The Space Physics Laboratory (SPL) of Vikram Sarabhai Space Centre (VSSC) has launched a science program called the Indian network for Space Weather Impact Monitoring (InSWIM) to monitor the effects of Space Weather events on the Indian low-latitude ionosphere-thermosphere system. This program aims to study the impact of Space weather on the Indian ionospheric region and develop an Ionospheric Model. The InSWIM network stations will be equipped with instruments such as Global Navigation Satellite Systems (GNSS) receivers, Low Earth Orbit (LEO) receivers, ionosondes, magnetometers, and airglow photometers/imagers. Currently, multi- frequency, multi-constellation GNSS Receiver systems are operational at various stations in India. This network will enable us to understand (a) the quiet-time variability of the ionosphere over the Indian low-latitude region, (b) comprehensively study the response of the low-latitude ionosphere specific to the Indian longitudes under different space weather conditions, with the goal of understanding the various physical mechanisms causing variability in the ionospheric regions, and (c) develop an ionospheric model to reduce ionospheric errors in GNSS systems. Additionally, this network will provide complementary information for rocket and satellite-based experiments. This paper aims to provide details of the InSWIM network to the scientific community for its possible use in monitoring and studying the impact of space weather on the near-Earth space environment. (c) 2024 COSPAR. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Atomic oxygen (AO) is the most abundant species in the terrestrial thermosphere at altitudes of 180 to 700 km and plays a crucial role in numerous upper atmospheric processes. The drag encountered by a spacecraft is directly influenced by the background atmospheric density, making AO a notable contributor in this regard. The objective of this study is to analyze the variability of the density of AO in the near-Earth space environment under different solar activity conditions, as well as during severe space weather events. The analysis is primarily focused on the altitude range of 300 to 500 km in the Earth's thermosphere across the globe, which is a crucial operating region for a large number of Low Earth Orbiting satellites. AO exerts significant drag on the Low Earth Orbit (LEO) satellites and reacts with the materials of the satellite body and in situ probes mounted therein, causing considerable degradation to both. Additionally, AO interferes with measurements made by optical instruments through phenomena such as 'shuttle glow'. Analysis based on NRLMSIS 2.0 model revealed that the concentration of AO displays significant seasonal variations primarily due to the asymmetry in solar insolation. The highest concentration of AO was observed during the spring equinoctial month of the solar maximum year, while the lowest concentration was noted during the winter solstitial month of the solar minimum year. The enhancement seen in AO density contours show an asymmetry in the zonal direction at approximately 50 degrees N in June and 50 degrees S in December. Similarly, during geomagnetically disturbed days, the AO density shows enhancements along various longitudes, as indicated by the elongated contours. During geomagnetic storms, the heightened Joule heating in the polar region modifies the meridional circulation, which in turn affects the global distribution of AO density. Similarly, an analysis of thermospheric neutral winds obtained from the HWM93 model reveals that the zonal winds exhibit a strong westward acceleration over the region where AO density is enhanced. Furthermore, such an increase in AO density during geomagnetic storms has been found to cause orbital degradation ranging from a few tens of meters to a few hundred meters, depending on the strength and geo-effectiveness of the storms. This study emphasizes the critical contribution of AO density to satellite drag force, necessitating in situ measurements for an accurate estimation of the altitude decay of the spacecraft. (c) 2024 COSPAR. Published by Elsevier B.V. All rights reserved.

Results on first systematic measurements of nocturnal thermospheric neutral winds and temperature using a Fabry-Perot Interferometer (FPI) from Trivandrum (8.5 & DEG;N, 77 & DEG;E, 0.5 & DEG; dip latitude), an Indian station close to the geomagnetic equator, are presented. These measurements were made during the low solar activity period of June 2017 to April 2018. The OI 6300 A thermospheric nightglow emission has been used as the tracer. Using these measurements, nocturnal variability of monthly mean temperature and wind have been obtained. These mean temperature and wind are compared with the predictions of the neutral atmospheric models, NRL (USA)'s Mass Spectrometer and Incoherent Scatter Radar 2000 (MSISE-00) and Horizontal Wind Model 2014 (HWM14). The FPI temperatures reveal an overall decreasing trend beginning post-sunset till the end of observation for all months. The lowest monthly mean temperature was estimated to be 601 & DEG;K in the pre-morning hours in March 2018, which is about 45 & DEG;K less than the corresponding model value. There are overall agreements between the FPI and the MSIS model temperatures. Although, in some months,measurements exhibit large deviations from the model predictions indicating its limitations as applied to the equatorial thermosphere. The mean meridional wind is found to agree with the predictions of HWM14 model remarkably well and also corroborates with the wind retrieved from the collocated ionosonde measurements reported earlier from Trivandrum. Meridional wind during solsticial months clearly exhibits signatures of interhemispheric flow. The mean zonal wind, however, deviates significantly from the model predictions on most of the occasions. The mean zonal wind is found to be less compared to that reported elsewhere in the African and Brazillian longitude sectors. It is ascribed to be due to the relatively less pre-reversal enhancement (PRE) and the consequent strong ion-drag over Trivandrum. Further, it appears that PRE is relatively stronger in winter solstice than in the equinoxes in agreement with an earlier investigation carried out from Trivandrum. & COPY; 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

This study presents the response of thermospheric O1D 630.0 nm dayglow emission to the variability associated with equatorial Counter Electrojet (CEJ) events. The analysis based on the data from a meridian scanning Dayglow Photometer, Digital Ionosonde and Proton Precession Magnetometer over Trivandrum (8.5 degrees N, 77 degrees E, 0.5 degrees dip lat.), indicates that the O1D 630.0 nm emission behave distinctly different during the CEJ events compared to that on normal days. It has been observed that O1D 630.0 nm emission shows enhancement during the negative excursion of the DH, followed by an unusual depletion during the peak CEJ time. The observed variability was found to be more pronounced in a latitudinal region of +/- 3 degrees centered at around the dip equator. In addition, the emission intensities also exhibit the presence of enhanced short period oscillations of periodicity 20-30 min during the CEJ events. Analysis of the data from the collocated ionosonde revealed that the F-region electron density showed enhancement during the early phase of the CEJ and a decrease during the peak CEJ. Further, the simulation studies using a Quasi 2 dimensional ionospheric model showed that the modified plasma fountain during the CEJ can alter the plasma density at the emission centroid. The study reveals a strong dynamical coupling between the E and F-region of the dip equatorial ionosphere.(c) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

This paper discusses the results from the aeronomy experiment carried out using a multiwavelength photometer operated onboard Sagar Kanya ship from the Indian region, which covered a latitude region from 14.5 N to 2S (7 degrees to -10 degrees Dip latitudes) along the same longitude region (-72-75E). It has been found that the thermospheric airglows, especially the OI 630.0 nm emission near to the geographic equator exhibits 'Midnight Brightness' on different days of observations. Though the magnitudes were found to be less, the OI 777.4 nm and OI 557.7 nm emissions also exhibited intensity variations on those days, indicating a common causative mechanism for the same. The simulation studies using a quasi-two dimensional ionospheric model, revealed that such an enhancement could be due to the F-region collapse associated with the Midnight Temperature Maximum (MTM) phenomenon. (c) 2022 COSPAR. Published by Elsevier B.V. All rights reserved.

This paper reports, for the “first time,” the delayed response of O1D 630.0 nm dayglow emissions over Trivandrum (8.5°N, 77°E, 0.5°N dip lat.), a geomagnetic dip equatorial station in India, to the noontime X‐class solar flare event of July 30, 2005. The dayglow measurements were made using a unique dayglow photometer operating at three wavelengths. The Equatorial Electrojet induced magnetic field, measured using a proton precession magnetometer, showed a magnetic spike having ∼90 nT enhancement during this flare with a time delay of ∼7.2 min. A noteworthy observation is that unlike to the conventional belief, the O1D 630.0 nm dayglow over the dip equator exhibited a fourfold enhancement during the noontime flare after a time delay of ∼45 min. Analysis of satellite measured electron density and modeling simulations indicate that the thermospheric O1D 630.0 nm dayglow emission over the dip equatorial region during a solar flare is primarily driven by the electrodynamics, rather than the direct solar control. This finding is new, unique and very important for the studies related to plasma‐neutral coupling and also for modeling studies on the equatorial thermosphere‐ionosphere region is concerned.

Airglows are weak emissions originating at different heights of earth’s atmosphere. Recombination of ions, ionized by sunlight during day, luminescence arose by cosmic rays striking the upper atmosphere, and the chemiluminescence initiated mainly by oxygen and nitrogen reacting with hydroxyl ions at heights of few hundreds of kilometers are the various process causing airglow. The Day and Night glow Photometer (DNPM),emerging in the optical aeronomy laboratory of space physics laboratory, VSSC, is designed to have the capability of producing real-time measurements of airglow emissions originating at various altitude region of the earth’s near space during daytime and night time. The DNPM has a complex daytime optical system for extracting the faint airglow mixed with numerous orders of intense solar radiation background and a night time optical system, which directly measures the night airglow emission as there is no solar background. This paper reports the design and implementation of data acquisition and control system for DNPM during night. The functioning of DNPM is automated at both software and hardware level by means of Laboratory Virtual Instrumentation Engineering Workbench (LabVIEW). National instrument PCI 6602 DAQ module is the data acquisition module used for this purpose. The functioning of the device is tested and verified successfully.

This paper presents the response of the equatorial ionosphere to the noon time annular solar eclipse of 15 January 2010. The observation has been made using a Digisonde, Meteor Wind Radar, and Proton Precession Magnetometer over Trivandrum, (8.5oE; 77oN; dip lat 0.5oN), a geomagnetic dip equatorial station in India. It has been found that the E, F1, and F2 regions of the equatorial ionosphere respond to solar eclipse with different time delays, F1 being responding faster and the E and F2 regions slower. Though there have been studies on the delayed response of F2 region during the solar eclipses, the delayed response of E region is quite unexpected since this region is dominated by the recombination chemistry. The plausible reasons for this have been explored, and it is suggested that the downward diffusion of atomic oxygen plays a major role for the observed phenomenon.

Climatology of lunar semidiurnal (LSD) oscillations is constructed using a decade of meteor radar observations over low and equatorial latitudes, and their response to sudden stratospheric warming (SSW) events is discussed. Meteor radar-measured hourly zonal and meridional winds in the 80-100 km region over Thumba (8.5 degrees N, 76.9 degrees E) and Kototabang (0.2 degrees S, 100.3 degrees E) have been used to extract the LSD amplitudes during the years 2006-2015 and 2002-2012, respectively. The similarities/discrepancies in the climatology of LSD over low and equatorial latitudes as compared to Vial and Forbes lunar tidal model outputs are discussed. The present results obtained using four individual cases over Thumba and Kototabang indicate that the amplitudes of LSD show pronounced enhancement during polar vortex weakening (PVW) events rather than the conventional SSW events. Case studies presented in this study are characterized by simultaneous occurrence of enhanced LSD amplitudes and reduced solar semidiurnal (SSD) amplitudes, together with the presence of a quasi 16 day (Q16D) wave. Thus, present results corroborate the existence of nonlinear interaction between the Q16D wave and the SSD in the equatorial latitude and low-latitude mesosphere lower thermosphere (MLT) region during PVW/SSW events and their potential contamination to LSD amplitudes. Further, using ground-based magnetometer measurements from the dip equatorial site Tirunelveli (8.7 degrees N, 77 degrees E, 0.23 degrees dip angle), LSD amplitudes in EEJ intensities are also estimated. An excellent association is found between the temporal evolution of LSD amplitudes in zonal winds at MLT region and in the intensity of EEJ, providing a paradigmatic example of MLT-ionosphere coupling. Plain Language Summary Periodic heating of the Earth's atmosphere by solar radiation and the gravitational attraction by the Sun and the Moon generate tides in the atmosphere, termed as thermal and gravitational tides, respectively. Gravitational tides in the atmosphere are primarily driven by the Moon, as it is closer to the Earth. During extreme polar stratospheric events such as sudden stratospheric warming (SSW), tides in the Earth's middle atmosphere are found to show large variabilities. These variabilities are often observed to affect the mesosphere-thermosphere-ionosphere region significantly. In this study, variability of lunar semidiurnal tides in the mesosphere lower thermosphere region over equatorial and low latitudes during SSW events is discussed in detail. The results suggest that daughter waves resulting from the interaction of the 12 hr solar tide with a planetary scale wave having a period of around 16 days can possibly be misinterpreted as an enhancement of the lunar semidiurnal tide, as reported by several studies in the past especially over the equatorial regions. The analysis also shows that lunar semidiurnal tides in ionospheric currents, called the equatorial electrojet, are very well correlated with those in winds present in the atmosphere below.

This study investigates the role of stratospheric quasi-biennial oscillation (QBO) in modulating the response of equatorial/low-latitude ionosphere over the Indian sector to the major sudden stratospheric warmings (SSWs) occurred during the period 2003 to 2013. The analysis based on the equatorial electrojet-induced surface magnetic field, total electron content, and mesospheric wind data reveals that the equatorial ionosphere responds in distinctly different ways to the SSWs occurred during different QBO phases. The peaking time of the equatorial electrojet and occurrence time of counter electrojet displays a shift toward morning/evening sector during the westward/eastward phase of the QBO as inferred from the zonal mean zonal wind at 10-hPa level (similar to 30 km). The analysis reveals that the enhancement and depletions seen in total electron content over both the equatorial and low-latitude ionosphere display a shift toward morning/evening during the westward/eastward phase of the QBO. The upper mesospheric tides (diurnal and semidiurnal) estimated using the meteor radar measured winds over the dip equator also exhibit similar shift in their phases. The occurrence of periodic counter electrojets during the SSW period is found to be coinciding with enhancement in polar stratospheric temperature and in close association with the increased amplitude of the semidiurnal tide. These observations clearly vindicate that the phase of the QBO plays a crucial role in structuring the equatorial electrodynamics and electron density distribution over the low-latitudes during the SSW events.

The L-band scintillations are one of the important manifestations of plasma density irregularities in the post sunset equatorial F-region, commonly known as Equatorial Spread-F (ESF). It has gained its importance mainly because of its effect on the satellite to ground communication links. The occurrence of ESF depends primarily on Rayleigh–Taylor instability mechanism aided by seed perturbations and a verity of other favorable ionospheric background conditions. After sunset, ionospheric F-region will be raised to higher altitudes due to pre-reversal enhancement and the ESF is found to occur any time after the layer reaches sufficient heights of low collision frequency. This means that high altitudes are favorable for the occurrence of plasma irregularities. However, observations show that scintillations can occur even if the base height of ionosphere is as low as ~225 km, if the plasma density scale heights are favorable. Perhaps, strong scintillations are present at all heights from 225-300 km, for plasma density scale heights ranging from 20-30 km, in the current period of analysis. The present study investigates the reason behind the presence of such strong scintillations even at altitudes as low as ~225 km. The data presented in this study is from a dip equatorial station Trivandrum (8.5 N, 77 E, 0.5 dip lat), India. Since the GPS satellites are moving, scintillation data is taken from Geostationary Satellites situated at 55° E and 83° E longitudes respectively. The data from equinoxial months of 2012 and 2013 are used for the current analysis. The base height (h’F), critical frequency of the F layer (foF2) and ionospheric scale heights (L) are obtained from the Digisonde at Trivandrum. Our analysis shows that, strong scintillations are present just before midnight. This is partly due to modification of ionospheric scale height and due to the presence of higher electron densities during this period. The mechanism behind this increased electron densities are discussed in this study. It can be shown that this high electron density is not from any other source, but due to the compression of ionosphere as the layer descends in altitude after the pre-reversal enhancement. Further, the increase in critical frequency of Fregion is not seen in GPS Total electron content (TEC) measurements, which corroborates with the argument. It can be also shown from the data that, the change in base height of Fregion is proportional to the increase in foF2. The study is important, as it give some information on how modification of background conditions can lead to strong ionospheric scintillations.

Understanding the coupling of ionosphere-thermosphere (IT) system from the lower atmospheric forcing is one of the primary challenges for the space weather community. The present paper deals with the role of two lower atmospheric processes over the Indian region ionosphere i.e., stratospheric Quasi-Biennial Oscillation (QBO) and Sudden Stratospheric Warming (SSW). The role of QBO in modulating the response of equatorial/low latitude ionosphere over Indian sector to the major SSW events of 2009 and 2013 has been investigated by using combined measurements from meteor wind radar operating over Trivandrum, Global Positioning System (GPS) derived total electron content (TEC), and magnetic field data. The time variation of Equatorial Electrojet (EEJ)-induced surface magnetic field show that the response of EEJ is distinctly different during different phases of the QBO. The peaking time of EEJ and occurrence time of counter electrojet (CEJ) were found to be shifted towards morning/evening sector during the westward/eastward phase of the QBO during the SSW years. The TEC over both the equatorial and low-latitude ionosphere exhibit a similar feature. The tidal components derived from horizontal winds using a meteor wind radar revealed similar shift in their peaking time. These observations clearly vindicate that the phase of QBO plays a crucial role in structuring the equatorial electrodynamics and electron density distribution over low-latitudes during the SSW evens. These results are unique and achieves significance as we are heading towards solar minimum period where forcing from the lower atmosphere is an import aspect of ionospheric variability.

This study examines the response of the nighttime thermosphere‐ionosphere system (TIS) over a dip equatorial station, Trivandrum (8.5°N, 77°E, 0.5° dip latitude) to prompt penetration electric field (PPEF) events occurred on 5 January 2016 and 6 March 2016. The investigation is based on nightglow emission measurements at wavelengths OI 777.4 nm, OI 630.0 nm, and OI 557.7 nm using a multiwavelength nighttime photometer. These emissions emanate from different altitude regions of the TIS, that is, 557.7 nm from ~100 km, 630.0 nm from ~220 km, and 777.4 nm from ~350 km. It has been observed that, during the westward PPEF event, the intensities of 777.4‐ and 630.0‐nm nightglow emissions enhance, whereas during the eastward PPEF event the intensities decrease. Similarly, though it is not very prominent, 557.7‐nm emission also exhibits a small enhancement/decrease during the westward/eastward PPEF events. The ionospheric base height obtained from a collocated digital ionosonde shows that during the eastward (westward) PPEF event the ionospheric layer moves upward (downward). The downward (upward) layer movement increases (decreases) the plasma density in the centroid of these airglow emissions, which in turn enhances (decreases) the emission rates. The study demonstrates the coupling between interplanetary medium and neutral TIS during nighttime PPEF events.

This paper presents a case study exploring the variability of daytime ionospheric sporadic E (Es) during Counter Equatorial Electrojet (CEJ) events over Trivandrum (8.5°N, 77°E, 0.5°N dip lat.), a dip equatorial station in India. It makes use of collocated measurements of (a) daytime mesopause temperature estimated using a Multiwavelength Dayglow Photometer, (b) base height of sporadic E-layer i.e. Esq obtained from a Digital Ionosonde, (c) Equatorial Electrojet (EEJ) induced magnetic field at the surface measured using a Proton Precession Magnetometer and (d) neutral meridional and zonal winds measured using meteor wind radar. The study addresses why, only on certain CEJ days, the Es layer turns into a blanketing Es layer, also referred to as Esb layer, appearing for a brief period and almost simultaneously as the CEJ induced magnetic field on the surface is either zero or very small, and not on all the CEJ days despite the reversal of the zonal winds in the dynamo region (∼98 km). Further, a simulation using the simultaneously measured zonal wind and temperature shows that during CEJ, especially when the zonal wind reverses and the mesopause temperature lowers, the conditions are in fact less favourable for the ionization to converge resulting in Esb. On the other hand, on such occasions, the meridional wind exhibits a tendency to turn equatorward with time with the changeover gradually happening as the altitude decreases. These observations have been discussed to allude to the possible role of the overall wind system and its spatio-temporal changes in the ionospheric altitudes which could influence the gradual transition of Esq to Esb over the dip equator.

Using the equatorial electrojet (EEJ)‐induced surface magnetic field and total electron content (TEC) measurements, we investigated the impact of the sudden stratospheric warming (SSW) of January 2009 on the equatorial electrodynamics and low‐latitude ionosphere over the Indian longitudes. Results indicate that the intensity of EEJ and the TEC over low latitudes (extending up to 30°N) exhibit significant perturbations during and after the SSW peak. One of the interesting features is the deviation of EEJ and TEC from the normal quiet time behavior well before the onset of the SSW. This is found to coincide with the beginning of enhanced planetary wave (PW) activity over high latitudes. The substantial amplification of the semidiurnal perturbation after the SSW peak is seen to be coinciding with the onset of new and full moons. The response of TEC to SSW is found to be latitude dependent as the near‐equatorial (NE) stations show the semidiurnal perturbation only after the SSW peak. Another notable feature is the presence of reduced ionization in the night sector over the NE and low‐latitude regions, appearing as an “ionization hole,” well after the SSW peak. The investigation revealed the existence of a quasi 16 day wave in the TEC over low latitudes similar to the one present in the EEJ strength. These results have been discussed in the light of changes in the dynamical background because of enhanced PW activity during SSW, which creates favorable conditions for the amplification of lunar tides, and their subsequent interaction with the lower thermospheric tidal fields.

This study presents the influence of stratospheric sudden warming (SSW) events in modulating the start time of the equatorial spread‐ F (ESF) through enhanced planetary wave (PW) activity during the winter months of the SSW years. The analysis based on the data from a digital ionosonde and proton precession magnetometer over Trivandrum (8.5°N, 77°E, 0.5°N dip lat.) revealed that the PWs of quasi‐16 day periodicity influence the start time of the ESF to a significant extent during the SSW years. On the other hand, during a normal year such effect is not very evidently present. It has been observed that the quasi‐16 day wave propagates to ionospheric dynamo region from the atmosphere below and modifies the electrodynamical processes like the equatorial electrojet and prereversal enhancement, which is more pronounced during both the SSW periods. Such a modification in the electrodynamics can modulate the equatorial plasma fountain and influence the F region neutral dynamics, which in turn can affect the occurrence of ESF by modifying the seeding conditions.