S. Fukao

A climatology of F region gravity wave propagation over the middle and upper atmosphere radar

By observing the ionospheric F region simultaneously in multiple beams with the middle and upper atmosphere radar, we have been able to track the passage of gravity waves and measure their propagation characteristics. Here we develop a climatology of wave propagation based on the observation of 58 daytime experiments conducted during 1986–1994. The thermosphere seems to be continuously swept by waves detectable by an incoherent scatter radar. These waves generally come for hours on end from a consistent or slowly varying direction, which can be any direction on a given day. Statistically, the waves show a moderate preference for southward travel, with this preference being reduced or shifted to southeastward travel during disturbed times. On average, the horizontal phase trace speed remains near 240 m/s for all periods inspected (40–130 min). This speed matches the behavior expected for lossless waves with 150–200 km vertical wavelength. We find small variability in this relation for different times of day, seasons, solar and magnetic conditions, and directions of wave travel, though waves on disturbed days seem to travel moderately faster on solar minimum mornings.

Equinoctial asymmetries in the ionosphere and thermosphere observed by the MU radar

Annual variations of the ionosphere and thermosphere studied with the middle and upper atmosphere (MU) radar during the solar maximum period 1988–1992 show that the well-known seasonal anomaly in the electron density Ne exists only during daytime and at altitudes near the ionospheric peak and below. The observations also reveal the existence of equinoctial asymmetries in the ionosphere and thermosphere, with the asymmetry in the ionosphere changing its character with altitude during daytime. In the bottomside ionosphere the electron density Ne is slightly greater in September equinox than in March equinox. At higher altitudes the asymmetry reverses and becomes strong; the values of Ne in March equinox exceed those in September equinox by up to 100%. The electron temperature Te exhibits equinoctial asymmetries almost opposite those in Ne. The ion temperature Ti shows a weak asymmetry, in phase with the asymmetry in Ne. The field-aligned and field-perpendicular plasma velocities V‖ and V⊥ are also different in the two equinoxes. In the thermosphere the neutral wind and composition show consistent equinoctial asymmetries. The meridional component of the daytime poleward wind velocity (Uθ) derived from the field-parallel plasma velocity is weaker in March equinox than in September equinox by up to 20 m s−1, and the values of the daytime [O]/[N2] ratio obtained from MSIS-86 are larger in September equinox than in March equinox by about 20%. Model calculations carried out by incorporating the measured V⊥ and Uθ into the Sheffield University plasmasphere-ionosphere model that uses MSIS-86 for neutral atmosphere show that the equinoctial asymmetries in the ionosphere arise mainly from the corresponding asymmetries in the thermosphere, with major contributions from neutral winds and minor contributions from composition.

A frequency domain radar interferometric imaging (FII) technique based on high-resolution methods

Abstract In the present work, we propose a frequency-domain interferometric imaging (FII) technique for a better knowledge of the vertical distribution of the atmospheric scatterers detected by MST radars. This is an extension of the dual frequency-domain interferometry (FDI) technique to multiple frequencies. Its objective is to reduce the ambiguity (resulting from the use of only two adjacent frequencies), inherent with the FDI technique. Different methods, commonly used in antenna array processing, are first described within the context of application to the FII technique. These methods are the Fourier-based imaging, the Capons and the singular value decomposition method used with the MUSIC algorithm. Some preliminary simulations and tests performed on data collected with the middle and upper atmosphere (MU) radar (Shigaraki, Japan) are also presented. This work is a first step in the developments of the FII technique which seems to be very promising.

A climatology of middle and upper atmosphere radar observations of thermospheric winds

Shigaraki middle and upper atmosphere (MU) radar observations of horizontal thermospheric winds in the magnetic meridian plane over the period September 1986 to September 1996 are reported as climatological averages in the form of time-of-day variations for several combinations of seasonal and solar activity conditions and are compared with winds predicted by the horizontal wind model (HWM) and with winds measured at Saint Santin and Millstone Hill. The dominant feature of the MU wind behavior is its mean diurnal variation of northward flow by day and southward flow by night, with the nighttime wind smoothly approaching and receding from a midnight maximum, while the daytime wind tends to show two peaks, a strong one in the early daylight hours and a weak one in the afternoon-evening. HWM shows the same unimodal nighttime and bimodal daytime behavior, but the HWM pattern is shifted about 2 hours later in time. The amplitude of the diurnal harmonic decreases from 78 m/s at solar minimum to 45 m/s at solar maximum, while HWM shows a corresponding increase from 53 to 62 m/s. The diurnal amplitude is remarkably stable with season but is superposed on a steady wind of 41 m/s southward in summer, 15 m/s northward in winter, and midway between these limits at the equinoxes. HWM shows a symmetric pattern of 30 m/s southward in summer and 30 m/s northward in winter. Ion drag appears to be the main regulator of wind speed, and the seasonal wind patterns have a profound effect on the seasonal behavior of the ionosphere.

The SEEK Chemical Release Experiment: Observed neutral wind profile in a region of sporadic E

Wind measurements made with the chemical release technique during a sporadic E layer event are presented. The data were obtained as part of the Sporadic E Experiment over Kyushu (SEEK) sounding rocket campaign. The winds show a strong maximum in the lower E region approaching 150 m s−1 near 105-km altitude and a large shear below the maximum. The large shear was within a few kilometers altitude of the peak in the electron densities measured on the downleg of the rocket trajectory. Calculations of the Richardson numbers for the wind profile show that the altitude range near the layer was highly unstable.

Traveling ionospheric disturbances observed in the OI 630‐nm nightglow images over Japan by using a Multipoint Imager Network during the FRONT Campaign

Pilot observations using a network of five all-sky imagers (ASIs) were conducted during the new moon period of May 19–22, 1998 as part of the F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign. The network observation enabled us to track propagation of medium-scale traveling ionospheric disturbances (TIDs) in the OI 630-nm nightglow over a distance of more than 2500 km. The TIDs were observed every night during the campaign period, but occurrence was limited from evening to midnight. They have horizontal wavelengths of 200–600 km, travel a horizontal distance of more than 1000 km, and last for more than three hours. In every case, the TIDs moved southwestward with a velocity of 83–137 m/s. Using dual-site TID images, the altitude of the TID structures in the 630-nm nightglow was calculated to be ∼260 km, which corresponds to the bottom side of the mid-latitude ionospheric F layer.

Observations of a Reversal in Long-Term Average Vertical Velocities near the Jet Stream Wind Maximum

Abstract Analysis of vertical velocity measurements made for four days each month over the period from 1986 to 1988 by the MU radar in Japan shows a reversal in direction near the peak in the zonal wind profile during the winter months. More specifically, the reversal is noted during periods when the peak horizontal wind speeds 60 m s−1. The vertical velocities associated with the circulation have magnitudes of 10-20 cm s−1, and the depth of the circulation is of the order of several kilometers. In 6 out of 14 cases when the feature was observed, the direction of the vertical circulation, although not the magnitude, could be explained by adiabatic ascent or subsidence along the average potential temperature surface slopes for the observation intervals. The direction of the circulation was such that it would tend to produce cooling and heating for the ascent and subsidence, respectively, that would tend to strengthen or at least maintain the jet. In the remaining eight cases, the direction of the vertical ...

Gravity wave generation in the lower stratosphere due to passage of the typhoon 9426 (Orchid) observed by the MU radar at Shigaraki (34.85°N, 136.10°E)

[1] Intense gravity wave activities were investigated in the lower stratosphere during the typhoon 9426. Strong vertical winds were observed just a few hours before the arrival of the typhoon-center at the MU radar site. About 30 min to 1 hour after the typhooncenter had passed, a considerable reduction in vertical wind amplitude was detected. Dominant gravity waves showed time period in the range of 7–8 min, 15 min, and 40– 60 min in the upper troposphere and lower stratosphere. In the vicinity of the central region of the typhoon, a gravity wave was observed, which was monochromatic in nature with a vertical wavelength � 3 km between 1.5 km and 23 km height. In the lower stratosphere, the horizontal wavelength for the prominent period was detected in the range of 10–15 km (for 15 min wave period) and 25–50 km (for 40–60 min wave period). The vertical wavelength of these waves was examined from 2.5 km to 4.0 km. In the horizontal direction, the intrinsic group velocity was estimated between 9 ± 2 and 11 ± 2 m/s. Near the tropopause, the average direction of group velocity was assessed at about 20� ±3 � from the horizontal. The generation of gravity wave like features, in the lower stratosphere, is believed induced by convection, as the low temperature of the clouds indicates a deep penetration over the radar region as seen in the satellite GMS images. INDEX TERMS: 3314 Meteorology and Atmospheric Dynamics: Convective processes; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; 3384 Meteorology and Atmospheric Dynamics: Waves and tides; 6952 Radio Science: Radar atmospheric physics; KEYWORDS: convective updrafts, gravity waves, troposphere, lower stratosphere

Longitudinal variations of the topside ionosphere at low latitudes: Satellite measurements and mathematical modelings

The longitudinal variations of the topside ionosphere at low latitudes observed by the Hinotori satellite during equinoctial periods at high solar activity are studied using the Sheffield University plasmasphere-ionosphere model (SUPIM). The model values show that both the neutral wind and E × B drift velocities make important contributions to the observed longitudinal variations in the topside ionosphere. The displacement of the geographic and magnetic equators and the magnetic declination angle, which give rise to conjugate-hemisphere differences in the neutral wind in the magnetic meridian, are the principal causes of the observed north-south asymmetries in the electron density about the magnetic equator. A comparison of the modeled and observed electron densities shows that the modeled longitudinal variations are, in general, in qualitative agreement with the observations when the neutral winds are taken from the HWM90 thermospheric wind model. Improved agreement in the magnitudes is achieved if HWM90 is modified so that at low latitudes (1) the eastward component of the zonal wind is increased during the day and decreased at night and (2) the diurnal variations of the meridional wind in the northern hemisphere at eastern longitudes and the equatorward wind at around midnight at western longitudes are reduced. The model reproduces the observed longitudinal variations in the development of the equatorial peak electron density during the day and in the equatorial trough and associated crests during the afternoon and postsunset periods. The trough and crests are most prominent at around 2000 LT, where the crest-to-trough ratio varies from about 1.15 at eastern longitudes to about 2.0 at western longitudes. Model calculations show that the longitudinal differences of these features can arise from a longitudinal variation in the vertical E × B drift velocity.

Plasma temperature variations in the ionosphere over the middle and upper atmosphere radar

The temperature variations in the ionosphere over the middle and upper atmosphere radar at Shigaraki (34.85°N, 136.10°E, magnetic latitude 25°N) in Japan are studied using the electron and ion temperature (Te and Ti, respectively) data measured by the radar during nearly a full solar cycle (1986–1995). A comprehensive picture of the diurnal, seasonal, and solar activity variations of Te and Ti is presented for the altitude range 200–550 km. The temperatures Te and Ti are found to have similar diurnal and altitude variations and different seasonal and solar activity dependence. With season, while daytime Te is highest in summer and lowest in equinox, daytime Ti is highest in equinox and lowest in summer. With solar activity, while daytime Te decreases, the corresponding Ti increases. The diurnal variation of Te is characterized by morning and evening peaks. The occurrence and strength of these peaks are found to depend on altitude, season, and solar activity. The peaks arise basically from the photoelectron heating of the morning and evening electron gas. However, neutral winds play a dominant role in the appearance of the peaks. A poleward wind, which reduces the electron density to a low value before sunset, is an essential requirement, especially for the evening peak. The mechanisms causing the morning and evening peaks in Te are illustrated through model calculations using the Sheffield University plasmasphere-ionosphere model.