Dip Equator Research Articles

Latitudinal Geomagnetic Variation in H on Quiet Days at Equatorial Stations

The latitudinal geomagnetic variation of the element H, D, and Z at eight Indian Observatories from about 0 to 220 dip latitude have been studied. It is observed that the day-day variability of Sq (H), Sq (D) and Sq (Z) exhibit latitudinal variation. The latitudinal variation is found to be dependent on the position of the station from the equator. The latitudinal variation peaking around the dip equator suggests that one of the possible causative factors is the electrojet strength at the station near the dip equator. The latitudinal variation is independent of local time. Nigeria Journal of Pure and Applied Physics VOLUME 1, AUGUST 2000, pp. 3-4

Global evolution of a substorm-associated DP2 current system observed by superDARN and magnetometers

This paper deals with an evolution of the electric field in the dayside auroral and equatorial ionosphere during a substorm on July 16, 1995. A southward turning of the IMF detected by WIND (171 Re) caused enhancements in the auroral electrojet intensity in the 7–10 MLT and 15.5–18.5 MLT sectors as observed by the IMAGE (74-56 cgmlat) and CANOPUS (70-58 cgmlat) magnetometer chains. SuperDARN detected an equatorward motion of the radar scattering region at speeds of several −10 degs/hour in the dayside (05–17 MLT), suggesting an increase in the flux of the open magnetic field in the polar cap. Furthermore, coherent magnetic variations are observed at subauroral to equatorial latitudes simultaneously with the auroral magnetic variations within a temporal resolution of 10 s. This suggests that the electric field increase during the growth phase is established instantaneously around the convection reversal in the 15.5–18.5 MLT sector, and furthermore penetrates instantaneously to mid and low latitudes. SuperDARN detected a continuous equatorward motion of the auroral oval during the expansion phase around the cusp, which implies a continuous magnetic merging at the day-side magnetopause during the expansion phase. A rapid decrease in the electric field is inferred from coherent auroral and equatorial magnetic field decreases during the recovery phase, which may have been caused by northward turning of the IMF. This magnetic field decrease resembles the change in magnetic field of the counter-electrojet at the dip equator in the afternoon sector.

New B0 and B1 models for IRI

The electron density profile in the F region bottomside is described in the International Reference Ionosphere (IRI) by two parameters: a thickness parameter B0 and a shape parameter B1. The models used for B0 and B1 in IRI are based on ionosonde data from magnetic mid-latitude stations. Comparisons with ionosonde data from several stations close to the magnetic equator show large discrepancies between the model and the data. We propose new models for B0 and B1 based on data from several ionosondes including low and mid latitude stations. Close to the magnetic dip equator the new B0 model provides an improvement over the current IRI model by a factor of up to 1.5.

Imprint of equatorial electrodynamical processes in the OI 630.0 nm dayglow

Results from coordinated measurements of OI 630.0 nm dayglow intensities (centered on ∼220 km altitude), along with VHF (50 MHz) coherent backscatter returns from Thiruvananthapuram, a dip equatorial station in India, revealed that the temporal variability at short periods (<4 h) of the Doppler frequency of the coherently backscattered 50 MHz radar signal in the electrojet region (∼101 km altitude) preceded the dayglow variations. The time delay was found to be inversely related to the electric field magnitude inferred from the Doppler frequency and also with the independently estimated electrojet strength inferred from the ground magnetic data. These results are presented as direct evidence for the prevailing electrodynamic coupling between the E- and F-region of the ionosphere over the dip equator.

Behaviour of magnetotelluric source fields within the equatorial zone

It is well known that equatorial electrojet (EEJ) currents can significantly affect the geomagnetic variations. However, in a recent study (Padilha et al., 1997) it was observed that magnetotelluric (MT) soundings carried out across the dip equator in the Brazilian equatorial zone were not affected significantly due to EEJ currents. By using new results from geomagnetic variation signals, measured simultaneously to the MT experiment at a chain of equatorial and mid-latitude stations, an attempt is made here to explain the MT results in terms of the behaviour of the primary inducing field during the survey. Most of the analysis is performed by considering a single frequency (0.885 mHz), representative of the MT frequency interval. It is observed that the amplitude of the geomagnetic variations appears horizontally homogeneous within the study area (from −3° to +3° of geomagnetic latitude), indicating that the primary field in the analysed frequency range may be considered sufficiently uniform in the horizontal direction thus satisfying the Tikhonov-Cagniard plane-wave criterion. The same geomagnetic data also show that, if any EEJ source effect exists, it would be restricted to the transition zone (between 3° and 5°, at both sides of the dip equator). Dmitriev-Berdichevsky’s constraints calculated at two different frequencies and a modelling exercise using EEJ parameters derived from a magnetometer array were able to explain the MT observations and have shown that source effects would just appear in frequencies lower than 1 mHz (resistive regions) and 0.1 mHz (conductive regions). Considering the characteristics of propagation and amplification of geomagnetic variations at the equatorial zone it is concluded that EEJ currents could be used as a source for lithospheric MT studies in these regions.

Equatorial enhancement of Pc5–6 magnetic storm time geomagnetic pulsations

Geomagnetic Pc5–6 (1–4 mHz) pulsations during three magnetic storms have been analyzed using ground observations at an array of 5 Indian stations ranging from 0.8° to 25° dip latitudes and two pairs of low-latitudes (~20°)—near equator (~4–5°) stations, located at ~4 hours to the West and East from this meridian. The pulsations are found to occur as individual wave packets that are characterized by similar spectra at all stations with two dominate bands of frequencies: ~1.5–2.0 mHz and ~2.5–4.0 mHz. Very strong daytime equatorial enhancement of the wave intensity has been noted. The higher enhancement is observed near noon while it is reduced towards the morning and evening sectors. The polarization of both frequency bands at the equator is found to be almost linear, mainly along the H-direction. It has been found that the amplitude of the D-component decreased with decrease in the latitude towards the equator. One of the spectral peaks is observed exclusively at the dip equator station and it has been attributed to the generation/amplification of MHD waves by the equatorial electrojet instabilities. The mechanism of day time equatorial enhancement of long period geomagnetic pulsations and the propagation characteristics are discussed.

Effects of solar activity variations on adiabatic heating and cooling effects in the nighttime equatorial topside ionosphere

Data from the Defense Meteorological Satellite Program are used to examine solar activity variations in ion temperature measured near 800 km altitude across the nighttime equatorial ionosphere. Evidence for ion cooling and heating by adiabatic expansion and compression are seen during high and moderate solar activity conditions. It is found that these effects are strongly dependent on the location of the O+/H+ transition height. During high solar activity a high transition height moves the region of maximum cooling at the dip equator well above 800 km. The region of maximum heating, however, appears in the winter hemisphere near 800 km, where strong longitude variability is seen, owing to the effects of F region neutral winds. During moderate solar activity conditions the transition height is lowered, bringing the region of maximum cooling closer to 800 km altitude. However, the cooling process is not as strong as that seen during high solar activity periods. The heating effects are also lowered during moderate solar activity periods. At 800 km altitude, only relatively small temperature peaks are seen in the winter hemisphere, closer to the dip equator than those seen during high solar activity periods.

On the Backus Effect--II

Recovering the internal geomagnetic vector field B on and outside the Earth's surface S from the knowledge of only its direction or its intensity IIBjl on S, and assessing the uniqueness of geomagnetic models computed in this way, have been long-standing questions of interest. In the present paper we address the second problem. Backus (1968, 1970) demonstrated uniqueness in some particular cases, but also produced a theoretical counterexample for which uniqueness could not be guaranteed. Using the same line of reasoning as Backus (1968), we show that adding the knowledge of the location of the dip equator on S to the knowledge of IIB 11 everywhere on S guarantees the uniqueness of the solution, to within a global sign, provided that the dip equator is made of one or possibly several closed curves on S, across which the normal component of the field changes sign (this component not being zero anywhere else).

Longitudinal and seasonal variations in nighttime plasma temperatures in the equatorial topside ionosphere during solar maximum

Latitude profiles of the ion and electron temperatures and total ion concentration across the equatorial region near 800 km altitude are routinely obtained from Defense Meteorological Satellite Program (DMSP) spacecraft. We have examined these profiles at 2100 hours local time to discover the influences of field‐aligned plasma transport induced by F region neutral winds. Such dependencies are readily seen by contrasting observations at different seasons and different longitudes distinguished by different magnetic declinations. These data show strong evidence for adiabatic heating produced by interhemispheric plasma transport. This heating manifests itself as a local temperature maximum that appears in the winter hemisphere during the solstices and is generally absent during equinox. A longitudinal variation in the appearance of this maximum is consistent with the roles of meridional and zonal winds in modulating the field‐aligned plasma velocities. The data also show a local temperature minimum near the dip equator. However, it is not so easy to attribute this minimum to adiabatic cooling since transport of plasma from below and the latitude variation in the flux tube content may also produce such a minimum.

Characteristics of the Equatorial Electrojet determined from an array of magnetometers in N-NE Brazil

An array of 29 vector magnetometers was operated in N-NE Brazil from November 1990 until March 1991. We present the analysis of 16 selected quiet days, for which a simple model of an equivalent current distribution for the Sq and EEJ, fits the observed maximum amplitude of the daily variation at midday. In equatorial regions the precise latitude profile of the Sq field is masked by the EEJ. This uncertainty is resolved by assuming that the EEJ, obtained after subtracting the Sq from the daily ranges, should present a ratio of 0.3 for the westward to eastward current. With this constraint, a combined non-linear least squares inversion of Sq and EEJ was used to estimate the parameters of Onwumechili’s model of the EEJ current distribution. The H and Z components of the EEJ are jointly inverted and good agreement obtained between the calculated and observed data for all 16 days. The EEJ’s main parameters averaged for 16 quiet days were: A total positive current intensity equal to 67 ± 20 (103 A) for diurnal range M4 (or 80 ± 20 (103 A) for M3) and a half-width of 403 ± 67 km. The EEJ centre was located at 21 ± 16 km south of the dip equator. The Sq was estimated from several permanent observatories and found to be centred at a mean latitude of 5.5 ± 2 degrees south.

Magnetic field aligned modeling for the upper F region and for the plasmasphere

Existing empirical models, e.g., the IRI and the PRIME model, have shortcomings for the upper-most F region and usually have no realistic formulation for the plasmasphere. These shortcomings can be overcome by replacing purely height oriented modeling by magnetic field aligned approaches.A magnetic field approximation is presented which uses dipole field lines with apexes above the dip equator. Modeling along these field lines can be based on diffusive equilibrium. For a single ion plasma (e.g., an H+ plasma) the integrations which are necessary to model along the field lines in a realistic way can be carried out by means of series expansions. For a multiple ion plasma and in case of arbitrary dependence of electron and ion temperatures on the coordinates one has to apply numerical integration.The principles of joining a field aligned model to a height oriented one are discussed including a method to cross the dip equator in a consistent way.A practical example is presented with a plasmasphere model added to the global model NeUoG which was developed at the University of Graz. The future development aims at replacing all of the topside F region of the model by a magnetic field aligned approach.

Occurrence of equatorial F region irregularities: Evidence for tropospheric seeding

We present a new gap‐free version of the seasonal and longitudinal (s/l) variations of PEFI, the equatorial F region irregularity (EFI) occurrence probability, based on data from the AE‐E spacecraft. The agreement of this and three earlier partial PEFI patterns verifies all four. We reinterpret another earlier gap‐ridden pattern, that of a topside ionogram index of average darkening by range spread F. We comparent with PEFI and, using ionosonde radio science considerations, we conclude that = PEFI times a factor depending on the average number of topside plasma bubbles visible to the ionosonde. The s/l variations of thus imply s/l variations in the average spacing of bubbles, whose seeds have an occurrence probability pattern Pseed. For discussion we assume PEFI = PinstPseed, where Pinst is the pattern of F region instability. The PEFI pattern, which is by definition independent of seed and/or bubble spacing, is far too complex to be explained by the dominant paradigm, that of changes in Pinst by simple changes in the F region altitude and/or north–south asymmetry. We examine evidence behind this dominance, and find it unconvincing. Both the asymmetry and sunset‐node/altitude hypotheses of 1984 and 1985, respectively, seem to be partly based on misunderstood data, and their features appear displaced in time and space from those of our repeat able PEFI pattern. In contrast, if Pseed variations influence the PEFI pattern and depend on thermospheric gravity waves from tropospheric convection near the dip equator, then the seasonal maxima (minima) of PEFI could be explained, since they all occur above relatively warm (cold) surface features, where convection is maximal (minimal). Also, the hypothesis of the dominance of the Pseed term could explain an unusual December/January PEFI maximum in the deep, wide, normal Pacific minimum in the one data set obtained in El Niño years. Based on the experiments we consider, we predict that the s/l variations of Pseed will be found to be similar to those of PEFI, and largely to explain them. Finally, we find reasons, based on the similarity of the variations to s/l patterns of the average scintillation index, for not using, as is commonly done, such scintillation patterns as substitutes for PEFI or Pinst patterns.

A simple model of low‐latitude electric fields

The low‐latitude electric fields are generally modeled by numerically solving a two‐dimensional potential equation developed from assumptions of equipotential field lines and current continuity. Additional assumptions are used to derive simple formulas for the vertical electric field EL and zonal electric field Eϕ in the plane of the magnetic dip equator. Electric fields obtained from the simple model will be compared against the electric fields of the more complete numerical model to demonstrate the validity of the simple model solution and to identify the range of validity. The simple electric field model is used to directly identify the primary causes of the low‐latitude electric field structure. The observed structure has sources that can be divided into components: (1) polarization fields created by divergences in neutral‐wind dynamo currents and (2) curl‐free effects demanded by ∇ × E = 0. It is shown that the evening prereversal enhancement of Eϕ is caused by “curl‐freeness” demands of the rapidly changing EL near sunset. This supports cursory statements of Rishbeth [1971] on the cause of the postsunset enhancement. Finally, the resulting altitude variation of the zonal electric field depends on the type of source. For polarization field sources the altitude variation is approximately proportional to L2. For curl‐free sources the altitude variation is related to the rate of change of EL with respect to longitude.

Joule heating due to vertical ion currents in the lower thermosphere over the dip equator

The theory of equatorial electrojet predicts the presence of vertical ion currents (Pedersen currents) as a part of the electrojet current system. The vertical ion current density profile over the dip equator, that forms a part of the meridional current system is derived from an electrojet model. The joule heating due to these currents flowing upward during daytime for a local time for 1100 hrs has been estimated. The primary east-west current density of the model is kept at the same value as that measured by means of rocket-borne magnetometer on one occasion. The electrical power dissipated as heat in the narrow belt in the height region of 100–180 km is estimated and found to be significant. The height of maximum power dissipation coincides with the altitude of maximum ion velocity i.e. 122 km. By solving the heat conduction equation we obtain a maximum temperature increase of 8°K around 135 km. The importance of this localized heating in the lower thermosphere around ±2° of the dip equator is discussed.

Rocket measurements of high‐altitude spread F irregularities at the magnetic dip equator

We present the first rocket measurements of equatorial spread F irregularities at altitudes significantly above 600 km, in what is believed to be the inertially dominated regime of spread F. On October 14, 1994, at 1955 LT (2255 UT) a Black Brant X rocket instrumented to measure plasma density and electric fields was launched from Alcântara, Brazil (2° 18.75'S, 44°22.3'W, dip angle 1.5° at 300 km altitude), into active topside spread F plumes. On upleg the payload remained continuously in these plumes up to an altitude of 822 km. Power spectra of electron density fluctuations show characteristic dual‐power law behavior with spectral indices of −1.7 at frequencies below 60 Hz and −5 at frequencies above 60 Hz. Shocklike coherent structures characterized the density waveforms. The electric field fluctuations had spectral indices ranging from −3 to −4. We observe no evidence of altitude variation in the spectral indices or in the density fluctuation waveforms. Over the entire altitude range of the observations, 350–822 km, the data remain in agreement with previous measurements of collisional spread F below 600 km.

Day-to-day variability of geomagnetic hourly amplitudes at low latitudes

SUMMARY A study of the variability of the amplitude of Sq at a fixed hour from one day to the next at nine stations from the dip equator to about 22° north of it has produced interesting results. The amplitude and sign of the variability change virtually randomly, making the mean practically zero. The variability occurs at all hours of the day. Its magnitudes in the components D, H and Z have the same diurnal variation, which peaks in the noon period like Sq(H) in low latitudes, and a weak seasonal variation that peaks at the June solstice (local summer). It is demonstrated that changes in the current intensities of the equatorial electrojet (EEJ) and the worldwide part of the Sq (W Sq) current layers have contrasting phases and can sometimes be in antiphase. Indeed, the changes are mostly independent. Inclusion of the magnetic element D revealed that the EEJ current system has not only an east‐west but also a north‐south component. The study shows that the meridional component of the EEJ current intensity evidenced at the Kodaikanal and Annamalainagar stations is an integral part of the zonal component at Trivandrum. This confirms the results of Rastogi (1996) and validates those of Onwumechili (1997). The results suggest that ionospheric conductivity mainly controls the magnitude, while the electric field and ultimately winds mainly control the phase and randomness of the day-to-day variability of the hourly amplitudes of Sq. The random component is attributed to local and/or regional atmospheric winds, probably of gravity wave origin.

Fabry-Perot interferometer measurements of neutral winds and F2 layer variations at the magnetic equator

Abstract. This letter presents some night-time observations of neutral wind variations at F2 layer levels near the dip equator, measured by the Fabry-Perot interferometer set up in 1994 at Korhogo (Ivory Coast, geographic latitude 9.25°N, longitude 355°E, dip latitude –2.5°). Our instrument uses the 630 nm (O1D) line to determine radial Doppler velocities of the oxygen atoms between 200 and 400 km altitude. First results for November 1994 to March 1995 reveal persistent eastward flows, and frequent intervals of southward winds of larger than 50 ms–1 velocity. Compared with the simultaneous ionospheric patterns deduced from the three West African equatorial ionosondes at Korhogo, Ouagadougou (Burkina-Faso, dip latitude +1.5°) and Dakar (Sénégal, dip latitude +5°), they illustrate various impacts of the thermospheric winds on F2 layer density: (1) on the mesoscale evolution (a few 103 km and a few 100 minutes scales) and (2) on local fluctuations (hundreds of km and tens of minutes characteristic times). We report on these fluctuations and discuss the opportunity to improve the time-resolution of the Fabry-Perot interferometer at Korhogo.Key words. Ionosphere (Equatorial ionosphere; Ionosphere-atmosphere interaction) · Meteorology and Atmospheric Dynamics (General circulation)

Equatorial electrojet as part of the global circuit: a case-study from the IEEY

Abstract. Geomagnetic storm-time variations often occur coherently at high latitude and the day-side dip equator where they affect the normal eastward Sq field. This paper presents an analysis of ground magnetic field and ionospheric electrodynamic data related to the geomagnetic storm which occured on 27 May 1993 during the International Equatorial Electrojet Year (IEEY) experiment. This storm-signature analysis on the auroral, mid-latitude and equatorial ground field and ionospheric electrodynamic data leads to the identification of a sensitive response of the equatorial electrojet (EEJ) to large-scale auroral return current: this response consists in a change of the eastward electric field during the pre-sunrise hours (0400-0600 UT) coherently to the high-, mid-, and equatorial-latitude H decrease and the disappearance of the EEJ irregularities between the time-interval 0800-0950 UT. Subsequent to the change in h'F during pre-sunrise hours, the observed foF2 increase revealed an enhancement of the equatorial ionization anomaly (EIA) caused by the high-latitude penetrating electric field. The strengthening of these irregularities attested by the Doppler frequency increase tracks the H component at the equator which undergoes a rapid increase around 0800 UT. The ∆H variations observed at the equator are the sum of the following components: SR, DP, DR, DCF and DT.Keywords. Equatorial electrojet · Magnetosphere-ionosphere interactions · Electric fields and currents · Auroral ionosphere · Ionospheric disturbances

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

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: firstly, to study the magnetic signature of the equatorial electrojet on the one hand, and secondly, to characterize the induced electric field variations on the other hand. The first results of the magnetic field investigations were presented by Doumouya and coworkers. Those of the electric field 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 effects of the source characteristics on the induced field of internal origin, and to the bias they may consequently cause to the results of electromagnetic probing of the Earth; the source effect in electromagnetic induction studies. During quiet magnetic situations, our results demonstrated the existence of two different sources. One of these, the SRE 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 SR*E 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 different ionospheric sources, and interpreting the electric diurnal variation as induced by the magnetic field 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 effect 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 stratified Earth corresponding to the average resistive structure of the two tectonic provinces crossed by the profile: a sedimentary basin and a cratonic shield. The apparent resistivity curves were found to depend significantly on both the model used and the distance to the center of the electrojet. These numerical results confirm the existence of a daytime source effect related to the equatorial electrojet. Furthermore, we show that the results account for the observed differences 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.Key words. Electromagnetics (Transient and time do- main) Geomagnetism and paleomagnetism (geomagne- tic induction) Ionosphere (equatorial ionosphere)

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

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: firstly, to study the magnetic signature of the equatorial electrojet on the one hand, and secondly, to characterize the induced electric field variations on the other hand. The first results of the magnetic field investigations were presented by Doumouya and coworkers. Those of the electric field 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 effects of the source characteristics on the induced field of internal origin, and to the bias they may consequently cause to the results of electromagnetic probing of the Earth; the source effect in electromagnetic induction studies. During quiet magnetic situations, our results demonstrated the existence of two different sources. One of these, the SRE 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 SR*E 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 different ionospheric sources, and interpreting the electric diurnal variation as induced by the magnetic field 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 effect 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 stratified Earth corresponding to the average resistive structure of the two tectonic provinces crossed by the profile: a sedimentary basin and a cratonic shield. The apparent resistivity curves were found to depend significantly on both the model used and the distance to the center of the electrojet. These numerical results confirm the existence of a daytime source effect related to the equatorial electrojet. Furthermore, we show that the results account for the observed differences 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.Key words. Electromagnetics (Transient and time do- main) Geomagnetism and paleomagnetism (geomagne- tic induction) Ionosphere (equatorial ionosphere)