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• The long-term characteristics of Equatorial F-region Irregularities (EFIs) is investigated along the Indian longitude sector. • The dependence of the occurrence of EFIs on seasons and solar activity is highlighted. • Enhancement and/or suppression of EFIs during two prominent geomagnetic storms are probed. • Storm associated enhanced electric fields extended ionospheric irregularities to mid-latitudes. • The analysis outcomes substantiate the spatiotemporal EFI characteristics and its predictions over the region. The occurrence characteristics of Equatorial F-region Irregularities (EFIs) along the Indian longitude sector are investigated using GPS-derived ROTI observations from 2010 to 2023, under both quiet and disturbed geomagnetic conditions. The analysis of the results emphasizes a strong dependence of EFIs occurrence on solar cycle variation. Additionally, the EFIs are more intense at the EIA regions under maximum solar activity periods attributed to the high ambient electron density in the F-layer. The occurrence of EFIs is found to be severe and frequently observed in equinox seasons in contrast to solstice seasons. Also, the occurrence of EFIs exhibits equinoctial asymmetry with more intense EFIs in the spring equinox than autumn equinox during the peak and declining periods of the solar cycle. Conversely, severe EFIs are observed during autumn equinox than spring equinox in the ascending phase of the solar cycle. The seasonal occurrence probability of EFIs is highest in the equinox seasons, following the winter solstice, and summer solstice in high solar activity conditions. Investigations of EFIs during two prominent geomagnetic storm events (17 March 2015 and 08 September 2017) during the study period show that the storm-induced Prompt Penetration Electric Fields (PPEFs) modulated the regular intensity of dusk time Pre-Reversal Enhancement (PRE), resulting in poleward expansion of ionospheric irregularities up to mid-latitude regions with spatial time lag. However, storm-induced westward electric fields and oscillating IMF Bz created Counter Electrojet (CEJ) like conditions, suppressing the dusk time ionospheric irregularities over dip equatorial and low-latitude regions on 17 March 2013. This study provides a comprehensive understanding of how ionospheric irregularities manifest under varying conditions, thereby supporting further research on substantiating the spatiotemporal EFIs characteristics and their predictions over the region. [ABSTRACT FROM AUTHOR]

Deformation in active convergent wedges like the Himalaya occurs over different time and space scales requiring integrated datasets for holistic insights into wedge-scale deformation. A new long-time series (1998–2009) of geodetic, static Global Positioning System (GPS) dataset suggests that active deformation in the Darjiling-Sikkim Himalaya (DSH) is not confined to the Himalayan front as previously believed. Instead, the ∼9 mm/yr N-S active convergence here is divided almost equally within the Lesser Himalayan Duplex (LHD) and the Higher Himalaya due to aseismic reverse slip of ∼18 mm/yr on the Himalayan décollement (Main Himalayan thrust (MHT)) dipping 6° → 006° and locked at ∼16 km depth. Slip transfer along the MHT causes minor to moderate earthquakes at the brittle-ductile transition in the DSH. Measured East velocities show no systematic variation indicating that strike-slip seismicity, though dominant in the region, does not alleviate the décollement (MHT) seismic hazard. Continued slip along the MHT, therefore, will cause a great décollement earthquake in the region. Also, GPS velocities vary in the eastern and western DSH suggesting segmentation of the Himalayan arc along a transverse fault over geological to decadal time scales. Our integrated approach can provide better insight into active deformation in convergent wedges worldwide. [ABSTRACT FROM AUTHOR]

The present work evaluates eleven existing empirical models to estimate Precipitable Water Vapor ( P W V ) over a high-altitude (4500 m amsl), cold-desert environment. These models are tested extensively and used globally to estimate P W V for low altitude sites (below 1000 m amsl). The moist parameters used in the model are: water vapor scale height ( H c ), dew point temperature ( T d ) and water vapor pressure ( E s 0 ). These moist parameters are derived from surface air temperature and relative humidity measured at high temporal resolution from automated weather station. The performance of these models are examined statistically with observed high-resolution GPS ( G P S P W V ) data over the region (2005–2012). The correlation coefficient (R) between the observed G P S P W V and Model P W V is 0.98 at daily data and varies diurnally from 0.93 to 0.97. Parameterization of moisture parameters were studied in-depth (i.e., 2 h to monthly time scales) using G P S P W V , T d , and E s 0 . The slope of the linear relationships between G P S P W V and T d varies from 0.073°C −1 to 0.106°C −1 (R: 0.83 to 0.97) while G P S P W V and E s 0 varied from 1.688 to 2.209 (R: 0.95 to 0.99) at daily, monthly and diurnal time scales. In addition, the moist parameters for the cold desert, high-altitude environment are examined in-depth at various time scales during 2005–2012. [ABSTRACT FROM AUTHOR]

Integrated precipitable water vapor (PWV) retrieved from Moderate Resolution Imaging Spectro-radiometer (MODIS) in near-infrared channels is examined with the PWV estimated from the Global Positioning System (GPS) data and measured surface temperature and pressure values over Hanle, located at a high-altitude (∼4500 m) trans-Himalayan region. Since the accuracy of MODIS derived PWV over the large elevated topography of the region is not well studied, the present work focus on the validation of MODIS PWV using the GPS data during 2005–2012. The study reveals that MODIS PWV compares well with the GPS PWV data with bias −0.018 cm, root mean square error (RMSE) 0.137 cm and coefficient of determination ( R 2 ) 0.91. The two types of data products are compared on short (daily) and long temporal (monthly-seasonal) scales in order to evaluate the seasonal dependence of PWV. The peak values of PWV from MODIS and GPS data at the site varied from 1.7 to 2.05 cm and 1.2 to 1.4 cm as the daily and the monthly means, respectively, which occurred during August. However, the seasonal peak of PWV occurred during summer as 0.88 and 0.97 cm for MODIS and GPS data, respectively. The present study noticed that about 85% of the total PWV estimated from MODIS as well as GPS data lies below 1.0 cm over the site. The seasonal study of PWV reveals that MODIS data is found to be underestimating the PWV when compared to GPS data with bias −0.095 cm and RMSE 0.215 cm particularly during summer. This is due to dry and high-altitude terrain of Hanle particularly during summer months. However, there are marginal differences (bias varied from 0.007 to 0.017 cm) during the remaining seasons and seasonally R 2 varies from 0.62 to 0.87. GPS PWV exhibits distinct diurnal cycle with minimum in morning as well as in night and peak in the late afternoon (16–18 h) irrespective of the seasons. However, the diurnal cycle is very prominent during summer than the rest of the seasons. [ABSTRACT FROM AUTHOR]

Abstract: Estimates of interseismic deformation in northeastern India based on GPS measurements at eight permanent stations (2003–2006) and six campaign sites (1997–2006) are presented here. The Euler pole of rotation of Indian tectonic plate in ITRF2000 determined from the present data set is located at 51.7±0.5 °N, −15.1±1.5 °E with angular velocity of 0.469±0.01 Myr−1. The results show that there is a statistically insignificant present-day active deformation within the Shillong Plateau and in the foreland spur north of the plateau in the Brahmaputra valley. Convergence rate of the northeastern GPS sites with respect to the IGS station Lhasa along baselines that are normal to the Himalayan arc in this region is 16±0.5 mm/yr. This represents the arc-normal Indo-Eurasian convergence rate across the northeastern boundary, similar to arc-normal convergence rates determined in central Nepal along the Himalayan arc. However, unlike central Nepal, in the Arunachal Himalaya the 16 mm/yr shortening is distributed between the Lesser as well as Higher and Tethyan Himalayas. Baselines between sites on the Indo-Burmese Fold and Thrust Belt (IBFTB) and Shillong Plateau indicate variations in the shortening rate from 1.5 mm/yr on the Tripura–Mizoram salient (TRS) south of the plateau, to 6 mm/yr in the Imphal Recess (IR) to the east and 8 mm/yr in Naga salient (NS) to the northeast. This suggests that the deformation in the IBFTB is segmented into N–S blocks along E–W transverse zones exhibiting dextral slip between NS–IR and sinistral slip between IR and TRS. Baselines between the IBFTB sites also show 10±0.6 mm/yr convergence pointing to the existence of an active transverse zone between Aizawl and Imphal. [Copyright &y& Elsevier]

Abstract: Water vapour plays a dominant role in the high-energy thermodynamics of the atmosphere, notably, the genesis of storm systems. However, its distribution is difficult to resolve by conventional means, since water vapour exhibits very high spatial and temporal variability. The growing networks of continuously operating GPS systems, however, offer the possibility of estimating the integrated water vapour (IWV) or, equivalently precipitable water vapour (PW). These estimates constitute critical inputs in operational weather forecasting and fundamental research to model atmospheric storm systems, atmospheric chemistry, and the hydrological cycle. This paper presents the results of IWV estimates from GPS data from continuously operating GPS stations established by C-MMACS at Bangalore, Kodaikanal, Hanle and Shillong over the 3-year period (2001–2003). These are the first results of such an endeavor, towards the study of PW at four different geographical locations in the Indian subcontinent. [Copyright &y& Elsevier]

Precipitable Water Vapor (PWV) is estimated using MODIS (Moderate Resolution Imaging Spectro-radio-meter) Terra level 3 data with daily resolution i. e MOD08_D3 at 64 cGPS (continuous Global Positioning System) stations spatially spread over Indian subcontinent between geodetic latitude 5° to 35° N and geodetic longitude of 70° to 96° E. MODIS-PWV is compared with GPS-PWV estimated at these cGPS stations to check the validity of water vapor retrieved from MODIS data in Indian subcontinent. Correlation coefficient (R2) between daily values of MODIS and GPS water vapor is above 0.9 with RMSE (root mean square error) of 2–5 mm for 22 cGPS in peninsular India, above 0.9 with RMSE of 3–6 mm for 5 cGPS in northeast India and above 0.8 with RMSE of 1–9 mm for 26 cGPS in Himalayas. PWV time series at all the cGPS stations indicated distinct seasonal cycle for both MODIS and GPS PWV with high RMSE (~6 mm) in wet months and low RMSE (~3 mm) during dry months. Taking advantage of broad spatial spread of stations and long span of data, model for spatial variability of GPS-PWV for Indian subcontinent is proposed. Inter-annual and seasonal variability of GPS-PWV is discussed in detail for peninsular India, northeast India and Himalayas. • Validation of MODIS-PWV using long data span at 64cGPS sites for Indian region. • MODIS-PWV has high RMSE, bias for coastal, islands and varying topography regions. • MODIS data fails to capture the seasonal variability of PWV for Indian sub-continent. • Spatial variability function of GPS-PWV is given for Indian Sub-continent. • Relation between the seasonal variability of PWV and temperature variation is given. [ABSTRACT FROM AUTHOR]

The Kachchh paleo-rift basin of western India which recently experienced Mw 7.6, 26th January 2001 Bhuj earthquake, is one of the most active and vulnerable intra-cratonic zones in the world categorized as seismic zone V, i.e. having potential to produce earthquakes of intensity IX or higher. We analysed the geodetic and seismic principal strain rates to reveal the seismic hazard potential of the Kachchh paleo-rift basin. The geodetic strain rates are calculated from the reported surface motion in the region using a modified least-square inversion scheme that utilizes distance and spatial coverage dependent weight factor on the uncertainty in the plate motion to get a reliable estimate of principal strain rates and their orientations at regular grid nodes. An optimal weighting factor yields a mean rate of compression of ~ − 16 ± 6 nano strain/yr towards north-northeast in the region. The seismic strain rates are estimated based on fault plane solutions of the significant past earthquakes (Mw ≥ 2.5) of the region using the Kostrov formulations. The seismic strain rate and stress inversion estimate indicate that the region released elastic strain at an average rate of ~ − 85 nano strain/yr towards north-south in the past 350 years. A composite analysis of the geodetic and seismic strain rates indicates 900–3000 years of recurrence interval of major (Mw 7.8–8) earthquakes in the region. Our analysis indicates a high rate of strain build-up on the reactivated major faults in the region that have the potential to generate devastating (Mw ≥6) earthquakes. [Display omitted] • A high rate of tectonic compression with a mean strain build-up rate of ~ 16 nano strain/yr towards NNE is obtained. • Three major earthquakes in past 350 years contribute >98% in measure of average seismic strain rate of ~85 nano strain/yr. • 5°-40° change in orientation of both (geodetic & seismic) principal strain rates axes show a rather complex tectonic zone. • Composite analysis of geodetic and seismic strain rates yields a recurrence period of >1000 years for Mw >7.5 earthquakes. • Zones with higher strain build-up compared to strain release rate have higher probability to produce next >Mw 6 earthquakes. [ABSTRACT FROM AUTHOR]

• Performance of IRI-2016 model is probed through geodetic GPS TEC during 2002–2019. • Large discrepancy is observed in high solar activity phases in both solar cycles. • IRI model fails to manifest the strength of ionization around EIA crest regions. • Differences are observed more during day time hours in all the phases of solar activity. • Winter anomaly is not reflected in IRI outputs during high solar activity years. We evaluate the performance of the latest version of International Reference Ionosphere (IRI-2016) by comparing the estimated Total Electron Content (TEC) with the observed values from four geodetic Global Positioning System (GPS) receivers, latitudinally aligned from the equator to low latitudes (−5° to 20° Geomagnetic) in the Indian longitudes (75–95°) from 2002 to 2019. It is observed that IRI-2016 model underestimates and overestimates the GPS TEC depending on the season, solar activity, and geographic location of observation. On a decadal scale, the monthly mean of IRI TEC and GPS TEC show distinct seasonal variation trends for all years with seasonal asymmetry. The bias and RMSE values are low in the ascending and descending phases of solar cycles 23 and 24 compared to high values with significant fluctuations observed during the peak solar activity phase. For the declining phase of solar cycle 24, IRI TEC underestimates the GPS TEC values at EIA crest regions. On an annual scale, IRI TEC agrees better with GPS TEC during the low solar activity years (2010 and 2019) than the high solar activity years (2002 and 2014). Moreover, IRI model is unable to manifest the winter anomaly characteristics during the high solar activity years. On a diurnal scale, the performance of IRI model is poor with high RMSE during daytime hours and it underestimates the GPS TEC at equatorial and low latitude regions during the high solar activity phases. On a seasonal scale, high bias and RMSE are observed during the spring equinox compared to other seasons under the high solar activity phases. On a spatial scale, the bias and RMSE are high with low yearly coefficient of determination ( R 2) in the EIA crest regions as compared to the equatorial and low latitude regions. High underestimation of IRI model was also observed in November 2011 due to high solar indices. The proportionality relationship between RMSE and solar indices is observed in all the phases of solar activity. Additionally, the mean annual RMSE and R 2 values indicate solar activity predominantly affecting the performance of IRI model. This study suggests that the influence of solar and magnetic indices inputs in the present IRI model could be revisited for reflecting the solar activity effects distinctly in the model outputs. In addition, IRI-2016 model requires further improvements for equatorial, low latitude, and EIA crest regions of the Indian sub-continent, specifically during the high solar activity phases. [ABSTRACT FROM AUTHOR]

The present work reports atmospheric opacity at 220 GHz over Indian Astronomical Observatory (IAO)-Hanle during 2006 to 2018. The opacity data were derived from the radiometric measurement of sky-brightness temperature as a function of zenith angle for every 10 min during day and night. Since the operating frequency range is not completely opaque during rainfall or cloudy atmosphere, observations are performed even during rainfall or cloudy sky conditions. The minimum opacities observed at the site varied from 0.06-0.07 at 1st quartile and 0.08-0.09 at 2nd quartile during December-January. Opacity at Hanle is linearly correlated with high temporal resolution GPS (Global Positioning System) Precipitable Water Vapor (PWV) with a correlation coefficient of about 0.8. The observed median opacity at Hanle has increased by 44% during the best observing months (October–March) and 24% over the annual data (October-September) with reference to the opacity observed during 2000–2003. Such increasing opacity trends are apparently due to effects of the dynamics of the regional and global hydrological cycle. In order to assess the impacts of the regional and global hydrological cycle, PWV obtained from AErosol RObotic NETwork (AERONET), satellite and reanalysis data were studied at nine high-altitude astronomical observatories spread spatially across the globe such as Atacama desert in Chile, Mauna Loa Observatory in Hawaii, Hanle and Merak in India, Ali and Yangbajing Observatories in Tibet, LLAMA in Argentina, and Dome A and Dome C in the Antarctic Plateau during 2003-2018. On an average, PWV trend over Atacama desert is increasing by 0.01 to 0.03 mm per year and at IAO-Hanle, Merak and Ali by 0.01 to 0.05 mm per year, indicating non uniform dynamic hydrological cycles. Interestingly, there is no noticeable PWV trend for the sites located in the Antarctic Plateau. During the four best observing months, PWV observed at Hanle, Merak and Ali sites have values lower than the Mauna Loa observatory by 0.14 mm and 0.56 mm at 1st and 2nd quartiles, respectively. • Median opacities at Hanle during 2006–2018 varied from 0.08–0.09 in winter. • Increasing opacity trends of 24% are noticed at Hanle using multi-year data sets. • Median precipitable water vapor (PWV) at Merak and Hanle varied from 2.16–2.23 mm. • Increasing opacity trends are attributed to increasing PWV of 0.01–0.05 mm yr-1. • Increasing PWV (0.01–0.03 mm yr-1) is observed at driest site in Atacama desert. [ABSTRACT FROM AUTHOR]