Equatorial Counter Electrojet Research Articles

The impact of the Hunga Tonga–Hunga Ha’apai volcanic eruption on the Peruvian atmosphere: from the sea surface to the ionosphere

The eruption of the Hunga Tonga Hunga Ha’apai volcano on 15 January 2022 significantly impacted the lower and upper atmosphere globally. Using multi-instrument observations, we described disturbances from the sea surface to the ionosphere associated with atmospheric waves generated by the volcanic eruption. Perturbations were detected in atmospheric pressure, horizontal magnetic field, equatorial electrojet (EEJ), ionospheric plasma drifts, total electron content (TEC), mesospheric and lower thermospheric (MLT) neutral winds, and ionospheric virtual height measured at low magnetic latitudes in the western South American sector (mainly in Peru). The eastward Lamb wave propagation was observed at the Jicamarca Radio Observatory on the day of the eruption at 13:50 UT and on its way back from the antipodal point (westward) on the next day at 07:05 UT. Perturbations in the horizontal component of the magnetic field (indicative of EEJ variations) were detected between 12:00 and 22:00 UT. During the same period, GNSS-TEC measurements of traveling ionospheric disturbances (TIDs) coincided approximately with the arrival time of Lamb and tsunami waves. On the other hand, a large westward variation of MLT winds occurred near 18:00 UT over Peru. However, MLT perturbations due to possible westward waves from the antipode have not been identified. In addition, daytime vertical plasma drifts showed an unusual downward behavior between 12:00 and 16:00 UT, followed by an upward enhancement between 16:00 and 19:00 UT. Untypical daytime eastward zonal plasma drifts were observed when westward drifts were expected. Variations in the EEJ are highly correlated with perturbations in the vertical plasma drift exhibiting a counter-equatorial electrojet (CEEJ) between 12:00 and 16:00 UT. These observations of plasma drifts and EEJ are, so far, the only ground-based radar measurements of these parameters in the western South American region after the eruption. We attributed the ion drift and EEJ perturbations to large-scale thermospheric wind variations produced by the eruption, which altered the dynamo electric field in the Hall and Pedersen regions. These types of multiple and simultaneous observations can contribute to advancing our understanding of the ionospheric processes associated with natural hazard events and the interaction with lower atmospheric layers.Graphical

Prompt penetration effects on Equatorial Electrojet from the Indian sector

The morphology of prompt penetration effects (PP) in the Equatorial Electrojet (EEJ) at Vencode (VEN, Geomagnetic latitude 0.29ºN) was studied using 5-min samples of EEJ and interplanetary Ey component data from 2011 to 2015. In contrast to previous studies that mainly focused on storm-time PP events, our study investigated the characteristics of PP effects on EEJ during both quiet and disturbed periods. Our findings reaffirm earlier reports of high PP efficiency during local noon for both quiet and disturbed days. Moreover, PP amplitudes were greater when IMF-Bz was oriented northward compared to southward, indicating the presence of overshielding effects even during quiet conditions. Additionally, we examined seasonal variations in PP amplitudes and observed weaker PP during the solstice compared to the equinox. Furthermore, our study analysed the equatorial counter electrojet (CEJ) caused by PP events associated with northward-turning IMF-Bz. We report 15% of CEJs at VEN associated with PP effects; a few PP events were observed while the equatorial electric field at VEN was already westward (i.e. PP within CEJ), indicating the combined effects of magnetospheric and ionospheric perturbations. Identifying CEJs caused by PP during quiet days helps to differentiate between magnetospheric and ionospheric mechanisms/processes. Additionally, we investigated the spatial variability in PP amplitudes at closely spaced sites, utilizing one year of concurrent data from Minicoy (MNC, Geomagnetic latitude 0.19ºN), VEN, and Campbell Bay (CBY, Geomagnetic latitude 0.29ºN) situated at geographic longitudes 72º, 77º and 93º, respectively. Our results showed significant differences in PP amplitudes between VEN-CBY and MNC-CBY, separated by 15º and 20º longitude, respectively. Observations from the three equatorial sites demonstrated a longitudinal trend, with PP amplitudes increasing westward towards MNC, inverse to the amplitudes of EEJ.Graphical

Validation of equatorial electrojet derived from Swarm observations using ground based magnetometers

This paper presents the analysis of the Equatorial electrojet (EEJS) and Counter equatorial electrojet (CEJS) at quiet times (Kp⩽3) deduced from the absolute scalar magnetometers (ASM) of the Swarm satellites. The EEJS and CEJS were compared with their corresponding values, EEJG and CEJG respectively, from ground magnetometers in the South American, West African, and Asian sectors during the years 2014–2018. The results show that EEJS highly correlated with EEJG, over South American, West African and Asian sectors with correlation coefficients of 0.91, 0.77 and 0.92 respectively. CEJS with CEJG also showed a reasonably good correlation of 0.78, 0.98 and 0.83 over South American, West African and Asian sectors respectively. For the first time, the Swarm observations of CEJ and EEJ are validated against CEJ and EEJ from ground magnetometers, over South America, West Africa and Asia. Both the occurrence rates of CEJS and CEJG show similar diurnal and seasonal trends, with the highest occurrence rates in the morning (7:00–9:00 LT) and evening (15:00–19:00 LT) times and the lowest occurrence rates around noon (11:00 to 12:00 LT) time. The seasonal occurrence rates of CEJS and CEJG were high in the solstice months and the lowest rates were during equinox months for all the sectors. The longitudinal variability of CEJS and CEJG show highest occurrence rates in the West African sector and the lowest occurrence rates in the Asian sector. This longitudinal variability of CEJS and CEJG occurrence rates were attributed to the effect of the non-migrating tides, migrating tides and stationary planetary waves. The annual occurrence rates of CEJS and CEJG seemed anti-correlated with F10.7.

On the behavior of thermospheric O1D 630.0 nm dayglow emission during counter electrojet events

This study presents the response of thermospheric O1D 630.0 nm dayglow emission to the variability associated with equatorial Counter Electrojet (CEJ) events. The analysis based on the data from a meridian scanning Dayglow Photometer, Digital Ionosonde and Proton Precession Magnetometer over Trivandrum (8.5°N, 77°E, 0.5°dip lat.), indicates that the O1D 630.0 nm emission behave distinctly different during the CEJ events compared to that on normal days. It has been observed that O1D 630.0 nm emission shows enhancement during the negative excursion of the ΔH, followed by an unusual depletion during the peak CEJ time. The observed variability was found to be more pronounced in a latitudinal region of ±3° centered at around the dip equator. In addition, the emission intensities also exhibit the presence of enhanced short period oscillations of periodicity 20–30 min during the CEJ events. Analysis of the data from the collocated ionosonde revealed that the F-region electron density showed enhancement during the early phase of the CEJ and a decrease during the peak CEJ. Further, the simulation studies using a Quasi 2 dimensional ionospheric model showed that the modified plasma fountain during the CEJ can alter the plasma density at the emission centroid. The study reveals a strong dynamical coupling between the E and F-region of the dip equatorial ionosphere.

Effects of Subauroral Polarization Streams on the Equatorial Electrojet During the Geomagnetic Storm on 1 June 2013: 2. The Temporal Variations

AbstractUsing ground‐based magnetic field measurements and numerical simulations from the Thermosphere‐Ionosphere Electrodynamic General Circulation Model (TIEGCM), a first paper (Zhang, Yamazaki, et al., 2021, doi: https://doi.org/10.1029/2021JA029681; under review) introduced the potential roles of disturbance dynamo electric field due to subauroral polarization streams (SAPS) on the equatorial electrojet (EEJ) during a moderate geomagnetic storm on 1 June 2013. Our second study investigated the temporal responses of equatorial electrojet to SAPS. At noon, the residual EEJ (ΔEEJ) owing to SAPS flows westward, that is, counter equatorial electrojet (CEJ). The temporal variation of CEJ excited by the dynamo electric field was basically consistent with that by SAPS, and the effects of zonal wind were larger than those of meridional wind. The relative time delay of CEJ and SAPS was related to the propagation time of disturbance wind from mid‐latitudes to low‐latitudes. It took 2–3 hr for SAPS‐related disturbance wind to propagate to the equatorial region and change the polarity of EEJ. The influence of meridional winds on the temporal variations of ΔEEJ is related to the generation of eastward currents at mid‐latitudes, which can accumulate the positive charges at dusk terminator and then generate a westward electric field at lower latitudes.

Ionospheric Electrodynamic Response to Solar Flares in September 2017

AbstractsIn this work, the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model is used to investigate the responses of ionospheric electrodynamic processes to the solar flares at the flare peaks and the underlying physical mechanisms on September 6 and 10, 2017. Simulations show that solar flares increased global daytime currents and reduced the eastward electric fields during the daytime from the equator to middle latitudes. Furthermore, westward equatorial electric fields and equatorial counter electrojets occurred in the early morning. At the flare peak, these electrodynamic responses are predominantly related to the enhanced E‐region conductivity by flares, as the responses of neutral winds and F‐region conductivity to flares are negligible. Specifically, the Cowling conductance enhancement is not the major process causing the reduction of zonal electric fields. This electric field reduction is primarily associated with the decrease of the ratio between the field line‐integrated wind‐driven currents and the conductance. The flare‐induced conductivity enhancement is larger but the background wind speed is smaller in the E‐region than in the F‐region, as a result, the increase of total integrated wind‐driven currents is less than the conductance enhancement.

Dependence of the Equatorial Electrojet on Auroral Activity and In Situ Solar Insulation

AbstractBy using 10 years of Challenging Minisatellite Payload satellite observations, we investigate the average conditions of the interplanetary magnetic field (IMF) prevailing during the westward counter equatorial electrojet (CEJ). Equally, we compared the average IMF conditions accompanying high‐latitude field‐aligned currents of the Region 1 (R1) and Region 2 (R2). It shows that both CEJ and high‐latitude field‐aligned currents events when R2 is greater than R1 tend to happen preferably during the northward turning of the IMF Bz and the substorm recovery phase. Sunlight has an important influence on the longitudinal distribution of the equatorial electrojet (EEJ), and the effect is opposite to the tidal electric field at region. The anticorrelation between cos (SZA) (solar zenith angle effect) average values during CEJ events and EEJ intensity is most prominent around June solstice. By using combined measurements from Challenging Minisatellite Payload and DMSP satellites, it is found that before the occurrence and in the initial phase of a subauroral polarization stream the EEJ gets enhanced, and after about 30 min it reduces in intensity. The CEJ occurrence rate more than doubles during subauroral polarization stream periods compared to normal conditions.

Longitudinal variability of the equatorial counter electrojet during the solar cycle 24

Ground and space-based geomagnetic data were used in the investigation of the longitudinal, seasonal and lunar phase dependence of the equatorial counter electrojet (CEJ) occurrence in the Peruvian, Brazilian, African, Indian and Philippine sectors during geomagnetically quiet days from the solar cycle 24 (2008 to 2018). We found that CEJ events occur more frequently during the morning (MCEJ) than in the afternoon (ACEJ). The highest MCEJ and ACEJ occurrence rates were observed for the Brazilian sector. Distinct seasonal dependence was found for each longitudinal sector under investigation. The lunar phase dependence was determined for the first time for the Philippine sector (longitude 125°E), and it was shown to be less pronounced than in the Peruvian, Brazilian and African sectors. We demonstrate that differences in CEJ rates derived from ground-based and satellite data can arise from the longitudinal separation between low-latitude and equatorial stations that are used to determine the signal and its consequent time delay in their sunrise/sunset times at ionospheric heights.

Features of horizontal magnetic field intensity over northern island of Malaysia

This study utilized magnetic field records of the horizontal component obtained from magnetic data acquisition system at northern island of Malaysia to delineate the diurnal variations of SqH and their monthly mean MSqH for a period of 4 years. The results show that for the entire period of study, the daytime SqH amplitudes increase from $$\sim 20$$ to $$\sim 160$$ nT and their MSqH amplitudes increased from $$\sim 40$$ to about 135 nT in 2008 to 2013, respectively. These variabilities of SqH and MSqH reached peak amplitudes between 10:00 and 12:00 LT hrs. The standard deviation fairly responds to the daily variability of SqH with a phase shift in January 2008 and 2013 which may likely have connection to the sudden stratospheric warming event that occurred in these months. We observed the extension of eastward field that continued beyond the sunset (18:00 LT) hours is more prevalent with higher magnitudes during the deep minimum solar activity and the probable causes are discussed. The observed counter equatorial electrojet associated with late reversal of night-time westward electric field (WEF) shows inverse relationship with increasing solar activity and those that are not associated to late reversal of night-time WEF seems to indicate linear relationship exception of deep minimum solar activity year (2008). We also observed a gradual shift of ECEJ to the morning sector with increasing solar activity. Throughout the years, days with stronger morning counter equatorial electrojet magnitudes generally weaken the expected strong eastward current and shifted its peak to earlier local time. The annual and seasonal variations of SqH show semiannual variation with equinoctial maxima and solstitial minima. About 40% of the SqH shows positive night-time to pre-sunrise variations which suggest the existence of weak electric currents not necessarily of ionospheric origin with appreciable influence at this longitude sector. The extension of eastward field beyond the sunset hours suggests likely modification of the evening side ionosphere over this region.

Relationship between plasma bubble and ionospheric current, equatorial electrojet, and equatorial counter electrojet

In recent years, it has been clarified from previous studies that plasma bubbles and equatorial electrojets (EEJs) are related. In general, EEJs are calculated by subtracting the magnetic field H component of the magnetic equator from that at low latitude. However, in this study, EE-index data at Langkawi (magnetic equator), which includes all local current systems, were used for the analysis during the period from January 1, 2011, to November 8, 2014. By using the EE-index, it was found that plasma bubbles tend to occur for larger EEJ strengths. This result differs from the previous studies. In addition, if an equatorial counter electrojet (CEJ) occurs, it is understood that plasma bubbles will rarely occur due to the westward current; however, we found that when the lunar tidal effect is strong, plasma bubbles can occur even in conjunction with CEJs. Finally, we want to find the relationship between plasma bubbles and ionospheric current to predict them.

Equatorial Counter Electrojet Longitudinal and Seasonal Variability in the American Sector

AbstractThe equatorial electrojet occasionally reverses during morning and afternoon hours, leading to periods of westward current in the ionospheric E region that are known as counter electrojet (CEJ) events. We present the first analysis of CEJ climatology and CEJ dependence on solar flux and lunar phase for the Brazilian sector, based on an extensive ground‐based data set for the years 2008 to 2017 from the geomagnetic observatory Tatuoca (1.2°S, 48.5°W), and we compare it to the results found for Huancayo (12.0°S, 75.3°W) observatory in the Peruvian sector. We found a predominance of morning CEJ events for both sectors. The afternoon CEJ occurrence rate in the Brazilian sector is twice as high as in the Peruvian sector. The afternoon CEJ occurrence rate strongly depends on season, with maximum rates occurring during the northern‐hemisphere summer for the Brazilian sector and during the northern‐hemisphere winter for the Peruvian sector. Significant discrepancies between the two sectors are also found for morning CEJ rates during the northern‐hemisphere summer. These longitudinal differences are in agreement with a CEJ climatology derived from contemporary Swarm satellite data and can be attributed in part to the well‐known longitudinal wave‐4 structure in the background equatorial electrojet strength that results from nonmigrating solar tides and stationary planetary waves. Simulations with the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model show that the remaining longitudinal variability in CEJ during northern summer can be explained by the effect of migrating tides in the presence of the varying geomagnetic field in the South Atlantic Anomaly.

Vertical coupling between the mesopause region and sporadic-E layer during equatorial counter electrojet events – A case study

This paper presents a case study exploring the variability of daytime ionospheric sporadic E (Es) during Counter Equatorial Electrojet (CEJ) events over Trivandrum (8.5°N, 77°E, 0.5°N dip lat.), a dip equatorial station in India. It makes use of collocated measurements of (a) daytime mesopause temperature estimated using a Multiwavelength Dayglow Photometer, (b) base height of sporadic E-layer i.e. Esq obtained from a Digital Ionosonde, (c) Equatorial Electrojet (EEJ) induced magnetic field at the surface measured using a Proton Precession Magnetometer and (d) neutral meridional and zonal winds measured using meteor wind radar. The study addresses why, only on certain CEJ days, the Es layer turns into a blanketing Es layer, also referred to as Esb layer, appearing for a brief period and almost simultaneously as the CEJ induced magnetic field on the surface is either zero or very small, and not on all the CEJ days despite the reversal of the zonal winds in the dynamo region (∼98 km). Further, a simulation using the simultaneously measured zonal wind and temperature shows that during CEJ, especially when the zonal wind reverses and the mesopause temperature lowers, the conditions are in fact less favourable for the ionization to converge resulting in Esb. On the other hand, on such occasions, the meridional wind exhibits a tendency to turn equatorward with time with the changeover gradually happening as the altitude decreases. These observations have been discussed to allude to the possible role of the overall wind system and its spatio-temporal changes in the ionospheric altitudes which could influence the gradual transition of Esq to Esb over the dip equator.

Comprehensive Analysis of the Counter Equatorial Electrojet: Average Properties as Deduced From CHAMP Observations

AbstractTen years of geomagnetic field observations by the CHAMP satellite are used for a systematic investigation of the counter equatorial electrojet (CEJ). For the first time a comprehensive characterization of CEJ is presented. CEJs occur preferably during early morning, and their occurrence rate is down to 4% at noon. The CEJ occurrence rate shows a clear annual variation with a peak around July–August and a secondary peak in January. The late summer peak is related to the effect of meteor dust ablation. The CEJ amplitude is closely controlled by magnetic activity, showing a good correlation with the aP index. Nonmigrating solar tides are the main reason for longitudinal patterns of occurrence rate. The most prominent wavenumber 1 longitudinal structures during all seasons can be attributed to the tidal components SW3 and SPW1. The wavenumber 4 becomes largest during late summer‐autumn season, which is related to the DE3 component. Also, the influence of lunar tides is evident in the CEJ occurrence rate. Strongest modulation of the CEJ by the semidiurnal lunar M2 tide is observed around January, which is responsible for the secondary CEJ occurrence peak in January. Both the solar and lunar tidal waves appear out of phase between CEJ and equatorial electrojet, implying that the same tidal mechanisms influence both current systems. Changes of solar wind conditions can also influence the CEJ. Immediately after a sudden increase of solar wind input CEJs are depressed, but about 3 hr after that event CEJs are strongly enhanced because of the disturbance wind dynamo.

Evidence for the Influence of DE3 Tide on the Occurrence of Equatorial Counterelectrojet

AbstractThe long‐term Challenging Minisatellite Payload magnetometer data are analyzed to derive the equatorial counterelectrojet (CEJ) signatures globally over a range of local times. The resultant local time‐longitude map of CEJ occurrence rate shows a strong influence of the diurnal eastward propagating wave number 3 (DE3) nonmigrating tide during July–September months. DE3 is also shown to account for the reduction of CEJ occurrence at certain longitudes. These aspects of DE3 tide‐CEJ relationship have implications for understanding the seasonal and longitudinal variations of CEJ. Observations of zonal winds from the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) Doppler Interferometer instrument on board the TIMED satellite are used to establish these findings.

On the Occurrence of Afternoon Counter Electrojet Over Indian Longitudes During June Solstice in Solar Minimum

AbstractStudies made earlier using ground‐based observations of geomagnetic field over the Indian longitudes revealed that the occurrence of equatorial counter electrojet (CEJ) events in afternoon hours is more frequent during June solstice (May‐June‐July‐August) in solar minimum than in other periods. In general, the June solstice solar minimum CEJ events occur between 1500 local time (LT) and 1800 LT with peak strength of about −10 nT at around 1600 LT. In order to understand the frequent occurrence of these CEJ events, an investigation is carried out using an equatorial electrojet model (Anandarao, 1976, https://doi.org/10.1029/GL003i009p00545) and the empirical vertical drift model by Fejer et al. (2008, https://doi.org/10.1029/2007JA012801). The strength, duration, peak value, and the occurrence time of CEJ obtained using electrojet model match remarkably well with the corresponding observation of average geomagnetic field variations. The occurrence of CEJ is found to be due to solar quiet (Sq) electric field in the westward direction which is manifested as downward drift in Fejer et al. (2008, https://doi.org/10.1029/2007JA012801) model output during 1500–1800 LT. Further, the occurrence of afternoon reversal of Sq electric field in this season is shown to be consistent with earlier studies from Indian sector. Therefore, this investigation provides explicit evidence for the role of westward Sq electric field on the generation of afternoon CEJ during June solstice in solar minimum periods over the Indian sector indicating the global nature of these CEJ events.

Comparison of quiet time vertical plasma drifts with global empirical models over the Indian sector: Some insights

The threshold vertical plasma drifts and their polarities are inferred from E-region irregularities reported earlier using different experiments conducted from India during geomagnetically quiet time. In addition, the hourly variations of magnetometer data sets are used in conjunction with an equatorial electrojet model (Anandarao, 1976) to deduce the vertical drifts around noon time (10:00–140:00 LT). These results are then compared with the vertical drifts presented by empirical models (Scherliess and Fejer, 1999; Fejer et al., 2008a) corresponding to the 60°E longitude sector. In general, the vertical drifts presented by empirical models are consistent with those inferred from snapshot measurements of E-region irregularities at different local times of the day except around sunrise hours. Further, the vertical drifts presented by Fejer et al. (2008a) model match fairly well with seasonally averaged vertical drifts deduced (within 1σ variation) using magnetometer data. A time difference is noticed between occurrence of pre-reversal enhancement in vertical drifts over India reported earlier using different techniques and Scherliess and Fejer (1999) model. Probable reason for the time difference is discussed. The occurrence characteristic of afternoon equatorial counter electrojet in June solstice during low solar epoch is consistent with the drifts obtained from empirical models. The seasonally averaged vertical drifts during nighttime reported earlier using ionosonde/HF radar experiments are not consistent with the presence of streaming plasma waves on a few occasions. Further, nocturnal vertical drifts are systematically under-estimated and probable reason for this is discussed.

Variability of equatorial counter electrojet signatures in the Indian region

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

First observational evidence for the connection between the meteoric activity and occurrence of equatorial counter electrojet

This paper presents the first direct observational evidence for the possible role of meteoric activity in the generation of the equatorial Counter Electrojets (CEJ), an enigmatic daytime electrodynamical process over the geomagnetic equatorial upper atmosphere. The investigation carried out using the data from Proton Precession Magnetometer and Meteor Wind Radar over a geomagnetic dip equatorial station, Trivandrum (8.5°N, 77°E, 0.5°N dip lat.) in India, revealed that the occurrence of the afternoon CEJ events during a month is directly proportional to the average monthly meteor counts over this location. The observation is found to be very consistent during the considered period of study, i.e the years 2006 and 2007. The study vindicates that the meteor showers play a major role in setting up the background condition conducive for the generation of CEJ by reducing the strength of the upward polarization field.

Study of equatorial E region irregularities using rare daytime VHF scintillation observations

AbstractScintillations on VHF radio signal are sparsely observed during daytime due to unavailability of strong electron density irregularities in equatorial E or F region. Type I/II irregularities observed at E region altitudes during the daytime are linked with either two‐stream or gradient drift instability. The occurrence of these irregularities in presence of strong blanketing Es (Esb) can produce weak‐moderate scintillations on VHF signal during daytime. Such sparse daytime VHF scintillations are used in the present study to retrieve information about E region irregularities, which are generally examined with radar observations. We use spaced receiver scintillation observations on 251 MHz signal transmitted from geostationary satellite UFO2 (71.2°E) and recorded at Tirunelveli (8.5°N, 77.8°E, dip latitude 0.6°N). Ionosonde data from Trivandrum (8.5°N, 76.6°E, dip latitude 0.5°N) during 2003–2005 is used to confirm the association of daytime scintillations with Esb. The daytime scintillations last for 15–45 min during postnoon hours. Their occurrence closely matches the peak occurrence time of Esb. For the first time, spatial scale lengths of E region irregularities are obtained using the technique introduced by Bhattacharyya et al. (2003). The observed spatial scales are validated using theoretical model. The theoretical model manifests 6–19% density fluctuations in the E region to produce weak scintillations (0.15 ≤S4≤ 0.4) on 251 MHz. The study reveals that scale lengths of E region irregularities are smaller on counter equatorial electrojet (CEEJ) days than non‐CEEJ days, which could be resulting from lower electron temperatures in E region on CEEJ days.

EEJ and EIA variations during modeling substorms with different onset moments

This paper presents the simulations of four modeling substorms with different moment of substorm onset at 00:00UT, 06:00UT, 12:00UT, and 18:00UT for spring equinoctial conditions in solar activity minimum. Such investigation provides opportunity to examine the longitudinal dependence of ionospheric response to geomagnetic substorms. Model runs were performed using modified Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP). We analyzed GSM TIP simulated global distributions of foF2, low latitude electric field and ionospheric currents at geomagnetic equator and their disturbances at different UT moments substorms. We considered in more detail the variations in equatorial ionization anomaly, equatorial electrojet and counter equatorial electrojet during substorms. It is shown that: (1) the effects in EIA, EEJ and CEJ strongly depend on the substorm onset moment; (2) disturbances in equatorial zonal current density during substorm has significant longitudinal dependence; (3) the observed controversy on the equatorial ionospheric electric field signature of substorms can depend on the substorm onset moments, i.e., on the longitudinal variability in parameters of the thermosphere–ionosphere system.