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Electron density and Neutral Wind (ENWi) probe and/or Langmuir probe were flown on three different rocket flights from equatorial station Thumba (8.5°N, 77°E), during daytime, eclipse time, and post sunset time respectively, of low solar activity period, to study ionization irregularities in the E region of the ionosphere. The spectral characteristics observed using the payloads were analyzed during the three different rocket flights. The 15 January 2010 solar eclipse event and “SOUREX 1” wind and electron density measurements confirm the validity of the wind shear theory for equatorial regions during the eclipse and post sunset periods, when electrojet is weak, using wind and electron density irregularity measurements from the same instrument (ENWi) for the first time. The spectral indices in the range of −1.2 to −1.78 and the strong wind shear due to gravity wave winds during the eclipse time as well as the post sunset time, confirm the role of neutral turbulence in the generation of irregularities in the 95–112 km altitude region. The spectral index of −2.0 to −2.7 and the presence of irregularities in the negative gradient regions establishes the role of wind‐driven gradient drift instability (GDI) in generating irregularities during the SOUREX 1 post sunset flight for altitudes below 93 km. Further, the role of gradient drift instability in triggering ionization irregularities during the daytime on 14 January 2010 is unraveled by the spectral index of −2.03 and the presence of irregularities in the positive gradient regions alone. The spectral characteristics of ionospheric plasma irregularities were studied using rocket‐borne in situ measurements of electron density and neutral winds. The Electron density and Neutral Wind probe and/or Langmuir probe were flown during different geophysical conditions from equatorial station, Thumba. The validity of the wind shear theory for equatorial region and the role of neutral turbulence in the generation of irregularities were verified by the electron density and neutral wind measurements during daytime for the 15 January 2010 solar eclipse event and post sunset time for the “SOUREX 1” experiment. The presence of irregularities in the positive electron density gradient regions alone and the spectral index value of −2.03 together confirm the role of gradient drift instability in the generation of daytime E region irregularities. The irregularities generated through wind‐driven GDI during post sunset time is characterized by the spectral index of −2.0 to −2.7 and the irregularities were present in the negative electron density gradient regions. In situ measurements of electron density and neutral wind as well as spectral analysis of irregularitiesRole of neutral turbulence in irregularity generation above 95 km during eclipse time and post sunset time unraveledRole of wind‐driven gradient drift instability in post sunset irregularity generation below 93 km is deduced In situ measurements of electron density and neutral wind as well as spectral analysis of irregularities Role of neutral turbulence in irregularity generation above 95 km during eclipse time and post sunset time unraveled Role of wind‐driven gradient drift instability in post sunset irregularity generation below 93 km is deduced

The model‐generated spatiotemporal maps to forecast the occurrence pattern of plasma density irregularities in the nightside equatorial Fregion that are responsible for the L‐band scintillations have been put to test, in both space and time, by comparing it with actual observations by the Communication/Navigation Outage Forecasting System satellite. The forecast model is based on (i) the temporal variations of the density perturbations during daytime in the Nmaxregion and (ii) the a priori knowledge of zonal velocity of the perturbations in the postsunset hours. The present study not only substantiates the hypothesis used for the generation of the scintillation forecast but also suggests that the equatorial plasma bubbles remain tied‐up with the initial perturbations which trigger the primary Rayleigh‐Taylor instability. The outcome highlights the need to take into account the altitudinal profile of the topside Fregion electron density as it could modify the zonal extent of the plasma bubbles that support the generation of the density irregularities and the consequent L‐band scintillations. The present study takes us one more step closer toward the realization of an operational forecast system for satellite‐based navigation. Generation of spatiotemporal maps to forecast the occurrence pattern of ionospheric irregularitiesForecast model has been compared with the in situ measurements by the C/NOFSZonal extent of a plasma bubble as it evolves plays a crucial role in affecting the forecasting model

Before the onset of a geomagnetic storm on 22 January 2012 (Ap= 24), an enhancement in solar wind number density from 10/cm3to 22/cm3during 0440–0510 UT under northward interplanetary magnetic field (IMF Bz) condition is shown to have enhanced the high‐latitude ionospheric convection and also caused variations in the geomagnetic field globally. Conspicuous changes in ΔXare observed not only at longitudinally separated low‐latitude stations over Indian (prenoon), South American (midnight), Japanese (afternoon), Pacific (afternoon) and African (morning) sectors but also at latitudinally separated stations located over high and middle latitudes. The latitudinal variation of the amplitude of the ΔXduring 0440–0510 UT is shown to be consistent with the characteristics of prompt penetration electric field disturbances. Most importantly, the density pulse event caused enhancements in the equatorial electrojet strength and the peak height of the Flayer (hmF2) over the Indian dip equatorial sector. Further, the concomitant enhancements in electrojet current and Flayer movement over the dip equator observed during this space weather event suggest a common driver of prompt electric field disturbance at this time. Such simultaneous variations are found to be absent during magnetically quiet days. In absence of significant change in solar wind velocity and magnetospheric substorm activity, these observations point toward perceptible prompt electric field disturbance over the dip equator driven by the overcompression of the magnetosphere by solar wind density enhancement. Prompt penetration electric field driven by solar wind densityEquatorial electric field disturbance under northward IMF BzSignatures in global magnetic data and geosynchronous count rate

Measurements of electron density/irregularities and ion drift velocity/neutral wind were undertaken with Electron Density and Neutral Wind (ENWi) payload on‐board an RH300 flight on 6 April 2018 as part of the Sounding Rocket Experiment (SOUREX1). The presence of a clearly structured ENWi measured electron density profile is an interesting feature observed on this day. Concurrently electron density/irregularities are measured by a Langmuir probe payload which confirmed the presence of stratifications in electron density. The neutral wind measurements by ENWi also reveal a structured pattern. The hodograph analysis of the neutral wind components reveals clockwise rotation indicating downward phase propagation characteristics associated with the gravity waves. Analysis of the requisite ion convergence at the magnetic equator for occurrence of electron density stratifications using the observed electron density structures during the post sunset hours in relation to the ion convergence due to gravity wave winds is performed according to earlier theoretical simulations. This analysis reveals that conditions favoring the formation of stratification related ion convergence are present therein. Thus the present study for first time uses actual measurements of neutral winds and electron densities from same instrument to demonstrate the role of gravity wave induced winds in producing ion convergence at the magnetic equatorial region Trivandrum in the post sunset hours. Electron density and neutral wind (ENWi) rocket payload based in situ measurements were under taken at the dip equatorial Eregion on 6 April 2018. The ENWi profiles observed during this experiment were remarkably structured. The wavelet analysis of the electron density fluctuations and hodograph analysis of the neutral wind components reveal the gravity wave origin of the observed fluctuations. The assessed gravity wave induced convergence rate based on the in situ measurements is considerably greater than the calculated requisite convergence rate for the formation of electron density stratifications, thus confirming earlier theoretical simulations. Rocket payload based in‐situ measurements of electron density and neutral wind at the dip equatorial Eregion on 6 April 2018Presence of gravity waves with 2–3 km vertical wavelengthAnalysis on the requisite ion convergence at the magnetic equator for the occurrence of post sunset electron density stratifications Rocket payload based in‐situ measurements of electron density and neutral wind at the dip equatorial Eregion on 6 April 2018 Presence of gravity waves with 2–3 km vertical wavelength Analysis on the requisite ion convergence at the magnetic equator for the occurrence of post sunset electron density stratifications

The earlier evolved method for the forecast of the spatiotemporal variation of L band scintillation based on the expected variation of the perturbations, under favorable ionospheric/thermospheric conditions, has been refined by duly accounting for the local time variation of the zonal velocity of the perturbations. The unique combination of the two geostationary satellites (GSAT‐8 and GSAT‐10) over the Indian zone has been used to estimate the typical local time dependence of the perturbation velocities by closely following identifiable features in the scintillation pattern. The measured velocities, that registered a steady decrease with the progression of night, had been shown to significantly alter the forecast pattern of the scintillations with respect to longitude and local time. The significant improvement in the forecast pattern has been demonstrated through a case study putting the forecast method on a firmer footing. Perturbation zonal velocityImpact of varying zonal velocity of the perturbationsL band scintillation forecast

Coordinated digisonde and OI 630.0 nm airglow observations from Thumba (TVM), an Indian dip equatorial station, in conjunction with magnetic and geosynchronous particle flux measurements, reveal three different types of electric field disturbances in the equatorial ionosphere‐thermosphere system (ITS) occurring in succession over a period of 6 h on a single night (22–23 January,2012; Ap= 24). These include (1) westward electric field perturbations owing to a pseudo‐breakup and a substorm event, each lasting for about 30 min; (2) eastward electric field perturbations continuing for about an hour, owing to the southward excursion of Zcomponent of interplanetary magnetic field (Bz); and (3) DP2‐type fluctuating (period ∼40 min) electric field perturbation sustaining for about 4 h. The pseudo‐breakup and the fully grown substorm events are found to be longitudinally localized and different in terms of response in the westward auroral electrojet index (AL) as well as geosynchronous electron/proton injections. The polarity of the prompt penetration of interplanetary electric field that affects the equatorial ionosphere is observed to be eastward during 2100–2200 IST (Indian Standard Time) which is observationally sparse but consistent with modeling studies. Interestingly, on the same night, DP2‐type electric field fluctuations with ∼40 min periodicity and occasional eastward polarity (akin to daytime) are also found to affect the equatorial ITS for about 4 h (2200–0200 IST). The case study, thus, brings out different processes that constitute a long duration prompt penetration event which, otherwise, would have been categorized as a single event. Different types of electric field disturbances on a given nightThese include substorm, prompt penetration, and DP2‐type eventsImpact on equatorial vertical drift and 630.0 nm airglow intensity

The present work pertains to the eclipse‐induced gravity wave modulations in the ionosphere‐thermosphere region over Trivandrum (8.5°N, 77°E, dip 2°N) during the annular solar eclipse of 15 January 2010. Electron density and neutral wind rocket payload measured horizontal winds and electron densities at Eregion altitudes, and ionosonde‐derived foF1and foF2parameters are used to analyze the characteristics of the eclipse‐induced gravity waves. The analysis reveals an intensification of gravity waves with periods around 30–100 min during the peak phase of the eclipse. The vertical wavelength of the prevalent wave is found to be around 2 km. The role of gravity wave‐induced winds in generating blanketing Esover the equator is also examined. Eclipse induced gravity wave modulations in ionosphere‐thermosphere regionIntensification of 30‐100 min gravity waves of vertical wavelength of ~2 kmRole of gravity wave induced winds in generating blanketing Es is examined

We report the first observation of Argon‐40 (Ar‐40) in the mid latitude regions (−60°to +60°) of the lunar exosphere from CHandra’s Atmospheric Composition Explorer‐2 (CHACE‐2) experiment aboard Chandrayaan‐2 orbiter. The number density of Ar‐40 shows pre‐sunrise, sunrise and sunset peaks as well as nightside minima, typical of a condensable gas, which is similar to the features seen at the low latitudes in previous observations. The CHACE‐2 observed number densities of Ar‐40 and its diurnal variation at low latitudes (−30°to +30°) is consistent with LACE/Apollo observations. CHACE‐2 observations show Ar‐40 enhancements over certain longitude sectors. In addition to KREEP region, Ar‐40 bulges are observed at other longitudes, including the South Pole Aitken (SPA) terrain. The global distribution of Ar‐40 shows that the sunrise peak is observed at the same local time over highlands and mare regions. These observations call for a deeper understanding of the surface‐exosphere interactions and source distribution. The Moon is known to possess a tenuous atmosphere, known as surface bound exosphere. Lunar exosphere exists as a result of a dynamic equilibrium between several sources and sink processes. Noble gases serves as important tracers to understand such processes. Though, Argon‐40 (Ar‐40) is known to exist in lunar exosphere, the knowledge on its distribution at higher latitudes is lacking. For the first time, CHandra’s Atmospheric Composition Explorer‐2 (CHACE‐2) experiment aboard Chandrayaan‐2 orbiter has continuously observed Ar‐40 in latitude range of −60°to +60°. It is found that the Ar‐40 density variation with local solar time shows the behavior of a condensable gas, which is similar to that observed earlier at low latitudes. The distribution of Ar‐40 shows spatial heterogeneity with localized enhancements over KREEP and South Pole Aitken terrain. This suggests that there may be other regions with lower activation energy as the source of Ar‐40. The observed global distribution indicates that the interaction of Ar‐40 with the surface are similar in low and mid latitude regions. The CHACE‐2 observations hint at requirement for improvement in our understanding of the surface‐exosphere interactions and source distributions of Ar‐40. First observation of Argon‐40 in the mid latitude exosphere of the MoonObserved nightside minimum and sunrise and sunset peaks in Ar‐40 abundance is similar to that at low latitudesEnhanced Ar‐40 number density is observed at few longitudes, including South Pole Aitken terrain, in addition to KREEP First observation of Argon‐40 in the mid latitude exosphere of the Moon Observed nightside minimum and sunrise and sunset peaks in Ar‐40 abundance is similar to that at low latitudes Enhanced Ar‐40 number density is observed at few longitudes, including South Pole Aitken terrain, in addition to KREEP

Airglow emissions serve as good tracers of the altitudinal regions at which they originate. OI 630 nm (red line) nocturnal emissions originate from around 250 km altitude. A total of 142 nights of data corresponding to the months of January, February, and March in the years 2013, 2014, and 2016, obtained from Mt. Abu (24.6°N, 72.7°E, 16°N Mag), Gurushikhar, India, a low‐latitude location, are investigated. These are compared with the column integrated emission rates calculated using, as inputs, the measured electron density profiles obtained from a digisonde from Ahmedabad (AMD, 23.0°N, 72.6°E, 15°N Mag), India. Following the expected monotonic decrement in the emissions after sunset, an enhancement is observed on several nights that peaks at around 20–21 local time (LT). The cause for this enhancement has been investigated in detail and it is found that the neutral winds, as obtained using digisondes at two locations, show almost a very good correlation between a poleward directed wind or cessation of equatorward wind over AMD and the observed airglow emission enhancement in the post‐sunset time. Further, the percentage enhancement in emissions also shows a decrease in magnitudes from January to March which has a broad similarity to the decrease in the model climatological meridional wind magnitudes in the same duration. Based on the data spanning over different years, it is inferred that, during geomagnetic quiet periods, almost all of the nocturnal variability in OI 630 nm emissions is due to the variations in the neutral wind. Some atoms in the Earth’s upper atmosphere emit light due to internal processes known as airglow. The processes prevalent at those altitudes can be understood by remotely observing the modulations/variations of this airglow brightness. One such emission at 630 nm, known as redline airglow showed large enhancements in post‐sunset time over a low‐latitude location. While such redline emission enhancements can, on occasions, possibly occur due to a process at magnetic equator, known as pre‐reversal enhancement in electric field, systematic measurements carried out in this study over a three year duration in the months of January to March revealed that the meridional wind at thermospheric altitudes is the dominant cause of such enhancements. This conclusion has been made by measuring the relative variations in winds in the post‐sunset times that vary from night‐to‐night and the climatological model output that shows a broad month‐to‐month variation. We use the fact that poleward winds bring in electrons, one of the ingredients that produces redline emission, to low‐latitudes thereby causing these emission enhancements. Enhancement in OI 630 nm red line emissions observed during the post‐sunset time from a low‐latitude locationColumn integrated volume emission rate and relative meridional wind variations have been calculated using digisondes at two locationsPoleward meridional winds play an important role in causing the observed enhancements in the post‐sunset OI 630 nm emissions Enhancement in OI 630 nm red line emissions observed during the post‐sunset time from a low‐latitude location Column integrated volume emission rate and relative meridional wind variations have been calculated using digisondes at two locations Poleward meridional winds play an important role in causing the observed enhancements in the post‐sunset OI 630 nm emissions

This paper focuses on the linkage between reverse fountain effect and anomalous enhancement in the nighttime electron density over the equatorial ionosphere often referred as the ionospheric nighttime enhancement (INE). The Global Positioning System (GPS)‐derived total electron content (TEC) measurements over two Indian equatorial stations and ionosonde measurements are used to investigate the occurrence characteristics and mechanism of the premidnight and postmidnight INE events. The high‐resolution spatiotemporal two‐dimensional (2D) TEC maps generated by using satellite‐based augmentation system are used to visualize the reverse fountain effect. These TEC maps are found to be effective in envisaging the motion of equatorial ionization anomaly (EIA) in nighttime ionosphere and its impact on the distribution of electron density over the equatorial region. Salient features of this case study illustrate that, accompanying enhancements in the nighttime TEC, there exist decrease in the F2 layer height, prevalence of downward vertical plasma drift, and dramatic increase in the F2 region electron density. The 2D‐TEC maps reveal the persistence of well‐developed EIA until late evening hours and its gradual shift toward equator during the events of postmidnight INEs. It is found out that the premidnight INE is not invariably linked to the prereversal enhancement of the zonal electric field and could be associated with the redistribution of electron density caused by the reverse fountain effect. The present observations indicate that although the westward electric field serves as a prime driver, the INEs over the equatorial regions are associated with plasma motions caused by the retreating anomaly. We investigated the occurrence characteristics and mechanism of ionospheric nighttime enhancement over the equatorial regionSBAS‐generated 2D‐TEC maps are used to visualize the reverse fountain effect over the Indian regionIonospheric nighttime enhancement over the equatorial regions are associated with the plasma motions caused by the retreating anomaly

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. The patterns of EEJ‐induced field and TEC were found to be disrupted significantly before, during, and after the peak SSWSubstantial influence of lunar tides was observed during the SSW eventAnomalous depletion in nighttime TEC was observed over low latitudes after the SSW peak