Exploring polar mesos pheric summer echoes

Year (1932–33), Sir Edward Appleton made early radar observations of the polar atmosphere from Simavik and Tromso, Norway (Appleton et al. 1937). Radar remains an important means of studying phenomena, including aurorae, in the polar mesosphere and ionosphere. Less widely known than aurorae, PMSEs are radar features produced in regions where a fascinating combination of physical and chemical processes occur (e.g. Rapp and Lubken 2004). They involve: global atmospheric dynamics and thermodynamics; chemical and smaller scale thermodynamical processes leading to the formation of ice particles at altitudes of 80–90 km, far above ordinary rain clouds; plasma mechanisms affected by the presence of charged particles with sizes up to about 0.1 μm. Some have speculated that monitoring of PMSEs over several decades will provide diagnostics of industrial impact on the mesosphere and perhaps regions in the lower atmosphere as well. The troposphere, stratosphere and mesosphere and their boundaries are associated with structure in the plot of temperature versus altitude (e.g. Salby 1996). The stratopause and mesopause bound the mesosphere at altitudes of about 50 km and 90 km, respectively. In standard models for non-polar latitudes the temperature of the mesosphere decreases nearly linearly with altitude from around 270 K to about 180 K. The temperature increases above the mesopause to somewhat less than 200 K at 100 km. The number density of electrons increases from 10–10 cm at the mesopause by over an order of magnitude in less than 10 km (e.g. Kazil et al. 2003). Solar activity causes variations in the solar wind and, hence, in the intensity and spectrum of non-thermal particles in the atmosphere. These variations can result in significant modifications of the electron altitude profile in the mesosphere. The properties and location of the mesopause usually change more markedly with time at polar Polar mesospheric summer echoes (PMSEs) are radar features observed around 80–90 km, much higher than ordinary clouds. The ways in which nanoparticles affect radar reflectivity near the summer polar mesopause have been elucidated through the use of rocket-borne “dust” detectors and observations of scattered laser light. Artificial heating of the radar scattering regions by powerful microwave transmitters and simultaneous radar observations have also provided important data. Further efforts to understand the mechanisms producing mesospheric radar echoes and their seasonal variations will help refine our knowledge of large-scale atmospheric dynamics. AbstrAct HArtquist et al.: Polar mesosPheric summer echoes