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Using NASA's Global‐scale Observations of the Limb and Disk (GOLD) imager, we report nightside ionospheric changes during the G5 super geomagnetic storm of 10 and 11 May 2024. Specifically, the nightside southern crest of the Equatorial Ionization Anomaly (EIA) was observed to merge with the aurora near the southern tip of South America. During the storm, the EIA southern crest was seen moving poleward as fast as 450 m/s. Furthermore, the aurora extended to mid‐latitudes reaching the southern tips of Africa and South America. The poleward shift of the equatorial ionospheric structure and equatorward motion of the aurora means there was no mid‐latitude ionosphere in this region. These observations offer unique insights into the ionospheric response to extreme geomagnetic disturbances, highlighting the complex interplay between solar activity and Earth's upper atmosphere. Plain Language Summary: On Earth's nightside during the super geomagnetic storm that occurred on 10 May 2024, NASA's GOLD imager saw something new: a part of Earth's ionosphere, the southern peak of what typically appears as a double‐peaked structure in the ionospheric density at equatorial and low latitudes, merged with the aurora near the southern tip of South America. This has never been reported before. Additionally, the boundary of the aurora expanded further equatorward than usual. These observations of what happened in the Earth's ionosphere during this super storm are reported for the first time in this study. Key Points: EIA crests between ∼70° and 35°W moved poleward, with northern and southern crest reaching ∼38°N and ∼35°S Mlat in the American sectorSouthern EIA crest moved poleward with a speed of ∼450 m/s near ∼55°W Glon during strong IMF Bz and d(Dst)/dtFirst observation of the merging of an EIA crest with the aurora indicating no mid‐latitude ionosphere [ABSTRACT FROM AUTHOR]

A coronal mass ejection erupted from the Sun on 21 April 2023 and created a G4 geomagnetic storm on 23 April. NASA's global‐scale observations of the limb and disk (GOLD) imager observed bright equatorial ionization anomaly (EIA) crests at ∼25° Mlat, ∼11° poleward from their average locations, computed by averaging the EIA crests during the previous geomagnetic quiet days (18–22 April) between ∼15°W and 5°W Glon. Reversed C‐shape equatorial plasma bubbles (EPBs) were observed reaching ∼±36° Mlat (∼40°N and ∼30°S Glat) with apex altitudes ∼4,000 km and large westward tilts of ∼52°. Using GOLD's observations EPBs zonal motions are derived. It is observed that the EPBs zonal velocities are eastward near the equator and westward at mid‐latitudes. Model‐predicted prompt penetration electric fields indicate that they may have affected the postsunset pre‐reversal enhancement at equatorial latitudes. Zonal ion drifts from a defense meteorological satellite program satellite suggest that westward neutral winds and perturbed westward ion drifts over mid‐latitudes contributed to the observed latitudinal shear in zonal drifts. Plain Language Summary: A severe geomagnetic storm occurred on 23 April 2023. The effects of the storm on the nighttime equatorial and mid‐latitude ionosphere are investigated using NASA's global‐scale observations of the limb and disk (GOLD) measurements. GOLD observed bright, widely separated equatorial ionization anomaly (EIA) crests between ∼15°W and 5°W Glon. Extreme equatorial plasma bubbles (EPBs) reaching mid latitudes at ∼40 N and ∼30°S Glat with apex altitudes of ∼4,000 km at the magnetic equator were observed over these longitudes. The EPBs velocities were eastward at low latitudes, between the EIA crests, and reversed to westward near mid‐latitudes. At mid‐latitudes EPBs showed westward tilts that were larger than previous observations during similar geomagnetic conditions. Key Points: Maximum poleward shift (∼11°) of equatorial ionization anomaly crests was observed between ∼15°W and 5°W GlonReversed C‐shape Equatorial Plasma Bubbles (EPBs) extended to ∼±36° Mlat (∼40°N and ∼30°S Glat) with apex altitudes reaching ∼4,000 kmShear in EPBs zonal drifts is formulated as a function of magnetic apex heights based on their tilts [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: The post‐sunset ionosphere becomes conducive to the formation of plasma irregularities associated with depleted plasma densities. In the images obtained from space or ground, these plasma depleted regions appear as latitudinally elongated dark bands, which are known as "equatorial plasma bubbles (EPBs)." Satellite communication and navigation systems are adversely affected when the trans‐ionospheric radio signals travel through the EPBs. Thus, investigations of EPBs' formation and development are important. In 2‐D airglow images, the EPBs appear to be straight (aligned along the magnetic field lines). However, sometimes the poleward extensions of the EPBs can be tilted eastward or westward from the magnetic field line resembling either a C‐shape or reversed C‐shape, respectively. These differently shaped EPBs have been observed before. But, NASA's Global‐scale Observations of the Limb and Disk (GOLD) mission observed them simultaneously within ∼12° and 6° longitudes on 12 October 2020 and 26 December 2021 which is unusual. Such occurrence of opposite‐shaped EPBs within a small longitude range indicates small‐scale longitudinal variations in the E‐region density, neutral winds, electric fields, or a combination of them. We present detailed observations of these two unique events with possible explanations. Key Points: Differently shaped equatorial plasma bubbles are observed simultaneously within a small longitude of ∼10°Observations indicate longitudinal variations in EPBs' zonal drift velocities at the magnetic equator and equatorial ionization anomaly (EIA) crestsIndication of the effect of small spatial scale E‐region density, electric field, neutral wind variations, or a combination of them [ABSTRACT FROM AUTHOR]

The nighttime ionospheric response to a geomagnetic storm that occurred on 23–29 September 2020 is investigated over the South American, Atlantic, and West African longitude sectors using NASA's Global‐scale Observations of the Limb and Disk measurements. On 27 September the solar wind conditions were favorable for prompt penetration electric fields to influence the equatorial ionosphere over extended longitudes. The equatorial ionization anomaly (EIA) crests were shifted 8°–10° poleward compared to the quiet time monthly mean across ∼65°–35°W during the main phase. Ionosonde hmF2 (peak electron density height) measurements from Fortaleza (GG: 3.9°S and 38.4°W) indicated a stronger prereversal enhancement this evening than other nights. As a result, equatorial plasma bubbles (EPB) occurred at these longitudes on this evening. This is the first simultaneous investigation of EIA morphology and EPB occurrence rate over an extended longitude range from geostationary orbit during a geomagnetic storm. Plain Language Summary: The effects of a geomagnetic storm that occurred during 23–29 September 2020 on the nighttime equatorial ionospheric behavior is investigated using NASA's Global‐scale Observations of the Limb and Disk (GOLD) measurements. On each evening, the equatorial ionization anomaly (EIA) crests locations and brightnesses and equatorial plasma bubbles (EPB) occurrence rates are obtained over the South American, Atlantic, and West African longitude sectors. On 27 September the solar wind conditions were favorable for the penetration of interplanetary electric fields to the equatorial ionosphere over the dusk longitude sectors (∼35°W Lon). The PPEF strengthened the pre‐reversal enhancement and thereby enhanced the plasma fountain effect. Ionospheric F2 layer height increase is confirmed by digisonde measurements at Fortaleza (GG: 3.9°S and 38.4°W). On this day, the maximum poleward shifts compared to the quiet time monthly mean values are observed across ∼65°–35°W longitude during the storm's main phase. EPBs occurrence rate was maximum on this night. The present study reports the first simultaneous investigation of EIA morphology (crests locations and brightnesses) and EPB occurrence rates over an extended longitude range during a geomagnetic storm from a geostationary orbit. Key Points: First simultaneous observations of geomagnetic storm effects on equatorial ionization anomaly (EIA) morphology and equatorial plasma bubble (EPB) occurrence rate from the geostationary orbitMaximum poleward shift of the EIA crests and increase in EPB occurrence rate is observed during storm's main phase on 27 September 2020Concurrent increase of hmF2 from digisonde observation confirms the strengthening of the plasma fountain effect during postsunset hours [ABSTRACT FROM AUTHOR]

The Global‐scale Observation of Limb and Disk (GOLD) mission, for the first time, provides synoptic two‐dimensional (2D) maps of OI 135.6 nm observations. These maps describe the unambiguous and dynamic evolution of nighttime ionospheric F2‐peak electron densities (NmF2) as the 135.6 nm airglow emission radiance correlates well with NmF2 at night. On November 19, 2018, the 135.6 nm radiance measured by GOLD, NmF2 measured by a digisonde, and GPS total electron content (TEC) measurements at Cachoeira Paulista (CP) all showed a postsunset enhancement, with an increase near 22:30 universal time. The 135.6 nm radiance map showed that this enhancement was due to the southward movement of the southern equatorial ionization anomaly (EIA) crest. Therefore, the GOLD observation showed the linkage between postsunset enhancement of NmF2 and EIA movement. Furthermore, unlike the southward movement of the southern crest, the corresponding EIA northern crest, however, did not show northward motion. This is the first time that the EIA hemispheric asymmetry, which included both different densities and movement of two crests in a short time period (<2‐h), was captured. The cause of this asymmetric movement of the two crests is not clear and requires further investigation.Plain Language Summary: The postsunset enhancement of equatorial ionospheric NmF2 has been observed by low Earth orbit satellites and ground‐based equipment. However, these observations can only provide variations along satellite orbits or at a single location. In this paper, we provide, for the first time, two‐dimensional (2D) maps of nighttime OI 135.6 nm airglow observations by GOLD. These maps describe unambiguous and dynamic evolution of nighttime NmF2 as the 135.6 nm radiance is correlated well with nighttime NmF2. On the night of November 19, 2018, GOLD observed 135.6 nm radiance, NmF2 measured by digisonde, and total electron content at Cachoeira Paulista (CP) all exhibited a postsunset enhancement. This is the first time that the two dimensional (2D) measurements of the ionosphere by a geostationary satellite are compared with local observations in the same time range. The evolution of 135.6 nm radiance map suggests that this enhancement was mainly due to the southward movement of the southern equatorial ionization anomaly (EIA) crest. The corresponding northern EIA crest of CP is over the Ocean, which can be observed by GOLD and found to keep stationary, but not easily by other means of observations. This asymmetric movement of the two EIA crests is unknown and requires further investigation in the future.Key Points: The GOLD nighttime observations match well with digisonde and total electron content at both single location and large areaThe observed postsunset OI 135.6 nm radiance enhancement at Cachoeira Paulista is due to the southward motion of southern Equatorial ionization anomaly (EIA) crestAn event of EIA asymmetry motion occurred during the same time period (<2 h) [ABSTRACT FROM AUTHOR]