Abstract. High-resolution radiosondes and calibrated radars operating close to 50 MHz, are used to examine the relationship between the strength of radar scatter and refractive index gradient. Three radars are used, in Kiruna in Arctic Sweden, at Gadanki in southern India and at the Swedish/Finnish base Wasa/Aboa in Queen Maud Land, Antarctica. Calibration is accomplished using the daily variation of galactic noise measured at each site. Proportionality between radar scatter strength and the square of the mean gradient of potential refractive index, M 2 , is found in the upper troposphere and lower stratosphere at all three sites, confirming previously reported results from many VHF radars. If the radar scatter is interpreted as Fresnel scatter, the constant of proportionality between radar scatter and M 2 is found to be the same, within the calibration uncertainties, for all three radars. The radiosondes show evidence of distinct layering with sharp gradients, extending over 10s of kilometers horizontally, but the scatter is found to be two orders of magnitude weaker than would be expected from true Fresnel scatter from such layers. Using radar reflectivities resolved to a few 100 ms, we show that this is due to strong temporal variability in the scattering conditions, possibly due to undulations of the scattering layers. The constancy of the radar scatter -M 2 relationship between the different sites suggests an unexpected uniformity in these perturbations between very different regions of the globe.

Analysis of one year measurements of in situ radon (222 Rn) and its progenies along with surface air temperature, relative humidity and pressure near to the Earth's surface has been carried out for the first time at the National Atmospheric Research Laboratory (NARL, 13.5• N and 79.2 • E) located in a rural site in Gadanki, south India. The dataset was analysed to understand the behaviour of radon in relation to the surface air temperature and relative humidity at a rural site. It was observed that over a period of the 24 hours in a day, the activity of radon and its progenies reaches a peak in the morning hours followed by a remarkable decrease in the afternoon hours. Relatively, a higher concentration of radon was observed at NARL during fair weather days, and this can be attributed to the presence of rocky hills and dense vegetation surrounding the site. The high negative correlation between surface air temperature and activity of radon (R = -0.70, on an annual scale) suggests that dynamical removal of radon due to increased vertical mixing is one of the most important controlling processes of the radon accumulation in the atmospheric surface layer. The annual averaged activity of radon was found to be 12.01±0.66 Bq m −3 and 4.25±0.18 Bq m −3 for its progenies, in the study period.

We present the results of a search for the ground‐state hyperfine transition of the OH radical near 53 MHz using the National Mesosphere–Stratosphere–Troposphere (MST) Radar Facility at Gadanki, India. The observed position was G48.4−1.4 near the Galactic plane. The OH line is not detected. We place a 3σ upper limit for the line flux density at 39 Jy from our observations. We also did not detect recombination lines (RLs) of carbon, which were within the frequency range of our observations. The 3σ upper limit of 20 Jy obtained for the flux density of carbon RLs, along with observations at 34.5 and 327 MHz, are used to constrain the physical properties of the line‐forming region. Our upper limit is consistent with the line emission expected from a partially ionized region with electron temperature, density and path lengths in the range 20–300 K, 0.03–0.3 cm−3 and 0.1–170 pc, respectively.

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Meteoroids are responsible for deposition of thousands of kilograms of annual mass flux in the Earth's upper atmosphere but the disintegration mechanisms of these bodies, and hence their composition, still remains a subject of debate in the meteor radar community. The role and significance of fragmentation as a meteoroid disintegration mechanism has been of particular interest in the past few years but in contrast to the head echoes, relatively little work has been done to study the effect and extent of fragmentation on trail echoes observed by the high power large aperture radars. Using the 53 MHz Gadanki MST radar, we present examples of radar meteor trails whose evolution cannot be explained with just the aid of classical meteor ablation theory. These examples are analyzed and discussed on a case-by-case basis and it is reported that the evolution of these trails can be explained with the help of fragmentation. This study will form the basis for future modeling efforts of fragmenting meteor trails and has important implications on the form in which the meteoroid mass is deposited in the upper atmosphere.