Nandini Nagarajan

Evidence for short spatial correlation lengths of the noontime equatorial electrojet inferred from a comparison of satellite and ground magnetic data

[1] The current density of the noontime equatorial electrojet (EEJ) as determined from CHAMP data is highly variable between successive passes of the satellite, which are separated by 23 in distance and 93 min in time. An open question is to which extent this variability is caused by temporal or spatial variations in the ionosphere. Another important question is the connection between EEJ and global solar-quiet (Sq) current systems. We try to answer these questions by comparing the EEJ current density estimated from highquality scalar magnetic field measurements of the CHAMP satellite with the magnetic horizontal intensity variations at six equatorial observatory pairs distributed across the globe. Data taken during the period 2000–2002 were used for the present study. We apply corrections for the effect of local time (LT) and Sq fields. By estimating the correlation coefficients between the ground and satellite data as a function of distances between measurements, new insights into the spatial structure of the EEJ have been obtained. The high correlation, when CHAMP passes directly over an observatory, decays quickly in eastern and western directions. Typically, within ±15 of longitudinal separation between satellite and observatory, the correlation falls well below the statistical significance level. This observation holds for all longitude sectors. Interestingly, the correlation between CHAMP-inferred EEJ strength and observatory differences breaks down for the observatory pairs, outside of a ±4 latitudinal band. This implies that the EEJ and Sq variations are uncorrelated for periods up to 1 hour. Additionally, it was found that monitoring of the EEJ can be performed best if the reference observatory is 4 to 5 apart from the dip equator.

Characterization of seasonal and longitudinal variability of EEJ in the Indian region

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  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.

Estimation of the regional Bouguer gravity field over the Indian peninsula using two-dimensional filtering

Abstract The regional Bouguer gravity field for India has been obtained objectively by digitising the gravity data at 0.25° interval and by subjecting it to two-dimensional symmetrical filtering with cutoff of 0.004 cycles/km (>200 km). The data at continental edges were appended with co-sinusiods of appropriate wave number to produce a uniform rectangular grid. This process also avoids Gibbs phenomenon. The data set thus obtained was divided into four overlapping independent rectangles for the purpopse of filtering. This process also provided a check on the filtered values. The low-pass filtered (deeper?) and the residual (shallower?) maps so obtained are correlated with geology and tectonic regimes of the India subcontinent. The high gradients in the contours of the residual map and the gentle gradients in the regional map reflect the effects of shallower and mainly deeper structures, respectively. The regional field indicates: 1. (1) a characteristic difference between the eastern and the western coasts; 2. (2) a central high encompassing the Godavari graben and Satpura fold belt which bends syntaxially towards Aravalli trend; and 3. (3) a NW-SE-trending low in southern peninsular India. The salient observations from the residual map are: 1. (1) a clear rift-like structural signature with negative gravity values in the centre flanked by positive gravity values over the Godavari graben; 2. (2) the NE-SW Aravalli trend merges with the E-W Satpura trend in the south; 3. (3) the gravity signature of the Narmada-Son lineament is not very conspicuous; and 4. (4) an isolated patch of negative gravity values south of Bangalore is prominent.

Variability of equatorial counter electrojet signatures in the Indian region

[1] The limited longitudinal extent of equatorial counter electrojet (CEJ) has been inferred by several workers based on the analysis of ground data. However the scale length of CEJ characteristics at 2 hours or less has not been estimated so far. The present study seeks to characterize the longitudinal variability of CEJ phenomena at a longitudinal separation of ~15° using hourly averaged variations at two equatorial electrojet (EEJ) pairs of stations: Hyderabad & Vencode at 77°E and Port Blair & Campbell Bay at 93°E. The nature of CEJ events are classified by time of occurrence and studied using 12 months of concurrent data at the two longitudes. From examination of 323 CEJ events at VEN (Vencode) and 239 at CBY (Campbell Bay) over a period of 346 days, the observations are: i) the occurrence of CEJ is not simultaneous at VEN and CBY for about 40% of events; (ii) the amplitude and occurrence frequency of CEJ events is greater at VEN than CBY during both Kp < 2 and Kp ≥ 2; (iii) the influence of westward currents on the EEJ peak was evidenced by early or late peak occurrences comprising about 175 days at VEN and 89 days at CBY. It is established here that considerable variability of CEJ signatures is observed between the two longitudes at 15° separation revealing the impact of local electrodynamics. These local processes therefore significantly influence the characteristics of EEJ.