Shoichiro Fukao

Mid-latitude E region field-aligned irregularities observed with the MU radar

Fine structures E region field-aligned irregularities were observed on June 24–25, 1989, with the MU radar at Shigaraki, Japan (34.9°N, 136.1°E; geomagnetic latitude 25.0°N). The 3.2-m scale irregularities were observed with the MU radar in five main beam directions, each of which was nearly perpendicular to the geomagnetic field at 100 km altitude. Doppler spectra were obtained every 20 s with a range resolution of 600 m. Field-perpendicular echoes appeared from 2130 to 2330 LT and from 0400 to 1100 LT, times that correspond to postsunset and postsunrise period in the E region. A preliminary examination of the Doppler spectra indicates spectral widths of 50–120 m s−1 and the mean Doppler velocities are well below the ion acoustic speed. These spectral characteristics are consistent with those obtained in the equatorial and auroral electrojets, and have been attributed to the gradient drift instability. The echoes observed during the postsunset and postsunrise periods showed quite different morphologies in the time-height distribution. For this reason, they are classified into two types, ‘continuous’ and ‘quasi-periodic.’ The appearance of the ‘continuous’ echoes was mainly continuous in time and situated between 90 and 100 km altitude during the postsunrise period. The appearance of the ‘quasi-periodic’ echoes was intermittent with periods of 5–10 min and situated above 100 km altitude during the postsunset period. The quasi-periodic echoes showed phase propagation toward the radar, while the averaged mean Doppler velocity was away from the radar. By measuring the time delays in echo regions from five directions, an apparent westward motion (approximately 120 m s−1) of the irregularity regions was estimated.

High resolution mapping of TEC perturbations with the GSI GPS network over Japan

Two-dimensional total electron content (TEC) perturbations over Japan are mapped with the Geographical Survey Institute (GSI) GPS network, GEONET (GPS Earth Observation Network). Its spatial resolution is 0.15° latitude × 0.15° longitude and temporal resolution is 30 seconds. Two-dimensional TEC observations with these high resolutions revealed spatial structures and temporal evolutions of traveling ionospheric disturbances (TIDs) in the nighttime mid-latitude ionosphere on July 03, 1997. A preliminary result of the TEC perturbation mapping indicates that it would be a strong tool to investigate the ionospheric structures in detail. Coordinated observations with other ionospheric observation techniques, such as incoherent scatter (IS) radars, airglow imagers, and ionosondes, are needed to clarify the vertical structure of the ionosphere.

A new technique for mapping of total electron content using GPS network in Japan

The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15° × 0.15° in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.

Turbulent upwelling of the mid‐latitude ionosphere: 1. Observational results by the MU radar

In this paper we present the detailed results of a series of experiments designed to study the coherent backscatter of 50-MHz radar waves from the mid-latitude F region. Data were obtained with the active phased-array MU radar in Japan and include some auxiliary E region coherent echoes as well. As in other turbulent ionospheric phenomena the intense nonthermal scatter comes from irregularities oriented parallel to B. The strongest echoes correspond to irregularities at least 20 dB stronger than thermal backscatter at the same frequency from typical F region densities at the same range. Simultaneous observations with ionosondes show that these echoes occur during strong mid-latitude spread F. As defined by ionosondes, the latter phenomenon is certainly much more widespread than the turbulent upwelling events described here, but we believe that in some sense these correspond to the most violent mid-latitude spread F. The strongest echoes occur in large patches which display away Doppler shifts corresponding to irregularity motion upward and northward from the radar. At the edges of these patches there is often a brief period of toward Doppler before the echoing region ceases. On rare occasions comparable patches of strong away and toward Doppler are detected, although in such cases the Doppler width of the toward echoes is much narrower than that of the away echoes. The away patches often are characterized by mean velocities well over 250 m/s and Doppler widths (full width at half maximum) of 50 m/s. The multiple beam capability at MU allowed us to track the patches in the zonal direction on two days. The patches moved east to west in both cases at velocities of 125 m/s and 185 m/s, respectively. There is a distinct tendency for the bottom contour of the scattering region to be modulated at the same period as the patch occurrence frequency as well as at higher frequencies. This higher-frequency component may correspond to substructures in the large patches and to the E region coherent scatter patches which were detected simultaneously in several multiple beam experiments. In the companion paper (Kelley and Fukao, this issue), we explore a number of possible explanations for this phenomenon in more detail.

Seasonal variability of vertical eddy diffusivity in the middle atmosphere: 1. Three‐year observations by the middle and upper atmosphere radar

The vertical eddy diffusivity K due to atmospheric turbulence with spatial scales of 100–102 m has been computed from the echo power spectral width observed by the middle and upper atmosphere radar for almost every month from January 1986 to December 1988. The method of analysis follows Lilly et al. [1974], Sato and Woodman [1982], and Hocking [1983a, 1985, 1988], and the contamination due to beam broadening, vertical shear, and transience has been removed. Although observations for horizontal wind speeds larger than approximately 40 m/s, such as occur near the tropopause jet stream in winter, have been omitted because of excessive beam broadening, sufficient numbers of observations have been accumulated to produce a reasonable climatology for the upper troposphere and lower stratosphere (6–20 km altitude) and for the mesosphere (60–82 km altitude). The monthly median of K shows a local maximum near the tropopause jet stream altitude. It becomes larger in the mesosphere, increasing gradually with height. Maxima of K are observed in winter near the tropopause and in summer in the mesosphere, and the seasonal variability of K reaches approximately an order of magnitude. A semiannual variability is apparent in the mesosphere with minima in the equinoctial seasons.

MST Radar Observations of a Saturated Gravity Wave Spectrum

Abstract We present vertical wavenumber spectra of mesoscale wind fluctuations using data observed in the troposphere, lower stratosphere and mesosphere by the MU radar at 35°N in Japan in October 1986 and June 1987, as well as lower stratospheric spectra obtained by the Arecibo UHF radar at 18°N in Puerto Rico in June 1983. These spectra are much more homogeneous than previously available spectra since all of the data were observed by the same radar technique, the data in the different atmospheric regions were taken essentially simultaneously, and all of the spectra were analyzed using very similar methods. In the large-wavenumber ranges of the observed spectra, the asymptomatic slopes and amplitudes agree well with the saturated gravity wave spectral model developed by Dewan and Good (1986) and Smith et al. (1987), which has a slope of −3 and a spectral amplitude proportional to the buoyancy frequency squared. The good agreement between the model spectrum and the observed spectra from different altitude...

Gravity wave modulation of gradient drift instabilities in mid‐latitude sporadic E irregularities

Recent E region VHF backscatter echoes observed by the MU radar at mid-latitudes show quasi-periodic striations with a fairly constant range vs. time tilt in a RTI display. These features are explained in terms of gravity waves with frequencies close to the Brunt-Vaisala frequency which modulate the shape of sporadic E layers. The conditions of instability, when the magnetic field has a significant dip angle, is revised. Differing from previous work, we argue that conditions of local gradient drift instability are not sufficient and one has to consider the integrated properties of each magnetic filed tube. Stratified sporadic E layers are stable using this new criteria, unless they are distorted to produce unstable integrated gradients. Gravity waves with phase fronts parallel to the magnetic dip angle are capable of producing such distortion, imposing its own temporal and spatial periodicity on the echoes.

Observational Evidence of a Saturated Gravity Wave Spectrum in the Troposphere and Lower Stratosphere

Abstract Radial velocity and temperature data obtained at the MU Radar Observatory during October and November 1986 are used to examine the character of the motion spectrum in the troposphere and lower stratosphere. It is found that the spectrum is dominated by low-frequency gravity waves with an upward sense of propagation in the lower stratosphere and both upward and downward propagation in the troposphere. Vertical wavenumber spectra of velocity and temperature are used to examine the consistency of the motion spectrum with the saturated spectrum of gravity waves proposed by Smith et al. Results indicate excellent agreement of the observed and predicted velocity and temperature spectra in both amplitude and slope. Vertical wavenumber spectra in area-preserving form reveal a dominant vertical wavelength of ∼2.5 km, systematic variations in energy density and the dominant vertical scale with time, and consistency between the temporal variations of velocity and temperature variance. Taken together, our re...

A direct method for deriving drop-size distribution and vertical air velocities from VHF Doppler radar spectra

Abstract In precipitation environments, sensitive VHF Doppler radars have a capability to detect echoes from both refractive index irregularities and precipitation particles. The purpose of this paper is to propose a direct method to estimate the drop-size distribution N(D), the mean vertical air velocity and turbulence using Doppler spectra obtained by VHF Doppler radars. Bemuse the new method directly estimates turbulence as well as the mean vertical air velocity, the N(D) parameters, deduced from a least-squares fit approach, are free from cmrs inherent in conventional measurements using microwave Doppler radars. Temporal and spatial variations of N(D) and mean vertical air velocity during a cold front passage are then studied to demonstrate the capability of the present method.

Turbulent upwelling of the mid‐latitude ionosphere: 2. Theoretical framework

In a companion paper, data from the MU radar have been presented which show that during sunspot minimum conditions in Japan the mid-latitude ionosphere is sometimes characterized by regions of rapid and turbulent upwelling. In this paper we explore possible mechanisms for these events. The most likely process seems to be the instability of the equilibrium which occurs when the mid-latitude plasma is supported against gravity either by an eastward electric field component or by a southward neutral wind, as was proposed by Perkins (1973). We show, for example, that the growth rate determined by Perkins is considerably higher in sunspot minimum conditions than at sunspot maximum for comparable altitudes of the ionospheric F layer. The growth rate is not very large, however, and we argue here that the observed structures must evolve from preexisting undulations of the bottomside of the F region which are generated by gravity waves. That is, the gravity waves create finite amplitude structures which are amplified by the plasma instability. An intriguing feature of the gravity wave role in this process is that the echoing patches detected by the MU radar and the height bands detected by the Arecibo radar, which we believe to be related phenomena, all seem to propagate to the west. This is the same direction reported for the angle-of-arrival measurements of classic mid-latitude spread F by a number of researchers using ionosonde techniques. Since the MU radar detects the upwelling regions from nonthermal 3-m irregularities there must be mechanism to create such tracers. We propose that secondary structures are created at intermediate scales via the E×B instability operating on the dome of the upwelling structure and by the neutral wind-driven process on either the west or the east wall of the structure, depending on the direction of the zonal wind. The 3-m waves are then created in a cascade process which brings energy into a range of k space in which the structures are linearly damped. Finally, we discuss the fine structure of the echoing patches and suggest several plausible mechanisms, two of which involve E region coupling and one which deals with the vertical structure of the gravity wave seeding process.