Major Sudden Stratospheric Warming Events Research Articles

Influence of temperature changes and vertically transported trace species on the structure of MLT region during major SSW events

Sudden stratospheric warming (SSW) events are large-scale dynamic phenomena which can significantly affect the circulation, temperature, and composition of different atmospheric layers. The circulation changes during these events induce variations in the atmospheric neutral and ion densities and cause the vertical transport of various trace species. The severity of effects induced by major SSW events in the mesosphere and lower thermosphere (MLT) region has received less attention than that in the lower atmosphere. The major influence of temperature and vertically transported trace species on the energetics, thermal and compositional structure of the MLT region has been investigated during two major SSW events with elevated stratopause. Variations in the nitric oxide volume emission rates (NO-VER), a measure of infrared radiative cooling by NO, are reported for the first time in the context of the dynamical changes during SSW events. This study investigates the role of temperature and NO variability on the energetics of the MLT region, particularly during the formation of elevated stratopause. The effects of supplemented NO density on the secondary ozone layer has also been investigated during these events, the anti-correlation between secondary ozone and NO does not conclude on the role of NO in the secondary ozone peak density variations. Notwithstanding the similarity in terms of defining characteristics, both SSW events impact the secondary ozone layer differently. In contrast to earlier studies, it is suggested that along with temperature, the availability of atomic oxygen is the major factor for the observed variation in secondary ozone during the SSW events.

Quasi‐5‐Day Oscillations During Arctic Major Sudden Stratospheric Warmings From 2005 to 2021

AbstractTraveling quasi‐5‐day oscillations (Q5DOs) with different wavenumbers are independently observed in the mesosphere and the lower thermosphere during many recent sudden stratospheric warming (SSW) events, but their common activities during SSWs are still unclear. Based on the geopotential height data measured by the Aura/Microwave Limb Sounder (MLS) from August 2004 to March 2021, we statistically investigate the characteristics of the Q5DOs during eight Arctic major SSW events. The amplitudes of the Q5DOs are obtained by a new fitting method, which inhibits the effect of rapid changes in stationary planetary waves during SSWs. Our results reveal a robust feature that Q5DOs were enhanced during all interested SSW events. Our analysis indicates that eastward and westward propagating Q5DOs can be simultaneously enhanced during SSWs. Additionally, the wavenumbers of the enhanced Q5DOs are found to be associated with the geometry of polar vortices. Extremely strong westward Q5DOs with wavenumber 2 are consistently observed during split‐type major SSWs.

Effects of the Northern Hemisphere sudden stratospheric warmings on the Sporadic-E layers in the Brazilian sector

Tidal and Planetary Wave (PWs) amplitudes are strongly influenced by Sudden Stratospheric Warming (SSW) events. A nonlinear interaction between the tidal winds and planetary waves during the SSW may contribute to the intensification of sporadic-E (Es) layers in the lower thermosphere. This work investigated the relationship between SSW events in the Northern Hemisphere and the Es layer occurrence at low latitudes in the Brazilian sector. We used data from digital ionosondes installed in the observatories of Araguatins (ARA, 5.65° S; 48.12° W; dip lat. −5.44°) and São José dos Campos (SJC, 23.18° S; 45.89° W; dip lat. −21.37°) to analyze the Es layers. Additionally, we used the temperature, zonal wind, and PWs data at high latitudes in the Northern Hemisphere during the major SSW event that occurred in February/2018 and during the events of Dec/2018–Jan/2019 and Dec/2020–Jan/2021. The results showed a maximum frequency peak of 20 MHz (∼5 × 106 electrons.cm−3) at ARA and SJC during these SSW events. The large values of ftEs, fbEs, and electronic densities were observed between 100 and 115 km height in the Esf/l type layers during daytime or nighttime periods. The results also showed that the number of large values of ftEs, fbEs, and electronic density of the Es layer was much higher in ARA than in SJC, in general. The wavelet power spectrum analyses of the ftEs and fbEs showed a periodicity of 2-days before and after the central day of the SSWs events at the station of ARA, with three prominent peaks in the 2018/2019 event. At the SJC station the quasi-2-day periodicity in the wavelet analyses of the ftEs was observed after the central day in all three SSW events, with a peak before the central day during the 2020/2021 event.

The Evolution of Solar Tide‐Like Signatures in the Ionospheric Total Electron Content During Major Sudden Stratospheric Warming Events

AbstractBy using total electron content (TEC) from the global ionospheric maps, the solar tide‐like signatures in the ionosphere are explored. We further study how the main components evolve during the 2009 and 2018 major stratospheric sudden warming events (SSWs). The method of a least‐square fit is applied for extracting the solar tide‐like components. The three predominant components are SW2, D0, and SW1, amplitude of which increase in the equatorial ionization anomaly region after the onset of both SSW events, especially in the northern hemisphere. During the SSW events, the solar tide‐like components vary significantly with latitude and their evolutions show hemispherical asymmetries. Each same component presents similar evolutionary trend during both events. The characteristics of ionospheric tide‐like signatures and related mechanisms are discussed. The SW2 tide‐like responses in TEC closely follow the changes in the tidal wind fields in the lower thermosphere, which provide evidences of the atmosphere‐ionosphere coupling during SSW events. In addition, the latitudinal asymmetric distributing of stratospheric ozone and ionospheric TEC background during the SSW may be influence factors for the hemispherical asymmetries in tide‐like signatures responses in TEC.

Mid-latitude ionospheric disturbances during the major Sudden Stratospheric Warming event of 2018 observed by sub-ionospheric VLF/LF signals

This study examines the impact of a major sudden stratospheric warming (SSW) event in February 2018 on the ionosphere of the mid-to–high latitudes in the Northern Hemisphere. The SSW event commenced on February 11 and persisted for about 18 days. The polar stratospheric temperature at 10 hPa increased by about 50 K during February 17 (daytime) and February 18 (nighttime). To detect lower ionospheric variations, VLF/LF signals from four transmitters received by three receivers in the European sector and one receiver in the eastern USA are analyzed along the five propagation paths. The analysis of the diurnal variations in the European sector indicates a significant fluctuation in signal amplitude exceeding ∼5 dB during nighttime on the most disturbed day (February 13). The 1-h averaged nighttime amplitude also showed a maximum deviation of more than ±2σ on February 13. Notably, this occurrence took place two days after the onset of the SSW event at the European receiver locations. No delay in the occurrence of the maximum anomaly was observed in the westward propagation path in the eastern USA. We also analyzed the critical frequency of the F2 layer (foF2) of the ionosphere during February 2018, recorded by Lowell digisonde stations located near the VLF/LF receivers. The foF2 data shows a significant maximum anomaly of ∼1 MHz during nighttime on February 13 at the European stations, coinciding with the SSW period. This anomaly is consistent with the observed VLF/LF anomaly and aligns with the observed temperature variations. However, the deviations in foF2 at the USA stations are not as prominent as those observed at the European stations.

A Case Study of the Solar and Lunar Semidiurnal Tide Response to the 2013 Sudden Stratospheric Warming

AbstractThis study investigates the response of the semidiurnal tide (SDT) to the 2013 major sudden stratospheric warming (SSW) event using meteor radar wind observations and mechanistic tidal model simulations. In the model, the background atmosphere is constrained to meteorological fields from the Navy Global Environmental Model—High Altitude analysis system. The solar (thermal) and lunar (gravitational) SDT components are forced by incorporating hourly temperature tendency fields from the ERA5 forecast model, and by specifying the M2 and N2 lunar gravitational potentials, respectively. The simulated SDT response is compared against meteor wind observations from the CMOR (43.3°N, 80.8°W), Collm (51.3°N, 13.0°E), and Kiruna (67.5°N, 20.1°E) radars, showing close agreement with the observed amplitude and phase variability. Numerical experiments investigate the individual roles of the solar and lunar SDT components in shaping the net SDT response. Further experiments isolate the impact of changing propagation conditions through the zonal mean background atmosphere, non‐linear wave‐wave interactions, and the SSW‐induced stratospheric ozone redistribution. Results indicate that between 80 and 97 km altitude in the northern hemisphere mid‐to‐high latitudes the net SDT response is driven by the solar SDT component, which itself is shaped by changing propagation conditions through the zonal mean background atmosphere and by non‐linear wave‐wave interactions. In addition, it is demonstrated that as a result of the rapidly varying solar SDT during the SSW the contribution of the lunar SDT to the total measured tidal field can be significantly overestimated.

Whether Sudden Stratospheric Warming Effects Are Seen in the Midlatitude Thermosphere of the Opposite Hemisphere?

AbstractThe reaction of midlatitude daytime foF2 and thermospheric parameters in two Hemispheres has been analyzed for a minor Arctic Sudden Stratospheric Warming (SSW) in January 2008, three major Arctic SSWs in January 2006, 2009, 2013, and a minor Antarctic SSW in September 2019. Arctic SSWs always result in a simultaneous foF2 depression observed at some stations in the Northern Hemisphere but not necessary in the opposite Hemisphere, that is, not all SSWs have global appearance. Thermospheric parameters retrieved from ionospheric observations in two Hemispheres manifest a dependence on the type of SSW (major/minor) and its magnitude. Pronounced thermospheric SSW effects in the opposite (Southern) Hemisphere may be expected only for strong major Arctic SSW events. Retrieved exospheric temperature does not manifest any visible reaction to SSWs both for major and minor SSW events. The duration of foF2 and atomic oxygen decrease related to SSW is 3–5 days in the vicinity of the SSW peak. Both observed neutral gas density and retrieved thermospheric parameters do not manifest a significant difference comparing western and eastern regions of North America during the minor Antarctic SSW in September 2019. Therefore, the previously reported (80–100%) increase in TEC in the western region of North America was not related with variations of neutral composition and winds.

On the Different Quasi-2-Day Wave Behaviors during Sudden Stratospheric Warming Periods

The temporal variations in the sudden stratospheric warming (SSW) events in the winter stratosphere always coincide with the quasi-2-day wave (Q2DW) in the summer mesosphere, and the impact of SSW on Q2DW is interesting but still a mystery. Major SSWs occurred in both 2006 and 2009, while the Q2DW activity was quite different. The Second Modern Era-Retrospective Analysis for Research and Applications (MERRA-2) reanalysis dataset was used to comparatively analyze these two major SSW events and elucidate the reasons for the different Q2DW behaviors. We noticed that the summer easterly jet shows a large interannual variability. We conclude that the summer mesospheric Q2DWs are modulated by the winter SSW, whereas the modulation process is also affected by the interannual variability of the summer easterly flow itself. The effects of the SSW on the Q2DWs may differ from year to year due to the variability of the summer easterly flow itself, resulting in different anomalous Q2DW behavior. This conclusion may also be true for the interannual variability of other phenomena during the SSW period.

Possible influence of sudden stratospheric warmings on the atmospheric environment in the Beijing–Tianjin–Hebei region

Abstract. Using European Centre for Medium-Range Weather Forecasts fifth-generation (ERA5) and second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) reanalysis and surface meteorological observation data, this study explores the possible impact of sudden stratospheric warming (SSW) events on air quality in the Beijing–Tianjin–Hebei (BTH) region. Major SSW events are divided into polar vortex displacement SSW and polar vortex split SSW. As the duration of split SSW events is longer and the stratospheric signal pulses propagate further downward than displacement SSWs, subseasonal variability of the atmospheric particulates in the BTH is larger during split SSWs. The air particulate concentration is light before the SSW onset due to the enhanced perturbation in the troposphere associated with strengthened planetary waves. The air particulate concentration around the SSW onset dates begins to rise due to weakening of the tropospheric disturbance as the enhanced planetary waves enter the stratosphere. In the decaying period of the SSW, the air particulate concentration decreases as the stratospheric negative northern annular mode (NAM) signal propagates downward. Specifically, in the pre-SSW period of displacement (split) SSW events, a wavenumber-1-like (wavenumber-2-like) anomaly pattern is strengthened. The East Asian winter monsoon intensifies as the East Asian trough is deepened, especially before the split SSW event onset, leading to a cleaning period. Around the SSW onset period as the tropospheric perturbation diminishes and the East Asian winter monsoon weakens, a surge of air particulate concentration is observed. After the SSW onset, due to the downward propagation of the stratospheric negative NAM signal, cold anomalies form in northeastern East Asia, especially for split SSWs, corresponding to a cleaning period in the BHT region. The local meteorological conditions during the SSWs are also discussed.

Arctic Stratosphere Dynamical Processes in the Winter 2021–2022

The Arctic stratosphere winter season of 2021–2022 was characterized by a stable, cold stratospheric polar vortex with a volume of polar stratospheric clouds (PSC) close to the maximum values since 1980, before the beginning of minor sudden stratospheric warming (SSW) events in the late February and early March and major SSW on 20 March. Analysis of dynamical processes of the Arctic stratosphere using reanalysis data indicates that the main reasons for the strengthening of the stratospheric polar vortex in January–February are the minimum propagation of planetary wave activity from the troposphere to the stratosphere over the past 40 years and its reflection in the upper stratosphere–lower mesosphere in the second half of January. The first minor SSW was limited to the upper polar stratosphere, whereas the second one propagated to the middle and lower stratosphere and led to the disappearance of the PSC, which prevented significant ozone depletion. Both minor and major SSW events led to a weakening of the residual meridional circulation in the upper Arctic stratosphere and its intensification in the middle and lower stratosphere, which contributed to additional warming of the subpolar region and weakening of the polar vortex.

Evaluation of Polar Winter Mesopause Wind in WACCMX+DART.

This work evaluates zonal winds in both hemispheres near the polar winter mesopause in the Whole Atmosphere Community Climate Model (WACCM) with thermosphere‐ionosphere eXtension combined with data assimilation using the Data Assimilation Research Testbed (DART) (WACCMX+DART). We compare 14 years (2006–2019) of WACCMX+DART zonal mean zonal winds near 90 km to zonal mean zonal winds derived from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) geopotential height measurements during Arctic mid‐winter. 10 years (2008–2017) of WACCMX+DART zonal mean zonal winds are compared to SABER in the Antarctic mid‐winter. It is well known that WACCM, and WACCM‐X, zonal winds at the polar winter mesopause exhibit a strong easterly (westward) bias. One explanation for this is that the models omit higher order gravity waves (GWs), and thus the eastward drag caused by these GWs. We show for the first time that the model winds near the polar winter mesopause are in closer agreement with SABER observations when the winds near the stratopause are weak or reversed. The model and observed mesosphere and lower thermosphere winds agree most during dynamically disturbed times often associated with minor or major sudden stratospheric warming events. Results show that the deceleration of the stratospheric and mesospheric polar night jet allows enough eastward GWs to propagate into the mesosphere, driving eastward zonal winds that are in agreement with the observations. Thus, in both hemispheres, the winter polar night jet speed and direction near the stratopause may be a useful proxy for model fidelity in the polar winter upper mesosphere.

Modulation of a long-lasting extreme cold event in Siberia by a minor sudden stratospheric warming and the dynamical mechanism involved

Previous studies have paid much attention to the impact of major sudden stratospheric warming (SSW) events on the tropospheric circulation. However, the attention to the modulation of minor SSW events on the extreme cold events is limited. In this study, the extreme cold event in Siberia in the winter of 2000/2001, the longest-lasting one from 1980/1981 to 2019/2020, and its linkages to the minor SSW event have been examined. Our results show that the largest cooling occurred in Siberia during 30 December 2000 − 10 January 2001, and then the cooling weakened and migrated to Northeast China from 11 to 18 January 2001. During the recovery stage of this minor SSW event, the stratospheric polar vortex gradually strengthened, along with strengthening of the zonal winds over the Ural region. The vertical distribution of positive zonal wind anomalies in the Ural region favored the reflection of stratospheric planetary wave in the Atlantic-Euro and Siberia region. The changes of planetary wave propagation were beneficial to the strengthening of the trough in the Atlantic-Euro and Siberia region during 26 December − 10 January, which facilitated the growth and maintenance of the Ural ridge in the same period by strengthening the meridional flow. The strengthened Ural ridge resulted in the extreme cold event breaking out and lasting from 30 December to 10 January. Because the stratospheric polar vortex did not continue to strengthen and a new ridge generated in the Atlantic region during 11 − 18 January, the Ural ridge decayed and the cold air moved to Northeast China.

Dynamical evolution of a minor sudden stratospheric warming in the Southern Hemisphere in 2019

Abstract. A major strong sudden stratospheric warming (SSW) occurred in the Southern Hemisphere (SH) stratosphere in 2002 (hereafter referred to as SSW2002), which is one of the most unusual winters in the SH. Following several warmings, the polar vortex broke down in midwinter. Eastward-traveling waves and their interaction with quasi-stationary planetary waves played an important role during this event. This study analyzed the Japanese 55-year reanalysis (JRA-55) dataset to examine the SSW event that occurred in the SH in 2019 (hereafter referred to as SSW2019). In 2019, a rapid temperature increase and decelerated westerly winds were observed at the polar cap, but since there was no reversal of westerly winds to easterly winds at 60∘ S in the middle to lower stratosphere, the SSW2019 was classified as a minor warming event. The results showed that quasi-stationary planetary waves of zonal wavenumber 1 developed during the SSW2019. The strong vertical component of the Eliassen–Palm flux with zonal wavenumber 1 is indicative of pronounced propagation of planetary waves to the stratosphere. The wave driving in September 2019 was larger than that of the major SSW event in 2002. Major SSWs tend to accompany preceding minor warmings, preconditioning, which changes the zonal flow that weaken the polar night jet as seen in SSW2002. A similar preconditioning was hardly observed in SSW2019. The strong wave driving in SSW2019 occurred in high latitudes. Waveguides (i.e., positive values of the refractive index squared) were found at high latitudes in the upper stratosphere during the warming period, which provided favorable conditions for quasi-stationary planetary waves to propagate upward and poleward.

Long-term studies of mesosphere and lower-thermosphere summer length definitions based on mean zonal wind features observed for more than one solar cycle at middle and high latitudes in the Northern Hemisphere

Abstract. Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (∼ 54∘ N) and northern Norway (∼ 69∘ N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower-thermosphere summer length (MLT-SL) using SMR and PRR winds and (2) the mesosphere summer length (M-SL) using the PRR and MLS. Under both definitions, the summer begins around April and ends around middle September. The largest year-to-year variability is found in the summer beginning in both definitions, particularly at high latitudes, possibly due to the influence of the polar vortex. At high latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity as well as large-scale atmospheric influences (e.g., quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at middle latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO.

Responses of the African and American Equatorial Ionization Anomaly (EIA) to 2014 Arctic SSW Events

AbstractAside from the influence of forcing from above on the ionosphere during space weather, forcing from below also have significant influence on the ionosphere. This study investigates responses of the equatorial ionization anomaly (EIA) in the African and American longitudinal sectors to the combined effects of 2014 Sudden Stratospheric Warming (SSW) events and geomagnetic storms that coexisted with them. The study locations cover ±40° geomagnetic latitudes in both sectors. A multiinstrument approach with models was adopted. During the SSW events, a hemispherical asymmetry in TEC distribution was observed, with higher plasma ionization in the Northern Hemisphere (NH). Generally, in both sectors, EIA crests locations shifted to higher latitudes during peak phases of SSW, except in the SH of the African sector, where crests locations shifted to lower latitudes. Reversal of stratospheric zonal mean wind direction supported reversed fountain effect. TEC responded positively to SSW peak phases and daytime or nighttime orientation of Prompt Penetration Electric Field (PPEF) and PPEF strength played major role on TEC responses to storms. PPEF values were generally weak, but comparatively higher in the American sector. TEC were predominant in the American sector than the African sector due to the comparative higher electrodynamics over the American sector. EIA crests were generally located at higher latitudes on the days of SSW peaks than on the days of geomagnetic storms, except in the NH of the American sector. In both sectors, geomagnetic storms modified ionospheric irregularities by weakening or enhancing them, while the major SSW event weakened irregularities.

Ionospheric Response to Sudden Stratospheric Warming Events Across Longitudes During Solar Cycle 24

AbstractSolar cycle 24 from December 2008 to December 2019 had an unusual progression and was the weakest cycle in a century, providing an ideal scenario to study perceivable effect of wave forcing from lower atmosphere. This study examines the ionospheric response to meteorological phenomenon of sudden stratospheric warming (SSW) during solar cycle 24 (Arctic winter 2008/2009 to 2018/2019) using Global Positioning System‐derived total electron content (VTEC) and its deviation from monthly median (ΔVTEC) for four longitudinal chains. Each chain comprises eight stations, covering varied latitudes in both hemispheres. A longitude‐ and latitude‐dependent response of VTEC is observed within ∼3 weeks of peak polar stratospheric temperature anomaly (ΔTmax), with maximum variations seen for low‐latitude stations at American and Australian‐East Asian sectors. It is seen that variations during minor warming events are comparable or at times even larger than during major warming events. These responses increase with an increase in solar activity, with major SSW events primarily seen during moderate and low solar activity periods and minor SSW during high solar activity period. Under similar ionizing conditions, a quite similar ionospheric response is observed irrespective of ΔTmax and of SSW type (major or minor); however, under similar SSW strength, no prominent pattern in ionospheric response is observed. Latitude‐dependent semidiurnal ionospheric behavior is observed for only a few events. This vertical coupling between lower and upper atmosphere during SSW is predominantly influenced by 13–16 days periodicities in VTEC, along with other periodicities of 9, 7, 5, and 3 days.

Eastward‐Propagating Planetary Waves Prior to the January 2009 Sudden Stratospheric Warming

AbstractEastward‐propagating planetary waves (EPWs) were investigated prior to the boreal January 2009 major sudden stratospheric warming (SSW) event simulated by the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model with specified dynamics. About 22 days before SSW onset, a background flow with jet maxima around the upper polar stratosphere and subtropical mesosphere developed due to the net forcing by gravity and planetary waves. The mesospheric wind structure was largely unstable and supported a wave geometry conducive to overreflection. With a zonal phase speed of ∼10 m s−1, EPWs appeared near their turning and critical layers as wavenumber‐2 perturbations in the stratosphere and mesosphere. Accompanied by upward EPW activity from the lower stratosphere, EPW growth exhibited characteristics of wave instability and overreflection.

Statistical Characteristics and Long-Term Variations of Major Sudden Stratospheric Warming Events

Using NCEP/NCAR reanalysis data, we investigate the statistical characteristics and the long-term variations of major sudden stratospheric warming (SSW) events in the Northern Hemisphere. We find that the strength and duration of major SSW events has increased from 1958 to 2019 and that this is due to the strengthening of the winter planetary wave activity. We find that the frequency of displacement and split SSW events differs between early, middle and late winter. Early and middle winter are dominated by displacement and split SSW events, respectively, but the frequency of the two types of event is almost equal in late winter. This is due to the differences in the relative strength of wavenumber-1 and wavenumnber-2 planetary wave activity in the three winter periods. As a result of the increase in upward planetary wave activity and the decrease in westerly winds around the polar vortex in middle winter, a shift in the timing of SSW events toward middle winter is detected. In addition, we revealed the influence of the downward propagation of different types of SSW event on the surface temperature anomaly. There were surface cold centers in Russia and northern China after the middle split SSW events; by contrast, there were more cold events in North America after the middle split SSW events.

Observation of the Ionosphere in Middle Latitudes during 2009, 2018 and 2018/2019 Sudden Stratospheric Warming Events

The ionospheric weather is affected not only from above by the Sun but also from below by processes in the lower-lying atmospheric layers. One of the most pronounced atmospheric phenomena is the sudden stratospheric warming (SSW). Three major SSW events from the periods of very low solar activity during January 2009, February 2018, and December 2018/January 2019 were studied to evaluate this effect of the neutral atmosphere on the thermosphere and the ionosphere. The main question is to what extent the ionosphere responds to the SSW events with focus on middle latitudes over Europe. The source of the ionospheric data was ground-based measurements by Digisondes, and the total electron content (TEC). In all three events, the ionospheric response was demonstrated as an increase in electron density around the peak height of the F2 region, in TEC, and presence of wave activity. We presume that neutral atmosphere forcing and geomagnetic activity contributed differently in individual events. The ionospheric response during SSW 2009 was predominantly influenced by the neutral lower atmosphere. The ionospheric changes observed during 2018 and 2018/2019 SSWs are a combination of both geomagnetic and SSW forcing. The ionospheric response to geomagnetic forcing was noticeably lower during time intervals outside of SSWs.

The Latitudinal Variation and Hemispheric Asymmetry of the Ionospheric Lunitidal Signatures in the American Sector During Major Sudden Stratospheric Warming Events

AbstractDuring sudden stratospheric warming events, the ionosphere exhibits phase‐shifted semidiurnal perturbations, which are typically attributed to vertical coupling associated with the semidiurnal lunar tide (M2). Our understanding of ionospheric responses to M2 is limited. This study focuses on fundamental vertical coupling processes associated with the latitudinal extent and hemispheric asymmetry of ionospheric M2 signatures using total electron content data from the American sector. Our results illustrate that the asymmetry maximizes at 15°N and 20°S magnetic latitudes. In the Southern Hemisphere, the M2‐like signatures extend deep into midlatitude and encounter the Weddell Sea Anomaly. The time evolution of the Anomaly exhibits a distortion, which is attributed to an M2 modulation. The hemispheric asymmetry of M2 signatures in the low latitude can be primarily explained by the transequatorial wind modulation of the equatorial plasma fountain. Other physical processes could also be relevant, including hemispheric asymmetry of the M2 below the F‐region, the ambient thermospheric composition and ionospheric plasma distribution, and the geomagnetic field configuration.