Equatorial Electric Field Research Articles

Global Assimilation of Ionospheric Measurements (GAIM)

The ionosphere is a highly dynamic medium that exhibits weather disturbances at all latitudes, longitudes, and altitudes, and these disturbances can have detrimental effects on both military and civilian systems. In an effort to mitigate the adverse effects, we are developing a physics‐based data assimilation model of the ionosphere and neutral atmosphere called the Global Assimilation of Ionospheric Measurements (GAIM). GAIM will use a physics‐based ionosphere‐plasmasphere model and a Kalman filter as a basis for assimilating a diverse set of real‐time (or near real‐time) measurements. Some of the data to be assimilated include in situ density measurements from satellites, ionosonde electron density profiles, occultation data, ground‐based GPS total electron contents (TECs), two‐dimensional ionospheric density distributions from tomography chains, and line‐of‐sight UV emissions from selected satellites. When completed, GAIM will provide specifications and forecasts on a spatial grid that can be global, regional, or local. The primary output of GAIM will be a continuous reconstruction of the three‐dimensional electron density distribution from 90 km to geosynchronous altitude (35,000 km). GAIM also outputs auxiliary parameters, including NmF2, hmF2, NmE, hmE, and slant and vertical TEC. Furthermore, GAIM provides global distributions for the ionospheric drivers (neutral winds and densities, magnetospheric and equatorial electric fields, and electron precipitation patterns). In its specification mode, GAIM yields quantitative estimates for the accuracy of the reconstructed ionospheric densities.

Equatorial counterelectrojets during substorms

Equatorial counterelectrojet (CEJ) events are analyzed in association with changes in the interplanetary magnetic field (IMF), polar cap potential (PCP), and electric field measured in the equatorial ionosphere. In one event on 16 July 1995, the equatorial CEJ was observed at the afternoon dip equator during the recovery phase of the substorm when the IMF turned northward. Rapid decreases in the PCP and in the auroral electrojet occurred simultaneously with the equatorial CEJ, suggesting instantaneous equatorward penetration of the rapid decrease in the electric field associated with the region 1 field‐aligned currents (R1 FACs) under the condition of a well‐developed shielding electric field due to the R2 FACs. In the other event on 8 April 1993, the equatorial CEJ associated with the northward turning of the IMF was directly related to a rapid decrease in the equatorial electric field measured by the Jicamarca incoherent scatter radar as well as to a decrease in the PCP. We confirm the scenario for the substorm‐associated equatorial CEJ as caused by the dominant R2 FACs when the R1 FACs decrease abruptly because of the northward turning of the IMF. We also suggest that the DP 1 current system is composed of the Hall currents surrounding the R2 FACs and the equatorial CEJ closing with the R2 FACs, which are superposed on the DP 2 currents caused by the R1 FACs, being dominant when the IMF turns northward. The coherent occurrence of the electric field in the F region with the electric current in the E region at the equator is explained by applying the Earth‐ionosphere waveguide model of Kikuchi and Araki [1979b] as a most promising transmission mechanism. All the conditions for the equatorial CEJ most likely occur during the substorm, but the northward turning of the IMF and the resultant decrease in the PCP play a crucial role under the condition of well‐developed R2 FACs.

On the estimates of the ring current injection and decay

In the context of the space weather predictions, forecasting ring current strength (and of the Dst index) based on the solar wind upstream conditions is of specific interest for predicting the occurrence of geomagnetic storms. In the present paper, we have studied separately its two components: the Dst injection and decay. In particular, we have verified the validity of the Burton’s equation for estimating the ring current energy balance using the equatorial electric merging field instead of the original parameter V Bs (V is the solar wind speed and Bs is the southward component of the Interplanetary Magnetic Field, IMF). Then, based on this equation, we have used the phasespace method to determine the best-fit approximations for the ring current injection and decay as functions of the equatorial merging electric field (Em). Results indicate that the interplanetary injection is statistically higher than in previous estimations using V Bs. Specifically, weak but not-null ring current injection can be observed even during northward IMF, when previous studies considered it to be always zero. Moreover, results about the ring current decay indicate that the rate of Dst decay is faster than its predictions derived by using V Bs. In addition, smaller quiet time ring current and solar wind pressure corrections are contributing to Dst estimates obtained by Em instead of V Bs. These effects are compensated, so that the statistical Dst predictions using the equatorial electric merging field or using V Bs are about equivalent.

Ionospheric data assimilation: recovery of strong mid-latitudinal density gradients

The September 1999 Caribbean Ionospheric Campaign (CIC99), spanning the period 15–17 September 1999, was a period of repeated moderate geomagnetic storms. Both incoherent scatter radar (ISR) and digisonde F-layer measurements were made in the vicinity of Arecibo, Puerto Rico by the Arecibo ISR and Ramey Digisonde, which is one of the Digital Ionospheric Sounding System instruments. These data sets showed significant day-to-day F-layer variability, especially in the evening sectors. Local ionospheric data assimilation with the Assimilation Ionospheric Model (AIM1.06L) was able to reproduce this ionospheric day-to-day weather. However, during this time the Arecibo ISR was able to determine that strong latitude density gradients existed in the evening sector. These latitude gradients ranged from 5% to over 10% per degree of latitude, increasing equatorward. The climatological ionospheric forecast model (IFM) predicts gradients of the correct sign but significantly less than 5% per degree. Using the observed fact that the strong local vertical plasma drift was produced by an eastward electric field, an enhanced equatorial electric field model was created. This model was used to drive the IFM-E model. The effect of the enhanced electric field is to drive plasma flux tubes to higher latitudes and effectively move the poleward shoulder of the equatorial anomalies to higher latitudes. In this simulation, the simulated Arecibo density gradients range from 10% to 20% per degree. In addition, the local densities at Arecibo are now higher than the observed ones in the evening sector. The irony of this study is that the local AIM1.06L assimilation provides the correct local densities in the F region but is based upon the assumption of corotating field lines. When the plasma flux tubes are free to be E × B/B 2 convected, as in the IFM equator model simulations, the local densities are too high. However, gradients in latitude are more realistic although somewhat larger than observed. This latter simulation is based on “better” physics than the AIM1.06L, but because the latitude (apex altitude) distribution of the eastward electric field driver is uncertain, it produces poor assimilation results. This study points to the need for more extended latitude coverage in the CIC campaigns in order to address the issue of a minimum data requirement for the assimilation.

Quasi‐steady drift paths in a model magnetosphere with AMIE electric field: Implications for ring current formation

Recent measurements show that magnetospheric convection electric fields during the main phases of magnetic storms are much more complicated in spatial structure than the electric fields that have generally been used to model formation of the stormtime ring current. To investigate the transport effects of such more realistic stormtime electric fields on magnetospheric charged particles, we map the Assimilative Model of Ionospheric Electrodynamics (AMIE) electric potential functions analytically with a simple magnetic field model at selected times of interest during several magnetic storms from 1997–1998 and during the extremely large storm of July 2000. We calculated corresponding contours of constant Hamiltonian (kinetic plus potential energy) for first invariant values that range from 0 to 30 MeV/G (representative of equatorially mirroring cold plasma, ring current, and radiation‐belt ions and electrons). These equatorial contours would constitute drift paths if the electric field were truly constant in time. We thus calculated how far along such quasi‐drift paths the corresponding particles would have drifted after specific amounts of time, and we compare these quasi‐drift characteristics with those obtained from a simple semiempirical model of the convection electric field. We find considerable variability among stormtime equatorial quasi‐drift paths, reflecting the known variability of AMIE equipotentials. Patterns of equatorial quasi‐drift in the simplified electric field model are (by construction) symmetric about the dawn‐dusk meridian. During the main phases of storms, the equatorial electric field derived from AMIE tends to be strongest in an MLT sector several hours wide on the night side. This concentration of AMIE equipotentials provides a channel for rapid transport (requiring ∼20–30 min) for ions with first invariant values representative of the ring current population from the nightside neutral line to low L values (∼3–4) near dusk, where the partial ring current forms. During the extremely large “Bastille Day” storm of 15 July 2000 (minimum Dst = −300 nT) the drift patterns derived from AMIE show penetration of ions to as low as L ∼ 2. This deep penetration of ring current ions could help to account for the very strong ring current that was observed during this storm. The quasi‐steady state drift properties help us anticipate the implications of a more realistic electric field model for the particle drifts that lead to the formation of the stormtime ring current.

Long‐lasting disturbances in the equatorial ionospheric electric field simulated with a coupled magnetosphere‐ionosphere‐thermosphere model

The Magnetosphere‐Thermosphere‐Ionosphere‐Electrodynamics General Circulation model of Peymirat et al. [1998] is used to investigate ionospheric‐wind‐dynamo influences on low‐latitude ionospheric electric fields during and after a magnetic storm. Simulations are performed with time‐varying polar cap electric potentials and an expanding and contracting polar cap boundary. Three influences on equatorial electric fields can be of comparable importance: (1) global winds driven by solar heating; (2) direct penetration of polar cap electric fields to the equator that are partially shielded by the effects of Region‐2 field‐aligned currents; and (3) disturbance winds driven by high‐latitude heating and ion‐drag acceleration. The first two influences tend to have similar magnetic local time (MLT) variations in a steady state, while the disturbance‐wind influence tends to have the opposite MLT variations. The nighttime disturbance winds at upper midlatitudes that affect the global ionospheric wind dynamo are predominantly westward after the simulated magnetic storm. The nighttime winds drive an equatorward dynamo current that tends to charge the low‐latitude ionosphere positively around midnight, which can lead to reductions or reversals of the normal equatorial night‐side east‐west electric fields. The simulations partly support the theories of the so‐called “disturbance dynamo” [Blanc and Richmond, 1980] and “fossil wind” [Spiro et al., 1988], both of which predict long‐lasting disturbances in the equatorial eastward electric field associated with magnetic storms. However, the simulations do not support the element of fossil wind theory that links the disturbance‐wind influence on equatorial electric fields to polar cap contraction following the storm. The simulations show a stronger wind‐produced enhancement of steady state shielding than predicted by the model of Forbes and Harel [1989], due to the fact that the disturbance winds extend well equatorward of the Region‐2 currents.

Global ENA observations of the storm mainphase ring current: Implications for skewed electric fields in the inner magnetosphere

The local time distribution of the ring current in the 27–119 keV range during several geomagnetic storm main phases have been investigated. The data was obtained by the high energy neutral atom (HENA) imager onboard IMAGE. Global proton distributions are derived from the observed energetic neutral atom (ENA) images using a linear inversion technique. During storms with low IMF Bythe peak of the proton distribution is around 01 MLT. For storms with high IMF Bythe peak can rotate to dawn. The rotation angle depends on solar wind velocity and interplanetary magnetic field (IMF) By, but less on IMF Bz. We discuss how this morphology implies the existence of strong and skewed equatorial electric fields in the inner magnetosphere. Our results are consistent with in‐situ ring current measurements, radar observations and with kinetic models that self‐consistently calculate the electric field produced by the closure of the partial ring current.

By‐dependent prompt penetrating electric fields at the magnetic equator

We propose a new type of prompt penetrating electric field effect which seems to be related to the y‐component of the interplanetary magnetic field and the NBZ current system. We report an equatorial electric field pattern which is not consistent with present understanding of prompt penetrating electric fields. The associated unbalanced excess NBZ current in one hemisphere partially flows across the nighttime equatorial zone. We find that a current system closes perpendicular to the ecliptic plane in the solar wind with a direction opposite to the interplanetary electric field. This is required for an MHD generator located in the solar wind.

Electrodynamic coupling of high and low latitudes: Simulations of shielding/overshielding effects

The penetration of the electric field and associated magnetic perturbations from high latitudes to low latitudes is studied with the Magnetosphere‐Thermosphere‐lonosphere‐Electrodynamics General Circulation Model (MTlEGCM) of Peymirat et al. [1998] in response to variations of the polar cap potential drop. For a sudden decrease of the polar cap potential of ∼ 40 kV, the model reproduces the well‐known overshielding phenomenon corresponding to a sudden reversal of the low‐latitude electric field. For quasi‐periodic oscillations of the polar cap potential drop of ∼ 40 min period, the model predicts that the poleward electric field and the eastward Hall current in the auroral zone lag slightly in phase (< 1 min), while the eastward electric field and current at the magnetic equator are advanced slightly in phase, with respect to the potential‐drop oscillations. These phase differences are interpreted as the consequence of the succession of shielding and overshielding episodes induced by the response of the region‐2 field‐aligned currents to the polar cap variations. The phase differences among the polar cap potential and the auroral and equatorial electric fields and currents increase with the plasma sheet pressure. The amplitude of the associated magnetic perturbations is very dependent on the distribution of the potential along the polar cap boundary. The model predictions are tested against the observations of Kobea et al. [this issue] for two events, one representing simple overshielding and the other associated with polar cap potential oscillations. The model underestimates the decay time of the magnetic perturbations by a factor of 2 during the overshielding event, and the model gives results compatible with the observations during the second event. The disagreements may be due to limitations of the model and uncertainties of the input parameters.

Self‐consistent quasi‐static radial transport during the substorm growth phase

We develop a self‐consistent description of the slowly changing magnetic configuration of the near‐Earth plasma sheet (NEPS) during substorm growth phase. This new approach is valid for quasi‐static fluctuations ω < k‖vA (vA being the Alfvén velocity), with characteristic frequency lower than the bounce frequencies of electrons and ions (ω < ωbi,ωbe), and for spatial scales larger than the ion Larmor radius. The basic equations are obtained from a linearization of the cyclotron and bounce‐averaged Vlasov equation, together with Maxwell equations. The Vlasov‐Maxwell system of equations is solved for the quasi‐dipolar NEPS region. Using a 2‐D dipole for the equilibrium, we calculate analytically the perturbed components of the electromagnetic field as a function of an external forcing current. The quasi‐neutrality condition (QNC) is solved via an expansion in the small parameter Te/Ti (Te/Ti is the ratio between the electronic and ionic temperatures). To the lowest order in Te/Ti, we find that the enforcement of QNC implies the presence of a global electrostatic potential which is constant for a given magnetic field line but varies across the magnetic field. The corresponding electric field shields the effect of the inductive component of the electric field, thereby producing a partial reduction of the motion that would correspond to the inductive electric field. Furthermore, we show that enforcing the QNC implies a field‐aligned potential drop which is computed to the next order in Te/Ti in a companion paper [Le Contel et al., this issue]. In the present paper, we show that the direction of the azimuthal electric field varies along the field line, thus the equatorial electric field cannot be mapped onto the ionosphere. Furthermore during the growth phase, the (total) azimuthal electric field is directed eastward, close to the equator, and westward, off‐equator. Thus large equatorial pitch angle particles drift tailward, whereas small pitch angle particles drift earthward.

Disturbance daily variation of electrojet current at Indian longitude sector

Using the hourly mean data of the horizontal ( H) and vertical ( Z) components of the geomagnetic field at the set of nine observatories in India, it has been found that the Disturbance Daily Variation (SD) of H shows a prominent midday depression over the magnetic equator of the H field and a midday increase of Z field at stations near the northern fringe of the equatorial electrojet belt. The magnetic disturbance introduces a westward electric field over the equatorial region, causing a band of westward ionospheric current over the magnetic equator during the day time hours. The latitudinal extent of the disturbance time counter electrojet current seems to be larger than that of quiet time normal electrojet current. This suggests a systematic westward electric field superposed on the normal Sq field at low latitude ionosphere during disturbed periods, the source of which has to be clearly defined. Further correlative data analysis is required to isolate these sources of the disturbed equatorial electric fields.

Letter to the editor: Electric field fluctuations (25-35 min) in the midnight dip equatorial ionosphere

Abstract. Measurements with a HF Doppler sounder at Kodaikanal (10.2°N, 77.5°E, geomagnetic latitude 0.8°N) showed conspicuous quasi-periodic fluctuations (period 25-35 min) in F region vertical plasma drift, Vz in the interval 0047-0210 IST on the night of 23/24 December, 1991 (Ap = 14, Kp < 4-). The fluctuations in F region vertical drift are found to be coherent with variations in Bz (north-south) component of interplanetary magnetic field (IMF), in geomagnetic H/X components at high-mid latitude locations both in the sunlit and dark hemispheres and near the dayside dip equator, suggestive of DP2 origin. But the polarity of the electric field fluctuations at the midnight dip equator (eastward) is the same as the dayside equator inferred from magnetic variations, contrary to what is expected of equatorial DP2. The origin of the coherent occurrence of equatorial electric field fluctuations in the DP2 range of the same sign in the day and night hemispheres is unclear and merits further investigations.Key words: Ionosphere (electric fields and currents; equatorial ionosphere; ionosphere-magnetosphere interactions)

Electric field fluctuations (25–35 min) in the midnight dip equatorial ionosphere

Measurements with a HF Doppler sounder at Kodaikanal (10.2∞N, 77.5∞E, geomagnetic latitude 0.8∞N) showed conspicuous quasi-periodic fluctuations (period 25-35 min) in F region vertical plasma drift, Vz in the interval 0047-0210 IST on the night of 23/24 December, 1991 (Ap = 14, Kp <4 ) ). The fluctuations in F region vertical drift are found to be coherent with variations in Bz (north-south) component of interplan- etary magnetic field (IMF), in geomagnetic H/X com- ponents at high-mid latitude locations both in the sunlit and dark hemispheres and near the dayside dip equator, suggestive of DP2 origin. But the polarity of the electric field fluctuations at the midnight dip equator (eastward) is the same as the dayside equator inferred from magnetic variations, contrary to what is expected of equatorial DP2. The origin of the coherent occurrence of equatorial electric field fluctuations in the DP2 range of the same sign in the day and night hemispheres is unclear and merits further investigations.

Analysis and modeling of the GLO-1 observations of meteoric metals in the thermosphere

The GLO-1 experiment measured the radiance of ambient atmospheric species and of meteoric metals, including Na, Mg, Mg + and Ca +, along lines of sight with tangent altitudes between 120–350 km. The results confirm earlier observations of a strong dawn/dusk asymmetry in thermospheric ion density and the concentration of ion density near the geomagnetic equator. The data also show a substantial amount of neutral Na in the thermosphere, with a dawn/dusk asymmetry similar to the ions. We observe the presence of Ca + and Mg + in the relative abundance comparable to the abundance ratio in meteoric material and in the sun. We are also able to simultaneously observe Mg and Mg + in a few cases, over a range of altitudes. We have developed a model in an attempt to better understand these features. The one–dimensional model realistically and comprehensively incorporates the deposition of cosmic dust, transport by the equatorial electric field and diffusion, and ion and neutral chemistry specific to the three metal species studied. The model is driven by a single source function for dust influx, so that it can be used to predict the relative amounts of one metal species to another. These features of the model allow for diurnal variations and variations in latitude, reproducing the observations reasonably well on average.

Global transport and localized layering of metallic ions in the upper atmospherer

Abstract. A numerical model has been developed which is capable of simulating all phases of the life cycle of metallic ions, and results are described and interpreted herein for the typical case of Fe+ ions. This cycle begins with the initial deposition of metallics through meteor ablation and sputtering, followed by conversion of neutral Fe atoms to ions through photoionization and charge exchange with ambient ions. Global transport arising from daytime electric fields and poleward/ downward di.usion along geomagnetic field lines, localized transport and layer formation through de- scending convergent nulls in the thermospheric wind field, and finally annihilation by chemical neutralization and compound formation are treated. The model thus sheds new light on the interdependencies of the physical and chemical processes a.ecting atmospheric metallics. Model output analysis confirms the dominant role of both global and local transport to the ion's life cycle, showing that upward forcing from the equatorial electric field is critical to global movement, and that diurnal and semidiurnal tidal winds are responsible for the forma- tion of dense ion layers in the 90±250 km height region. It is demonstrated that the assumed combination of sources, chemical sinks, and transport mechanisms actually produces F-region densities and E-region layer densities similar to those observed. The model also shows that zonal and meridional winds and electric fields each play distinct roles in local transport, whereas the ion distribution is relatively insensitive to reasonable variations in meteoric deposition and chemical reaction rates.Key words. Ionosphere (ion chemistry and composition; ionosphere-atmosphere interactions).

Global transport and localized layering of metallic ions in the upper atmosphere

A numerical model has been developed which is capable of simulating all phases of the life cycle of metallic ions, and results are described and interpreted herein for the typical case of Fe+ ions. This cycle begins with the initial deposition of metallics through meteor ablation and sputtering, followed by conversion of neutral Fe atoms to ions through photoionization and charge exchange with ambient ions. Global transport arising from daytime electric fields and poleward/ downward di.usion along geomagnetic field lines, localized transport and layer formation through de- scending convergent nulls in the thermospheric wind field, and finally annihilation by chemical neutralization and compound formation are treated. The model thus sheds new light on the interdependencies of the physical and chemical processes a.ecting atmospheric metallics. Model output analysis confirms the dominant role of both global and local transport to the ion's life cycle, showing that upward forcing from the equatorial electric field is critical to global movement, and that diurnal and semidiurnal tidal winds are responsible for the forma- tion of dense ion layers in the 90±250 km height region. It is demonstrated that the assumed combination of sources, chemical sinks, and transport mechanisms actually produces F-region densities and E-region layer densities similar to those observed. The model also shows that zonal and meridional winds and electric fields each play distinct roles in local transport, whereas the ion distribution is relatively insensitive to reasonable variations in meteoric deposition and chemical reaction rates.Key words. Ionosphere (ion chemistry and composition; ionosphere-atmosphere interactions).

Coupling and energetics of the equatorial ionosphere-thermosphere system: advances during the STEP period

The equatorial ionosphere-thermosphere system (EITS) investigations during the STEP period (1990-1997) were focused on improving our understanding of the dymanic and electrodynamic-coupling process and energetics that govern the climatology of the system, as well as the variabilities of the system arising from forcing by magnetospheric and high latitude processes and by atmospheric waves from below. Thermosphere dynamics and dynamo electric fields serving as drivers of the coupling process are responsible for the major EITS phenomena and their variabilities at different time scales. Especially, the day-to-day variabilities of the equatorial spread F (ESF) have recieved specific attention because of interest in space application areas as well Significant advances were achieved in our understanding and representation of quiet and disturbed electric fields. The STEP period also marked notable improvement in the experimental diagnostic facilities available in the equatorialregions as well as in theoretical modeling of the interactive process that control the major EITS phenomena. Data from coordinated observational campaigns have contributed to a better understanding of the EITS global responses to magnetospheric disturbances. The advances to be briefly discussed in this paper concern most of the major phenomena and the inherent characteristics of the EITS: the equatorial electric field and its sunset enhancement; thermospheric winds and temperature including the MTM (midnight temperature maximum); peculiar features of the ionosphere in the immediate vicinity of the magnetic equator; equatorial spread F/plasma bubble erregularities, including the energy requirement for large scale field aligned irregularities; equatorial anomalies in ion/neutral densities and temperatures. Also presented briefly is an overview of the results on the disturbance characteristics of the key EITS parameters and driving forces under magnetospheric disturbances and atmospheric wave forcing.

Typical disturbances of the daytime equatorial F region observed with a high-resolution HF radar

Abstract. HF radar measurements were performed near the magnetic equator in Africa (Korhogo 9°24'63''N–5°37'38''W) during the International Equatorial Electrojet Year (1993–1994). The HF radar is a high-resolution zenithal radar. It gives ionograms, Doppler spectra and echo parameters at several frequencies simultaneously. This paper presents a comparative study of the daytime ionospheric structures observed during 3 days selected as representative of different magnetic conditions, given by magnetometer measurements. Broad Doppler spectra, large echo width, and amplitude fluctuations revealed small-scale instability processes up to the F-region peak. The height variations measured at different altitudes showed gravity waves and larger-scale disturbances related to solar daytime influence and equatorial electric fields. The possibility of retrieving the ionospheric electric fields from these Doppler or height variation measurements in the presence of the other possible equatorial ionospheric disturbances is discussed.

A substorm model with onset location tied to an auroral arc

Two important observational constraints on substorm onset theories are that (1) substorm onset occurs on field lines of a pre‐existing auroral arc, and (2) reduction of the cross‐tail current occurs in the near‐Earth magnetotail. Recent progress in linking generation of at least some auroral arcs to field line resonances (FLRs) and current disruption to possibly the cross‐field current instability (CCI) suggests a synergistic combination of these two theories. We suggest that as the cross‐tail current sheet thins to the extent that ions become unmagnetized at the end of substorm growth phase, the equatorial electric field associated with FLRs can amplify significantly the cross‐tail current to exceed the instability threshold. This combined theory naturally accounts for the observational constraints of substorm onset starting on a pre‐existing auroral arc and linking closely with current reduction in the near‐Earth magnetotail. It is compatible with the observation of the most equatorward auroral arc being the most likely one for initial brightening because the cross‐tail current is most intense in its near‐Earth end and is therefore most likely to exceed the unstable threshold for CCI. It is also consistent with auroral precipitation fading to be a common feature before an auroral breakup. Other predictions yet to be tested are that (1) the onset is more favorable for the phase of the resonance in which the resonance‐associated field‐aligned current is directed from the ionosphere to the magnetosphere, and (2) the onset is more favorable poleward of the resonance shell or auroral arc.

Typical disturbances of the daytime equatorial F region observed with a high-resolution HF radar

HF radar measurements were performed near the magnetic equator in Africa (Korhogo 9°24'63''N–5°37'38''W) during the International Equatorial Electrojet Year (1993–1994). The HF radar is a high-resolution zenithal radar. It gives ionograms, Doppler spectra and echo parameters at several frequencies simultaneously. This paper presents a comparative study of the daytime ionospheric structures observed during 3 days selected as representative of different magnetic conditions, given by magnetometer measurements. Broad Doppler spectra, large echo width, and amplitude fluctuations revealed small-scale instability processes up to the F-region peak. The height variations measured at different altitudes showed gravity waves and larger-scale disturbances related to solar daytime influence and equatorial electric fields. The possibility of retrieving the ionospheric electric fields from these Doppler or height variation measurements in the presence of the other possible equatorial ionospheric disturbances is discussed.