New narrow‐beam meteor radar results at Christmas Island: Implications for diurnal wind estimation

Abstract

Abstract. Solar heating of the atmosphere is responsible for most of the diurnal oscillations in the neutral wind velocities and temperatures of the mesosphere and lower thermosphere. When these oscillations are global-scale waves, they are called atmospheric tides. Excited in the lower atmosphere, tides can propagate up into the upper atmosphere where they can break and deposit considerable energy and momentum into the mean flow. Ground-based estimates of the diurnal winds over Christmas Island have been made using a narrow-beam meteor radar system. Previously, data collected using the three-beam antenna configuration of the meteor radar were processed under the assumption that the vertical wind component was at least 2 orders of magnitude smaller than the horizontal component for all temporal scales of motion. The addition of two oblique beams to the radar configuration in late 1993 made it possible to estimate the horizontal wind field without applying the negligible vertical wind assumption, by using a coplanar analysis technique. Not only did the diurnal fits of the horizontal coplanar winds agree better with the results of the collocated mediumfrequency radar and model predictions, but also the Christmas Island meteor radar appears to be measuring a significant vertical velocity. This velocity has a diurnal amplitude of 10-15 m s -1 and maximizes at midnight across all heights. Under the assumption that this strong vertical motion is produced by geophysical phenomena, two hypotheses to explain this velocity are presented: the vertical motion associated with gravity wave breaking and the influence of strong electric fields on the ionized meteor trails.

Middle atmosphere dynamical studies at Resolute Bay over a full representative year: Mean winds, tides, and special oscillations

“…However, these studies were performed at an altitude of 105 km, well within the E region, and were performed for equatorial conditions. Only one study has yet shown any support for such contamination [Chang et al, 1999], and that was also at the equator. Thus both such studies were for high-altitude equatorial conditions, and any similar effects at the poles are unknown.…”

Comment on “Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system” by Karanam Kishore Kumar et al.

“…In this scenario, the same caution that Briggs [1977] made for spaced receiver experiments applies to the meteor systems operating in the vicinity of the magnetic equator. In the light of the previous report based on the meteor radar observations made from Christmas Island by Chang et al [1999], we remind the readers of the potential problems in making reliable neutral wind measurements with radar systems probing the electrojet heights.…”

“…MF, meteor and MST radars are being used to measure neutral winds in the mesosphere–lower thermosphere (MLT) region (80–100 km) with the advantage of continuous observation over a particular site. Radars operating near the magnetic equator have complexities in interpreting the measurements in terms of neutral wind, since the plasma drifts at electrojet heights are expected to contaminate neutral wind measurements [ Briggs , 1977; Chang et al , 1999; Gurubaran and Rajaram , 2000]. In this context we revisit the interpretations made by Kumar et al [2007], in particular, the affirmation that the meteor system makes reliable wind measurements at heights above 90 km.…”

“…Previous reports imply that nonspecular echoes detected by meteor radars operating in the vicinity of the magnetic equator are related to plasma irregularities undergoing E × B drift, since in this case the radar target is not the meteor trail itself but the irregularities that grow on the trail as a result of plasma instabilities [ Chapin and Kudeki , 1994; Chang et al , 1999; Oppenheim et al , 2000]. With theoretical simulations favoring the role of plasma instabilities in the generation of plasma waves within a meteor trail that could cause backscatter for the meteor radars, it needs to be ascertained whether the echoes detected over Trivandrum are not associated with plasma waves generated by any of the instability mechanisms that can operate within the meteor trail.…”

Reply to comment by R. Dhanya and S. Gurubaran on “Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system”

“…DG09 refer to Chapin and Kudeki [1994], Chang et al [1999], and Oppenheim et al [2000] in the context of irregularities that grow on the trail as a result of plasma instabilities. The study carried out by Chapin and Kudeki [1994] deals with long‐lived nonspecular meteor echoes, which are not considered for the wind estimation by the meteor radars.…”