Abstract
Geomagnetic storms of G1-class were observed on 3 and 4 February 2022, which caused the loss of 38 out of 49 SpaceX satellites during their launch due to enhanced neutral density. The ...effects of storm-time neutral dynamics and electrodynamics over the American sector during this minor storm have been investigated using Global Positioning System—total electron content (TEC) and Global‐scale Observations of the Limb and Disk (GOLD) mission measured thermospheric composition and temperature. Results revealed an unexpected feature in terms of increase in O/N
2
and depletion in TEC over the American low-latitudes. This feature is in addition to the classic storm time ionospheric variations of enhancement in ionospheric electron density in presence of enhanced O/N
2
and an intense equatorial electrojet (EEJ). Further, significant morning-noon electron density reductions were observed over the southern mid-high latitudes along the American longitudes. Results from Multiscale Atmosphere-Geospace Environment (MAGE) model simulations elucidated storm-induced equatorward thermospheric wind which caused the strong morning counter electrojet by generating the disturbance dynamo electric field. This further explains the morning TEC depletion at low-latitudes despite an increase in O/N
2
. Sub-storm related magnetospheric convection resulted in significant noon-time peak in EEJ on 4 February. Observation and modelling approaches together suggested that combined effects of storm-time neutral dynamic and electrodynamic forcing resulted in significant ionospheric variations over the American sector during minor geomagnetic storms.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Oxygen dayglow emissions at multiple wavelengths that emanate from different heights (from around 130 km to peak altitude of the ionospheric F region) over a low‐latitude location showed systematic ...enhancements in intensities throughout the daytime hours during four sudden stratospheric warming (SSW) events that occurred in the years 2010–2013. The lower thermospheric temperatures at 120 km obtained from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument are found to be enhanced during SSW events at arctic latitudes and show a gradient with a decrease toward low‐latitudes. During these events, the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics Doppler Interferometer measurements showed equatorward winds in the mesosphere lower thermosphere (MLT) altitudes over high‐latitudes. Both the high‐latitude lower thermospheric temperature enhancements and the MLT region equatorward winds occur nearly simultaneously with the observed enhancements in the atomic oxygen dayglow emission intensities at all the wavelengths over low‐latitudes. Based on these measurements and other supporting information, it is proposed that a new cell of meridional circulation in the MLT winds is set up during SSW events, which enables transport of atomic oxygen from high‐to‐low latitudes. Such an additional contribution of oxygen density over low‐latitudes interacts with lower thermospheric daytime dynamics in that region and is attributed to be the cause for the observed enhancement in the oxygen daytime optical emission intensities over low‐latitudes. The results presented here provide experimental evidence to such circulation alluded to by earlier simulation studies.
Key Points
Enhancement in low‐latitude oxygen dayglow intensity during SSW
Mesosphere‐thermospheric meridional circulation during SSW is proposed
Meridional transport of oxygen due to circulation explains dayglow enhancement
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The OI 630.0 nm airglow emission variability provides salient information on the dynamical changes taking place in the upper atmosphere at around 250 km. The emission rates vary with the changes in ...the ambient electron densities and the neutral constituents that are associated with these emissions. On several occasions, enhancements in these emissions are observed during post‐sunset hours, around 21 local time (LT), as measured from Mt. Abu (24.6°N, 72.7°E, 19°N Mag), a low‐latitude location at Indian longitudes. These enhancements occur following the typical monotonic decrease in emission intensity after sunset. The presence of poleward meridional wind preceded by cessation and reversal of equatorward wind at the post‐sunset hours was shown to be the cause for such observed emission enhancements in an earlier study. In this study, the cause of such reversal in meridional winds during post‐sunset hours has been investigated using the variation in electron densities and meridional winds simulated by the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X), which also shows enhancements in electron densities similar to those observed in the post‐sunset OI 630.0 nm nightglow emissions, and simultaneous reversal in meridional winds as well. The amplitudes and phases of different components of tides obtained from WACCM‐X meridional winds reveal a significant contribution of higher‐order tides, especially, quarter‐diurnal tides, to the observed reversal in the meridional winds during post‐sunset hours.
Plain Language Summary
Airglow emissions occur when atoms and molecules are deexcited after being excited by the solar energetic photons. OI 630.0 nm emissions (redline), which peak around 250 km and have a width of emissions around 100 km, act as tracers of that altitudinal region. Typically, the redline emissions decrease monotonically with time after sunset in the absence of solar radiation. However, on several occasions, an enhancement in redline emissions has been measured during post‐sunset hours over a low‐latitude location in the Indian longitudes. A previous study attributed this enhancement to the increase in electron density at the emission altitudes in the presence of poleward meridional winds, despite usually being equatorward during such times. The cause of such reversal in the equatorward winds and simultaneous increase in electron density at post‐sunset hours has been examined in this study using the variation in electron densities and winds simulated by the Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (WACCM‐X). Remarkably, the simulation results also show enhancements in electron density similar to those observed in redline emissions. A strong tidal contribution, especially quarter‐diurnal component, has been found to be the cause behind the poleward reversal of meridional winds after sunset that causes an enhancement in redline emissions.
Key Points
OI 630.0 nm post‐sunset emission enhancement over low‐latitudes is consistent with the presence of poleward meridional winds
On some days, WACCM‐X simulated meridional winds show a poleward wind reversal during post‐sunset hours
Quarter‐diurnal tides seem to play a significant role in reversing the meridional winds after sunset
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Although sudden stratospheric warming (SSW) is mainly a northern high‐latitude phenomena, there are several reports of a concomitant global dynamical response throughout the mesosphere and lower ...thermosphere. Published reports based on model simulations so far attributed such variabilities to changes in global circulation; however, there is no clear explanation of how all these regions are physically connected during SSW events. The present investigation uses wind observations from two ground‐based specular meteor radars over northern high latitudes and midlatitudes and global winds from a high‐altitude meteorological analysis system to characterize global mesospheric circulation anomalies for major SSW events during 2010 and 2013. During these events radar observations and the reanalysis winds exhibited strong southward winds over the two northern midlatitude and high‐latitude stations. By removing seasonal variability from the high‐altitude meteorological analyses, we show that these southward wind anomalies are indeed part of a larger global‐scale circulation, which gets set up during SSW and extend from the Northern pole to low‐latitude regions of Southern Hemisphere in the mesosphere and lower thermosphere altitudes. These results also offer a possible explanation of how low‐latitude ionospheric electrodynamics are influenced by the changes in the circulation set in during SSW at high latitudes.
Key Points
SMR and high‐altitude assimilation show southward winds at Northern high latitudes and midlatitudes during SSW
The southward feature is seen to extend over the whole Northern hemisphere in assimilated zonal mean meridional wind
Direct and indirect roles of this meridional circulation on low‐latitude ionosphere‐thermosphere are discussed
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Comprehensive behavior of the low-latitude upper atmosphere during sudden stratospheric warming (SSW) events at varying levels of solar activity has been investigated. The equatorial electrojet (EEJ) ...strength and the total electron content (TEC) data from low latitudes over Indian longitudes during the mid-winter season in the years 2005 to 2013 are used in this study. Five major and three minor SSW events occurred in the observation duration, wherein the solar activity had varied from minimum (almost no sunspots) to mini-maximum (approximately 50 sunspots of the solar cycle 24). Spectral powers of the large-scale planetary wave (PW) features in the EEJ and the TEC have been found to be varying with solar activity and SSW strengths. Specially, the spectral powers of quasi-16-day wave variations during the three very strong SSW events in the years 2006, 2009, and 2013 were found to be very high in comparison with those of other years. For these major events, the amplitudes of the semi-diurnal tides and quasi-16-day waves were found to be highly correlated and were maximum around the peak of SSW, suggesting a strong interaction between the two waves. However, this correlation was poor and the quasi-16-day spectral power was low for the minor events. A strong coupling of atmospheres was noted during a relatively high solar activity epoch of 2013 SSW, which was, however, explained to be due to the occurrence of a strong SSW event. These results suggest that the vertical coupling of atmospheres is stronger during strong major SSW events and these events play an important role in enabling the coupling even during high solar activity.
A total solar eclipse occurred in the Southern Hemisphere on 2 July 2019 from approximately 17 to 22 UT. Its effect in the thermosphere over South America was imaged from geostationary orbit by ...NASA's Global‐scale Observation of Limb and Disk (GOLD) instrument. GOLD observed a large brightness reduction (>80% around totality) in OI 135.6 nm and N2 LBH band emissions compared to baseline measurements made 2 days prior. In addition, a significant enhancement (with respect to the baseline) in the ΣO/N2 column density ratio (~80%) was observed within the eclipse's totality. This enhancement suggests that the eclipse induced compositional changes in the thermosphere. After the eclipse passed, a slight enhancement in ΣO/N2 column density ratio (~7%) was also seen around the totality path when compared to measurements before the eclipse. These observations are the first synoptic imaging measurements of an eclipse's thermospheric effects with the potential to drastically improve and test our understanding of how the thermosphere responds to rapid, localized changes in solar short wavelength radiation.
Plain Language Summary
During total solar eclipses, solar radiation incident on parts of the Earth is blocked by the Moon. At Earth's surface, this creates almost night‐like conditions within totality. In the upper atmosphere, a reduction in solar radiation (>90% around totality) significantly decreases ionization, and the heating of the atmosphere is also reduced. These reductions also change the airglow emissions in the thermosphere. Since airglow emissions are produced by physical and photochemical processes in the upper atmosphere, changes in their brightnesses act as a proxy to these processes. In this paper, we present imaging observations of the eclipse's effects in the thermosphere. These effects are presented as brightness changes in prominent thermospheric ultraviolet emissions observed by NASA's GOLD mission from a geostationary orbit during the 2 July 2019 total solar eclipse.
Key Points
First synoptic global‐scale observations of a solar eclipse's effect on the thermosphere are presented based on FUV imaging
Greater than 80% reduction around eclipse totality is seen in OI 135.6 nm and N2 LBH emission brightnesses
Enhanced ΣO/N2 column density ratio (~80%) is observed within eclipse's totality and after the eclipse (~7%) suggesting compositional changes
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We present the first horizontal divergence and relative vorticity measurements at polar mesospheric altitudes measured from the ground. Our technique relies on combining information from two specular ...meteor radars (SMRs) separated 130 km at polar latitudes, specifically, the Andenes and Tromsø radars in northern Norway. The resulting values are obtained over a region that spans an approximate area of 400 km diameter at mesospheric altitudes. The temporal and vertical resolution are 1 h and 2 km in altitude. The technique not only allows to obtain the gradient terms of the horizontal wind, that in turn are used to derive the horizontal divergence and relative vorticity, but also improves the horizontal sampling compared to single SMRs. Synthetic data are used to qualitatively test the technique and identify potential sources of biases on the resulting measurements. For example, we have found that an apparent large mean vertical velocity is obtained, after averaging many days, if there is a persistent divergent field. We present a climatology of the resulting wind field parameters from 12 years of continuous observations and focus on the summer results. We found a persistent altitudinal pattern in both the horizontal divergence and relative vorticity fields during all northern hemispheric summers. The horizontal divergence is mainly positive decreasing in magnitude below ∼86 km, and the relative vorticity is negative/positive below/above ∼88 km over northern Norway.
Plain Language Summary
We present a novel approach to measure new parameters of the wind field at altitudes between 80 and 100 km at polar latitudes. The approach consists of using closely located radars. These radars measure the winds from observing the time evolution of meteor trails. Since the echos from meteor trails are strong, the radars used are relatively small. We have analyzed data from radars in northern Norway that have been operating for more than 12 years. This is the first time that such data sets are combined to explore new parameters like horizontal divergence and relative vorticity. The climatology of this new parameters shows that there are persistent and well‐identified patterns at the polar summer mesosphere. We expect that the new parameters will be useful in future efforts to improve the understanding of processes in this part of the atmosphere.
Key Points
First direct measurements of mesoscale horizontal divergence and relative vorticity in the polar MLT region
The height transition from negative to positive horizontal divergence in the polar summer indicates the height of maximum mean vertical wind
The transition height of horizontal divergence is slightly higher than for the relative vorticity in the polar summer mesosphere
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The seasonal and interannual variabilities of mesospheric semidiurnal tides (SDT) are investigated using specular meteor radar‐based winds. The horizontal wind observations during 2003 to 2014 from a ...high‐latitude station, Andenes (69°N, 16°E), and during 2008 to 2014 from a midlatitude station, Juliusruh (54°N, 13°E), are used. It has been observed that the amplitudes of mesospheric SDTs are enhanced at both stations during August–September of all the years. These enhancements show a systematic behavior with that of the low‐latitude stratospheric quasi‐biennial oscillation (QBO), which is characterized based on winds from radiosonde data. The SDT amplitude values during enhancement are below/above mean level for those years in which the QBO wind at 50 hPa is westward/eastward (QBOw/QBOe). The average SDT amplitudes during the August–September enhancement duration are found to vary hand in hand with the low‐latitude QBO wind, suggesting QBO modulation of SDT. Stratospheric and lower mesospheric zonal wind perturbations from MERRA reanalysis data show weak local forcing in the Northern Hemisphere and indication of enhanced quasi‐stationary planetary waves (SPW) in the Southern Hemisphere. Based on these observations and some earlier results, we hypothesize that the QBOw/QBOe wind damp/enhance the southern hemispheric SPW of wave number 1 (SPW1). This modulated SPW1 then interacts with the northern midlatitude and high‐latitude SDTs to imprint the signature of QBO on them.
Key Points
Enhanced mesospheric semidiurnal tide in northern midlatitudes and high latitudes during August‐September
Semidiurnal tidal amplitudes during August‐September enhancement are above/below mean level for QBO eastward/westward wind
QBO modulation of northern semidiurnal tides is teleconnected via interhemispheric coupling of southern planetary waves
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
We present a study of the semidiurnal solar tide (S2) during the fall and spring transition times in the Northern Hemisphere. The tides have been obtained from wind measurements provided by three ...meteor radars located at Andenes (69∘ N, 16∘ E), Juliusruh (54∘ N, 13∘ E) and Tavistock (42∘ N, 81∘ W). During the fall, S2 is characterized by a sudden and pronounced decrease occurring every year and at all height levels. The spring transition also shows a decrease in S2, but not sudden and that ascends from lower to higher altitudes during an interval of ∼ 15 to 40 days. To assess contributions of different semidiurnal tidal components, we have examined a 20-year free-run simulation by the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA). We found that the differences exhibited by the S2 tide between equinox times are mainly due to distinct behaviors of the migrating semidiurnal and the non-migrating westward-propagating wave number 1 tidal components (SW2 and SW1, respectively). Specifically, during the fall both SW2 and SW1 decrease, while during the springtime SW2 decreases but SW1 remains approximately constant or decreases only slightly. The decrease shown by SW1 during the fall occurs later than that of SW2 and S2, which indicates that the behavior of S2 is mainly driven by the migrating component. Nonetheless, the influence of SW1 is necessary to explain the behavior of S2 during the spring. In addition, a strong shift in the phase of S2 (of SW2 in the simulations) is also observed during the fall. Our meteor radar wind measurements show more gravity wave activity in the fall than during the spring, which might be indicating that the fall decrease is partly due to interactions between SW2 and gravity waves.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The Saura radar is designed and used to measure winds and electron densities at polar latitudes (69
∘
N) within the D region, namely between 50 and 100 km altitude. A relatively narrow radar beam can ...be generated and steered into distinct pointing directions as a rather large antenna array is used. From the observed radial velocities of the individual pointing directions, the horizontal and vertical wind fields can be obtained using the Doppler beam swinging (DBS) method. With recent upgrades to the radar, the interferometric capabilities are largely improved allowing simultaneous application of different wind estimation techniques now, and also echo localization. In recent studies, Saura DBS winds assuming isotropic scattering were found to be underestimated in comparison with highly reliable winds observed with the MAARSY MST radar in the presence of polar mesospheric summer echoes (PMSE). This underestimation has been investigated by analyzing the scattering positions as well as applying the imaging Doppler interferometry technique. Besides this, Saura winds derived with the classical DBS method seem to be error prone at altitudes above 90 km and even below this altitude for periods of enhanced ionization, e.g., particle precipitations. Various methods taking into account the scattering positions have been used to correct the wind underestimation. These winds are compared to MST radar winds during PMSE, and an optimal combination of these methods for the Saura radar is presented. This combined wind data appears to be reliable; it shows reasonable amplitudes as well as tidal structures for the entire altitude region.
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