The causes of the ionospheric annual asymmetry, which refers to a larger averaged electron density at geomagnetic conjugate latitudes in December than in June, remain an unresolved problem that still ...generates considerable interest. The ionospheric annual asymmetry in the peak electron density of the F2 layer (NmF2) is typically 20–40%, which cannot be explained by the 7% annual asymmetry in photoionization caused by the shorter Sun‐Earth distance in December. Mikhailov and Perrone (2011, 2015) suggested that the annual asymmetry in atomic oxygen production due to O2 dissociation is sufficient to explain the ionospheric annual asymmetry at middle latitudes. In our study, a series of the Global Mean Model (GMM) simulations have been conducted to test this hypothesis. Although O2 dissociation and eddy diffusion processes are included in the GMM, the simulated annual asymmetry of NmF2 is only 13%. Furthermore, the annual asymmetry increase in neutral composition in our GMM simulations can only explain about one fifth of the ionospheric annual asymmetry. Therefore, the atomic oxygen production mechanism is unlikely to be a major contributor to the ionospheric annual asymmetry.
Key Points
The causes of the ionospheric annual asymmetry remain an unresolved problem
Neutral composition can only explain one fifth of the ionospheric annual asymmetry in the GMM
Atomic oxygen production mechanism is unlikely to be the major contributor to the ionospheric annual asymmetry
The total electron content (TEC) data measured by the Jason, CHAMP, GRACE, and SAC‐C satellites, the in situ electron densities from CHAMP and GRACE, and the vertical E × B drifts from the ROCSAT, ...have been utilized to examine the ionospheric response to the October 2003 superstorms. The combination of observations from multiple satellites provides a unique global view of ionospheric storm effects, especially over the Pacific Ocean and American regions, which were under sunlit conditions during the main phases of the October 2003 superstorms. The main results of this study are as follows: (1) There were substantial increases in TEC in the daytime at low and middle latitudes during both superstorms. (2) The enhancements were greater during the 30 October superstorm and occurred over a wider range of local times. (3) They also tended to peak at earlier local times during this second event. (4) These TEC enhancement events occurred at the local times when there were enhancements in the upward vertical drift. (5) The strong upward vertical drifts are attributed to penetration electric fields, suggesting that these penetration electric fields played a significant role in the electron density enhancements during these superstorms. Overall, the main contribution of this study is the simultaneous view of the storm time ionospheric response from multiple satellites, and the association of local time differences in ionospheric plasma response with measured vertical drift variations.
Key Points
Simultaneous view of ionospheric storm effects from multiple satellites
Greater and overwider LTs seen in the TEC enhancements during the 2nd storm
Good correspondence of LT differences in plasma response with vertical drifts
Recently Mikhailov and Perrone (2020, https://doi.org/10.1029/2019ja027122) proposed the mechanism of neutral atomic oxygen reduction, which is associated with the upper atmospheric circulation, to ...explain the reduction of total thermospheric mass density during the storm recovery phase. The authors further concluded arbitrarily that there is no need to introduce the storm‐time overcooling concept. However, their methodology is fundamentally flawed and their results and conclusions are incorrect. Instead, the overcooling scenario remains a viable concept in understanding thermospheric variations in the storm recovery phase.
Key Points
The methodology, model applicability and hypothesis in Mikhailov and Perrone (2020) are problematic
A self‐consistent ionosphere‐thermosphere simulation system is essential for storm studies
The overcooling scenario remains a viable concept in understanding thermospheric variations in the storm recovery phase
So far studies of the effect of geomagnetic storms on thermospheric density and satellite orbits have been mainly focused on severe storm events caused by Coronal Mass Ejections (CMEs). The effect of ...long‐duration, less intensive geomagnetic activity that is related to Corotating Interaction Regions (CIRs) has not been fully explored. In this paper, thermospheric densities observed by the CHAMP satellite and its orbit parameters are used to compare the responses of satellite orbital altitudes to geomagnetic activity caused by CMEs and CIRs. Three cases are investigated in this paper. Each case had one or two CME storm(s) and one CIR storm that occurred successively. In these cases three out of four CME‐storms were stronger than their corresponding CIR‐storms, but the durations of these CME‐storms were much shorter. Thus, the satellite orbit decay rates during CME‐storms are usually larger than those during CIR‐storms. However, CIR‐storms often had long durations that perturbed satellite orbits for longer periods of time. As a result, the total thermospheric density changes and satellite orbit decays for the entire periods of CIR‐storms were much greater than those for the CME‐storms since these parameters were related to the total energy deposited into the thermosphere/ionosphere, which depended on both the strengths and the durations of the storms. This study indicates that more attention should be paid to CIR storms during the declining phase and during solar minimum, when they occur frequently and periodically. Whereas fewer CME storms occurring under these conditions. We also found that changes in thermospheric densities and CHAMP orbit decay rates correlated well with variations of auroral hemispheric power, but lagging by about 3–6 h.
Key Points
The responses of satellite orbits to CME and CIR storms are studied
Total satellite orbit decays during CIR storms should be paid attention to
Changes in orbit decay rates correlated with those in auroral hemispheric power
The Global‐scale Observations of the Limb and Disk (GOLD) is a National Aeronautics and Space Administration mission of opportunity designed to study how the Earth's ionosphere‐thermosphere system ...responds to geomagnetic storms, solar radiation, and upward propagating atmospheric tides and waves. GOLD employs an instrument with two identical ultraviolet spectrographs that make observations of the Earth's thermosphere and ionosphere from a commercial communications satellite owned and operated by Société Européenne des Satellites (SES) and located in geostationary orbit at 47.5° west longitude (near the mouth of the Amazon River). They make images of atomic oxygen 135.6 nm and N2 Lyman‐Birge‐Hopfield (LBH) 137–162 nm radiances of the entire disk that is observable from geostationary orbit and on the near‐equatorial limb. They also observe occultations of stars to measure molecular oxygen column densities on the limb. Here, we provide an overview of the instrument and compare its prelaunch and early flight measurement performance. Direct comparison of LBH spectra of an electron lamp taken before launch with spectra on orbit provides evidence that both cascade and direct excitation are important sources of thermospheric LBH emission.
Plain Language Summary
The Global‐scale Observations of the Limb and Disk (GOLD) is a National Aeronautics and Space Administration mission of opportunity designed to study how the Earth's ionosphere‐thermosphere system responds to geomagnetic storms, solar radiation, and upward propagating tides on time scales as short as 30 min. GOLD employs two identical ultraviolet spectrographs that make observations of the Earth's thermosphere and ionosphere from a commercial communications satellite owned and operated by SES and located in geostationary orbit at 47.5° west longitude (near the mouth of the Amazon River). They make images of atomic oxygen 135.6 nm and N2 LBH radiances of the entire disk that is observable from geostationary orbit and on the near‐equatorial limb. They also observe occultations of stars to measure molecular oxygen column densities on the limb. Here we describe the GOLD instrument including its optical system and detector. Its performance was characterized in the lab before launch. We compare measurements of laboratory sources made then to observations of the thermosphere after launch and find good agreement.
Key Points
GOLD makes thermospheric images of OI and N2 LBH emissions, ionospheric images of OI emission and observes O2 absorption on the limb
An overview of the instrument design and performance based on laboratory characterization is provided
Imaging and spectroscopic performance confirm laboratory results. Radiometric sensitivity using stars is ~20% less than ground measurement
Solar flare enhancements to the soft X‐ray (XUV) and extreme ultraviolet (EUV) spectral irradiance depend on the location of the flare on the solar disk. Most emission lines in the XUV region (∼0.1 ...to ∼25 nm) are optically thin and are weakly dependent on the location of the flare, but in the EUV region (∼25 to ∼120 nm), many important lines and continua are optically thick, so enhancements are relatively smaller for flares located near the solar limb, due to absorption by the solar atmosphere. The flare irradiance spectral model (FISM) was used to illustrate these location effects, assuming two X17 flares that are identical except that one occurs near disk center and the other near the limb. FISM spectra of these two flares were used as solar input to the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) to investigate the ionosphere/thermosphere (I/T) response. Model simulations showed that in the E region ionosphere, where XUV dominates ionization, flare location does not affect I/T response. However, flare‐driven changes in the F region ionosphere, total electron content (TEC), and neutral density in the upper thermosphere, are 2–3 times stronger for a disk‐center flare than for a limb flare, due to the importance of EUV enhancement. Flare location did not affect the timing of the ionospheric response, but the thermospheric response was ∼20 min faster for the disk‐center flare. Model simulations of I/T responses to an X17 flare on 28 October 2003 were consistent with measurements of TEC and neutral density changes.
Ionospheric observations from the ground‐based GPS receiver network, CHAMP and GRACE satellites and ionosondes were used to examine topside and bottomside ionospheric variations at low and middle ...latitudes over the Pacific and American sectors during the October 2003 superstorms. The latitudinal variation and the storm time response of the ground‐based GPS total electron content (TEC) were generally consistent with those of the CHAMP and GRACE up‐looking TEC. The TECs at heights below the satellite altitudes during the main phases were comparable to, or even less than, the quiet time values. However, the storm time CHAMP and GRACE up‐looking TECs showed profound increases at low and middle latitudes. The ground‐based TEC and ionosonde data were also combined to study the TEC variations below and above the F2 peak height (hmF2). The topside TECs above hmF2 at low and middle latitudes showed significant increases during storm time; however, the bottomside TEC below hmF2 did not show so obvious changes. Consequently, the bottomside ionosphere made only a minor contribution to the ionospheric positive phase seen in the total TEC at low and middle latitudes. Moreover, at middle latitudes F2 peak electron densities during storm time did not have the obvious enhancements that were seen in both the ground‐based and topside TECs, although they were accompanied by increases of hmF2. Therefore, storm time TEC changes are not necessarily related to changes in ionospheric peak densities. Our results suggest that TEC increases at low and middle latitudes are also associated with effective plasma scale height variations during storms.
Key Points
Bottomside ionosphere made a minor contribution to ionospheric positive storm
TEC changes are not necessarily related to ionospheric peak densities
TEC increases are also associated with effective plasma scale height variations
Changes in the thermosphere‐ionosphere system caused by high‐speed streams in the solar wind, and the co‐rotating interaction regions they engender, are studied using a combination of model ...simulations and data analysis. The magnetospheric responses to these structures and consequent ionospheric drivers are simulated using the numerical Coupled Magnetosphere‐Ionosphere‐Thermosphere model and the empirical Weimer 2005 model, finding that the interplanetary magnetic field (IMF) is more important than solar wind speed and density per se in controlling magnetosphere‐ionosphere coupling. The NCAR Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model is then employed to calculate neutral density, nitric oxide cooling, and electron density, for comparison to space‐based measurements from the STAR instrument on the CHAMP satellite, the SABER instrument on the TIMED satellite, and GPS occultations from the COSMIC mission, respectively. The recurrent, periodic changes observed under solar minimum conditions during 2008, and particularly during the Whole Heliospheric Interval (March–April of 2008), are simulated by the model and compared to these measurements. Numerical experiments were conducted to elucidate the mechanisms of solar wind and IMF forcing, setting the solar wind speed and density to nominal values, smoothing the IMF, and also setting it to zero. The results confirm the importance of IMF variations, particularly its north‐south component (Bz), but also show that when the average Bzvalues are negative (southward), the interaction with increased solar wind speed amplifies the magnetosphere‐ionosphere‐thermosphere response. Conversely, during events whenBz is on average positive (northward), even large increases in solar wind speed have small effects on the system.
Key Points
Thermosphere‐ionosphere impacts during HSS/CIR are mostly caused by IMF changes
Amplification due to solar wind speed is most significant when Bz is southward
Numerical modeling describes the diversity of thermosphere‐ionosphere response
The sudden increase of X‐ray and extreme ultra‐violet irradiance during flares increases the density of the ionosphere through enhanced photoionization. In this paper, we use model simulations to ...investigate possible additional contributions from electrodynamics, finding that the vertical E × B drift in the magnetic equatorial region plays a significant role in the ionosphere response to solar flares. During the initial stage of flares, upward E × B drifts weaken in the magnetic equatorial region, causing a weakened equatorial fountain effect, which in turn causes lowering of the peak height of theF2 region and depletion of the peak electron density of the F2 region. In this initial stage, total electron content (TEC) enhancement is predominantly determined by solar zenith angle control of photoionization. As flares decay, upward E × B drifts are enhanced in the magnetic equatorial region, causing increases of the peak height and density of the F2 region. This process lasts for several hours, causing a prolonged F2‐region disturbance and TEC enhancement in the magnetic equator region in the aftermath of flares. During this stage, the global morphology of the TEC enhancement becomes predominantly determined by these perturbations to the electrodynamics of the ionosphere.
Key Points
Flares have significant impacts on ionosphere electrodynamics
Upward ExB drifts initially weaken causing weakening of the Appleton anomaly
Upward ExB drifts strengthen for extended time causing extended TEC disturbance
Electron density in the topside ionosphere has significant variations with latitude, longitude, altitude, local time, season, and solar cycle. This paper focuses on the global and seasonal features ...of longitudinal structures of daytime topside electron density (Ne) at middle latitudes and their possible causes. We used in situ Ne measured by DEMETER and F2 layer peak height (hmF2) and peak density (NmF2) from COSMIC. The longitudinal variations of the daytime topside Ne show a wave number 2‐type structure in the Northern Hemisphere, whereas those in the Southern Hemisphere are dominated by a wave number 1 structure and are much larger than those in the Northern Hemisphere. The patterns around December solstice (DS) in the Northern Hemisphere (winter) are different from other seasons, whereas the patterns in the Southern Hemisphere are similar in each season. Around March equinox (ME), June solstice (JS), and September equinox (SE) in the Northern Hemisphere and around ME, SE, and DS in the Southern Hemisphere, the longitudinal variations of topside Ne have similar patterns to hmF2. Around JS in the Southern Hemisphere (winter), the topside Ne has similar patterns to NmF2 and hmF2 does not change much with longitude. Thus, the topside variations may be explained intuitively in terms of hmF2 and NmF2. This approach works reasonably well in most of the situations except in the northern winter in the topside not too far from the F2 peak. In this sense, understanding variations in hmF2 and NmF2 becomes an important and relevant subject for this topside ionospheric study.
Key Points
The longitudinal variations of topside Ne have similar patterns to hmF2, except in the northern winter (DS) and southern winter (JS)
In the southern winter (JS) the topside Ne has similar patterns to NmF2, whereas hmF2 does not change much with longitude
The patterns of topside Ne in winter are different from other seasons in the Northern Hemisphere
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