Proceedings with Extended Abstracts (single PDF file) - Radio ...

the upper region are similar to those observed at other radars [Yamamoto et al, 1991; Pan etal., 1998; Tsunoda et al., 1998] and the maximum occurrence period is between 23 and 05LT.The lower region echoes consists of QP and continuous features just like thoseobserved in the daytime ones. Again, we find that there is 45 % characterized QP featuresthat is the night LQP echoes [Pan and Rao, 2002]. The occurrence ranges of the night timeLQP echoes are slightly lower than the daytime LQP echoes. We note that 57 % of themappeared between 95 to 97 km ranges, 24 % between 92 and 94 km ranges and 13 % of themoccurred at other ranges. The periodicity features are similar to those of the daytime LQPechoes that the majority is around 2 minutes period. Unlike the horizontally distributeddaytime LQP echoes, most of the nighttime LQP present significant descending rate withabout 1.5 km/hour.Figure 3 presents the histograms of the mean radial velocity of the echoes for threecategories: (left) daytime echoes at 90-110 km ranges (central) nighttime echoes at 90 to 105km ranges, and (right) nighttime echoes at 105 to 140 km ranges. We notice the figure 3presents wider ranges of value from the left to the right plots. The mean radial velocities atthe lower E region vary between –30 and +30 m/s for daytime and –50 to +50 m/s during thenighttime and the values may vary from –100 to +100 m/s during the nighttime upper Eregion. Furthermore, the mean radial velocities of the lower E region echoes are close to zerofor both daytime and nighttime but a downward velocity is noticed at the upper E region.Figure 4 shows the histograms of the spectral widths of the similar regions as inFigure 3. It is clearly shown in Figure 4 that the daytime and the nighttime (for both upperand lower regions) echo present different distributions. The mean spectral widths in thedaytime lower E region present a bell shape distribution with the mean value of about 30 m/s.On the other hand, nighttime echoes are non-symmetric and are wider than those detected inthe daytime. The majority of the spectral width in the lower region is narrower (about 50 to60 m/s) than the one in the upper region (about 70 m/s).Figure 3. The histograms of the mean radial velocity of the echoes.Figure 4. The histograms of the spectral widths of the echoes.Summary and DiscussionsOver all, the following characteristics of the E region FAI observed by the Gadankiradar based on this data set are notified:(1) There are two echoing regions: the lower region between 90 and 100 km ranges and theupper region between 105 and 120 km ranges.(2) Echoes observed in both the upper and lower regions are similar to type 2 echoes reportedby other radars in the mid-latitudes.(3) Echoes of the lower region may occur in daytime as well as in nighttime. Although thereis an observation break between 06 and 09 LT, noontime (11 to 14 LT) seems to be theminimum period of occurrence. QP echoes are commonly detected at the lower region no116