A VHF backscatter radar with operating frequency 30.8？MHz has been operated at Kototabang (0.20°S, 100.32°E; dip latitude 10.36°S), Indonesia, since February 2006. We analyzed E-region field-aligned irregularities (FAIs) observed by this radar through a year of 2007 and found that the E-region FAI observed at Kototabang can be classified into two groups. One is “descending FAI”. Altitude of the FAI echo region descends with time from 102？km to 88？km altitude during 0700–1000 and 1900–0000？LT in June solstice season. The other is “low-altitude FAI”, which is observed in an altitude range from 88 to 94？km mainly during nighttime. The observed Doppler velocity show distinct local time and altitude dependence. The seasonally averaged zonal velocity above (below) approximately 94？km altitude is westward (eastward) during daytime and eastward (westward) during nighttime. Meridional/vertical velocity perpendicular to the geomagnetic fields is upward during daytime and downward during nighttime. The direction of the FAI velocity above approximately 94？km altitude is consistent with that of the background E × B plasma drifts reported previously. 1. Introduction VHF, UHF, and L-band radars are used for observations of the Bragg scatter echo from field-aligned irregularities (FAIs) with a spatial scale of one half the radar wavelength. Regarding the E-region FAIs at mid-latitudes, using a powerful radar, middle and upper atmosphere (MU) radar at Shigaraki, Japan, Yamamoto et al. [1] have discovered two types of the E-region FAI echoes: quasiperiodic (QP) and continuous echoes. The QP echoes appearing intermittently at altitudes of sporadic E (Es) layer (above 100？km) with periods of 5–20？min. Morphological features of the QP echoes have been reported by Yamamoto et al. [2]. Occurrence rate of the QP echoes shows distinct seasonal variation with a maximum in summer and rises abruptly after the local sunset and decreases after midnight. Numerous experimental and theoretical studies have been made to disclose the mechanisms generating the QP echoes (e.g.,[3–7]). On the other hand, the continuous FAI echoes appear continuously in time at low-altitude of the E-region (90–100？km), mainly around sunrise and occasionally at postmidnight [8]. However, studies of the E-region FAIs at low latitudes are limited compared to those at the mid-latitude regions. Using the Piura VHF radar in Peru (5.2°S, 80.6°W; dip latitude 7°N) and the Gadanki MST radar in India (13.5°N, 79.2°E; dip latitude 6.3°N), Chau et al. [9] and Patra et al. [10] have reported statistical characteristics
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