The solar minimum period during 2008–2009 was characterized by lower thermospheric density than the previous solar minimum and lower than any previously measured. Recent work used the NCAR ...Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model to show that the primary cause of density changes from 1996 to 2008 was a small reduction in solar extreme ultraviolet (EUV) irradiance, causing a decrease in thermospheric temperature and hence a contracted thermosphere. There are similar effects in the ionosphere, with most measurements showing an F region ionosphere that is unusually low in density, and in peak altitude. This paper addresses the question of whether model simulations previously conducted, and their solar, geomagnetic, and anthropogenic inputs, produce ionospheric changes commensurate with observations. We conducted a 15 year model run and obtained good agreement with observations of the global mean thermospheric density at 400 km throughout the solar cycle, with a reduction of ~30% from the 1996 solar minimum to 2008–2009. We then compared ionosonde measurements of the midday peak density of the ionospheric F region (NmF2) to the model simulations at various locations. Reasonable agreement was obtained between measurements and the model, supporting the validity of the neutral density comparisons. The global average NmF2 was estimated to have declined between the two solar minima by ~15%. In these simulations, a 10% reduction of solar EUV plays the largest role in causing the ionospheric change, with a minor contribution from lower geomagnetic activity and a very small additional effect from anthropogenic increase in CO2.
Key Points
The ionosphere and thermosphere were anomalously low in density during 2008‐2009
Model simulations comparing 2008‐2009 to 1996 generally agree with observations
The primary cause was lower solar EUV irradiance than the previous solar minimum
The period of 6–11 September 2017 was an active period in which multiple solar flares and a major geomagnetic storm occurred. The two largest flares, an X9.3 and an X8.2, were a disk flare and a limb ...flare, respectively. We conducted model simulations and data analysis to examine solar flare effects on the coupled thermosphere and ionosphere (TI) system in connection with flare location effects and to investigate the occurrences of large‐scale traveling atmosphere disturbances (TADs) due to flares and storms. Soft X‐ray enhancement, which dominates E region ionization, is essentially not affected by the location of a flare on the Sun; solar extreme ultraviolet enhancement, which dominates ionization above ~150 km, is much weaker for a limb flare compared to a disk flare with the same magnitude. Consequently, flare responses in the lower thermosphere, and ionosphere E region are not affected by flare locations, but above ~150 km, the TI system responds more strongly to disk flares than to limb flares. However, our studies show that during the space weather events in September 2017, these flare location effects were masked by other factors including local time and longitude. Large‐scale TADs occurred when there were both flares and storms. The presence of the flares changed the magnitudes and propagation speeds of the large‐scale TADs. However, there was no evidence that large‐scale TADs occurred when there were only flares and not storms, indicating that solar flares alone were not sufficient to excite large‐scale TADs.
Key Points
Flare location effects were masked by geophysical conditions including local time and longitude
Solar flares changed the amplitudes and propagation speeds of TADs excited by storms
Solar flares alone were not the sufficient conditions to excite large‐scale TADs
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
There is still an inadequate understanding of how the interplanetary magnetic field (IMF) east‐west component (By) affects thermospheric composition, and other ionospheric and thermospheric fields in ...a systematic way. Utilizing the state‐of‐art first‐principles Coupled Magnetosphere Ionosphere Thermosphere (CMIT) modeling and TIMED/Global Ultraviolet Imager (GUVI)‐observed ΣO/N2 covering an entire solar cycle (year 2002–2016), as well as a neutral parcel trajectory tracing technique, we emphasize that not only the direction of By, but also its strength relative to the IMF north‐south component (Bz) that has important effects on high latitude convection, Joule heating, electron density, neutral winds, and neutral composition patterns in the upper thermosphere. The Northern Hemisphere convection pattern becomes more twisted for positive By cases than negative cases: the dusk cell becomes more rounded compared with the dawn cell. Consequently, equatorward neutral winds are stronger during postmidnight hours in negative By cases than in positive By cases, creating a favorable condition for neutral composition disturbances (characterized by low ΣO/N2) to expand to lower latitudes. This may lead to a more elongated ΣO/N2 depletion area along the morning‐premidnight direction for negative By conditions compared with the positive By conditions. Backward neutral parcel trajectories indicate that a lower ΣO/N2 parcel in negative By cases comes from lower altitudes, as compared with that for positive By cases, leading to larger enhancements of N2 in the former case.
We investigated trends of carbon dioxide (CO2) in the upper atmosphere, using data from the Atmosphere Chemistry Experiment Fourier Transform Spectrometer and from the Sounding of the Atmosphere ...using Broadband Emission Radiometry. Recent analyses of these measurements had indicated that CO2 above approximately 90 km appeared to be increasing about twice as fast as it was in the lower atmosphere. Models could not reproduce this differential CO2 trend, calculating instead that the proportional CO2 increase is approximately constant with altitude. We found three issues with the methodologies used to derive trends from CO2 profiles: the way that seasonal changes and sampling are accounted for in the analysis, referred to as deseasonalizing; the registration of profiles in pressure versus altitude coordinates; and data quality indicators. Each of these can have significant effects on the derivation of trends. We applied several deseasonalizing procedures, using both pressure and altitude coordinates, also used a time series fit without deseasonalizing, and applied data quality filters. The derived trends were approximately constant with pressure or altitude, about 5.5% per decade, consistent with lower atmosphere CO2 trends, and consistent with model calculations. We conclude that the difference between the trend of CO2 above the CO2 homopause and the trend in the lower, well‐mixed atmosphere is not statistically significant.
Key Points
Deseasonalizing methods can have significant effects on the derivation of trends
The registration of profiles in pressure versus altitude coordinates and data quality indicators can also affect the derivation of trends
The difference between the trend of CO2 above its homopause and the trend in the lower atmosphere is not statistically significant
Plain Language Summary
Carbon dioxide (CO2) has been increasing in the atmosphere where we live, at an average rate of about 5.5% per decade in the past several decades. This increase of CO2 causes a global warming effect here, but this same increase of CO2 causes a global cooling effect in the space where low Earth orbit satellites fly. Is the rate of CO2 increase the same in the space, or is CO2 increasing twice as fast in the space as it does down here, as suggested by some recent research? This question is important since the rate of CO2 increase determines the rate of cooling it causes in the space. The more cooling, the less drag that space objects encounter; consequently, more space junks accumulate in the space environment. We investigated the rate of CO2 increase in the space based on recent satellite observations. We found that the rate of CO2 increase is approximately constant with altitude, at about 5.5% per decade in the space, the same as the rate of CO2 increase in the atmosphere where we live.
Topside ionospheric total electron content (TEC) observations from multiple low‐Earth orbit (LEO) satellites have been used to investigate the local time, altitudinal, and longitudinal dependence of ...the topside ionospheric storm effect during both the main and recovery phases of the March 2015 geomagnetic storm. The results of this study show, for the first time, that there was a persistent topside TEC depletion that lasted for more than 3 days after the storm main phase at most longitudes, except in the Pacific Ocean region, where the topside TECs during the storm recovery phase were comparable to the quiet time ones. The observed depletion in the topside ionospheric TEC was relatively larger at higher altitudes in the evening sector and greater at local times closer to midnight. Moreover, the topside TEC patterns observed by MetOp‐A (832 km) were different from those seen by other LEO satellites with lower orbital altitudes during the storm main phase and at the beginning of the recovery phase, especially in the evening sector. This suggests that the physical processes that control the storm time behavior of topside ionospheric response to storms are altitude‐dependent.
Key Points
Long‐duration depletion in topside TEC after storm main phase except in the Pacific Ocean region
Greater depletion at higher altitudes in the evening and at local times closer to midnight
Storm time topside TEC patterns showed strong altitudinal dependence
Annual/semiannual variation of the ionosphere Qian, Liying; Burns, Alan G.; Solomon, Stanley C. ...
Geophysical research letters,
28 May 2013, Letnik:
40, Številka:
10
Journal Article
Recenzirano
Odprti dostop
We investigated the relationship between the systematic annual and semiannual variations in the ionosphere and thermosphere using a combination of data analysis and model simulation. A climatology of ...daytime peak density and height of the ionospheric F2 layer was obtained from GPS radio occultation measurements by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) during 2007–2010. These measurements were compared to simulations by the NCAR Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIE‐GCM). Model reproduction of the ionospheric annual and semiannual variations was significantly improved by imposing seasonal variation of eddy diffusion at the lower boundary, which also improves agreement with thermospheric density measurements. Since changes in turbulent mixing affect both the thermosphere and ionosphere by altering the proportion of atomic and molecular gases, these results support the proposition that composition change drives the annual/semiannual variation in both the neutral and ionized components of the coupled system.
Key Points
Turbulent mixing in the mesopause drives composition change in the thermosphereThe turbulent mixing shows seasonal changesComposition change drives the annual/semiannual variation in the ionosphere
Enhanced energy input from the magnetosphere to the upper atmosphere during geomagnetic storms has a profound effect on thermospheric density and consequently near‐Earth satellite orbit decay. These ...geomagnetic storms are caused by two different processes. The first is coronal mass ejections (CMEs) and the second is corotating interaction regions (CIRs). CME‐driven storms are characterized by large maximum energy input but relatively short duration, whereas CIR‐driven storms have relatively small maximum energy input but are of a considerably longer duration. In this paper we carried out a statistical study to assess the relative importance of each kind of storm to satellite orbital decay. The results demonstrate that CIR storms have a slightly larger effect on total orbital decay than CME storms do in a statistical sense. During the declining phase and the minimum years of a solar cycle, CIR storms occur frequently and quasiperiodically. These storms have a large effect on thermospheric densities and satellite orbits because of their relatively long duration. Thus, it is important to fully understand their behavior and impact.
Key Points
The importance of storms to satellite orbital decay is assessed statisticallyCIR storms have a slightly larger effect on total orbital decay than CME stormsStrong CME storms often produce large instantaneous changes in satellite orbit
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 upward looking ionospheric total electron content (TEC) from the MetOp‐A and TSX satellites during 2008–2015 has been used to systematically study the longitudinal variations of the topside ...ionosphere and plasmasphere. The results of this study are summarized as follows: (1) There are significant longitudinal variations in the topside ionosphere and plasmasphere at low latitudes. The TEC maximum during the June solstice over the Western and Central Pacific Ocean corresponds to a TEC minimum at the same location during the December solstice, but the opposite behavior occurs over South America and the Atlantic Ocean. (2) During the solstices, the relative longitudinal variations in the geomagnetic equatorial region do not have a strong dependence on local time and solar activity. (3) The TEC in the winter hemisphere decreases with increasing solar activity, especially at higher altitudes and at night. The topside TEC depletion with solar activity depends on longitude. (4) The solstice‐like longitudinal pattern lasts much longer than the equinox‐like patterns, with the June solstice pattern lasting the longest. Furthermore, the equinox‐like longitudinal patterns occur in March when expected, whereas they extend from the autumnal equinox until the end of October. (5) The longitudinal variations of upward looking TEC are different from the corresponding longitudinal variations of electron densities around the F2 peak and orbital altitudes. This indicates that the topside ionosphere structure is strongly influenced by the physical processes in the topside region, rather than being a pure reflection of the ionospheric F2 peak structure.
Key Points
Opposite longitudinal patterns of topside TEC between June and December
Durations of solstice‐like patterns are much longer than the equinoctial ones
Longitudinal patterns of topside TEC are generally independent of solar activity
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