The kinematic precise point positioning (KPPP) is one of the best techniques aiming for zero-difference precise positioning. Recent studies have shown a growing use of KPPP in several kinematic platform-based applications for precision approach. This study therefore aims to explore the impact of the ionospheric plasma density perturbations on the KPPP, which bear significant effect over the global equatorial and low latitude belt. With a unique data set of simultaneous observations from a reference station and a close-by scintillation monitoring station located within Hyderabad in India, a study of impact of post-midnight scintillations on KPPP is performed. A 4-year duration around peak of the solar cycle 24 between 2013 and 2016 is considered for KPPP analysis by GipsyX software using continuous observations. The daily ENU (east, north, up) residuals are obtained by subtracting the monthly mean precise position from the daily 30-s sampled KPPP estimates. Then the nighttime ENU residuals (between 18:00 and 06:00 Indian standard time, IST) are segregated into pre-midnight (18:00 to 00:00 IST) and post-midnight (00:00 to 06:00 IST) intervals for all the nights. Owing to a different nature of post-midnight irregularities over the equatorial and low latitudes, the results are obtained for post-midnight KPPP. The amplitude scintillation index S4 is obtained for the corresponding nights for all the days and simultaneous records are used to obtain a few case studies of very high ENU residuals reaching up to 20 m in some cases. Also, the statistical analysis of the ENU shows a decreasing trend in the amplitude of ENU perturbations from midnight to morning hours, wherein, maximum perturbations are observed from 00:00 to 01:00 local time. Conclusions are presented in comparison with the previous literature, and it is found that this could possibly be the first report of the post-midnight impact of the ionospheric scintillation on the KPPP over Indian region.

Utilizing the advantage of the almost stable NavIC (Navigation with Indian constellation) satellite links over the Indian region, this study, for the first time, reports latitudinal and seasonal variations in the NavIC total electron content (TEC) across the Indian region during the lowest solar activity period of the year 2020. Characteristics of the observed TEC are notably obtained from a NavIC satellite (stationed at 83°E longitude) using the dual frequency transmission in the S-band (2492.028 MHz) and L5-band (1176.45 MHz). Significantly, the measurements are collected from the NavIC receivers placed at six sites located at different latitudes along a longitude of 77°E thus covering the latitudinal Indian region from geodetic latitude 8°N up to 32°N (geomagnetic latitude 0°N up to 24°N). Major findings from this study show a strong winter and equinoctial anomaly, which are found to switch their character when the stations are either dip-equatorward or poleward from the crest location of the equatorial ionization anomaly (EIA). Another significant result elucidates the night-time VTEC enhancement as well as large night-to-night background variability, which is mainly observed during the summer months of 2020. Further, this study cross-examines the morphology of the ionosphere over the Indian region using a stable link from the NavIC satellite and presents a lucid comparison of results with the Vertical TEC (VTEC) estimated using the standard International Reference Ionosphere (IRI)-2016 model. Most of the morphological characters of VTEC variations are found in conformity with previous studies, with a few significant latitudinal changes noted above. However, even during the lowest solar activity, large differences are found between the observed and IRI-model VTEC values, which show an unusually strong seasonal and latitudinal dependence and stress for further needful investigation, both in the modelling and the observation domains. With the advent of India’s own satellite navigation system, known as NavIC, a new opportunity for ionospheric observations with a very stable satellite link is created. This study, for the first time, gives a detailed account of continuous and highly accurate TEC (total electron content) observations from the high-elevation link using a ground chain of 6 NavIC receiver stations along the longitude of 77°E. A comprehensive analysis focused on resolving different patterns in TEC variability has revealed that the present understanding of the equinoctial asymmetry, winter anomaly and the summer night-time TEC enhancements compared to the nocturnal background TEC of other seasons falls short to explain newly found features. The contrasting latitudinal changes between the dip-equator and the crest of the anomaly versus those beyond the crest latitudes in the context of anomalous variations are discussed and explained to the extent possible in light of the existing literature on these features.

The equatorial post-sunset ionospheric irregularities induce rapid fluctuations in the phase and amplitude of global navigation satellite system (GNSS) signals which may lead to the loss of lock and can potentially degrade the position accuracy. This study presents a new analysis of L-band scintillation from a low latitude station at Guntur (Geographic 16.44°N, 80.62°E, dip 22.18°), India, for the period of 18 months from August 2021 to January 2023. The observations are categorized either in the medium Earth-orbiting (MEO) or geosynchronous orbiting (GSO) satellites (GSO is considered as a set of the geostationary and inclined geosynchronous satellites) for L1, L2, and L5 signals. The results show a higher occurrence of moderate (0.5 < S4 ≤ 0.8) and strong (S4 > 0.8) scintillations on different signals from the MEO compared to the GSO satellites. Statistically, the average of peak S4 values provides a higher confidence in the severity of scintillations on a given night, which is found to be in-line with the scintillation occurrences. The percentage occurrence of scintillation-affected satellites is found to be higher on L1 compared to other signals, wherein a contrasting higher percentage of affected satellites over GSO than MEO is observed. While a clear demarcation between the L2/L5 signals and L1 is found over the MEO, in the case of GSO, the CCDF over L5 is found to match mostly with the L1 signal. This could possibly originate from the space diversity gain effect known to impact the closely spaced geostationary satellite links. Another major difference of higher slopes and less scatter of S4 values corresponding to L1 versus L2/L5 from the GSO satellite is found compared to mostly non-linear highly scattered relations from the MEO. The distribution of the percentage of scintillation-affected satellites on L1 shows a close match between MEO and GSO in a total number of minutes up to ~60%. However, such a number of minutes corresponding to higher than 60% is found to be larger for GSO. Thus, the results indicate the possibility of homogeneous spatial patterns in a scintillation distribution over a low latitude site, which could originate from the closely spaced GSO links and highlight the role of the number of available satellites with the geometry of the links, being the deciding factors. This helps the ionospheric community to develop inter-GNSS (MEO and GSO) operability models for achieving highly accurate positioning solutions during adverse ionospheric weather conditions.

Some of the extreme space weather events like the geomagnetic storm of 17–18 March 2015 have produced dramatic effects on low-latitude ionosphere as reported in various studies. Similarly, some of the extreme storms are studied to emphasize the anomalous deviations in ionospheric delay; however, the effects of excess ionospheric delay on GPS-based positioning are rarely reported from India. This paper presents a robust analysis including estimation of GPS receiver position during 6 major geomagnetic storms using dual frequency GPS signals in L1 and L2 bands. Epoch-wise solution estimates are computed using precise orbit SP3 files for satellite orbit and clock corrections and receiver specific differential code biases are obtained from IGS-CODE center. A range domain Kalman filter has also been developed to smooth the 30 s sampled code pseudorange using carrier phase data and resolve the initial ambiguity. Then, the precise positioning residuals are estimated in two different cases of ionospheric corrected and ionospheric uncorrected error models. We then compare the results with ionospheric corrected position estimates to finally obtain the effect of excess daytime ionospheric variations during main phase of some extreme geomagnetic storms. Results from 5 GPS receivers located within 77°–80° E longitude sector are obtained for two 2-hour windows covering local noon during 6 storms including the St. Patrick’s Day storm. The effect of severity of the storms and their impact on static precise positioning are brought out by comparing the results with performance on a quiet day. It is found that the magnitude of error in estimated altitude exhibits maximum deviations due to ionospheric variations during storms, and the dip latitude of station is important in terms of magnitude of ionospheric error in positioning over the equatorial ionization anomaly region.

Tropospheric disturbances like tropical cyclones produce a spectrum of gravity waves which can propagate to higher altitudes and can even perturb the ionosphere -thermosphere system. The observations of a severe cyclonic storm Hudhud during 7-14 October 2014 using vertical total electron content (VTEC) from 11 stations and the brightness temperature from AIRS (atmospheric infrared sounder) imagery are analysed. Main results show ring -arc structures resembling concentric gravity waves (CGWs) in the brightness temperature and widespread, synchronized and episodic perturbations in VTEC during 10, 11 and 12 October. The stratospheric perturbations are found to spread in a range of radial distance between - 730 and 2500 km with horizontal wavelengths between 307 and 541 km. A wavelet analysis of the daytime band-pass (5-60 min) filtered dVTEC perturbations shows enhancements in the spectral power associated with periods of - 30-50 min on different days following the cyclone movement from east to west. The radial phase speed of the dVTEC for different temporal bands is found to vary between 114 and 320 m/s at different intervals during 10-11 October. Additionally, a comparative analysis of the past and present results is performed using the established dispersion relations which shows a compatible range of variability of wave parameters among all the studies including the present results. The results substantiate strong signatures of the Hudhud cyclone at multiple altitudes and depict vertical coupling of the atmosphere-ionosphere system through a theoretical framework. (c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

The third strongest geomagnetic storm of solar cycle 24 occurred from 25 to 31 August 2018 (minimum SYM-H index = –206 nT on 26 August) and was associated with a weak coronal mass ejection (CME) in the low solar activity period. The storm itself has shown some very unusual characteristics in interplanetary space, which unraveled further surprises in its ionospheric and thermospheric responses. This study provides a detailed analysis of the effects of this storm over the vast African-Asian longitude region over both hemispheres. The longitudinal differences in the equatorward boundary of the auroral oval and the ionospheric convection patterns are presented using the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard the Defense Meteorological Satellite Program (DMSP) satellite and SuperDARN observations, respectively. The global ionospheric map (GIM)-based hourly vertical TEC (total electron content) variations show large enhancements and depletions over different regions at different phases of storms. It is found that during the morning hours on 26 August, a large TEC enhancement in the Asian low latitude region is observed, but such an effect is not observed over the African region, establishing a clear longitudinal difference. The space-time structure of the enhanced ion-density observations from Swarm satellites is used to confirm the VTEC differences and their sustenance. Furthermore, a latitudinal variation in the [O/N2] ratio during 25-27 August is analyzed for a UT-dependent response and the north–south asymmetry in the O density during the main and recovery phases of the storm. A competing effect of the prompt penetration electric field and disturbance dynamo is found to dominantly modulate the longitudinal patterns, which created positive/negative ionospheric storms over the vast African-Asian region.

A recent study by Younas et al. (2020, https://doi.org/10.1029/2020ja027981 , referred to hereafter as Y20) has shown incorrect results of the variations in the level‐3 data of [O/N2] ratio obtained from global ultra violet imager on board TIMED satellite. Y20 have shown the variations in [O/N2] ratio during 25–28 August 2018 but they have wrongly mapped the corresponding longitudes of the orbits in east‐west direction. So, the decrements and enhancements in the [O/N2] ratio are misinterpreted by Y20 in combination with their results on TEC (total electron content). Also, they have derived hemispheric asymmetry in [O/N2] ratio, the variations of which are not reproducible in similar shape from the same data set and method. The results of Y20 on wrong mapping of [O/N2] ratio are further repeated by Bolaji et al. (2021, https://doi.org/10.1029/2020ja029068 ). We provide a correct method, validate it and call for the needful corrections in both the papers.

Magnetic reconnection is a very important energy conversion process that plays a vital role in solar wind-magnetosphere coupling. This study presents a unique analysis of the coordinated observations from MMS, Cluster and THEMIS spacecraft during a geomagnetic storm on 31 December 2015. The analyses are aimed to understand the intricacies of the short-scale processes during 4 different short (20 min) encounters and a long duration (8 h) traversal of all the satellites covering the main and the recovery phase of the storm. The main results show that at the reconnection sites in the magnetotail, the FAC density is majorly contributed by the electrons moving anti-parallel to the magnetic field. Significantly, the FACs estimated from the Curlometer method and plasma (or local) method agree well and authenticate the interpretations. The majority of the electron population is found to shift towards the mid (0.2-2 keV) and highenergy (2-30 keV) range during the reconnection, wherein higher (-one order more) ion and electron energy flux is observed under the effect of the intense storm compared to non-storm cases. The Hall electric field is found to be the major contributor to the total electric field during magnetic reconnection whereas, we observe the growing significance of the pressure divergence term during later phases of the storm. Longer duration (8-hour) continuous observations from the unique alignment of the four spacecrafts show a dominance of magnetic (plasma) properties over plasma (magnetic) properties in the magnetotail (magnetopause). The magnetotail (magnetopause) is found to be highly dynamic with higher (lower) levels of the electric and magnetic field, ion and electron temperature, and simultaneous lower (higher) levels of plasma density, energy flux, and FACs. The drastic enhancements of the plasma and field parameters happen in the sun-earth plane and during different crossings at the magnetopause and magnetotail. The simultaneous observations from magnetopause and magnetotail during a moderate geomagnetic storm indicate further needful coordinated investigations on the nature of reconnections on the day and night side and variations during the intense category of storms.(c) 2023 COSPAR. Published by Elsevier B.V. All rights reserved.

Abstract Four supersubstorms of the solar cycle 24 are analyzed to investigate the global variations in the H‐component and geomagnetically induced currents (GICs). The response to the storm sudden commencement (SSC) of the 2012 and 2017 events is observed differently over different latitude bands. Initially, the latitude band of 0°–45° shows a step‐like preliminary positive impulse (PPI), 45°–65° shows a Gaussian‐kind of PPI and 65°–90° shows a preliminary reverse impulse, followed by a main impulse all over. The H‐component variations during the supersubstorm periods show a strong north‐south asymmetry over the high latitudes which is attributed to the seasonal dependence of the growth and decay of ionospheric currents respectively in the summer and winter hemispheres. The co‐latitude band of ∼55°–65° shows a complete reversal of phase (i.e., global positive peak) of the H‐component compared to the maximum depression observed from the close‐by latitude bands and peak depressions in the SYM‐H, SML, and AE indices. The low latitude variations exhibit a dominant local time‐dependent control of the perturbation electric fields over the short and long terms during the geomagnetic events. The D‐component variations reflect complex ionospheric contributions during the SSC and the main phase with more asymmetries and variability. The GIC threat represented by the dB/dt peaks during the supersubstorms shows the highest magnitude (∼900 nT/min) in the latitude band of 60°–75° with a secondary peak over the dip equatorial regions. Further, some scattered and prominent peaks over the mid and high latitudes outside the supersubstorm periods are also observed.

&lt;p&gt;The forecast of the occurrence of scintillation for end-use predictions over the Indian region is a challenging task. In the context of this challenge, the understanding of the day-to-day spatial and temporal variability of the post-sunset equatorial F-region ionospheric irregularities represents a very important problem. Notably, the spatial (zonal) variations in the scintillation occurrence depends upon the day-to-day perturbations in the equatorial vertical ExB drift and the zonal movement of scintillation patches follow the zonal ExB drift patterns. A conceptual framework that combines information from scintillation indices (as derived from the GNSS receivers) with modelled background information (as derived from physics based ionospheric models) is proposed. The idea is to understand the dependence of local morphology on the physical mechanisms responsible for the formation of the equatorial F-region ionospheric irregularities in the range of 9&lt;sup&gt;o &lt;/sup&gt;N to 18&lt;sup&gt;o&lt;/sup&gt; N geographical latitudes and 74&lt;sup&gt;o &lt;/sup&gt;E to 82&lt;sup&gt;o&lt;/sup&gt; E geographic longitudes respectively. The Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM) is chosen as a background model to provide several parameters of significance in this context. A day-wise correlation analysis for the year 2014 (high solar activity year) is performed between all the TIEGCM model outputs and observed S&lt;sub&gt;4&lt;/sub&gt; index variations within the above mentioned pre-defined geographical boundary. The output parameters (e.g., equatorial zonal ExB drift, vertical ExB drift, critical height) show positive correlation with the observed post sunset variations in the S&lt;sub&gt;4&lt;/sub&gt; index. Moreover, the zonal ExB drift is also ingested with observations from the Communication/Navigation Outage Forecast System (C/NOFS) satellite. A method to infer the zonal and vertical ExB drifts from the combination of TIEGCM outputs, C/NOFS in-situ data, and GNSS S&lt;sub&gt;4&lt;/sub&gt; observations is introduced on the basis of a two-dimensional image evaluation approach. This framework establishes a basis for the prediction of spatial ionospheric irregularities over the region of interest.&lt;/p&gt;

The lengthiest GPS observations from a reference station during 2002–2018 and a colocated scintillation monitoring receiver during 2013–2018 at Hyderabad, India, are used to analyze the degradation in kinematic precise point positioning (KPPP) due to ionospheric irregularities. The GipsyX software is employed to estimate the KPPP of the reference station and the residuals for each coordinate are computed against the monthly mean values of the daytime observations. The results establish the long-term variability of the residuals of KPPP under the climatological and event-specific effect of the post-sunset equatorial plasma bubbles (EPBs). The extreme post-sunset fluctuations in the residuals during a few quiet and disturbed nights obtained from the 18 years of continuous observations are emphasized. The outstanding and persistent patterns in the residuals are majorly found during extended post-sunset hours with striking seasonal contrast. The large and rapid fluctuations in the residuals are found to exhibit long-term day-to-day, seasonal, and solar cycle variations, which correspond to the occurrence of the amplitude and phase scintillations. Thus, uniquely, the colocated observations of scintillations corroborate the impact of the simultaneously affected number of satellites (S4 index > 0.17) and indicate the possibility of cycle slips eventually affecting the KPPP solutions. The seasonal climatology of the scintillations is shown to translate over the 3-D position error, which modulates with change in the solar F10.7 cm flux. This study, thus probably for the first time, reports the long-term effect of solar cycle variability on the amplitude of perturbations in KPPP for the solar cycles 23 and 24.

Ionospheric maps from an optimal GNSS (global navigation satellite system) receiver network may lead to better understanding and prediction of the equatorial ionospheric gradients. An unsupervised machine learning framework is proposed using the k-means clustering approach to optimize the available number of GNSS receivers. Beginning with k = 15 receivers, the simulations are performed to obtain distribution of the ionospheric pierce points (IPPs) in different grid resolutions up to k = 1,000. The algorithm uses a reference grid and subsequently optimizes the locations according to the available IPPs in a cluster. The IPP cluster centers are iteratively optimized based on minimizing the "within cluster sum of squares" error. The IPPs from the optimal receiver locations are used to generate a 2-D vertical total electron content (VTEC) map using NeQuick-G model. A one-sigma confidence limit derived from hourly histogram of the IPPs from the k locations is used as decisive criteria for useable grid cells. The number of empty bins thus obtained in a given scenarios provides a percentage normalized empty grid ratio (NEG(ratio)). A NEG(ratio) below 10% is used as a threshold to make a decision over eligibility of a scenario to produce a desired VTEC map. Several scenarios are thus tested for varying temporal and spatial grid resolutions, geographical boundaries, GNSS and NavIC satellite configurations, considering the diurnal and annual variation in year 2019. The framework is found to be successful in designing a GNSS network for any geographical region to obtain high resolution 2-D VTEC maps for space weather applications.

A novel GNU radio-based software-defined radio receiver using universal serial radio peripheral (USRP) front end and Yagi antenna is developed to record meteor echoes in passive mode using FM broadcast. Pilot observations are made for four days in August–September 2020 to record FM signals at 103.2, 104, and 107.5 MHz with a varying sample rate of 8 and 200 kHz. The in-phase and quadrature signals are processed to obtain the amplitude and phase of the received signals for identification of the meteor echoes. A new algorithm is developed to automatically detect the echoes using a normalized amplitude threshold and a minimum temporal separation between two echoes. Overdense meteor echoes with a plateau-like amplitude response are found using a ratio test and remaining echoes are rejected. Thus, the system can detect almost all types of meteor echoes reported earlier. Notably, signatures of fragmentation and background wind are also observed in the form of different phase patterns. Histogram of the meteor counts shows a peak for shorter durations indicating larger populations of the smaller size of meteoroids. The log–log plot of the meteor duration versus count is found to exhibit a characteristic transition of the slope. An empirical estimate of the height of the echoes is obtained. The echoes show micrometeoroid populations, which may depend upon the transmitting–receiving setup and the geometry. Hence, an inexpensive, low-power, portable, passive and automated meteor detection system is demonstrated, which can be highly useful for understanding the meteor phenomena and estimation of atmospheric parameters.

The solar cycle 24 exhibited only four supersubstorms (SSSs) during three of the geomagnetic storms of May 2011, March 2012, and September 2017. A robust and quantitative analyses of these events is performed and the results show that multiple hits by the magnetic clouds with fast moving background solar wind plasma provided the conditions for occurrence of these events. The available solar wind energy, the magnetospheric input energy and the sink energies are estimated. The input energy dominates the total dissipation for shorter intervals of the SSSs, wherein, a dominance of Joule heating sink over the auroral particle precipitation and ring current sinks is revealed. However, for longer time intervals (6 months and more), the total dissipation supersedes the total input energy wherein, the auroral precipitation sink dominates. The contribution of the ring current sink is found at the lowest levels against a high percentage share (79%–91%) of ionospheric sink during the SSSs. Intriguingly, when the SSS expansion and the recovery phases are compared on dissipation terms, percentage share of the Joule heating (ring current) is found more during the later (former) phase. The input efficiencies are found at ∼3.05%, 4.95%, 3.98%, and 1.15%, and the dissipation efficiencies at 73.95%, 58.56%, 37.72%, and 77.94%, respectively during the four SSSs. The dissipation efficiency and the ratio of the sink energies are found to be inversely proportional to the intensity of the substorms. The energy partitioning in the magnetosphere‐ionosphere system also respond in a contrasting fashion to the different levels of substorms.

Loss of lock (LoL) may occur due to receiver dynamics, multipath, and interference apart from the ionospheric irregularities for a given receiver configuration. This study makes a robust analysis of the LoL using multi-frequency GNSS observations during 2014–2017 from a low latitude station at Waltair, India. With possibly the first such results from India, this study shows a strong seasonal, local time, and solar activity dependence of the LoL. While the LoL occurrence is found to increase with the severity of scintillation and maximize for strong scintillations, it is also found that the LoL may occur at any level of S4 due to other dominating factors. It is found that while the weak and moderate S4 events weigh the distribution, they correspond to fewer than 15% of the total LoL. A combination of 0.5 < S4 < 0.7 and 0.4 < $$\sigma_{\varphi }$$ < 0.6 is found to produce LoL for most of the seasons for all the signals. A limitation of the scintillation data is highlighted wherein spurious values of 60-s $$\sigma_{\varphi }$$ (i.e. $$\sigma_{\varphi }$$ > 1.5) are found to constitute more than 53% of total LoL events against the widely accepted $$\sigma_{\varphi }$$ < 1.5. The LoL events are found to increase over latitudes away from the dip equator for most of the signals, whereas they decrease with an increasing elevation angle of IPP, possibly due to multiple refractions over excess path lengths at lower elevation angles and a latitudinal gradient in the electron density. A combination is found composed of S4 > 0.6 and $$\sigma_{\varphi }$$ > 1.5 with lower elevation angles that produce more LoL events between 20:00 and 22:00 local times. New probabilistic forecast models for the occurrence of the LoL are finally given for the different seasons against S4, $$\sigma_{\varphi }$$ and local time that would be useful for different applications.

Abstract The present study provides a multi‐instrument analysis of the ionospheric response to the effects of the St. Patrick's Day storm of 17–18 March 2015. Simultaneous observations from 85 global positioning system stations and multiple satellites along the past observations from the literature survey are utilized to ascertain the spatio‐temporal confinements between −20°E and 150°E longitudes. The main results include longitudinal differences in the episodic equatorward expansions of the auroral oval which is shown to relate with varying high‐latitude Joule heating and equatorward wind surges at different local times. The zonal movement of equatorial ion density depletions from initially east to west followed by a sluggish westward pattern on the night of 17 March is observed. The zonal movement is found to be better explained by the longitudinally decreasing impact of the disturbance dynamo in the nightside. The enhancements in total electron content during morning on 17 March are observed over mid‐latitudes, which are found to move equatorward in the noon time. These enhancements could be associated with the enhanced O density bulge resulting from the thermospheric expansion during the main phase. A daytime sustained and confined electron density depletion between about 100°E and 170°E is observed on 18 March which is being discussed in detail for the first time. Interestingly, large longitudinal differences are observed in the response of the disturbance dynamo, including no observable effect during morning (−20°E–20°E) but dominant afternoon and westward progression during the day of 18 March.