Panel (a) shows the height-log|$(1/\tau _{1/2})$| distribution for an example date (25 July) after applying the cleaning criteria (see the text). The scale height, |$H_{kt}$|⁠, is adopted from MSIS model atmosphere. The contours are showing the number of detections relative to that of the peak height (⁠|$\sim 89$| km) and the gradient |$H_{D}$| is estimated at the contour level 0.4 which correspond to the height interval |$85-95$| km. Panel (b) shows distribution of |$\tau _{1/2}$| in logarithmic scale, while panel (c) shows the average annual variation (year: 2010–2020) of the ratio |$H_{D}/H_{kt}$|⁠.