T. Tsuda

Characteristics of gravity waves in the mesosphere observed with the middle and upper atmosphere radar: 1. Momentum flux

We observed wind motions from 60- to 90-km altitudes with the MU radar (35°N, 136°E) in the four observation periods: October 1986, June 1987, July 1990, and January/February 1991. Mean wind profiles were fairly consistent with results of Kyoto meteor radar observations (35°N, 136°E) collected in 1983–1985 at 80- to 110-km altitudes, and zonal mean winds generally agreed well with CIRA 1986 except for the profiles below 70 km in January/February 1991, although discrepancy of amplitudes sometimes ranged up to 10 to 20 m/s. Characteristics of frequency spectra of radial wind velocities were basically similar among results determined in four observation periods. That is, oblique spectra had a logarithmic slope of about −5/3 in the frequency range lower than 4–5 × 10−4 s−1. Gravity waves with periods longer than 20–50 min to 5 hours (the lowest limit of the spectral analysis) were found to carry a large part of the zonal momentum flux, while a dominant frequency component was not detected for the meridional flux. Zonal and meridional drag of mean winds induced by the gravity wave with periods from 5 min to 8–10 hours were +51 m/s/d and −4 m/s/d in June at 75 km and −4.0 m/s/d and +7.4 m/s/d in October at 70 km, respectively. The major part of the drag force was also found in the wave component with periods larger than 30 min. The day-to-day variation in the zonal momentum flux showed a good correlation with the vertical shear of the zonal mean wind, which suggests effects of the gravity wave activities on mean wind fields, or the effect of shear variation on the gravity wave activity.

Characteristics of gravity waves in the mesosphere observed with the middle and upper atmosphere radar: 2. Propagation direction

We studied the characteristics of dominant component of gravity waves (λz ∼ 10 km) in the vertical profile of the wind velocities observed in the mesosphere (at around 70–75 km) with the middle and upper atmosphere (MU) radar in the four observation campaigns (June 1987, July 1990, October 1986, and January/February 1991) and the monthly mesosphere observations carried out 4–5 days a month in December 1985 to December 1988. By assuming linear dispersion relations of gravity waves, both vertical and horizontal propagation directions of gravity waves were determined. Most of the dominant waves propagated upward, except that they sometimes propagated downward mainly in summer. A typical vertical wavelength and intrinsic period of the dominant gravity waves were 4–15 km and 4–15 hours, respectively. The mean values of the vertical wavelength, horizontal wavelength, and intrinsic periods were 8.2 km, 1200 km, and 10.2 hours, respectively, which did not show significant seasonal variations. Most of the waves were found to be dissipated by the eddy diffusion of 15–100 m2/s, which does not agree with MF radar observations (10- to 120-min observed periods) but is rather consistent with sodium lidar observations (36–300 m2/s). The mean horizontal phase velocity was 33 m/s. The horizontal propagation direction was generally eastward throughout a year except for the westward propagation in early winter (November to December). This suggests that the dominant gravity waves are less sensitive to a change in the mean zonal wind direction in the middle atmosphere than the short-period gravity waves (2 hours to 5 min). Momentum flux carried by the dominant gravity waves was estimated to be 0.5–1 m2/s2, which was generally smaller than the momentum flux of gravity waves with periods ranging from 8 hours to 5 min. The amplitude of the momentum flux of the dominant gravity waves had an autumn maximum, while the momentum flux with periods of 8 hours to 5 min had equinoctial minima. Thus in September and October the momentum flux due to the dominant gravity waves became significant. We further pointed out that the possibility that / depends on the wave periods, and therefore the major frequency component of is not easily determined from simple dispersion relation only.

Height‐dependent ionospheric variations in the vicinity of nightside poleward expanding aurora after substorm onset

High-latitude ionospheric variations at times near auroral substorms exhibit large temporal variations in both vertical and horizontal extents. Statistical analysis was made of data from the European Incoherent Scatter UHF radar at Tromso, Norway, and International Monitor for Auroral Geomagnetic Effects magnetometer for finding common features in electron density, ion and electron temperatures and relating these to currents and associated heating. This paper particularly focused on the height dependencies. Results show clear evidences of large electric field with corresponding frictional heating and Pedersen currents located just outside the front of the poleward expanding aurora, which typically appeared at the eastside of westward traveling surge. At the beginning of the substorm recovery phase, the ionospheric density had a large peak in the E region and a smaller peak in the F region. This structure was named as C form in this paper based on its shape in the altitude-time plot. The lower altitude density maximum is associated with hard auroral electron precipitation probably during pulsating aurora. We attribute the upper F region density maximum to local ionization by lower energy particle precipitation and/or long-lived plasma that is convected horizontally into the overhead measurement volume from the dayside hemisphere.

Statistical analysis of gravity waves observed with the middle and upper atmosphere radar in the middle atmosphere: 2. Waves propagated in different directions

The statistical method developed in the first part of the paper is applied to obtain parameters of internal gravity wave (IGW) harmonics with periods from 5 min to 6 hours propagating upward and downward, and in the direction of the mean wind and opposite to it, from the observations of motions in the middle atmosphere with the middle and upper atmosphere radar. Usually, wave amplitudes and modulus of momentum fluxes are larger for IGWs having upward energy fluxes and propagated opposite to the mean wind (eastward propagation in summer and westward propagation in winter). Most of IGWs have momentum fluxes directed to the northeast in winter and to the east in summer.

Variations of the neutral temperature and sodium density between 80 and 107 km above Tromsø during the winter of 2010–2011 by a new solid‐state sodium lidar

[1]xa0A new solid-state sodium lidar installed at Ramfjordmoen, Tromso (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.

Lower thermospheric wind variations in auroral patches during the substorm recovery phase

Measurements of the lower thermospheric wind with a Fabry-Perot interferometer (FPI) at Tromso, Norway, found the largest wind variations in a night during the appearance of auroral patches at the substorm recovery phase. Taking into account magnetospheric substorm evolution of plasma energy accumulation and release, the largest wind amplitude at the recovery phase is a fascinating result. The results are the first detailed investigation of the magnetosphere-ionosphere-thermosphere coupled system at the substorm recovery phase using comprehensive data sets of solar wind, geomagnetic field, auroral pattern, and FPI-derived wind. This study used three events in November 2010 and January 2012, particularly focusing on the wind signatures associated with the auroral morphology, and found three specific features: (1) wind fluctuations that were isolated at the edge and/or in the darker area of an auroral patch with the largest vertical amplitude up to about 20u2009m/s and with the longest oscillation period about 10u2009min, (2) when the convection electric field was smaller than 15u2009mV/m, and (3) wind fluctuations that were accompanied by pulsating aurora. This approach suggests that the energy dissipation to produce the wind fluctuations is localized in the auroral pattern. Effects of the altitudinal variation in the volume emission rate were investigated to evaluate the instrumental artifact due to vertical wind shear. The small electric field values suggest weak contributions of the Joule heating and Lorentz force processes in wind fluctuations. Other unknown mechanisms may play a principal role at the recovery phase.

Height and time characteristics of seasonal and diurnal variations in PMWE based on 1 year observations by the PANSY radar (69.0°S, 39.6°E)

We report height and time variations in polar mesosphere winter echoes (PMWE) based on the Program of the Antarctic Syowa mesosphere-stratosphere-troposphere/incoherent scatter (PANSY) radar observations. PMWE were identified for 110 days from March to September 2013. PMWE occurrence frequency increased abruptly in May when two solar proton events occurred. PMWE were also observed even during periods without any solar proton events, suggesting that a possible cause of the PMWE is ionization by energetic electron precipitations. The monthly mean PMWE characteristics showed that occurrence of PMWE were mainly restricted to sunlit time. This fact indicates that electrons detached from negatively charged particles play an important role. While PMWE below 72u2009km in altitude completely disappeared before sunset, it was detected above that altitude for a few hours even after sunset. This height dependence in the altitude range of 60–80u2009km can be explained qualitatively by empirical effective recombination rates.

A sporadic sodium layer event detected with five‐directional lidar and simultaneous wind, electron density, and electric field observation at Tromsø, Norway

A sporadic sodium layer (SSL) was detected with five-directional lidar observation on 15 December 2012 at Tromso, Norway. We have derived the horizontal velocity of the SSL front from the SSL onset times at the five positions and compared it with the background wind velocity from the collocated meteor radar and European Incoherent Scatter (EISCAT) radar. As a result, both velocities were fairly consistent. The increase rate in the height-integrated sodium density around the SSL onset was 3–6xa0×1010xa0m−2xa0s−1, which was comparable to relatively large cases in the previous studies. However, the EISCAT-observed electric field was too small to induce such a rapid sodium atom production. In addition, the amounts of the sodium atom increases at the five positions were mostly same. Thus, there were no clear signatures for the sodium atom production. These results strongly indicate that the observed SSL was just advected by the background wind.

Depletion of mesospheric sodium during extended period of pulsating aurora

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°N, 19.2°E) during a 2xa0h 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 100xa0km 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 100xa0km. These Na density depletions lasted for 8xa0min 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 O2+ while changing the R value which is defined as the ratio of NO+ to O2+ densities, from 1 to 10. The correlation coefficients between observed and modeled Na density depletion calculated with typical value R = 3 for time intervals T1, T2, and T3 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.

Statistical investigation of Na layer response to geomagnetic activity using resonance scattering measurements by Odin/OSIRIS

We have performed a statistical investigation of the global response of the Na layer to geomagnetic activity using Na density data from 2004 to 2010 obtained using the Optical Spectrograph and Infrared Imager System (OSIRIS) on board the Odin satellite. In the analysis, we categorized the Na density data according to the auroral electrojet (AE) index, and then compared the resulting datasets. Regarding the results, we found a significant decrease in the Na density above a height of ∼95 km in both the southern and northern polar regions with an increase in the AE index. The cause of the decrease in the Na density is discussed, and we conclude that the decrease in the Na density was mainly due to the effect of energetic particle precipitation.