[1] The penetration of magnetospheric electric fields to the magnetic equator has been investigated for two intense magnetic storms that occurred on 31 March 2001 and 6 November 2001. The digital ground magnetic data from equatorial station Tirunelveli (TIR, 0.17°S geomagnetic latitude (GML)) and low-latitude station Alibag (ABG, 10.17°N GML) have been used to identify the storm time electrojet index, EEJ(Dis), which is the difference of the magnetic field variations between TIR and ABG after removing the quiet day variations. The appearance of enhanced DP 2 currents and counterelectrojets (CEJ) during the main and recovery phases of the magnetic storms is possibly due to prompt penetration of electric fields from the high latitudes. These signatures can be interpreted as a clear indicator of the eastward and westward electric fields at the equator. The observed results suggest that the magnitude of the equatorial ionospheric currents driven by the penetrating electric fields is very sensitive to ionospheric conductivity (which depends on local time). Moreover, the intensity of the DP 2 currents started decreasing during the end of the main phase of the storm despite the large negative southward IMF Bz, indicating the dominance of a well-developed shielding electric field for 1 h. As an effect of penetrating electric fields at the equator, the equatorial ionization anomaly is enhanced during the main phase (because of strong eastward electric field) and is inhibited or reduced due to the strong CEJ (because of westward electric field) during the recovery phase.Citation: Veenadhari, B., S. Alex, T. Kikuchi, A. Shinbori, R. Singh, and E. Chandrasekhar (2010), Penetration of magnetospheric electric fields to the equator and their effects on the low-latitude ionosphere during intense geomagnetic storms,
We apply continuous wavelet transform technique to the full decade (2001–2010) of CHAMP vector magnetic data from 55417 tracks, to search for evidences of external source field signatures that are either misunderstood or ignored in modern magnetic field models. We show that satellite magnetic time-series, after subtracting the main and lithospheric field contributions, exhibit several external source signals. Other than the diurnal, 27-day, and annual periodicities, we also have observed the 210-day periodicity, in the external magnetic field. Central to these observations is the local time dependency of 27-day variations, which suggests that the purely zonal source model, generally considered for the estimation of electromagnetic induction response is inadequate. We discuss the origin and characteristics of 210-day periodicity, although its geophysical significance needs to be fully ascertained. A better understanding of the external fields as seen at satellite altitude is a prerequisite for an optimum exploitation of Swarm multi-satellite mission, which is scheduled for launch in 2013.
SUMMARY
We provide a new hypothesis for the deep subsurface structures near the Bhuj 2001 earthquake region based on magnetotelluric (MT) investigations carried out close to the epicentre zone. 2‐D inversion of broad‐band MT data of two profiles of lengths 32 km (AA′) and 52 km (BB′) revealed a thick (∼3 km) highly conductive (1–4 Ω‐m) surface layer of fluvio‐marine Mesozoic–Cenozoic sediments. The models delineate the hypocentre zone located at ∼20–25 km depth that manifests the high resistivity–conductivity transition zone. The accumulation of compressive stresses post‐rifting along this weak zone has resulted in the reverse slip of Bhuj 2001 earthquake. The reverse fault (F1) associated with the earthquake is believed to be an ancient normal fault formed during the rifting phase. Contrary to earlier suggested theories, we suggest that F1 got initiated along the high resistivity–conductivity transition zone causing the Bhuj 2001 event. The geoelectric models revealed a laterally extending partially resistive zone at 20–30 km depth range showing a tendency to extend further deep. Model calculations using synthetic data also support this observation. Therefore, we hypothesize the presence of a basal detachment, marking the transition zone between the continental crust and the lithospheric upper mantle at ∼40 km depth, intersected by the F1. The geoelectric models suggest that the crustal thinning caused the asthenospheric upwelling and/or serpentinization leading to the ascent of volatiles and melts. The subsurface geometry in Kachchh basin suggests the thick‐skinned deformation.
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