Equatorial electrojet at African longitudes: first results from magnetic measurements

Doumouya, V.; Vassal, J.; Cohen, Y.; Fambitakoye, O.; Menvielle, M.

doi:10.1007/s005850050637

Cited by 44 publications

(83 citation statements)

References 25 publications

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“…In section 6 we discuss the longitudinal and seasonal structure of the EEF. [Doumouya et al, 1998]. At each station, a fluxgate magnetometer records the three components of the geomagnetic field at a sampling rate of 1 Hz.…”

Section: Deriving a Time Series Of Eej Current Profiles From Ground Mmentioning

confidence: 99%

Longitudinal and seasonal structure of the ionospheric equatorial electric field

Alken¹,

Chulliat

Maus³

2013

JGR Space Physics

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[1] The daytime eastward equatorial electric field (EEF) in the ionospheric E-region plays an important role in equatorial ionospheric dynamics. It is responsible for driving the equatorial electrojet (EEJ) current system, equatorial vertical ion drifts, and the equatorial ionization anomaly. Due to its importance, there is much interest in accurately measuring and modeling the EEF. In this work we propose a method of estimating the EEF using CHAMP satellite-derived latitudinal current profiles of the daytime EEJ along with Δ H measurements from ground magnetometer stations. Magnetometer station pairs in both Africa and South America were used for this study to produce time series of electrojet current profiles. These current profiles were then inverted for estimates of the EEF by solving the governing electrostatic equations. We compare our results with the Ion Velocity Meter (IVM) instrument on board the Communication/Navigation Outage Forecasting System satellite. We find high correlations of about 80% with the IVM data; however, we also find a constant offset of about 0.3 mV/m between the two data sets in Africa. Further investigation is needed to determine its cause. We compare the EEF structure in Africa and South America and find differences which can be attributed to the effect of atmospheric nonmigrating tides. This technique can be extended to any pair of ground magnetometer stations which can capture the day-to-day strength of the EEJ.Citation: Alken, P., A. Chulliat, and S. Maus (2013), Longitudinal and seasonal structure of the ionospheric equatorial electric field,

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Section: Deriving a Time Series Of Eej Current Profiles From Ground Mmentioning

confidence: 99%

Longitudinal and seasonal structure of the ionospheric equatorial electric field

Alken¹,

Chulliat

Maus³

2013

JGR Space Physics

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“…Given the latitudinal extent of the equatorial electrojet (see this study, Sect. 3.3, and e.g., Fambitakoye and Mayaud, 1976a;Onwumechilli and Ozoemena, 1985;Doumouya et al, 1997), the plane wave approximation no longer holds for frequencies smaller than a few cph, such that interpretation of daytime electromagnetic observations must take into account the actual distribution of ionospheric currents at equatorial latitudes. The variation of the E-W current density with latitude has been studied for decades.…”

Section: Induction By the Irregular Variationsmentioning

confidence: 99%

“…The variation of the E-W current density with latitude has been studied for decades. Chapman (1951), followed by Fambitakoye and Mayaud (1976a) and Doumouya et al (1997) modeled the equatorial electrojet as a 1-dimensional symmetrical distribution of E-W ionospheric currents scaled by its latitudinal extent and its intensity at the center: Fig. 6.…”

Section: Induction By the Irregular Variationsmentioning

confidence: 99%

“…The aim of this experiment was twofold: ®rstly, to study the magnetic signature of the equatorial electrojet, and compare the results to those established in Chad and the Central African Republic, and secondly, to characterize the induced electric ®eld variations on the other hand. The ®rst results of the magnetic ®eld investigations were presented by Doumouya et al (1997). Those of the electric ®eld experiment will be discussed in this study.…”

Section: Introductionmentioning

confidence: 96%

“…The magnetic S R E variation has been extensively studied for decades, and its behavior is now relatively well understood (see e.g., Forbes, 1981 for a review, and Doumouya et al, 1997). In contrast, the behavior of the electric ®eld S R E has been matter of debate for decades.…”

Section: Introductionmentioning

confidence: 99%

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A study of transient variations in the Earth's electromagnetic field at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast)

Vassal

Menvielle

Cohen³

et al. 1998

Ann. Geophys.

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Abstract. In the framework of the French-Ivorian participation to the IEEY, a network of 10 electromagnetic stations were installed at African longitudes. The aim of this experiment was twofold: ®rstly, to study the magnetic signature of the equatorial electrojet on the one hand, and secondly, to characterize the induced electric ®eld variations on the other hand. The ®rst results of the magnetic ®eld investigations were presented by Doumouya and coworkers. Those of the electric ®eld experiment will be discussed in this study. The electromagnetic experiment will be described. The analysis of the electromagnetic transient variations was conducted in accordance with the classical distinction between quiet and disturbed magnetic situations. A morphological analysis of the recordings is given, taking into consideration successively quiet and disturbed magnetic situations, with the results interpreted in terms of the characterization of external and internal sources. Particular attention was paid to the e ects of the source characteristics on the induced ®eld of internal origin, and to the bias they may consequently cause to the results of electromagnetic probing of the Earth; the source e ect in electromagnetic induction studies. During quiet magnetic situations, our results demonstrated the existence of two di erent sources. One of these, the S R E source, was responsible for most of the magnetic diurnal variation and corresponded to the well-known magnetic signature of the equatorial electrojet. The other source (the S *E R source) was responsible for most of the electric diurnal variation, and was also likely to be an ionospheric source. Electric and magnetic diurnal variations are therefore related to di erent ionospheric sources, and interpreting the electric diurnal variation as induced by the magnetic ®eld diurnal variation is not relevant. Furthermore, the magnetotelluric probing of the upper mantle at dip equator latitudes with the electromagnetic diurnal variation is consequently impossible to perform. In the case of irregular variations, the source e ect related to the equatorial electrojet is also discussed. A Gaussian model of equatorial electrojet was considered, and apparent resistivities were computed for two models of strati®ed Earth corresponding to the average resistive structure of the two tectonic provinces crossed by the pro®le: a sedimentary basin and a cratonic shield. The apparent resistivity curves were found to depend signi®cantly on both the model used and the distance to the center of the electrojet. These numerical results con®rm the existence of a daytime source e ect related to the equatorial electrojet. Furthermore, we show that the results account for the observed di erences between daytime and night-time apparent resistivity curves. In particular, it was shown that electromagnetic probing of the Earth using the classical Cagniard-Tikhonov magnetotelluric method is impossible with daytime recordings made at dip latitude stations.

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

2014

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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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Equatorial electrojet at African longitudes: first results from magnetic measurements

Cited by 44 publications

References 25 publications

Longitudinal and seasonal structure of the ionospheric equatorial electric field

Longitudinal and seasonal structure of the ionospheric equatorial electric field

A study of transient variations in the Earth's electromagnetic field at equatorial electrojet latitudes in western Africa (Mali and the Ivory Coast)

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

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