Equatorial Vertical Drift Research Articles

Deep Electron Density Depletion Near Sunset Terminator on St. Patrick's Day Storm and Its Impacts on Skywave Propagation

AbstractThe plasma density distribution in the equatorial and low latitude ionosphere can often be severely disturbed during active space weather events that can have paramount impacts on the long‐distance high frequency (HF) radio wave (Skywave) communications through the ionosphere. On the St. Patrick's Day storm of 17 March 2015, a deep depletion of plasma with more than two orders of magnitude was observed over a narrow longitudinal sector near the sunset terminator. The controlled SAMI2 (Sami2 is Another Model of the Ionosphere) simulations indicate that a large equatorial vertical drift around sunset terminator can produce such a deep electron density depletion and strong reinforcement of equatorial ionization anomaly. The impacts of these ionospheric density disturbances on the Skywave communication systems have been investigated using an HF propagation simulator that solves the propagation path of radio waves under given background ionospheric conditions. The results clearly demonstrate that the usable HF spectrum for Skywave communications is reduced by more than 50% over the region of depletion. Further, large areas of skip zones, where the Skywave signals are not receivable, are produced due to low ionospheric densities over this region. This study can have important applications in the planning and operation of Skywave systems during the active space weather periods.

Penetrating Electric Field Simulated by the MAGE and Comparison With ICON Observation

AbstractUsing the newly developed, Multiscale Atmosphere‐Geospace Environment (MAGE) model, we simulated the penetrating electric field in the equatorial region under different interplanetary magnetic field (IMF) BZ conditions during September 2020. Two intervals were selected for detailed analysis and the latter one was compared with the vertical ion drift data from the NASA Ionospheric Connection Explorer (ICON) satellite. The MAGE simulations show that in southward IMF (S‐IMF) cases, the dawn‐dusk electric potential drop at the equator is about 12% of the cross polar cap potential difference. Based on the MAGE simulation, the dawn‐dusk potential drop at the equator varies nearly instantaneously on the order of a few minutes with the changes in the IMF BZ or interplanetary electric field, which in turn alters the vertical ion drift. The daytime changes of the equatorial vertical ion drift in response to the penetrating electric field related to the IMF BZ are only half of that during the nighttime. ICON data, though not inconsistent with the simulation, were not able to verify the occurrence of penetrating electric field because of its unfavorable location at the time. The MAGE simulation shows a pre‐reversal enhancement (PRE) during southward IMF cases, but the PRE was absent in the ICON IVM observations. Further observations and modeling are needed to resolve this discrepancy.

Equatorial Plasma Drifts During the Magnetic Storm on November 7–11, 2004: Identifications of the Roles of Penetration and Disturbance Dynamo Electric Fields With Multi‐Instrumental Measurements

AbstractPenetration electric fields and disturbance dynamo electric fields are the two major disturbance electric fields in the equatorial ionosphere during geomagnetic storms. In this study, we analyze equatorial ion drifts measured by radar and satellites during the geomagnetic storm on November 7–11, 2004. The magnetic storm had two main phases, with minimum Dst values of −373 and −289 nT, respectively. The equatorial vertical ion drifts in the dusk‐evening sector were continuously enhanced in the upward direction for 13 h on November 7–8, including 7 h of the first main phase and 6 h of the early stage of the recovery phase. On November 10, the equatorial vertical ion drifts were continuously enhanced in the upward direction for ∼10 h, including ∼9 h of the second main phase and ∼1.5 h of the early stage of the recovery phase. Enhanced thermospheric nitric oxide 5.3 μm emission and westward disturbance winds were measured in the equatorial region during the two main phases of the magnetic storm, indicating occurrence of disturbance dynamo electric fields. The observations show that penetration electric fields are the dominant electric fields in the equatorial ionosphere for 13 and 10 h with continuous southward IMF even if disturbance winds already occur. Very large westward ion drifts are generated in the dusk‐evening equatorial region during the magnetic storm. It is suggested that the westward drifts are caused by penetration of electric fields associated with subauroral polarization streams.

Dependence of Penetration Electric Field Efficiency on the Interplanetary Electric Field

AbstractSouthward interplanetary magnetic field (IMF) causes the generation of penetration electric fields in the ionosphere at middle and low latitudes. Penetration efficiency, which is defined to be the ratio of the change of the equatorial ionospheric zonal electric field to the change of the dawn‐dusk (Ey) component of the interplanetary electric field, is used to represent the quantitative relationship between interplanetary and ionospheric electric fields. We have searched equatorial vertical ion drift data measured by the Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere (JULIA) radar over 19 years (2001–2019) for penetration electric fields when the correlation between interplanetary and ionospheric electric fields is greater than 0.8. In total, 450 penetration electric field events are identified. It is found that the penetration efficiency peaks near noon and decreases afterward. The penetration efficiency depends on the rate of change of Ey with time, the initial Ey when the IMF starts to turn southward, and the change in Ey between the beginning of the IMF southward turning and the subsequent northward turning. The initial Ey is well‐correlated with the change in Ey. A positive initial Ey or a smaller change in Ey results in a higher penetration efficiency. The quantitative dependence of penetration efficiency on the structures of Ey is derived for the first time. The results of this study are expected to improve the accuracy of prediction of penetration electric fields.

Atmosphere-Ionosphere (A-I) Coupling as Viewed by ICON: Day-to-Day Variability Due to Planetary Wave (PW)-Tide Interactions.

Coincident Ionospheric Connections Explorer (ICON) measurements of neutral winds, plasma drifts and total ion densities (:=Ne, electron density) are analyzed during January 1-21, 2020 to reveal the relationship between neutral winds and ionospheric variability on a day-to-day basis. Atmosphere-ionosphere (A-I) connectivity inevitably involves a spectrum of planetary waves (PWs), tides and secondary waves due to wave-wave nonlinear interactions. To provide a definitive attribution of dynamical origins, the current study focuses on a time interval when the longitudinal wave-4 component of the E-region winds is dominated by the eastward-propagating diurnal tide with zonal wavenumber s = -3 (DE3). DE3 is identified in winds and ionospheric parameters through its characteristic dependence on local solar time and longitude as ICON's orbit precesses. Superimposed on this trend are large variations in low-latitude DE3 wave-4 zonal winds (±40 ms-1) and topside F-region equatorial vertical drifts at periods consistent with 2-days and 6-days PWs, and a ~3-day ultra-fast Kelvin wave (UFKW), coexisting during this time interval; the DE3 winds, dynamo electric fields, and drifts are modulated by these waves. Wave-4 variability in Ne is of order 25%-35%, but the origins are more complex, likely additionally reflecting transport by ~20-25 ms-1 wave-4 in-situ winds containing strong signatures of DE3 interactions with ambient diurnal Sun-synchronous winds and ion drag. These results are the first to show a direct link between day-to-day wave-4 variability in contemporaneously measured E-region neutral winds and F-region ionospheric drifts and electron densities.

Radio and optical investigations of storm time evolution of post-midnight equatorial plasma bubbles (EPBs) and their drifts over Indian sector

We investigated the spatio-temporal evolution of disturbed time post mid-night Equatorial Plasma Bubbles (EPBs) using Canadian Advanced Digital Ionosonde (CADI) located at dip equatorial station, Tirunelveli (8.73°N, 77.7°E, 0.23°N Dip. Lat.), an all-sky imager (ASI) observations at low latitude station Panhala (16.48°N, 74.6°E, 11.1°N Dip. Lat.) and Gadanki Ionospheric Radar Interferometer (GIRI) at Gadanki (13.5°N, 79.2°E; 6.5°N Dip. Lat.) which is situated at few degrees towards east and south of Panhala on 02–03 February 2017 night. During this night, IMF Bz showed its periodic variation starting from 16:00 UT to 23:00 UT accompanied by decrease in SYM-H to as low as −35 nT indicating the onset of weak magnetic storm. The analyzed results suggested that cause of post-midnight EPBs could be due to manifestation of fluctuating eastward/westward electric field due to combined under-shielding/over-shielding Electric Fields and disturbance dynamo electric fields that led to rise and fall of the F-layer over dip equator. Interestingly, the EPBs over Panhala showed eastward motion initially that quickly reversed to westward later. Along with westward motion they also started growing until 21:30 UT. However, most of these EPBs disappeared with time except the one that started descending/shrinking towards southern side (i.e. towards equator). The rising and shrinking of EPBs is found to be fairly correlated with the equatorial vertical drifts. The westward drift of EPBs at Panhala and its anti-correlation with vertical drifts has been confirmed from CADI zonal/vertical drifts. Accordingly, the study also investigated the role of storm induced vertical Hall electric field as a possible cause for westward drifts and its anti-correlation with vertical drifts. However, GIRI observations do not show any significant westward drift on this night at Gadanki suggesting that there is a longitudinal gradient in the zonal drift of these EPBs. In addition to longitudinal drift reversal, the latitudinal gradient in zonal drifts also has been noticed. The present work highlights the role of storm induced disturbances in the generation and evolution of post-midnight EPBs which is believed to be triggered by weak magnetic disturbances in the deep low solar minimum.

On Estimation of Daytime Equatorial Vertical (E × B) Plasma Drifts Using Optical Neutral Dayglow Emission Measurements

AbstractVertical drift (E × B) over the geomagnetic equator of the Earth is one of the fundamental parameters that governs the equatorial electrodynamics, which is responsible for the formation of large‐scale phenomena in low and equatorial latitudes. We present a novel approach using ground‐based optical neutral oxygen dayglow emission intensity measurements to estimate the daytime equatorial vertical drifts. The diurnal patterns of the dayglow emission intensities at OI 557.7, 630.0, and 777.4 nm wavelengths obtained from a low‐latitude location, Hyderabad (78.4°E, 17.5°N; 8.9°N MLAT), India, are observed to be symmetric with respect to local noon on some days and asymmetric on some other. The extent of asymmetric nature is characterized by a parameter called Asymmetricity in Time (AT), which shows similar variations with the integrated Equatorial Electrojet (EEJ) strength. AT values obtained from the dayglow measurements are used to infer the vertical drifts. Vertical drifts derived by this method match well with the global climatological model estimates and with radar measurements. This new approach provides an alternative method for obtaining vertical drifts to enable systematic investigations of equatorial electrodynamics.

Spatial and Temporal Features of the Topside Ionospheric Electron Density by a New Model Based On GPS Radio Occultation Data

AbstractA new method for probing the spatial and temporal features of the topside ionosphere is presented. The Vary‐Chap model given by linear functions was used to discover and report features of the topside scale height and its gradient. Based on the global coverage of the radio occultation data, spherical harmonic functions were applied to detect some spatial features of the estimated topside. In addition, a Fourier time‐dependent method was applied in 10 consecutive years to both estimate and predict the temporal evolution of the topside. As a result, the temporal variation of the peak height showed high correlation with the scale height. And on the other hand, the electron density peak showed a strong anticorrelation with the gradient of the scale height. This suggests that the equatorial topside was mainly controlled by the E × B equatorial vertical drift, which increases the scale height in the equatorial region, and the diffusion of the electrons along the geomagnetic field lines, which reduces the gradient of the scale height. Also, a 1 year prediction with a reasonable accuracy showed that the proposed model can be considered a practical and useful tool for predicting features of the topside ionosphere, which may have special interest for the development of climatological models.

Nighttime thermospheric meridional winds as inferred from ionosonde parameters over Indian region and their plausible effects on plasma irregularities

In this paper, we present nighttime thermospheric meridional winds at Indian low latitude station namely Hyderabad during March–May 2013 derived using (a) h’F method and (b) hpF2 method based on ionosondes. The estimation of meridional winds using h’F method employs the basic principle that the vertical drift at magnetic equator is purely due to ExB drift, while for a station slightly away from magnetic equator, meridional wind also contributes for the vertical drift in addition to diffusion along the field lines. Assuming the EXB drifts are the same at both the locations, the vertical drift contribution is subtracted from the equatorial vertical drifts and then meridional winds are derived, whereas in hpF2 method, winds are derived using the SERVO theory. The winds so derived are compared with HWM07 wind model. The comparison suggests that magnitude of winds derived using h’F method are in better agreement with HWM07 model than hpF2 derived winds where their polarities are better comparable to HWM07 model than hpF2 method. The poor agreement of hpF2 method could be due to uncertainties involved in estimating various ionospheric parameters. Also the winds derived using h’F method showed signatures of the well known phenomenon of Midnight Temperature Maximum (MTM). Based on these results, we suggest that h’F method is better suited for meridional wind estimates over low latitudes than hpF2 method. Accordingly, we further investigated the role of thermospheric meridional winds in equatorial spread F (ESF) occurrence. Our results suggest that equatorward winds along with post sunset height increase could enhance the Rayleigh Taylor (RT) instability growth rate and thereby leading to the generation of ESF/scintillations. The Superposed Epoch Analysis (SEA) technique has been applied to understand the general trend of h’F and meridional winds around the ESF onset time. The study showed reduction in poleward winds, half an hour prior to the onset of ESF. Hence indicating the role of meridional winds in the generation of plasma irregularities/scintillations in addition to post sunset height rise.

Equatorial vertical drift modulation by the lunar and solar semidiurnal tides during the 2013 sudden stratospheric warming

AbstractDuring the 2013 stratospheric sudden warming (SSW) period the Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere (JULIA) radar at Jicarmarca, Peru, observed low‐latitude vertical drift modulation with lows of 0–12 m/s daytime maximum drifts between 6–13 and 22–25 January and enhanced drifts up to 43 m/s between 15 snd 19 January. The NCAR thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model reproduces the prevailing vertical drift feature and is used to examine possible causes. The simulations indicate that the modulation of the vertical drift is generated by the beating of the semidiurnal solar SW2 and lunar M2 tides. During the SSW period the beating is observable since the magnitudes of lunar and solar semidiurnal tidal amplitudes are comparable. The theoretical beating frequency between SW2 and M2 is 1/(15.13 day) which may be modified due to phase changes. This study highlights the importance of the lunar tide during SSW periods and indicates that the equatorial vertical drift modulation should be observable at other longitudes as well.

Causes of the longitudinal differences in the equatorial vertical E × B drift during the 2013 SSW period as simulated by the TIME‐GCM

AbstractDuring stratospheric sudden warming (SSW) periods large changes in the low‐latitude vertical drift have been observed at Jicamarca as well as in other longitudinal sectors. In general, a strengthening of the daytime maximum vertical drift with a shift from prenoon to the afternoon is observed. During the January 2013 stratospheric warming significant longitudinal differences in the equatorial vertical drift were observed. At Jicamarca the previously reported SSW behavior prevails; however, no shift of the daytime maximum drift was exhibited in the African sector. Using the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) the possible causes for the longitudinal difference are examined. The timing of the strong SSW effect in the vertical drift (15–20 January) coincides with moderate geomagnetic activity. The simulation indicates that approximately half of the daytime vertical drift increase in the American sector may be related to the moderate geophysical conditions (Kp = 4) with the effect being negligible in the African sector. The simulation suggests that the wind dynamo accounts for approximately 50% of the daytime vertical drift in the American sector and almost 100% in the African sector. The simulation agrees with previous findings that the migrating solar tides and the semidiurnal westward propagating tide with zonal wave number 1 (SW1) mainly contribute to the daytime wind dynamo and vertical drift. Numerical experiments suggest that the neutral wind and the geomagnetic main field contribute to the presence (absence) of a local time shift in the daytime maximum drift in the American (African) sector.

Daytime ionospheric equatorial vertical drifts during the 2008–2009 extreme solar minimum

AbstractOne of the most interesting observations made by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite mission was the detection of average equatorial ionospheric vertical drifts that largely differed from model predictions. C/NOFS measurements showed, in particular, downward drifts in the afternoon sector, and upward drifts around local midnight hours during the 2008 and 2009 extreme solar minimum. The unexpected behavior of the drifts has important implications for ionospheric modeling and suggests the necessity for a better understanding of the low‐latitude electrodynamics. We used ground‐based radar measurements to quantify the seasonal and solar flux variability of daytime equatorial drifts at lower altitudes (∼150 km) than those probed by C/NOFS (above ∼400 km). We found that average vertical drifts at 150 km altitude are in good agreement with model predictions of F region drifts and did not show the signatures of an enhanced semidiurnal pattern, as seen by C/NOFS. Comparison of the 150 km echo drifts with model predictions also shows that the increase (decrease) with height of the vertical drifts in the morning (afternoon) hours is a regular feature of the equatorial ionosphere. It occurred in all seasons and solar flux conditions between 2001 and 2011.

Ionospheric response to sudden stratospheric warming events at low and high solar activity

AbstractThe sensitivity of the ionospheric response to a sudden stratospheric warming (SSW) event has been examined under conditions of low and high solar activity through simulations using the whole atmosphere model (WAM) and the global ionosphere plasmasphere model (GIP). During non‐SSW conditions, simulated daytime mean vertical drifts at the magnetic equator show similar solar activity dependence as an empirical model. Model results of ionospheric total electron content (TEC) and equatorial vertical drift for the January 2009 major SSW, which occurred at very low solar activity conditions, show reasonable agreement with observations. The simulations demonstrate that the E region dynamo is capable of creating the semidiurnal variation of vertical drift. WAM and GIP were also run at high solar activity conditions, using the same lower atmosphere conditions as present in the January 2009 SSW event. The simulations indicate that the amplitude and phase of migrating tides in the dynamo region during the event have similar magnitudes for both solar flux conditions. However, comparing the ionospheric responses to a major SSW under low and high solar activity periods, it was found that the changes in the ionospheric vertical drifts and relative changes in TEC decreased with increasing solar activity. The simulations indicate that the F region dynamo becomes more important throughout the daytime and contributes to the upward drift in the afternoon during the event when the solar activity is higher. Our test simulations also confirm that the increase of the ionospheric conductivity associated with increasing solar activity is responsible for the decrease of vertical drift changes during an SSW. In particular, first, the increase in F region conductivity allows the closure of E region currents through the F region, reducing the polarization electric field before noon. Second, the F region dynamo contributes an upward drift postnoon, maintaining upward drifts till after sunset. The direct changes of the thermospheric wind at higher solar activity due to increased dissipation of the tides from the lower atmosphere are relatively minor and do not contribute greatly to the changes of ionospheric responses in the low‐latitude region.

TIME‐GCM study of the ionospheric equatorial vertical drift changes during the 2006 stratospheric sudden warming

This modeling study quantifies the daytime low‐latitude vertical E×B drift changes in the longitudinal wave number 1 (wn1) to wn4 during the major extended January 2006 stratospheric sudden warming (SSW) period as simulated by the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM), and attributes the drift changes to specific tides and planetary waves (PWs). The largest drift amplitude change (approximately 5 m/s) is seen in wn1 with a strong temporal correlation to the SSW. The wn1 drift is primarily caused by the semidiurnal westward propagating tide with zonal wave number 1 (SW1), and secondarily by a stationary planetary wave with zonal wave number 1 (PW1). SW1 is generated by the nonlinear interaction of PW1 and the migrating semidiurnal tide (SW2) at high latitude around 90–100 km. The simulations suggest that the E region PW1 around 100–130 km at the different latitudes has different origins: at high latitudes, the PW1 is related to the original stratospheric PW1; at midlatitudes, the model indicates PW1 is due to the nonlinear interaction of SW1 and SW2 around 95–105 km; and at low latitudes, the PW1 might be caused by the nonlinear interaction between DE2 and DE3. The time evolution of the simulated wn4 in the vertical E×B drift amplitude shows no temporal correlation with the SSW. The wn4 in the low‐latitude vertical drift is attributed to the diurnal eastward propagating tide with zonal wave number 3 (DE3), and the contributions from SE2, TE1, and PW4 are negligible.

Swarm SCARF equatorial electric field inversion chain

The day-time eastward equatorial electric field (EEF) in the ionospheric E-region plays a crucial 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 (EIA). Due to its importance, there is much interest in accurately measuring and modeling the EEF for both climatological and near real-time studies. The Swarm satellite mission offers a unique opportunity to estimate the equatorial electric field from measurements of the geomagnetic field. Due to the near-polar orbits of each satellite, the on-board magnetometers record a full profile in latitude of the ionospheric current signatures at satellite altitude. These latitudinal magnetic profiles are then modeled using a first principles approach with empirical climatological inputs specifying the state of the ionosphere. Since the EEF is the primary driver of the low-latitude ionospheric current system, the observed magnetic measurements can then be inverted for the EEF. This paper details the algorithm for recovering the EEF from Swarm geomagnetic field measurements. The equatorial electric field estimates are an official Swarm level-2 product developed within the Swarm SCARF (Satellite Constellation Application Research Facility). They will be made freely available by ESA after the commissioning phase.

Equatorial 150 km echoes and daytime F region vertical plasma drifts in the Brazilian longitude sector

Abstract. Previous studies showed that conventional coherent backscatter radar measurements of the Doppler velocity of the so-called 150 km echoes can provide an alternative way of estimating ionospheric vertical plasma drifts during daytime hours (Kudeki and Fawcett, 1993; Chau and Woodman, 2004). Using observations made by a small, low-power 30 MHz coherent backscatter radar located in the equatorial site of São Luís (2.59° S, 44.21° W; −2.35° dip lat), we were able to detect and monitor the occurrence of 150 km echoes in the Brazilian sector. Using these measurements we estimated the local time variation of daytime vertical ionospheric drifts in the eastern American sector. Here, we present a few interesting cases of 150 km-echoes observations made by the São Luís radar and estimates of the diurnal variation of vertical drifts. These cases exemplify the variability of the vertical drifts in the Brazilian sector. Using same-day 150 km-echoes measurements made at the Jicamarca Radio Observatory in Peru, we also demonstrate the variability of the equatorial vertical drifts across the American sector. In addition to first estimates of the absolute vertical plasma drifts in the eastern American (Brazilian) sector, we also present observations of abnormal drifts detected by the São Luís radar associated with the 2009 major sudden stratospheric warming event.

Longitudinal and day‐to‐day variability in the ionosphere from lower atmosphere tidal forcing

AbstractSimulations with the global ionosphere plasmasphere model driven by whole atmosphere model winds show significant longitudinal and day‐to‐day variations in the ionospheric parameters. Under fixed solar and geomagnetic activity levels, the contributions of lower atmosphere tides to the longitudinal and day‐to‐day variability in the upper atmosphere are estimated. Larger relative variability is found in the nighttime than in the daytime, which is consistent with observations. The perturbations from the lower atmosphere contribute about half of the observed variability in the ionospheric F2 peak plasma density under moderate solar activity and geomagnetic quiet conditions. The daily variability of the equatorial vertical plasma drifts is primarily driven by the day‐to‐day amplitude changes of the migrating semidiurnal tide, while the wave‐4 and wave‐3 longitudinal variations during September are dominated by the nonmigrating diurnal eastward propagating tides with zonal wave numbers 3 and 2, respectively.

Longitudinal and seasonal structure of the ionospheric equatorial electric field

AbstractThe 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.

High‐speed stream impacts on the equatorial ionization anomaly region during the deep solar minimum year 2008

To demonstrate high‐speed stream effects during the recent deep solar minimum year 2008, we have analyzed manually scaledfoF2 and hmF2 at Jicamarca and total electron content (TEC) in the equatorial ionization anomaly (EIA) region over the America longitudinal sector. Our results reveal that a prominent 9 day oscillation appears in the hmF2 and foF2 at the dip equator. The 9 day oscillation amplitudes of foF2are not always positively correlated with TEC in the equatorial ionosphere, and they show nonlinear dependence on the intensity of geomagnetic disturbances. With the outputs of Fejer and Scherliess's (1997) empirical model, we found that this periodicity is also present in equatorial vertical drifts caused by disturbance dynamoelectric field (DDEF) but absent in the drifts due to prompt penetration electric field (PPEF). DDEF effects on the equatorial periodic variations alone are not sufficient to explain the observed phenomena; other mechanisms, such as thermal expansion/contraction and neutral composition changes, are also the plausible causes of the periodic oscillation in the equatorial ionosphere. Further, the complicated patterns appear in the 9 day band‐pass‐filtered TEC perturbations in the EIA region, and they are quite different from the patterns of global coherent thermospheric oscillations triggered by high‐speed streams. We also found that the latitudinal variations of band‐passed‐filtered TEC present different behaviors involving tilt latitudinal configuration, antiphased correlation between the crests and trough, and south‐north asymmetry, which vary as a function of season, local time, or even from event to event.

Equatorial vertical plasma drifts and the measured and IRI model-predicted F 2-layer parameters above Ouagadougou during solar minimum

In this paper, hourly median value of ionosonde measurements: peak height F2-layer (hmF2), F2-layer critical frequency ( foF2) and propagation factor M(3000)F2 made at near-equatorial dip latitude, Ouagadougou, Burkina Faso (12°N, 1.5°W; dip: 1.5°N) and relevant F2-layer parameters: thickness parameter (Bo), electron temperature (Te), ion temperature (Ti), total electron content (TEC) and electron density (Ne) (at the fixed altitude of 300 km) provided by the International Reference Ionosphere (IRI) model for the longitude of Ouagadougou are contrasted with the IRI vertical drift model to explore in detail the monthly climatological behavior of equatorial ionosphere and the effects of equatorial electrodynamics on the diurnal structure of F2-layer parameters. The analysis period covers four months representative of solstitial and equinoctial seasonal periods during solar minimum year of 1987 for geomagnetically quiet-day. It is demonstrated that the month-by-month morphological patterns between vertical E × B drifts and F2-layer parameters range from worst to reasonably good and are largely seasonally dependent. A cross-correlation analysis conducted between equatorial drift and F2-layer characteristics yield statistically significant correlations for equatorial vertical drift and IRI-Bo, IRI-Te and IRI-TEC, whereas little or no acceptable correlation is obtained with observational evidence. Examination of the association between measured foF2, hmF2 and M(3000)F2 illustrates consistent much more smaller correlation coefficients with no systematic linkage.