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An unseasonal development of Equatorial Plasma Bubble (EPB)/Equatorial Spread-F (ESF) activity in a wide longitudinal sector over India and Southeast Asia during the post-sunset hours of 28 July 2014 has been investigated in detail using the multiple independent observations in this paper and a companion paper by (Prog Earth Planet Sci 5:10, 2018). This post-sunset ESF/EPB event was preceded by a substantial elevation of the equatorial F-layer due to strong pre-reversal enhancement (PRE) during the season which is climatologically unfavorable for strong PRE and post-sunset EPBs. In this paper, it is found that a strong equatorward meridional neutral wind at off-equatorial E-region and enhanced longitudinal gradient of equatorial electrojet appear to be the responsible candidates for the enhanced PRE and EPB development on 28 July 2014. These changes in the local wind system and PRE are found to have an apparent connection with the planetary wave forces of lower atmospheric origin. Both the post-sunset height rise (PSSR) and lower thermospheric meridional neutral wind consistently exhibit quasi-2-day planetary wave-like oscillations during 20-31 July 2014 with maximum amplitudes on 28 July 2014. This study emphasizes the role of planetary wave forces from the lower atmosphere that can alter the local neutral wind system and E-region conductivities that may lead to the development of strong PRE and EPB activity which were otherwise climatologically unfavorable for post-sunset EPB/ESF development. [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]

The observational data on OI 630 nm thermospheric nightglow from the Indian dip equatorial station, Tirunelveli (8.7°N, 77.8°E geographic, 1.6°N dip latitude) obtained between January 2013 and January 2015 using an All-Sky Airglow Imager (ASAI) are utilized in this work to examine the presence of gravity waves in the thermosphere. Two types of wave signatures were observed: (1) quasi-periodic waves consisting of alternating crests and troughs and (2) single bands of enhanced intensity (SBEI). The phase speed, wavelength and time period of the quasi-periodic waves were in the range of 70–160 m/s, 130–575 km and 25–75 min, respectively, while the phase speed and scale size of the SBEI features were found to be in the range of 150–250 m/s and 230–470 km, respectively. During equinoxes, quasi-periodic waves propagated towards north-northwest direction, while during winter they were observed to propagate towards northwest, south-southwest and southeast directions. [ABSTRACT FROM AUTHOR]

Investigations on the variations of the virtual height ( h ' F ) of the base of the F-layer over Thumba (8.5°N, 77°E, dip lat 0.5°N) in 2002 (high solar activity) and 2008 (very low solar activity) under quiet geomagnetic conditions show characteristic pre-midnight rise of h ' F in the June solstice of 2008. Comparison of the h ' F variations in 2008 over Thumba and Fortaleza (3.9°S, 38.4°W, dip lat 1.8°S), Brazil, reveals that the pre-midnight rise of h ' F is significantly more over Thumba during the June solstice. Drift measurements on-board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite elicit that the midnight upward drift over the Indian sector during the northern summer months of 2009 is the largest, a feature that significantly weakens in 2010. C/NOFS measurements also confirm the electro-dynamical nature of the pre-midnight h ' F rise over the Indian sector in the June solstice during the low solar activity. As the equatorial F-region vertical drifts during nighttime are controlled by E-region dynamo driven by tidal wind system, systematic wind measurements at upper mesospheric heights by an MF radar (1.98 MHz) from Tirunelveli (8.7°N, 77.8°E, dip lat 0.5°S), India, during 2000–2011 are used to derive the tidal components. This reveals that the phases of both the meridional and zonal components of the diurnal tide regress while the phase of the meridional component of the semidiurnal tide significantly advances with decreasing solar activity with concomitant increases in amplitudes during the summer months. These observations suggest the possible semidiurnal tidal influence on the pre-midnight h ' F rise over the Indian sector in the June solstice during low solar activity. [ABSTRACT FROM AUTHOR]

Abstract: The present study demonstrates how the relationship between the high latitude northern hemispheric major sudden stratospheric warming (SSW) events of 2006 and 2009 and low-latitude mesospheric tidal variability in zonal winds observed by the MF radar at Tirunelveli (8.7°0N, 77.8°E) exists. It is found that the ozone mixing ratio increases at low latitudes during the SSW and it could probably be due to the SSW induced reversal of meridional circulation towards southward, which may aid the transport of ozone from high to low latitudes, but prevent the same from low to high latitudes. As semi-diurnal tide is produced due to solar insolation absorption of ozone and the increase in the ozone mixing ratio could be a reason for the increase in the semi-diurnal tidal amplitude. The variabilities of diurnal tide appear to be governed mostly by variation of specific humidity at 300hPa over equator and intraseasonal variability dominates the variabilities in both the parameters. [Copyright &y& Elsevier]

The altitude profiles of middle atmospheric temperatures are examined to study the effects of the solar eclipse on 15 January 2010 as it passed through the Indian longitudes. It was found that thermal structures exhibited strong inversion layers on the eclipse day, with large temperature deviations around 45, 50, 60, 75, 82 and 92 km altitudes compared to normal days. Fourier analysis revealed no significant changes in the gravity wave characteristics; however, the amplitudes of the observed waves were significantly higher (∼ two times) compared to the normal days. [ABSTRACT FROM AUTHOR]

Vector measurements of middle atmospheric electric fields are mostly carried out using a double-Langmuir probe technique on platforms like balloons and rockets. High impedances (up to 1013 ohms) and low signal levels (~few millivolts) are the factors that are to be considered in the design of an electrometer for one such experiment. Here, we offer a simple circuit concept that invokes an instrumentation amplifier capable of meeting our several needs, namely generating the guard voltage, ultra-high impedance (1015 ohms) input terminals, femtoampere bias current, differencing, gain setting, etc. all in one package. A three-channel differential electrometer was developed based on this concept and flown on a high-altitude balloon platform. Results reveal that the instrument is capable of detecting weak electric field variations at stratospheric altitudes. [ABSTRACT FROM AUTHOR]

We analyzed the mesospheric winds and temperature data for investigating the waves scaling from the period of few hours to several hours (and few days) based on the airglow observations at Kolhapur (16.8°N, 74.2°E, 10.6°N dip. lat.). The data presented in this study are collected using medium frequency radar and Multispectral Scanning Photometer at low latitude station Kolhapur. We scrutinized the wind and temperature relation of these waves for the observed period from January to May 2011. The data of 56 clear nights were collected and out of which 22 nights of data shows a conspicuous wavelike features. The nocturnal variability reveals the prominent wave signatures with a period which range from 7 to 12 h (h) as a dominant nocturnal wave. The presence of quasi 2.8–4 days waves with significant amplitudes is also detected. The comparison of the winds and temperatures suggests the temperature waves to be near in phase with meridional wind component and a time delayed relation with the zonal wind component. [ABSTRACT FROM AUTHOR]

Recently, equatorial secondary cosmic ray observatory has been established at Equatorial Geophysical Research Laboratory (EGRL), Tirunelveli, (Geographic Coordinates: 8.71°N, 77.76°E), to study secondary cosmic rays (SCR) produced due to the interaction of primary cosmic rays with the Earth’s atmosphere. EGRL is a regional center of Indian Institute of Geomagnetism (IIG), located near the equator in the Southern part of India. Two NaI(Tl) scintillation detectors are installed inside the temperature controlled environment. One detector is cylindrical in shape of size 7.62 cm × 7.62 cm and another one is rectangular cuboid of 10.16 cm × 10.16 cm × 40.64 cm size. Besides NaI(Tl) detectors, various other research facilities such as the Geomagnetic observatory, Medium Frequency Radar System, Digital Ionosonde, All-sky airglow imager, Atmospheric electricity laboratory to measure the near-Earth atmospheric electric fields are also available at EGRL. With the accessibility of multi- instrument facilities, the objective is set to understand the relationship between SCR and various atmospheric and ionospheric processes, during space weather and terrestrial events. For gamma-ray spectroscopy, it is important to test the performance of the NaI(Tl) scintillation detectors and to calibrate the gamma-ray spectrum in terms of energy. The present article describes the details of the experimental setup installed near the equator to study cosmic rays, along with the performance testing and calibration of the detectors under various conditions. A systematic shift in the gain is observed with varying temperature of the detector system. It is found that the detector’s response to the variations in the temperature is not just linear or non-linear type, but it depends on the history of the variation, indicating temperature hysteresis effects on NaI detector and PMT system. This signifies the importance of isothermal environment while studying SCR flux using NaI(Tl) detectors, especially for the experiments conducted during daytime such as solar eclipses etc. [ABSTRACT FROM AUTHOR]

Identification and quantification of possible drivers of recent global temperature variability remains a challenging task. This important issue is addressed adopting a non-parametric information theory technique, the Transfer Entropy and its normalized variant. It distinctly quantifies actual information exchanged along with the directional flow of information between any two variables with no bearing on their common history or inputs, unlike correlation, mutual information etc. Measurements of greenhouse gases: $$\hbox {CO}_{2}$$ , $$\hbox {CH}_{4}$$ and $$\hbox {N}_{2}\hbox {O}$$ ; volcanic aerosols; solar activity: UV radiation, total solar irradiance ( TSI) and cosmic ray flux ( CR); El Niño Southern Oscillation ( ENSO) and Global Mean Temperature Anomaly ( GMTA) made during 1984-2005 are utilized to distinguish driving and responding signals of global temperature variability. Estimates of their relative contributions reveal that $$\hbox {CO}_{2}$$ ( $${\sim } 24 \%$$ ), $$\hbox {CH}_{4}$$ ( $${\sim } 19 \%$$ ) and volcanic aerosols ( $${\sim }23 \%$$ ) are the primary contributors to the observed variations in GMTA. While, UV ( $${\sim } 9 \%$$ ) and ENSO ( $${\sim } 12 \%$$ ) act as secondary drivers of variations in the GMTA, the remaining play a marginal role in the observed recent global temperature variability. Interestingly, ENSO and GMTA mutually drive each other at varied time lags. This study assists future modelling efforts in climate science. [ABSTRACT FROM AUTHOR]

The medium frequency radar deployed at Tirunelveli (8.7°N, 77.8°E), which is located near the southmost tip of peninsular India, have been providing continuous data from the year 1993 to the year 2012 that helped to study the long term tendencies in the lunar tidal variabilities over this geographic location. In the present paper we present the results of seasonal, interannual and long-term variabilities of lunar semi-diurnal tides in the upper mesosphere over Tirunelveli. The present study also includes comparison with model values. The study shows that the tidal amplitudes are larger in the meridional components of the mesospheric winds than the zonal winds. The seasonal variations of the tides are similar in both the components. The tides show maximum amplitudes of about ∼5 m/s in February/March, secondary maximum amplitudes of about ∼3 m/s in September and minimum amplitudes during summer months (May–August). The observed seasonal variation of the lunar tides do not compare well with Vial and Forbes (1994) model values, though it is consistent with earlier observations. The lunar tidal phase in meridional winds leads that in zonal winds from January to June and from September to November, while the latter leads the former during July/August. The lunar tides show large interannual variability. There are unusual amplitude enhancements in the lunar tide in meridional winds during the winters of 2006 and 2009, when major sudden stratospheric warmings (SSW) occurred at high latitude northern hemisphere, whereas zonal lunar tide does not show any clear association with the SSW. Vertical wavelengths of lunar tides in zonal and meridional wind are in the range of 20–90 km. The vertical wavelengths of lunar tides in both zonal and meridional component are smaller in June and larger in November and December. The monthly mean zonal and meridional winds are subjected to regression analysis to study the tidal response to long-period oscillations, namely, quasi-biennial oscillation (QBO), solar cycle variation and El-nino southern oscillation (ENSO). It is found the lunar tide in both zonal and meridional winds show significant QBO response, whereas zonal tide only shows significant negative response to solar cycle and positive response to ENSO. Besides, zonal tide only shows significant long-term increasing trend. [ABSTRACT FROM AUTHOR]

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 45–50 h 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. [ABSTRACT FROM AUTHOR]

This study presents comparison of low-latitude dynamical responses to boreal 2008/09 and austral 2002 winter Major Stratospheric Warming (MSW) events, as both events are of vortex split type. During these winters, planetary wave (PW) variability and changes in low-latitude circulation are examined using European Center for Medium Range Weather Forecasting (ECMWF) reanalysis (ERA)-interim data sets and mesospheric wind data acquired by the MF radars at Tirunelveli (8.7°N) and Rarotonga (22°S). Eliassen-Palm diagnostic is used to provide an evidence for the lateral PW energy propagation from high to low-latitudes during both the MSW events. The PW flux reaches much lower latitudes during the boreal event than during the austral event. The low-latitude westward winds at stratospheric heights are stronger (weaker) during the boreal (austral) MSW. Weak (strong) PW wave activity at low latitude mesospheric heights during boreal (austral) MSW indicates the influence of low-latitude stratospheric westward winds on the vertical propagation of PW to low-latitude mesosphere. [ABSTRACT FROM AUTHOR]

Schumann resonances (SRs) are the AC components of the global electric circuit and are excited by the lightning activity within the Earth-ionosphere waveguide. An induction magnetometer, which was operated from the Indian Antarctic station, Maitri (70.8°S, 11.7°E), served to examine the SR parameters, namely the amplitude and frequency, in the north-south (HNS) and east-west (HEW) magnetic components. The analysis for the first resonant mode presented in this work reveals a strong UT variation in its amplitude in seasonal as well as yearly timescales. The NS amplitude reveals a semi-diurnal variation with peaks at ~1000 and ~2100 UT, whereas the EW amplitude exhibits a strong diurnal variation with a pronounced peak at 1600 UT. The diurnal curves for the frequency for both components are similar in nature to those for the amplitude, but for a time shift. The diurnal trend in the amplitude is retained irrespective of seasons, whereas significant difference are noticed in the frequency behaviour between the summer and winter seasons, especially in the EW component. The observed diurnal variation in the SR intensity is explained in terms of the dominant thunderstorm activity centred over the three convectively active regions: Asia/ Maritime Continent (Indonesia), South America and Africa. The diurnal variation in frequency depends not only on the location of the thunderstorm region with respect to the observer, but also on the ionospheric day/night conditions and the Earth-ionosphere cavity thickness. [ABSTRACT FROM AUTHOR]

The photometric measurements of mesospheric OH and O(¹S) emission, carried out from Kolhapur (16.8°N, 74.2°E), Maharashtra during January-April 2005 are used to study the wave characteristics. The nocturnal variability reveals the dominant long-period wave signatures with significant amplitudes of embedded short-period waves. We carry out a sensitivity study on the vertical wavelength (VW) derived with the help of Krassovsky parameters (η = |η|eiφ) of the OH data, which reveals VW to vary from 38.9 to 110.2 km. This was compared with the VW estimates using the phase difference of the simultaneously observed waves in both OH and O(¹S) emission intensities. Results reveal that in the absence of attitudinally resolved measurements, the VW estimated using Krassovsky method can be used. [ABSTRACT FROM AUTHOR]

In this paper, we study the Equatorial Plasma Bubble (EPB) features using All sky imager (ASI) observations of O( 1 D) 630.0 nm night airglow emission from Kolhapur (16.8°N, 74.2°E, 10.6°N dip lat.) and Gadanki (13.5°N, 79.2°E, 6.5°N dip lat.) during March 2012. The optical data was supported by the ionosonde measurements from Tirunelveli (8.7°N, 77.8°E, 0.5°N dip lat.) which revealed the occurrence of equatorial spread-F. The EPBs were monitored at both locations as nearly north–south aligned intensity depleted regions. We computed east–west plasma drift velocity over Kolhapur and Gadanki for the nights having coordinated measurements. Also, the observed plasma bubble drift velocities are compared with the zonal neutral wind velocities obtained from the HWM-07 model and the empirical drift model of England and Immel (2012) . We observed that, generally, the mean zonal drift velocities of the plasma bubbles tend to decrease with local time (after midnight). Our results reveal the drift velocity noted in Kolhapur data varies from 124 m/s to 181.8 m/s, while from the Gadanki data show the drift velocity to range from 116.3 m/s to 160.3 m/s. [ABSTRACT FROM AUTHOR]

This paper reports the nightglow observations of OI 630.0nm emissions, made by using all sky imager operating at low latitude station Kolhapur (16.8°N, 74.2°E and dip lat. 10.6°N) during high sunspot number years of 24th solar cycle. The images are analyzed to study the nocturnal, seasonal and solar activity dependence occurrence of plasma bubbles. We observed EPBs in images regularly during a limited period 19:30 to 02:30LT and reach maximum probability of occurrence at 22:30LT. The observation pattern of EPBs shows nearly no occurrence during the month of May and it maximizes during the period October–April. The equinox and solstice seasonal variations in the occurrence of plasma bubbles show nearly equal and large differences, respectively, between years of 2010–11 and 2011–12. [ABSTRACT FROM AUTHOR]

This paper presents observations of plasma blobs by nightglow OI 630.0 nm emissions using ground-based techniques, all sky imager and photometer from Kolhapur. The nightglow observations have been made at low latitude station, Kolhapur (16.42°N, 74.2°E, and 10.6°N dip lat.) during clear moonless nights for period of October 2011-April 2012. Generally, these occur 3 h after sunset (18:00 IST). Herein we have calculated velocities of plasma blobs using scanning method, introduced by Pimenta et al. (Adv Space Res 27:1219-1224, ). The average zonal drift velocity (eastward) of the plasma blobs were found to be 133 ms and vary between 100 and 200 ms. The width (east-west expansion) and length (north-south expansion) of plasma blobs is calculated by recently developed method of Sharma et al. (Curr Sci 106(08):1085-1093, ). Their mean width and length were in the range of 70-180 and 500-950 km respectively. The study shows that localized eastward polarization electric field plays an important role in the generation of plasma blobs. [ABSTRACT FROM AUTHOR]

This article describes the possibility of using the fast image analysis technique for qualitative and quantitative analysis of equatorial plasma bubble obtained using All-Sky imager (ASI) data. Automated image processing (generally) is useful for identification of equatorial plasma bubbles (EPBs) and its parameters. We have developed a fast (and efficient) analysis technique essential to study the data of images. The present work reports the results of a statistical study of the zonal plasma bubble velocities using nightglow OI 630.0 nm emission data, acquired by ASI (FOV 140°) at the low-latitude station Kolhapur (16.42°N, 74.2°E and 10.6°N dip lat.). Based on the observations of 15 nights made in January 2012, we have determined the velocity of EPB using our new method. The daily mean values of the EPB velocity match well with those of the earlier studies made at Kolhapur. We have found that, generally, the mean zonal drift velocities of the plasma bubbles tend to decrease with local time (after midnight). The most significant finding from this work is that the calculated velocities of plasma bubble using fast and scanning methods are nearly equal. [ABSTRACT FROM AUTHOR]

Abstract: The relation between intrusions of stratospheric air into the upper troposphere and deep convection at equator during the stratospheric sudden warming (SSW) event of 2009 is examined using the ERA-interim reanalysis and NOAA outgoing longwave radiation (OLR) data sets. There is an intrusion of potential vorticity (PV) equatorward and westward, when the amplitude of planetary wave of zonal wavenumber 2 at 10hPa decreases drastically and polar stratospheric temperature increases simultaneously at 60°N. As a special case, the PV intrudes as narrow tongue at longitudes near 60°E (Indian ocean sector) even to latitudes less than 20°N during the SSW, whereas PV normally intrudes near 210°E (eastern Pacific) to equatorial latitudes. Decrease in OLR is observed east of these PV intrusions. Vertical velocity is largely upward at all pressure levels. As the PV intrusion can have profound influence on tropospheric convection and the latent heat release due to equatorial convection is an important source mechanism for the generation of gravity waves, we examined gravity wave activity in the daily radiosonde observations of winds and temperature at Gadanki (13.5°N, 79.2°E). It is observed that the potential energy per unit mass, estimated from the gravity wave temperature perturbations has considerably enhanced in relation with the deep convection. The predominant direction of propagation of the gravity waves is westward prior to the SSW, as a response to the active convection over Indonesia, turns to eastward during and after the SSW, as a response to the PV intrusion induced convection over west of India. [Copyright &y& Elsevier]

Simultaneous photometric measurements of mesospheric OH night airglow emissions have been made from two stations, Gadanki (13.5°N, 79.2°E) and Kolhapur (16.8°N, 74.2°E), India during February-March 2010 to study the differences in the observed wave characteristics. Our results reveal the wave periodicities to be similar at both locations, which ranged from 2 to 8 h. The inferred Krassovsky parameters (? = |?|eif) show large variability, with ranges of |?| varying from 2.4 to 4.2 over Gadanki and from 2.2 to 6.3 over Kolhapur. The phase values of Krassovsky parameter, f, exhibit variation from -101° to -202° in Kolhapur data and -38° to -93° in Gadanki data. The deduced vertical wavelengths indicate that the observed waves were propagating upwards with vertical wavelengths varying from -26 to -62 km. We note that the observed night-time OH emission profile shows the peak of emission at ~ 85 km in Kolhapur and at ~ 90 km in Gadanki in the corresponding months. These observed characteristics of waves and the cause of noted differences are discussed. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an overview of CAWSES—India program, highlighting some of the Indian coordinated efforts to understand the Sun−Earth system as a whole, with special emphasis to the equatorial and low latitude phenomena. Two multi-institutional, multi-instrument campaigns, one under ‘Space Weather: Science and Applications’ and the other emphasizing tides under ‘Atmospheric Coupling Processes’ were conducted during February–April 2006. The highlights of the campaigns include improved prediction of equatorial spread-F (ESF) based on a ‘new factor’ combining the strength and asymmetry of the equatorial ionization anomaly (EIA); simultaneous detection of quasi-2-day wave at both E and F region heights; tomographic image showing a Traveling Ionospheric Disturbance (TID) in association with a counter electrojet (CEJ) event—all highlighting the neutral and electrodynamical coupling of the equatorial ionosphere−thermosphere regions; an unusual lowering of mesospheric temperature during a moderate geomagnetic storm—highlighting a new space weather effect; diurnal tide showing a peak amplitude of 35m/s at 45km in zonal wind and a significantly lower amplitude (∼15m/s) in the meridional wind and also evidence of convective activity in the lower atmosphere influencing the tidal variability in the Mesosphere−Lower Thermosphere (MLT) region highlighting the vertical coupling of the atmospheric regions. The development of models for equatorial and low latitude ionosphere has been among the prime activities taken up under ‘Space Climate’. Two models have been developed towards this end: one based on Second Degree (SD) relationships of F region parameters to the mean sun spot number and the other a Multiple Regression Analysis (MRA) based model involving expressions relating F region parameters to that representing solar and geomagnetic activities. The two models, intended for long and short term predictions, have been found to represent the equatorial and low latitude ionosphere over Indian longitudes better than the commonly used International Reference Ionosphere (IRI). Under the theme of ‘Solar Influence on Atmospheric Climate’, a study has been made on the effects of solar variability on middle atmosphere using satellite data and model simulations, highlighting the significant differences between them. [Copyright &y& Elsevier]

The global component of fairweather electricity is subject to special attention to watch the solar-terrestrial effects and secular changes in climate. It is generally considered that the diurnal variation of atmospheric electricity parameters, if they are not following the Carnegie pattern, are not representative of the global thunderstorm activity. Some of the results obtained from Maitri (70°45′54″S, 11°44′03″), are discussed here in context with global thunderstorm activity and space weather influences. The diurnal pattern of the Potential Gradient and current density strongly deviate from the Carnegie curve. We have showed that this deviation is not due to the local electrical influence but due to the global thunderstorm activity. During fairweather condition the parameters are representing the global thunderstorm activity and to some extent they respond to the upper atmospheric electro dynamic phenomenon. The mean value of the potential gradient (77.7 V/m) and current density (2.13 pA/m) well below the expected global mean but close to the value reported from the same location and season in the past years. The mean conductivity, 3.34 × 10 mhom, is slightly at higher side and they exhibit a different diurnal trend comparing to the past measurements at this location. [ABSTRACT FROM AUTHOR]

Abstract: Spectral analysis of Tirunelveli (8.7°N, 77.8°E) MF radar winds for the year 2007 indicate the presence of long-period Kelvin waves with periods ∼23 and ∼16 days in the low-latitude mesosphere during Indian summer monsoon months. The dominant presence of these slow-phase speed waves at mesospheric altitudes motivated us to investigate their origin and vertical propagation characteristics. Space-time Fourier analysis of NCEP winds and OLR show the presence of these periodicities with zonal wavenumber 1 indicating that tropical convection is the potential source for these waves and westward phase of stratospheric QBO winds might have favoured these waves to reach the mesosphere. [Copyright &y& Elsevier]

The main aim of the paper is to describe mesosphere and lower thermosphere (MLT) wind field observed between June 2000 and May 2002 by medium frequency (MF) radars at two locations near the equatorial region and at tropical latitude. We have presented and compared observations of mean horizontal winds in the mesosphere and lower thermosphere (MLT) region over Kolhapur (16.8°N, 74.2°E) and Tirunelveli (8.7°N, 77.8°E). Our analysis reveals annual oscillation in the zonal wind and semi-annual oscillation in the meridional wind over Kolhapur. On the other hand, results over Tirunelveli reveal semi-annual oscillation (SAO) in the zonal wind and annual oscillation in the meridional wind. Also we have observed enhanced magnitude of wind speed in spring equinox period of 2002 exhibiting the signatures of the quasi-biennial oscillation (QBO) over Tirunelveli. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we are concerned with the variation of the atmospheric electric field and the air–earth current due to the excessive power generated by the solar wind-magnetosphere dynamo during geomagnetic storms, recorded at Maitri in Antarctica during 2004. A major part of the power generated by the solar wind-magnetosphere dynamo is used in the formation of the ring current and the rest is utilized for Joule heating and auroral particle precipitation. The method adopted by Frank-Kamenetsky et al. [Frank-Kamenetsky A.V., Troshichev O.A., Burns G.B., Papitashvili V.O., 2001. Variations of atmospheric electric field in the near-pole region related to the interplanetary magnetic field. J. Geophys. Res. 106, A1, 179–190.] was utilized to delineate the variations due to the signatures of tropical thunderstorm activity from the geoelectric data; while statistical methods used in our earlier studies were used to delimit variation due to the constant buffeting of the solar wind. We find that the solar wind-magnetosphere energy coupling function (ε) to be well correlated with the atmospheric electric parameters during the onset of geomagnetic disturbances. However the correlation breaks down during minor storms and sub-storm events. [Copyright &y& Elsevier]

Abstract: Vertical coupling in the low-latitude atmosphere–ionosphere system driven by the 5-day Rossby W1 and 6-day Kelvin E1 waves in the low-latitude MLT region has been investigated. Three different types of data were analysed in order to detect and extract the ∼6-day wave signals. The National Centres for Environmental Prediction (NCEP) geopotential height and zonal wind data at two pressure levels, 30 and 10hPa, were used to explore the features of the ∼6-day waves present in the stratosphere during the period from 1 July to 31 December 2004. The ∼6-day wave activity was identified in the neutral MLT winds by radar measurements located at four equatorial and three tropical stations. The ∼6-day variations in the ionospheric electric currents (registered by perturbations in the geomagnetic field) were detected in the data from 26 magnetometer stations situated at low latitudes. The analysis shows that the global ∼6-day Kelvin E1 and ∼6-day Rossby W1 waves observed in the low-latitude MLT region are most probably vertically propagating from the stratosphere. The global ∼6-day W1 and E1 waves seen in the ionospheric electric currents are caused by the simultaneous ∼6-day wave activity in the MLT region. The main forcing agent in the equatorial MLT region seems to be the waves themselves, whereas in the tropical MLT region the modulated tides are also of importance. [Copyright &y& Elsevier]