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.

The DE 2 satellite observed electric field fluctuations on the topside of the nighttime midlatitude ionosphere. They extended several hundred kilometers in the latitudinal direction with wavelengths of several tens of kilometers, and their amplitudes were a few millivolts per meter. Such fluctuations were often observed at magnetically conjugate points in the northern and southern hemispheres. These electric field fluctuations are perpendicular to the geomagnetic field. They are not accompanied by any significant plasma depletion or electron temperature v•riations. Magnetic field fluctuations are sometimes observed simultaneously with electric field fluctuations. We interpret that these fluctuations are caused by fieldaligned currents which flow from the ionosphere in one hemisphere to the conjugate point in the other hemisphere. The power spectrum of these midlatitude electric field fluctuations follows a power law of the form Power c• f-•, with the spectral index n of 3.5 to 4.5, which is steeper than that of the electric field fluctuations in the high-latitude ionosphere or in the equatorial ionosphere. This phenomenon may be related to other ionospheric phenomena, for example, the F region field-aligned irregularities or spre•d-F, observed by ground-b•sed methods such •s the MU r•d•r, but the relationship is not clear. (MEFs), which often appear simultaneously at magnetically conjugate points, and discuss the relation between these electric field fluctuations and the F region FAIs. In situ observations of the midlatitude ionospheric electric field by satellites play an important role in understanding the mechanism of the ionospheric irregularities which have been observed by ground-based techniques. Observation The DE 2 satellite flew in polar orbit at about 250-km to 900-km altitudes. It observed the ionospheric electric field at midlatitudes from August 1981 to February 21,439 21,440 SAITO ET AL.' MIDLATITUDE ELECTRIC FIELD FLUCTUATIONS a sampling rate of two samples per second [Krehbiel et al., 1981]. Vector magnetic field data were obtained by dinate (SPC) system, where the x axis is in the direction of the satellite velocity and the y axis is downward, with the z axis completing a right-handed coordinate system.

[1] A VHF Doppler radar with an active phased-array antenna system, called the Equatorial Atmosphere Radar (EAR), was established recently at the equator near Bukittinggi, West Sumatra, Indonesia (0.20°S, 100.32°E, 865 m above sea level). The EAR is a large monostatic radar which operates at 47.0 MHz with peak output power of 100 kW. The EAR uses a circular antenna array, approximately 110 m in diameter, which consists of 560 three-element Yagi antennas. Each antenna is driven by a solid-state transmitter-receiver module. This system configuration allows the antenna beam to be steered electronically up to 5,000 times per second. The scientific objective of the EAR is to advance knowledge of dynamical and electrodynamical coupling processes in the equatorial atmosphere from the near-surface region to the upper atmosphere. The equatorial atmosphere over Indonesia is considered to play an important role in global change of the Earth's atmosphere. This paper presents the system description of the EAR, including observational results of the equatorial atmosphere made for the first time with altitude resolution of 75-150 m.

Midlatitude field‐aligned irregularities (FAI) associated with sporadic E layers were continuously observed on June 17–19, 1989, with the middle and upper atmosphere radar to investigate spatial and temporal behaviors of the 3.2‐m‐scale FAI. Using Doppler spectrum, mean Doppler velocity, and echo intensity data, we present some features of the plasma turbulence and irregularity movement pertinent to two types of radar echoes: “continuous echoes” appearing at 90‐ to 100‐km altitudes at night and in the morning, and “quasi‐periodic (QP) echoes” appearing at 95‐ to 125‐km altitudes at night. For both types of echoes, echo intensities and Doppler velocities were modulated with periods of 5–20 min due to gravity wave activities. Most of the enhanced echoes were located within the region where Doppler velocities tend to be close to zero. On many occasions there were velocity shears across the enhanced echo region. The Doppler velocities and echoing‐patch movements for the continuous echoes are compared with the existing F region plasma drift and neutral wind models, respectively, to find a partial consistency. General characteristics of the Doppler velocity and spectral width associated with the QP echoes are consistent with previous observational results. Unusual movement of the FAI sheet related to the QP echoes is discussed in the light of recent theory.

Abstract. Regional variability of raindrop size distribution (DSD) along the Equator was investigated through a network of Parsivel disdrometers in Indonesia. The disdrometers were installed at Kototabang (KT; 100.32 • E, 0.20 • S), Pontianak (PT; 109.37 • E, 0.00 • S), Manado (MN; 124.92 • E, 1.55 • N) and Biak (BK; 136.10 • E, 1.18 • S). It was found that the DSD at PT has more large drops than at the other three sites. The DSDs at the four sites are influenced by both oceanic and continental systems, and majority of the data matched the maritime-like DSD that was reported in a previous study. Continental-like DSDs were somewhat dominant at PT and KT. Regional variability of DSD is closely related to the variability of topography, mesoscale convective system propagation and horizontal scale of landmass. Different DSDs at different sites led to different Z-R relationships in which the radar reflectivity at PT was much larger than at other sites, at the same rainfall rate.

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.

No abstract

[1] The Middle and Upper atmosphere Radar (MUR) was upgraded in March 2004 for radar imaging capability with 5 frequencies across a 1 MHz bandwidth and 25 digital receivers. Although digitization introduces problems of its own, the uniformity of digitization is a great benefit over the analogue system in place before. This increased reliability will help make the new system an important component of long-term atmospheric science programs. We demonstrate 3-D imaging with Capon's method, which can provide information about structure morphology. In addition, we demonstrate an experimental 0.5 ms pulse mode and compare this to Capon method imaging results.