JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. D6, PAGES 11,685-11,693, JUNE 20, 1995 On the lack of southern hemisphere polar mesosphere summer echoes B. B. Balsley, • R. F. Woodman, 2 M. Sarango, 3 R. Rodriguez, 3 J. Urbina, TM E. Ragaini,3, • J. Carey,• M. Huaman,• and A. Giraldez 6 Abstract. We report VHF radar observations of the southern high-latitude mesopause region using wind profilers that were installed recently on King George Island, Antarctica, and Ushuaia, Argentina. Briefly, our observations, which were made during January and February 1993, show almost no evidence of so-called polar mesosphere summer echoes, or PMSE. Since these echoes are a predominant feature of the northern high-latitude mesospherein summer, their absence in the southern hemisphere is both surprisingand intriguing. In this paper we present evidence demonstrating the virtual absence of the echoes and demonstrate that our systemswere capable of detecting them had they been present. We also outline some of the consequences of this intriguing result, which are supportedby observed hemisphericdifferencesin polar mesosphericclouds, mesospheric temperatures, upper atmospheric gravity wave activity, and mean circulation patterns. 1. Background Since they were first reported more than a decade ago by Ecklund and Balsley [1981], polar mesospheresummer ech- oes, or PMSE, have been a challenging and intriguing topic for middle atmosphericresearch. An excellent review of the status of both theory and observations of PMSE has been provided recently by Cho and Kelley [ 1993]. Another recent review by Thomas [1991] discusses the possible relationship between PMSE and both noctilucentclouds (NLC) and polar mesospheric clouds (PMC). As observed at VHF (•50 Mhz), northern-hemispheric PMSE exhibit the following characteristics: (1) They com- prise a thin but intense echoing region near the summer mesopause (•86 km), with a half power thickness of about 1 km. (2) While somerelatively strong, albeit sporadic, echoes have been reported at latitudes as low as 52øN, the strongest, most continuousechoesare observedat latitude poleward of about 65øN. (3) The echoes appear around mid-May, last until mid-August, and are relatively continuous, with a slight minimum of occurrence near 1700 LT. (4) The height range and seasonal variations of PMSE correlate very well with those of the cold summer mesopause (i.e., the coldest known atmospheric region). (5) Joint observations using VHF ra- dars and sounding rockets show that intense PMSE are often associated with sharp "biteouts" in the ambient electron density. Except for our short note published earlier [Balsley et al., 1993], there have been no directed studies of PMSE in the •Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder. 2jicamarca Radio Observatory, Lima, Peru. 3Departamento deFfsica, Universidad dePiura, Piura, Peru. 4Now at University of Illinois, Urbana. 5Now at Politecnico di Milano, Milan, Italy. 6Laboratorio Ionosf6rico dela Armada Argentina, Buenos Aires. Copyright 1995 by the American Geophysical Union. Paper number 95JD00510. 0148-0227/95/95JD-00510505.00 southern hemisphere. Although a few VHF radars have been in operation at high southern latitudes over the past decade or so, their primary mission has been directed to higher ionospheric heights to study the radar aurora australis. The purposeof this paper is to provide a detailed report of our initial attempts to observe PMSE at high southern latitudes. Most of our data were gathered using the VHF wind profiler recently installed at the Peruvian base "Machu Picchu" on King George Island, Antarctica (62o06 'S, 58ø28'W). Additional data were obtained from a VHF pro- filer temporarily installed at Ushuaia, Argentina (54ø47'S, 68ø18'W). The Machu Picchu profiler is a cooperative project between the University of Colorado at Boulder and a num- ber of Peruvian institutions (Peru's Geophysical Institute, the University of Piura (Peru), and the Peruvian Navy). The Ushuaia installation, in addition to the aforementioned groups, involves the cooperation of the Laboratorio Ionos- f6rico de la Armada Argentina (LIARA) of the Republic of Argentina. Our observations at Machu Picchu encompassthe period from early January to mid-February 1993. Mesosphere- lower thermosphere (MLT) observations between •70 km and 95 km were made typically between 1500 and 1800 UT. Exceptions include two long "runs" of 14 hours and 21 hours and a few other brief periods. Useable data periods comprisesome 20 separatedays. A similar observingsched- ule was used for the Ushuaia profiler. The remaining time at both stations was dedicated to observations of the tropo- sphere and lower stratosphere. The results of our initial southern hemispheric PMSE studiesare both surprisingand intriguing. As we will discuss below, our entire data set shows almost no evidence of PMSE. Both the latitude and the season were favorable for PMSE observations (January-February in the southern hemisphere corresponds in season to July-August in the northern hemisphere). The Machu Picchu radar, at •62øS, is comparable in latitude to Poker Flat, Alaska (65ø8'N), where the initial PMSE observations were made. The Ushuaia profiler, on the other hand, although it is some 7ø farther 11,685