藤原 均

The accidental reentry of 38 Starlink satellites occurred in early February 2022, associated with the occurrence of moderate magnetic storms. A poorly understood structure of Coronal Mass Ejections (CMEs) caused the magnetic storms at unexpected timing. Therefore, a better understanding of minor CME structures is necessary for the modern space weather forecast. During this event, the “up to 50%” enhancement of air drag force was observed at ~200 km altitude, preventing the satellites’ safety operations. Although the mass density enhancement predicted by the NRLMSIS2.0 empirical model is less than 25% under the present moderate magnetic storms, the real-time GAIA simulation showed a mass density enhancement of up to 50%. Further, the real-time GAIA simulation suggests that the actual thermospheric disturbances at 200 km altitude may occur with larger amplitude in a broader area than previously thought.

We surveyed the relationship between the scale of space weather events and their occurrence rate in Japan, and we discussed the social impact of these phenomena during the Project for Solar–Terrestrial Environment Prediction (PSTEP) in 2015–2019. The information was compiled for domestic users of space weather forecasts for appropriate preparedness against space weather disasters. This paper gives a comprehensive summary of the survey, focusing on the fields of electricity, satellite operations, communication and broadcasting, satellite positioning usage, aviation, human space activity, and daily life on the Earth’s surface, using the cutting-edge knowledge of space weather. Quantitative estimations of the economic impact of space weather events on electricity supply and aviation are also given. Some topics requiring future research, which were identified during the survey are also described. Graphic Abstract: [Figure not available: see fulltext.].

<title>Abstract</title>Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

Abstract A sporadic E layer has significant influence on radio communications and broadcasting, and predicting the occurrence of sporadic E layers is one of the most important issues in space weather forecast. While sporadic E layer occurrence and the magnitude of the critical sporadic E frequency (foEs) have clear seasonal variations, significant day-to-day variations as well as regional and temporal variations also occur. Because of the highly complex behavior of sporadic E layers, the prediction of sporadic E layer occurrence has been one of the most difficult issues in space weather forecast. To explore the possibility of numerically predicting sporadic E layer occurrence, we employed the whole atmosphere–ionosphere coupled model GAIA, examining parameters related to the formation of sporadic E layer such as vertical ions velocities and vertical ion convergences at different altitudes between 90 and 150 km. Those parameters in GAIA were compared with the observed foEs data obtained by ionosonde observations in Japan. Although the agreement is not very good in the present version of GAIA, the results suggest a possibility that sporadic E layer occurrence can be numerically predicted using the parameters derived from GAIA. We have recently developed a real-time GAIA simulation system that can predict atmosphere–ionosphere conditions for a few days ahead. We present an experimental prediction scheme and a preliminary result for predicting sporadic E layer occurrence.

Upper atmospheric conditions are crucial for the safe operation of spacecraft orbiting near Earth and for communication and positioning systems using radio signals. To understand and predict the upper atmospheric conditions, which include complex variations affected by both low altitude and upper surrounding environments, we have developed a quasi-real-time and forecast simulations using a physical global model, the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). The GAIA simulation system provides a global distribution of ionospheric total electron content (TEC) with background atmospheric and electric distributions including a few-days prediction. The prediction accuracy for the detection of significant ionospheric storms decreases with increasing lead time, i.e., the duration of the model simulation which is not constrained by realistic input parameters. Similar characteristic variations associated with sudden stratospheric warmings (SSWs) are reproduced with the full or limited input of meteorological data at least the prior 3 days. This is a first step toward the usage of GAIA for space weather forecasting.

©2020. American Geophysical Union. All Rights Reserved. We report a new set of stellar occultation measurements for nightside temperature profiles made by the Mars Atmosphere and Volatile EvolutioN/Imaging Ultraviolet Spectrograph that provide evidence for a recurring layer of warm air between 70 and 90 km altitudes in the nightside mesosphere of Mars during Ls = 0–180° in Martian Year 33–34. The nightside profiles reveal a recurring peak of atmospheric temperature around 80 km over the equator to the middle latitudes in the northern hemisphere. The predictions of the Mars Climate Database have a warm layer with much smaller amplitudes. The observed peak amplitudes are larger than those predicted by the model by up to 90 K. Wavenumber-3 structures are seen in the warm layer that are potentially signatures of thermal tides or stationary planetary waves, with amplitudes two times larger than predicted.

Magnetic variations calculated by the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) are compared with those observed at global magnetic observatory network in geomagnetic calm days in order to evaluate accuracy of the ionospheric current system calculated by GAIA. The calculated Y component magnetic variations can reproduce more than 50% of the observed variations at more than half observatories treated. In particular, GAIA can reproduce more than 75% of the observed Y component variations in the equinox, whereas there is tendency of low correlation of the waveform between the calculated and observed variations in the winter season. Next, GAIA reproduces so well of the X component variations at the low-latitude observatories. Low correlation between the calculated and observed X component variations at middle-latitude observatories seems to be caused by inaccurate determination of the position of the ionospheric Sq current vortex. Last, although the calculated Z component variations do not so well reproduce the observed ones compared with other component, GAIA can reproduce more than 50% of the observed Z component variation at about half observatories in general. Calculated amplitude of the horizontal magnetic variations (X and Y components) exhibit smaller than the observed one whereas that of the vertical variation (Z component) is larger than the observed one. This tendency is roughly explained by the induction effect of the Earth that is not considered in GAIA. Thus, GAIA considerably well reproduces the pure ionospheric current system that is not affected by the solid Earth.

Using a global atmosphere-ionosphere coupled model, the characteristics and excitation source of traveling ionospheric disturbances (TIDs) during geomagnetically quiet periods are studied. This is the first paper concerning the simulation of TIDs generated by upward propagating gravity waves (GWs) that are spontaneously generated in the model. The dominant horizontal wavelengths of the simulated TIDs range from 700 to 1,500 km. The dominant periods and horizontal phase velocities of TIDs are 45–90 min and 250–300 m s−1, respectively. These features are the same as those of GWs in the 250–300 km height region. The phase of the electron density variations due to TIDs descends with increasing time, which is characteristic of the upward propagation of GWs. These electron density variations that are caused due to TIDs are explained by the transport processes of a neutral wind along a geomagnetic field line. These results indicate that the electron density variations respond locally to the passage of neutral wind fluctuations associated with upward propagating GWs. The GWs that excite TIDs are secondary GWs, which are generated in the mesosphere and lower thermosphere via the dissipation/breaking of tropospheric GWs. The magnitudes of TIDs at middle latitudes are larger in winter than in summer. The mechanisms of seasonal and day-to-day variations in TIDs that are caused due to GWs are discussed in this study.

The linear growth rates of the Rayleigh-Taylor (R-T) instability in the ionosphere from 2011 to 2013 were obtained with a whole atmosphere-ionosphere coupled model GAIA (ground-to-topside model of atmosphere and ionosphere for aeronomy). The effects of thermospheric dynamics driven by atmospheric waves propagating from below on the R-T growth rate are included in the model by incorporating meteorological reanalysis data in the region below 30 km altitude. The daily maximum R-T growth rates for these periods are compared with the observed occurrence days of the equatorial plasma bubble (EPB) determined by the Equatorial Atmosphere Radar (EAR) and Global Positioning System (GPS) in West Sumatra, Indonesia. We found that a high R-T growth rate tends to correspond to the actual EPB occurrence, suggesting the possibility of predicting EPB occurrences with numerical models.

The effects of decreasing the intrinsic magnetic field on the upper atmospheric dynamics at low to middle latitudes are investigated using the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). GAIA incorporates a meteorological reanalysis data set at low altitudes (&lt;30 km), which enables us to investigate the atmospheric response to various waves under dynamic and chemical interactions with the ionosphere. In this simulation experiment, we reduced the magnetic field strength to as low as 10% of the current value. The averaged neutral velocity, density, and temperature at low to middle latitudes at 300 km altitude show little change with the magnetic field variation, while the dynamo field, current density, and the ionospheric conductivities are modified significantly. The wind velocity and tidal wave amplitude in the thermosphere remain large owing to the small constraint on plasma motion for a small field. On the other hand, the superrotation feature at the dip equator is weakened by 20% for a 10% magnetic field because the increase in ion drag for the small magnetic field prevents the superrotation.

There have been a number of papers reporting that the statistical occurrence rate of the sporadic E (E-s) layer depends not only on the local time and season but also on the geographical location, implying that geographical and seasonal dependence in vertical neutral wind shear is one of the factors responsible for the geographical and seasonal dependence in Es layer occurrences rate. To study the role of neutral wind shear in the global distribution of the Es layer occurrence rate, we employ a self-consistent atmosphere-ionosphere coupled model called GAIA (Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy), which incorporates meteorological reanalysis data in the lower atmosphere. The average distribution of neutral wind shear in the lower thermosphere is derived for the June-August and December-February periods, and the global distribution of vertical ion convergence is obtained to estimate the Es layer occurrence rate. It is found that the local and seasonal dependence of neutral wind shear is an important factor in determining the dependence of the Es layer occurrence rate on geographical distribution and seasonal variation. However, there are uncertainties in the simulated vertical neutral wind shears, which have larger scales than the observed wind shear scales. Furthermore, other processes such as localization of magnetic field distribution, background metallic ion distribution, ionospheric electric fields, and chemical processes of metallic ions are also likely to make an important contribution to geographical distribution and seasonal variation of the Es occurrence rate.

Using an atmosphere-ionosphere coupled model, the excitation source and temporal (seasonal and interannual) variations in non-migrating tides are investigated in this study. We first focus our attention on temporal variations in eastward moving diurnal tide with zonal wavenumber 3 (DE3), which is the largest of all the non migrating tides in the mesosphere and lower thermosphere (MLT). Our simulation results indicate that upward propagation of the DE3 excited in the troposphere is sensitive to the zonal mean zonal wind in the stratosphere and mesosphere. The DE3 amplitude is enhanced in the region where the vertical shear of the zonal mean zonal wind is positive (westerly shear). Quasi-2-year variation in the DE3 amplitude in the MLT region is generated by quasi-2-year variation in the zonal mean zonal wind between 40 and 70 km, which is modulated by the stratospheric QBO. The excitation mechanisms of SW3 (westward moving semidiurnal tide with zonal wavenumber 3) and SW1 (westward moving semidiurnal tide with zonal wavenumber 1) are also investigated. During equinoxes, the SW3 and SW1 are excited by tropospheric heating (latent heat release and solar radiative heating) associated with cumulus convection in the tropics, and propagate upward into the MLT region. On the other hand, during solstices, SW3 and SW1 are generated in the winter stratosphere and mesosphere through the nonlinear interaction between the stationary planetary wave and migrating semidiurnal tide, and propagate upward to the lower thermosphere. The excitation sources of other non-migrating tides are also discussed.

Wavelike perturbations in the Martian upper thermosphere observed by the Neutral Gas Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft have been analyzed. The amplitudes of small-scale perturbations with apparent wavelengths between similar to 100 and similar to 500 km in the Ar density around the exobase show a clear dependence on temperature (T-0) of the upper thermosphere. The average amplitude of the perturbations is similar to 10% on the dayside and similar to 20% on the nightside, which is about 2 and 10 times larger than those observed in the Venusian upper thermosphere and in the low-latitude region of Earth's upper thermosphere, respectively. The amplitudes are inversely proportional to T-0, suggesting saturation due to convective instability in the Martian upper thermosphere. After removing the dependence on T-0, dependences of the average amplitude on the geographic latitude and longitude and solar wind parameters are found to be not larger than a few percent. These results suggest that the amplitudes of small-scale perturbations are mainly determined by convective breaking/saturation in the upper thermosphere on Mars, unlike those on Venus and Earth.

We quantitatively evaluated the Na density depletion due to charge transfer reactions between Na atoms and molecular ions produced by high-energy electron precipitation during a pulsating aurora (PsA). An extended period of PsA was captured by an all-sky camera at the European Incoherent Scatter (EISCAT) radar Tromso site (69.6 degrees N, 19.2 degrees E) during a 2 h interval from 00:00 to 02 00 UT on 25 January 2012. During this period, using the EISCAT very high frequency (VHF) radar, we detected three intervals of intense ionization below 100 km that were probably caused by precipitation of high-energy electrons during the PsA. In these intervals, the sodium lidar at Tromso observed characteristic depletion of Na density at altitudes between 97 and 100 km. These Na density depletions lasted for 8 min and represented 5-8% of the background Na layer. To examine the cause of this depletion, we modeled the depletion rate based on charge transfer reactions with NO+ and O-2(+) while changing the R value which is defined as the ratio of NO+ to O-2(+) densities, from 1 to 10. The correlation coefficients between observed and modeled Na density depletion calculated with typical value R=3 for time intervals T-1, T-2, and T-3 were 0.66, 0.80, and 0.67, respectively. The observed Na density depletion rates fall within the range of modeled depletion rate calculated with R from 1 to 10. This suggests that the charge transfer reactions triggered by the auroral impact ionization at low altitudes are the predominant process responsible for Na density depletion during PsA intervals.

A one-dimensional full-particle model of the Martian upper thermosphere-exosphere has been developed, where the Direct Simulation Monte Carlo (DSMC) method is applied to both thermal and nonthermal components. Our full-particle model can self-consistently solve the transition from collisional to collisionless domains in the upper thermosphere, so that the energy deposition from nonthermal energetic components to thermal components in the transition region is properly described. For the solar EUV condition during the Viking 1 measurement (1 EUV case), computed density profiles are in good agreement with those observed by Viking 1 and with the conventional model. For a solar EUV flux 6 times the Viking 1 condition (6 EUV case), the computed heating efficiency is essentially the same as the 1 EUV case but slightly increases by about 10% below the exobase, and temperature deviates from the conventional model in and above the transition region. This result suggests that the conventional heating efficiency of 0.18 is a good approximation for low (1 EUV case) to moderately strong (6 EUV case) solar EUV conditions but would be inappropriate for an extremely strong solar EUV (up to similar to 100 times stronger flux) environment. We also find that applying different models of the CO2-O collisional energy transfer rate results in a difference in the calculated exobase temperature by 150 K for the 6 EUV case.

Impacts of sudden stratospheric warming (SSW) on the thermosphere were studied using a gravity wave (GW)-resolving whole atmosphere model. During an SSW event, the mesosphere at high latitudes cools, and the lower thermosphere becomes warm. At the peak of the SSW event, a temperature drop occurs above an altitude of 150 km at high latitudes. Our results indicate that the SSW event strongly affects meridional circulation and GW drag in the thermosphere. In the lower thermosphere, upward wind in the Arctic region, southward wind in the region between the North Pole and the South Pole, and downward wind in the Antarctic region are dominant before SSW occurs. The SSW event reverses meridional circulation at altitudes between 90 and 125 km in the Northern Hemisphere. During the SSW event, downward wind in the Arctic region and northward wind in the Northern Hemisphere prevail in the lower thermosphere. A detailed analysis revealed that during the SSW event, the change in meridional circulation is caused by the attenuation of the GW drag, and we identified the mechanism responsible for this attenuation. Moreover, we assessed the impacts of SSW on temperatures in the equatorial region and Southern Hemisphere.

Pulsating auroras show quasi-periodic intensity modulations caused by the precipitation of energetic electrons of the order of tens of keV. It is expected theoretically that not only these electrons but also subrelativistic/relativistic electrons precipitate simultaneously into the ionosphere owing to whistler mode wave-particle interactions. The height-resolved electron density profile was observed with the European Incoherent Scatter (EISCAT) TromsO VHF radar on 17 November 2012. Electron density enhancements were clearly identified at altitudes &gt;68km in association with the pulsating aurora, suggesting precipitation of electrons with a broadband energy range from similar to 10keV up to at least 200keV. The riometer and network of subionospheric radio wave observations also showed the energetic electron precipitations during this period. During this period, the footprint of the Van Allen Probe-A satellite was very close to TromsO and the satellite observed rising tone emissions of the lower band chorus (LBC) waves near the equatorial plane. Considering the observed LBC waves and electrons, we conducted a computer simulation of the wave-particle interactions. This showed simultaneous precipitation of electrons at both tens of keV and a few hundred keV, which is consistent with the energy spectrum estimated by the inversion method using the EISCAT observations. This result revealed that electrons with a wide energy range simultaneously precipitate into the ionosphere in association with the pulsating aurora, providing the evidence that pulsating auroras are caused by whistler chorus waves. We suggest that scattering by propagating whistler simultaneously causes both the precipitations of subrelativistic electrons and the pulsating aurora.

The advent of new satellite missions, ground-based instrumentation networks, and the development of whole atmosphere models over the past decade resulted in a paradigm shift in understanding the variability of geospace, that is, the region of the atmosphere between the stratosphere and several thousand kilometers above ground where atmosphere-ionosphere-magnetosphere interactions occur. It has now been realized that conditions in geospace are linked strongly to terrestrial weather and climate below, contradicting previous textbook knowledge that the space weather of Earth's near space environment is driven by energy injections at high latitudes connected with magnetosphere-ionosphere coupling and solar radiation variation at extreme ultraviolet wavelengths alone. The primary mechanism through which energy and momentum are transferred from the lower atmosphere is through the generation, propagation, and dissipation of atmospheric waves over a wide range of spatial and temporal scales including electrodynamic coupling through dynamo processes and plasma bubble seeding. The main task of Task Group 4 of SCOSTEP's CAWSES-II program, 2009 to 2013, was to study the geospace response to waves generated by meteorological events, their interaction with the mean flow, and their impact on the ionosphere and their relation to competing thermospheric disturbances generated by energy inputs from above, such as auroral processes at high latitudes. This paper reviews the progress made during the CAWSES-II time period, emphasizing the role of gravity waves, planetary waves and tides, and their ionospheric impacts. Specific campaign contributions from Task Group 4 are highlighted, and future research directions are discussed.

© Author(s) 2014. CC Attribution 3.0 License. This paper is primarily concerned with an event observed from 16:30 to 24:30 UT on 29 October 2010 during a very geomagnetically quiet interval (Kp &leq; 1). The sodium LIDAR observations conducted at Troms&oslash;, Norway (69.6° N, 19.2° E) captured a clearly discernible gravity wave (GW) signature. Derived vertical and horizontal wavelengths, maximum amplitude, apparent and intrinsic period, and horizontal phase velocity were about ∼ 11.9 km, ∼ 1.38 &times; 103 km, ∼ 15 K, 4 h, ∼ 7.7 h, and ∼ 96 ms-1, respectively, between a height of 80 and 95 km. Of particular interest is a temporal development of the uppermost altitude that the GW reached. The GW disappeared around 95 km height between 16:30 and 21:00 UT, while after 21:00 UT the GW appeared to propagate to higher altitudes (above 100 km). We have evaluated three mechanisms (critical-level filtering, convective and dynamic instabilities) for dissipations using data obtained by the sodium LIDAR and a meteor radar. It is found that critical-level filtering did not occur, and the convective and dynamic instabilities occurred on some occasions. MF radar echo power showed significant enhancements between 18:30 and 21:00 UT, and an overturning feature of the sodium mixing ratio was observed between 18:30 and 21:20 UT above about 95 km. From these results, we have concluded that the GW was dissipated by wave breaking and instabilities before 21:00 UT. We have also investigated the difference of the background atmosphere for the two intervals and would suggest that a probable cause of the change in the GW propagation was due to the difference in the temperature gradient of the background atmosphere above 94 km.

Changes of the zonal mean state of the thermosphere during the 2009 stratospheric sudden warming (SSW) have been investigated using the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) model. Both the zonal mean thermal and dynamical structure of the thermosphere exhibit pronounced changes during the SSW in terms of zonal mean temperature and winds. First, the zonal mean temperature above 100 km altitude drops at all latitudes except for in a narrow band around 60 degrees N. Such temperature perturbations are found to be dominantly caused by changes in direct heating/cooling processes related to solar radiation and thermal heat conduction at high latitudes, but by dynamical processes in tropical regions. Second, the zonal mean zonal wind experiences a strong westward perturbation in the tropical thermosphere, along with distinct change in the meridional circulation. This change consists of two parts. One is a global scale north-to-south flow accompanied with upwelling/downwelling in the northern/southern polar region, the other is a fountain-like flow in tropical lower thermosphere. The large enhancement of semidiurnal tides is suggested to be the primary cause for the fountain-like flow.

In order to study the dynamical role of gravity waves (GWs) propagating upward from the lower atmosphere to the thermosphere, numerical simulation using a high-resolution general circulation model that contains the region from the ground surface to the exobase (about 500 km height) has been performed. Our results indicate that the zonal momentum drag due to breaking/dissipation of GWs (GW drag) plays an important role not only in the mesosphere but also in the thermosphere. In particular, the GW drag at high latitudes in the 150-250 km height region exceeds 200 ms(-1) (d)(-1) and is important for the zonal momentum balance. The semidiurnal variation of the GW drag is dominant in the 100-200 km height region, while the diurnal variation of the GW drag prevails above a height of 200 km. The GW drag in the thermosphere is mainly directed against the background zonal wind, indicating the filtering effect by the background wind. A global view of the GW activity in the middle and upper atmosphere is also investigated. The global distribution of the GW activity in the thermosphere is not uniform, and there are some enhanced regions of the GW activity. The GW activity in the thermosphere is stronger in high latitudes than in low latitudes. The GW activity in the winter thermosphere is influenced by the mesospheric jet and the planetary wave activity in the mesosphere.

We report extreme enhancements of the nitric oxide (NO) column density observed with the ground-based millimeter-wave spectroscopic radiometer installed at Syowa Station, Antarctica, during a large geomagnetic storm in April 2012. From the NO spectrum line shape and NO column density relationship with solar radiation, we concluded that the NO was emitted in the altitude range between 75 km and 100 km. The column density of NO gradually increased during the recovery phase. In addition to variations on a time frame of several days, we found diurnal variations. The increase of NO was related to precipitated electrons in the energy range of 30-300 keV observed by Polar-orbiting Operational Environmental Satellite (POES)/The Meteorological Operational (METOP). We found a rapid response (within 1 h) and a one-to-one correspondence between them. For the first time, we show that a remarkable increase of the column density of NO is caused by dawn-dusk asymmetry of the plasma sheet electrons.

The present study compares simulations of the 2009 sudden stratospheric warming (SSW) from four different whole atmosphere models. The models included in the comparison are the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy, Hamburg Model of the Neutral and Ionized Atmosphere, Whole Atmosphere Model, and Whole Atmosphere Community Climate Model Extended version (WACCM-X). The comparison focuses on the zonal mean, planetary wave, and tidal variability in the middle and upper atmosphere during the 2009 SSW. The model simulations are constrained in the lower atmosphere, and the simulated zonal mean and planetary wave variability is thus similar up to approximate to 1 hPa (50 km). With the exception of WACCM-X, which is constrained up to 0.002 hPa (92 km), the models are unconstrained at higher altitudes leading to considerable divergence among the model simulations in the mesosphere and thermosphere. We attribute the differences at higher altitudes to be primarily due to different gravity wave drag parameterizations. In the mesosphere and lower thermosphere, we find both similarities and differences among the model simulated migrating and nonmigrating tides. The migrating diurnal tide (DW1) is similar in all of the model simulations. The model simulations reveal similar temporal evolution of the amplitude and phase of the migrating semidiurnal tide (SW2); however, the absolute SW2 amplitudes are significantly different. Through comparison of the zonal mean, planetary wave, and tidal variability during the 2009 SSW, the results of the present study provide insight into aspects of the middle and upper atmosphere variability that are considered to be robust features, as well as aspects that should be considered with significant uncertainty.

In order to study the dynamical role of gravity waves (GWs) propagating upward from the lower atmosphere to the thermosphere, numerical simulation using a high-resolution general circulation model that contains the region from the ground surface to the exobase (about 500 km height) has been performed. Our results indicate that the zonal momentum drag due to breaking/dissipation of GWs (GW drag) plays an important role not only in the mesosphere but also in the thermosphere. In particular, the GW drag at high latitudes in the150-250 km height region exceeds 200 ms-1 (d)-1 and is important for the zonal momentum balance. The semidiurnal variation of the GW drag is dominant in the 100-200 km height region, while the diurnal variation of the GW drag prevails above a height of 200 km. The GW drag in the thermosphere is mainly directed against the background zonal wind, indicating the filtering effect by the background wind. A global view of the GW activity in the middle and upper atmosphere is also investigated. The global distribution of the GW activity in the thermosphere is not uniform, and there are some enhanced regions of the GW activity. The GW activity in the thermosphere is stronger in high latitudes than in low latitudes. The GW activity in the winter thermosphere is influenced by the mesospheric jet and the planetary wave activity in the mesosphere. ©2014. American Geophysical Union. All Rights Reserved.

A new solid-state sodium lidar installed at Ramfjordmoen, Tromsø (69.6°N, 19.2°E), started observations of neutral temperature together with sodium density in the mesosphere-lower thermosphere (MLT) region on 1 October 2010. The new lidar provided temperature data with a time resolution of 10 min and with good quality between ∼80 and ∼105 km from October 2010 to March 2011. This paper aims at introducing the new lidar with its observational results obtained over the first 6 months of observations. We succeeded in obtaining neutral temperature and sodium density data of ∼255.5 h in total. In order to evaluate our observations, we compared (1) the sodium density with that published in the literature, (2) average temperature and column sodium density data with those obtained with Arctic Lidar Observatory for Middle Atmosphere Research Weber sodium lidar, and (3) the neutral temperature data with those obtained by Sounding of the Atmosphere with Broadband Emission Radiometry/Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. For the night of 5 October 2010, we succeeded in conducting simultaneous observations of the new lidar and the European Incoherent Scatter UHF radar with the tristatic Common Program 1 (CP-1) mode. Comparisons of neutral and ion temperatures showed a good agreement at 104 km between 0050 and 0230 UT on 6 October 2010 when the electric field strength was smaller, while significant deviations (up to ∼25 K) are found at 107 km. We evaluated contributions of Joule heating and electron-ion heat exchange, but derived values seem to be underestimated. Key Points Introduction of A new solid state sodium LIDAR installed at Tromsoe Evaluation the data obtained by the LIDAR Comparison of ion and neutral temperature in the polar MLT region ©2013. American Geophysical Union. All Rights Reserved.

Simultaneous measurements of the polar ionosphere with the European Incoherent Scatter (EISCAT) ultra high frequency (UHF) radar at Tromso and the EISCAT Svalbard radar (ESR) at Longyearbyen were made during 07:00-12:00 UT on 12 March 2012. During the period, the Advanced Composition Explorer (ACE) spacecraft observed changes in the solar wind which were due to the arrival of coronal mass ejection (CME) effects associated with the 10 March M8.4 X-ray event. The solar wind showed two-step variations which caused strong ionospheric heating. First, the arrival of shock structures in the solar wind with enhancements of density and velocity, and a negative interplanetary magnetic field (IMF)-Bz component caused strong ionospheric heating around Longyearbyen; the ion temperature at about 300 km increased from about 1100 to 3400K over Longyearbyen while that over Tromso increased from about 1050 to 1200 K. After the passage of the shock structures, the IMF-Bz component showed positive values and the solar wind speed and density also decreased. The second strong ionospheric heating occurred after the IMF-Bz component showed negative values again; the negative values lasted for more than 1.5 h. This solar wind variation caused stronger heating of the ionosphere in the lower latitudes than higher latitudes, suggesting expansion of the auroral oval/heating region to the lower latitude region. This study shows an example of the CME-induced dayside ionospheric heating: a short-duration and very large rise in the ion temperature which was closely related to the polar cap size and polar cap potential variations as a result of interaction between the solar wind and the magnetosphere.

We investigated generation mechanisms of the local time variation of the wind velocity in the Venusian mesosphere and thermosphere, which has been suggested from recent ground-based CO millimeter/submillimeter and CO 2 10 μm observations, using our general circulation model. Our model considers the momentum transport caused by gravity waves with the gravity wave parameterization developed by Medvedev and Klaassen (2000). Our results show that atmospheric circulation distinctly changes from the dayside to the nightside. The subsolar-to-antisolar (SSAS) wind is predominant in the dayside. On the other hand, the retrograde superrotating zonal (RSZ) wind is superposed on the SSAS wind in the nightside. These characteristics are consistent with the previous observations. The westward momentum, which drives the RSZ wind in the nightside at an altitude of 110 km, is supplied by gravity waves in the 115-130 km altitude region. The downward flow that originates in the SSAS wind transports the westward momentum downward in the nightside. Key Points The local time variation of Venus thermospheric wind by GCM The results showed different circulation between dayside and nightside Westward momentum driving RSZ wind is supplied by gravity waves © 2013. American Geophysical Union. All Rights Reserved.

The whole atmosphere model GAIA is employed to shed light on atmospheric response to the 2009 major stratosphere sudden warming (SSW) from the ground to exobase. Distinct features are revealed about SSW impacts on thermospheric temperature and density above 100 km altitude. (1) The effect is primarily quasi-semidiurnal in tropical regions, with warming in the noon and pre-midnight sectors and cooling in the dawn and dusk sectors. (2) This pattern exists at all altitudes above 100 km, with its phase being almost constant above 200 km, but propagates downward in the lower thermosphere between 100 and 200 km. (3) The northern polar region experiences warming in a narrow layer between 100 and 130 km, while the southern polar region experiences cooling throughout 100-400 km altitudes. (4) The global net thermal effect on the atmosphere above 100 km is a cooling of approximately -12 K. These characteristics provide us with an urgently needed global context to better connect and understand the increasing upper atmosphere observations during SSW events. Key Points Strong LT dependence in tropical regions Fast downward phase propagation between 100 and 200 km Global net cooling of -12 K on the atmosphere above 100 km ©2013. American Geophysical Union. All Rights Reserved.

The generation mechanism for the 4-peak longitudinal structure of the neutral density in the upper thermosphere is examined using an atmosphere-ionosphere coupled model. Our result indicates that the wave-4 structure of the neutral density in the upper thermosphere is caused by the upward propagation of the eastward diurnal tide with zonal wavenumber 3 (DE3) and the eastward semidiurnal tide with zonal wavenumber 2 (SE2) from the troposphere. The wave-4 structure of the neutral density in the equatorial region is mainly generated by the DE3, while the SE2 is important for the generation of the wave-4 structure in middle latitudes. Our simulation demonstrates that the wave-4 structure is evident at the height range from 150 km to 500 km. Furthermore, we examine the day-to-day variation of the wave-4 amplitude of the neutral density at 400 km height and its relation with the SE2 and DE3 amplitudes at various heights. (C) 2011 Elsevier Ltd. All rights reserved.

This paper compares results from a whole atmosphere-ionosphere coupled model, GAIA, with the COSMIC and TIMED/SABER observations during the 2008/2009 northern winter season. The GAIA model has assimilated meteorological reanalysis data by a nudging method. The comparison shows general agreement in the major features from the stratosphere to the ionosphere including the growth and decay of the major stratospheric sudden warming (SSW) event in 2009. During this period, a pronounced semidiurnal variation in the F region electron density and its local-time phase shift similar to the previous observations are reproduced by the model and COSMIC observation. The model suggests that the electron density variation is caused by an enhanced semidiurnal variation in the E x B drift, which is probably related to an amplified semidiurnal migrating tide (SW2) in the lower thermosphere. The model and TIMED/SABER observation show that the SW2 tide amplifies at low latitudes from the stratosphere to the thermosphere as well as the phase variation. Possible mechanisms for the SW2 variability in the low latitude stratosphere could be the change of its propagation condition, especially the (2, 2) mode, due to changing zonal background wind and meridional temperature gradient, and/or an enhancement of its source due to redistribution of stratospheric ozone. Present results also show a prominent long-term variation of the terdiurnal migrating component (TW3) in the ionosphere and atmosphere.

This paper for the first time presents a detailed comparison between simulated and observed global electron density responses to different atmospheric tides forced from below. The recently developed Earth's whole atmospheric model from the troposphere to the ionosphere, called GAIA, has been used for the simulation of the electron density tidal responses. They have been compared with the extracted from the COSMIC electron density data tidal responses for the period of time October 2007 to March 2009. Particular attention has been paid to the nonmigrating DE3/DE2 and migrating DW1, SW2 and TW3 electron density responses. The GAIA model reproduced quite well the COSMIC DE3/DE2 responses. Both simulations and observations revealed three altitude regions of enhanced electron density responses: (1) an upper level response, above 300 km height, apparently shaped mainly by the "fountain effect"; (2) a response located near altitudes of similar to 200-270 km, and (3) a lower thermospheric response situated near 120-150 km height. A possible mechanism is suggested for explaining the two lower level responses. For the first time the GAIA model simulations supported the observational evidence found in the COSMIC measurements that the ionospheric WN4 (WN3) longitude structure is not generated only by the DE3 (DE2) tide as it has been often assumed. As regards the comparison of the migrating DW1, SW2 and TW3 responses the obtained results clearly demonstrate that the GAIA model reproduce very well of the SW2 and TW3 COSMIC electron density responses. The only main discrepancy is seen in the migrating DW1 response; the observation does not support the splitting of the simulated response at both sides of the equator. This is due mainly to the difference between the SABER and GAIA SW2 tide in the lower thermosphere as it turned out that the DW1 electron density response strongly depends on the mean features of the lower thermospheric SW2 tide.

We have examined excitation mechanism of the fast jet of the neutral atmosphere along the dip equator in the upper thermosphere. The zonal momentum balance of the neutral atmosphere is estimated using an atmosphere-ionosphere coupled model. The coupled model used in this study is a self-consistent global model of the atmosphere and ionosphere covering the height range from the ground surface to the exobase. It can reproduce the observed equatorial fast jet above 250 km heights. The analysis of the zonal momentum balance reveals that the pressure gradient and ion drag play an important role on the formation of the fast jet near the dip equator. In particular, the fast jet near the equator is closely related with the latitudinal difference of the ion drag force. We also investigate the zonal momentum balance of the longitudinal wave-4 structure of the zonal wind in the fixed local time frame. Furthermore, significant day-to-day variations in the neutral zonal wind and the ion drift near the dip equator are obtained although the solar UV/EUV fluxes and the energy input from the magnetosphere are assumed to be constant during the numerical simulation. This result indicates the importance of the lower atmospheric variability on day-to-day variations in the thermosphere/ionosphere.

[1] This paper compares results from a whole atmosphere-ionosphere coupled model, GAIA, with the COSMIC and TIMED/SABER observations during the 2008/2009 northern winter season. The GAIA model has assimilated meteorological reanalysis data by a nudging method. The comparison shows general agreement in the major features from the stratosphere to the ionosphere including the growth and decay of the major stratospheric sudden warming (SSW) event in 2009. During this period, a pronounced semidiurnal variation in the F region electron density and its local-time phase shift similar to the previous observations are reproduced by the model and COSMIC observation. The model suggests that the electron density variation is caused by an enhanced semidiurnal variation in the E × B drift, which is probably related to an amplified semidiurnal migrating tide (SW2) in the lower thermosphere. The model and TIMED/SABER observation show that the SW2 tide amplifies at low latitudes from the stratosphere to the thermosphere as well as the phase variation. Possible mechanisms for the SW2 variability in the low latitude stratosphere could be the change of its propagation condition, especially the (2, 2) mode, due to changing zonal background wind and meridional temperature gradient, and/or an enhancement of its source due to redistribution of stratospheric ozone. Present results also show a prominent long-term variation of the terdiurnal migrating component (TW3) in the ionosphere and atmosphere. © 2012. American Geophysical Union. All Rights Reserved.

This paper for the first time presents a detailed comparison between simulated and observed global electron density responses to different atmospheric tides forced from below. The recently developed Earth's whole atmospheric model from the troposphere to the ionosphere, called GAIA, has been used for the simulation of the electron density tidal responses. They have been compared with the extracted from the COSMIC electron density data tidal responses for the period of time October 2007 to March 2009. Particular attention has been paid to the nonmigrating DE3/DE2 and migrating DW1, SW2 and TW3 electron density responses. The GAIA model reproduced quite well the COSMIC DE3/DE2 responses. Both simulations and observations revealed three altitude regions of enhanced electron density responses: (1) an upper level response, above 300 km height, apparently shaped mainly by the fountain effect (2) a response located near altitudes of ∼200-270 km, and (3) a lower thermospheric response situated near 120-150 km height. A possible mechanism is suggested for explaining the two lower level responses. For the first time the GAIA model simulations supported the observational evidence found in the COSMIC measurements that the ionospheric WN4 (WN3) longitude structure is not generated only by the DE3 (DE2) tide as it has been often assumed. As regards the comparison of the migrating DW1, SW2 and TW3 responses the obtained results clearly demonstrate that the GAIA model reproduce very well of the SW2 and TW3 COSMIC electron density responses. The only main discrepancy is seen in the migrating DW1 response the observation does not support the splitting of the simulated response at both sides of the equator. This is due mainly to the difference between the SABER and GAIA SW2 tide in the lower thermosphere as it turned out that the DW1 electron density response strongly depends on the mean features of the lower thermospheric SW2 tide. © 2012. American Geophysical Union. All Rights Reserved.

We have examined excitation mechanism of the fast jet of the neutral atmosphere along the dip equator in the upper thermosphere. The zonal momentum balance of the neutral atmosphere is estimated using an atmosphere-ionosphere coupled model. The coupled model used in this study is a self-consistent global model of the atmosphere and ionosphere covering the height range from the ground surface to the exobase. It can reproduce the observed equatorial fast jet above 250km heights. The analysis of the zonal momentum balance reveals that the pressure gradient and ion drag play an important role on the formation of the fast jet near the dip equator. In particular, the fast jet near the equator is closely related with the latitudinal difference of the ion drag force. We also investigate the zonal momentum balance of the longitudinal wave-4 structure of the zonal wind in the fixed local time frame. Furthermore, significant day-to-day variations in the neutral zonal wind and the ion drift near the dip equator are obtained although the solar UV/EUV fluxes and the energy input from the magnetosphere are assumed to be constant during the numerical simulation. This result indicates the importance of the lower atmospheric variability on day-to-day variations in the thermosphere/ionosphere.

The IPY long-run data were obtained from the European Incoherent Scatter Svalbard radar (ESR) observations during March 2007 and February 2008. Since the solar and geomagnetic activities were quite low during the period, this data set is extremely helpful for describing the basic states (ground states) of the thermosphere and ionosphere in the polar cap region. The monthly-averaged ion temperatures for 12 months show similar local time (or UT) variations to each other. The ion temperatures also show significant seasonal variations. The amplitudes of the local time and seasonal variations observed are much larger than the ones predicted by the IRI-2007 model. In addition, we performed numerical simulations with a general circulation model (GCM), which covers all the atmospheric regions, to investigate variations of the neutrals in the polar thermosphere. The GCM simulations show significant variations of the neutral temperature in the polar region in comparison with the NRLMSISE-00 empirical model. These results indicate that both the ions and neutrals would show larger variations than those described by the empirical models, suggesting significant heat sources in the polar cap region even under solar minimum and geomagnetically quiet conditions.

We report on the characteristics of tidal variations in the Martian lower atmosphere (&lt;45 km) using the Mars Express (MEX) Planetary Fourier Spectrometer (PFS) temperature data for about three Martian years (between the ends of MY26 and MY29). The PFS data, which widely cover local time, enable us to investigate diurnal variations in the atmospheric temperature at various altitudes. We focus on diurnal variations in the atmospheric temperature and on longitudinal temperature variability in a fixed local time frame. We find that the latitudinal and diurnal variations at 0.52 mbar (similar to 25 km) during the dust-clear period (Ls = 30 degrees-60 degrees) are consistent with general characteristics presented by previous numerical simulations. The characteristics of the diurnal variations as a function of altitude in the tropics are also explained as results from the propagation of the migrating diurnal tide. The longitudinal temperature variability in the dayside (14.36-14.94 LT) equatorial regions (10 degrees S-5 degrees S) near the northern summer solstice (Ls = 76 degrees-83 degrees) in MY28 are investigated. The longitudinal temperature structure has two local maxima at 2.85 mbar (similar to 10 km) but is relatively uniform at 0.52 mbar. We find that the wave-3 structure is apparent at 0.11 mbar (similar to 40 km) in the present case. This structure would be strongly dependent on activities of the atmospheric waves, e.g., the diurnal Kelvin wave 2 (DK2). Citation: Sato, T. M., H. Fujiwara, Y. O. Takahashi, Y. Kasaba, V. Formisano, M. Giuranna, and D. Grassi (2011), Tidal variations in the Martian lower atmosphere inferred from Mars Express Planetary Fourier Spectrometer temperature data, Geophys. Res. Lett., 38, L24205, doi:10.1029/2011GL050348.

The equatorial mass density anomaly (EMA) is a fascinating phenomenon in the equatorial upper atmosphere. In this study, we investigate the generation mechanism of the EMA using the ground-to-topside model of the atmosphere and ionosphere for aeronomy (GAIA). The GAIA model is a self-consistent global model of the atmosphere and ionosphere covering the height range from the ground surface to the exobase. It can reproduce the observed EMA structure at 300-400 km heights. Our results show that the EMA structure can extend down to 200 km height. The EMA during daytime is caused by the in situ diurnal tide and the upward propagating terdiurnal tide. About half of the magnitude of the EMA is generated by the upward propagating terdiurnal tide from the lower atmosphere. This is the first report concerning the importance of the upward propagating tide for EMA formation. The in situ diurnal tide in the thermosphere is also essential for EMA formation. The in situ diurnal tide is modified by the momentum exchange between the plasma and the neutral atmosphere. This is seen as the enhanced upward flow of the neutral atmosphere along the dip equator in the 200-400 km height region, which has a profound effect on the latitudinal distributions of the atmospheric composition, temperature, pressure, and density in the thermosphere.

The equatorial mass density anomaly (EMA) is a fascinating phenomenon in the equatorial upper atmosphere. In this study, we investigate the generation mechanism of the EMA using the ground-to-topside model of the atmosphere and ionosphere for aeronomy (GAIA). The GAIA model is a self-consistent global model of the atmosphere and ionosphere covering the height range from the ground surface to the exobase. It can reproduce the observed EMA structure at 300-400 km heights. Our results show that the EMA structure can extend down to 200 km height. The EMA during daytime is caused by the in situ diurnal tide and the upward propagating terdiurnal tide. About half of the magnitude of the EMA is generated by the upward propagating terdiurnal tide from the lower atmosphere. This is the first report concerning the importance of the upward propagating tide for EMA formation. The in situ diurnal tide in the thermosphere is also essential for EMA formation. The in situ diurnal tide is modified by the momentum exchange between the plasma and the neutral atmosphere. This is seen as the enhanced upward flow of the neutral atmosphere along the dip equator in the 200-400 km height region, which has a profound effect on the latitudinal distributions of the atmospheric composition, temperature, pressure, and density in the thermosphere. Copyright © 2011 by the American Geophysical Union.

We studied the occurrence characteristics and variability of the terdiurnal tide (8 hour period) in the equatorial mesosphere and lower thermosphere (MLT), using a meteor radar at Koto Tabang (0.2°S, 100.3°E) and MF radars at Tirunelveli (8.7°N, 77.8°E) and Pameungpeuk (7.4°S, 107.4°E). These locations, one being located right over the equator and the other two at conjugate points around the equator within 10, form a unique experimental setup to study equatorial MLT dynamics. The terdiurnal tide exists as a distinct wave signature at all three locations. While the daily amplitudes can be as large as 15 m s&lt sup&gt -1&lt /sup&gt , the monthly mean amplitudes lie between 1 and 10 m s &lt sup&gt -1&lt /sup&gt . The amplitude of the terdiurnal tide at Pameungpeuk is generally smaller than that observed at Tirunelveli and Koto Tabang. The seasonal variation in amplitude shows both annual and semiannual oscillations of ∼1 m s&lt sup&gt -1&lt /sup&gt at all three locations. The present observations combined with previous reports indicate that the timing of the primary maximum of the terdiurnal tide amplitude shifts from autumn to late spring and early summer as one moves from high latitudes to the equator (all with respect to the Northern Hemisphere). The amplitudes and seasonal variation in the present observations show good comparison with that simulated by the General Circulation Model (GCM) developed by Kyushu University, Japan. This study supports the occurrence of nonlinear interaction between diurnal and semidiurnal tides and shows that gravity waves play an important role in the generation of the terdiurnal tide. Copyright 2011 by the American Geophysical Union.

This paper introduces a new Earth's atmosphere-ionosphere coupled model that treats seamlessly the neutral atmospheric region from the troposphere to the thermosphere as well as the thermosphere-ionosphere interaction including the electrodynamics self-consistently. The model is especially useful for the study of vertical connection between the meteorological phenomena and the upper atmospheric behaviors. As an initial simulation using the coupled model, we have carried out a 30 day consecutive run in September. The result reveals that the longitudinal structure of the F-region ionosphere varies on a day-to-day basis in a highly complex way and that a four-peak structure of the daytime equatorial ionization anomaly (EIA) similar to the recent observations appears as an averaged feature. The simulation reproduces and thus confirms the vertical coupling processes proposed so far with respect to the formation of the averaged EIA longitudinal structure the excitation of solar nonmigrating tides in the troposphere, their propagation through the middle atmosphere, and the modulation of ionospheric dynamo, which in turn affects EIA generation. The simulation result indicates that not only the ionospheric averaged longitudinal structure but also the day-to-day variation can be modulated significantly by the lower atmospheric effect. Copyright 2011 by the American Geophysical Union.

We studied the occurrence characteristics and variability of the terdiurnal tide (8 hour period) in the equatorial mesosphere and lower thermosphere (MLT), using a meteor radar at Koto Tabang (0.2 degrees S, 100.3 degrees E) and MF radars at Tirunelveli (8.7 degrees N, 77.8 degrees E) and Pameungpeuk (7.4 degrees S, 107.4 degrees E). These locations, one being located right over the equator and the other two at conjugate points around the equator within +/- 10 degrees, form a unique experimental setup to study equatorial MLT dynamics. The terdiurnal tide exists as a distinct wave signature at all three locations. While the daily amplitudes can be as large as 15 m s(-1), the monthly mean amplitudes lie between 1 and 10 m s(-1). The amplitude of the terdiurnal tide at Pameungpeuk is generally smaller than that observed at Tirunelveli and Koto Tabang. The seasonal variation in amplitude shows both annual and semiannual oscillations of similar to 1 m s(-1) at all three locations. The present observations combined with previous reports indicate that the timing of the primary maximum of the terdiurnal tide amplitude shifts from autumn to late spring and early summer as one moves from high latitudes to the equator (all with respect to the Northern Hemisphere). The amplitudes and seasonal variation in the present observations show good comparison with that simulated by the General Circulation Model (GCM) developed by Kyushu University, Japan. This study supports the occurrence of nonlinear interaction between diurnal and semidiurnal tides and shows that gravity waves play an important role in the generation of the terdiurnal tide.