The National Aeronautics and Space Administration (NASA) Global‐scale Observations of the Limb and Disk (GOLD) satellite takes far‐ultraviolet images of the Earth from geostationary orbit. GOLD ...observes the complete structure of equatorial plasma bubbles (EPBs). Since there are repeated observations of the same regions of the Earth, the zonal drift velocities of EPBs are derived using GOLD data. EPBs observed within 60–25°W longitudes on 27–29 November 2018 are considered in the present analysis. The drift velocities obtained on 27 November 2018 are 116 ± 4, 118 ± 6, and 105 ± 9 m/s at the North and South crests of equatorial ionization anomaly (EIA) and the magnetic equator, respectively. While on 29 November the velocities 107 ± 10, 106 ± 8, and 110 ± 4 m/s are in agreement with the 27th (within the uncertainties), on 28 November the velocities are substantially lower: 80 ± 3, 95 ± 7, and 88 ± 11 m/s. This is the first simultaneous measurement of EPB zonal drift velocities at both crests of EIA and the magnetic equator. On 27–29 November 2018, the average spacing between adjacent EPBs is found to be ~377, 526, and 442 km, respectively.
Plain Language Summary
After the sunset, the ionosphere becomes conducive to the formation of plasma irregularities. These irregularities are associated with depletions in the plasma density. In the images obtained from ground or space, these depleted regions look like elongated dark bands and are known as “equatorial plasma bubbles (EPBs).” The transionospheric radio wave propagation, satellite communication, and navigation systems are adversely affected by these bubbles. Thus, it is important to understand and investigate the formation and development of the plasma bubbles. These have been extensively studied using ground‐based instruments and satellites. However, all of these techniques are limited to short time observations and small spatial coverage. NASA's Global‐scale Observations of the Limb and Disk (GOLD) satellite has brought the unique opportunity to observe the Earth's complete disk continuously from the geostationary orbit. In the present study, zonal drift velocities of EPBs are derived from the data obtained by GOLD from geostationary orbit. Further, for the first time, EPB drift velocities are derived simultaneously at crests of the EIA and at the geomagnetic equator.
Key Points
Derivation of zonal drift velocities of EPBs observed by GOLD from geostationary orbit
First simultaneous measurement of EPBs zonal drift velocities at the equator and both crests of the equatorial ionization anomaly (EIA)
Zonal drift velocities of EPBs at the equator and EIA crests are equal
We report a new ionosphere phenomenon: Equatorial ionization anomaly (EIA) discontinuity (EIAD), based on OI 135.6 nm radiance observations from the Global Observations of Limb and Disk (GOLD), ...ground‐based total electron content maps and in‐situ ion density data from Constellation Observing System for Meteorology, Ionosphere, and Climate‐2. The EIAD occurs when the OI radiance of the EIA crest has a local minimum, at a fixed UT, with the radiance in the local longitude region being weaker than that on the east and west sides. In the GOLD field‐of‐view, EIAD follows the seasonal variations of EIA. EIAD appears more often over the Atlantic Ocean and Africa than over South America. It occurs more in the southern crest during the December solstice, and more in the northern crest during both equinoxes. EIAD can occur under both quiet and disturbed times.
Plain Language Summary
The equatorial ionization anomaly (EIA) is very dynamic and can exhibit various structures. Here we report a newly discovered EIA structure: EIA discontinuity, namely the EIA crest shows a lower electron density in the middle longitude range than in east and west longitude ranges. We first show the observation of EIA discontinuity observed concurrently by a geo‐stationary orbit satellite, a low‐earth‐orbit satellite and ground‐based global positioning system receiver. Then a statistical study illustrates that the EIA discontinuity is mostly captured in field‐of‐view of the geo‐stationary satellite in one hemisphere. It obeys the seasonal variation of EIA. The occurrence is higher in the spring equinox than in the fall equinox. Near the December solstices, it appears more in the southern crest. In both equinoxes, it appears more often in the northern crest. In August, its occurrence increases with the increase of solar irradiance. The EIA discontinuity can occur under both geomagnetically quiet and disturbed times.
Key Points
Equatorial ionization anomaly (EIA) discontinuity is the EIA crest with a weaker electron density in a longitude region than longitudes to the east and west
Statistical study shows that its occurrence has a preference in Atlantic Ocean and Africa than America within the Global Observations of Limb and Disk field‐of‐view
EIA discontinuity can occur under both geomagnetically quiet and disturbed times
We conduct observational and modeling studies of thermospheric composition responses to weak geomagnetic activity (nongeomagnetic storms). We found that the thermospheric O and N2 column density ...ratio (∑O/N2) in part of the Northern Hemisphere measured by Global‐scale Observations of the Limb and Disk (GOLD) exhibited large and long‐lived depletions during weak geomagnetic activity in May and June 2019. The depletions reached 30% of quiet time values, extended equatorward to 10°N and lasted more than 10 hr. Furthermore, numerical simulation results are similar to these observations and indicate that the ∑O/N2 depletions were pushed westward by zonal winds. The ∑O/N2 evolution during weak geomagnetic activity suggests that the formation mechanism of the ∑O/N2 depletions is similar to that during a geomagnetic storm. The effects of weak geomagnetic activity are often ignored but, in fact, are important for understanding thermosphere neutral composition variability and hence the state of the thermosphere‐ionosphere system.
Plain Language Summary
The column density ratio of O and N2 (∑O/N2) has been used to monitor geomagnetic storm effects in the thermosphere, as well as providing valuable information about the ionosphere. This triggers an important question: Can weak geomagnetic activities cause changes in thermospheric composition too? Here, we conduct studies based on geostationary orbit observations and numerical simulations. Model outputs replicate the general morphology of this variability for the cases examined. This made it possible to understand the cause of the composition response to weak geomagnetic forcing. We found that the ∑O/N2 depletion observed was pushed westward by the zonal wind. During weak geomagnetic activity, the ∑O/N2 response is similar to the response during a geomagnetic storm, albeit it is weaker. In summary, our study suggests that weak geomagnetic activity can also generate strong and long‐lived responses in thermosphere composition during solar minimum and that this response can be important to understanding the thermosphere and ionosphere variability during the so‐called quiet times.
Key Points
The observed ∑O/N2 exhibits strong and long‐lived response to weak geomagnetic activity
The numerical simulation results resemble the observed ∑O/N2 responses during weak geomagnetic activity
Weak geomagnetic activity may have important effects on thermosphere‐ionosphere variability that cannot be simply ignored
The National Aeronautics and Space Administration (NASA) Global‐scale Observations of the Limb and Disk (GOLD) has been imaging the thermosphere and ionosphere since October 2018. It provides ...continuous measurements over a large area from its geostationary orbit. The unambiguous two‐dimensional (2‐D) maps of OI 135.6 nm radiance retrieved from GOLD after sunset are compared with the total electron content (TEC) maps measured by GPS receivers in the American sector. The OI 135.6 nm radiance observed by GOLD is an indicator of the peak electron density of the ionosphere F2 region, while the TEC is the total electron density in the column. Our comparisons show that the two data sets match each other very well in the equatorial ionization anomaly (EIA) morphology and its seasonal variability. Equatorial plasma bubbles (EPBs) are evident in GOLD nighttime OI 135.6 radiance. Corresponding depletions are shown in TEC maps, but without GOLD data as a reference, it is difficult to discern that the depletions are EPBs. In addition, both GOLD 135.6 radiance and TEC maps observed third peaks of electron density poleward of the southern EIA crests. Furthermore, both show that the ionosphere after sunset is quite dynamic and has strong day‐to‐day variability. In all, the GOLD and TEC have valuable synergy to allow us to gain a better understanding of the equatorial ionosphere.
Key Points
GOLD measured 135.6 nm radiance are similar to total electron content in morphology
GOLD had advantages in observing equatorial plasma bubbles
Following the 2022 Tonga Volcano eruption, dramatic suppression and deformation of the equatorial ionization anomaly (EIA) crests occurred in the American sector ∼14,000 km away from the epicenter. ...The EIA crests variations and associated ionosphere‐thermosphere disturbances were investigated using Global Navigation Satellite System total electron content data, Global‐scale Observations of the Limb and Disk ultraviolet images, Ionospheric Connection Explorer wind data, and ionosonde observations. The main results are as follows: (a) Following the eastward passage of expected eruption‐induced atmospheric disturbances, daytime EIA crests, especially the southern one, showed severe suppression of more than 10 TEC Unit and collapsed equatorward over 10° latitudes, forming a single band of enhanced density near the geomagnetic equator around 14–17 UT, (b) Evening EIA crests experienced a drastic deformation around 22 UT, forming a unique X‐pattern in a limited longitudinal area between 20 and 40°W. (c) Thermospheric horizontal winds, especially the zonal winds, showed long‐lasting quasi‐periodic fluctuations between ±200 m/s for 7–8 hr after the passage of volcano‐induced Lamb waves. The EIA suppression and X‐pattern merging was consistent with a westward equatorial zonal dynamo electric field induced by the strong zonal wind oscillation with a westward reversal.
Plain Language Summary
The extreme Tonga volcano eruption on 15 January 2022 triggered profound and prolonged disturbances in the equatorial ionosphere. Using ground‐based Global Navigation Satellite System total electron content and ionosonde data, low‐Earth orbiting Ionospheric Connection Explorer wind observations, and geostationary Global‐scale Observations of the Limb and Disk ultraviolet measurements of thermospheric and ionospheric airglow emission, we found volcano‐induced atmospheric Lamb waves as a plausible source of the observed pronounced deformation of equatorial ionization anomaly (EIA) in the American sector. This deformation was triggered by volcano‐induced wave passage through the region, even after these waves had traveled 12,000–16,000 km distance for over 10 hr. The typical double‐crested EIA morphology was severely deformed during both the daytime and nighttime. Daytime EIA crests exhibited a severe suppression of more than 10 TEC Unit (TECu) (1TECu = 1016 m−2) and an equatorward collapse around 14–17 UT (10–13 LT) especially for the southern crest. Nighttime EIA crests exhibited a unique X‐pattern with crests drastically merging in a narrow longitude region between 20 and 40°W around 22 UT (20 LT). These large EIA deformations were coincident, sharing spatial scales with long‐lasting oscillations of thermospheric zonal wind following the arrival of volcano‐induced waves in the American sector. This study sheds new light on the potential far‐reaching and long‐lasting atmosphere‐ionosphere‐thermosphere impacts from a catastrophic geological event.
Key Points
Daytime equatorial ionization anomaly (EIA) crests eroded severely by 10+ TEC Unit and collapsed equatorward over 10° latitude, following the arrival of volcano‐induced Lamb waves
EIA crests exhibited a unique deformation after sunset with an X‐pattern merging in a longitudinally‐confined area between 20° and 40°W
Thermospheric zonal wind showed post‐volcanic oscillations of ±200 m/s that may explain the EIA suppression and X‐pattern via dynamo effect
We conducted observational and modeling studies of thermospheric composition and ionospheric total electron content (TEC) variations during two geomagnetically quiet periods (maximum Kp = 1.7) at ...solar minimum. Daytime thermospheric O and N2 column density ratio (∑O/N2) observed by Global‐scale Observations of the Limb and Disk and TEC from a network of ground‐based Global Navigation Satellites System receivers both exhibited large (∼30% of reference values) and long‐lived (5–11 h) day‐to‐day variations in roughly the same mid‐latitude geographic regions. Numerical simulations replicated the observed variability, though not perfectly. Analysis of the simulations suggested that the variations were mainly generated in the high‐latitudes and were subsequently advected equatorward and westward. When high‐latitudes input was turned off in simulations, the variations were negligible. This suggested the potentially important role of high‐latitude geomagnetic forcing in thermospheric composition and ionospheric density variations at mid‐latitudes even during some “geomagnetically quiet” periods at solar‐minimum.
Plain Language Summary
This study presents two cases when geomagnetic forcing can be a plausible source of mid‐latitude thermospheric composition and ionosphere density variations even during what is typically considered as geomagnetically quiet times (magnetic activity index Kp < 2). The column density ratio of thermospheric O and N2 (∑O/N2) plays a major role in the daytime ionospheric F‐region plasma density at mid‐latitudes. In this study, thermospheric and ionospheric variations during geomagnetically quiet times are investigated with the two‐dimensional images of ∑O/N2 provided by a satellite located in geostationary orbit and the ground‐based total electron content (TEC) maps. Both ∑O/N2 and TEC displayed similar strong, long‐lived and localized depletions and enhancements at mid‐latitudes. Numerical simulations driven by an empirical model of geomagnetic activity, but with a climatological tide in the lower boundary, qualitatively produced the patterns of observed variations. Analysis of simulations revealed that ∑O/N2 variations were initially formed at high‐latitudes and then transported equatorward and westward. When geomagnetic forcing was turned off in simulations, the modeled ∑O/N2 and TEC variations were negligible. This study suggests the potentially important roles of high‐latitude forcing in thermosphere and ionosphere variations at mid‐latitudes even during some “geomagnetically quiet” periods at solar‐minimum.
Key Points
Observed ∑O/N2 and total electron content (TEC) exhibited ∼30% day‐to‐day variations at mid‐latitudes under geomagnetically quiet conditions (Kp < 2)
∑O/N2 and TEC variations are mainly due to the horizontal advection of thermospheric composition disturbances from high‐latitudes
Geomagnetic forcing in quiet time can play a crucial role in the mid‐latitude thermosphere and ionosphere variations during solar‐minimum
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 orbit
Maximum poleward shift of the EIA crests and increase in EPB occurrence rate is observed during storm's main phase on 27 September 2020
Concurrent increase of hmF2 from digisonde observation confirms the strengthening of the plasma fountain effect during postsunset hours
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) crests
Indication of the effect of small spatial scale E‐region density, electric field, neutral wind variations, or a combination of them
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 area
The observed postsunset OI 135.6 nm radiance enhancement at Cachoeira Paulista is due to the southward motion of southern Equatorial ionization anomaly (EIA) crest
An event of EIA asymmetry motion occurred during the same time period (<2 h)
Abstract
We conduct observational and modeling studies of thermospheric composition responses to weak geomagnetic activity (nongeomagnetic storms). We found that the thermospheric O and N
2
column ...density ratio (∑O/N
2
) in part of the Northern Hemisphere measured by Global‐scale Observations of the Limb and Disk (GOLD) exhibited large and long‐lived depletions during weak geomagnetic activity in May and June 2019. The depletions reached 30% of quiet time values, extended equatorward to 10°N and lasted more than 10 hr. Furthermore, numerical simulation results are similar to these observations and indicate that the ∑O/N
2
depletions were pushed westward by zonal winds. The ∑O/N
2
evolution during weak geomagnetic activity suggests that the formation mechanism of the ∑O/N
2
depletions is similar to that during a geomagnetic storm. The effects of weak geomagnetic activity are often ignored but, in fact, are important for understanding thermosphere neutral composition variability and hence the state of the thermosphere‐ionosphere system.
Plain Language Summary
The column density ratio of O and N
2
(∑O/N
2
) has been used to monitor geomagnetic storm effects in the thermosphere, as well as providing valuable information about the ionosphere. This triggers an important question: Can weak geomagnetic activities cause changes in thermospheric composition too? Here, we conduct studies based on geostationary orbit observations and numerical simulations. Model outputs replicate the general morphology of this variability for the cases examined. This made it possible to understand the cause of the composition response to weak geomagnetic forcing. We found that the ∑O/N
2
depletion observed was pushed westward by the zonal wind. During weak geomagnetic activity, the ∑O/N
2
response is similar to the response during a geomagnetic storm, albeit it is weaker. In summary, our study suggests that weak geomagnetic activity can also generate strong and long‐lived responses in thermosphere composition during solar minimum and that this response can be important to understanding the thermosphere and ionosphere variability during the so‐called quiet times.
Key Points
The observed ∑O/N
2
exhibits strong and long‐lived response to weak geomagnetic activity
The numerical simulation results resemble the observed ∑O/N
2
responses during weak geomagnetic activity
Weak geomagnetic activity may have important effects on thermosphere‐ionosphere variability that cannot be simply ignored
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