[1] The lithospheric structure of the Indian plate has been investigated using converted wave techniques (P and S receiver functions) and a novel stacking analysis technique (without using deconvolution) applied to a large seismological data set from permanent and temporary broadband seismic stations. We observe coherent energy from at least two seismic discontinuities, i.e., the crust-mantle (Moho) and lithosphere-asthenosphere boundary (LAB) in the uppermost mantle. Here we provide a novel seismic image of the Indian lithosphere showing definitive evidence of its flexure, which is interpreted to be primarily caused by the hard collision at~55 Myr resulting in the world's highest mountain chain-the Himalayas and the Tibetan plateau. Results from geoidal and gravity studies do suggest postcollisional flexuring of the Indian plate; however, the flexure lacks observational constraints. The observed wavelength of the flex is~1000 km with the thickness of the Indian shield lithosphere varying from~70 km to 140 km; such a low value for a continent implies that the Indian plate has been reworked in the past. The plate deepens in the Himalayan region to a depth of~170 km. Further, the converted phases are interpreted to be resulting from the bottom of the lithosphere. We clearly demonstrate that these are distinct and different from the midlithospheric discontinuity. For a large number of stations, the midlithospheric discontinuity and LAB are clearly separated in depth. Our observations suggest that the Archaean lithosphere is no longer intact and is prone to deformation.

Prompt penetration (PP) and ionospheric disturbance dynamo (Ddyn) are two expressions of the disturbed electric field developed during a geomagnetic storm in the Earth's ionosphere. Earlier investigations treated these two processes independently over longitudinal separation beyond 20°. In the present study, we investigate the conjunction of these phenomena on equatorial electrojet at 20° longitude separations using daytime magnetic field data from three equatorial sites viz., Minicoy, Vencode, and Campbell Bay and three low‐latitude sites viz., Alibag, Hyderabad, and Nabagram, located within 20° longitude over Indian region, during three intense storms (Dst <− 150 nT). We propose a statistical method to identify PP signatures from large data set. In addition, we distinguish the signatures of PP and Ddyn during early recovery phase of intense storms and compute pure Ddyn signature in the late recovery phase. It is seen that the effect of PP is similar at the three equatorial sites for each storm. We find that signatures of Ddyn are amplified at Minicoy and Vencode compared to Campbell Bay, and their magnitude decreases toward low‐latitude stations, also reflected in the current vector patterns. Our investigations of intense geomagnetic storms show that PP effect dominates the main phase, followed by a combined effect of PP and Ddyn in the early recovery phase. Eventually the Ddyn signature dominates in the late recovery phase with decreasing amplitudes of Ddyn with increasing time. Such investigations can provide new information about variations in ionospheric parameters.

This paper presents the seasonal and longitudinal variability of the equatorial electrojet (EEJ) based on geomagnetic variation data from two electrojet stations in the northern Indian Ocean at a longitudinal separation of~15°: i.e., at 77°E and at 93°E. One complete year of data is used (i.e., from November 2011 to October 2012) at the two longitudes and compared with the climatological model of the equatorial electrojet (EEJM-2.0). The results of our analysis show that (i) the monthly averaged hourly values of EEJ strength at 77°E and 93°E are overall in agreement with global characteristics of EEJ with significant departures over the year of study, (ii) the monthly average hourly daytime values at Campbell Bay and Vencode show poor correlation (r < 0.7) for 5 out of 10 months, (iii) comparison of observed EEJ strength at respective longitudes and the current densities derived from EEJM-2.0 show overall agreement with significant differences for monthly mean hourly daytime values at respective longitudes, and (iv) day-to-day variability in noontime EEJ amplitudes between the two longitudes is >10 nT, >20 nT for 30% of quiet days, sorted by planetary index (Kp) <1 and <2. This variability is reflected in monthly average values (V) mechanisms for persistent differences in EEJ on day-to-day basis are sought from perturbation of westward ion drifts by neutral winds caused by the upward propagation of gravity waves from troposphere/stratosphere into the mesosphere. These mechanisms have been identified theoretically and experimentally. The four-wave structure of ionospheric current densities obtained by EEJM-2.0 and other contemporary models closely resembles atmospheric tidal signatures and has a common origin. The magnitude and persistence of these differences, at short spatial scales (15°), are significant observation. These effects are reflected in the monthly and seasonal signatures of EEJ and contribute to the contemporary models.

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high-quality magnetotelluric data at 100 stations, provide both regional information about the thickness of the Deccan Traps and the occurrence of localized density heterogeneities and anomalous conductive zones in the vicinity of the hypocentral zone. Acquisition of airborne LiDAR data to obtain a high-resolution topographic model of the region has been completed over an area of 1,064 km 2 centred on the Koyna seismic zone. Seismometers have been deployed in the granitic basement inside two boreholes and are planned in another set of six boreholes to obtain accurate hypocentral locations and constrain the disposition of fault zones.