Hidekatsu Jin

The neutral dynamics during the 2009 sudden stratosphere warming simulated by different whole atmosphere models

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 ∼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.

A global view of gravity waves in the thermosphere simulated by a general circulation model

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.

Thermal and dynamical changes of the zonal mean state of the thermosphere during the 2009 SSW: GAIA simulations

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°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.

Nonlinear growth, bifurcation, and pinching of equatorial plasma bubble simulated by three‐dimensional high‐resolution bubble model

A new three-dimensional high-resolution numerical model to study equatorial plasma bubble (EPB) has been developed. The High-Resolution Bubble (HIRB) model is developed in a magnetic dipole coordinate system for the equatorial and low-latitude ionosphere with a spatial resolution of as fine as 1 km. Adopting a higher-order numerical scheme than those used in the existing models, the HIRB model is capable of reproducing the bifurcation, pinching, and turbulent structures of EPB. From a seeding perturbation resembling large-scale wave structure (LSWS), EPB grows nonlinearly from the crest of LSWS upwelling, bifurcates at the top of EPB, then becomes turbulent at the topside of the F region. One of the bifurcated EPB is pinched off from the primary EPB and stops growing after pinching. The narrow channel of EPB tends to have a wiggle due to the secondary instability along the wall of EPB. Because of the fringe field effect above and below the EPB, upward drifting low-density plasma converges toward the F peak altitude, forming a narrow-depleted channel, and diverges above the peak, forming a flattened top of the EPB. The flattened top which has a steep upward density gradient is so unstable that bifurcation can easily occur even from a very small thermal perturbation. A higher density region between the bifurcated EPB moves downward due to westward polarization electric field. The EPB is pinched off when it reaches the wall of the primary EPB. It is concluded that turbulent plume-like irregularities can be spontaneously generated only from large-scale perturbation at the bottomside F region.

West wall structuring of equatorial plasma bubbles simulated by three‐dimensional HIRB model

Plasma density depletions in the equatorial ionosphere, or so-called equatorial plasma bubbles (EPBs), are generated in the postsunset period and tend to have a very complex spatial structure. Especially, the east-west asymmetry of EPBs has been reported by various observations. Using a high-resolution bubble (HIRB) model, which is a newly developed three-dimensional numerical model for the equatorial ionosphere, small-scale structuring at the west wall of large-scale F layer upwelling is clearly reproduced for the first time. It is not an eastward neutral wind but a vertical shear of zonal plasma drift velocity at the bottomside of the F region that plays an important role in accelerating the instability growth at the west wall and generating the east-west asymmetry of EPBs.

Impacts of sudden stratospheric warming on general circulation of the thermosphere

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.

Polar cap ionosphere and thermosphere during the solar minimum period: EISCAT Svalbard radar observations and GCM simulations

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.

A new ionospheric storm scale based on TEC and foF2 statistics

In this paper, we propose the I-scale, a new ionospheric storm scale for general users in various regions in the world. With the I-scale, ionospheric storms can be classified at any season, local time, and location. Since the ionospheric condition largely depends on many factors such as solar irradiance, energy input from the magnetosphere, and lower atmospheric activity, it had been difficult to scale ionospheric storms, which are mainly caused by solar and geomagnetic activities. In this study, statistical analysis was carried out for total electron content (TEC) and F2 layer critical frequency (foF2) in Japan for 18 years from 1997 to 2014. Seasonal, local time, and latitudinal dependences of TEC and foF2 variabilities are excluded by normalizing each percentage variation using their statistical standard deviations. The I-scale is defined by setting thresholds to the normalized numbers to seven categories: I0, IP1, IP2, IP3, IN1, IN2, and IN3. I0 represents a quiet state, and IP1 (IN1), IP2 (IN2), and IP3 (IN3) represent moderate, strong, and severe positive (negative) storms, respectively. The proposed I-scale can be used for other locations, such as polar and equatorial regions. It is considered that the proposed I-scale can be a standardized scale to help the users to assess the impact of space weather on their systems.

Effect of intrinsic magnetic field decrease on the low‐ to middle‐latitude upper atmosphere dynamics simulated by GAIA

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 (<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 super-rotation 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 super-rotation.

Evaluation of the Sq Magnetic Field Variation Calculated by GAIA

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 does 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.