Thermosphere-ionosphere-mesosphere-electrodynamics General Circulation Model Research Articles

Understanding Nighttime Ionospheric Depletions Associated With Sudden Stratospheric Warmings in the American Sector

AbstractThis study focuses on understanding what drives the previously observed deep nighttime ionospheric hole in the American sector during the January 2013 sudden stratospheric warming (SSW). Performing a set of numerical experiments with the thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) constrained by a high‐altitude version of the Navy Global Environmental Model, we demonstrate that this nighttime ionospheric hole was the result of increased poleward and down magnetic field line plasma motion at low and midlatitudes in response to altered F‐region neutral meridional winds. Thermospheric meridional wind modifications that produced this nighttime depletion resulted from the well‐known enhancements in semidiurnal tidal amplitudes associated with stratospheric warming (SSWs) in the upper mesosphere and thermosphere. Investigations into other deep nighttime ionospheric depletions and their cause were also considered. Measurements of total electron content from Global Navigation Satellite System receivers and additional constrained TIME‐GCM simulations showed that nighttime ionospheric depletions were also observed on several nights during the January‐February 2010 SSW, which resulted from the same forcing mechanisms as those observed in January 2013. Lastly, the recent January 2021 SSW was examined using Modern‐Era Retrospective Analysis for Research and Applications, Version 2, COSMIC‐2 Global Ionospheric Specification electron density, and ICON Michelson Interferometer for Global High‐Resolution Thermospheric Imaging horizontal wind data and revealed a deep nighttime ionospheric depletion in the American sector was likely driven by modified meridional winds in the thermosphere. The results shown herein highlight the importance of thermospheric winds in driving nighttime ionospheric variability over a wide latitude range.

A Modeling Study on the Responses of the Mesosphere and Lower Thermosphere (MLT) Temperature to the Initial and Main Phases of Geomagnetic Storms at High Latitudes

AbstractJoule heating and radiative cooling usually play key roles in high‐latitude thermospheric temperature changes during geomagnetic storms. In the mesosphere and lower thermosphere (MLT), however, the causes of storm‐time temperature changes at high latitudes are still elusive. Here, we elucidate the nature and mechanisms of MLT temperature variations at high latitudes during the 10 September 2005 storm by diagnostically analyzing the MLT thermodynamics in the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) simulations. In the storm's initial and main phases, the MLT temperature decreases at 0:00 local time (LT)−12:00 LT, but increases in the 12:00 LT–24:00 LT sector at high latitudes. Afterward, the temperature decrease disappears and temperature increase occurs at all local times in the high latitudes. Adiabatic heating/cooling and vertical advection associated with vertical winds are the main drivers of high‐latitude temperature changes in the entire altitude range of the MLT region. However, around the auroral oval and above ∼100 km, the Joule heating rate is comparable to the heating caused by vertical advection and adiabatic heating/cooling associated with vertical winds and becomes one of the major contributors to total heating in the high‐latitude MLT region. The effects of Joule heating can penetrate down to ∼95 km. Horizontal advection also plays a key role in storm‐time MLT temperature changes inside the polar cap and becomes larger than the adiabatic heating/cooling above ∼105 km.

Temperature Variations in the Mesosphere and Lower Thermosphere during Geomagnetic Storms with Disparate Durations at High Latitudes

Using the temperature data observed from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER), we investigate the response of the mesosphere and lower thermosphere (MLT) to two medium geomagnetic storms with disparate durations, on 20 April 2018 and 10 April 2022. The high-latitude MLT temperature increase in the Southern hemisphere can reach 40 K during April 2018 geomagnetic storm with a longer duration (Kp values greater than 4 for 15 h), while the temperature variations are less than 10 K for the April 2022 event (Kp values greater than 4 for 6 h). To investigate the different temperature responses to disparate geomagnetic storm durations and understand what physical process results in this difference, we simulated the two events using the thermosphere ionosphere mesosphere electrodynamics general circulation model (TIMEGCM). The simulations show that more particles and energy input in longer-duration geomagnetic storms produce larger ion drag force and pressure gradient force at ~130 km, and then the enhanced two forces cause faster horizontal wind, leading to larger horizontal divergence. Subsequently, the stronger downward vertical wind is transported to the MLT region (below 110 km) and ultimately makes greater temperature increases through adiabatic heating/cooling and vertical advection. Therefore, the effects of the storm’s duration on the MLT temperature are also important.

Tests of a New Solar Flare Model Against D and E Region Ionosphere Data

AbstractWe present results from a suite of models designed to simulate solar flare effects on the D and E region of the ionosphere. This suite includes models of the solar spectrum, the ionosphere and of HF radiowave propagation. A central component of this system is the development of photoelectron ionization enhancement factors with higher energy resolution in the soft X‐ray spectral region that can be used to supplement existing ionization schemes currently implemented in upper atmospheric general circulation models. We tested this photoelectron model in the NCAR Thermosphere‐Ionosphere‐Mesosphere‐ Electrodynamics General Circulation Model (TIME‐GCM) and in a photochemical model of the D region. In both cases, we compared predicted flare response using two different input solar flare spectra. One is the Flare Irradiance Spectral Model (FISM) and the other is a physics based model called NRLFLARE. Our predictions for the E region were compared with incoherent scatter radar data and suggest that enhanced flux in the 1–2 nm spectral region, as indicated by NRLFLARE, is important for reproducing the observations. For the D region, we combined our theoretical results for the X1.3 flare of 7 September 2017 with ray tracing calculations that suggest 20–40 db of 6.4 MHz absorption. This agrees with previously published observations and model estimates, all of which suggest greater HF absorption than the operational D region absorption prediction model (swpc.noaa.gov/products/d‐region‐absorption‐predictions‐d‐rap). Finally, our theoretical comparison with previously published empirical models derived from very low frequency data was less clear due, in part, to large differences between the different empirical models.

Responses of Mesosphere and Lower Thermosphere Temperature to the Geomagnetic Storm on 7–8 September 2017

The variations of neutral temperature in the mesosphere and lower thermosphere (MLT) region, during the 7–8 September 2017 intense geomagnetic storm, are studied using observations by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite. They are also studied using simulations by the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIMEGCM). The neutral temperature data cover the altitudes from 80 km to 110 km between 83° N and 52° S latitude, obtained from both SABER observations and model simulations. The SABER observations reveal that temperature increases (the maximum increase is larger than 35 K at ~108 km) and decreases (the maximum decrease is larger than 20 K at ~105 km) during the geomagnetic storm. The storm effects penetrate down to ~80 km. In observations, temperature variations corresponding to the storm show hemispheric asymmetry. That is, the variations of temperature are more prominent in the northern hemisphere than in the southern hemisphere. Conversely, the TIMEGCM outputs agree with the observations in general but overestimate the temperature increases and underestimate the temperature decreases at high and middle latitudes. Meanwhile, the simulations show stronger temperature decreases and weaker temperature increases than observations at low latitudes. After analyzing the temperature variations, we suggest that vertical winds may play an important role in inducing these significant variations of temperature in the MLT region.

On the Effects of Mesospheric and Lower Thermospheric Oxygen Chemistry on the Thermosphere and Ionosphere Semiannual Oscillation

AbstractThis study quantifies mesosphere/lower thermosphere (MLT) oxygen chemical contributions to the global thermosphere‐ionosphere (T‐I) semiannual oscillation (SAO) using a series of numerical experiments from the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) that isolate essential chemical processes affecting O and O2 in the MLT region. We track the vertical dynamical, diffusive, and chemical fluxes of O and O2 in and out of two control volumes between ∼80 and 130 km using a finite volume approach to the individual species continuity equation to investigate their relative importance on the global T‐I SAO. TIME‐GCM results indicate that the global T‐I SAO amplitude and phase is fairly insensitive to significant changes in odd oxygen chemical reaction rates in the MLT. While chemistry has an appreciable effect on O in the MLT region, sensitivity to changes in odd oxygen and odd hydrogen chemical rates appear to be offset by a consequent adjustment in the vertical bulk wind and eddy diffusive transport of O locally, rendering their effects inconsequential to the global T‐I SAO aloft. The implications of our findings for reproducing a self‐consistent global T‐I SAO in the NCAR thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) with a lower boundary near ∼100 km are discussed. Specifically, including latitude‐season variations in O, O2, and N2 from NRLMSIS® 2.0 at the lower boundary of the TIE‐GCM near ∼100 km improves its representation of the climatological T‐I SAO. However, reformulating the TIE‐GCM temperature lower boundary condition could further improve its ability to simulate the T‐I SAO from first‐principles.

The nature and origins of the day-to-day variability in Earth’s surface magnetic field

Numerical experiments are performed with the National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) to reveal the characteristics and origins of daytime magnetic field variations on the ground (ΔB) at planetary-wave (PW) periods (2–20 days). Simulations are performed to separate the responses to forcing in the lower atmosphere from solar-magnetospheric forcing. Lower-atmosphere forcing is specified at the 97-km lower boundary of the TIE-GCM by NCAR's Thermosphere-Ionosphere-Mesosphere Electrodynamics General Circulation Model (TIME-GCM), which itself is forced at 30 km by MERRA (Modern Era Retrospective-analysis for Research and Applications) outputs. Solar and magnetospheric inputs to the TIE-GCM are specified according to parameterizations based on F10.7 and Kp. The study focuses on latitudes 0°–65°N during October 1–31, 2009, when F10.7 (range 68–80), Kp (range 0–4), and Ap (range 0–13) are typical of quiet-time “weather”. Neutral dynamics in the dynamo region (ca. 100–150 km) during this period is dominated by winds due to PW modulated tides, where the PW include the quasi-6, 10 and 16-day westward-propagating normal modes with zonal wavenumber s = 1, and eastward-propagating ultra-fast Kelvin waves (UFKW) with s = −1 and periods between 2 and 5 days.Results and conclusions are as follows. PW-period perturbations in daytime ΔB at the ground are dominated by variability originating in the lower atmosphere. The only exception is the 45°–65° latitude regime around noon, where the ΔB variability due to lower atmospheric forcing exceeds that due to solar-magnetospheric forcing by only about 50%. Broadband zonally-symmetric oscillations also occur in ΔB due to dissipation of the tidal spectrum at PW periods in the E-region. These results raise the possibility that some level of contamination from the lower atmosphere may exist in magnetic indices such as ap, Kp, and Ap that are used as measures solar-magnetosphere-ionosphere coupling strength, under levels of geomagnetic activity similar to that characterizing October 2009. It is also found that variations in conductivities play a minor role compared with neutral winds in producing PW-period variations in ΔB, and that there is not a robust one-to-one correspondence between spectral peaks in ΔB and those in the neutral winds. Several factors contribute to this latter result, which are explained in the text.

Simulation and Observation of Lunar Tide Effect on High‐Latitude, Mesospheric and Lower Thermospheric Winds During the 2013 Sudden Stratospheric Warming Event

AbstractWe investigate the high‐latitude mesospheric and lower thermospheric winds during the 2013 sudden stratospheric warming event using ground‐based optical Doppler remote sensing observations of the OH and O (557.7 nm) emission from Eureka (80°N, 86°W) and Thermosphere Ionosphere Mesosphere Electrodynamics‐General Circulation Model (TIME‐GCM) simulations. Simulations with and without lunar tidal forcing of the TIME‐GCM were performed. It has been found that the additional lunar tidal forcing only impacts slightly the semidiurnal tidal amplitude and phase at Eureka. The TIME‐GCM simulations still have noticeable discrepancies in the mean winds and the semidiurnal tidal amplitude and phase compared to the observations. The semidiurnal tidal phase shift during the stratospheric warming event may be associated with the sudden stratospheric warming related zonal mean wind reversal, which is similar to the seasonal change in the zonal mean wind from winter to summer. Accordingly, during the reversal, more modes of the semidiurnal tide propagate to the mesosphere, changing the phase of the semidiurnal tide.

Evaluating Different Techniques for Constraining Lower Atmospheric Variability in an Upper Atmosphere General Circulation Model: A Case Study During the 2010 Sudden Stratospheric Warming

AbstractWe analyze the effects specified dynamics (SD) and 4D Tendency nudging have on accurately reproducing the middle and upper atmospheric variability induced by the 2010 sudden stratospheric warming (SSW) event in the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM). TIME‐GCM numerical experiments were performed using constrained middle atmospheric winds and temperatures from a high‐altitude version of the Navy Global Environmental Model to compare the previously implemented SD scheme, with the newly implemented 4D Tendency scheme. Model comparisons focused on zonal mean winds, composition, planetary waves, and tides in the thermosphere‐ionosphere system. Through 4D Tendency nudging we reveal that coupling coefficients of the one‐way SD coupling approach between the TIME‐GCM and observed SSW conditions were too strong. Prior implementations produced unusually strong vertical shears in the zonal mean winds in the mesosphere and lower thermosphere (MLT), where the model is free running. Differences in zonal mean MLT winds between SD and 4D Tendency nudging simulations resulted in migrating diurnal (DW1) and semidiurnal (SW2) tidal amplitude differences at lower thermospheric altitudes. The consequences of simulating different MLT dynamics using SD and 4D Tendency nudging in the overlaying ionosphere are reported and validated using electron density data from the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites. Although we demonstrate that SD and 4D Tendency nudging techniques are approximately equivalent, results presented herein establish that 4D Tendency nudging has the added potential to identify physical model parameters that contribute to data‐model differences during the 2010 SSW.

Origins of the Thermosphere‐Ionosphere Semiannual Oscillation: Reformulating the “Thermospheric Spoon” Mechanism

AbstractWe demonstrate how Earth's obliquity generates the global thermosphere‐ionosphere (T‐I) semiannual oscillation (SAO) in mass density and electron density primarily through seasonally varying large‐scale advection of neutral thermospheric constituents, sometimes referred to as the “thermospheric spoon” mechanism (TSM). The National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) is used to isolate the TSM forcing of this prominent intraannual variation (IAV) and to elucidate the contributions of other processes to the T‐I SAO. An ∼30% SAO in globally averaged mass density (relative to its global annual average) at 400 km is reproduced in the TIME‐GCM in the absence of seasonally varying eddy diffusion, tropospheric tidal forcing, and gravity wave breaking. Artificially, decreasing the tilt of Earth's rotation axis with respect to the ecliptic plane to 11.75° reduces seasonal variations in insolation and weakens interhemispheric pressure differences at the solstices, thereby damping the global‐scale, interhemispheric transport of atomic oxygen (O) and molecular nitrogen in the thermosphere and reducing the simulated global mass density SAO amplitude to ∼10%. Simulated T‐I IAVs in mass density and electron density have equinoctial maxima at all latitudes near the F2 region peak; this phasing and its latitude dependence agree well with empirically inferred climatologies. When tropospheric tides and gravity waves are included, simulated IAV amplitudes and their latitudinal dependence also agree well with empirically inferred climatologies. Simulated meridional and vertical transport of O due to the TSM couples to the upper mesospheric circulation, which also contributes to the T‐I SAO through O chemistry.

Exploring Wave‐Wave Interactions in a General Circulation Model

AbstractNonlinear interactions involving Kelvin waves with (periods, zonal wave numbers) = (3.7d, s =− 1) (UFKW1) and = (2.4d, s =− 1) (UFKW2) and s = 0 and s = 1 quasi 9 day waves (Q9DW) with diurnal tides DW1, DW2, DW3, DE2, and DE3 are explored within a National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) simulation driven at its ∼30 km lower boundary by interpolated 3‐hourly output from Modern‐Era Retrospective Analysis for Research and Applications (MERRA). The existence of nonlinear wave‐wave interactions between the above primary waves is determined by the presence of secondary waves (SWs) with frequencies and zonal wave numbers that are the sums and differences of those of the primary (interacting) waves. Focus is on 10–21 April 2009, when the nontidal dynamics in the mesosphere‐lower thermosphere (MLT) region is dominated by UFKW and when identification of SW is robust. Fifteen SWs are identified in all. An interesting triad is identified involving UFKW1, DE3, and a secondary UFKW4 = (1.5d, s =− 2): The UFKW1‐DE3 interaction produces UFKW4, the UFKW4‐DE3 interaction produces UFKW1, and the UFKW1 interaction with UFKW4 produces DE3. At 120 km the dynamic range of the reconstructed latitude‐longitude zonal wind field due to all of the SW is roughly half that of the primary waves, which produced them. This suggests that nonlinear wave‐wave interactions could significantly modify the way that the lower atmosphere couples with the ionosphere.

Middle atmosphere dynamical sources of the semiannual oscillation in the thermosphere and ionosphere

AbstractThe strong global semiannual oscillation (SAO) in thermospheric density has been observed for five decades, but definitive knowledge of its source has been elusive. We use the National Center of Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) to study how middle atmospheric dynamics generate the SAO in the thermosphere‐ionosphere (T‐I). The “standard” TIME‐GCM simulates, from first principles, SAOs in thermospheric mass density and ionospheric total electron content that agree well with observed climatological variations. Diagnosis of the globally averaged continuity equation for atomic oxygen ([O]) shows that the T‐I SAO originates in the upper mesosphere, where an SAO in [O] is forced by nonlinear, resolved‐scale variations in the advective, net tidal, and diffusive transport of O. Contrary to earlier hypotheses, TIME‐GCM simulations demonstrate that intra‐annually varying eddy diffusion by breaking gravity waves may not be the primary driver of the T‐I SAO: A pronounced SAO is produced without parameterized gravity waves.

Numerical simulation of the 6 day wave effects on the ionosphere: Dynamo modulation

AbstractThe Thermosphere‐Ionosphere‐Mesosphere Electrodynamics General Circulation Model (TIME‐GCM) is used to theoretically study the 6 day wave effects on the ionosphere. By introducing a 6 day perturbation with zonal wave number 1 at the model lower boundary, the TIME‐GCM reasonably reproduces the 6 day wave in temperature and horizontal winds in the mesosphere and lower thermosphere region during the vernal equinox. The E region wind dynamo exhibits a prominent 6 day oscillation that is directly modulated by the 6 day wave. Meanwhile, significant local time variability (diurnal and semidiurnal) is also seen in wind dynamo as a result of altered tides due to the nonlinear interaction between the 6 day wave and migrating tides. More importantly, the perturbations in the E region neutral winds (both the 6 day oscillation and tidal‐induced short‐term variability) modulate the polarization electric fields, thus leading to the perturbations in vertical ion drifts and ionospheric F2 region peak electron density (NmF2). Our modeling work shows that the 6 day wave couples with the ionosphere via both the direct neutral wind modulation and the interaction with atmospheric tides.

Observations and simulations of midlatitude ionospheric and thermospheric response to the January 2013 stratospheric sudden warming event

AbstractUsing observations from midlatitudes, we examine the ionospheric and thermospheric responses to the 2013 stratospheric sudden warming event by comparing data with four simulations performed by the Whole Atmosphere Community Climate Model eXtended (WACCM‐X), Thermosphere‐Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM), and Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIEGCM). The WACCM‐X simulation was nudged by the GEOS‐5 data. The two TIMEGCM simulations were nudged by the Modern‐Era Retrospective analysis for Research and Applications data and by the aforementioned WACCM‐X outputs, respectively. The standard TIEGCM simulation was also performed. These four simulations were compared with Millstone Hill (42.6°N, 71.4°W) incoherent scatter radar data, Millstone Hill and Boulder (40.1°N, 105.2°W) upper and lower thermospheric wind data. The meteor radar data from Collm (51.3°N, 13°E) were also used to examine the zonal wave number of the semidiurnal tide (SD). We evaluate the model simulations of the mesospheric and thermospheric responses to the 2013 SSW. The TIMEGCM simulation nudged with the WACCM‐X output has suitable stratospheric input and ionospheric dynamics and can reproduce a sharp rise of hmf2 on January 12 observed by the Millstone Hill radar. The comparison of different models with the lower thermospheric SD tide yielded mixed results. The SD tide maintained mostly as a migrating tide for most of the time and matched the TIEGCM simulation very well. The WACCM‐X appeared to perform better when the observed SD tide displays the large phase shift. It also has larger and more variable SD tide amplitude. The two TIMEGCM simulations have smaller SD amplitudes in general. Observations showed complex SD tide patterns after 20 January, which was difficult to characterize as a migrating tidal mode.

Influence of the sudden stratospheric warming on quasi-2-day waves

Abstract. The influence of the sudden stratospheric warming (SSW) on a quasi-2-day wave (QTDW) with westward zonal wave number 3 (W3) is investigated using the Thermosphere–Ionosphere–Mesosphere Electrodynamics General Circulation Model (TIME-GCM). The summer easterly jet below 90 km is strengthened during an SSW, which results in a larger refractive index and thus more favorable conditions for the propagation of W3. In the winter hemisphere, the Eliassen–Palm (EP) flux diagnostics indicate that the strong instabilities at middle and high latitudes in the mesopause region are important for the amplification of W3, which is weakened during SSW periods due to the deceleration or even reversal of the winter westerly winds. Nonlinear interactions between the W3 and the wave number 1 stationary planetary wave produce QTDW with westward zonal wave number 2 (W2). The meridional wind perturbations of the W2 peak in the equatorial region, while the zonal wind and temperature components maximize at middle latitudes. The EP flux diagnostics indicate that the W2 is capable of propagating upward in both winter and summer hemispheres, whereas the propagation of W3 is mostly confined to the summer hemisphere. This characteristic is likely due to the fact that the phase speed of W2 is larger, and therefore its waveguide has a broader latitudinal extension. The larger phase speed also makes W2 less vulnerable to dissipation and critical layer filtering by the background wind when propagating upward.

Synthetic thermosphere winds based on CHAMP neutral and plasma density measurements

AbstractMeridional winds in the thermosphere are key to understanding latitudinal coupling and thermosphere‐ionosphere coupling, and yet global measurements of this wind component are scarce. In this work, neutral and electron densities measured by the Challenging Minisatellite Payload (CHAMP) satellite at solar low and geomagnetically quiet conditions are converted to pressure gradient and ion drag forces, which are then used to solve the horizontal momentum equation to estimate low latitude to midlatitude zonal and meridional “synthetic” winds. We validate the method by showing that neutral and electron densities output from National Center for Atmospheric Research (NCAR) Thermosphere Ionosphere Mesosphere Electrodynamics‐General Circulation Model (TIME‐GCM) can be used to derive solutions to the momentum equations that replicate reasonably well (over 85% of the variance) the winds self‐consistently calculated within the TIME‐GCM. CHAMP cross‐track winds are found to share over 65% of the variance with the synthetic zonal winds, providing further reassurance that this wind product should provide credible results. Comparisons with the Horizontal Wind Model 14 (HWM14) show that the empirical model largely underestimates wind speeds and does not reproduce much of the observed variability. Additionally, in this work we reveal the longitude, latitude, local time, and seasonal variability in the winds; show evidence of ionosphere‐thermosphere (IT) coupling, with enhanced postsunset eastward winds due to depleted ion drag; demonstrate superrotation speeds of ∼27 m/s at the equator; discuss vertical wave coupling due the diurnal eastward propagating tide with zonal wave number 3 and the semidiurnal eastward propagating tide with zonal wave number 2.

Impact of the semidiurnal lunar tide on the midlatitude thermospheric wind and ionosphere during sudden stratosphere warmings

AbstractVariability of the midlatitude ionosphere and thermosphere during the 2009 and 2013 sudden stratosphere warmings (SSWs) is investigated in the present study using a combination of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) observations and thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) simulations. Both the COSMIC observations and TIME‐GCM simulations reveal perturbations in the F region peak height (hmF2) at Southern Hemisphere midlatitudes during SSW time periods. The perturbations are ∼20–30 km, which corresponds to 10–20% variability of the background mean hmF2. The TIME‐GCM simulations and COSMIC observations of the hmF2 variability are in overall good agreement, and the simulations can thus be used to understand the physical processes responsible for the hmF2 variability. Through comparison of simulations with and without the migrating semidiurnal lunar tide (M2), we conclude that the midlatitude hmF2 variability is primarily driven by the propagation of the M2 into the thermosphere where it modulates the field‐aligned neutral winds, which in turn raise and lower the F region peak height. Though there are subtle differences, the consistency of the behavior between the 2009 and 2013 SSWs suggests that variability in the Southern Hemisphere midlatitude ionosphere and thermosphere is a consistent feature of the SSW impact on the upper atmosphere.

Study of the thermospheric and ionospheric response to the 2009 sudden stratospheric warming using TIME‐GCM and GSM TIP models: First results

AbstractThis paper presents a study of mesosphere and low thermosphere influence on ionospheric disturbances during 2009 major sudden stratospheric warming (SSW) event. This period was characterized by extremely low solar and geomagnetic activity. The study was performed using two first principal models: thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) and global self‐consistent model of thermosphere, ionosphere, and protonosphere (GSM TIP). The stratospheric anomalies during SSW event were modeled by specifying the temperature and density perturbations at the lower boundary of the TIME‐GCM (30 km altitude) according to data from European Centre for Medium‐Range Weather Forecasts. Then TIME‐GCM output at 80 km was used as lower boundary conditions for driving GSM TIP model runs. We compare models' results with ground‐based ionospheric data at low latitudes obtained by GPS receivers in the American longitudinal sector. GSM TIP simulation predicts the occurrence of the quasi‐wave vertical structure in neutral temperature disturbances at 80–200 km altitude, and the positive and negative disturbances in total electron content at low latitude during the 2009 SSW event. According to our model results the formation mechanisms of the low‐latitude ionospheric response are the disturbances in the n(O)/n(N2) ratio and thermospheric wind. The change in zonal electric field is key mechanism driving the ionospheric response at low latitudes, but our model results do not completely reproduce the variability in zonal electric fields (vertical plasma drift) at low latitudes.

Causes of the longitudinal differences in the equatorial vertical E × B drift during the 2013 SSW period as simulated by the TIME‐GCM

AbstractDuring stratospheric sudden warming (SSW) periods large changes in the low‐latitude vertical drift have been observed at Jicamarca as well as in other longitudinal sectors. In general, a strengthening of the daytime maximum vertical drift with a shift from prenoon to the afternoon is observed. During the January 2013 stratospheric warming significant longitudinal differences in the equatorial vertical drift were observed. At Jicamarca the previously reported SSW behavior prevails; however, no shift of the daytime maximum drift was exhibited in the African sector. Using the National Center for Atmospheric Research thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) the possible causes for the longitudinal difference are examined. The timing of the strong SSW effect in the vertical drift (15–20 January) coincides with moderate geomagnetic activity. The simulation indicates that approximately half of the daytime vertical drift increase in the American sector may be related to the moderate geophysical conditions (Kp = 4) with the effect being negligible in the African sector. The simulation suggests that the wind dynamo accounts for approximately 50% of the daytime vertical drift in the American sector and almost 100% in the African sector. The simulation agrees with previous findings that the migrating solar tides and the semidiurnal westward propagating tide with zonal wave number 1 (SW1) mainly contribute to the daytime wind dynamo and vertical drift. Numerical experiments suggest that the neutral wind and the geomagnetic main field contribute to the presence (absence) of a local time shift in the daytime maximum drift in the American (African) sector.

Evidence of nonlinear interaction between quasi 2 day wave and quasi‐stationary wave

AbstractThe nonlinear interaction between the westward quasi 2 day wave (QTDW) with zonal wave number s = 3 (W3) and stationary planetary wave with s = 1 (SPW1) is first investigated using both Thermosphere, Ionosphere, and Mesosphere Electric Dynamics (TIMED) satellite observations and the thermosphere‐ionosphere‐mesosphere electrodynamics general circulation model (TIME‐GCM) simulations. A QTDW with westward s = 2 (W2) is identified in the mesosphere and lower thermosphere (MLT) region in TIMED/Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature and TIMED/TIMED Doppler Imager (TIDI) wind observations during 2011/2012 austral summer period, which coincides with a strong SPW1 episode at high latitude of the northern winter hemisphere. The temperature perturbation of W2 QTDW reaches a maximum amplitude of ~8 K at ~30°S and ~88 km in the Southern Hemisphere, with a smaller amplitude in the Northern Hemisphere at similar latitude and minimum amplitude at the equator. The maximum meridional wind amplitude of the W2 QTDW is observed to be ~40 m/s at 95 km in the equatorial region. The TIME‐GCM is utilized to simulate the nonlinear interactions between W3 QTDW and SPW1 by specifying both W3 QTDW and SPW1 perturbations at the lower model boundary. The model results show a clear W2 QTDW signature in the MLT region, which agrees well with the TIMED/SABER temperature and TIMED/TIDI horizontal wind observations. We conclude that the W2 QTDW during the 2011/2012 austral summer period results from the nonlinear interaction between W3 QTDW and SPW1.