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We investigated the ionosphere response to the two severe geomagnetic storms in the ascending phase of solar cycle 25 which occurred during the 23-24 March 2023 (SYM-Hmin = −169 nT) and 23-24 April 2023 (SYM-Hmin = −233 nT) using a latitudinally aligned dense network of Global Navigation Satellite System (GNSS) receivers, magnetometers, and digisonde along the Indian longitude sector. The significant variations in TEC during the storm's main and recovery phases are mainly linked to the influence of westward Disturbance Dynamo Electric Fields (DDEFs). During the initial phase of the March 23-24 geomagnetic storm, no changes in daytime TEC were observed, despite the storm occurring at noon time with a southward IMF Bz due to the influence of electron density in the top side ionosphere. Furthermore, both pre-reversal enhancement (PRE) and ionosphere irregularities are suppressed within two hours of their onset during the main phase of the March 23-24 storm, owing to the westward transition of zonal electric fields from an eastward direction. During the recovery phase of both storms, a daytime positive storm effect is observed over the dip equatorial region, while the beyond equatorial ionization anomaly (EIA) and mid-latitude regions perceived a negative ionospheric storm effect. This phenomenon is attributed to the influence of the dominant westward DDEFs during the period. Moreover, these DDEFs effectively inhibited the equinoctial manifestation of PRE effects and post-sunset ionospheric irregularities during the recovery phase of both storms. These findings are further confirmed with supporting information from TEC recorded by Swarm satellites, model-derived ionospheric electric fields, and thermospheric O/N2. The results from this study may advance the understanding of ionospheric response to severe geomagnetic storms under the prevailing westward DDEFs during the dayside recovery phase, complementing the global efforts for more reliable space weather modeling and prediction services. [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]