Unique Combinations of Differently Shaped Equatorial Plasma Bubbles Occurring Within a Small Longitude Range

On 12 October 2020 and 26 December 2021, NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission observed differently shaped equatorial plasma bubbles (EPBs) simultaneously within ∼10° longitude, near the subsatellite point and over the Atlantic, respectively which is unusual. On 12 October 2020, three EPBs with differing curvatures were observed in a ∼12° longitude sector. The westside EPB was curved toward the east, in a C‐shape. The middle was straight. The eastside EPB was curved westward, in a reversed C‐shape. In the second case, 26 December 2021, in a smaller longitude range of ∼6° adjacent C‐shaped and reversed C‐shaped EPBs were observed. EPBs' zonal drift velocities at the magnetic equator and both equatorial ionization anomaly crests were compared. These occurrences of oppositely shaped EPBs simultaneously in a narrow longitude may indicate that small‐scale longitudinal variations in the E‐region density, electric field, neutral wind variations, or a combination of them were present.


10.1029/2023JA031625
2 of 8 C-shape EPBs were reported for the first time using the Jicamarca radar backscatter maps (Woodman & La Hoz, 1976); where no explanation for the formation mechanism was given.During the early night, zonal plasma drifts over the off-equatorial latitudes were observed to be stronger than the equatorial latitudes using electric field measurements from DE-2 (Aggson et al., 1987) and ground-based airglow measurements (Martinis et al., 2003).These latitudinal variations in the zonal plasma drifts can be related to their altitudinal variations at the magnetic equator.SAMI3 produced a C-shape EPB when HWM07 neutral wind was taken as input (Huba et al., 2009).In this case, the HWM07 zonal winds produced a strong westward drift at low altitudes which caused an eastward tilt of EPB (C-shape EPB) at higher altitudes.Haerendal (1980) deduced westward plasma flow in the lower F-region which reverses to eastward at higher levels, from vapor cloud release experiments.
Using a ground-based all-sky airglow imaging system, the westward tilt of the airglow depletions to the magnetic field lines was reported (Mendillo & Baumgardner, 1982;Mendillo & Tyler, 1983).A decrease of the eastward plasma drift velocity with increasing altitude (latitude) could be produced if the eastward neutral wind velocity also decreases with altitude (latitude) (Anderson & Mendillo, 1983;Rishbeth, 1972).This could form reversed C-shape EPB.Such a decrease in zonal neutral wind speed at the EIA crests due to larger ion drag has been observed by Dynamics Explorer-2 (DE-2) satellite (Raghavarao et al., 1991) and ground-based Fabry-Perot interferometer (Martinis et al., 2001(Martinis et al., , 2003)).Reversed C-shape EPBs were observed in the OI 135.6-nm emission images by the Global Ultraviolet Imager (GUVI) on board the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) satellite (Kelley et al., 2003).Kil et al. (2009) explained the reversed C-shape structure by (a) the reduction of the eastward plasma flow inside the EPB due to the development of a polarization electric field that retarded at higher apex height (indicated by Woodman and La Hoz (1976)) and (b) the latitudinal variation of the eastward drift of the background ionosphere, as explained by Anderson and Mendillo (1983) and Martinis et al. (2003).Even if the zonal neutral wind is constant with altitude, plasma drifts can vary with altitude due to the variations in the Pedersen conductivity.Zalesak et al. (1982) incorporated an eastward neutral wind in the equatorial F region and E region Pedersen conductivity effects in their two-dimensional numerical simulation and found that the rising EPBs were shaped by the vertical shears of the plasma motion, resulting in the reversed C-shape EPBs.Thus, the actual EPB shape depends strongly on both the zonal neutral wind profile and the Pedersen conductivity.Huba et al. (2009) have investigated the model (HWM93) zonal winds' effect on the EPBs' morphology through SAMI3, a three-dimensional modeling simulation.They found that stronger neutral zonal winds at lower altitudes and a decrease of velocity at latitudes away from the equator cause the reversed C-shape EPBs which agreed with the observations reported by Kelley et al. (2003) and Kil et al. (2009).
On 12 October 2020, the NASA Global-scale Observations of the Limb and Disk (GOLD) mission observed three consecutive EPBs; a C-shape, a straight, and a reversed C-shape EPBs, within ∼12° longitudes at magnetic equatorial latitudes.The observed longitude is close to the GOLD's subsatellite point (∼47°W).In another case on 26 December 2021, a reversed C-shape and C-shape EPB were observed sidewise within ∼6° longitude.While there are numerous images with EPBs having different shapes, these two examples of consecutive EPBs with reversing shapes within a small longitude range are unusual and the only two cases we have found after checking almost 4 years of GOLD data since its operation through September 2022.This points to small longitudinal variations in the E-region density, electric field, neutral wind variations, or a combination of them.We report detailed observations of these two unique and rare events and discuss their possible formation mechanisms.

Data
Nighttime OI 135.6 nm partial disk scans made by the GOLD imager are the data used in this study.The GOLD imager was launched on a commercial communications satellite on 25 January 2018 and it is in geostationary orbit at 47.5 o W. Nominal operations and observations started on 9 October 2018.It has two identical spectrographs that obtain the Earth images in the far-ultraviolet (FUV) range, at ∼134-162 nm wavelength.It measures the column-integrated emission rate along the line of sight.When geolocating the observations, an emission altitude of 300 km is assumed.GOLD can observe the American, Atlantic, and Western African longitudes, which provides a unique opportunity to unambiguously observe the spatial-temporal evolution of various ionospheric-thermospheric features in this active region of the equatorial ionization anomaly (EIA).The nighttime L1C disk images are obtained at a cadence of 15 min and binned to 90 × 80 km at the nadir.Detailed information about the GOLD instrument and observation modes are discussed in Eastes et al. (2017Eastes et al. ( , 2020Eastes et al. ( , 2023) ) and McClintock et al. (2020).Figure 1a shows simultaneous images obtained by CHA and CHB from the northern and southern hemispheres, respectively on 12 October 2020 at 23:10 UT.The black dashed line marks the geomagnetic equator.The emissions peaking on either side of the geomagnetic equator are the EIA crests.At 10°S GLat there is a faint data artifact.At that latitude, there appears to be a longitudinally extended region of slightly brighter emission, but EPBs are still apparent as depletions in the brightness across the EIA crests.This artifact is due to the high voltage being too low during the flat field measurements, which is explained in Section 3.1.18of the GOLD data release note Rev 4.6 (https://gold.cs.ucf.edu/wp-content/documentation/GOLD_Release_Notes_Rev4.6.pdf).Over the Eastern side of South America (∼50° −35°W longitudes) there are three distinct and differently shaped EPBs.This longitude range is close to the crossing of the geographic and geomagnetic equators and near the subsatellite location.The EPB on the left side (marked as B1) has a C-shape, the one in the middle (B2) is straight, whereas the right EPB (B3) has a reversed C-shape.B3 appears to be bifurcated at the south (S) EIA crests.Each of these EPB shapes has been observed previously by others and by GOLD, but in the observations on 12 October 2020, the three distinct EPB shapes occurred consecutively over a narrow longitude range (∼12°).Contrary to this case, the C-and reversed C-shape EPBs were observed in an opposite order within ∼30° and 20° W longitudes on 26 December 2021.Consecutive images taken by CHB at 22:40 and 22:55 UT on 26 December 2021 are combined and shown in Figure 1b.The EPBs at the west side of 30°W longitude appear to be straight.But the EPBs (B4 and B5) observed within ∼30° and 20° W longitudes are of opposite shapes and are the focus in this image.The reversed C-shape EPB (B4) is observed to the west of the C-shape EPB (B5) and the two are separated by ∼6° at their magnetic equators (Figure 1b).In both cases (Figures 1a and 1b) the EPBs are observed multiple times between ∼19:30 to 22.00 LT with initial observations around ∼2 hr after the local sunset.To investigate the different EPBs' shapes (B1-B5) observed at similar local times but within a small longitude range, we derived their zonal drift velocities at the magnetic equator and EIA crest latitudes.The EPB drift velocity derivation method is explained below for 12 October 2020 and is also used for 26 December 2021.For the investigation, images are first transferred into magnetic coordinates (Laundal & Richmond, 2017;Thébault et al., 2015) and is explained in Karan et al. (2020Karan et al. ( , 2023)).Figures 2a-2d show the images at 23:10, 23:25, 23:40, and 23:55 UT on 12 October 2020 in magnetic coordinates, respectively.At 23:10 UT (Figure 2a) the locations of B1, B2, and B3 at the magnetic equator are (∼47°W Glon, 0.5°N Glat, 26.5° Mlon), (∼40°W Glon, 2.5°N Glat, 33.2° Mlon), and (∼35.5°WGlon, 5.7°N Glat, 38.5° Mlon), respectively.Next, we obtain the EPBs' longitudes at three different magnetic latitude ranges; (10° to 15°), (−6° to 6°), and (−15° to −10°), shown by green, blue, and magenta boxes, respectively, in Figure 2.These latitude ranges distinguish the EIA crests from the magnetic equatorial region.Since B2 reaches lower latitudes than B1 and B3, the EIA crest latitude ranges for B2 are considered to be (6° to 12°) and (−12° to −6°).The brightness along the latitudes in each box is summed at each longitude.From the longitudinal variations of the summed brightness, the EPBs' longitudes in an image were obtained.This method is explained in detail by Karan et al. (2020).B1, B2, and B3 longitudes are obtained from all the nighttime partial disk images on 12 October 2020, which are shown in Figure 3a.Following the same method, B4 and B5 longitudes on 26 December 2021 are obtained and are shown in Figure 3b.
EPB longitudes obtained at the magnetic equator, N and S EIA crests latitudes are shown by blue dots, green plus, and red cross symbols, respectively in Figure 3.One of the advantages of the GOLD observations is that EPB locations are obtained multiple times.Earlier detection of B3 at 22:10 UT is due to the GOLD imager's observation sequence from east to west following the sunset terminator.All EPBs shift eastward with time at each latitude range.From the changes in the longitudes, EPB drift velocities are derived (following the method explained in Karan et al. ( 2020)) at the three latitude ranges and are listed in Table 1.From the EPB zonal drift velocities, it can be seen that for B1 and B5, the zonal drift velocities at the EIA crests are higher than at the magnetic equator which is consistent with their C-shape.Bubbles B3 and B4 show opposite behavior and higher drift velocities at the magnetic equator.On 12 October 2020, the drift velocities at both EIA crest latitudes decreased from west to east (B1 to B3), but increased at the magnetic equatorial latitudes.So, we calculated the inter-bubble separations at different observation times to better understand the connection between the drift velocities and EPBs shapes.Due to the lower zonal velocity of B1 (C-shape) at the magnetic equator and its location, west of B3, the separation of B1 from B3 increased from ∼11.1° to 12.8° during ∼22.5-24.0UT.On the other hand, B5 (C-shape) is east of B4.So, the separation between B5 and B4 (having higher zonal velocity) decreased from ∼5.5° to ∼4.3° during ∼21.8-22.8UT.
At the EIA crests, where the electron densities are largest, we would expect larger ion drag forces and lower zonal wind speeds than at the magnetic equator.As a result, EPB zonal drift velocity would be lowered at these EIA crests than at the equator (Martinis et al., 2001(Martinis et al., , 2003;;Raghavarao et al., 1991;Valladares et al., 2002) producing a reversed C-shape EPB (Huba et al., 2009).This is the case with B3 and B4.Since B2 did not reach the peak of the EIA crests, ion drag effects might not be too different at the three latitude ranges and hence, no latitudinal variation in the shape of B2 is observed.The ion drag force mechanism does not explain the formation of the C-shape EPBs (B1 and B5), where the drift velocities at the EIA crests are higher than at the equator.A possible mechanism for this could be due to the contribution of plasma flux tube integrated neutral zonal wind and F region Pedersen conductivity (Aggson et al., 1987;Huba et al., 2009;Martinis et al., 2003).The primary driver for the zonal plasma drifts is the zonal F region neutral wind, which drives vertical Pedersen currents.A downward polarization electric field is set up to balance the dynamo current in the integrated F region.The downward electric filed moves the plasma eastward with the neutral wind.At lower apex altitude below the F region, the E region winds dominate the electric fields and cause a westward drift or smaller eastward drift.Haerendal (1980) deduced westward plasma flow in the lower F-region which reverses to eastward at higher levels.A strong westward zonal wind in the lower F region producing an eastward tilt at higher altitudes was reported by Huba et al. (2009) and this caused the C-shape EPBs like B1 and B5.This is particularly important  during post-sunset conditions when any potential contribution from E-region could be important.Martinis et al. (2003) compared zonal drifts measured near the magnetic equator and away from it.The results showed that early in the night (∼20:00 to 22:30 LT) when E-region contribution can be important, drifts near the equator were slower than later when no significant E-region contribution exists.In the present case, the GOLD imager scanned the C-shape B1 apex at about 23:15 UT (∼20:00 LT) which was about 2 hr past the E-layer sunset.The C-shape EPB (B5) on 26 December 2021 was scanned at about 22:53 UT (∼21:15 LT) which was 2.5 hr past the E-layer sunset.So, during these early nights the slower zonal plasma drifts at the equatorial latitudes as suggested by Martinis et al. (2003) could have caused the C-shape of B1 and B5.
While the above-mentioned observations and mechanisms may explain the C and reversed C-shape EPBs, the occurrence of both shape EPBs within a small (∼12° and ∼6°) longitude range is significant here.This indicates the existence of small spatial scale E-region density, electric field, neutral wind variations, or a combination of them affecting the EPB shapes.Lower atmospheric gravity waves or in-situ generated secondary gravity waves can cause small-scale longitudinal variations in the E-region density (Mandal et al., 2019;Manju et al., 2023;Pallamraju et al., 2016;Yoshimura et al., 2003).The other potential factor is the longitudinal variation of the electric field.Before about 22 LT, the presence of a perturbation electric field associated with the R-T instability contributes to the vertical as well as zonal movement of an EPB (Huba & Joyce, 2010).The upward movement of an EPB causes an increase in its latitudinal extent, as it maps along geomagnetic field lines which can be seen for B1 and B2.The shorter latitudinal extent of B2 indicates that the EPB did not rise to the heights above the dip equator as reached by B1 and B3.This could be due to the prevalent ambient ionospheric-thermospheric conditions at the longitude where B2 was generated.Small spatial scale variations (∼3° longitude) in the daytime equatorial electric fields have been reported (Karan & Pallamraju, 2017, 2020).B1 and B3 are separated by ∼11.5° longitudes at 23:10 UT.Between the ∼46 min LT, the ambient zonal and vertical plasma drifts also change considerably (Fejer, 2011) which could affect the different EPBs shapes.But, B4 and B5 are separated by ∼5.5° longitudes (which is ∼22 min).So, the difference in the ambient zonal and vertical plasma drifts may not be significant.Narrow longitude variations in zonal winds could be caused by small scale perturbations but current models or observations have not identified such variations.We observed that over the magnetic equatorial latitudes, the zonal plasma drifts are slower at the longitudes of B1 and B5 as compared to the longitudes of other EPBs (see Table 1).Further, the zonal plasma drifts depend on both E and F region conductivities and neutral zonal wind shears; both vary in altitude and latitude.Overall the dynamics of the EPBs are quite complex, particularly in the initial phase after their development.So, the different shapes at such a small longitude range could be due to the combined effect of the factors mentioned above.
On 12 October 2020, the EPBs were observed close to the South Atlantic Anomaly (SAA) regions.Because of particle radiation in the SAA, uncertainties in the ICON (Ionospheric Connection Explorer) measurements are too large for them to be useful.On 26 December 2021, ICON concurrent measurements were not available in the same spatial area of B4 and B5.One potential effect that may be excluded is geomagnetic activity, since in both cases, the geomagnetic conditions were quiet.Unfortunately, it is not possible to conclusively identify the mechanisms responsible for the occurrence of different EPB shapes within a small longitude range.However, observations reported in this study provide a challenge for numerical simulations and an opportunity for advancing our understanding of the I-T system.Numerical simulation efforts are required to understand such events.Considering the adverse impact of ionospheric plasma irregularities on trans-ionospheric satellite communication and navigation, the present observation needs immediate attention in the space science research community for further investigations.

Conclusions
The C-shape and reversed C-shape EPBs have been observed previously.However, in this paper, we report two cases when NASA's GOLD imager observed these differently shaped EPBs within ∼10° longitudes which have not been reported before.Three consecutive EPBs with C-shape, straight, and reversed C-shape were observed within the ∼12° longitude range over the eastern side of South America on 12 October 2020.In another case on 26 December 2021, a reversed C-shape and C-shape EPB were observed within the ∼6° longitudes over the Atlantic sector.These are observed close to the crossing of geographic and geomagnetic equators and, close to the subsatellite point of the GOLD imager.The calculated EPB drift velocities at the magnetic equator and EIA crest latitudes are different which could be due to the latitudinal variations of the zonal wind speeds that drive their motions.Also, the inter-bubble separations corroborated these findings.Further, different EPBs' shapes in such small longitude ranges indicate small-scale longitudinal differences in the E-region density, electric field, neutral wind variations, or a combination of them.These rare and unique observations are crucial for a better understanding of plasma irregularities and provide a challenge for numerical simulations to advance our understanding of the I-T system.
GOLD takes nighttime disk observations using both channels A and B (CHA and CHB hereafter).Most individual scans cover ∼45° in longitude, ∼3 hr in Local Time (LT), just to the east of the sunset terminator.Starting from 20:10 UT, CHB takes nighttime partial disk images, alternating between the Northern and Southern hemispheres until 23:10 UT.From 23:10 to 00:09 UT (the next day) simultaneous observations of the Northern and Southern hemispheres are made using CHA and CHB.The observation sequence is described in detail by Karan et al. (2020).

Figure 1 .
Figure 1.(a) The nighttime 135.6 nm images obtained by GOLD simultaneously by CHA and CHB at 23:10 UT on 12 October 2020 are combined.The black dashed line shows the geomagnetic equator.The two bright emission patches seen at all longitudes on either side of the magnetic equator are the equatorial ionization anomaly (EIA) crests.Equatorial plasma bubbles (EPBs) are the depletions in brightness across the EIA crests.The C-shape, straight, and reversed C-shape EPBs are marked as B1, B2, and B3, respectively.(b) Similar to Figure A but the CHB images obtained on 26 December 2021 at 22:40 UT and 22:55 UT are combined.The reversed C-shape and C-shape EPBs are marked as B4 and B5.

Figure 2 .
Figure 2. Panels (a-d) show the nighttime images observed by GOLD at 23:10, 23:25, 23:40, and 23:55 UT on 12 October 2020, respectively in geomagnetic coordinates.The green, blue, and magenta boxes mark the N-crest, magnetic equator, and S-crest of equatorial ionization anomaly latitudes (common on each panel).

Figure 3 .
Figure 3. Equatorial plasma bubble longitudes at different times of observations on (a) 12 October 2020 and (b) 26 December 2021.Blue dot, green plus, and red cross symbols indicate to longitudes as obtained at the magnetic equator, N and S equatorial ionization anomaly crests latitudes, respectively.