Mesospheric winds and waves over Jicamarca on May 23-24,1974

Abstract

The VHF radar at Jicamarca (12.0°S, 76.9°W) was used to probe the mesosphere for 24 hours on May 23–24, 1974. The inferred zonal wind shows a strong eastward prevailing component below 75 km for these winter conditions, as would be expected from the annual and semiannual oscillations. The zonal winds are in good agreement in their region of overlap with rocket observations made at Ascension Island (8.0°S, 14.4°W) for the same period. This is the first direct confirmation that Jicamarca VHF observations are measuring mesospheric winds. Substantial wind oscillations are present, but the lack of nighttime echoes precludes a decomposition into tidal components. The dominant periodicity in the short-period oscillations changes with altitude, with the short-period cutoff decreasing from around 10 min at 70 km to 4 min at 80 km. This suggests both a local energy source for the oscillations and the importance of the background temperature structure in determining the wave characteristics. The variation of echo power with height in the two antennas that were used shows that 2–10 times more power was received on the average in the nearly vertical antenna than in the antenna offset from the vertical by 3.45° at heights below 75 km, suggesting the possibility that a ‘partial reflection’ mechanism is important in the 55- to 75-km region at a 50-MHz operating frequency. The powers in the two antennas become nearly equal above 75 km, with the possibility that slightly more power is received in the off-vertical antenna. The continuous scattered power becomes very small above 80 km; however, meteor echoes are frequently observed. Equatorial electrojet echoes dominate the echo returns above 85 km.