Severe Positive Ionospheric Storm at American Low Latitudes During an Intense Long-Lasting CEJ Period of the August 2018 Geomagnetic Storm

Highlights

Generally, the westward electric field is thought to be closely related to the ionospheric negative storms at the crest latitudes of the equatorial ionization anomaly (EIA). However, studies have shown that the westward electric field could cause the postmidnight enhancement of F2-layer peak density (NmF2) accompanied by decreases in the F2-layer peak height (hmF2) and scale height (Hm) in the low-latitude ionosphere. Can a prominent positive ionospheric storm also occur during geomagnetic storms when the electric field is westward? In this study, we focused on the ionospheric responses in the American sector during the intense long-lasting counter equatorial electrojet (CEJ) period on 26 August 2018, as an intense long-lasting CEJ indicates a significant westward electric field during daytime at low latitudes. Using comprehensive observations from ground-based and space-borne instruments, we confirmed that the westward electric field at low latitudes is an important local candidate mechanism for the severe positive ionospheric storm; the combined effects of the westward electric field and thermospheric disturbances significantly influence ionospheric morphologies.

What are the main findings?

• During the long-lasting CEJ event, total electron content and NmF2 showed unexpectedly large enhancements (exceeding 150%) in the southern EIA region of the American sector.
• Enhancements were accompanied by pronounced decreases in hmF2 and Hm, along with higher O/N₂.

What is the implications of the main finding?

• Westward electric field is an important mechanism for positive ionospheric storms, even at EIA crest latitudes.

Abstract

The westward electric field plays an important role in the hmF2-declining type (where hmF2 denotes the peak height of the F2 layer) of postmidnight electron density (Ne) enhancements at equatorial ionization anomaly (EIA) latitudes through the compression effects, whereas its effectiveness in causing ionospheric storms has not been carefully investigated. As the counter equatorial electrojet (CEJ) is an indicator of the westward electric field, in this study, we focused on the American sector during the long-lasting (~10 h) intense CEJ event on 26 August 2018, based on observations from magnetometers, ionosonde-derived Ne profiles, ground-based and satellite-borne total electron content (TEC) measurements, and ∑O/N₂. The peak density of the F2 layer (NmF2) and TEC at the ionosondes presented severe long-duration enhancements at southern EIA latitudes, accompanied by decreases in hmF2 and scale height (Hm) of the Ne profile, similar to the hmF2-declining type of postmidnight NmF2 enhancements. The dominant contribution to the TEC enhancements came from the topside ionosphere. Moreover, the TEC map displayed prominent enhancements spanning a wide range of longitudes in the American sector. This study suggests that the electric-field-driven compression and convergence played important roles in the F-region Ne enhancements by changing the shape of the F layer. Meanwhile, elevated O/N₂ and equatorward winds may also have contributed to the TEC enhancements.

1. Introduction

The $\overrightarrow{E}\times \overrightarrow{B}$ plasma drift plays an important role in the formation of the equatorial ionization anomaly (EIA). The daytime eastward electric field (i.e., upward $\overrightarrow{E}\times \overrightarrow{B}$ plasma drifts) in the low-latitude ionosphere lifts plasma to higher altitudes; meanwhile, field-aligned diffusion driven by gravity and pressure gradients transports plasma toward higher latitudes in both hemispheres [1,2,3]. This “fountain effect” produces a distinct EIA structure, namely an electron density (Ne) trough at the magnetic equator and two Ne crests near ±15° magnetic latitudes [4,5]. In contrast, the westward electric field generally corresponds to the reversal fountain effect and consequently weakens the plasma transport from equator to low latitudes, resulting in Ne enhancements and reductions at the trough and crests of EIA, respectively [1,6,7].

However, the westward electric field near the EIA crests sometimes causes local Ne enhancements rather than depletions. Liu et al. [8] reported the postmidnight enhancements of the F2-layer peak density (NmF2) accompanied by a declining F2-layer peak height (hmF2) and thinning F-layer thickness at the Sanya ionosonde (18°N lat.). They explained these Ne enhancements in terms of the compression effects driven by the westward electric field; the resulting drift transports plasma from higher to lower altitudes/latitudes, driving a large downward plasma flux while simultaneously decreasing the ionospheric height and thinning the ionospheric thickness (represented by the decrease in Ne scale height). The downward flux has a large gradient near the F2 peak and in the bottomside ionosphere, resulting in strong plasma convergence which exceeds the local chemical loss.

The geomagnetic storm is a unique natural magnifier, and all physical and chemical processes are enlarged or represented in special forms. During geomagnetic storms, the $\overrightarrow{E}\times \overrightarrow{B}$ plasma drift (or equatorial electrojet [EEJ]) in the low-latitude ionosphere could be significantly disturbed by the magnetospheric convection electric field transmitted via closed geomagnetic field lines and by the ionospheric disturbance dynamo driven by the disturbed thermospheric winds [9,10,11,12,13,14]. In addition, the low-latitude thermospheric composition and winds are also severely perturbed by the equatorward propagating thermospheric circulation disturbance [15,16,17,18]. Positive ionospheric storms at EIA crest latitudes, characterized by significant enhancements in electron density or total electron content (TEC) relative to quiet-time levels, are generally driven by eastward prompt penetration electric fields, equatorward neutral winds, and elevated O/N₂ ratios induced by equatorward-propagating thermospheric circulation disturbances [19,20,21,22,23,24]. Thus, several interesting and important questions emerge as follows: (1) Does the ionosphere exhibit severe Ne enhancements (i.e., a positive ionospheric storm) at EIA crest latitudes during an interval of intense westward electric field? (2) What do the corresponding topside and bottomside ionospheric responses as well as their latitude and longitude variations look like? (3) What are the influences of thermospheric disturbance?

From 11:00 to 21:30 UT on 26 August 2018 during the early recovery phase of the August 2018 geomagnetic storm (yellow shading in Figure 1), the Bz component of the interplanetary magnetic field (IMF) exhibited three intense south–north turnings, resulting in an intense, long-lasting counter equatorial electrojet (CEJ) during daytime in the American sector, owing to the transmission of the magnetospheric convective electric field [25,26,27,28,29,30]. CEJ is the reversed flow of the equatorial electrojet [31]. Intense daytime CEJ indicates a significant westward electric field in the low-latitude ionosphere; therefore, this period offers an excellent opportunity to study the effects of the westward electric field on ionospheric storms, as well as other questions mentioned above. Actually, during the same CEJ period, Li et al. [26] reported severe enhancements in the NmF2 observed at the Cachoeira Paulista ionosonde (18°S mag. lat.), and they further suggested that these enhancements might be related to the local compression driven by the westward electric field [1,32]. Moreover, Moro et al. [27] also reported NmF2 enhancements at Sao Luis (mag. equator) and Santa Maria (20°S mag. lat.) during the same period and indicated the importance of the thermospheric composition. However, it remains unclear whether the westward electric field influences the positive ionospheric storm and what the relative effects of plasma drift and thermospheric disturbances are.

In this study, we investigated the ionospheric responses in the American sector during the intense long-lasting CEJ period of the 26 August 2018 geomagnetic storm, using comprehensive observations from ground-based magnetometers and ionosondes, TEC measurements from ground-based and Swarm-borne GNSS receivers (Global Navigation Satellite System), Swarm in situ Ne, and GUVI (Global Ultraviolet Imager) O/N₂ column density ratios. This study is an extension of Liu et al. [8] and Li et al. [26] with a focus on the reversed fountain effects in the low-latitude ionosphere during the geomagnetic storm as well as the combined effects of electrodynamics and thermospheric disturbance. The current study could strengthen our understanding of ionospheric morphologies and corresponding mechanisms in both quiet and disturbed geomagnetic conditions, thereby helping to elucidate the Solar–Terrestrial energy coupling and transmission and to mitigate and forecast space weather hazards to radio communication and navigation [33].

2. Materials and Methods

2.1. Geophysical Conditions During the August 2018 Storm

The 25–26 August 2018 storm was the third strongest storm in Solar Cycle 24 [34]. As indicated by the yellow shading in Figure 1, the solar wind and interplanetary conditions displayed several large oscillations during 11:00–21:30 UT on 26 August 2018. IMF Bz (Figure 1b) displayed three intense north-to-south turnings and varied from −13 to 16 nT. Meanwhile, the solar wind dynamic pressure (Pdy, Figure 1a), IMF By (Figure 1c), and interplanetary electric field (Ey, Figure 1d) also exhibited strong oscillations. The Bz turnings induced an intense and long-duration CEJ (red line in Figure 1g). The EEJ is a good indicator of the daytime zonal electric field and the vertical plasma drift in the equatorial ionospheric F region, under the condition that the conductivity in the E region does not change significantly [35,36,37]. Thus, the intense and long-duration CEJ, marked by the yellow shading in Figure 1g, indicated the dominant period of the westward electric field on 26 August 2018. 23 August was selected as a quiet-time reference because the IMF and solar wind velocity were only weakly disturbed.

2.2. Horizontal Component of the Geomagnetic Field Measured by Ground-Based Magnetometers

Given the proportional relation between EEJ and the differential magnetic horizontal component (∆H) between equatorial and low-latitude magnetometers [38], the EEJ in the American sector (near 48°W long.) is represented by the ∆H between TTB (Tatuoca, 1.21°S, 48.5°W, 1.67°N mag. lat.) and KOU (Kourou, 5.21°S, 52.7°W, 9.75°N mag. lat.). The quiet reference of EEJ was determined by averaging the international geomagnetic quiet days in August 2018 [6, 14, 10, 13, 23, http://isgi.unistra.fr/data_download.php (accessed on 23 August 2026)]. Figure 2 displays the locations of the ground-based magnetometers and ionosondes used in this study.

2.3. Electron Density (Ne) Observed by Ground-Based Ionosondes and Swarm Langmuir Probe

To investigate the variations in ionospheric F region, we collected Ne profiles, NmF2 (in units of electrons cm⁻³) and hmF2 (in units of km) for the F2 layer from a chain of ionosondes at Sao Luis (2.6°S, 44.2°W, 0.8°N mag. lat.), Cachoeira Paulista (22.7°S, 45°W, −17.6°S mag. lat.), and Santa Maria (29.7°S, 53.8°W, −19.7°S mag. lat.), derived from the manually scaled ionograms using the SAO-Explorer software (version 3.6.1) [39]. The Ne scale height describes the ionospheric altitude gradient and is closely related to ionospheric dynamics [40,41,42,43]; therefore, α-Chapman scale height (Hm) was retrieved by fitting the Ne profiles below hmF2 using functions specified in Equations (1) and (2). Moreover, we collected in situ Ne observations from the Langmuir probe on board the Swarm B spacecraft at an altitude of approximately 515 km. To focus on the dayside, only orbits that crossed the equator around 9:30 LT were used.

$$\mathrm{N}\mathrm{e}=\mathrm{NmF}2\exp \{{\displaystyle \frac{1}{2}}\times [1-\mathrm{z}-\exp (-\mathrm{z})]\}$$

(1)

$$\mathrm{z}={\displaystyle \frac{\mathrm{h}-\mathrm{hmF}2}{\mathrm{Hm}}}$$

(2)

2.4. Vertical Total Electron Content (TEC) Derived from Ground-Based and Satellite-Borne GNSS Receivers as Well as Ionosondes

To examine the overall ionospheric response over the American sector, we collected the ground-based GNSS network-derived TEC processed by the Massachusetts Institute of Technology Haystack Observatory (MIT-TEC) with a resolution of 5 min and 1° × 1° (lat. × long.). At the locations of ionosondes, the GNSS, bottom (Bot-), and topside (Top-) TECs were also obtained to examine the ionospheric responses over different altitude ranges. The GNSS-TEC value at each ionosonde location was retrieved by binning the MIT-TEC data. Bot-TEC was calculated by integrating the ionosonde-derived Ne profile below hmF2. Top-TEC was calculated by subtracting Bot-TEC from GNSS-TEC. Moreover, upward-looking TEC data obtained from GNSS receivers on board Swarm B (515 km) and MetOp-A (832 km) satellites were also used to study the topside ionospheric responses. The orbits of Swarm B and MetOp-A crossed the equator at about 9:30 LT. The vertical Low Earth Orbit TECs (LEO-TEC) were calculated by removing satellite and receiver bias and mapping the slant TEC to vertical TEC. Detailed procedures for retrieving vertical LEO-TEC are described by Yue et al. [44] and Zhong et al. [45].

2.5. ∑O/N₂ from Global Ultraviolet Imager (GUVI) Instruments Onboard the TIMED Satellite

The column density ratio of O to N₂ (∑O/N₂) was used to examine the influence of thermospheric composition changes on plasma density in the ionospheric F region. ∑O/N₂ is defined as the ratio of height-integrated O to N₂ column densities from the top of the atmosphere down to the altitude where the column number density of N₂ is 10¹⁷ cm⁻² [46], based on simultaneous measurements of GUVI disk-viewing airglow of OI 135.6 nm and N₂ Lyman–Birge–Hopfield radiances in the nadir direction [47].

3. Results

3.1. Ionosondes

Figure 3a–c show the variations in Ne, NmF2, hmF2, and Chapman scale height derived from ionosondes at Sao Luis, Cachoeira Paulista and Santa Maria, respectively. During the strong CEJ period on 26 August 2018 (i.e., dominated by a westward electric field), the three stations all displayed pronounced NmF2 enhancements during three intervals, accompanied by obvious changes in the shapes of the Ne profiles. The enhancements occurred in the morning, noon, and afternoon local times, and they are marked by the yellow, green, and gray shaded regions, respectively. During the shaded periods, the minimum and maximum NmF2 changes were 4 × 10⁵ and 10 × 10⁵ cm⁻³, respectively. Except for the morning and noon NmF2 enhancements at Sao Luis, the Ne enhancements displayed morphologies similar to those of characteristic postmidnight NmF2 enhancements. That is, NmF2 showed a large increase, while hmF2 decreased greatly and thickness (as indicated by the Hm) declined. Moreover, all NmF2 values during the enhancement periods were predominantly larger than their geomagnetic quiet-time levels (23 August 2018), except for the noon and afternoon enhancements at Sao Luis, which fell within quiet-time day-to-day variability [48,49]. Compared with the quiet-time reference, the percentages of NmF2 enhancements reached approximately 92.7% at Sao Luis (Figure 3(a3)), 306.9% at Cachoeira Paulista (Figure 3(b3)), and 258.6% at Santa Maria (Figure 3(c3)).

To further examine the temporal development of the enhancements, Figure 4 is a reorganized version of Figure 3, showing the variations in NmF2, hmF2, and Hm ordered by latitudes. The results show that, the lower the latitude, the earlier the NmF2 enhancements occurred. Figure 4(a1) displays that NmF2 enhancements at the magnetic equator occurred at 11:00, 14:00, and 17:50 UT and then reached their peaks at 11:50, 14:50, and 18:50 UT. Figure 4(a2,a3) show that the onset and peak times of the NmF2 enhancements occurred 1–2 h later at 18°S and 20°S than those at the magnetic equator.

Based on the mechanism proposed by Liu et al. [8] for the effect of the westward electric field on the postmidnight NmF2 enhancement, we would anticipate pronounced Ne enhancements within the EIA region during the intense CEJ period on 26 August compared with geomagnetic quiet time (namely strong positive ionospheric storm). The three NmF2-enhancement intervals observed by ionosondes are consistent with this expectation. The Ne enhancements at Sao Luis may also be related to reduced plasma transport from magnetic equatorial region toward low latitudes driven by the westward electric field.

3.2. TEC

3.2.1. TEC Variations in Different Altitude Ranges at Ionosonde Locations

Figure 5 depicts the variations in GNSS TEC, topside TEC, and bottomside TEC at the ionosonde locations during the CEJ period. The three NmF2 enhancements at the ionosondes are also marked by the yellow, green, and gray shaded regions in Figure 5. The corresponding peak time of each NmF2 enhancement is marked by the vertical dash-dot lines. Figure 5a,b demonstrate that, during the period of NmF2 enhancements, total, topside, and bottomside TECs all showed significant increases; however, the dominant contribution was from the topside ionosphere, whose response time differed from that of the bottomside ionosphere. As shown in Figure 5(b1), the total TEC enhancements at the magnetic equator reached values of 10 (160%), 8 (55%), 2 TECu (20%) compared with the quiet reference on 23 August. As shown in Figure 5(b2,b3), the TEC enhancements near the southern crests of EIA reached values of 14 (160%), 10 (70%), and 5.8 (65%) TECu. Figure 5(b1–b3) show that the topside ionosphere, which increased by 80%, contributed the largest portion to the TEC enhancements. Moreover, the TEC enhancements peaked approximately 1 h earlier in the bottomside than in the topside ionosphere, whereas the difference between topside and total TECs was negligible at all ionosondes. Figure 5(c1–c3) show that the magnitudes of all TEC enhancements exceeded 35% (i.e., above the day-to-day variability of the quiet-time ionosphere [48,49]), except for the afternoon enhancements at Sao Luis. The relative daytime increase in total TEC during the storm reached 160%, far exceeding the quiet-time postmidnight NmF2 enhancements near the EIA crest, which were less than 15%, as reported in simulations by Le et al. [50].

3.2.2. TEC Spatial Responses over the American Sector

The hmF2-declining type of nighttime NmF2 enhancements shows complex latitude variations due to modulation by meridional winds [50,51,52], and the ionosphere–thermosphere system also presents substantial spatial variations during geomagnetic storms. Therefore, it is necessary to examine the TEC spatial responses in the American sector during the CEJ period. Figure 6 displays the latitude versus longitude variations of the MIT-TEC at different UT values. The black squares mark the ionosonde locations of Sao Luis, Cachoeira Paulista and Santa Maria.

The TEC over the American sector displayed extremely widespread and pronounced enhancements, and the TEC pattern was different from that during the quiet-time reference on 23 August 2018. Figure 6a demonstrates that, under the quiet geomagnetic condition, the TEC within 90–30°W did not show a typical EIA double-peak structure until 17:00 UT. In contrast, during the storm day (Figure 6b), the TEC within 90–45°W displayed an evident narrow double-peak structure at equatorial and low latitudes before 17:00 UT, with larger magnitude on the western side than the eastern side, whereas it showed a single-peak structure within 90–30°W after 17:00 UT. As shown in Figure 6c, the differential TEC (DTEC) over the American sector generally displayed positive values between 40°S and 40°N before 17:00 UT. After 17:00 UT, positive ∆TEC was mainly confined to the Southern hemisphere, while the Northern hemisphere displayed strong depletions due to the disturbance of ∑O/N₂ [25,26].

It is interesting that, in Figure 6(b2,b3), the morning TEC displayed a narrow double-peak structure in the southern hemisphere within 20° mag. lat. and between 90°W and 30°W, because the typical EIA double-peak structure usually appears after or near local noon (shown in Figure 6(a4–a6)). The morning narrow double-peak structure was more evident in the west than east of 45°W, namely, the structure was stronger at earlier local times. Figure 7 displays the latitude profiles of the TEC at 75°W. The observations indicate that the narrow double-peak structure persisted for about 2.5 h (12:30–15:00 UT), and the corresponding ∆TEC between the crest and trough was about 2.5 TECu (17%). Furthermore, we also examined the latitude distribution of Ne obtained from the onboard Langmuir probe and the upward-looking TEC measurements from LEO satellites. Figure 8(a2) shows that the Ne at 75°W from Swarm presented a clear double-peak structure at low latitudes around 14:10 UT (09:35 LT) on 26 August, whereas no double-peak could be identified in the preceding and following Swarm orbits (Figure 8(a1,a3)). Figure 8c shows that the upward-looking LEO-TEC from Swarm (515 km) and MetOp-A (832 km) displayed negligible variations during 12:29–15:52 UT between 26 and 23 August, indicating that the increase in ionospheric plasma occurred mainly below 515 km. The observations from both MIT-TEC (Figure 6(b2) and Figure 7b–g) and Swarm in situ Ne (Figure 8(a2)) showed an evident narrow double-peak structure at low latitudes with clear longitudinal and local-time difference, which indicates that the westward electric field may play important roles in modulating ionospheric latitude variation over a wide range of altitudes and its effect depends on the background ionospheric conditions.

4. Discussion

Under strong westward electric field conditions, previous investigations have shown that daytime Ne usually decreases at the EIA crests and increases near the magnetic equator. However, during the intense CEJ period on 26 August 2018, the most striking feature is that the ionosphere in the American sector presented a severe long-lasting (~10 h) and widespread daytime positive storm in the southern EIA, accompanied by significant changes in ionospheric morphology in height and latitude. Liu et al. [8] first suggested that the westward electric field could induce strong nighttime Ne and NmF2 enhancements in the F region at the EIA crest regions. In addition, besides electrodynamics, complex thermospheric disturbance is also a competitive driver during the ionospheric storm. In this section, to uncover several important mechanisms of the severe positive ionospheric storm in the American sector, we will discuss the effects of the westward electric field and thermospheric disturbances, including neutral composition, winds, and waves. Figure 9 presents a brief summary of the possible physical processes for the severe positive ionospheric storm during the CEJ period. The westward electric field contributed dominantly to the NmF2 enhancement; it transported plasma from higher to lower altitudes/latitudes, while simultaneously lowering the ionospheric height and reducing the ionospheric thickness. The thermospheric disturbance (i.e., high ∑O/N₂ and strong equatorward winds) played an essential role in the TEC enhancement and also contributed to the NmF2 enhancement by intensifying the compression effect driven by the westward electric field.

4.1. Effects of the Downward $\overrightarrow{\mathit{E}}\times \overrightarrow{\mathit{B}}$ Plasma Drift

Liu et al. [8] first associated the hmF2-declining and Hm-thinning NmF2 enhancements with the mechanism of compression effects driven by the rapid westward electric field. Le et al. [50] further confirmed this mechanism through theoretical ionospheric simulations. In this study, during the intense long-lasting CEJ period, the ionosondes at 0°, 18°S, and 20°S mag. lat. in the American sector presented characteristics similar to those of typical nighttime Ne enhancements driven by the westward electric field; NmF2 displayed pronounced enhancements accompanied by a significant drop in hmF2 and a decrease in thickness (represented by the decrease in Hm). Such characteristics indicate the essential roles of the westward electric field, which transports the ionospheric plasma from higher to lower heights, and indicate that the Ne increase caused by the significant downward plasma flux exceeded its loss due to the chemical recombination at low altitudes [8,53]. These electrodynamic processes caused NmF2 enhancements accompanied by decreases in ionospheric height and thickness.

Besides the direct downward plasma transport at low latitudes, the westward electric field also creates a region of low plasma pressure at ionospheric F layer altitudes and, consequently, the surrounding three-dimensional (3D) plasma flux would flow to fill this low-pressure region due to the strong gradients [7,32]. We suggest that the plasma flux gradient (convergence) induced by the westward electric field is essential for the Ne enhancements. Firstly, the unbalanced plasma pressure gradients caused by the westward electric field would induce a 3D plasma inflow, which could strengthen the convergent effect of the westward electric field and cause TEC enhancements in the low-pressure region. The 3D plasma inflow includes convergent field-aligned ambipolar diffusion and horizontal plasma movements (as shown in Figure 9). Secondly, the convergent 3D plasma inflow also influences the changes in Ne profiles. The ionospheric Ne profiles displayed a significant decrease in Hm. On one hand, Hm reflects the ionospheric thickness in the F region and is proportional to ionospheric slab thickness ${\displaystyle \frac{TEC}{NmF2}}$ [54]. The decrease in Hm indicates that TEC increases at a lower rate than NmF2. On the other hand, previous studies revealed that the plasma diffusion/transport significantly influences the topside ionospheric vertical scale height (VSH) [40,41,42,43]. Their relationship is given by Equation (3) [43]:

$${\displaystyle \frac{1}{\mathrm{VSH}}}=-{\displaystyle \frac{\mathrm{d}\ln \mathrm{Ne}}{\mathrm{dh}}}={\displaystyle \frac{{\mathrm{K}}_{\mathrm{b}}({\mathrm{T}}_{\mathrm{i}}+{\mathrm{T}}_{\mathrm{e}})}{{\mathrm{m}}_{\mathrm{i}}\mathrm{g}}}+{\displaystyle \frac{{\mathrm{m}}_{\mathrm{i}}{\mathrm{\nu }}_{\mathrm{i}\mathrm{n}}{\mathrm{W}}_{\mathrm{D}}}{{\mathrm{K}}_{\mathrm{b}}({\mathrm{T}}_{\mathrm{i}}+{\mathrm{T}}_{\mathrm{e}})}}+{\displaystyle \frac{\mathrm{d}\ln ({\mathrm{T}}_{\mathrm{i}}+{\mathrm{T}}_{\mathrm{e}})}{\mathrm{d}\mathrm{h}}}$$

(3)

where h is the height, K_b is the Boltzmann constant, T_i and T_e are the ion and electron temperatures, m_i is the O⁺ mass, g is the gravity acceleration, $v_{in}$ is the ion-neutral collision frequency, and W_D is the vertical plasma drift velocity. VSH is proportional to Hm. The intense westward electric field corresponds to a negative W_D with a large absolute magnitude. If the plasma temperature changes are neglected, VSH and Hm should increase according to Equation (3); however, the results showed a decreasing pattern. It is worth noting that the underlying assumption of Equation (3) is the neglect of the ionospheric horizontal gradient and horizontal plasma drift. However, the assumption is invalid during the period of intense westward electric field, because the westward electric field would cause large 3D plasma pressure gradients at low latitudes in the ionospheric F region and, consequently, the surrounding 3D plasma flux would flow inward to fill the low-pressure region. The horizontal plasma drift velocity is usually larger than the vertical drift velocity in the low-latitude F region [10]. Thus, the horizontal Ne gradient and plasma drift may have significant effects on the decrease in Hm. Further 3D simulations are needed to analyze the effects of the ionospheric horizontal variations on the Ne enhancements during this CEJ period.

During the NmF2 enhancements, the topside ionosphere contributed dominantly to the increase in total TEC, which is consistent with the variation in hmF2, since a lower hmF2 indicates the longer (shorter) height range for the topside (bottomside) ionosphere. The topside TEC enhancements peaked later than the bottomside TEC enhancements, which may be caused by the height variation of the chemical recombination rates. The dominant increase in the topside ionosphere as well as the negligible difference between topside and total TEC during the three enhancements indicates that the 3D convergent plasma flow (including horizontal plasma flow, field-aligned convergent ambipolar diffusions, and downward plasma flow driven by westward electric field) caused by the westward electric field mainly caused the plasma convergence/increase in the topside ionosphere.

The above analysis indicates the essentially comprehensive roles of the westward electric field in the height morphology of Ne profile during the severe positive ionospheric storm in the American sector. We next examined its influence on the latitudinal distribution. Observations show that the occurrence and peak times of the three NmF2 enhancements were about 1–2 h earlier at the magnetic equator than 18°S magnetic latitudes, which suggested the equatorial ionosphere experienced the dominant and maximum plasma influx earlier than at higher low latitudes. The $\overrightarrow{E}\times \overrightarrow{B}$ plasma drift may play an important role in the latitude difference of the NmF2 response time due to the modulation by the geomagnetic field configuration. On one hand, the vertical $\overrightarrow{E}\times \overrightarrow{B}$ plasma drift velocity is proportional to ${\displaystyle \frac{E}{B}}$ cosI, where E is the zonal electric field, B is the geomagnetic field, and I is the geomagnetic inclination. If we neglect the latitude variation of the electric field, the drift velocity varies positively with cosI. That means the lower (smaller) the latitude (I) is, the stronger the plasma drift. In addition, “vertical compression” at low latitudes is more difficult than at the magnetic equator, due to its strong field-aligned plasma diffusion. Thus, the latitude variation in magnetic inclination makes compression easier near the magnetic equator, which may consequently cause the latitudinal variation in the response time of the NmF2 enhancements.

4.2. Effects from the Thermospheric Disturbance

It is well known that, during the geomagnetic storm, the equatorward propagating disturbed thermospheric circulation originating from high latitudes significantly disturbs meridional winds, changes composition, and stimulates waves [18,55,56]. All these thermospheric disturbances have latitude-dependent time delays and complex spatial variations [6]. Thus, besides the westward electric field, the thermospheric disturbances may also have played important roles in the severe positive storms in this study.

The widespread and intensive morning TEC enhancements during the intense long-duration CEJ period (11–15 UT, Figure 5c and Figure 6(c1–c3)) are unique, which is not consistent with the general feature due to the westward electric field. The TEC increase caused by the westward electric field is not enough to produce such severe and widespread TEC enhancements in the American sector. The thermospheric composition and winds may play essential roles. The downward plasma flow driven by the westward electric field undoubtedly caused TEC decrease due to the high recombination rate in the low altitudes. In contrast, the surrounding three-dimensional plasma influx may have produced a certain degree of TEC enhancements in the low-pressure region at low latitudes. Thus, the westward electric field alone cannot adequately explain the widespread and serious TEC enhancements. The ∑O/N₂ data from GUVI were used to examine the chemical processes associated with the thermospheric composition changes. As shown in Figure 10, ∑O/N₂ presented an obvious increase of about 0.2 (larger than 40%) in the southern hemisphere in the American sector during 14:55–19:00 UT. Considering the thermospheric atomic oxygen and molecular compositions largely determine the electron production and loss rates in the ionospheric F region, the elevated ∑O/N₂ may have contributed to the observed severe TEC and NmF2 enhancements by providing a larger plasma reservoir for electric-field-driven compression and convergence.

In addition to thermospheric composition, the meridional winds may also play an important role in the severe Ne enhancements, especially equatorward winds. Figure 10 shows that the regions of low ∑O/N₂ at high latitudes in both hemispheres extended westward and equatorward over time, and Figure 4b displays that the hmF2 at 18° and 20°S mag. lat. during the shaded region (NmF2 enhancements) was significantly higher on 26 August (red lines) than on 23 August (black lines). These observations of ∑O/N₂ and hmF2 are consistent with strong equatorward winds during the severe storm. At the low latitudes, the equatorward winds can increase Ne by the combined effects of reducing/stopping the field-aligned downward plasma diffusion and decreasing chemical loss by lifting the ionosphere to higher altitudes at low latitudes, and the two effects are optimized at latitudes near ±16° [7]. Simulations by Le et al. [50] also showed that stronger equatorward winds produced larger nighttime Ne enhancements and delayed the NmF2 peaks. The equatorward winds take time to reach low latitudes, and the low-latitude ionosphere responds more slowly to winds than to electric fields. Thus, besides the westward electric field, the equatorward winds may also contribute to the latitude difference of the occurrence and peak times of NmF2 at the ionosondes. Under the condition in which the equatorward winds lifted the ionosphere on the bottomside, meanwhile the westward electric field compressed the ionosphere on the topside, combined with the elevated ∑O/N₂, and the injected 3D plasma inflow due to the pressure gradient caused by the westward electric field, the compression effect driven by the westward electric field would be largely strengthened. Thus, the equatorward winds may play essential roles in the widespread and severe TEC enhancements and Ne spatial morphological variations during the severe positive ionospheric storm. In addition, the morning narrow double-peak structure in the American sector within 20° mag. lat. at 90–30°W may be the combined effects mentioned before.

Atmospheric gravity waves may also have contributed to the severe ionospheric storm. Different from the compression effect theory of Liu et al. [8], in which the compression is driven by the westward electric field, Pezzopane et al. [57] attributed the hmF2-declining nighttime NmF2 enhancements to traveling ionospheric disturbances (TIDs) caused by atmospheric gravity wave propagation. During the hmF2-declining nighttime NmF2 enhancements, the Ne increase generally occurs earlier at high than at low altitude. This characteristic is well consistent with the downward phase propagation of atmospheric gravity wave from high to low altitudes. To check the gravity wave effects on the severe ionospheric storm in the current study, the temporal variations in Ne at different altitudes of the three ionosondes are shown in Figure 11. The result shows that only three of the nine cases of Ne enhancements presented the gravity-wave-like structure (namely, the Ne increase occurs earlier at high than at low altitude). Moreover, in the current study, the Ne enhancements happened earlier at the magnetic equator than at the southern crest of EIA. Therefore, if there was a gravity wave, it should propagate from north to south. Figure 12 plots the differential TEC variation at 65–55°W longitudes. There are evident equatorward propagating large-scale TIDs. Their propagation speed is about 678 m/s (40° lat./2 h). Figure 3(a3,b3) show that NmF2 reached its peak about 1–2 h later at 22.7°S than at 2.6°S, indicating a meridional propagation speed of 340–680 m/s. These response times are slower than the propagation of TIDs. Large-scale TIDs can cause the F-region Ne to deviate from its background value by 20–30%. Thus, atmospheric gravity waves may also have contributed to the severe positive ionospheric storm in the current study, but they were unlikely to have been the dominant mechanism.

5. Conclusions

In this paper, we investigated the ionospheric responses in the American sector during the daytime CEJ period of the August 2018 geomagnetic storm using comprehensive observations. The major results are outlined as follows:

(1) In the southern low latitudes, the NmF2 and TEC at ionosondes displayed severe enhancements (reaching 300% and 160% relative to the quiet-time reference) accompanied by decreases in the ionospheric height and thickness. The Ne enhancements occurred and reached their peaks earlier at the magnetic equator than at low latitudes. Total TEC as well as its bottomside and topside portions at those ionosonde sites showed significant increases, but the dominant TEC enhancement originated from the topside ionosphere. Moreover, the MIT-TEC maps display extremely widespread and pronounced enhancements in the American sector. To the best of our knowledge, this is the first time that such a severe and widespread positive ionospheric storm has been observed during the intense westward electric field.

(2) The unique positive ionospheric storm likely resulted from a combination of the chemical and plasma transport effects driven by thermospheric disturbance and westward electric field. First, the westward electric field mainly caused the NmF2 enhancements by changing the shape of the F layer through the compression effects and causative injected convergent 3D plasma flow. The plasma flow includes field-aligned ambipolar diffusion and horizontal movements. Our study indicates that the horizontal plasma gradient and movements are important for the profile thinning during the Ne enhancement. Second, the thermospheric composition with high ∑O/N₂ and equatorward winds played important roles in the TEC and Ne enhancements. Equatorward winds may have contributed to the enhancements by lifting the ionosphere to higher altitudes, thereby reducing downward field-aligned diffusion and chemical recombination. Third, the reversed latitude dependence of the Ne enhancements was caused by the magnetic inclination modulating transport driven by the westward electric field and neutral winds. Fourth, gravity waves also contributed to the positive storm, but they were not the dominant mechanism.

Our studies have shown that the westward electric field is an important candidate mechanism for positive ionospheric storms, not only due to its compression effects but also due to the causative 3D plasma inflow caused by the pressure changes. The combined effects of the westward electric field and complex thermospheric disturbance significantly influenced the ionospheric morphology across altitudes, latitudes, and longitudes. Further 3D theoretical simulations considering ionospheric horizontal gradient and movements are needed to quantify the relative contributions of multiple thermospheric influences and the westward electric field.