Hitoshi Fujiwara

Characteristics are presented of the lower thermospheric wind from a long run data set obtained by the EISCAT UHF radar at Tromso (69.6 degrees N, 19.2 degrees E) over similar to 23 days, from September 6 to 29, 2005. The derived semidiurnal amplitude exhibited day-to-day variations (similar to 5-30 ms(-1)) at and above 109 km, while the phase varied little with the day. We have found a mode change of the semidiurnal tide occurring during September 17-22, 2005. Between September 6 and 16, the vertical wavelengths were estimated to be similar to 58 km and similar to 76 km for the meridional and zonal components, respectively, while between September 23 and 29, they became less than similar to 24 km. The day to day variability of the diurnal tide was less obvious than that of the semidiurnal tide. The diurnal amplitude of the meridional component increased with height except for 8 days between September 13 and 20, when the diurnal amplitudes were smaller values (<40 ms(-1)) at and above 111 km than those for the other intervals. Furthermore, the shapes of the altitude profiles of the meridional phase differ from those for the other intervals. We have evaluated contributions due to the electric field and the ion drag acceleration and showed that they were not the causes. From the analysis of 22.5 days of wind data, we found about 5-6 day oscillations in the lower thermosphere, probably where there were planetary wave activities in the lower thermosphere.

We developed a new numerical model of the Jovian magnetosphere-ionosphere coupling current system in order to investigate the effects of diurnal variation of ionospheric conductance. The conductance is determined by ion chemical processes that include the generation of hydrogen and hydrocarbon ions by solar EUV radiation and auroral electrons precipitation. The model solves the torque equations for magnetospheric plasma accelerated by the radial currents flowing along the magnetospheric equator. The conductance and magnetospheric plasma then change the field-aligned currents (FACs) and the intensity of the electric field projected onto the ionosphere. Because of the positive feedback of the ionospheric conductance on the FAC, the FAC is the maximum on the dayside and minimum just before sunrise. The power transferred from the planetary rotation is mainly consumed in the upper atmosphere on the dayside, while it is used for magnetospheric plasma acceleration in other local time (LT) sectors. Further, our simulations show that the magnetospheric plasma density and mass flux affect the temporal variation in the peak FAC density. The enhancement of the solar EUV flux by a factor of 2.4 increases the FAC density by 30%. The maximum density of the FAC is determined not only by the relationship between the precipitating electron flux and ionospheric conductance, but also by the system inertia, i.e., the inertia of the magnetospheric plasma. A theoretical analysis and numerical simulations reveal that the FAC density is in proportion to the planetary angular velocity on the dayside and to the square of the planetary angular velocity on the nightside. When the radial current at the outer boundary is fixed at values above 30 MA, as assumed in previous model studies, the peak FAC density determined at latitude 73 degrees-74 degrees is larger than the diurnal variable component. This result suggests large effects of this assumed radial current at the outer boundary on the system. (C) 2009 Elsevier Ltd. All rights reserved.

Characteristics are presented of the lower thermospheric wind from a long run data set obtained by the EISCAT UHF radar at Troms (69.6N, 19.2E) over ∼23 days, from September 6 to 29, 2005. The derived semidiurnal amplitude exhibited day-to-day variations (∼5-30 ms-1) at and above 109 km, while the phase varied little with the day. We have found a mode change of the semidiurnal tide occurring during September 17-22, 2005. Between September 6 and 16, the vertical wavelengths were estimated to be ∼58 km and ∼76 km for the meridional and zonal components, respectively, while between September 23 and 29, they became less than ∼24 km. The day to day variability of the diurnal tide was less obvious than that of the semidiurnal tide. The diurnal amplitude of the meridional component increased with height except for 8 days between September 13 and 20, when the diurnal amplitudes were smaller values (<40 ms-1) at and above 111 km than those for the other intervals. Furthermore, the shapes of the altitude profiles of the meridional phase differ from those for the other intervals. We have evaluated contributions due to the electric field and the ion drag acceleration and showed that they were not the causes. From the analysis of 22.5 days of wind data, we found about 5-6 day oscillations in the lower thermosphere, probably where there were planetary wave activities in the lower thermosphere. Copyright © 2010 by the American Geophysical Union.

The thermosphere is the transition region from the atmosphere to space. Both the solar ultraviolet radiation and the solar wind energy inputs have caused significant thermospheric variations from past to present. In order to understand thermospheric/ionospheric disturbances in association with changes in solar activity, observational and modelling efforts have been made by many researchers. Recent satellite observations, e.g., the satellite CHAMP, have revealed mass density variations in the upper thermosphere. The thermospheric temperature, wind, and composition variations have been also investigated with general/global circulation models (GCMs) which include forcings due to the solar wind energy inputs and the lower atmospheric effects. In particular, we have developed a GCM which covers all the atmospheric regions, troposphere, stratosphere, mesosphere, and thermosphere, to describe variations of the thermospheric temperature and density caused by both effects from the lower atmosphere and the magnetosphere. GCM simulations represent global and localized temperature and density structures, winch vary from hour to hour, depending on forcings due to the lower atmosphere, solar and geomagnetic activities. This modelling attempt will enable us to describe the thermospheric weather influenced by solar activity in cooperation with ground-based and satellite observations.

In order to illustrate morphological features and variations of temperature in the upper thermosphere, we performed numerical simulations with a whole atmosphere general circulation model (GCM) for the solar minimum and geomagnetically quiet conditions in March, June, September, and December. In previous GCMs, tidal effects were imposed at the lower boundaries assuming dominant diurnal and semi-diurnal tidal modes. Since the GCM used in the present study covers all the atmospheric regions, the atmospheric tides with various modes are generated within the GCM. The global temperature distributions obtained from the GCM are in agreement with ones obtained from NRLMSISE-00. In addition, the GCM also represents localised temperature structures which are superimposed on the global day-night distributions. These localised structures, which vary from hour to hour, would be observed as variations with periods of about 2-3 h at a single site. The amplitudes of the 2-3 h variations are significant at high-latitude, while the amplitudes are small at low-latitude. The diurnal temperature variation is more clearly identified at low-latitude than at high-latitude. When we assume the same high-latitude convection electric field in each month, the temperature calculated in the polar cap region shows diurnal variation more clearly in winter than in summer. The midnight temperature maximum (MTM), which is one of the typical low-latitude temperature structures, is also seen in the GCM results. The MTMs in the GCM results show significant day-to-day variation with amplitudes of several 10s to about 150 K. The wind convergence and stream of warm air are found around the MTM. The GCM also represent the meridional wind reversals and/or abatements which are caused due to local time variations of airflow pattern in the low-latitude region.

In order to clarify the role of neutral dynamics in the Jovian magnetosphere-ionosphere-thermosphere coupling system, we have developed a new numerical model that includes the effect of neutral dynamics on the coupling current. The model calculates axisymmetric thermospheric dynamics and ion composition by considering fundamental physical and chemical processes. The ionospheric Pedersen current is obtained from the thermospheric and ionospheric parameters. The model simultaneously solves the torque equations of the magnetospheric plasma due to radial currents flowing at the magnetospheric equator, which enables us to update the electric field projected onto the ionosphere and the field-aligned currents (FACs) depending upon the thermospheric dynamics. The self-consistently calculated temperature and ion velocity are consistent with observations. The estimated neutral wind field captures the zonally averaged characteristics in previous three-dimensional models. The energy extracted from the planetary rotation is mainly used for magnetospheric plasma acceleration below 73.5 degrees latitude while consumed in the upper atmosphere, mainly by Joule heating at above 73.5 degrees latitude. The neutral wind dynamics contributes to a reduction in the electric field of 22% compared with the case of neutral rigid corotation. About 90% of this reduction is attributable to neutral winds below the 550-km altitude in the auroral region. The calculated radial current in the equatorial magnetosphere is smaller than observations. This indicates that the enhancement of the background conductance and/or the additional radial current at the outer boundary would be expected to reproduce the observed current.

Observations by the CHAMP satellite have recently revealed wave structure of the neutral density near the solar terminator in the upper atmosphere. The amplitude of the neutral density perturbation associated with a solar terminator wave is of order +/- 3-6%, and the horizontal wavelength is of order 3000 km. In this study, we examine the excitation mechanism of the solar terminator wave using a general circulation model (GCM) that extends from the ground surface to the exobase. Our result reveals that waves similar to the solar terminator wave observed by the CHAMP satellite are found not only in the thermosphere but also in the stratosphere and mesosphere. The terminator wave is excited in the stratosphere and/or troposphere and propagates upward into the upper thermosphere. Specifically, the solar terminator wave is mainly generated by superposition of the upward propagating migrating tides from zonal wave number 4 to zonal wave number 6. The terminator wave is more prominent during solstice than during equinox. This seasonal variability of the terminator wave is also studied using a GCM simulation. In addition, the maximum temperature perturbation at low latitudes occurs near midnight, indicating a contribution to the midnight temperature maximum in the upper thermosphere. The effect of the terminator wave on the generation of the midnight temperature maximum is examined. The relation between the midnight temperature maximum and the upward propagating migrating tides is also discussed.

In order to clarify the role of neutral dynamics in the Jovian magnetosphere-ionosphere-thermosphere coupling system, we have developed a new numerical model that includes the effect of neutral dynamics on the coupling current. The model calculates axisymmetric thermospheric dynamics and ion composition by considering fundamental physical and chemical processes. The ionospheric Pedersen current is obtained from the thermospheric and ionospheric parameters. The model simultaneously solves the torque equations of the magnetospheric plasma due to radial currents flowing at the magnetospheric equator, which enables us to update the electric field projected onto the ionosphere and the field-aligned currents (FACs) depending upon the thermospheric dynamics. The self-consistently calculated temperature and ion velocity are consistent with observations. The estimated neutral wind field captures the zonally averaged characteristics in previous three-dimensional models. The energy extracted from the planetary rotation is mainly used for magnetospheric plasma acceleration below 73.5° latitude while consumed in the upper atmosphere, mainly by Joule heating at above 73.5° latitude. The neutral wind dynamics contributes to a reduction in the electric field of 22% compared with the case of neutral rigid corotation. About 90% of this reduction is attributable to neutral winds below the 550-km altitude in the auroral region. The calculated radial current in the equatorial magnetosphere is smaller than observations. This indicates that the enhancement of the background conductance and/or the additional radial current at the outer boundary would be expected to reproduce the observed current. Copyright 2009 by the American Geophysical Union.

Observations by the CHAMP satellite have recently revealed wave structure of the neutral density near the solar terminator in the upper atmosphere. The amplitude of the neutral density perturbation associated with a solar terminator wave is of order ± 3-6%, and the horizontal wavelength is of order 3000 km. In this study, we examine the excitation mechanism of the solar terminator wave using a general circulation model (GCM) that extends from the ground surface to the exobase. Our result reveals that waves similar to the solar terminator wave observed by the CHAMP satellite are found not only in the thermosphere but also in the stratosphere and mesosphere. The terminator wave is excited in the stratosphere and/or troposphere and propagates upward into the upper thermosphere. Specifically, the solar terminator wave is mainly generated by superposition of the upward propagating migrating tides from zonal wave number 4 to zonal wave number 6. The terminator wave is more prominent during solstice than during equinox. This seasonal variability of the terminator wave is also studied using a GCM simulation. In addition, the maximum temperature perturbation at low latitudes occurs near midnight, indicating a contribution to the midnight temperature maximum in the upper thermosphere. The effect of the terminator wave on the generation of the midnight temperature maximum is examined. The relation between the midnight temperature maximum and the upward propagating migrating tides is also discussed. Copyright 2009 by the American Geophysical Union.

筆頭

Using a general circulation model that contains the region from the ground surface to the upper thermosphere, we have examined characteristics of gravity waves in the equatorial thermosphere. At an altitude of 150 km. the dominant periods of gravity waves for zonal wave number 20 (zonal wavelength lambda(x) approximate to 2000 km), 40 (lambda(x) approximate to 1000 km) and 80 (lambda(x) approximate to 500 km) are 3, 1.5 and 1h, respectively. For individual zonal wave numbers, the corresponding dominant period becomes shorter at higher altitudes Clue to dissipation processes in the thermosphere. such as molecular viscosity and ion drag force, indicating that gravity waves with a larger horizontal phase velocity (larger vertical wavelength) can penetrate into the thermosphere. The longitudinal variation of gravity wave activity in the equatorial thermosphere and upward propagation of gravity waves from the lower atmosphere were also studied. The longitudinal distribution of gravity wave activity in the thermosphere is quite similar to that of gravity wave activity in the lower atmosphere and the cumulus convective activity in the tropical troposphere. Our results indicate that the strong, energy flux due to gravity waves from the enhanced cumulus convective activity propagates upward into the upper thermosphere. The relation between the wind fluctuation associated with gravity waves and the ionospheric variation is discussed. Fluctuations of the neutral zonal wind with periods of 1-2 h are significant in the 200- to 300-km height region, and its amplitude sometimes exceeds 50 m s(-1). These results suggest that upward propagating gravity waves call affect the ionospheric variation in the F-region.

It is well-known that low-latitude ionospheric/thermospheric disturbances are sometimes generated in association with the passage of traveling ionospheric/atmospheric disturbances (TIDs/TADs) produced in the high-latitude region and that the low-latitude ionosphere/thermosphere should be strongly coupled with the lower atmosphere. These facts suggest that the appearance of thermospheric disturbances with complex structures in the low-latitude region are the result of a superposition of disturbances which have different origins. We have investigated the lower atmospheric effects oil the morphology of the thermospheric disturbances in response to changes in the geomagnetic activity by Using a whole atmosphere general circulation model (GCM). In order to suppress the lower atmospheric effects, we set the global mean temperature and zero-wind below about 80-km altitude in the GCM. The simulation results show that the lower atmospheric effects call produce latitudinal and longitudinal structures in the low-latitude thermosphere. These lower atmospheric effects also modulate the amplitudes and structures of TADs propagating from the high- to low-latitude regions. Our results suggest that the lower atmospheric effects can produce variability in the TIDs/TADs, which in turn would create conditions Conducive to plasma instabilities in the low-latitude ionosphere.

The global wave number 4 longitudinal structure of ionospheric density has been observed recently by a number of satellite measurements and considered as a signature of dynamical coupling from the deep atmosphere to the ionosphere. By using a numerical model of atmospheric electrodynamics with input fields from a whole atmosphere general circulation model and an ionosphere-thermosphere model, we investigated the generation mechanism for the longitudinal structure of the F-region zonal electric field (vertical E × B drift) as a possible driver of the ionospheric density variation, especially with respect to the eastward zonal wave number 3 diurnal tide (DE3) that originates from the convective activities in the troposphere and propagates upward. The simulation showed that the longitudinal profile of zonal perturbation electric field is largely influenced by the zonal DE3 wind around the height of peak Hall conductivity during the daytime, and that it is by the zonal DE3 wind in the F-region during the nighttime. The daytime zonal electric field is a direct result from charge separation induced by the Hall dynamo current, whereas the nighttime zonal electric field is rather produced to satisfy the electrostatic condition. Copyright 2008 by the American Geophysical Union.

The global wave number 4 longitudinal structure of ionospheric density has been observed recently by a number of satellite measurements and considered as a signature of dynamical coupling from the deep atmosphere to the ionosphere. By using a numerical model of atmospheric electrodynamics with input fields from a whole atmosphere general circulation model and an ionosphere-thermosphere model, we investigated the generation mechanism for the longitudinal structure of the F-region zonal electric field (vertical E x B drift) as a possible driver of the ionospheric density variation, especially with respect to the eastward zonal wave number 3 diurnal tide (DE3) that originates from the convective activities in the troposphere and propagates upward. The simulation showed that the longitudinal profile of zonal perturbation electric field is largely influenced by the zonal DE3 wind around the height of peak Hall conductivity during the daytime, and that it is by the zonal DE3 wind in the F-region during the nighttime. The daytime zonal electric field is a direct result from charge separation induced by the Hall dynamo current, whereas the nighttime zonal electric field is rather produced to satisfy the electrostatic condition.

A solar terminator wave is discovered in neutral thermosphere densities. The data originate from the accelerometer experiment on the CHAMP satellite between 2001 and 2007. During solar minimum conditions the phase fronts of the dusk terminator wave during Northern Hemisphere summer extend from about -60 degrees to almost +30 degrees latitude, at an angle of about 30 degrees with respect to the terminator. The density amplitudes are of order +/-3-6%, and the horizontal wavelength is of order 3,000 km. The dusk terminator wave is generally more well-defined than that near dawn, is more prominent during solar minimum than solar maximum, and during solstice as opposed to equinox. This wave is also found in similarly-analyzed output from the Kyushu University General Circulation Model that extends from the surface to the exobase. At solar minimum the model wave amplitude is similar to that observed, but with a horizontal wavelength close to 2,000 km. Analytic theory predicts a typical horizontal wavelength of 1,000 km. While there have been several reports of ionospheric waves in connection with the solar terminator, this appears to be the first such observation in the neutral thermosphere. In addition, the orientations of the dawn and dusk terminator waves are such that the maximum equatorial density perturbation occurs near midnight; therefore, some previous observations of the so-called "midnight temperature maximum'' may contain contributions attributable to the terminator wave.

By using a general circulation model that contains the region from the ground surface to the upper thermosphere, characteristics of gravity waves in the mesosphere and thermosphere are examined. At 100 km height, the dominant periods of gravity waves for zonal wave number 20 (zonal wavelength λ&lt inf&gt x&lt /inf&gt ≈ 2000 km), 40(λ&lt inf&gt x&lt /inf&gt ≈ 1000 km) and 80 (λ&lt inf&gt x&lt /inf&gt ≈ 500 km) are 6 h, 3 h and 1.5-2 h, respectively. For the individual zonal wave numbers, the corresponding dominant period becomes shorter at higher altitudes due to dissipation processes in the thermosphere, such as molecular viscosity and ion drag force. This means that gravity waves with larger horizontal phase velocity (larger vertical wavelength) can penetrate into the lower thermosphere. The vertical energy flux due to gravity waves indicates that upward propagation from the lower atmosphere to the thermosphere is dominant. Fluctuations of the horizontal wind associated with gravity waves and its relation to the ionospheric variation are discussed. Short-period g ravity waves into the thermosphere induce day-to-day variations of the zonal wind in the upper thermosphere. This result suggests that day-to-day variability of the zonal wind caused by gravity waves is expected to contribute to the ionospheric variability. Copyright 2008 by the American Geophysical Union.

By using a general circulation model that contains the region from the ground surface to the upper thermosphere, characteristics of gravity waves in the mesosphere and thermosphere are examined. At 100 km height, the dominant periods of gravity waves for zonal wave number 20 (zonal wavelength lambda(x) approximate to 2000 km), 40(lambda(x) approximate to 1000 km) and 80 (lambda(x) approximate to 500 km) are 6 h, 3 h and 1.5 - 2 h, respectively. For the individual zonal wave numbers, the corresponding dominant period becomes shorter at higher altitudes due to dissipation processes in the thermosphere, such as molecular viscosity and ion drag force. This means that gravity waves with larger horizontal phase velocity ( larger vertical wavelength) can penetrate into the lower thermosphere. The vertical energy flux due to gravity waves indicates that upward propagation from the lower atmosphere to the thermosphere is dominant. Fluctuations of the horizontal wind associated with gravity waves and its relation to the ionospheric variation are discussed. Short-period gravity waves into the thermosphere induce day-to-day variations of the zonal wind in the upper thermosphere. This result suggests that day-to-day variability of the zonal wind caused by gravity waves is expected to contribute to the ionospheric variability.

From simultaneous observations of the European incoherent scatter Svalbard radar (ESR) and the Cooperative UK Twin Located Auroral Sounding System (CUTLASS) Finland radar on 9 March 1999, we have derived the height distributions of the thermospheric heating rate at the F region height in association with electromagnetic energy inputs into the dayside polar cap/cusp region. The ESR and CUTLASS radar observations provide the ionospheric parameters with fine time-resolutions of a few minutes. Although the geomagnetic activity was rather moderate (Kp=3(+)similar to 4), the electric field obtained from the ESR data sometimes shows values exceeding 40 mV/m. The estimated passive energy deposition rates are also larger than 150 W/kg in the upper thermosphere over the ESR site during the period of the enhanced electric field. In addition, enhancements of the Pedersen conductivity also contribute to heating the upper thermosphere, while there is only a small contribution for thermospheric heating from the direct particle heating due to soft particle precipitation in the dayside polar cap/cusp region. In the same period, the CUTLASS observations of the ion drift show the signature of poleward moving pulsed ionospheric flows with a recurrence rate of about 10-20 min. The estimated electromagnetic energy deposition rate shows the existence of the strong heat source in the dayside polar cap/cusp region of the upper thermosphere in association with the dayside magnetospheric phenomena of reconnections and flux transfer events.

We have investigated characteristics of the large-scale traveling atmospheric disturbances (LS-TADs) generated during geomagnetically quiet and disturbed periods using a whole atmosphere general circulation model (GCM). The GCM simulations show that various TADs appear in association with passages of regions with large temperature gradients near the solar terminator, midnight temperature anomaly, and auroral oval which move with the Earth&apos;s rotation. These TADs, which are superimposed on each other, appear even when a geomagnetically quiet period. The TADs generated during a geomagnetically quiet period show structures extending in the longitudinal direction at high-latitude and in the latitudinal direction at mid- and low-latitude. These structures disappear after their short-range propagations. The TADs generated during a geomagnetically disturbed period show structures extending widely in the longitudinal direction and propagate from high- to low-latitude. These simulation results suggest the different generation mechanisms and features between the TADs generated during geomagnetically quiet and disturbed periods.

By using a general circulation model, excitation mechanism of intraseasonal oscillation of the zonal mean zonal wind in the equatorial mesosphere and lower thermosphere (MLT) is investigated. Out results show that wave-mean flow interaction with ultrafast Kelvin waves and diurnal tides is important for driving the intraseasonal oscillation. Not only the migrating diurnal tide but also the nonmigrating, diurnal tide plays an important role in excitation of the intraseasonal oscillation. Intraseasonal variations of amplitudes of the ultrafast Kelvin waves and the diurnal tides are found at height range from 20 to 90 km heights, indicating dynamical coupling between the MLT region and the lower atmosphere. Intraseasonal variations of amplitudes of these waves and their relations with the tropospheric intraseasonal oscillation are discussed. Copyright 2006 by the American Geophysical Union.

[1] By using a general circulation model, excitation mechanism of intraseasonal oscillation of the zonal mean zonal wind in the equatorial mesosphere and lower thermosphere (MLT) is investigated. Our results show that wave-mean flow interaction with ultrafast Kelvin waves and diurnal tides is important for driving the intraseasonal oscillation. Not only the migrating diurnal tide but also the nonmigrating diurnal tide plays an important role in excitation of the intraseasonal oscillation. Intraseasonal variations of amplitudes of the ultrafast Kelvin waves and the diurnal tides are found at height range from 20 to 90 km heights, indicating dynamical coupling between the MLT region and the lower atmosphere. Intraseasonal variations of amplitudes of these waves and their relations with the tropospheric intraseasonal oscillation are discussed.

A high-sensitive technique to detect O(S-1) atoms using vacuum ultraviolet laser-induced fluorescence (VUV-LIF) spectroscopy has been applied to study the O(S-1) production process from the UV photodissociation of O-3, N2O, and H2O2. The quantum yields for O(S-1) formation from O-3 photolysis at 215 and 220 nm are determined to be (1.4 +/- 0.4) x 10(-4) and (5 +/- 3) x 10(-5), respectively. Based on thermochemical considerations, the O(S-1) formation from O-3 photolysis at 215 and 220 nm is attributed to a spin-forbidden process of O(S-1) + O-2(X-3 Sigma(-)(g)). Analysis of the Doppler profile of O(S-1) produced from O-3 photolysis at 193 nm also indicates that the O(S-1) atoms are produced from the spin-forbidden process. In the photolysis of N2O and H2O2 at 193 nm, no discernible signal of O(S-1) atoms has been detected. The upper limit values of the quantum yields for O(S-1) production from N2O and H2O2 photolysis at 193 nm are estimated to be 8 x 10(-5) and 3 x 10(-5), respectively. Using the experimental results, the impact of the O(l S) formation from O-3 photolysis on the atmospheric OH radical formation through the reaction of O(S-1) + H2O has been estimated. The calculated results show that the contribution of the O(S-1) + H2O reaction to the OH production rate is similar to 2% of that of the O(D-1) + H2O reaction at 30 km altitude in mid-latitude. Implications of the present laboratory experimental results for the terrestrial airglow of O(S-1) at 557.7 nm have also been discussed.

We investigate the vertical and latitudinal structure of the migrating diurnal tide in a low dust condition ( dust optical depth of 0.3 at 0.67 mu m) in the Martian atmosphere by using a general circulation model ( GCM) and a linear response model ( LRM). The migrating diurnal tide simulated in our Mars GCM well represents general characteristics of the migrating diurnal tide which have been reported in previous observational and GCM studies. The GCM simulation shows that the vertical wavelength of the migrating diurnal tide in the low latitude region at equinox is similar to 45 km which is larger than that of the major propagating mode predicted from the classical tidal theory ( similar to 25 - 35 km). The Hough function decomposition and the numerical experiments using the LRM reveal that the large vertical wavelength of the migrating diurnal tide is caused by the effects of the zonal mean vorticity (zeta) over bar. It is suggested that the vertical wavelength of the migrating diurnal tide increases through the changes of the effects of the planetary rotation in the presence of non-zero zonal mean vorticity (zeta) over bar . Such a strong dependence of the vertical wavelength of the migrating diurnal tide on (zeta) over bar is not observed in the Earth's atmosphere. The Martian radius, about the half of the Earth's radius, would be one of the important factors to cause more effective (zeta) over bar in the Martian atmosphere than that in the Earth's atmosphere.

The ion and neutral temperatures in the auroral region have been compared with those in the polar cap based upon simultaneous measurements with the EISCAT-UHF radar at Tromso and the EISCAT Svalbard radar (ESR) at Longyearbyen from March 8 to 12 in 1999. The E region ion temperature at Tromso does not show any characteristic difference between dayside and nightside except for a tendency to increase slightly during the nights. At Longyearbyen, however, the E region ion temperature shows the opposite sense of the day-night asymmetry, i.e., being higher in the dayside than in the nightside. The day-night asymmetry of the neutral temperature estimated from the measured ion temperature has the same feature as the day-night asymmetry of the ion temperature. The dayside E region neutral temperature at Longyearbyen is higher than that at Tromso, particularly at and above 110 km. The difference between them increases with increasing altitude. The E region neutral temperatures derived from the two EISCAT radars have been compared with the temperature predicted by the mass spectrometer/ incoherent scatter (MSISE-90) model. The dayside neutral temperature at Tromso is consistent to the MSIS temperature. In contrast, the dayside E region neutral temperature at Longyearbyen is significantly higher than the MSISE-90 model temperature. Energetics which account for the distributions of the ion and neutral temperatures are also discussed.

[1] The ion and neutral temperatures in the auroral region have been compared with those in the polar cap based upon simultaneous measurements with the EISCAT-UHF radar at Tromso and the EISCAT Svalbard radar (ESR) at Longyearbyen from March 8 to 12 in 1999. The E region ion temperature at Tromsø does not show any characteristic difference between dayside and nightside except for a tendency to increase slightly during the nights. At Longyearbyen, however, the E region ion temperature shows the opposite sense of the day-night asymmetry, i.e., being higher in the dayside than in the nightside. The day-night asymmetry of the neutral temperature estimated from the measured ion temperature has the same feature as the day-night asymmetry of the ion temperature. The dayside E region neutral temperature at Longyearbyen is higher than that at Tromsø, particularly at and above 110 km. The difference between them increases with increasing altitude. The E region neutral temperatures derived from the two EISCAT radars have been compared with the temperature predicted by the mass spectrometer/incoherent scatter (MSISE-90) model. The dayside neutral temperature at Tromso is consistent to the MSIS temperature. In contrast, the dayside E region neutral temperature at Longyearbyen is significantly higher than the MSISE-90 model temperature. Energetics which account for the distributions of the ion and neutral temperatures are also discussed. Copyright 2005 by the American Geophysical Union.

Turbulent and electromagnetic energy dissipation rates in the altitude range of similar to98-116 km have been estimated using data obtained from the European Incoherent Scatter Svalbard radar (ESR) observations on 22 September 1998 and 12 March 1999. Solar and geomagnetic activities were moderate and quiet during the observational periods, respectively. The horizontal neutral wind fields derived from the ESR observations show strong vertical shears and temporal variations. The Richardson numbers for the wind fields sometimes show values less than 0.25. The turbulent energy dissipation rates estimated from the wind shears have wavy structures depending on the shear distributions and decrease with height rapidly from similar to110 km. The turbulent energy dissipation rates also have maxima, which sometimes exceed 1 W/kg, in the region similar to102-108 km. The electromagnetic energy dissipation rates are strongly controlled by the neutral wind in the region below similar to110 km, while the Pedersen conductivity affects the energy dissipation above similar to110 km. On average, the two energy dissipation rates change their relative importance at similar to109 km on 22 September 1998 and at similar to112 km on 12 March 1999, depending on the wind shear, the electric field, and the Pedersen conductivity. When the electromagnetic energy fluxes are small, the contribution of the neutral wind to the electromagnetic and turbulent energy dissipation becomes large and the region, where turbulent energy dissipation is dominant, also expands upward. The magnitude and the dissipation height of the energy poured from the upper and lower regions seem to be strongly dependent on the neutral wind particularly below similar to110 km. The variations of the two energy dissipation rates may have significant effects on energetics in the polar lower thermosphere.

Turbulent and electromagnetic energy dissipation rates in the altitude range of ∼98-116 km have been estimated using data obtained from the European Incoherent Scatter Svalbard radar (ESR) observations on 22 September 1998 and 12 March 1999. Solar and geomagnetic activities were moderate and quiet during the observational periods, respectively. The horizontal neutral wind fields derived from the ESR observations show strong vertical shears and temporal variations. The Richardson numbers for the wind fields sometimes show values less than 0.25. The turbulent energy dissipation rates estimated from the wind shears have wavy structures depending on the shear distributions and decrease with height rapidly from ∼110 km. The turbulent energy dissipation rates also have maxima, which sometimes exceed 1 W/kg, in the region ∼102-108 km. The electromagnetic energy dissipation rates are strongly controlled by the neutral wind in the region below ∼110 km, while the Pedersen conductivity affects the energy dissipation above ∼110 km. On average, the two energy dissipation rates change their relative importance at ∼109 km on 22 September 1998 and at ∼112 km on 12 March 1999, depending on the wind shear, the electric field, and the Pedersen conductivity. When the electromagnetic energy fluxes are small, the contribution of the neutral wind to the electromagnetic and turbulent energy dissipation becomes large and the region, where turbulent energy dissipation is dominant, also expands upward. The magnitude and the dissipation height of the energy poured from the upper and lower regions seem to be strongly dependent on the neutral wind particularly below ∼110 km. The variations of the two energy dissipation rates may have significant effects on energetics in the polar lower thermosphere. Copyright 2004 by the American Geophysical Union.

By using a general circulation model, we examine behavior of the migrating semidiurnal tide for equinox during solar cycle minimum and geomagnetically quiet conditions. We investigate day-to-day variations of the migrating semidiurnal tide in the mesosphere and thermosphere, and their relation with the migrating semidiurnal tide generated in the lower atmosphere. The results show that day-to-day variations of the migrating semidiurnal tide are evident from the upper troposphere to the thermosphere. Fluctuations of the migrating semidiurnal tide amplitude with periods of 17-18 and 25 days are found at altitudes from 20 to 200 km height, indicating dynamical coupling between the mesosphere and thermosphere and the lower atmosphere.

[1] A new general circulation model (GCM) which contains the region from the ground surface to the exobase has been developed and used to investigate behavior of the migrating diurnal tide for equinox during solar cycle minimum and geomagnetically quiet conditions. In particular, we study day-to-day variations of migrating diurnal tides in the mesosphere and thermosphere and their relation with migrating diurnal tides generated in the lower atmosphere. The results show that day-to-day variations of the migrating diurnal tide are evident from the upper troposphere to the thermosphere. Fluctuations of the migrating diurnal amplitude with periods of 12 and 25 days are found in the height range from 20 to 300 km, indicating dynamical coupling between the mesosphere and thermosphere and the lower atmosphere.

[1] A Martian atmospheric general circulation model is developed to investigate the effect of topographic elevation in the meridional circulation of the Martian atmosphere. It is confirmed that, even at equinoxes, the meridional circulation below similar to20 km altitude has an asymmetric pattern with respect to the equator. Sensitivity experiments reveal that the topographic elevation difference between the northern and southern hemispheres is the most dominant factor for producing such an asymmetric circulation. Contributions from variations of surface thermal inertia and surface albedo are weak compared with that of surface elevation. Thermal budget analyses show that the mean meridional circulation below similar to20 km altitude is driven by convective heating whose magnitude is controlled by the potential temperature of the surface mixed layer. Since the Martian atmosphere is optically and thermally thin, the potential temperature of the surface mixed layer is directly influenced by the geometric height of the surface. The elevated southern hemisphere induces the upward motion of the Martian Hadley circulation to be located in the southern hemisphere even at equinoxes when the surface temperature distribution is symmetric with respect to the equator. At southern summer solstice, when the potential temperature of the southern surface mixed layer is highest, the convective activity there becomes most active, and thus the Martian Hadley circulation becomes most intensive. This seasonal intensification of the convective activity and the Hadley circulation may account for the frequent occurrence of dust loading in the summer southern hemisphere.

Transient production of F-region plasma irregularities due to traveling convection vortices (TCVs) was investigated using the Super Dual Auroral Radar Network (SuperDARN) combined with ground magnetometer networks and the POLAR ultraviolet imager. We selected two large-amplitude (100-200 nT) TCV events that occured on 22 May 1996 and 24 July 1996. It is found that the TCV-associated HF backscatter arises in blobs with spatial scale of a few hundreds km. They traveled following tailward bulk motion of the TCV across the three fields-of-view of the SuperDARN HF radars in the prenoon sector. The spectra in the blobs showed unidirectional Doppler velocities of typically 400-600 m/s, with flow directions away from the radar. These unidirectional velocities correspond to the poleward and/or eastward convective flow near the leading edge of upward field-aligned current. The backscatter blobs overlapped the poleward and westward part of the TCV-related transient aurora. It is likely that the transient backscatter blobs are produced by the three-dimensional gradient drift instabilities in the three-dimensional current system of the TCV. In this case, nonlinear rapid evolution of irregularities would occur in the upward field-aligned current region. The spectral width of the backscatter blob is typically distributed between 50 and 300 m/s, but sometimes it is over 400 m/s. This suggests that the temporal broad spectra over 400 m/s are produced by Pc1-2 bursts, while the background spectral width of 50-300 m/s are produced by the velocity gradient structure of convection vortices themselves.

Simultaneous Common Program Two experiments by the EISCAT UHF radar at Tromso and the EISCAT Svalbard radar at Longyearbyen from 00:00 to 15:00 UT on 22 September 1998 and 9 March 1999 have been utilized to investigate distributions of the ion and neutral temperatures in the E-region between 105 and 115 km. During the experiments, soft particle precipitations in the dayside cusp were observed over the Svalbard radar site by the Defense Meteorological Satellite Program (DMSP) F11 satellite. It is found that the dayside electric field in the regions of the low-latitude boundary of the polar cap and the cusp was greater and more variable than that in the auroral region. The ion temperature, parallel to the geomagnetic field at Longyearbyen, was higher than that at Tromso during the daytime from 06:00 to 12:00UT. The steady-state ion energy equation has been applied to derive neutral temperature under the assumption of no significant heat transport and viscous heating. The estimated neutral temperature at Longyearbyen was also higher than that at Tromso. The ion and neutral energy budget was discussed in terms of the ion frictional heating and the Joule heating. The results indicate two possibilities: either the neutral temperature was high in the low latitude boundary of the polar cap and the cusp, or the heat transport by the polar cap neutral winds toward the dayside sector was significant.

An intent of this paper is to study dayside quiet time E region neutral momentum balance in the auroral ionosphere. Neutral wind vectors were obtained by using the European Incoherent Scatter (EISCAT) Common Program 2 (CP-2) data between 98 and 119 km height from 1100 to 1300 UT (from 1200 to 1400 LT and from 1400 to 1600 MLT) on June 15 and 16, 1993. Individual forcing terms in the neutral momentum equation were determined from the derived neutral winds with the MSISE-90 model. In order to calculate the velocity change with time including advection, designated as the time-dependent term, and the viscous force in the neutral momentum equation, a polynomial fitting procedure was applied to perform the vertical and time differentiation of;ht: derived neutral winds. Below 106 km height, the time-dependent term was greater than the pressure gradient force except for the meridional component at 98 km height on June 15. Between 110 and 116 km height, the pressure gradient force was in the same order of magnitude as that of the Coriolis force and the time-dependent term was smaller than these two. The neutral wind flow was nearly perpendicular to the pressure gradient force. At 119 km height, the viscous drag was in the same order of magnitude as those of the pressure gradient and Coriolis forces and greater than the Pedersen and Hall drags. The magnitude of ion drag was similar to 7% of the Coriolis force at this height level.

Two numerical simulations of the thermospheric response to magnetospheric energy injection have been performed using a zonally averaged, time-dependent model of neutral composition, dynamics, and energy budget. The simulations are distinguished by the duration of the source. The first simulation has an energy injection of 1 hour, representative of substorm type forcing, and the second one has a 12-hour energy injection, representative of main storm type forcing. They were performed under the condition of equinox at solar minimum, In the first simulation, large-scale atmospheric gravity waves (AGWs), generated by the substorm energy via Joule heating of ionospheric currents, are clearly identified in the wind-field in a meridional plane as well as in the temporal and spacial variations of the total energy density of air above about 130 km height. These waves reach the equator after about 3 hours and propagate into the opposite hemisphere, The horizontal propagation speed is close to the speed of sound (for example, roughly 440 m/s at about 150 km altitude and 670 m/s at about 260 km altitude). Snapshots of the wind system affected by the substorm energy injection show a ''four-cell'' pattern between the poles. Above 260 km the cells have the opposite rotational direction to those below. These small-scale features in the wind system are indicative of the internal atmospheric gravity waves with the vertical phase propagation. From a term analysis of the energy conservation equation, it is identified that the dominant energy process associated with the propagation of AGWs is adiabatic compressional heating and/or expansive cooling process. If call be concluded that the energy oscillations at middle and low latitude are mainly produced by AGWs propagating from high latitude during the substorm. The second simulation indicated that horizontal and vertical advections. vertical heat conduction, and infrared radiative cooling by nitric oxide are important in addition to adiabatic compressional heating and/or expansive cooling. It is suggested that short-duration energy injection preferentially generates AGWs which dominate the energy oscillations at low latitudes through adiabatic heating and cooling. Long-duration energy injection is more effective in generating a meridional circulation which transfers energy by both advective and adiabatic processes.