained between Arecibo Observatory spectrometer and Lidar temperatures, and results are similar to previous studiesThe temperature comparison improves when time‐varying weighting functions are used instead of a single weighting functionRotational temperature estimation from the spectrometer data depends strongly on the choice of Einstein coefficients for the P1 line transitions [ABSTRACT FROM AUTHOR]